The renin gene and aldosterone-producing adenomas

A Genetic Variant in the Distal Enhancer Region of the Human Renin Gene Affects Renin Expression

Background

The high heritability of plasma renin activity was confirmed in recent investigations. A variation located near the strong enhancer of the human renin gene (REN), C-5312T, has been shown to have different transcription activity levels depending on its allele: the 5312T allele shows transcription levels that are 45% greater than those of the 5312C allele. The purpose of this study was to confirm the hypothesis that variations in the enhancer region of the REN gene are involved in regulating renal expression of renin.

Methods

Sixty-four subjects with biopsy-proven renal diseases were included in this study (male/female: 35/29, age 41.9 ± 20.9 years, SBP/DBP 123.1 ± 23.7/73.4 ± 14.8 mmHg, s-Cr 0.93 ± 0.63 mg/dl). A genetic variant of REN, C-5312T, was assayed by PCR-RFLP and the TaqMan method. Total RNAs from a small part of the renal cortex were reverse-transcribed and amplified for REN and GAPDH with a real-time PCR system.

Results

Logarithmically transformed expression values of the relative ratio of REN to GAPDH (10⁻³) were as follows (mean ± SE): CC (26 cases), 0.016 ± 0.005; CT (33 cases), 0.047 ± 0.021 (p = 0.41 vs. CC); TT (5 cases), 0.198 ± 0.194 (p = 0.011 vs. CC, p < 0.031 vs. CT). Thus, significant differences in REN expression were observed among the genetic variants.

Conclusion

The results suggest that variants in the enhancer region of the human renin gene have an effect on the expression levels of renin in renal tissue; this observation is in good accordance with the results of the transcriptional assay.

Down-regulation of D2 dopamine receptor and increased protein kinase Cmu phosphorylation in aldosterone-producing adenoma play roles in aldosterone overproduction

CONTEXT

The mechanism associated with the overproduction of aldosterone by aldosterone-producing adenomas (APA) is unknown.

OBJECTIVE

The objective of the study was to explore the role of the D2 dopamine receptor (D2R) on aldosterone synthesis and secretion and clarify the clinical importance of this role on aldosterone overproduction in APA.

RESULTS

D2R expression in APA was examined in 24 patients and was much less than that in the nontumorous adrenal cortex. D2R mRNA levels in APA were inversely correlated with CYP11B2 mRNA levels and the patient's plasma aldosterone concentration. Angiotensin II (AII)-stimulated aldosterone secretion and CYP11B2 mRNA expression in human adenocarcinoma cells (H295R) was attenuated by the D2 agonist, bromocriptine (BMC). BMC selectively attenuated AII-induced protein kinase C (PKC)-mu phosphorylation and its translocation to the cell membrane. PKCmu-specific short-hairpin RNA significantly decreased AII-induced CYP11B2 mRNA expression and aldosterone secretion. BMC also attenuated the AII-induced increase in cytoplasmic calcium, partially through an inhibition of cytoplasmic inositol 1,4,5 triphosphate production. Despite similar total PKCmu levels in APA and the nontumorous adrenal cortex, expression of phosphorylated PKCmu in APA was much higher.

CONCLUSION

This is the first study to demonstrate that the D2R modulated aldosterone secretion and synthesis through a specific attenuation of PKCmu activity, as well as the intracellular calcium level. Down-regulation of the D2R in APA, in turn, increased PKCmu activity and led to overproduction of aldosterone in affected patients. The D2R may thus serve as a potential treatment target for primary aldosteronism.

Pathophysiological Roles of the Adrenal Renin-Angiotensin System in Patients with Primary Aldosteronism

The mechanism of overproduction of aldosterone in primary aldosteronism is unclear. The intraadrenal renin-angiotensin system (RAS) has been suggested to possess the functional role of the synthesizing aldosterone and regulating blood pressure. In order to clarify the pathophysiological roles of adrenal RAS in aldosterone-producing adenoma (APA), we studied the expressions of the messenger RNAs (mRNAs) of renin, angiotensinogen, type 1 (AT1R) and type 2 angiotensin II receptor (AT2R), CYP11B1 (11 beta-hydroxylase gene) and CYP11B2 (aldosterone synthase gene) in 8 patients with angiotensin II-responsive (ATII-R) APA and compared them with the expressions of the same mRNAs in 8 patients with angiotensin II-unresponsive (ATII-U) APA. Quantification of the mRNA of each gene was done using a real-time polymerase chain reaction with specific primers. There were no significant differences between ATII-R APA and ATII-U APA in the mRNA levels of renin, angiotensinogen, AT1 R, CYP11B1 and CYP11B2. The amount of AT2R mRNA was significantly higher in the patients with ATII-R APA than in those with ATII-U APA (p<0.05). These results may suggest that AT2R partially contributes to the overproduction of aldosterone in ATII-R APA.

Pathophysiology and diagnosis of primary aldosteronism

The pathophysiology of primary aldosteronism still remains unknown. In mRNA and protein levels, overexpression of aldosterone synthase (P-450aldo) is recognized, although abnormalities and defects of DNA and its upper stream have not been detected. Several candidate genes responsible for pathogenesis of primary aldosteronism, such as renin, angiotensin receptor type II, etc., have been proposed, but no decisive genes have been found. A relatively reliable screening for hyperaldosteronism is a determination of the ratio of the plasma aldosterone level to the plasma renin activity. For differentiating several types of aldosteronisms, the simplest test is the response of plasma aldosterone to two hours in an upright posture: plasma aldosterone rises in most patients with idiopathic hyperaldosteronism. In contrast, in cases of autonomous aldosterone-producing tumor, most patients show no response or even a decrease in plasma aldosterone concentration. The size and location of the aldosterone-producing adenoma are determined by using computed tomography.

Primary aldosteronism: a new understanding

Primary aldosteronism (PAL) may always have a genetic basis. This leads to either abnormally regulated, increased biosynthesis (Familial Hyperaldosteronism Type I, FHI) or to unrestrained hyperplasia and neoplasia, usually benign. The distinction between diffuse hyperplasia, nodular hyperplasia and adenoma may be relatively unimportant in functional and etiological terms. The genetic basis must be understood before diagnosis of disease (FHI) or of predisposition (all other PAL) can be made at birth and appropriate surveillance commenced. The natural history of PAL other than FHI is for progressive increase in severity, with both adrenals eventually involved. Long-term follow-up of PAL is therefore mandatory, and postoperative assessment of residual non-suppressible aldosterone production by fludrocortisone suppression testing useful in defining biochemical cure or improvement, and the need for specific medical treatment.

Adrenaline cells of the rat adrenal cortex and medulla contain renin and prorenin

The distribution and content of renin in Sprague-Dawley (SD) and transgenic (mREN-2)27 rats (TG) were compared to further define the cellular basis and function of the adrenal renin-angiotensin system. Antibody binding (to rat and mouse renin protein and prosequence) was visualised in serial paraffin sections using an avidin-biotin peroxidase technique. Chromaffin and adrenaline cells were identified by tyrosine hydroxylase (TH) and phenylethanolamine N-methyltransferase immunoreactivity, respectively. In SD zona glomerulosa (ZG), renin and its prosequence localised to small steroid cells while in homozygous (receiving lisinopril) and heterozygous (untreated) TG, steroid cells labelled in all cortical zones. In addition, throughout the cortex of each strain, large polyhedral adrenaline chromaffin cells occurring singly or in small groups and occasionally in rays labelled for renin and prosequence. Similar large adrenaline cells immunolabelled for all antisera in medulla while other cells were only TH-positive. Total adrenal renin content was 53 times higher in heterozygous transgenics than SD rats and was mainly (74%) prorenin. In SD, 37% of cortical renin was prorenin but in adrenal medulla only active renin was detected. Thus, from present and previous work both renin and prorenin occur not only in mitochondrial dense bodies of the ZG, but also in secretory granules of adrenaline chromaffin cells in both cortex and medulla implying in situ synthesis and paracrine functions.

PCR-SSCP analysis of the angiotensin II type 1 receptor gene in patients with aldosterone-producing adenomas

1. In patients with primary aldosteronism due to angiotensin-responsive and angiotensin-unresponsive aldosterone-producing adenomas, no differences in the coding region of the angiotensin II type 1 (AT1) receptor gene were observed compared to normal subjects in peripheral blood leucocyte DNA. 2. Furthermore, no differences in the AT1 receptor gene were observed in DNA extracted from tumour tissue of either subgroup. 3. Genotypic and allelic frequencies for an RFLP detected in the coding region of the AT1 receptor gene were not significantly different between normal subjects and patients with aldosterone-producing adenomas as a group, nor between normal subjects and patients of either subgroup when compared with each other. 4. In those patients heterozygous in peripheral blood at the RFLP site, tumour DNA showed the same allelic pattern. 5. In patients with aldosterone-producing adenomas either responsive or unresponsive to the renin-angiotensin system, no differences were detected using SSCP analysis in the coding region of the AT1 receptor gene in peripheral blood or tumour tissue.