Characterization of the hamster CYP11B2 gene regulatory regions

Extramitochondrial OPA1 and adrenocortical function

We have previously described that silencing of the mitochondrial protein OPA1 enhances mitochondrial Ca(2+) signaling and aldosterone production in H295R adrenocortical cells. Since extramitochondrial OPA1 (emOPA1) was reported to facilitate cAMP-induced lipolysis, we hypothesized that emOPA1, via the enhanced hydrolysis of cholesterol esters, augments aldosterone production in H295R cells. A few OPA1 immunopositive spots were detected in ∼40% of the cells. In cell fractionation studies OPA1/COX IV (mitochondrial marker) ratio in the post-mitochondrial fractions was an order of magnitude higher than that in the mitochondrial fraction. The ratio of long to short OPA1 isoforms was lower in post-mitochondrial than in mitochondrial fractions. Knockdown of OPA1 failed to reduce db-cAMP-induced phosphorylation of hormone-sensitive lipase (HSL), Ca(2+) signaling and aldosterone secretion. In conclusion, OPA1 could be detected in the post-mitochondrial fractions, nevertheless, OPA1 did not interfere with the cAMP - PKA - HSL mediated activation of aldosterone secretion.

Angiotensin II-activated protein kinase D mediates acute aldosterone secretion

Dysregulation of the renin-angiotensin II (AngII)-aldosterone system can contribute to cardiovascular disease, such that an understanding of this system is critical. Diacylglycerol-sensitive serine/threonine protein kinase D (PKD) is activated by AngII in several systems, including the human adrenocortical carcinoma cell line NCI H295R, where this enzyme enhances chronic (24h) AngII-evoked aldosterone secretion. However, the role of PKD in acute AngII-elicited aldosterone secretion has not been previously examined. In primary cultures of bovine adrenal glomerulosa cells, which secrete detectable quantities of aldosterone in response to secretagogues within minutes, PKD was activated in response to AngII, but not an elevated potassium concentration or adrenocorticotrophic hormone. This activation was time- and dose-dependent and occurred through the AT1, but not the AT2, receptor. Adenovirus-mediated overexpression of constitutively active PKD resulted in enhanced AngII-induced aldosterone secretion; whereas overexpression of a dominant-negative PKD construct decreased AngII-stimulated aldosterone secretion. Thus, we demonstrate for the first time that PKD mediates acute AngII-induced aldosterone secretion.

Control of CYP11B2 gene expression through differential regulation of its promoter by atypical and conventional protein kinase C isoforms

We reported previously that the protein kinase C (PKC) inhibitor GF109203X stimulated the hamster CYP11B2 promoter activity in transfected NCI-H295 cells. PKCalpha, -epsilon, and -zeta were detected in hamster adrenal zona glomerulosa and NCI-H295 cells, and PKCtheta in NCI-H295 cells. 12-O-Tetradecanoylphorbol-13-acetate (TPA) inhibited basal and stimulated cytochrome P450 aldosterone synthase mRNA expression by angiotensin (AII), dibutyryl cyclic adenosine 3':5'-monophosphate (Bt2cAMP), or KCl in NCI-H295 cells. Basal CYP11B2 promoter activity was inhibited in cells cotransfected with constitutively active (CA) PKCalpha, -epsilon, and -theta mutants, whereas it was increased with CA-PKCzeta. Dominant negative (DN) PKCalpha, -theta, -epsilon, and -zeta mutants stimulated the promoter activity. AII-, KCl-, and Bt2cAMP-stimulatory effects were abolished in cells cotransfected with CA-PKCalpha, -epsilon, or -theta. The effect of Bt2cAMP was abolished by CA-PKCzeta but AII and KCl were still able to enhance the promoter activity. DN-PKCalpha, -epsilon, -theta, or -zeta did not inhibit these effects. Gö6976 enhanced promoter activity, providing further evidence that PKCalpha was involved. Various CYP11B2 promoter constructs were used to identify the area associated with TPA and PKC inhibition. TPA and CA-PKCalpha, -epsilon, or -theta abolished the effects of AII, KCl, and Bt2cAMP on the activity of -102 and longer constructs. In summary, our findings suggest that the hamster CYP11B2 gene is under differential control by conventional (alpha) and atypical (zeta) PKC.

Regulation of CYP11B2 gene expression by protein kinase C

Bisindolylmaleimide, a protein kinase C (PKC) inhibitor, was shown to stimulate the hamster CYP11B2 promoter activity in transfected NCI-H295 cells. In this study we have found that TPA, an activator of PKC, also inhibited the hamster CYP11B2 promoter activity. DAG-dependent PKC alpha and PKC epsilon, and atypical PKC zeta were detected in hamster adrenal zona glomerulosa, whereas the isoforms alpha, epsilon, zeta and theta were found in NCI-H295 cells. CYP11B2 promoter activity was inhibited in cells co-transfected with constitutively active PKC alpha and epsilon mutants, whereas it was increased with the constitutively active PKC zeta mutant. Dominant negative PKC alpha, epsilon and zeta mutants stimulated the promoter activity. Gö6976, a specific inhibitor of classical PKCs, enhanced promoter activity, providing further evidence that PKC alpha, the only classical PKC revealed in hamster adrenal and NCI-H295 cells, was involved in the promoter inhibition.

Characterization of the hamster CYP11B2 gene encoding adrenal cytochrome P450 aldosterone synthase

A CYP11B2 gene encoding cytochrome P450 aldosterone synthase (P450aldo) was isolated from a hamster genomic library. The gene, which contained 9 exons, was composed of 9,045 bp, of which 3,722 bp were located in the 5' untranslated region (5' UTR). A TATA box sequence (gataaa) and other putative cis elements, previously named Ad1 to Ad6, were identified in the 5' UTR of the hamster gene comparable to the CYP11B2 gene of other animal species. Footprint analysis showed protection by nuclear protein extracts from hamster adrenal zona glomerulosa (ZG) in the regions containing the above mentioned cis elements. In addition, a new protected cis element, between -143 and -161 bp, was demonstrated, and gel-shift assays revealed that the sequence of this new cis element was specifically retarded by factors in the nuclear extracts of hamster adrenal ZG. We then examined the transcriptional activity of the 5' UTR of the CYP11B2 gene, using chloramphenicol acyltransferase (CAT) as the reporter gene. Ten deletion plasmids were constructed using a modified pCAT vector. Transient transfections of the chimeric reporter constructs into Y1 cells showed that the highest basal promoter activity was obtained with the construct containing up to -134 bp. Increasing the length of the regulatory region of CYP11B2 gene to -167 bp resulted in less than two-thirds of the maximal activity, indicating the probability of putative inhibitory cis elements in this area of the gene. Forskolin stimulated the expression of the reporter gene of deletion plasmids excepting the construct containing only the TATA box, and the highest activity also occurred with the -134 bp construct. TPA had no stimulatory effects on any of the constructs, and interestingly it slightly inhibited CAT activity. In contrast to TPA, staurosporine, an inhibitor of the PKC pathway, stimulated CAT activity. To conclude, the promoter region of the hamster CYP11B2 gene transfected in Y1 cells is responsive to forskolin, indicating that the gene is controlled by the PKA signaling pathway. Paradoxically, staurosporine, but not TPA, stimulates the promoter activity of the CYP11B2 gene, indicating that PKC might, at least in Y1 cells, act as a negative regulator on the aldosterone synthase promoter. Moreover, a new cis element was shown to exert a negative effect on basal as well as on stimulated activities of the hamster promoter CYP11B2 gene.