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Retinoid X Receptor Signalling in the Specification of Skeletal Muscle Lineage

Written By

Melanie Le May and Qiao Li

Submitted: 02 April 2012 Published: 22 August 2012

DOI: 10.5772/45879

From the Edited Volume

Skeletal Muscle - From Myogenesis to Clinical Relations

Edited by Julianna Cseri

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1. Introduction

Pluripotent stem cells have the capacity to develop into different cell lineages, and can be promoted into skeletal muscle lineage through the use of small molecule inducers. Retinoic acid (RA) signaling through the retinoic acid receptor (RAR) and retinoid X receptor (RXR), is important for embryonic development, and is able to enhance myogenic differentiation in vitro if used in combination with other small molecule inducers. Nevertheless, it only yields moderate results in promoting the differentiation of embryonic stem (ES) cells into skeletal myocytes. RXR isalso known to be essential for embryonic development, but it is generally considered to act as a silent partner for other nuclear receptors such as RAR. We recently discovered that RXR selective ligand efficiently induces myogenic differentiation in mouse ES cells which respond poorly to RA. In addition, myogenic differentiation, enhanced by the RXR ligand, is mediated through a RAR independent mechanism, and recapitulates closely the sequential events observed in vivo. Since ES cell differentiation represents the properties of early developing embryo, efficiently generating skeletal myocytes with RXR selective ligand provides means to further scrutinize signaling pathways in skeletal myogenesis, in view of developing cell-based therapies for skeletal muscle-related diseases.In this chapter, we attempt to provide an in-depth analysis of recent research findings and the current stage of knowledge in the field of skeletal myogenesis.

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2. The retinoid X and retinoic acid receptors

RXR belongs to the nuclear hormone receptor superfamily, such as steroid hormone, thyroid hormone, vitamin D receptors, and nuclear receptors including RAR, PPAR, LXR and PXR (Szanto et al., 2004). It is a very unique protein with the ability to form heterodimers with one third of the 48 other nuclear receptors (Mangelsdorf et al., 1995) giving it the potential to converge a large array of signaling pathways. The RXR can form homodimers, permissive heterodimers, and non-permissive heterodimers in a ligand-dependent or -independent manner (Tanaka and De Luca, 2009). When RXR forms homodimers or permissive heterodimers (with PPAR, LXR, PXR etc.), it is amenable to RXR ligand-dependant activation since the activation domain of the partner receptor is placed in proximity to RXR helixes. Once RXR is activated by the ligand, conformational changes cause direct stabilization of the activation domain of its partner (Gampe, Jr. et al., 2000b). When RXR forms non-permissive heterodimers (with RAR, VDR, TR etc.), it is not activated by ligand, as the binding of the partner receptor to RXR allosterically inhibits it (Kurokawa et al., 1994; Tanaka et al., 2009). Furthermore, the activation domain of the partner is not located in proximity to ligand activated residues in the RXR interface (Bourguet et al., 2000; Gampe, Jr. et al., 2000b).

2.1. DNA binding

The receptor dimers of RXR and its partner, constitutively bind to specific DNA response elements in the promoters or enhancers of the genes they govern. DNA binding specificity is determined by the number of spacer nucleotides present between two direct repeats of the canonical binding sequence 5’-PuGGTCA (Leid et al., 1992; Umesono and Evans, 1989). The RXR/RAR heterodimers bind to the retinoic acid response element (RARE) with a consensus half site separated by 2 or 5 nucleotides (DR2 or DR5), whereas the RXR homodimers bind to the retinoid X response element (RXRE) separated by only one nucleotide (DR1) (Tanaka et al., 2009) (Figure 1). Selective response element recognition is due to a short sequence (the P box) located at the C-terminal base of the N-terminal C1 finger of the DNA binding domain (DBD) which interacts with the binding motif, and also due to a weak dimerization function which encompasses the N-terminal base of the CII finger (D-box) of the DBD (Danielsen et al., 1989; Green et al., 1988; Kumar and Chambon, 1988; Luisi et al., 1991; Mader et al., 1989; Umesono et al., 1989). While RXR/RAR heterodimers bind more effectively to the RAREs than RXR homodimers, RXRs homodimers can bind RXREs with high affinity (Zhang et al., 1992). RAREs can overlap with RXREs, and since RXR/RAR heterodimers bind with a higher affinity than RXR homodimers, (Tanaka et al., 2009), this may interfere with RXR signaling.

Figure 1.

The Binding of RXR/RAR Heterodimer and RXR Homodimer to DNA. RXR/RAR heterodimers (left) and RXR homodimers (right) bind via the DNA binding domain to two direct repeats of the canonical binding sequence 5’-PuGGTCA separated by 2 or 5 nucleotides, or 1 nucleotide respectively.

2.2. Ligands of RXR and RAR

While RXR and RAR constitutively bind to DNA, they require agonist binding to activate gene transcription. Several endogenous ligands are well characterized and many synthetic ligands have been developed.

RA, the active derivative of vitamin A, can exist as two isoforms: all-trans RA and 9-cis RA. RAR bind and are activated by all-trans RA as well as its 9-cis isomer, while the RXR bind and are activated only by 9-cis RA (Ricaud et al., 2005). However, due to the considerable difficulty of detecting 9-cis RA endogenously in embryos or in adult tissue (Niederreither and Dolle, 2008), there has been debate about the in vivo role of activated RXR, and has led to the belief that RXR serves only to orient and position the heterodimers properly on the DNA (Perlmann and Jansson, 1995; Willy et al., 1995; Willy and Mangelsdorf, 1997)

In the last two decades, a wide range of RXR selective compounds has been engineered. The synthetic RXR ligands can act as agonists and activate both homodimers and permissive heterodimers. Conversely, they can also act antagonistically of homodimers, as is the case for the synthetic ligand LG100754, and promote only the activation of non-permissive heterodimers (Lala et al., 1996). Bexarotene (LGD1069) is a synthetic RXR selective compound used in the treatment of cancer. It is unable to transactivate the RXR-RAR heterodimer (Lehmann et al., 1992) and will not activate RARs (Nau et al., 1999).

There are conflicting interpretations of RXR participation in the activation of RXR/RAR heterodimers. Some studies demonstrate that allosteric inhibition of RXR in the RXR/RAR heterodimer only occurs when the RAR is unliganded and that this inhibition is relieved once RAR is liganded(Forman et al., 1995; Lala et al., 1996). Other reports indicate that both receptors bind their ligands independently and that their effectsare additive (Kersten et al., 1995). The discrepancy between varying reports can possibly be reconciled by the fact that different ligands interact with distinct side chains in the ligand binding domain and thus mediate differential activation of the receptor complex. The exact response is therefore highly dependent on the identity of the ligand and cannot simply be classified as agonistic versus antagonistic. Although RXR can engage in ligand binding when RAR is ligand occupied and/or if a suitable synthetic ligand is present (Chen et al., 1996; Kersten et al., 1996; Lala et al., 1996; Minucci et al., 1997; Roy et al., 1995), bexarotene is unable to transactivate the RXR/RAR heterodimer (Lehmann et al., 1992). In fact, bexarotene has been reported to reduce interactions between RXRs and RARs whereas ligand such as 9-cis increases the binding of RXRs to RARs (Dong and Noy, 1998).

All-trans RA does not bind RXR (Mangelsdorf et al., 1992), and more importantly, although all-trans RA has the ability to isomerize to 9-cis RA, pharmacological doses of all-trans RA are required to generate enough 9-cis to activate the RXRs (Mic et al., 2003). Optimal enhancement of skeletal myogenic differentiation requires low concentrations of all-trans RA. Thus, all-trans RA isomerization is simply not a feasible explanation to the similar enhancement of myogenic differentiation by RA and bexarotene observed in P19 stem cells(Le May et al., 2011). Finally, while RA metabolites, such as 4-oxo-RA, were originally believed to play a role in RA signaling, they have more recently been shown as physiologically not required (Niederreither et al., 2002; Pijnappel et al., 1993).

2.3. The interaction of RXR and RAR with their cofactors

In response to ligand activation, RXR and RAR bind co-activators and the respective binding of cofactors again depends on the identity of the ligand. Agonist binding induces large conformational changes within the receptor causing helix 11 and 12 (the AF-2 domain) to close the lid of the ligand binding pocket and generate high affinity co-activator binding sites. This charged surface has a high affinity for a specific amino acid motif, LXXLL, which mediates the binding of co-activators to nuclear receptors (Westin et al., 1998). Alternatively, if an antagonist or partial agonist binds, helix 12 is repositioned to an adjacent groove on the LBD and a charged surface that favors the co-repressor binding motif is formed (Perissi et al., 1999).

Co-activators, as their name implies, have the ability to activate transcription and interact with the basal transcriptional machinery, bridge and direct the assembly of transcriptional pre-initiation complexes, and induce chromatin remodeling (Rosenfeld et al., 2006; Bastien and Rochette-Egly, 2004). Co-activators such as p300, CREB Binding Protein (CBP), and p300/CBP-Associated Factor (PCAF) can all act as histone acetyltransferases (HATs) (Niederreither et al., 2008; Ogryzko et al., 1996) and form large multimolecular complexes.

Interestingly, co-activators p300 and CBP are also able to acetylate proteins other than histones, such as transcription factors (Gu and Roeder, 1997; Li et al. 1998; Li et al. 1999). CBP and p300 are heavily autoacetylated and upon recruitment to the receptors, can acetylate more of themselves in an intermolecular fashion (Karanam et al., 2006). In addition to this, they have the ability to recruit PCAF(Yang et al., 1996), a coactivator involved in myogenesis. p300 influences RXR activity as RXR are subjects for p300 acetylation, which promotes their binding to RXRE and increases their transcriptional activity as well (Zhao et al., 2007). Co-activators play crucial roles in gene activation, however, those recruited by particular RXR dimers at specific genetic loci in response to ligand have yet to be identified.

Alternatively, in the absence of ligand, the co-repressors, such as the nuclear receptor corepressor (NCoR) and silencing mediator of retinoic acid and thyroid hormone receptor (SMRT) family, bind and recruit a mulitprotein complex containing the histone deacetylase HDAC3 (Guenther et al., 2000; Li et al., 2000). The recruited histone methyl-transferases and histone deacetylases stabilize the nucleosome structure so that the DNA is inaccessible for transcription (Niederreither et al., 2008).

When RXR forms permissive heterodimers (i.e.: RXR/PPAR), neither receptor binds the co-repressors under normal circumstances (DiRenzo et al., 1997). Ligand binding to one receptor recruits the co-activators and although the other receptor may be unliganded, the high local concentration of bound co-activators favor the docking of the second LXXLL motif with the co-activator binding sites of the other receptor. If ligand is present for both receptors of the permissive heterodimer, they can synergistically recruit co-activators (Ahuja et al., 2003).

Non-permissive heterodimers (i.e.: RXR/RAR) do bind co-repressors and this binding to unliganded RXR and its partner is stabilized by both receptors. Transactivation requires ligand binding to the RXR partner (i.e.: RAR) to convert it into the agonist conformation, displace co-repressors, and recruit co-activators (Zhang et al., 1999; Vivat et al., 1997). As with permissive heterodimers, synergistic recruitment of co-activators occurs when ligands are present for both receptors (Ahuja et al., 2003).

2.4. RXR and RAR in development

Gene mutation studies have determined that both RXR and RAR are essential for proper development, and delineated roles and tissue expression patterns for the different isoforms of the two receptors (α, β, and γ). The different RAR and RXR isotypes are encoded by different genes and their isoforms differ in their NH2-terminal regions which are generated by differential promoter usage and alternative splicing (Chiba et al., 1997). While RXR-α null embryos show defects in RXR/PPARγ (Peroxisome Proliferator Activated Receptor) signaling, the RARs appear to be the most important partners for RXRs (Ahuja et al., 2003).RXR/RAR non-permissive heterodimers have been extensively studied in the context ofdevelopment.

During development, RXR-α and β are ubiquitously expressed with the highest levels of RXR-α present in the liver, heart, intestines, kidney, spleen, placenta, and the epidermis (Ahuja et al., 2003; Pratt et al., 1998). RXR-γ is expressed in all developing skeletal and cardiac muscles, the anterior pituitary, and the brain. The expression of RXR isoforms is tissue specific and often overlaps,yet occasionally certain isoforms are uniquely expressed.(Mangelsdorf et al., 1992) RXR-α is the primary isoform and supports the activity of all three RARs. Furthermore, RXR-α may be important in the expression of RXRγ since the RXR gene contains a RXRE(Barger and Kelly, 1997).

Studies with mice lacking expression of RXR-α have found that these mice die in utero as a result of hypoplastic myocardium (Kastner et al., 1994; Sucov et al., 1994) and RXR-α null mutations exhibit growth retardation, webbed digits (Mark et al., 2006) and defects in the chorioallantoic placenta (Sapin et al., 1997). Loss of RXR-β and RXR-γ is not as severe since they can be compensated for by RXR-α (Tanaka et al., 2009), which may explain why the RXRγ-/- mouse mutants are viable and have no muscular defects even in compound mutant combinations (Dolle, 2009).

Similarly, animals lacking RAR-α or RAR-γ result in postpartum lethality (Lohnes et al., 1993). In RAR knock-out studies where two RARs are deleted, the mutants display a spectrum of defects that resemble vitamin A deficiency syndrome (Lohnes et al., 1993; Lufkin et al., 1993) and the function of the residual RAR is highly dependent on RXR-α (Ahuja et al., 2003).

Even in normal development, the RARs are highly dependent on the RXRs. Homodimer formation of RARs is energetically unfavored, because of the limited contact between the interfaces. Pairing with RXR creates an extended area of intermolecular contact that stabilizes the heterodimer formation. This substantially larger surface area and consequent stability, results in the preferential formation of RXR/RAR heterodimers(Bourguet et al., 2000; Gampe, Jr. et al., 2000a).

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3. Skeletal myogenesis

Skeletal muscle forms in the embryo from paraxial mesoderm, which segments into somites on either side of the neural tube and notochord (Christ and Ordahl, 1995). Extracellular signals from surrounding tissues play a significant role in muscle development. These signals include members of the Wnt family, specifically Wnt1 and Wnt7asecreted from the neural tube and surface ectoderm(Cossu and Borello, 1999), Sonic Hedge Hog (Shh) secreted by notochord and floor plate cells and which acts in conjunction with Wnt1 (Cossu et al., 1999), bone morphogenetic protein4 (BMP4) secreted by the lateral plate mesoderm cells (Borycki et al., 1999; Dietrich et al., 1998; Munsterberg et al., 1995; Pourquie et al., 1996; Tajbakhsh et al., 1998), and RA which is under tight regulatory control for its synthesis, degradation, and transport(Rohwedel et al., 1999). These act on downstream targets such as HOX genes, which controls specification of the body axis (Rohwedel et al., 1999), Brachyury T, a protein required for posterior mesoderm and notochord differentiation (Skerjanc, 1999), and the myogenic regulatory factors (MRFs) including Myf5, MyoD, myogenin, and Mrf4 which are required for the commitment and maturation of skeletal muscle (Cossu et al., 1999; Rohwedel et al., 1999; Skerjanc, 1999).

3.1. Myogenic regulatory factors and their cofactors

The formation of myoblasts from myogenic progenitors and their successive cell cycle arrest and differentiation into mature skeletal muscle involves two key families of transcription factors. The MyoD family of basic Helix-Loop-Helix (bHLH) proteins which includes the four master transcriptional regulators (also referred to as MRFs): Myf5, MyoD, myogenin, and Mrf4 (Arnold and Braun, 2000; Braun et al., 1989; Braun et al., 1990; Davis et al., 1987; Edmondson and Olson, 1990; Froeschle et al., 1998) and the myocyte enhancer factor 2 (MEF2) family of MADS-box transcription factors which includes MEF2A, -B, -C, and –D (Naya and Olson, 1999).

Myf5 and MyoD are involved in skeletal muscle specification and commitment and have the capacity of remodeling chromatin and opening gene loci that participate in further muscle differentiation (Bergstrom and Tapscott, 2001), whereas terminal differentiation is governed by myogenin and MRF4. Each MRF is sufficient to dominantly induce myogenesis when introduced into a variety of non-muscle cells (Olson, 1990; Weintraub, 1993), and ectopic expression of MyoD can inhibit cell cycle before the S phase independently of its DNA binding and the induction of myogenic differentiation (Crescenzi et al., 1990; Sorrentino et al., 1990).

Members of the MEF2 family alone are not sufficient to induce myogenesis, however the ability of the MRFs to convert cells is reliant on the function of the MEF2 family. MEF2 proteins bind as homodimers and heterodimers to the consensus sequence YTA(A/T)4TAR found in the promoter region of nearly every known muscle-specific gene (Black and Olson, 1998), and together with the myogenic bHLH proteins, synergistically activate the transcription of myogenic genes. Unlike the MRFs, MEF2 genes are also expressed outside the skeletal muscle lineage in tissues such as cardiac and smooth muscle (Black et al., 1998; Edmondson et al., 1994; Leifer et al., 1993; Lyons et al., 1995).

Figure 2.

Involvement of Myogenic Regulatory Factors in Myogenesis. Myf5 and MyoD are involved in specification and commitment of muscle progenitors into skeletal muscle lineage. Mrf4 also plays a role as a determination gene in addition to directing terminal differentiation along with myogenin.

The bHLH domain of the MRFs is responsible for DNA binding and for dimerization with the ubiquitously expressed bHLH E protein (Hu et al., 1992; Murre et al., 1989; Parker et al., 2006). The resulting myogenic bHLH-E heterodimers bind to DNA at the consensus sequences known as an E-box (CANNTG), specific DNA motifs present at muscle gene enhancers and/or promoters, where they regulate gene expression (Sartorelli and Caretti, 2005). These genes include cytoskeletal, sarcomeric, metabolic, and cell signaling proteins (Angus et al., 2001; Gramolini and Jasmin, 1999; Kraner et al., 1999; Li and Capetanaki, 1993; Lin et al., 1991; Marsh et al., 1998; Shield et al., 1996; Simon and Burden, 1993; Wheeler et al., 1999). A requirement for the MyoD family of transcription factors in this combinatorial complex is demonstrated by the fact that the E protein homodimers bind to the same DNA sequences as the MyoD-E protein heterodimers,yet only the MyoD-E protein complex can cooperate with MEF2 factors (Naya et al., 1999). Furthermore, the MRFs and MEF2 factors activate and repress each others transcription in a complex network (Arnold and Winter, 1998; Bergstrom et al., 2002; Cserjesi and Olson, 1991; Olson and Klein, 1994; Wong et al., 1994). For example, expression of myogenin requires MEF2, while myogenin activates the expression of MEF2 independently of other skeletal gene products (Cserjesi et al., 1991; Ridgeway et al., 2000). Similarly, MRFs can positively regulate their own transcription and transcription of each other, creating positive auto- and cross-regulatory loops (Braun et al., 1989; Thayer et al., 1989)

3.2. Roles of meox and pax in the specification of myogenic progenitors

Signals from surrounding tissues activate the premyogenic program, and result in the expression of transcription factors such as Pax3, and Meox1/2 that specify cells into the skeletal muscle lineage and mediate the induction of the MRFs (McDermott et al., 2005; Petropoulos and Skerjanc, 2002; Petropoulos et al., 2004; Ridgeway and Skerjanc, 2001; Williams and Ordahl, 1994).

Pax3, a transcription factor with homeo and paired domain motifs, is thought to be activated by Wnt6a (Fan et al., 1997) and is responsible for both delamination and migration of muscle progenitors to the limb bud (Tajbakhsh et al., 1997). Pax3 is initially expressed throughout the somite before becoming restricted to the dermomyotome and subsequently the migratory muscle progenitor cells (Goulding et al., 1994; Williams et al., 1994). The importance of Pax3 in the delamination and migration of muscle progenitor cells is highlighted by the fact that mice which are Pax3 null have severe muscle loss (Alvares et al., 2003; Bladt et al., 1995; Dietrich et al., 1999; Epstein et al., 1996; Grifone et al., 2005).

Pax3 directly regulates the expression of Myf5 through the limb bud enhancer of Myf5 gene(Bajard et al., 2006) and acts with Myf5, upstream of MyoD which cannot be properly expressed in the Pax3/Myf5 double knockout (Tajbakhsh et al., 1997). It is when the migrating cells reach the limb bud that they begin to express Myf5 and MyoD, and it is both before and after activation of these genes that the cells undergo extensive proliferation (Buckingham et al., 2003; Tajbakhsh and Buckingham, 1994). Pax3, along with additional factors such as Myf5, c-met, Msx1 and the fibroblast growth factor (FGF) family of receptors promote myoblast proliferation. Proliferation is arrested by inhibitory signals which promote differentiation by inducing cell cycle arrest proteins such as MyoD (Alric et al., 1998).

Meox1 and Meox2 are closely related homeobox genes with mesoderm and mesenchyme specific expression during mouse development (Candia et al., 1992). Meox1 is expressed in the dermomyotome whereas after delamination and migration to the limb bud, Meox2 is predominantly expressed (Candia et al., 1992; Candia and Wright, 1996). In Meox2 deficient limb buds, Pax3 and Myf5 are downregulated and mice homozygous for a null mutation in Meox2 have defects in limb muscle differentiation resulting in an overall reduction in muscle mass and absence of specific muscles (Mankoo et al., 1999). It is only the compound mutant embryos of Meox1-/-; Meox2-/- that display a dramatic phenotype associated with disrupted somite development. In these embryos, the axial skeleton fails to develop and most skeletal muscles are absent or reduced in size (Mankoo et al., 2003).Interestingly, in cell cultures, overexpression of Meox1 does not induce myogenesis and while a dominant negative Meox1 has been shown to downregulate Pax3 and Gli2 expression and inhibit myogenesis in the P19 stem cells (Petropoulos et al., 2004), Meox1 mutant mice exhibit mild defects in sclerotome-derived vertebral and rib bones (Mankoo et al., 2003) rather than showing any overt muscle defects.

3.3. Roles of histone acetyltransferases in myogenic expression

Not only are extracellular signals crucial for the proper induction of MRFs, but intracellular prompts involving acetyletransferases play a fundamental role as well. CBP and p300 are required for growth arrest and apoptosis (Vo and Goodman, 2001), and along with PCAF are required for terminal differentiation of myoblasts and transactivation of muscle specific promoters such as myosin heavy chain (MHC) and muscle creatine kinase (MCK) (Eckner et al., 1996; Polesskaya et al., 2001; Puri et al., 1997a; Puri et al., 1997b; Yuan et al., 1996). Embryonic stem cells lacking p300 or its HAT activity are strongly impaired in their ability to activate Myf5 and MyoD (Roth et al., 2003). When properly expressed, Myf5 and MyoD, in cooperation with MEF2 transcription factors and with p300 and CBP, mediate the activation of the secondary MRFs, myogenin and Mrf4. p300 has been shown to bind directly to MyoD (Sartorelli et al., 1997; Yuan et al., 1996), and both p300 and PCAF play a critical role in the maximal MyoD dependant transactivation; p300 acetylates histones H3 and H4 and recruits PCAF to the promoter whereas PCAF acetylates MyoD to enhance transcription initiation, increase its affinity for DNA binding, and facilitate heterodimer formation with E proteins (Dilworth et al., 2004; Puri et al., 1997a; Sartorelli et al., 1999). However, MyoD has also been found to be acetylated in proliferating myoblasts where it is inactive, therefore further mechanisms besides simply acetylation are required for MyoD activation (Polesskaya et al., 2000).

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4. Impact of extracellular cues on MRF expression

Ligands of RAR and RXR play important roles in the activation of myogenesis and this activation is highly dependent on the identity of the ligand. RA is required for proper somite formation (Maden et al., 1996; Maden et al., 2000; Niederreither et al., 1999), induction of specification genes Meox1, Meox2, and Pax3, and counteracts inhibitory signals such as BMP4 (Kennedy et al., 2009). RA signaling intersects with that of BMP4, as BMP4 and RA function antagonistically and have the capacity to counteract each other's inhibition of entry into skeletal and cardiac muscle lineages (Kennedy et al., 2009). Low concentrations of RA can regulate the levels of Myf5 implying the existence of a RARE in the Myf5 regulatory region (Carnac et al., 1993). RA also enhances MyoD and myogenin expression (Carnac et al., 1993), and RA receptors and MyoD have been found to upregulate each other’s transcriptional activity; their transcriptional co-activation requires a RA receptor-MyoD complex that binds to MyoD DNA binding sites in muscle cells (Froeschle et al., 1998). RA is capable of inhibiting proliferation of myoblasts through inducing cell cycle arrest proteins (Alric et al., 1998) and in vitamin A deficient embryos, myogenin is downregulated (Maden et al., 2000) providing a link between RA and myoblast maturation.

RA and bexarotene are both capable of inducing skeletal myogenesis in the P19 stem cells, however, they do so through differential activation of crucial specification genes (Le May et al., 2011). Bexarotene primarily activates Meox1 while RA mainly activates Pax3.Nontheless, both ligands are equally capable of inducing later target genes such as MyoD and myogenin. Alternatively, only bexarotene is capable of inducing myogenesis in ES cells to a significant level (Le May et al., 2011).Furthermore, treatment of these cells with bexarotene gives long, mature, multinucleated myofibers.

4.1. Stem cell as a model for study of myogenic differentiation

It is highly advantageous to use stem cell tissue cultures to study the importance of specification genes in a controlled environment to understand their relationship with each other and their regulation by extracellular signaling molecules. Specification factors exist in a very complex relationship and have the ability to autoregulate and cross-regulate one another (Petropoulos et al., 2004).

In P19 stem cell cultures, Pax3 overexpression can induce Meox1 but is unable to activate Gli2 and a dominant negative Pax3 mutation does not affect Gli2 levels. Conversely, Gli2, which also has the ability to upregulate Meox1, can upregulate Pax3 while the dominant negative Gli2 P19 cells downregulate Meox1, Pax3, and MyoD expression and inhibits myogenesis. Lastly, Meox1 can activate the expression of Gli2 but overexpression of this protein is insufficient to induce Pax3 or skeletal myogenesis (Petropoulos et al., 2004). The ability of each of these factors to induce each other, or, in their absence, completely abolish myogenesis underlines the importance of these factors in the specification process.

Wnt signaling via β-catenin is also essential and sufficient for the induction of specification factors Pax3, Meox1, and Gli2 and in P19 stem cells, a dominant negative β-catenin inhibits Pax3, Gli2, Meox1 and MyoD expression and abolishes myogenesis (Petropoulos et al., 2002). This is not surprising since mutations of either Gli2, Meox1, or Pax3 in these cells will abrogate myogenesis (Petropoulos et al., 2004). Pax3 expression is essential and sufficient for the expression of the transcription factor Six1 and the induction of skeletal myogenesis (Ridgeway et al., 2001). Its overexpression induces Myf-5, MyoD, and myogenin expression (Maroto et al., 1997) whereas a dominant negative Pax3 in P19 cells results in a loss of MyoD and myogenin expression and subsequent myogenesis (Ridgeway et al., 2001).

4.2. Significance of a separate RXR signaling pathway

The importance of a separate, rexinoid signaling pathway in skeletal muscle development non-overlapping with RA signal transduction is demonstrated by the fact that an RXR selective ligand, bexarotene effectively enhances skeletal myogenesis in mouse ES cells that respond poorly to RA (Le May et al., 2011). This difference in the two signaling pathways stems from differential activation of very early genes involved in crucial lineage specification, although both bexarotene and RA are dependent on functional β-catenin signaling(Le May et al., 2011). It is intriguing that a cell type such as ES cells, that has thus far been relatively resistant to RA-induced skeletal muscle differentiation develops so well in the presence of an RXR selective ligand, especially considering these cells do not posse the necessary machinery to synthesize 9-cis RA, the purported endogenous ligand (Chen and Khillan, 2010). It appears that P19 cells have the ability to differentiate by both retinoid and rexinoid signaling instigated pathways while ES cells respond well only to rexinoid mediated pathways. Similarly, RAC65 cells resistant to RA-induced skeletal muscle and neuronal conversion (Costa and McBurney, 1996)demonstrate efficient skeletal differentiation when treated with RXR selective ligand (Le May et al., 2011). The ability of rexinoid to bypass the dominant negative RAR inhibition in RAC65 cells is not unique to skeletal muscle and has also been documented for neuronal differentiation as well (Yokota and Ohkubo, 1996). Finally, RXR is able to activate target genes involved in RA signaling that cannot be induced by RARs as is the case with the response element in the CRBPII (Cellular Retinol Binding Protein Type II) gene which contains a DR1, underscoring the possibility of RXR/RXR and RXR/RAR independent pathways (Mangelsdorf et al., 1991).

It remains to be determined which specific co-activators are recruited by RXR in the enhancers or promoters of target genes during skeletal myogenesis. RXR homodimers or RXR permissive heterodimers might recruit a separate set of co-activators and therefore differentially control gene expression. It could be that the unique ability of bexarotene versus RA to control the transcription factor’s interactions with co-activators is the method by which distinct and even competing signaling pathways can be distinguished.

4.3. Unsaturated fatty acids activate RXR

The physiological significance of 9-cis RA signaling is debated due to a lack of consensus on its existence in the developing embryo. However, the enzymes that contribute to its biosynthesis are well documented (Mertz et al., 1997; Romert et al., 1998) in addition to its ability to induce the formation of homodimers that bind to DR1 sequences (Zhang et al., 1992). The lack of a known ligand is hardly reason to exclude RXR as physiologically significant in vivo and a major factor that supports the presence of an active endogenous ligand is the fact that RXR tetramers cannot dissociate without agonist binding.

Studies using RXR ligand-detector mice have identified specific regions of the spinal cord as major sites of endogenous rexinoid production and classify naturally occurring poly-unsaturated fatty acids, including docosahexaenoic acid (DHA) as a major endogenous ligand for RXR in the mouse brain(Ahuja et al., 2003; de Urquiza et al., 2000). When characterized in the ligand binding domain of RXR-α, DHA has a significantly higher number of ligand-protein contacts than 9-cis and certain synthetic ligands and also has the ability to activate RXR homodimers as well as synergistically activate the RXR-RAR heterodimers in combination with all-trans RA(Lengqvist et al., 2004). It remains to be determined if this ligand is functional in all tissues or whether there are other yet undiscovered ligands. Presently, additional unsaturated fatty acids, including docosapentaenoic, arachidonic, and oleic acids, also have been found to bind and activate RXR, suggesting that this ability is not exclusive for DHA. Irrespective of whether an endogenous RXR ligand does indeed exist, the ability to control cell growth and differentiation through targeting RXR with highly selective ligands confers many therapeutic applications to this unique receptor.

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5. Therapeutic potentials of rexinoids

It is unknown whether RXR homodimer or RXR permissive heterodimer signaling is the main mechanism governing skeletal muscle differentiation. Regardless, controlling cell processes using RXR selective ligands underlines the fact that two distinct and possibly overlapping pathways exist. Moreover, RAR-independent rexinoid signaling provides another route of achieving cell cycle arrest and differentiation when RA signaling is aberrant, a situation frequently seen in cancer where differentiation often appears to result in loss of a malignant phenotype(Gokhale et al., 2000).

RXR-α overexpression sensitizes tumors to rexinoid-induced anti-growth effects, cellular differentiation, decreased cell proliferation, apoptosis of some type of cancer cells, and prevention of angiogenesis and metastasis (Qu and Tang, 2010).Bexarotene, has been approved by the FDA for use in the treatment of refractory or persistent cutaneous T-cell lymphoma and has the ability to reduce tumor development in several other cancers (Duvic et al., 2001; Wu et al., 2002). However, the use of this compound in the treatment of lung and breast carcinomas has yielded disappointing results (Tanaka et al., 2009) demonstrating our lack of understanding of the molecular mechanisms underlying rexinoid-induced antitumoreffects and RXR-induced multi-pathway activation.

One of the reasons rexinoids seem such promising chemotherapeutic compounds compared to retinoids, is that retinoids have numerous side effects which severely limit the dosage and efficacy while rexinoids display mild toxicity. Furthermore, RXR expression is rarely lost in human tumors whereas RAR expression is frequently lost or reduced in various cancers (Sun and Lotan, 2002; Umesono et al., 1989). Since p53 abnormalities are reported in more than 50% of human cancers, and p21 is rarely mutated (Shiohara et al., 1994; Tanaka et al., 2007), RXR mediated induction of p21 is a promising therapeutic target for these cancers. The study of myogenic differentiation may provide some answers to new target genes as the development and progression of cancer involves aberrations in the same mechanisms that regulate cell differentiation during embryogenesis. It remains to be revealed which other genes can also be targeted by rexinoids and which specific interactions take place that we can study and apply to our development of more potent and effective therapeutics.

Pluripotent stem cells closely simulate embryonic development and present a model system with which to dissect signaling pathways of target receptors in controlled environments. They hold a tremendous potential for cell-based therapies through their capacity to grow and regenerate new tissues. Many diseases including muscular dystrophies, cancer, AIDS, and even normal conditions such as aging show prominent muscle loss that would benefit enormously from regenerative cell-based therapies. However, our ability to use stem cells in muscle-wasting disorders has been limited due to the low rate of myogenic differentiation in ES cell cultures and the difficulty in identifying and isolating progenitor cells. To harvest the full potential of these cells in therapies, it is imperative that we find small molecule inducers capable of efficiently directing stem cells into skeletal muscle lineage. Attempts at using RA in ES cell cultures have thus far yielded disappointing results; however, the ability of rexinoids to induce these cells has not yet been fully explored. Understanding the myogenic pathway in vivo as well as deciphering differentiation cues to culture pure populations of myogenic progenitors will prove a vital tool in the treatment of such devastating diseases.

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6. Conclusion

RXR selective ligand is an effective inducer of skeletal myogenesis not only in the P19 pluripotent stem cells, but also in the mouse ES cells which have thus far been relatively resistant to RA induction. RXR specific signaling plays animportant role in this process through a separate RAR-independent mediated pathway. It appears that RA and rexinoid enhance skeletal myogenesis through differential activation of early developmental genes. Our study demonstrates that activation of RXR causes an increase in the mesodermal Meox1 gene while RA induces the skeletal specific gene Pax3. It will be interesting to uncover other novel genes targeted by rexinoid. Determining the molecular mechanism by which rexinoid exerts its effects to enhance skeletal myogenesis is challenging due in part to the complexity of the developmental systems in which it exerts its effects as well as the intricate relationship of protein complexes and gene regulation.Since ES cells closely recapitulate the properties of the developing embryo, elucidating these molecular pathways will be imperative in the manipulation of stem cell progenitors and aid in the generation of pure populations of skeletal myocytes to use in the treatment of muscle-related diseases.

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Acknowledgement

This work was supported by grant from the Natural Sciences and Engineering Research Council of Canada.

References

  1. 1. AhujaH. S.SzantoA.NagyL.DaviesP. J. 2003 The retinoid X receptor and its ligands: versatile regulators of metabolic function, cell differentiation and cell death. J Biol Regul Homeost Agents, 17 2945 .
  2. 2. Alric S, Froeschle A, Piquemal D, Carnac G, and Bonnieu A (1998) Functional specificity of the two retinoic acid receptor RAR and RXR families in myogenesis. Oncogene, 16, 273-282.
  3. 3. AlvaresL. E.SchubertF. R.ThorpeC.MootoosamyR. C.ChengL.ParkynG.LumsdenA.Dietrich 2003chubert FR, Thorpe C, Mootoosamy RC, Cheng L, Parkyn G, Lumsden A, and Dietrich S (2003) Intrinsic, Hox-dependent cues determine the fate of skeletal muscle precursors. Dev Cell, 5 379390 .
  4. 4. Angus LM, Chan RY, and Jasmin BJ 2001 Role of intronic E- and N-box motifs in the transcriptional induction of the acetylcholinesterase gene during myogenic differentiation. J Biol Chem, 276 1760317609 .
  5. 5. Arnold HH and Braun 2000 (2000) Genetics of muscle determination and development. Curr Top Dev Biol, 48 129164 .
  6. 6. Arnold HH and Winter 1998 (1998) Muscle differentiation: more complexity to the network of myogenic regulators. Curr Opin Genet Dev, 8 539544 .
  7. 7. BajardL.RelaixF.LaghaM.RocancourtD.DaubasP.MEBuckingham 2006 A novel genetic hierarchy functions during hypaxial myogenesis: Pax3 directly activates Myf5 in muscle progenitor cells in the limb. Genes Dev, 20 24502464 .
  8. 8. Barger PM and Kelly DP 1997 Identification of a retinoid/chicken ovalbumin upstream promoter transcription factor response element in the human retinoid X receptor gamma2 gene promoter. J Biol Chem, 272 27222728 .
  9. 9. BastienJ.Rochette-Egly 2004 (2004) Nuclear retinoid receptors and the transcription of retinoid-target genes. Gene, 328 116 .
  10. 10. BergstromD. A.PennB. H.StrandA.PerryR. L.MARudnickiTapscottS. J. 2002 Promoter-specific regulation of MyoD binding and signal transduction cooperate to pattern gene expression. Mol Cell, 9 587600 .
  11. 11. Bergstrom DA and Tapscott SJ 2001 Molecular distinction between specification and differentiation in the myogenic basic helix-loop-helix transcription factor family. Mol Cell Biol, 21 24042412 .
  12. 12. Black BL and Olson EN 1998 Transcriptional control of muscle development by myocyte enhancer factor-2 (MEF2) proteins. Annu Rev Cell Dev Biol, 14 167196 .
  13. 13. BladtF.RiethmacherD.IsenmannS.AguzziA.Birchmeier 1995 (1995) Essential role for the c-met receptor in the migration of myogenic precursor cells into the limb bud. Nature, 376 768771 .
  14. 14. BoryckiA. G.BrunkB.TajbakhshS.BuckinghamM.ChiangC.CPEmerson Jr 1999 Sonic hedgehog controls epaxial muscle determination through Myf5 activation. Development, 126 40534063 .
  15. 15. BourguetW.VivatV.WurtzJ. M.ChambonP.GronemeyerH.Moras 2000 (2000) Crystal structure of a heterodimeric complex of RAR and RXR ligand-binding domains. Mol Cell, 5 289298 .
  16. 16. BraunT.BoberE.WinterB.RosenthalN.ArnoldH. H. 1990 Myf-6, a new member of the human gene family of myogenic determination factors: evidence for a gene cluster on chromosome 12. EMBO J, 9 821831 .
  17. 17. BraunT.Buschhausen-DenkerG.BoberE.TannichE.ArnoldH. H. 1989 A novel human muscle factor related to but distinct from MyoD1 induces myogenic conversion in 10T1/2 fibroblasts. EMBO J, 8 701709 .
  18. 18. BuckinghamM.BajardL.ChangT.DaubasP.HadchouelJ.MeilhacS.MontarrasD.RocancourtD.Relaix 2003 (2003) The formation of skeletal muscle: from somite to limb. J Anat, 202 5968 .
  19. 19. CandiaA. F.HuJ.CrosbyJ.LalleyP. A.NodenD.NadeauJ. H.WrightC. V. 1992 Mox-1 and Mox-2 define a novel homeobox gene subfamily and are differentially expressed during early mesodermal patterning in mouse embryos. Development, 116 11231136 .
  20. 20. Candia AF and Wright CV 1996 Differential localization of Mox-1 and Mox-2 proteins indicates distinct roles during development. Int J Dev Biol, 40 11791184 .
  21. 21. CarnacG.Albagli-CurielO.LevinA.Bonnieu 1993lbagli-Curiel O, Levin A, and Bonnieu A (1993) 9-cis-retinoic acid regulates the expression of the muscle determination gene Myf5. Endocrinology, 133 21712176 .
  22. 22. ChenJ. Y.CliffordJ.ZusiC.StarrettJ.TortolaniD.OstrowskiJ.ReczekP. R.ChambonP.Gronemeyer 1996 (1996) Two distinct actions of retinoid-receptor ligands. Nature, 382 819822 .
  23. 23. ChenL.KhillanJ. S. 2010 A novel signaling by vitamin A/retinol promotes self renewal of mouse embryonic stem cells by activating PI3K/Akt signaling pathway via insulin-like growth factor-1 receptor. Stem Cells, 28 5763 .
  24. 24. ChibaH.CliffordJ.MetzgerD.Chambon 1997 Specific and redundant functions of retinoid X Receptor/Retinoic acid receptor heterodimers in differentiation, proliferation, and apoptosis of F9 embryonal carcinoma cells. J Cell Biol, 139, 735-747.
  25. 25. ChristB.CPOrdahl 1995 Early stages of chick somite development. Anat Embryol (Berl), 191 381396 .
  26. 26. CossuG.Borello 1999 (1999) Wnt signaling and the activation of myogenesis in mammals. EMBO J, 18 68676872 .
  27. 27. Costa SL and McBurney MW 1996 Dominant negative mutant of retinoic acid receptor alpha inhibits retinoic acid-induced 19 cell differentiation by binding to DNA. Exp Cell Res, 225, 35-43.
  28. 28. CrescenziM.FlemingT. P.LassarA. B.WeintraubH.AaronsonS. A. 1990 MyoD induces growth arrest independent of differentiation in normal and transformed cells. Proc Natl Acad Sci U S A, 87 84428446 .
  29. 29. CserjesiP.OlsonE. N. 1991 Myogenin induces the myocyte-specific enhancer binding factor MEF-2 independently of other muscle-specific gene products. Mol Cell Biol, 11 48544862 .
  30. 30. DanielsenM.HinckL.RingoldG. M. 1989 Two amino acids within the knuckle of the first zinc finger specify DNA response element activation by the glucocorticoid receptor. Cell, 57 11311138 .
  31. 31. DavisR. L.WeintraubH.LassarA. B. 1987 Expression of a single transfected cDNA converts fibroblasts to myoblasts. Cell, 51 9871000 .
  32. 32. de UrquizaA. M.LiuS.SjobergM.ZetterstromR. H.GriffithsW.SjovallJ.Perlmann 2000 (2000) Docosahexaenoic acid, a ligand for the retinoid X receptor in mouse brain. Science, 290 21402144 .
  33. 33. DietrichS.Abou-RebyehF.BrohmannH.BladtF.Sonnenberg-RiethmacherE.YamaaiT.LumsdenA.Brand-SaberiB.Birchmeier 1999 (1999) The role of SF/HGF and c-Met in the development of skeletal muscle. Development, 126 16211629 .
  34. 34. DietrichS.SchubertF. R.HealyC.SharpeP. T.Lumsden 1998 (1998) Specification of the hypaxial musculature. Development, 125 22352249 .
  35. 35. Dilworth FJ, Seaver KJ, Fishburn AL, Htet SL, and Tapscott SJ 2004 In vitro transcription system delineates the distinct roles of the coactivators pCAF and 300 during MyoD/E47-dependent transactivation. Proc Natl Acad Sci U S A, 101, 11593-11598.
  36. 36. Di RenzoJ.SoderstromM.KurokawaR.OgliastroM. H.RicoteM.IngreyS.HorleinA.RosenfeldM. G. (1997) Peroxisome proliferator-activated receptors and retinoic acid receptors differentially control the interactions of retinoid X receptor heterodimers with ligands, coactivators, and corepressors. Mol Cell Biol, 17, 2166-2176.
  37. 37. Dolle 2009 Developmental expression of retinoic acid receptors (RARs). Nucl Recept Signal, 7, e006.
  38. 38. DongD.Noy 1998oy N (1998) Heterodimer formation by retinoid X receptor: regulation by ligands and by the receptor’s self-association properties. Biochemistry, 37 1069110700 .
  39. 39. DuvicM.MartinA. G.KimY.OlsenE.WoodG. S.CACrowleyYocumR. C. 2001 Phase 2 and 3 clinical trial of oral bexarotene (Targretin capsules) for the treatment of refractory or persistent early-stage cutaneous T-cell lymphoma. Arch Dermatol, 137 581593 .
  40. 40. EcknerR.YaoT. P.OldreadE.LivingstonD. M. 1996 Interaction and functional collaboration of 300 CBP and bHLH proteins in muscle and B-cell differentiation. Genes Dev, 10, 2478-2490.
  41. 41. Edmondson DG, Lyons GE, Martin JF, and Olson EN 1994 Mef2 gene expression marks the cardiac and skeletal muscle lineages during mouse embryogenesis. Development, 120 12511263 .
  42. 42. Edmondson DG and Olson EN 1990 A gene with homology to the myc similarity region of MyoD1 is expressed during myogenesis and is sufficient to activate the muscle differentiation program. Genes Dev, 4, 1450.
  43. 43. EpsteinJ. A.ShapiroD. N.ChengJ.LamP. Y.MaasR. L. 1996 Pax3 modulates expression of the c-Met receptor during limb muscle development. Proc Natl Acad Sci U S A, 93 42134218 .
  44. 44. Fan C1997, Lee CS, and Tessier-Lavigne M (1997) A role for WNT proteins in induction of dermomyotome. Dev Biol, 191 160165 .
  45. 45. BMFormanUmesono. K.ChenJ.EvansR. M. 1995 Unique response pathways are established by allosteric interactions among nuclear hormone receptors. Cell, 81 541550 .
  46. 46. FroeschleA.AlricS.KitzmannM.CarnacG.AuradeF.Rochette-EglyC.Bonnieu 1998, Alric S, Kitzmann M, Carnac G, Aurade F, Rochette-Egly C, and Bonnieu A (1998) Retinoic acid receptors and muscle b-HLH proteins: partners in retinoid-induced myogenesis. Oncogene, 16 33693378 .
  47. 47. Gampe RT, Jr., Montana VG, Lambert MH, Miller AB, Bledsoe RK, Milburn MV, Kliewer SA, Willson TM, and Xu HE 2000a Asymmetry in the PPARgamma/RXRalpha crystal structure reveals the molecular basis of heterodimerization among nuclear receptors. Mol Cell, 5 545555 .
  48. 48. Gampe RT, Jr., Montana VG, Lambert MH, Wisely GB, Milburn MV, and Xu HE 2000b Structural basis for autorepression of retinoid X receptor by tetramer formation and the AF-2 helix. Genes Dev, 14 22292241 .
  49. 49. Gokhale PJ, Giesberts AM, and Andrews PW 2000 Brachyury is expressed by human teratocarcinoma cells in the absence of mesodermal differentiation. Cell Growth Differ, 11 157162 .
  50. 50. GouldingM.LumsdenA.PaquetteA. J. 1994 Regulation of Pax-3 expression in the dermomyotome and its role in muscle development. Development, 120 957971 .
  51. 51. Gramolini AO and Jasmin BJ 1999 Expression of the utrophin gene during myogenic differentiation. Nucleic Acids Res, 27 36033609 .
  52. 52. GreenS.KumarV.TheulazI.WahliW.Chambon 1988 The N-terminal DNA-binding ‘zinc finger’ of the oestrogen and glucocorticoid receptors determines target gene specificity. EMBO J, 7, 3037-3044.
  53. 53. GrifoneR.DemignonJ.HoubronC.SouilE.NiroC.MJSellerHamard. G.Maire 2005 Six1 and Six4 homeoproteins are required for Pax3 and Mrf expression during myogenesis in the mouse embryo. Development, 132, 2235-2249.
  54. 54. GuW.RoederR. G. 1997 Activation of 53 sequence-specific DNA binding by acetylation of the p53 C-terminal domain. Cell, 90, 595-606.
  55. 55. GuentherM. G.LaneW. S.FischleW.VerdinE.MALazarShiekhattar 2000 (2000) A core SMRT corepressor complex containing HDAC3 and TBL1, a WD40-repeat protein linked to deafness. Genes Dev, 14 10481057 .
  56. 56. Hu JS, Olson EN, and Kingston RE 1992 HEB, a helix-loop-helix protein related to E2A and ITF2 that can modulate the DNA-binding ability of myogenic regulatory factors. Mol Cell Biol, 12 10311042 .
  57. 57. KaranamB.JiangL.WangL.KelleherN. L.ColeP. A. 2006 Kinetic and mass spectrometric analysis of 300 histone acetyltransferase domain autoacetylation. J Biol Chem, 281, 40292-40301.
  58. 58. KastnerP.GrondonaJ. M.MarkM.GansmullerA.Le MeurM.DecimoD.VoneschJ. L.DolleP.Chambon 1994, Grondona JM, Mark M, Gansmuller A, LeMeur M, Decimo D, Vonesch JL, Dolle P, and Chambon 1994 Genetic analysis of RXR alpha developmental function: convergence of RXR and RAR signaling pathways in heart and eye morphogenesis. Cell, 78, 987-1003.
  59. 59. KennedyK. A.PorterT.MehtaV.RyanS. D.PriceF.PeshdaryV.KaramboulasC.SavageJ.DrysdaleT. A.LiS. C.BennettS. A.SkerjancI. S. 2009 Retinoic acid enhances skeletal muscle progenitor formation and bypasses inhibition by bone morphogenetic protein 4 but not dominant negative beta-catenin. BMC Biol, 7, 67.
  60. 60. KerstenS.DawsonM. I.BALewisNoy 1996oy N (1996) Individual subunits of heterodimers comprised of retinoic acid and retinoid X receptors interact with their ligands independently. Biochemistry, 35 38163824 .
  61. 61. KerstenS.KelleherD.ChambonP.GronemeyerH.Noy 1995oy N (1995) Retinoid X receptor alpha forms tetramers in solution. Proc Natl Acad Sci U S A, 92 86458649 .
  62. 62. KranerS. D.MMRichSholl.MAZhouH.ZorcC. S.KallenR. G.BarchiR. L. 1999 Interaction between the skeletal muscle type 1 Na+ channel promoter E-box and an upstream repressor element. Release of repression by myogenin. J Biol Chem, 274 81298136 .
  63. 63. KumarV.Chambon 1988 The estrogen receptor binds tightly to its responsive element as a ligand-induced homodimer. Cell, 55, 145-156.
  64. 64. KurokawaR.Di RenzoJ.BoehmM.SugarmanJ.GlossB.RosenfeldM. G.HeymanR. A.GlassC. K. 1994 Regulation of retinoid signalling by receptor polarity and allosteric control of ligand binding. Nature, 371 528531 .
  65. 65. DSLalaMukherjee. R.SchulmanI. G.KochS. S.DardashtiL. J.NadzanA. M.CrostonG. E.EvansR. M.HeymanR. A. 1996 Activation of specific RXR heterodimers by an antagonist of RXR homodimers. Nature, 383 450453 .
  66. 66. Le MayM.MachH.LacroixN.HouC.ChenJ.Li 2011 (2011) Contribution of retinoid X receptor signaling to the specification of skeletal muscle lineage. J Biol Chem, 286 2680626812 .
  67. 67. Lehmann JM, Jong L, Fanjul A, Cameron JF, Lu XP, Haefner P, Dawson MI, and Pfahl M (1992) Retinoids selective for retinoid X receptor response pathways. Science, 258, 1944-1946.
  68. 68. LeidM.KastnerP.Chambon 1992, and Chambon 1992 Multiplicity generates diversity in the retinoic acid signalling pathways. Trends Biochem Sci, 17, 427-433.
  69. 69. LeiferD.KraincD.YuY. T.Mc DermottJ.BreitbartR. E.HengJ.NeveR. L.KosofskyB.Nadal-GinardB.LiptonS. A. 1993 MEF2C, a MADS/MEF2-family transcription factor expressed in a laminar distribution in cerebral cortex. Proc Natl Acad Sci U S A, 90 15461550 .
  70. 70. LengqvistJ.MataD. U.BergmanA. C.WillsonT. M.SjovallJ.PerlmannT.GriffithsW. J. 2004 Polyunsaturated fatty acids including docosahexaenoic and arachidonic acid bind to the retinoid X receptor alpha ligand-binding domain. Mol Cell Proteomics, 3 692703 .
  71. 71. LiH.Capetanaki 1993 (1993) Regulation of the mouse desmin gene: transactivated by MyoD, myogenin, MRF4 and Myf5. Nucleic Acids Res, 21 335343 .
  72. 72. LiJ.WangJ.WangJ.NawazZ.LiuJ. M.QinJ.Wong 2000, Wang J, Wang J, Nawaz Z, Liu JM, Qin J, and Wong J (2000) Both corepressor proteins SMRT and N-CoR exist in large protein complexes containing HDAC3. EMBO J, 19 43424350 .
  73. 73. LiQ.HerrlerM.LandsbergerN.KaludovN.OgryzkoV. V.NakataniY.WolffeA. P. 1998 Xenopus NF-Y pre-sets chromatin to potentiate 300 and acetylation- responsive transcription from the Xenopus hsp70 promoter in vivo. EMBO J, 17, 6300-6315.
  74. 74. LiQ.ImhofA.CollingwoodT. N.UrnovF. D.WolffeA. P. 1999 300 stimulates transcription instigated by ligand-bound thyroid hormone receptor at a step subsequent to chromatin disruption. EMBO J, 18, 5634-5652.
  75. 75. LinH.YutzeyK. E.KoniecznyS. F. 1991 Muscle-specific expression of the troponin I gene requires interactions between helix-loop-helix muscle regulatory factors and ubiquitous transcription factors. Mol Cell Biol, 11 267280 .
  76. 76. LohnesD.KastnerP.DierichA.MarkM.Le MeurM.Chambon 1993, Dierich A, Mark M, LeMeur M, and Chambon 1993 Function of retinoic acid receptor gamma in the mouse. Cell, 73, 643-658.
  77. 77. LufkinT.LohnesD.MarkM.DierichA.GorryP.GaubM. P.Le MeurM.Chambon 1993, Gaub MP, LeMeur M, and Chambon 1993 High postnatal lethality and testis degeneration in retinoic acid receptor alpha mutant mice. Proc Natl Acad Sci U S A, 90, 7225-7229.
  78. 78. LuisiB. F.XuW. X.OtwinowskiZ.FreedmanL. P.YamamotoK. R.SiglerP. B. 1991 Crystallographic analysis of the interaction of the glucocorticoid receptor with DNA. Nature, 352 497505 .
  79. 79. LyonsG. E.MicalesB. K.SchwarzJ.MartinJ. F.OlsonE. N. 1995 Expression of mef2 genes in the mouse central nervous system suggests a role in neuronal maturation. J Neurosci, 15 57275738 .
  80. 80. Maden M, Gale E, Kostetskii I, and Zile M (1996) Vitamin A-deficient quail embryos have half a hindbrain and other neural defects. Curr Biol, 6, 417-426.
  81. 81. MadenM.GrahamA.ZileM.Gale 2000 (2000) Abnormalities of somite development in the absence of retinoic acid. Int J Dev Biol, 44 151159 .
  82. 82. MaderS.KumarV.de VerneuilH.Chambon 1989 Three amino acids of the oestrogen receptor are essential to its ability to distinguish an oestrogen from a glucocorticoid-responsive element. Nature, 338, 271-274.
  83. 83. MangelsdorfD. J.BorgmeyerU.HeymanR. A.ZhouJ. Y.OngE. S.OroA. E.KakizukaA.EvansR. M. 1992 Characterization of three RXR genes that mediate the action of 9-cis retinoic acid. Genes Dev, 6 329344 .
  84. 84. MangelsdorfD. J.ThummelC.BeatoM.HerrlichP.SchutzG.UmesonoK.BlumbergB.KastnerP.MarkM.ChambonP.EvansR. M. 1995 The nuclear receptor superfamily: the second decade. Cell, 83 835839 .
  85. 85. MangelsdorfD. J.UmesonoK.KliewerS. A.BorgmeyerU.OngE. S.EvansR. M. 1991 A direct repeat in the cellular retinol-binding protein type II gene confers differential regulation by RXR and RAR. Cell, 66 555561 .
  86. 86. BSMankooCollins. N. S.AshbyP.GrigorievaE.PevnyL. H.CandiaA.WrightC. V.RigbyP. W.PachnisV, Rigby PW, and Pachnis V (1999) Mox2 is a component of the genetic hierarchy controlling limb muscle development. Nature, 400, 69-73.
  87. 87. BSMankooSkuntz. S.HarriganI.GrigorievaE.CandiaA.WrightC. V.ArnheiterH.PachnisV, Arnheiter H, and Pachnis V (2003) The concerted action of Meox homeobox genes is required upstream of genetic pathways essential for the formation, patterning and differentiation of somites. Development, 130, 4655-4664.
  88. 88. MarkM.GhyselinckN. B.Chambon 2006 Function of retinoid nuclear receptors: lessons from genetic and pharmacological dissections of the retinoic acid signaling pathway during mouse embryogenesis. Annu Rev Pharmacol Toxicol, 46, 451-480.
  89. 89. MarotoM.ReshefR.MunsterbergA. E.KoesterS.GouldingM.LassarA. B. 1997 Ectopic Pax-3 activates MyoD and Myf-5 expression in embryonic mesoderm and neural tissue. Cell, 89 139148 .
  90. 90. Marsh DR, Carson JA, Stewart LN, and Booth FW 1998 Activation of the skeletal alpha-actin promoter during muscle regeneration. J Muscle Res Cell Motil, 19 897907 .
  91. 91. Mc DermottA.GustafssonM.ElsamT.HuiC. C.CPEmerson JrBoryckiA. G. 2005 Gli2 and Gli3 have redundant and context-dependent function in skeletal muscle formation. Development, 132 345357 .
  92. 92. MertzJ. R.ShangE.PiantedosiR.WeiS.WolgemuthD. J.BlanerW. S. 1997 Identification and characterization of a stereospecific human enzyme that catalyzes 9-cis-retinol oxidation. A possible role in 9-cis-retinoic acid formation. J Biol Chem, 272 1174411749 .
  93. 93. MicF. A.MolotkovA.BenbrookD. M.Duester 2003 (2003) Retinoid activation of retinoic acid receptor but not retinoid X receptor is sufficient to rescue lethal defect in retinoic acid synthesis. Proc Natl Acad Sci U S A, 100 71357140 .
  94. 94. MinucciS.LeidM.ToyamaR.Saint-JeannetJ. P.PetersonV. J.HornV.IshmaelJ. E.BhattacharyyaN.DeyA.DawidI. B.Ozato 1997 (1997) Retinoid X receptor (RXR) within the RXR-retinoic acid receptor heterodimer binds its ligand and enhances retinoid-dependent gene expression. Mol Cell Biol, 17 644655 .
  95. 95. MunsterbergA. E.KitajewskiJ.BumcrotD. A.Mc MahonA. P.LassarA. B. 1995 Combinatorial signaling by Sonic hedgehog and Wnt family members induces myogenic bHLH gene expression in the somite. Genes Dev, 9 29112922 .
  96. 96. MurreC.Mc CawP. S.VaessinH.CaudyM.JanL. Y.JanY. N.CabreraC. V.BuskinJ. N.HauschkaS. D.LassarA. B.and 1989 Interactions between heterologous helix-loop-helix proteins generate complexes that bind specifically to a common DNA sequence. Cell, 58 537544 .
  97. 97. NauH.BlanerW. S.AgadirA.and 1999 Retinoids: the biochemical and molecular basis of vitamin A and retinoid action. Germany: Springer-Verlag Berlin HeidelBerg New York.
  98. 98. Naya FJ and Olson 1999 (1999) MEF2: a transcriptional target for signaling pathways controlling skeletal muscle growth and differentiation. Curr Opin Cell Biol, 11 683688 .
  99. 99. NiederreitherK.Abu-AbedS.SchuhbaurB.PetkovichM.ChambonP.Dolle 2002etkovich M, Chambon P, and Dolle 2002 Genetic evidence that oxidative derivatives of retinoic acid are not involved in retinoid signaling during mouse development. Nat Genet, 31, 84-88.
  100. 100. NiederreitherK.Dolle 2008 Retinoic acid in development: towards an integrated view. Nat Rev Genet, 9, 541-553.
  101. 101. NiederreitherK.SubbarayanV.DolleP.Chambon 1999, and Chambon 1999 Embryonic retinoic acid synthesis is essential for early mouse post-implantation development. Nat Genet, 21, 444-448.
  102. 102. OgryzkoV. V.SchiltzR. L.RussanovaV.HowardB. H.Nakatani 1996 (1996) The transcriptional coactivators 300 and CBP are histone acetyltransferases. Cell, 87, 953-959.
  103. 103. Olson EN 1990 MyoD family: a paradigm for development? Genes Dev, 4 14541461 .
  104. 104. Olson EN and Klein WH 1994 bHLH factors in muscle development: dead lines and commitments, what to leave in and what to leave out. Genes Dev, 8 18 .
  105. 105. Parker MH, Perry RL, Fauteux MC, Berkes CA, and Rudnicki MA 2006 MyoD synergizes with the E-protein HEB beta to induce myogenic differentiation. Mol Cell Biol, 26 57715783 .
  106. 106. PerissiV.StaszewskiL. M.Mc InerneyE. M.KurokawaR.KronesA.RoseD. W.LambertM. H.MilburnM. V.GlassC. K.RosenfeldM. G. 1999 Molecular determinants of nuclear receptor-corepressor interaction. Genes Dev, 13 31983208 .
  107. 107. PerlmannT.Jansson 1995 (1995) A novel pathway for vitamin A signaling mediated by RXR heterodimerization with NGFI-B and NURR1. Genes Dev, 9 769782 .
  108. 108. PetropoulosH.GianakopoulosP. J.RidgewayA. G.SkerjancI. S. 2004 Disruption of Meox or Gli activity ablates skeletal myogenesis in 19 cells. J Biol Chem, 279, 23874-23881.
  109. 109. PetropoulosH.SkerjancI. S. 2002 Beta-catenin is essential and sufficient for skeletal myogenesis in 19 cells. J Biol Chem, 277, 15393-15399.
  110. 110. PijnappelW. W.HendriksH. F.FolkersG. E.van denBrink.CEDekkerE. J.EdelenboschC.van der SaagP. T.DurstonA. J. 1993 The retinoid ligand 4-oxo-retinoic acid is a highly active modulator of positional specification. Nature, 366 340344 .
  111. 111. PolesskayaA.DuquetA.NaguibnevaI.WeiseC.VervischA.BengalE.HuchoF.RobinP.Harel-Bellan 2000, Duquet A, Naguibneva I, Weise C, Vervisch A, Bengal E, Hucho F, Robin P, and Harel-Bellan A (2000) CREB-binding protein/300 activates MyoD by acetylation. J Biol Chem, 275, 34359-34364.
  112. 112. PolesskayaA.NaguibnevaI.FritschL.DuquetA.Ait-Si-AliS.RobinP.VervischA.PritchardL. L.ColeP.Harel-Bellan 2001, Naguibneva I, Fritsch L, Duquet A, Ait-Si-Ali S, Robin P, Vervisch A, Pritchard LL, Cole P, and Harel-Bellan A (2001) CBP/300 and muscle differentiation: no HAT, no muscle. EMBO J, 20, 6816-6825.
  113. 113. PourquieO.FanC. M.ColteyM.HirsingerE.WatanabeY.BreantC.Francis-WestP.BrickellP.Tessier-LavigneM.Le DouarinN. M. 1996 Lateral and axial signals involved in avian somite patterning: a role for BMP4. Cell, 84 461471 .
  114. 114. MAPrattCrippen. C.HubbardK.Menard 1998A, Crippen C, Hubbard K, and Menard M (1998) Deregulated expression of the retinoid X receptor alpha prevents muscle differentiation in 19 embryonal carcinoma cells. Cell Growth Differ, 9, 713-722.
  115. 115. PuriP. L.AvantaggiatiM. L.BalsanoC.SangN.GraessmannA.GiordanoA.LevreroM. 1997a 300 is required for MyoD-dependent cell cycle arrest and muscle-specific gene transcription. EMBO J, 16, 369-383.
  116. 116. PuriP. L.SartorelliV.YangX. J.HamamoriY.OgryzkoV. V.HowardB. H.KedesL.WangJ. Y.GraessmannA.NakataniY.LevreroM. 1997b Differential roles of 300 and PCAF acetyltransferases in muscle differentiation. Mol Cell, 1, 35-45.
  117. 117. QuL.Tang 2010 (2010) Bexarotene: a promising anticancer agent. Cancer Chemother Pharmacol, 65 201205 .
  118. 118. RicaudS.VernusB.Bonnieu 2005 (2005) Response of human rhabdomyosarcoma cell lines to retinoic acid: relationship with induction of differentiation and retinoic acid sensitivity. Exp Cell Res, 311 192204 .
  119. 119. Ridgeway AG and Skerjanc IS 2001 Pax3 is essential for skeletal myogenesis and the expression of Six1 and Eya2. J Biol Chem, 276 1903319039 .
  120. 120. RidgewayA. G.WiltonS.SkerjancI. S. 2000 Myocyte enhancer factor 2C and myogenin up-regulate each other’s expression and induce the development of skeletal muscle in 19 cells. J Biol Chem, 275, 41-46.
  121. 121. RohwedelJ.GuanK.WobusA. M. 1999 Induction of cellular differentiation by retinoic acid in vitro. Cells Tissues Organs, 165 190202 .
  122. 122. RomertA.TuvendalP.SimonA.DenckerL.Eriksson 1998 (1998) The identification of a 9-cis retinol dehydrogenase in the mouse embryo reveals a pathway for synthesis of 9-cis retinoic acid. Proc Natl Acad Sci U S A, 95 44044409 .
  123. 123. Rosenfeld MG, Lunyak VV, and Glass CK 2006 Sensors and signals: a coactivator/corepressor/epigenetic code for integrating signal-dependent programs of transcriptional response. Genes Dev, 20 14051428 .
  124. 124. Roth JF, Shikama N, Henzen C, Desbaillets I, Lutz W, Marino S, Wittwer J, Schorle H, Gassmann M, and Eckner R (2003) Differential role of p300 and CBP acetyltransferase during myogenesis: p300 acts upstream of MyoD and Myf5. EMBO J, 22, 5186-5196.
  125. 125. RoyB.TanejaR.Chambon 1995 Synergistic activation of retinoic acid (RA)-responsive genes and induction of embryonal carcinoma cell differentiation by an RA receptor alpha (RAR alpha)-, RAR beta-, or RAR gamma-selective ligand in combination with a retinoid X receptor-specific ligand. Mol Cell Biol, 15, 6481-6487.
  126. 126. SapinV.DolleP.HindelangC.KastnerP.Chambon 1997, Hindelang C, Kastner P, and Chambon 1997 Defects of the chorioallantoic placenta in mouse RXRalpha null fetuses. Dev Biol, 191, 29-41.
  127. 127. SartorelliV.Caretti 2005 (2005) Mechanisms underlying the transcriptional regulation of skeletal myogenesis. Curr Opin Genet Dev, 15 528535 .
  128. 128. SartorelliV.HuangJ.HamamoriY.Kedes 1997 (1997) Molecular mechanisms of myogenic coactivation by 300 direct interaction with the activation domain of MyoD and with the MADS box of MEF2C. Mol Cell Biol, 17, 1010-1026.
  129. 129. 1999, Hamamori Y, Ogryzko V, Chung G, Nakatani Y, Wang JY, and Kedes L (1999) Acetylation of MyoD directed by PCAF is necessary for the execution of the muscle program. Mol Cell, 4 725734 .
  130. 130. Shield MA, Haugen HS, Clegg CH, and Hauschka SD 1996 E-box sites and a proximal regulatory region of the muscle creatine kinase gene differentially regulate expression in diverse skeletal muscles and cardiac muscle of transgenic mice. Mol Cell Biol, 16 50585068 .
  131. 131. ShioharaM.el -DeiryW. S.WadaM.NakamakiT.TakeuchiS.YangR.ChenD. L.VogelsteinB.KoefflerH. P. 1994 Absence of WAF1 mutations in a variety of human malignancies. Blood, 84 37813784 .
  132. 132. Simon AM and Burden SJ 1993 An E box mediates activation and repression of the acetylcholine receptor delta-subunit gene during myogenesis. Mol Cell Biol, 13 51335140 .
  133. 133. Skerjanc IS 1999 Cardiac and skeletal muscle development in 19 embryonal carcinoma cells. Trends Cardiovasc Med, 9, 139-143.
  134. 134. Sorrentino V, Pepperkok R, Davis RL, Ansorge W, and Philipson L (1990) Cell proliferation inhibited by MyoD1 independently of myogenic differentiation. Nature, 345, 813-815.
  135. 135. SucovH. M.DysonE.GumeringerC. L.PriceJ.ChienK. R.EvansR. M. 1994 RXR alpha mutant mice establish a genetic basis for vitamin A signaling in heart morphogenesis. Genes Dev, 8 10071018 .
  136. 136. Sun SY and Lotan 2002 (2002) Retinoids and their receptors in cancer development and chemoprevention. Crit Rev Oncol Hematol, 41 4155 .
  137. 137. SzantoA.NarkarV.ShenQ.UrayI. P.DaviesP. J.Nagy 2004 (2004) Retinoid X receptors: X-ploring their (patho)physiological functions. Cell Death Differ, 11 Suppl 2, S126 -S143.
  138. 138. TajbakhshS.BorelloU.VivarelliE.KellyR.PapkoffJ.DuprezD.BuckinghamM.Cossu 1998 (1998) Differential activation of Myf5 and MyoD by different Wnts in explants of mouse paraxial mesoderm and the later activation of myogenesis in the absence of Myf5. Development, 125 41554162 .
  139. 139. TajbakhshS.MEBuckingham 1994 Mouse limb muscle is determined in the absence of the earliest myogenic factor myf-5. Proc Natl Acad Sci U S A, 91 747751 .
  140. 140. TajbakhshS.RocancourtD.CossuG.Buckingham 1997 (1997) Redefining the genetic hierarchies controlling skeletal myogenesis: Pax-3 and Myf-5 act upstream of MyoD. Cell, 89 127138 .
  141. 141. TanakaT.De LucaL. M. 2009 Therapeutic potential of "rexinoids" in cancer prevention and treatment. Cancer Res, 69 49454947 .
  142. 142. TanakaT.SuhK. S.LoA. M.De LucaL. M. 2007 21WAF1 CIP1 is a common transcriptional target of retinoid receptors: pleiotropic regulatory mechanism through retinoic acid receptor (RAR)/retinoid X receptor (RXR) heterodimer and RXR/RXR homodimer. J Biol Chem, 282, 29987-29997.
  143. 143. Thayer MJ, Tapscott SJ, Davis RL, Wright WE, Lassar AB, and Weintraub 1989 (1989) Positive autoregulation of the myogenic determination gene MyoD1. Cell, 58 241248 .
  144. 144. UmesonoK.EvansR. M. 1989 Determinants of target gene specificity for steroid/thyroid hormone receptors. Cell, 57 11391146 .
  145. 145. VivatV.ZechelC.WurtzJ. M.BourguetW.KagechikaH.UmemiyaH.ShudoK.MorasD.GronemeyerH.Chambon 1997 A mutation mimicking ligand-induced conformational change yields a constitutive RXR that senses allosteric effects in heterodimers. EMBO J, 16, 5697-5709.
  146. 146. VoN.GoodmanR. H. 2001 CREB-binding protein and 300 in transcriptional regulation. J Biol Chem, 276, 13505-13508.
  147. 147. Weintraub 1993 (1993) The MyoD family and myogenesis: redundancy, networks, and thresholds. Cell, 75 12411244 .
  148. 148. WestinS.KurokawaR.NolteR. T.WiselyG. B.Mc InerneyE. M.RoseD. W.MilburnM. V.RosenfeldM. G.GlassC. K. 1998 Interactions controlling the assembly of nuclear-receptor heterodimers and co-activators. Nature, 395 199202 .
  149. 149. Wheeler MT, Snyder EC, Patterson MN, and Swoap SJ 1999 An E-box within the MHC IIB gene is bound by MyoD and is required for gene expression in fast muscle. Am J Physiol, 276, C1069 -C1078.
  150. 150. Williams BA and Ordahl CP 1994 Pax-3 expression in segmental mesoderm marks early stages in myogenic cell specification. Development, 120 785796 .
  151. 151. Willy PJ and Mangelsdorf DJ 1997 Unique requirements for retinoid-dependent transcriptional activation by the orphan receptor LXR. Genes Dev, 11 289298 .
  152. 152. WillyP. J.UmesonoK.OngE. S.EvansR. M.HeymanR. A.MangelsdorfD. J. 1995 LXR, a nuclear receptor that defines a distinct retinoid response pathway. Genes Dev, 9 10331045 .
  153. 153. WongM. W.PisegnaM.LuM. F.LeibhamD.Perry 1994W, Pisegna M, Lu MF, Leibham D, and Perry M (1994) Activation of Xenopus MyoD transcription by members of the MEF2 protein family. Dev Biol, 166 683695 .
  154. 154. WuK.KimH. T.RodriquezJ. L.HilsenbeckS. G.MohsinS. K.XuX. C.LamphW. W.KuhnJ. G.GreenJ. E.BrownP. H. 2002 Suppression of mammary tumorigenesis in transgenic mice by the RXR-selective retinoid, LGD1069. Cancer Epidemiol Biomarkers Prev, 11 467474 .
  155. 155. Yang XJ, Ogryzko VV, Nishikawa J, Howard BH, and Nakatani Y (1996) A p300/CBP-associated factor that competes with the adenoviral oncoprotein E1A. Nature, 382, 319-324.
  156. 156. YokotaY.Ohkubo 1996 (1996) 9-cis-retinoic acid induces neuronal differentiation of retinoic acid-nonresponsive embryonal carcinoma cells. Exp Cell Res, 228 17 .
  157. 157. YuanW.CondorelliG.CarusoM.FelsaniA.Giordano 1996, and Giordano A (1996) Human 300 protein is a coactivator for the transcription factor MyoD. J Biol Chem, 271, 9009-9013.
  158. 158. ZhangJ.HuX.MALazar 1999 A novel role for helix 12 of retinoid X receptor in regulating repression. Mol Cell Biol, 19 64486457 .
  159. 159. ZhangX. K.LehmannJ.HoffmannB.DawsonM. I.CameronJ.GraupnerG.HermannT.TranP.Pfahl 1992I, Cameron J, Graupner G, Hermann T, Tran P, and Pfahl M (1992) Homodimer formation of retinoid X receptor induced by 9-cis retinoic acid. Nature, 358 587591 .
  160. 160. ZhaoW. X.TianM.ZhaoB. X.LiG. D.LiuB.ZhanY. Y.ChenH. Z.Wu 2007 (2007) Orphan receptor TR3 attenuates the 300 acetylation of retinoid X receptor-alpha. Mol Endocrinol, 21, 2877-2889.

Written By

Melanie Le May and Qiao Li

Submitted: 02 April 2012 Published: 22 August 2012