Aldosterone, the principal human mineralocorticoid, acts mainly for sodium reabsorption with potassium and hydrogen excretion. The adrenal cortex is the main site of aldosterone synthesis; however, extra-adrenal tissues such as the nervous, the cardiovascular, and the adipose tissues may be involved. Therefore, its action is mediated via endocrine as well as paracrine or autocrine mode. Aldosterone receptors are distributed extensively in the renal distal nephron and other sites, such as the heart, brain, vessels, and liver. The aldosterone synthase catalyzes the conversion of deoxycorticosterone finally to aldosterone. CYP11B2 gene occupies human chromosome 8q21-22 with nine exons and eight introns. Alteration of aldosterone synthase gene that is attributable to genetic polymorphisms can affect its transcription leading to several cardiovascular disorders such as essential hypertension, myocardial infarction, cardiomyopathies, and atrial fibrillations. Accordingly, it is important to illustrate these polymorphisms and the mechanisms by which they alter the aldosterone synthase gene and produce cardiovascular dysfunctions.
2.1 Aldosterone biosynthesis
Aldosterone is the main human mineralocorticoid. The main site of aldosterone synthesis is the zona glomerulosa (ZG) in the adrenal cortex. However, it can be produced by extra-adrenal tissues such as the central nervous system, the cardiovascular system, and the adipose tissue with a non-detectable physiological relevance and a small contribution to circulating aldosterone levels .
It is synthesized from cholesterol by a group of enzymatic cascade (Figure 1). First, cholesterol is translocated into the mitochondria across its wall mediated by steroidogenic acute regulatory protein (StAR). Cholesterol is then converted to pregnenolone, through three reactions, a 20α-hydroxylation, a 22-hydroxylation, and cleavage of the bond between C-20 and C-22 catalyzed by the
The produced pregnenolone is then released into the cytoplasm where it undergoes dehydrogenation of the 3β-hydroxyl group and isomerization of the double bond at C-5 to Δ4 by 3β-hydroxysteroid dehydrogenase (3β-HSD) and converted to progesterone . Furthermore, progesterone undergoes 21-hydroxylation by the
2.2 Mechanism of action of aldosterone
The action of aldosterone is mediated though mineralocorticoid receptor (MR), a specific nuclear receptor that comprises N-terminal domain, DNA-binding domain, and C-terminal ligand-binding domain. It is present as a hetero-oligomeric complex with heat-shock proteins in the cell. Once aldosterone binds to its receptor, it undergoes a conformational change resulting in dissociation of the associated proteins, dimerization, and nuclear translocation . This hormone receptor complex combines with the steroid responsive elements in the 5′-UTR of aldosterone-responsive genes with the release of aldosterone-induced proteins (AIP) that enhances or suppresses gene transcription .
The serine threonine glucocorticoid regulated kinase 1 (sgk1), which is one of the AIP proteins, has been phosphorylated and activated by the aldosterone, which in turn phosphorylates epithelial sodium channel (ENaC) regulatory protein, known as Nedd4–2, reducing its binding to ENaC  with subsequent increase in ENaC density and stability at the apical membrane resulting in increased ENaC-dependent Na+ reabsorption . The glucocorticoid-induced leucine zipper (GILZ) and the corticosteroid hormone-induced factor (CHIF) are also AIP proteins. GILZ interacts with aldosterone inhibiting the ERK signaling pathway, thus liberating ENaC from Nedd4 proteins; accordingly, its action in blocking sodium reabsorption is inhibited . CHIF may affect the baso-lateral Na/K-pump, resulting in increased sodium reabsorption with potassium or hydrogen ion excretion (Figure 2) . Aldosterone also exerts a genomic action via modulating the gene expression and subsequent protein production that result in a lag time of 1–2 h before a noticeable change in target cell activity occur .
2.3 Physiological action of aldosterone
As the kidneys are the main site of action for aldosterone, MRs are confined mainly in high concentration to the renal distal convoluted tubules and collecting duct controlling the apically located epithelial sodium channels at their luminal cells. Mineralocorticoid receptors also exist in other epithelial sites, such as the colon, sweat gland ducts, salivary glands , and non-epithelial sites including myocytes, endocardium of the heart, brain, vascular smooth muscle, liver, and leukocytes . The main action of aldosterone is stimulation of sodium reabsorption in the kidney and at other secretory epithelial sites with excretion of potassium and hydrogen ions  possibly mediated via increasing the opening periods of the existing ion channels or increasing their number .
In cardiovascular system, aldosterone promotes myocardial hypertrophy and fibrosis via increasing collagen I synthesis in cardiac fibroblasts and also elevating endothelin receptor numbers that further increases collagen synthesis . So, increased expression of mineralocorticoid receptors in the heart may result in left-ventricular hypertrophy in normotensive subjects . Aldosterone also stimulates vascular constriction via enhancing the pressor response to catecholamines and impairing the vasodilatory response to acetylcholine or by upregulation of angiotensin II receptors . Also, aldosterone excess can trigger collagen deposition in blood vessels, enhancing vascular remodeling and reducing compliance . In the CNS, it appears to regulate blood pressure, salt appetite, and sympathetic tone .
Under normal circumstances, it is likely that cardiac MRs are occupied by glucocorticoid due to its higher circulating concentration exerting antagonistic effect attenuating the rise in blood pressure and the cardiac fibrosis caused by aldosterone alone .
3. Cytochrome P450 11B2 (
3.1 Reaction catalyzed
3.2 Molecular structure
3.3 Protein structure
3.4 Tissue-specific expression
3.5 Regulation of
Adrenal steroidogenesis is under both acute and chronic regulation by tropic hormones. The acute response occurs within minutes and involves the mobilization of cholesterol from intracellular stores to the mitochondrial membrane in response to ACTH, angiotensin II, K+, and their respective intracellular messenger pathways. The chronic response takes several hours and involves the transcription of the genes encoding the steroidogenic enzymes .
3.5.1 Signaling pathways that regulate aldosterone production
Adrenocorticotrophin (ACTH) is a 39 amino acid peptide released from the anterior pituitary in pulsatile and diurnal rhythm with the highest levels in the morning and the lowest at night . ACTH exerts its effects by binding to its receptor (ACTH-R), a G-protein-coupled receptor. ACTH acutely stimulates aldosterone secretion but in the long term has an inhibitory effect on
Acute stimulation of aldosterone production has suggested to be mediated via activation of StAR protein production. Also, ACTH after binding to its receptor can activate adenylate cyclase, resulting in an increased intracellular cAMP concentration, activation of protein kinase A (PKA), and calcium influx via calcium channels (Figure 5) .
On the other hand, chronic ACTH stimulation may depress serum aldosterone level as cyclic AMP, the second messenger for ACTH, desensitizes adrenocortical cells to angiotensin II by causing a reduction in the expression of angiotensin II receptors. ACTH may also decrease aldosterone production by stimulating the expression of
Under normal circulating ACTH levels, the glomerulosa maintains
220.127.116.11 Angiotensin II
Angiotensin II is thought to stimulate aldosterone synthesis as result of sodium depletion and extracellular fluid volume reduction through various intracellular signaling pathways. However, the best characterized pathway is the activation of phospholipase C (Figure 5). It is mediated by acting on angiotensin 1 (AT1) receptor, a specific G-protein-coupled receptor that activates phospholipase C. Once activated, phospholipase C hydrolyses phosphatidyl inositol 4,5-biphosphate (PIP2) to 1,4,5 inositol triphosphate (IP3) and 1,2-diacylglycerol (DAG) resulting in release of Ca2+ from intracellular stores and activation of protein kinase C (PKC), respectively. The increased intracellular Ca2+ concentration activates calmodulin and Ca2+/calmodulin-dependent protein kinases (CaM kinases) [28, 29] to phosphorylate and activate transcription factors as activating transcription factor 1 (ATF-1), cAMP-response-element binding protein (CREB), nerve growth factor IB (NGFIB), and nuclear receptor related 1 protein (
The level of potassium affects renin secretion as well as having a direct effect on the adrenal cortex to increase aldosterone secretion. Increased extracellular K+ (like angiotensin II) stimulates aldosterone secretion through an increase in intracellular Ca2+ and activation of calmodulin kinases with consequent phosphorylation of transcription factors leading to stimulation of
3.5.2 Transcriptional regulation of
As mentioned previously, chronic regulation of steroidogenesis involves transcription of the genes encoding the necessary steroidogenic enzymes. This is mediated by alteration of trans-acting factors that bind to the cis-regulatory elements within the 5′ regulatory regions of the target genes. Investigation of the 5′ regulatory regions of the
18.104.22.168 Ad1 (CRE)
The Ad1 element closely resembles a consensus cAMP regulatory element (CRE) site. CREs plays an essential role in cAMP-dependent gene expression of a wide variety of genes. Proteins, such as the CRE-binding protein (CREB) and the highly related activating transcription factors (ATF), bind to CRE sites to initiate transcription. CREB binds to DNA as a dimer and has a conserved region of leucine residues (leucine zipper) at its C terminus that enables dimerization and sequence specific DNA binding .
CREB functions not only as a component of a variety of signaling pathways, particularly PKA, but also mitogen-activated protein kinases (MAPKs) and CaMKs. All these pathways mediate CREB-induced transcription by phosphorylating CREB at residue serine 133 . The phosphorylated serine 133 binds another protein referred to as the CREB-binding protein (CBP). CBP is a 265-kDa nuclear protein, which binds to phosphorylated CREB and allows recruitment and stabilization of the RNA polymerase II transcription complex on the promoter of CREB target genes .
Using electrophoretic mobility shift assay, the
22.214.171.124 AD4 (SF-1)
The Ad4 site (CCAAGGTC) is also found to be important in the regulation of the
SF-1 is an orphan member of the nuclear hormone receptor superfamily, with potential phosphorylation sites for cAMP-dependent kinases, CaMK or PKC, suggesting a role for SF-1 in cAMP-dependent transcription. SF-1 is a 53 kDa protein consisting of a zinc finger domain and ligand binding/dimerization domain. The hydroxyl cholesterols enhance SF-1-dependent transcriptional activity in vitro, suggesting that SF-1 is a ligand-activated receptor. However, this is a controversial finding and needs further clarification .
SF-1 is expressed exclusively in steroidogenic tissues and plays an essential role in the development and function of the primary steroidogenic tissues . Within the adrenal, SF-1 has been found to play a key role in the transcriptional regulation of most of the steroid hydroxylase genes (
The Ad4 or SF-1 site has been identified in all steroid P450s based on sequence alignments, including at position −351/−343 (AGGTCC) of
Electrophoretic mobility shift assay (EMSA) analysis of the −129/−114 (Ad5) element (CTCCAGCCTTGACCTT) has shown that it binds several nuclear proteins, including SF-1 and another orphan nuclear receptor, chicken ovalbumin upstream promoter transcription factor (COUP-TF). On the bovine
The transcription factors nerve growth factor IB (NGFIB) and nuclear receptor-related 1 protein (NURRl) are members of the NGFIB family of orphan nuclear receptors that bind to a consensus sequence NGFI-B response element (NBRE) (AAAGGTCA). These factors can also bind to the Ad5 element as well as a novel NBRE-1 site at −766/−759. Both these transcription factors increase
4. Genotypic variants and haplotypes in
Several polymorphisms have been identified in
The second polymorphism is a gene intronic conversion (IC) whereby a fragment of the “wild-type (Wt)” intron 2 of
Beside the previously described polymorphisms of
CYP11B2genotypic variants and hypertension
Hypertension is a complex progressive cardiovascular disorder, not just a scale of threshold blood pressure values, with many causes that result in both functional and structural cardiac and vascular system abnormalities that damage the heart, kidneys, brain, vasculature, and other organs, and lead to premature morbidity and death . Essential hypertension is a multifactorial disorder, predisposed by genetic and environmental factors . Growing evidences suggest the potential role of genetic alteration affecting genes encoding the aldosterone synthesis pathway enzymes with associated enhanced aldosterone production resulting in progression to essential hypertension .
Several polymorphic variants of
On the other hand,
CYP11B2genotypic variants and atrial fibrillation
Atrial fibrillation (AF) is the most clinically prevalent type of cardiac arrhythmia which may be precipitated due to the presence of underlying heart disease such as valvular dysfunction, ventricular dysfunction, and hypertension. However, AF does not exist in some patients with one or more of these risk factors and presents in others without any risk factors. Accordingly, the genetic role had been emerged in the predisposition for AF. A positive family history of AF in at least one parent was suggested by a recent Framingham Heart study on 2243 participants to be associated with an 85% increased relative risk for AF .
Taking together this genetic role, two types of AF have been identified regarding the heredity characteristics as familial and non-familial AF. Recent studies have detected several candidate genes which were suggested to be associated with the familial AF type such as genes encoding for subunits of potassium or sodium channels, sarcolipin, connexin 40, endothelial nitric oxide synthase, interleukin 10, and RAAS .
Association of genetic variants of renin angiotensin aldosterone system (RAAS) system with non-familial AF was suggested by Tsai et al.  using a risk-factor matched design.
The possible mechanisms for the association of
CYP11B2genotypic variants and coronary artery disease
Coronary artery disease (CAD) is a complex disorder comprised two major subsequent events: coronary atherosclerosis and myocardial infarction (MI). Despite the major progress in diagnosis of CAD, the pathogenesis and possible risk factors need further evaluation. The classical risk factors including positive family history, smoking, high body mass index, and disorders as hyperlipidemia, hypertension, and diabetes mellitus have been reported to be responsible for no more than 50% of total risk factors for CAD. Accordingly, genetic background seems also to participate in the predisposition for CAD .
Among the different polymorphisms described for
The underlying mechanism by which the
CYP11B2genotypic variants and hypertrophic cardiomyopathy
Hypertrophic cardiomyopathy (HCM) is a clinical cardiac dysfunction characterized mainly by hypertrophy of left ventricle. HCM is an autosomal dominant disorder and its diagnosis needs to exclude other cardiac or systemic causes of increased ventricular wall thickening. HCM comprises different histological features as cardiomyocyte hypertrophy, myofibrillar disarray, and fibrosis. Several factors are involved in the pathogenesis of HCM, with the genetic element has an upper hand. Mutations and polymorphisms in genes encoding the sarcomere proteins and renin-angiotensin-aldosterone system (RAAS) seem to be related to the predisposition for left ventricular hypertrophy (LVH) and HCM .
Evidences suggested that aldosterone seems to play a major role in the progression of LVH and HCM as it is produced locally in the heart and
Several polymorphisms in the
CYP11B2genotypic variants and heart failure
Heart failure (HF) is a cardiac disorder characterized by cardiac remodeling with subsequent cardiac dysfunction . It is a multifactorial disease, which is precipitated due to underlying cardiac disorders as hypertension, CAD, valvular dysfunction, arrhythmia, and cardiomyopathy. Integrated neuronal and hormonal elements are involved in the pathogenesis of HF including activation of the sympathetic nervous system and the renin-angiotensin-aldosterone system, and other mediators as endothelin, vascular endothelial growth factor, and inflammatory cytokines. Based on the previously described neurohormonal factors, therapeutic approaches for HF have developed to block these factors including angiotensin-converting enzyme inhibitors (ACEI), and aldosterone antagonists . Several risk and prognostic factors have influenced the progression of chronic congestive heart failure (CHF) including age, New York Heart Association (NYHA) class, renal function, and comorbidities such as atrial fibrillation, diabetes mellitus, and ischemic heart disease . However, genetic elements seem to affect the risk, severity, and therapeutic response of HF .
A study by McNamara et al.  was performed on a total of 354 subjects from A-HeFT (African American Heart Failure Trial). They detected a higher frequency of −344 TT genotypes carriers (61%) than CC genotypes carriers (6.2%) in African Americans. Also, the −344 C allele was associated with significantly poorer HF hospitalization-free survival and a higher rate of death. Moreover, the therapeutic influence of nitric oxide donor (isosorbide dinitrate and hydralazine) was augmented in the presence of the TT genotype.
Another study by Feola et al.  on 175 patients from the European continental ancestry congestive heart failure (CHF) population and revealed that no significant differences between −344 CC and −344 TT genotypes carriers regarding cardiac output, end-systolic or diastolic left ventricle diameter, left ventricular ejection fraction (LVEF), and pro-natriuretic peptide (BNP). Also, −344 C allele carriers have a higher degree of disability (Barthel Index), NYHA class, and a lower cardiac index. Moreover, the two groups showed a similar clinical outcomes either death or re-hospitalization after 4 years follow up period.
In 107 Black South African patients with dilated cardiomyopathy in functional class II–III, Tiago et al.  detected that the −344C allele was associated with improvement of LVEF after traditional therapy despite it is not related to the baseline LVEF as well as left ventricular dimension. Also, the allele distribution has proven to be different among races as the C allele was significantly more frequent in non-African-American HF patients if compared with African-American patients and associated only in the African-American with a lower end-systolic left ventricular diameter at 1 year follow-up . Moreover, the association of −344C allele was confirmed as a risk factor for the progression of AF in a cohort of 194 African-American HF patients .
First of all thanks to gracious ALLAH who guided me to the frank clear way. Second, I dedicate this work to my parents and my dear professor Murad Ahmad Murad for their encouragement and sustained support. And finally, I am grateful to my lovely niece, Marium.