Growth Hormone Gene Family and Its Evolution

Abstract

In this review, we will analyze the family of growth hormone (GH) genes, the territories where they are produced, the proteolytic generation of GH isoforms, both at the pituitary and tissue levels, the biological activity of these molecular forms, and we will describe the new variant GH-V2 and its effects biological. Finally, we will analyze the evolution of the hormone from its starting point with a common gene with PRL to its actions in the most evolved organisms as a true prohormone.

1. Introduction

Just one century ago (1921), it was reported that the administration of bovine anterior pituitary gland extracts to rats induced greater growth in these thus treated animals [1]. This fact led to the assumption that a pituitary factor had to be responsible of the longitudinal growth of the organism, something proven some years later when it was shown that human dwarfs could grow when treated with human pituitary extracts. This pioneer treatment was introduced by Dr. Maurice Raben at Tufts New England Medical Center in 1956 [2]. One of the first dwarfs treated in this way, who reached normal height, was in good health 62 years later [3]. However, the discovery of the human growth hormone (GH) took place a year earlier, 1955, by Choh Hao Li, who worked at the University of California, within his studies on the isolation of pituitary hormones. Li himself developed a method for isolating GH from human cadavers and purifying the hormone so that it could be administered to children with GH-deficiency dwarfism. At the program’s peak in 1973, 82,500 pituitary glands had been collected for treatment of about 3,000 children. Too many pituitary glands removed from human cadavers for so few GH-deficient children treated, so that the program declined. Furthermore, the extraction techniques could not avoid the fact that some of the pituitaries were contaminated with nerve tissue from the posterior pituitary lobe. This led to the appearance of several cases of iatrogenic Creutzfeldt-Jacob disease in patients who had received pituitary extracts in which pathogenic prions were present in the posterior pituitary lobe, something a priori undetectable. In 1962, the Li group made the first approach to the amino acid composition of human pituitary GH [4], a very important finding that allowed him to determine the structure of human GH and synthesize it in small quantities in 1971. This was a very important step forward as it paved the way for the production of the hormone in genetically engineered bacteria (Escherichia Coli), greatly expanding the offer. The product was approved in the United States for sale in October 1985, and a new world began for GH-deficient children, because the advances in the field of genetically engineering permitted to produce practically unlimited quantities of the hormone, pure and safe, by DNA recombinant technology. The hormone was first produced in prokaryotes (Escherichia Coli) and later in eukaryotes (murine fibroblasts). Apparently, the hormones produced in these different type of cells were the same in terms of their effectiveness in producing further growth in GH-deficient children. However, we suspected that there must be some differences depending on the type of cells used for production of GH. In fact, similar doses expressed in international units (IU), corresponded to different weight of the theoretically identical product. To analyze the reason for these differences, we carried out an electrophoretic study of the different GHs existing on the market, and we saw that the hormone produced in murine fibroblasts contained not only the main GH variant GH 22 kDa but also the minor variant 20 kDa. This is logical, since during the expression of human GH, an alternative splicing occurs in 10% of the primary transcript, giving rise to the 20 kDa GH, but this alternative splicing cannot take place in prokaryotes. Initially, these differences were not considered important in terms of effectiveness at the growth level; and it took many years until a utility or any physiological action could be found for this GH 20 kDa. However, different studies carried out in the last years reveal that this isoform in vivo possesses very important properties. For example, it has been seen that GH 20 kDa has several GH-like activities in male high-fat-fed rats, lacking, unlike GH, diabetogenic and lactogenic effects, and failing to increase plasma IGF-I or body length. Instead, treating mice with GH gene deletion (−/−) leads to clear increases in plasma IGF-I, femur length, body length, body weight, and lean body mass and decreased fat mass. These effects are similar to those observed in mice receiving GH treatment. That is, GH 20 kDa can stimulate IGF-I and longitudinal body growth in GH-deficient mice in a similar way to GH 22 kDa, but unlike this main form, the GH isoform promotes growth without inhibiting the actions of insulin and without promoting (in vitro) growth of cancers presenting prolactin receptor (PRLR). The authors conclude that this GH isoform may improve current GH therapies especially in patients at risk for metabolic syndrome or PRLR-positive cancers [5]. Another recent study indicates that GH 20 kDa can internalize into the cytoplasm, as GH 22 kDa does, but its functions appear to be different from those exerted by the main form [6]. From these and other studies, it is expected that in the coming years, the possible beneficial effects of GH 20 kDa on the human body will be better known.

From 1985, when treatments with recombinant GH began, the spectrum of therapeutic applications of this hormone, initially restricted to children with proven GH-deficiency, increased significantly, especially in the pediatric population [7]. This is the case in children suffering from idiopathic short stature, or children with growth retardation caused by chronic renal failure, Turner syndrome, SHOX gene deficiency, Noonan syndrome, but also in adults with GH deficiency as long as they have any other hormonal pituitary deficiency (with the sole exception of Prolactin), and in patients with acquired immunodeficiency syndrome (AIDSS) wasting [8, 9]. This led to GH being considered a hormone that, in addition to its metabolic properties (hyperglycemic by counteracting the effects of insulin, lipolytic and protein anabolic), was considered as the growth hormone. Consequently, the study of its effects was practically restricted to pediatric doctors, whose main interest in GH was to prescribe it for growth in children with short stature.

1.1 The new concepts about GH

At that time, relatively few researchers tried to investigate whether the hormone might could have other actions in the body far beyond those described [10]. Perhaps this situation changed when a previous study was analyzed in which it had been discovered that Insulin and Epidermal Growth Factor (EGF) could be internalized in living cells [11], contrary to the classical concept that protein hormones only carry out their biological actions after interacting with specific membrane receptors, that is, without entering cells, unlike what happens with steroid or thyroid hormones whose receptors are located in the cytoplasm or cell nucleus. That finding led to the Waters group, in Queensland, to investigate whether the same could happen with GH [12, 13, 14]. They demonstrated that this hormone, after interacting with its membrane receptor (GHR), is also internalized along with its GHR through the endosomal pathway. This mechanism allows the translocation of GH and GHR to the nucleus of the cell where they induce the transcription of many different genes. This was a key discovery, as it allegedly implied that GH could perform many diverse, tissue-specific functions, far beyond its classical known effects. Therefore, the detection of GHR in the nucleus of a cell indicates that there has been a previous interaction GH-GHR at the cell membrane. Another important finding that reinforced the idea that GH exerted many different effects in the organism was the demonstration by our group that internalized GH also undergoes specific tissue proteolytic processing, influenced by sex and age (in mice), which gives rise to different molecular forms, whose actions are unknown still [15]. Figure 1 schematizes these concepts.

Figure 1.

Interaction GH-GHR and translocation to the nucleus of the cell. (1) Once GH interacts with its membrane receptor GHR, it is activated (red color) and induces a cascade of phosphorylation (2) that represents GH signaling pathways. (3) GH and GHR are internalized through the endosomal pathway. In endosomes, parts of GH and GHR are proteolytically degraded, but another part translocates to the nucleus (4), where they induce gene transcription (5). In endosomes, GH gives rise (6) to different tissue-specific molecular forms (7), whose actions are still unknown.

According to these concepts, it is logical to understand that GH is a hormone whose actions can affect practically the entire organism, instead of being a mere metabolic hormone and the hormone for growth. Throughout this chapter, we will analyze the role of the genes that make up the GH family, as well as how this hormone is now considered a prohormone given its pleiotropic actions.

2. Family of GH genes

2.1 The GH gene locus

The locus of the human genome where the genes belonging to the GH family are found is located on the long arm of chromosome 17, in the region q22–24 [16, 17, 18] (Figure 2), spanning 48 kb of DNA in the order: 5′-(hGH-1/-hCS-5/hCS-1/hGH-2/hCS-2)-3′ [19, 20]. These genes encode normal human GH (GH-N) and its variant (GH-V) as well as the group of chorionic genes: hCS-L, hCS-A, and hCS-B. These are genes highly conserved in evolution, being the homology among them 5%. The hGH/hCS gene locus seems that it has been evolved by duplication mechanisms [20]. They come from a common ancestral gene with that of Prolactin (PRL) from which they diverged about 300–400 million years ago, although the gene coding for PRL is found on chromosome 6. All these genes have five exons separated by four introns [20] (Figure 2). Among them, GH-1 (now known as GH-N) is expressed in the pituitary gland and peripheral tissues, while the other four seem to be expressed only in the placenta, although N-glycosylated GH-related peptides have been found in human pituitary extracts, suggesting that the hGH-2 gene (now known as GH-variant [GH-V), or other unknown GH-related genes, could be expressed too at the pituitary level, and perhaps in other tissues, since the hGH-N gene lacks the consensus sequence for N-glycosylation observed in some nonplacental GH-related products [21, 22].

Figure 2.

hGH/hCS gene locus localization, expression of the genes of this family and products resulting from it. As noted above, the hGH/hCS gene family locus is located on the long arm of chromosome 17 (q22–q24). The figure also represents the 5′—3′ arrangement of the genes that make up this family, of which GH-N is outlined in the upper rectangle (indicated by a black arrow). In this gene, it can be seen the organization in exons (I–V) separated by introns (A–D). Note that, although the GH-N gene is expressed practically throughout the body (pituitary and multiple tissues and organs), the other four genes seem to be expressed only in the placenta, although as we will see, this is not exactly the case, at least for the GH-V gene.

2.2 Molecular GH heterogeneity

Figure 2 is only a schematic description of the hGH gene family, but pituitary and tissue GH heterogeneity is really high. For example, in the human pituitary, many isoforms of GH-N can be found. Apart from the main GH products, 22 kDa and the 20 kDa form of GH, the latter resulting from the alternative splicing of the mRNA of the GH-N gene, there are several posttranslationally modified forms of GH (N-acylated, deamidated, and O-glycosylated forms of monomeric GH), as well as non-covalent and disulfide-linked oligomers up to pentameric GH [23, 24]. The high heterogeneity of pituitary GH can be seen in Figure 3.

Figure 3.

Heterogeneity of human pituitary (PIT) GH. Western blot showing the high heterogeneity of GH-N obtained from a human pituitary extract. Note the dimer 44 kDa formed by GH 22 kDa, as well as the number of GH-N variants whose biological significance is unknown. The 5 kDa GH form is not shown in this figure.

Furthermore, in human pituitary extracts, some significant amounts of GH variants of lower molecular weight (17 kDa and 5 kDa) can be found. These originate from the selective cleavage of the bond between amino acids 43 and 44, leading to the production of fragments 1–43 and 44–191 in the somatotrophic cells themselves and have also been found in human plasma and tissues and in some different species [15, 25, 26, 27, 28, 29, 30]. Since these forms can be originated by specific cleavage of the main 22 kDa GH form, it is likely that they appear in tissues as a result of posttranslational modification of the hormone. Interestingly, while human GH 22 kDa has both insulin-like and diabetogenic effects, these proteolytically generated fragments show opposite effects: the short 5 kDa GH form potentiates the effects of the insulin, while the 17 kDa form shows diabetogenic activity [28, 29, 30]. Since these two isoforms can be generated under acidic conditions, it has been postulated that they may play a significant physiological role because the hormone in cells is exposed to an acidic environment [25]. Similar conclusions have been drawn regarding the effects of these 17 and 5 kDa GH variants at the end of the last century. According to their data [29, 30, 31], these GH fragments, obtained by recombinant DNA technology, have potent in vivo effects on glucose homeostasis in rodents, but cannot stimulate body growth.

The great heterogeneity of forms produced from GH-N is due to the fact that in the main molecule, there are a series of points susceptible of hydrolysis by proteases, which would be another point in favor of considering GH as a prohormone, which undergoes selective proteolysis, both in the pituitary and in specific tissues, giving rise to peptides with specific biological activities. For example, there are two fragments of GH, GH 177–191 and GH 95–133, of which the first shows a lipolytic activity identical to that of the 22 kDa GH-N form, while the second is a potent mitogen. For its part, the 24 kDa GH-N form caused by retention of signal peptide, like the 12 kDa form originating from the previous one, shows greater somatogenic and lactogenic potency than the 22 kDa GH-N itself.

The alternative splicing of the major pituitary GH-N form 22 kDa occurs in approximately 10% of the transcripts in eukaryotes (Figure 4).

Figure 4.

Alternative splicing of the GH-N gene. After transcription of the GH-N gene, a part of intron III is cleaved (in 10% of transcripts, white rectangle), giving rise to a shorter GH-N form: The 20 kDA GH-N form.

As a consequence of the cleavage of part of intron III in the 20 kDa GH-N form, the amino acids located from position 32 to 46 have disappeared (Figure 5).

Figure 5.

Schematic description of the primary structure of GH-N 22 kDa. The molecule is a polypeptide formed by 191 amino acids arranged in a single chain in which there are two disulfide bridges (S–S) that join the cysteines located at positions 53 and 182 with those located at positions 165 and 189 (red lines). The alternative splicing removes amino acids 32–46 (represented by yellow circles) giving rise to the GH-N 20 kDa form.

The pituitary expression of these two proteins (GH-N 22 kDa and 20 kDa) reflects the alternative utilization of a major (B) and a minor (B′) splice acceptor site in exon 3 of the hGH-N transcript, although it has been postulated that, in addition to the importance of sequences in the immediate vicinity of the two alternative splice acceptor sites, additional sequences further away in the transcript also contribute to this alternative splicing selection. These more distal sequences would not act individually, but instead would interact in such a way that the net level of alternative splicing in exon 3 would be dictated by the overall higher-order structure of the hGH-N transcript [32]. Curiously, a recent study concludes that although there is an important GH-N expression in retina, no splicing variants have been detected [33]. Our group found expression of GH and its receptor in hippocampal neural stem cells, but we did not differentiate whether the main GH form or the GH 20 kDa or both [34] were expressed. In the case of retina, it seems to be a particular situation, because both GH-N 22 kDa and 20 kDa bind and activate the same receptor (GHR); both forms exhibit similar, but not identical physiological activities, although the kinetics of signaling by GH-N 22 kDa and 20 kDa is different being weaker that of 20 kDa than of the major form, which can justify the different biological activities exhibited by these GH-N forms [35]. Furthermore, in humans, 20 kDa GH is a weaker agonist for the receptor of PRL than 22 kDa GH [36]. Other differences between both GH-N forms are related to their half-life in plasma. GH 20 kDa has a lower affinity for GH-binding proteins (GHBP) than GH 22 kDa [37] and fails to form a 1:1 complex with this binding protein [37]. The internalization rate of GH 20 kDa is less than that of GH 22 kDa [38]. Lastly, GH 20 kDa shows a stronger tendency to aggregate than GH 22 kDa [38, 39, 40]. The amplitude of GH secretory bursts is negatively correlated to percentage of body fat while testosterone plasma levels positively affect the secretory burst mass of the hormone [41]. Regarding the existence of the C-terminal disulfide bridge of GH, we know that it has been conserved throughout the evolution, although its role is unknown [42], but they seem to be fundamental for the maintenance of the active conformation of the hormone.

GH has a complex tertiary structure, like other polypeptide hormones. Tertiary structure plays a role in how the hormone regulates receptor activation. GH is a long chain four α-helix bundle proteins (Figure 6).

Figure 6.

Structure of 22 kDa GH-N showing the disposition of its 4 α-helix. Site 1 (outlined in a circle) is the one that first binds to the extracellular domain (ECD) of GHR-1, allowing the receptor to dimerize and Site 2 (outlined in a dotted line circle) of GH can bind to this second receptor (GHR-2). After that, the biological effects of the hormone begin.

A notable feature of their tertiary structure is that it contains no symmetry that might support equivalent binding environments for the receptors. How the two receptors bind to the asymmetric hormone was first revealed from the crystal structure of human growth hormone bound to the ECD of its receptor (hGHR). The characteristic arrangement of the four antiparallel α-helix is essential when it comes to produce the binding of GH to its receptor. Since this binding occurs in 1:2 ratio (one GH molecule and two receptor molecules) in each molecule of GH there are two receptor recognition epitopes, located at opposite ends of the nucleus of α-helix, site 1, and site 2. The structure shows that the two ECDs binding to site 2 and site 1, respectively, use essentially the same set of residues to bind to two sites on opposite faces of the hormone (Figure 6). This binding is characterized by extraordinary local and global plasticity at the binding surfaces. The two binding sites have distinctly different topographies and electrostatic character, leading to different affinities for the receptor ECDs. The high-affinity site, site 1, is always occupied first by ECD1. This sequence of events is required because productive binding of ECD2 at site 2 of the hormone requires additional contacts to a patch of the C-terminal domain of ECD1. The binding of ECD2 is the programmed regulatory step for triggering biological action, and it involves a set of highly tuned interactions among binding interfaces in two spatially distinct binding sites. The energetic relationships between the ECD1-ECD2 contacts and the hormone-ECD2 site 2 interactions are known to be important.

In the case of the 20 kDa GH-N, the reduced affinity exhibited by the receptor of the 22 kDa form suggests that conformational changes occurring in it affecting recognition epitopes.

As Figure 2 shows, another form of GH, the GH variant or GH-V is expressed primarily in the placenta. This GH-V has an identical size that the pituitary 22 kDa GH-N, although both differ from each other by 13 amino acids dispersed throughout GH-V. However, the differences between GH-N and GH-V in terms of their somatogenic and metabolic activities are small. While secretion of pituitary GH-N is pulsatile under control from the hypothalamus, the secretion of placental GH-V is tonic and increases progressively in maternal blood during the second and third trimester [24]. Despite its placental expression, GH-V has been found in human testis [43, 44, 45], where most likely it plays an auto/paracrine function in reproduction (spermatogenesis). In addition to the testis it is likely, as indicated above, that GH-V, or some unknown gene related to it, can be expressed in other territories, including the pituitary.

2.3 The new GH-V variant

As with 22 kDa GH-N, the GH-V also undergoes alternative splicing during gene transcription, resulting from a 45 bp deletion produced by the use of an alternative acceptor site within exon 3 [46]. This group studied the effects of a 7-day treatment with 22 kDa hGH-N or 20 kDa hGH-V, administered subcutaneously (sc) in the same dose on the body composition and the endocrine and metabolic profiles in young male Wistar rats fed either with chow or a high-fat (HF) diet for 4 weeks post-weaning. Total body growth in the 20 kDa hGH-V-treated animals was intermediary between that of control and hGH-N-treated animals. Both 22 kDa hGH-N and 20 kDa hGH-V significantly reduced total body fat mass compared with control animals, and there were no differences between the GH isoforms in anti-lipogenic activity in animals fed the HF diet. Fasting plasma insulin and C peptide were significantly increased in animals on the HF diet and further increased by hGH-N but were unchanged in 20 kDa hGH-V-treated animals compared with saline-treated controls. Plasma volume was increased in hGH-N-treated animals but was unchanged in 20 kDa hGH-V-treated animals compared with controls. Furthermore, 20 kDa hGH-V had reduced lactogenic activity characteristic of hGH-N as tested in vitro compared with the 20 kDa hGH-N and 22 kDa hGH-N variants [46]. In summary, placental 20 kDa hGH-V retains some of the growth-promoting and all antilipogenic activities of pituitary 22 kDa hGH-N but has diminished diabetogenic and lactogenic properties compared with the native 22 kDa hGH-N [46]. These results indicate that some clear differences exist between 20 kDa GH-N and 20 kDa GH-V, perhaps indicating the different metabolic needs existing between not pregnant and pregnant women. A further very recent study was conducted to better characterize the in vivo activities of GHv (current name for this 20 kDa GH-V variant) in both sexes of a GH-deficient model of mice (GH−/− mice). GHv-treated GH−/− mice had significant increases to serum IGF-1, femur length, body length, body weight, and lean body mass and reduced body fat mass similar to mice receiving usual GH treatment. GH-N treatment increased circulating insulin levels and impaired insulin sensitivity; in contrast, both measures were unchanged in GHv-treated mice. The study also tested the ability of GH-N and GHv to stimulate the proliferation of human cancer cell lines and found that GHv has a decreased proliferative response in cancers with high PRLR (GHv is unable to bind to prolactin receptor) [5]. Their findings demonstrate that GHv can stimulate IGF-I and subsequent longitudinal body growth in GH-deficient mice similar to GH-N, but unlike it, GHv promoted growth without inhibiting insulin action and without promoting the growth of PRLR-positive cancers in vitro. Thus, GHv may represent improvements to current GH therapies especially for individuals at risk for metabolic syndrome or PRLR-positive cancers [5]. These results are surprising, especially if we think that this GHv comes from the placenta. What is the reason for showing such improved effects compared with those produced by the classic pituitary GH-N? Is GHv also expressed in tissues other than the placenta? Hopefully in the coming years we will have the answer to this question and, perhaps, many more surprises regarding this variant of GH, although it must be taken into account that these two studies have been carried out in two species, rats and mice, quite different from humans, including the GH that they produce.

3. The evolution of GH gene family

Growth hormone is a classic molecule in the study of the molecular clock, exhibiting a relatively constant rate of evolution in most orders of mammals, except primates and artiodactyls, where a dramatically enhanced rate of evolution (25–50-fold) has been reported. The rapid evolution of primate growth hormone occurred after the divergence of tarsiers (prosimians from Southeast Asia, with four extant species, living in tropical rain forests and included in the prosimian group, although some researchers see them as a link between prosimians and simians) and simians, but before the separation of old world monkeys (OWM) from new world monkeys (NWM). This event of rapid sequence evolution coincided with multiple duplications of the growth hormone gene, suggesting gene duplication as a possible cause of the accelerated sequence evolution. Multiple gene duplications and several gene conversion events both occurred in the evolutionary history of this gene family in OWM/hominoids. GHN genes in both hominoids and OWM are under strong purifying selection, while GHV genes in OWM and hominoids evolved at different evolutionary rates and underwent different selective constraints [47]. A key question is how hormone-receptor preferences have arisen among the duplicates, given that GH and PRL came from the same common gene from which they split 300–400 million years ago, moreover when both receptor genes (GHR and PRLR) show a surprising asynchrony in hormone and receptor gene duplications [48, 49].

Growth hormone exhibits a rare episodic pattern of molecular evolution characterized by sustained burst of rapid changes that are imposed very slow evolution on long periods. For example, there was a remarkable period of rapid change in the evolution of GH in primates or an ancestor and gave rise to the species-descending specificity [50, 51]. This pattern, also seen in placental growth hormones, are a consequence of selection, may reflect changes in the functions of GH additional to its basic growth-inducer effects [50, 51].

The biological specificity of GH is accompanied by significant differences in amino acid sequences, signifying an unusual episodic pattern of molecular evolution in which long periods wherein the sequence is markedly conserved (near-stasis) are interrupted by occasional bursts of rapid changes [50, 51, 52, 53]. This is likely caused by positive Darwinian selection [52].

The burst of rapid changes on GH in the lineage leading to higher primates is particularly marked with substitution about 35% of all amino acid residues. This burst occurred before the separation of lineages leading to man and OWM (in fact, sequences of GHs of man and rhesus monkey are very similar) (Figure 7). However, there are not evidences for duplications of the GH gene in non-primate mammals.

Figure 7.

Phylogenetic tree for mammalians GH (modified from ref. [53]). Y-axis represents million years in evolution; 0 is the current moment. Numbers of substitutions are indicated along the branches of the tree. Thick line indicates a period of rapid evolution for GH. There was another period of rapid evolution, which occurred earlier and gave rise to GH in alpaca, deer, and sheep, but this was omitted to simplify the figure, as a number of species were also omitted. Note the differences between rat and mouse, as well as between slow Loris, marmoset, and rhesus and man.

The episode of rapid change seen for GH evolution in primates appears to be specific to the coding sequence for the mature protein hormone; the pattern of evolution seen for other components of the GH gene is different. Thus, when sequences of the signal peptide, 5′ untranslated region (Figure 2), or introns are analyzed, the burst of rapid evolution seen for the hormone is not present. The burst of change is specific to the protein-coding component of the gene indicating that its cause relates to the protein, resulting from adaptive change in response to selection, or loss of selective constraints due to loss of function. The episode of rapid acceleration is particularly marked for residues associated with receptor binding. There are evidences for an episode of rapid change in the GH receptor during primate evolution [53].

Of interest is the fact that regulatory sequences involved in the transcription of the gene are also affected by evolutionary changes. For example, a negative regulatory element (NRE3) is conserved in most mammals, including slow loris and marmoset. Two binding sites for the transcription factor Pit-1 are in the corresponding position for other mammalian GH genes [53]. It is notable that the distal and proximal sites in slow loris are much closer than in all other mammalian species, reflecting a deletion of about 14 nucleotides in the slow loris gene, which removes the site of the cyclic ANP response element existing in the GH gene. This region is not deleted in other mammalian GH genes, with the exception of marmoset and rabbit, but its different sequence makes it unlikely its functioning as a CRE [54]. A glucocorticoid response element, present in the first intron of the human GH gene, does not appear to be present in the marmoset or the slow loris gene. However, a number of putative negative thyroid hormone responsive elements exist in the human GH gene appear in the marmoset gene, but not in the slow loris gene. Therefore, these differences between these regulatory elements in human, marmoset, and slow loris GH genes indicate that the regulation of GH in these primate species shows significant differences. Of the many differences between non-primate and human GHs, that at position 169 (His in in pig and other non-primates, Asp at the equivalent position, 171, in man) appears to be most important in determining species specificity [55].

As we have seen, although in summary form, there are many changes in the molecular form of GH throughout the evolution, much more marked the more phylogenetically the species are apart from each other [56].

These changes seem to be related to the acquisition of new properties of GH, which leads the original gene to participate only in the regulation of growth. This would explain why GH has such high multiple actions in the body, rather than simply acting for growth, for which the initial gene seemed to emerge.

4. Conclusions

GH is a pleiotropic hormone produced mainly in the pituitary gland and the placenta, although the molecular forms of the hormone depend on the territory where they are produced. From an evolutionary point of view, GH comes from a common gene with PRL, from which it diverged millions of years ago, this may explain why GH and PRL share some physiological actions. All GH genes have five exons separated by four introns. Although GH has traditionally been considered a product of pituitary expression, today we know that the hormone can be produced in practically any territory and cell, the brain, retina, and gonads being particularly important. Currently, we can consider GH as a true prohormone, given the large number of forms that can be generated by proteolytic processing, both in the pituitary itself and in the tissues, processing that is tissue and sex-specific. Although it has been considered that only the classic GH-N can be generated in the pituitary, GH N-glycosylated products have been identified in this gland, suggesting that they may have similarities with placental GH-V. Regarding this, a new placental variant has been identified, GH-V2, which seems to play important roles, even antagonistic to those of pituitary GH-N. The original ancestral GH has undergone many structural changes throughout evolution, which has allowed the number of actions of the hormone to increase as the complexity of organisms increases. This leads to the fact that its initial effect as merely a growth hormone has been increased in a species-specific way.

Conflict of interest

The authors declare no conflict of interest.

Thanks

We want to thank the Foltra Foundation for the facilities given for the writing of this chapter. We also want to thank Diego de Souza (Infirmary, Foltra Medical Center) for his help in formatting the figures and references in this manuscript.