Synthesis of 6 beta-hydroxyaldosterone by A6 (toad kidney) cells in culture

Aldosterone metabolism in cultures of rat renal cortical and medullary cells

The metabolism of 4-(14)C-d-aldosterone (at 3 nM) was studied in the primary target organ of the hormone, in renal cortical and medullary cell cultures obtained from Wistar rats. Larger amounts of aldosterone were metabolized in medullary cells than in cortical cells, as measured by a decreased 4-(14)C-d-aldosterone radioactivity concentration (26+/-9% and 12+/-7% of the initial aldosterone added, respectively (n=5, p<0.05)). The 14C radiometabolites of aldosterone in both cultures co-chromatographed with 5alpha dihydro- (DHA) and 3alpha,3beta tetrahydroaldosterone (THA). Aldosterone metabolism was totally inhibited by a mineralocorticoid receptor antagonist canrenoat (Soldactone) (at 10(-5) to 10(-3) M), while the glucocorticoid receptor antagonist RU 38486 (Roussel UCLAF) (at 10(-5) to 10(-4) M) had no effect. Thus, the study confirmed that, in rat kidney, aldosterone can be converted to its reduced metabolites by a metabolism which is inhibited by a mineralocorticoid receptor antagonist. This indicated that the metabolism might play some role in modulation of the intracellular response to aldosterone in the kidney.

The microsomal mixed function oxidase system of amphibians and reptiles: components, activities and induction

This article reviews current research in amphibian and reptilian cytochromes P450, important to the overall understanding of xenobiotic metabolism in the ecosystem and the evolution of P450s. Amphibians and reptilians contain the normal mixed function oxidase system (MFO). In general the MFO content and activities are less than those found in mammals, but only a few of the known activities have been examined in these vertebrate classes. Research to date has focused on two families of cytochromes P450, CYP1 and 2. The isoforms examined catalyze the classic activities but there have been notable absences. The total number of isoforms present and the breadth of substrates metabolized are yet unknown. Induction by foreign compounds (xenobiotics) is lengthier and yields lower levels of induced activity than is typically found in mammals. When these animals are pretreated with 3-methylcholanthrene (3MC) and beta-naphthaflavone (BNF), which are known to induce the same isoform in mammals, multiple isoforms are induced with different activities. Phenobarbital-pretreatment in turtles and alligators induces cytochromes P450 and suggestive data indicates induction in the lizard Agama lizard and the newt Pleurodeles waltl. In amphibians and reptiles a CYP2B protein does appear to be present along with constitutive activities associated with the 2 family of cytochromes P450. The markedly different response to classic inducers combined with lower or absent activities alters the view of how amphibians and reptilians respond to xenobiotic challenges.

A second enzyme protecting mineralocorticoid receptors from glucocorticoid occupancy

We have confirmed that A6 cells (derived from kidney of Xenopus laevis), which contain both mineralocorticoid and glucocorticoid receptors, do not normally possess 11 beta-hydroxysteroid dehydroxgenase (11 beta-HSD1 or 11 beta-HSD2) enzymatic activity and so are without apparent "protective" enzymes. A6 cells do not convert the glucocorticoid corticosterone to 11-dehydrocorticosterone but do, however, possess steroid 6 beta-hydroxylase that transforms corticosterone to 6 beta-hydroxycorticosterone. This hydroxylase is cytochrome P-450 3A (CYP3A). We have now determined the effects of 3 alpha,5 beta-tetrahydroprogesterone and chenodeoxycholic acid (both inhibitors of 11 beta-HSD1) and 11-dehydrocorticosterone and 11 beta-hydroxy-3 alpha,5 beta-tetrahydroprogesterone (inhibitors of 11 beta-HSD2) and carbenoxalone, which inhibits both 11 beta-HSD1 and 11 beta-HSD2, on the actions and metabolism of corticosterone and active Na+ transport [short-circuit current (Isc)] in A6 cells. All of these 11 beta-HSD inhibitory substances induced a significant increment in corticosterone-induced Isc, which was detectable within 2 h. However, none of these agents caused an increase in Isc when incubated by themselves with A6 cells. In all cases, the additional Isc was inhibited by the mineralocorticoid receptor (MR) antagonist, RU-28318, whereas the original Isc elicited by corticosterone alone was inhibited by the glucocorticoid receptor antagonist, RU-38486. In separate experiments, each agent was shown to significantly inhibit metabolism of corticosterone to 6 beta-hydroxycorticosterone in A6 cells, and a linear relationship existed between 6 beta-hydroxylase inhibition and the MR-mediated increase in Isc in the one inhibitor tested. Troleandomycin, a selective inhibitor of CYP3A, inhibited 6 beta-hydroxylase and also significantly enhanced corticosterone-induced Isc at 2 h. These experiments indicate that the enhanced MR-mediated Isc in A6 cells may be related to inhibition of 6 beta-hydroxylase activity in these cells and that this 6 beta-hydroxylase (CYP3A) may be protecting the expression of corticosterone-induced active Na+ transport in A6 cells by MR-mediated mechanism(s).

Androgen metabolism in cultured rat renal inner medullary collecting duct (IMCD) cells

Although the stimulations of renal hypertrophy and renal erythropoietin production have been well-known androgen effects in the kidney, recent investigative progresses are further providing good evidences for androgen-regulated gene productions of key enzymes or local hormone substrates important to renal cell metabolisms and tubular functions in mouse or rat proximal tubules, respectively. It has been also reported that testosterone restores vasopressin receptors in medullary collecting ducts of the ageing rat and improves a urinary concentrating ability. Therefore in the present study we examined a metabolic pathway of androgens in cultured rat renal IMCD cells, which finally determine a urinary composition and volume. IMCD cells cultured from kidneys of male Wistar rats weighing about 200 g were incubated with serum-free culture media containing 4 nM [3H] testosterone or [3H] androstenedione for 2-48 h. Radioactive compounds in incubation media were then separated by reverse-phase high-pressure liquid chromatography (HPLC) and identified mainly on the basis of comparison of retention times of standard materials on HPLC. The main metabolites identified in testosterone or androstenedione incubation experiment were 5 alpha-dihydrotestosterone or 5 alpha-androstanedione, respectively. 5 alpha-Reductase inhibitor, MK 906, effectively inhibited the formations of these Ring A reduced metabolites. These results may suggest that rat renal IMCD cells possess 5 alpha-reductase activity, thereby converting androgens into their biologically active forms in vivo.

Formation of 6 beta-OH-deoxycorticosterone from deoxycorticosterone by A6 cells

We have investigated the metabolism of deoxycorticosterone in A6 cells, a continuous cell line derived from the kidney of Xenopus laevis. A6 cells at confluence were incubated with serum-free culture media containing 2.5 microM [3H] deoxycorticosterone. When radioactive compounds in incubation media were separated by reverse-phase high-pressure liquid chromatography, the formation of polar metabolites was observed. One component of polar metabolites cochromatographed with 6 beta-hydroxydeoxycorticosterone. In order to identify this component more rigorously, large scale cultures were performed and this compound was separated and purified by reverse-phase high-pressure liquid chromatography. The purified material was analyzed by gas chromatography-mass spectrometry, secondary ion mass spectrometry, and nuclear magnetic resonance spectroscopy. These procedures revealed that this material was 6 beta-hydroxydeoxycorticosterone.