Potassium channel mutant KCNJ5 T158A expression in HAC-15 cells increases aldosterone synthesis

Novel genes in primary aldosteronism

PURPOSE OF REVIEW

Novel high-throughput genetic techniques have increased the pace of discoveries in the field of primary aldosteronism. Mutations in the potassium channel gene KCNJ5 are a cause of familial and sporadic forms of primary aldosteronism with around 30-40% of aldosterone-producing adenomas being affected by somatic mutations.

RECENT FINDINGS

Exome sequencing of tumors without KCNJ5 mutations revealed genetic alterations in the ATPases ATP1A1 and ATP2B3, with a combined prevalence of 5-7%. Mutations in the gene encoding a subunit of the Ca channel Cav1.3 (CACNA1D) were described with a prevalence of 5-8%. In addition, a new syndrome consisting of primary aldosteronism, seizures, and neuromuscular disease with germline CACNA1D mutations could be identified. All these genetic variants enhance Ca-mediated signalling and steroidogenesis in affected glomerulosa cells and provide the molecular basis for autonomous aldosterone secretion. Furthermore, the pattern of genetic alterations allows for subgrouping of patient cohorts with potentially distinct clinical features including sex and age distribution as well as endocrine and cardiovascular endpoints.

SUMMARY

Altogether in around 50% of aldosterone-producing adenomas, a somatic point mutation can be identified as the underlying genetic cause. These findings will provide the framework for potential identification of new biomarkers and therapeutic targets of this most common form of secondary hypertension.

Genetic spectrum and clinical correlates of somatic mutations in aldosterone-producing adenoma

Primary aldosteronism is the most common form of secondary hypertension. Somatic mutations in KCNJ5, ATP1A1, ATP2B3, and CACNA1D have been described in aldosterone-producing adenomas (APAs). Our aim was to investigate the prevalence of somatic mutations in these genes in unselected patients with APA (n=474), collected through the European Network for the Study of Adrenal Tumors. Correlations with clinical and biochemical parameters were first analyzed in a subset of 199 patients from a single center and then replicated in 2 additional centers. Somatic heterozygous KCNJ5 mutations were present in 38% (180/474) of APAs, whereas ATP1A1 mutations were found in 5.3% (25/474) and ATP2B3 mutations in 1.7% (8/474) of APAs. Previously reported somatic CACNA1D mutations as well as 10 novel CACNA1D mutations were identified in 44 of 474 (9.3%) APAs. There was no difference in the cellular composition of APAs or in CYP11B2, CYP11B1, KCNJ5, CACNA1D, or ATP1A1 gene expression in APAs across genotypes. Patients with KCNJ5 mutations were more frequently female, diagnosed younger, and with higher minimal plasma potassium concentrations compared with CACNA1D mutation carriers or noncarriers. CACNA1D mutations were associated with smaller adenomas. These associations were largely dependent on the population structure of the different centers. In conclusion, recurrent somatic mutations were identified in 54% of APAs. Young women with APAs are more likely to be KCNJ5 mutation carriers; identification of specific characteristics or surrogate biomarkers of mutation status may lead to targeted treatment options.

Autoimmune mechanisms activating the angiotensin AT1 receptor in 'primary' aldosteronism

CONTEXT

The mechanisms causing excessive aldosterone production and hypertension in primary aldosteronism (PA) are complex and often incompletely recognized. Autoantibodies to the angiotensin AT1 receptor (AT1R) have been reported in some PA patients with an aldosterone-producing adenoma but not with idiopathic adrenal hyperplasia.

OBJECTIVE

We investigated whether these autoantibodies will activate AT1R and thereby potentially contribute to the pathophysiology of PA.

DESIGN

AT1R autoantibody activity in sera and/or IgG purified from 13 biochemically confirmed PA patients was measured using AT1R-transfected cells, and their contractile effects were assayed using perfused rat cremaster arterioles. Aldosterone stimulation was measured in vitro using isolated human adrenal carcinoma (HAC15) adrenal cells. These data were compared with sera obtained from a group of normotensive control subjects who were expected to have negligible AT1R autoantibodies.

RESULTS

Sera from each of the 13 PA patients significantly increased AT1R activation in AT1R-transfected cells compared with 20 control subjects, and this activity was inhibited by the selective AT1R blocker losartan. Sera and IgG purified from AT1R autoantibody-positive sera demonstrated significant vasoconstrictive effects in isolated rat cremaster arterioles and were blocked by losartan. Moreover, the AT1R autoantibody-positive IgG directly stimulated aldosterone production in the cultured adrenal cells and enhanced angiotensin-induced aldosterone production in these cells, and these effects were blocked by candesartan.

CONCLUSIONS

These data support a probable pathophysiological role for AT1R autoantibodies in PA and thereby raise important etiological and therapeutic implications.

Overview of the genetic determinants of primary aldosteronism

Primary aldosteronism is the most common cause of secondary hypertension. The syndrome accounts for 10% of all cases of hypertension and is primarily caused by bilateral adrenal hyperplasia or aldosterone-producing adenoma. Over the last few years, the use of exome sequencing has significantly improved our understanding of this syndrome. Somatic mutations in the KCNJ5, ATP1A1, ATP2B3 or CACNA1D genes are present in more than half of all cases of aldosterone-producing adenoma (~40%, ~6%, ~1% and ~8%, respectively). Germline gain-of-function mutations in KCNJ5 are now known to cause familial hyperaldosteronism type III, and an additional form of genetic hyperaldosteronism has been reported in patients with germline mutations in CACNA1D. These genes code for channels that control ion homeostasis across the plasma membrane of zona glomerulosa cells. Moreover, all these mutations modulate the same pathway, in which elevated intracellular calcium levels lead to aldosterone hyperproduction and (in some cases) adrenal cell proliferation. From a clinical standpoint, the discovery of these mutations has potential implications for patient management. The mutated channels could be targeted by drugs, in order to control hormonal and overgrowth-related manifestations. Furthermore, some of these mutations are associated with high cell turnover and may be amenable to diagnosis via the sequencing of cell-free (circulating) DNA. However, genotype-phenotype correlations in patients harboring these mutations have yet to be characterized. Despite this recent progress, much remains to be done to elucidate the yet unknown mechanisms underlying sporadic bilateral adrenal hyperplasia.

Lower expression of the TWIK-related acid-sensitive K+ channel 2 (TASK-2) gene is a hallmark of aldosterone-producing adenoma causing human primary aldosteronism

CONTEXT

The molecular mechanisms of primary aldosteronism, a common cause of human hypertension, are unknown, but alterations of K(+) channels can play a key role.

OBJECTIVE

The objective of the study was to investigate the following: 1) the expression of the Twik-related acid-sensitive K(+) channels (TASK) in aldosterone producing adenomas (APAs); 2) the role of TASK-2 in aldosterone synthesis; and 3) the determinants of TASK-2-blunted expression in APAs.

DESIGN

We analyzed the transcriptome and the microRNA profiles of 32 consecutive APAs and investigated the protein expression and localization of TASK-2 in APA and adrenocortical cell lines (H295R and HAC15) using immunoblotting and confocal microscopy. The functional effect of TASK-2 blunted activity caused by a dominant-negative mutation on steroidogenic enzymes, and aldosterone production was also assessed. TASK-2 regulation by selected microRNA was studied by a luciferase assay.

RESULTS

TASK-2 was consistently less expressed at the transcript and protein levels in APAs than in the normal human adrenal cortex. H295R cell transfection with a TASK-2 dominant-negative mutant construct significantly increased the aldosterone production by 153% and the gene expression of aldosterone synthase (CYP11B2, gene expression fold change 3.1 vs control, P < .05) and the steroidogenic acute regulatory protein (gene expression fold change 1.8 vs control, P < .05). Two microRNAs, hsa-miR-23 and hsa-miR-34, were found to decrease the TASK-2 expression by binding to the 3' untranslated region of the TASK-2 gene.

CONCLUSIONS

The TASK-2 channel lower expression represents a hallmark of APA and is associated with a higher expression of hsa-miR-23 and hsa-miR-34. The ensuing blunted TASK-2 activity increased the production of aldosterone in vitro and the expression of steroidogenic acute regulatory protein and CYP11B2. Hence, the lower expression of TASK-2 channel in APA cells can explain high aldosterone secretion in human primary aldosteronism despite the suppression of angiotensin II, hypertension, and hypokalemia.

Pharmacology and pathophysiology of mutated KCNJ5 found in adrenal aldosterone-producing adenomas

Somatic mutations of the potassium channel KCNJ5 are found in 40% of aldosterone producing adenomas (APAs). APA-related mutations of KCNJ5 lead to a pathological Na(+) permeability and a rise in cytosolic Ca(2+), the latter presumably by depolarizing the membrane and activating voltage-gated Ca(2+) channels. The aim of this study was to further investigate the effects of mutated KCNJ5 channels on intracellular Na(+) and Ca(2+) homeostasis in human adrenocortical NCI-H295R cells. Expression of mutant KCNJ5 led to a 2-fold increase in intracellular Na(+) and, in parallel, to a substantial rise in intracellular Ca(2+). The increase in Ca(2+) appeared to be caused by activation of voltage-gated Ca(2+) channels and by an impairment of Ca(2+) extrusion by Na(+)/Ca(2+) exchangers. The mutated KCNJ5 exhibited a pharmacological profile that differed from the one of wild-type channels. Mutated KCNJ5 was less Ba(2+) and tertiapin-Q sensitive but was inhibited by blockers of Na(+) and Ca(2+)-transporting proteins, such as verapamil and amiloride. The clinical use of these drugs might influence aldosterone levels in APA patients with KCNJ5 mutations. This might implicate diagnostic testing of APAs and could offer new therapeutic strategies.

Somatic ATP1A1 , ATP2B3 , and KCNJ5 Mutations in Aldosterone-Producing Adenomas

Aldosterone-producing adenomas (APAs) cause a sporadic form of primary aldosteronism and somatic mutations in the KCNJ5 gene, which encodes the G-protein–activated inward rectifier K + channel 4, GIRK4, account for ≈40% of APAs. Additional somatic APA mutations were identified recently in 2 other genes, ATP1A1 and ATP2B3 , encoding Na + /K + -ATPase 1 and Ca 2+ -ATPase 3, respectively, at a combined prevalence of 6.8%. We have screened 112 APAs for mutations in known hotspots for genetic alterations associated with primary aldosteronism. Somatic mutations in ATP1A1 , ATP2B3 , and KCNJ5 were present in 6.3%, 0.9%, and 39.3% of APAs, respectively, and included 2 novel mutations (Na + /K + -ATPase p.Gly99Arg and GIRK4 p.Trp126Arg). CYP11B2 gene expression was higher in APAs harboring ATP1A1 and ATP2B3 mutations compared with those without these or KCNJ5 mutations. Overexpression of Na + /K + -ATPase p.Gly99Arg and GIRK4 p.Trp126Arg in HAC15 adrenal cells resulted in upregulation of CYP11B2 gene expression and its transcriptional regulator NR4A2. Structural modeling of the Na + /K + -ATPase showed that the Gly99Arg substitution most likely interferes with the gateway to the ion binding pocket. In vitro functional assays demonstrated that Gly99Arg displays severely impaired ATPase activity, a reduced apparent affinity for Na + activation of phosphorylation and K + inhibition of phosphorylation that indicate decreased Na + and K + binding, respectively. Moreover, whole cell patch-clamp studies established that overexpression of Na + /K + -ATPase Gly99Arg causes membrane voltage depolarization. In conclusion, somatic mutations are common in APAs that result in an increase in CYP11B2 gene expression and may account for the dysregulated aldosterone production in a subset of patients with sporadic primary aldosteronism.

Minireview: potassium channels and aldosterone dysregulation: is primary aldosteronism a potassium channelopathy?

Primary aldosteronism is the most common form of secondary hypertension and has significant cardiovascular consequences. Aldosterone-producing adenomas (APAs) are responsible for half the cases of primary aldosteronism, and about half have mutations of the G protein-activated inward rectifying potassium channel Kir3.4. Under basal conditions, the adrenal zona glomerulosa cells are hyperpolarized with negative resting potentials determined by membrane permeability to K(+) mediated through various K(+) channels, including the leak K(+) channels TASK-1, TASK-3, and Twik-Related Potassium Channel 1, and G protein inward rectifying potassium channel Kir3.4. Angiotensin II decreases the activity of the leak K(+) channels and Kir3.4 channel and decreases the expression of the Kir3.4 channel, resulting in membrane depolarization, increased intracellular calcium, calcium-calmodulin pathway activation, and increased expression of cytochrome P450 aldosterone synthase (CYP11B2), the last enzyme for aldosterone production. Somatic mutations of the selectivity filter of the Kir3.4 channel in APA results in loss of selectivity for K(+) and entry of sodium, resulting in membrane depolarization, calcium mobilization, increased CYP11B2 expression, and hyperaldosteronism. Germ cell mutations cause familial hyperaldosteronism type 3, which is associated with adrenal zona glomerulosa hyperplasia, rather than adenoma. Less commonly, somatic mutations of the sodium-potassium ATPase, calcium ATPase, or the calcium channel calcium channel voltage-dependent L type alpha 1D have been found in some APAs. The regulation of aldosterone secretion is exerted to a significant degree by activation of membrane K(+) and calcium channels or pumps, so it is not surprising that the known causes of disorders of aldosterone secretion in APA have been channelopathies, which activate mechanisms that increase aldosterone synthesis.

Gene mutations that promote adrenal aldosterone production, sodium retention, and hypertension

Primary aldosteronism (PA) is the most common form of secondary hypertension, found in about 5% of all hypertension cases, and up to 20% of resistant hypertension cases. The most common forms of PA are an aldosterone-producing adenoma and idiopathic (bilateral) hyperaldosteronism. Rare genetic forms of PA exist and, until recently, the only condition with a known genetic mechanism was familial hyperaldosteronism type 1, also known as glucocorticoid-remediable aldosteronism (FHA1/GRA). FHA type 3 has now been shown to derive from germline mutations in the KCNJ5 gene, which encodes a potassium channel found on the adrenal cells. Remarkably, somatic mutations in KCNJ5 are found in about one-third of aldosterone-producing adenomas, and these mutations are likely to be involved in their pathogenesis. Finally, mutations in the genes encoding an L-type calcium channel (CACNA1D) and in genes encoding a sodium–potassium adenosine triphosphatase (ATP1A1) or a calcium adenosine triphosphatase (ATP2B3) are found in other aldosterone-producing adenomas. These findings provide a working model, in which adenoma formation and/or aldosterone production in many cases derives from increased calcium entry, which drives the pathogenesis of primary aldosteronism.

Mutations in KCNJ5 determines presentation and likelihood of cure in primary hyperaldosteronism

INTRODUCTION

Primary hyperaldosteronism (PA) is a common cause of secondary hypertension. Two recurrent mutations (G151R and L168R) in the potassium channel gene KCNJ5 have been identified that affect the Kir3.4 potassium channel found in the cells of the zona glomerulosa of the adrenal gland. The aim of this study was to determine the prevalence of KCNJ5 mutations in an Australian cohort of patients and to correlate these findings with clinical outcome data, in order to describe the clinical impact on patients who harbour this mutation.

METHODS

Direct Sanger sequencing for KCNJ5 on DNA from adrenal tumour tissue of 83 patients with PA in a cohort study was undertaken and mutation status correlated with clinical outcome data.

RESULTS

Seventy-one of 83 patients (86%) had adrenocortical adenomas and 12 patients (14%) had bilateral adrenal hyperplasia. A total of 34 (41%) patients were found to have heterozygous somatic mutations in KCNJ5, G151R and L168R. No germ line mutations were identified. Patients with mutations were predominately female (68% versus 49%) and significantly younger at presentation (48 versus 55 years). When correlated with clinical data, our results demonstrated that patients with KCNJ5 mutations were more likely to be cured following surgery without the requirement for ongoing medications.

CONCLUSIONS

Our findings in a large Australian cohort show that patients with mutations in KCNJ5 present earlier with the signs and symptoms of PA benefit from surgical intervention. Moreover, our results highlight the importance of a thorough workup and management plan for younger patients who present with hypertension.

a Novel Y152C KCNJ5 mutation responsible for familial hyperaldosteronism type III

CONTEXT

Primary aldosteronism is a heterogeneous group of disorders comprising both sporadic and familial forms. Mutations in the KCNJ5 gene, which encodes the inward rectifier K(+) channel 4 (G protein-activated inward rectifier K(+) channel 4, Kir3.4), cause familial hyperaldosteronism type III (FH-III) and are involved in the pathogenesis of sporadic aldosterone-producing adenomas.

OBJECTIVE

The objective of the study was to characterize the effects of a newly described KCNJ5 mutation in vitro.

PATIENTS AND METHODS

The index case is a 62-year-old woman affected by primary aldosteronism, who underwent left adrenalectomy after workup for adrenal adenoma. Exon 1 of KCNJ5 was PCR amplified from adrenal tissue and peripheral blood and sequenced. Electrophysiological and gene expression studies were performed to establish the functional effects of the new mutation on the membrane potential and adrenal cell CYP11B2 expression.

RESULTS

KCNJ5 sequencing in the index case revealed a new p.Y152C germline mutation; interestingly, the phenotype of the patient was milder than most of the previously described FH-III families. The tyrosine-to-cysteine substitution resulted in pathological Na(+) permeability, cell membrane depolarization, and disturbed intracellular Ca(2+) homeostasis, effects similar, albeit smaller, to the ones demonstrated for other KCNJ5 mutations. Gene expression studies revealed an increased expression of CYP11B2 and its transcriptional regulator NR4A2 in HAC15 adrenal cells overexpressing KCNJ5(Y152C) compared to the wild-type channel. The effect was clearly Ca(2+)-dependent, because it was abolished by the calcium channel blocker nifedipine.

CONCLUSIONS

Herein we describe a new germline mutation in KCNJ5 responsible for FH-III.

Genetics of mineralocorticoid excess: an update for clinicians

Aldosterone plays a major role in the regulation of sodium and potassium homeostasis and blood pressure. More recently, aldosterone has emerged as a key hormone mediating end organ damage. In extreme cases, dysregulated aldosterone production leads to primary aldosteronism (PA), the most common form of secondary hypertension. However, even within the physiological range, high levels of aldosterone are associated with an increased risk of developing hypertension over time. PA represents the most common and curable form of hypertension, with a prevalence that increases with the severity of hypertension. Although genetic causes underlying glucocorticoid-remediable aldosteronism, one of the three Mendelian forms of PA, were established some time ago, somatic and inherited mutations in the potassium channel GIRK4 have only recently been implicated in the formation of aldosterone-producing adenoma (APA) and in familial hyperaldosteronism type 3. Moreover, recent findings have shown somatic mutations in two additional genes, involved in maintaining intracellular ionic homeostasis and cell membrane potential, in a subset of APAs. This review summarizes our current knowledge on the genetic determinants that contribute to variations in plasma aldosterone and renin levels in the general population and the genetics of familial and sporadic PA. Various animal models that have significantly improved our understanding of the pathophysiology of excess aldosterone production are also discussed. Finally, we outline the cardiovascular, renal, and metabolic consequences of mineralocorticoid excess beyond blood pressure regulation.

Primary aldosteronism and potassium channel mutations

PURPOSE OF REVIEW

To summarize and discuss data from recent studies implicating mutations in potassium channel genes in the pathogenesis of primary aldosteronism.

RECENT FINDINGS

Potassium channel gene variants are associated with the primary aldosteronism phenotype in animals (Kcnma1, TASK-1, and TASK-3) and humans (HERG and KCNJ5). Germline KCNJ5 mutations cause bilateral, familial primary aldosteronism with variable severity and genotype:phenotype correlations. Somatic KCNJ5 mutations occur in approximately 40% of aldosterone-producing adenomas, and are associated with younger age, female sex, more severe primary aldosteronism, lack of responsiveness of plasma aldosterone to upright posture, and zona fasciculata histology. Of five so far described, G151R and L168R are by far the most common. KCNJ5 mutations lead to reduced K⁺/Na⁺ channel selectivity and Na⁺ influx, predisposing to cell membrane depolarization, increased calcium influx, increased expression of genes promoting aldosterone synthesis, and increased aldosterone production by adrenocortical cells. How they lead to adrenal cell proliferation and tumor development is less well understood.

SUMMARY

These findings shed considerable light on the pathophysiology of primary aldosteronism with the potential to lead to new diagnostic approaches and treatments.

KCNJ5 mutations in aldosterone producing adenoma and relationship with adrenal cortex remodeling

Somatic mutations of KCNJ5, coding for the potassium channel GIRK4, have recently been implicated in the formation of aldosterone producing adenoma (APA). While a causal link between KCNJ5 mutations, membrane depolarization and aldosterone production has been established, the precise mechanism by which these mutations promote cell proliferation and APA formation remains unclear. The aim of our study was to correlate KCNJ5 mutation status with morphological and functional characteristics of the adrenal cortex adjacent to APA. While GIRK4 was expressed in APA and in the zona glomerulosa of the adjacent cortex, significantly lower levels were detected in APA harboring a KCNJ5 mutation. There was no correlation between KCNJ5 mutation status and the morphological measures of adrenal cortex remodeling, including nodulation, vascularization and expression of CYP11B2. The cell composition of APA was not significantly different between groups. These results indicate that KCNJ5 mutations are not correlated with adrenal cortex remodeling in APA.

[Update on endocrine hypertension]

Endocrine hypertension is the most common cause of secondary hypertension affecting ~3 % of the population, with primary hyperaldosteronism and pheochromocytoma being the principal conditions. Both diseases share an increased cardiovascular risk in comparison with essential hypertension patients (at the same blood pressure level). This augmented cardiovascular risk as well as the availability of specific treatment emphasize the importance of timely and correct diagnosis. Primary hyperaldosteronism, representing one tenth of hypertensive patients, is an under-diagnosed disease partly because of difficult diagnostic steps and absence of standard criteria. Recently, the description of somatic mutations in KCNJ5 gene in Conn adenomas had precipitated a resurgence of research activity to understand the pathophysiology of this common disease. Research had confirmed the role of these mutations in aldosterone hypersecretion; however, its role in adenoma formation is still to be elucidated. Elsewhere, much remains to be done in order to understand the pathogenesis of bilateral idiopathic hyperaldosteronism, the other common subtype of primary hyperaldosteronism. In pheochromocytoma, the revolution of genetics has led to major advances in the characterization of this rare disease. It is now clear that up to 50 % of patients with pheochromocytoma have a genetic abnormality and that different pheochromocytomas segregate into two clusters with distinct genotypes, signal transduction pathways and expression of biomarkers (phenotype). This continuing progress has huge effects on patient's management and follow-up. In this article we will shed light on the recent developments in both diseases with emphasis on their role in patient care.

Role of KCNJ5 in familial and sporadic primary aldosteronism

Primary aldosteronism is characterised by the dysregulation of aldosterone production and comprises both sporadic forms, caused by an aldosterone-producing adenoma or bilateral adrenal hyperplasia, and familial forms (familial hyperaldosteronism types I, II and III). The two principal physiological regulators of aldosterone synthesis are angiotensin II and serum K(+), which reverse the high resting K(+) conductance and hyperpolarized membrane potential of adrenal glomerulosa cells. The resulting membrane depolarization causes the opening of voltage-gated Ca(2+) channels and an increase in intracellular Ca(2+) that stimulates aldosterone biosynthesis. Point mutations in the KCNJ5 gene, which encodes the G-protein-activated inward rectifier K(+) channel 4 (GIRK4), have been implicated in the pathogenesis of both sporadic and familial forms of primary aldosteronism. These mutations interfere with the selectivity filter of GIRK4 causing Na(+) entry, cell depolarization and Ca(2+) channel opening, resulting in constitutive aldosterone production. Seven families with familial hyperaldosteronism caused by KCNJ5 germline mutations have so far been described, and multicentre studies have reported KCNJ5 mutations in approximately 40% of sporadic aldosterone-producing adenomas. Herein, we review the role of GIRK4 in adrenal pathophysiology and provide an overview of the clinical and biochemical phenotypes resulting from KCNJ5 mutations in patients with sporadic and familial primary aldosteronism.

Angiotensin II and III metabolism and effects on steroid production in the HAC15 human adrenocortical cell line

Aldosterone is synthesized in the zona glomerulosa of the adrenal cortex under primary regulation by the renin-angiotensin system. Angiotensin II (A-II) acts through the angiotensin types 1 and 2 receptors (AT1R and AT2R). A-II is metabolized in different tissues by various enzymes to generate two heptapeptides A-III and angiotensin 1-7, which can then be catabolized into smaller peptides. A-II was more potent than A-III in stimulating aldosterone secretion in the adrenocortical cell line HAC15, and A-II, but not A-III, stimulated cortisol secretion. A-II stimulated mRNA expression of steroidogenic acute regulatory protein, 3β-hydroxysteroid dehydrogenase, CYP11B1, and CYP11B2, whereas A-III stimulated 3β-hydroxysteroid dehydrogenase, CYP11B1, and CYP11B2 but decreased the expression of CYP17A1 required for cortisol synthesis. The stimulation of aldosterone secretion by A-II and A-III was blocked by the AT1R receptor blocker, losartan, but not by an AT2R blocker. A-II was rapidly metabolized by the HAC15 cells to mainly to angiotensin 1-7, but not to A-III, and disappeared from the supernatant within 6 h. A-III was metabolized rapidly and disappeared within 1 h. In conclusion, A-II was not converted to A-III in the HAC15 cell and is the more potent stimulator of aldosterone secretion and cortisol of the two. A-III stimulated aldosterone secretion but not cortisol secretion.

Primary aldosteronism: comparison between guidelines of the Japanese and the US Endocrine Society

The US and Japanese Endocrine Societies recently both developed a therapeutic guideline for primary aldosteronism (PA). These guidelines indicate that differential diagnosis between PA caused by aldosterone-producing adenoma (APA) and bilateral adrenal hyperplasia (or idiopathic hyperaldosteronism [IHA]) is important and critical for treatment choice, while they show some inconsistency due to different frequencies of disorders and medical circumstances in respective countries. Japanese Endocrine Society guidelines have a unique description on the adrenal vein sampling (AVS) against microAPA, detection of which is difficult with current imaging tools and thus is frequently misdiagnosed as IHA. We reported that AVS performed in combination with adrenocorticotropic hormone stimulation was useful for diagnosing laterality of PA lesions, and APA underlay 74% of our cases with significant reduction of IHA compared with those previously reported. Laterality of PA should be precisely evaluated with AVS and imaging studies, as the results strongly influence the choice of therapeutic strategies.

The potassium channel, Kir3.4 participates in angiotensin II-stimulated aldosterone production by a human adrenocortical cell line

Angiotensin II (A-II) regulation of aldosterone secretion is initiated by inducing cell membrane depolarization, thereby increasing intracellular calcium and activating the calcium calmodulin/calmodulin kinase cascade. Mutations in the selectivity filter of the KCNJ5 gene coding for inward rectifying potassium channel (Kir)3.4 has been found in about one third of aldosterone-producing adenomas. These mutations result in loss of selectivity of the inward rectifying current for potassium, which causes membrane depolarization and opening of calcium channels and activation of the calcium calmodulin/calmodulin kinase cascade and results in an increase in aldosterone secretion. In this study we show that A-II and a calcium ionophore down-regulate the expression of KCNJ5 mRNA and protein. Activation of Kir3.4 by naringin inhibits A-II-stimulated membrane voltage and aldosterone secretion. Overexpression of KCNJ5 in the HAC15 cells using a lentivirus resulted in a decrease in membrane voltage, intracellular calcium, expression of steroidogenic acute regulatory protein, 3-β-hydroxysteroid dehydrogenase 3B2, cytochrome P450 11B1 and cytochrome P450 11B2 mRNA, and aldosterone synthesis. In conclusion, A-II appears to stimulate aldosterone secretion by depolarizing the membrane acting in part through the regulation of the expression and activity of Kir3.4.

Minireview: aldosterone biosynthesis: electrically gated for our protection

Aldosterone produced by adrenal zona glomerulosa (ZG) cells plays an important role in maintaining salt/water balance and, hence, blood pressure homeostasis. However, when dysregulated, aldosterone advances renal and cardiovascular disease states. Multiple steps in the steroidogenic pathway require Ca(2+), and the sustained production of aldosterone depends on maintained Ca(2+) entry into the ZG cell. Nevertheless, the recorded membrane potential of isolated ZG cells is extremely hyperpolarized, allowing the opening of only a small fraction of low-voltage-activated Ca(2+) channels of the Ca(v)3.x family, the major Ca(2+) conductance on the ZG cell membrane. As a consequence, to activate sufficient Ca(2+) channels to sustain the production of aldosterone, aldosterone secretagogs would be required to affect large decreases in membrane voltage, a requirement that is inconsistent with the exquisite sensitivity of aldosterone production in vivo to small changes (0.1 mm) in extracellular K(+). In this review, we evaluate the contribution of membrane voltage and voltage-dependent Ca(2+) channels to the control of aldosterone production and consider data highlighting the electrical excitability of the ZG cell. This intrinsic capacity of ZG cells to behave as electrical oscillators provides a platform from which to generate a recurring Ca(2+) signal that is compatible with the lengthy time course of steroidogenesis and provides an alternative model for the physiological regulation of aldosterone production that permits both amplitude and temporal modulation of the Ca(2+) signal.

Comprehensive re-sequencing of adrenal aldosterone producing lesions reveal three somatic mutations near the KCNJ5 potassium channel selectivity filter

Background

Aldosterone producing lesions are a common cause of hypertension, but genetic alterations for tumorigenesis have been unclear. Recently, either of two recurrent somatic missense mutations (G151R or L168R) was found in the potassium channel KCNJ5 gene in aldosterone producing adenomas. These mutations alter the channel selectivity filter and result in Na⁺ conductance and cell depolarization, stimulating aldosterone production and cell proliferation. Because a similar mutation occurs in a Mendelian form of primary aldosteronism, these mutations appear to be sufficient for cell proliferation and aldosterone production. The prevalence and spectrum of KCNJ5 mutations in different entities of adrenocortical lesions remain to be defined.

Materials and Methods

The coding region and flanking intronic segments of KCNJ5 were subjected to Sanger DNA sequencing in 351 aldosterone producing lesions, from patients with primary aldosteronism and 130 other adrenocortical lesions. The specimens had been collected from 10 different worldwide referral centers.

Results

G151R or L168R somatic mutations were identified in 47% of aldosterone producing adenomas, each with similar frequency. A previously unreported somatic mutation near the selectivity filter, E145Q, was observed twice. Somatic G151R or L168R mutations were also found in 40% of aldosterone producing adenomas associated with marked hyperplasia, but not in specimens with merely unilateral hyperplasia. Mutations were absent in 130 non-aldosterone secreting lesions.

KCNJ5 mutations were overrepresented in aldosterone producing adenomas from female compared to male patients (63 vs. 24%). Males with KCNJ5 mutations were significantly younger than those without (45 vs. 54, respectively; p<0.005) and their APAs with KCNJ5 mutations were larger than those without (27.1 mm vs. 17.1 mm; p<0.005).

Discussion

Either of two somatic KCNJ5 mutations are highly prevalent and specific for aldosterone producing lesions. These findings provide new insight into the pathogenesis of primary aldosteronism.

The genetic basis of primary aldosteronism

Primary aldosteronism is commonly regarded as largely sporadic, but both germline and somatic mutations are increasingly recognized as underlying the condition. Three germline mutations causing familial hyperaldosteronism have been described, dubbed FH I (due to a CYP11B1/CYP11B2 chimera), FH II (localized to chromosome 7p22, exact location of mutation[s] unknown to date), and FH III (reflecting a T158A mutation in the potassium channel subunit KCNJ5). Major contributions (FH I, FH III) have been by Lifton and his associates; more recently they have also described somatic mutations (G151R, L168R) in KCNJ5 in over a third of aldosterone-producing adenomas, with results confirmed, refined, and extended in a much larger study from Europe. These findings have sparked considerable interest, and over the next 12 months a number of additional reports can be confidently expected to throw light on both normal and abnormal adrenocortical zonation and the genesis of primary aldosteronism.