Subcellular localization of 3 beta hydroxysteroid dehydrogenase isomerase in testis of Bufo arenarum H

Effect of glucocorticoids on androgen biosynthesis in the testes of the toad Rhinella arenarum (Amphibia, Anura)

In rat Leydig cells, glucocorticoids (GCs) inhibit testosterone production through the interaction with the glucocorticoid receptor (GR). However, the sensitivity of those cells to GCs is regulated by the enzyme 11β-hydroxysteroid dehydrogenase Type 1 (11β-HSD1). In the testes of the toad Rhinella arenarum, the presence of an 11β-HSD similar to type 2 and a cytosolic GR has also been described. However, there is a lack of information regarding the effects of GCs on amphibian testicular steroidogenesis. In this study, the effects of corticosterone on androgen production, and the activity of two steroidogenic enzymes in toad testes were reported. Corticosterone inhibits androgen production via the GR because the GR antagonist RU486 prevents corticosterone-induced inhibition of testosterone. Corticosterone also reduced the activity of the cytochrome P450 17-hydroxylase, C17,20-lyase (Cyp450 _c17 ) without affecting the 3β-hydroxysteroid dehydrogenase/isomerase activity. This effect on Cyp450 _c17 was likewise inhibited by RU486. On the other hand, corticosterone had no effect on the amount of steroidogenic acute regulator protein. These results suggest that GCs inhibit steroidogenesis in toad testes by reducing of Cyp450 _c17 activity via a GR-mediated mechanism.

Effect of estradiol on apoptosis, proliferation and steroidogenic enzymes in the testes of the toad Rhinella arenarum (Amphibia, Anura)

Estrogens inhibit androgen production and this negative action on amphibian steroidogenesis could be related to the regulation of steroidogenic enzymes. Estrogens are also involved in the regulation of amphibian spermatogenesis by controlling testicular apoptosis and spermatogonial proliferation. The Bidder's organ (BO) is a structure characteristic from the Bufonidae family and in adult males of Rhinella arenarum it is one of the main sources of plasma estradiol (E2). The purpose of this study is to analyze the effect of E2 on testicular steroidogenic enzymes, apoptosis and proliferation in the toad R. arenarum. For this purpose, testicular fragments were treated during 24h with or without 2 or 20nM of E2. After treatments, the activities of cytochrome P450 17α-hydroxylase-C17-20 lyase (CypP450c17) and 3β-hydroxysteroid dehydrogenase/isomerase (3β-HSD/I) were measured by the transformation of radioactive substrates into products, and CypP450c17 expression was determined by Western blot analysis. Apoptosis in testicular sections was detected with a commercial fluorescent kit based on TUNEL method, and proliferation was evaluated by BrdU incorporation. Results indicate that E2 has no effect on CypP450c17 protein levels or enzymatic activity, while it reduces 3β-HSD/I activity during the post reproductive season. Furthermore, although E2 has no effect on apoptosis during the pre and the post reproductive seasons, it stimulates testicular apoptosis during the reproductive season, mostly in spermatocytes. Finally, E2 has no effect on testicular proliferation all year long. Taken together, these results suggest that E2 is involved in the regulation of testicular steroidogenesis and spermatogenesis.

5α-Reductase, an enzyme regulating glucocorticoid action in the testis of Rhinella arenarum (Amphibia: Anura)

The reduction of A-ring of glucocorticoids to produce 5α-dihydro-derivatives by 5α-reductases has been considered as a pathway of irreversible inactivation. However, 5α-reduced metabolites of corticosterone and testosterone have significant biological activity. In this paper, we investigated whether toad testicular 5α-reductase (5α-Red) is able to transform corticosterone into 5α-dihydrocorticosterone. Furthermore, we studied the role of 5α-reduced metabolite of corticosterone as a glucocorticoid receptor (GR) agonist. The activity of 5α-Red was assayed in subcellular fractions with [(3)H]corticosterone or [(3)H]testosterone as substrate. The enzyme localizes in microsomes and its optimal pH is between 7 and 8. The activity is not inhibited by finasteride. These results support the conclusion that toad 5α-Red resembles mammalian type 1 isoenzyme. Kinetic studies indicate that neither K(m) nor V(max) for both corticosterone and testosterone were significantly different among reproductive periods. The K(m) value for testosterone was significantly higher than that for corticosterone, indicating that the C-21 steroid is the preferred substrate for the enzyme. Studies of the binding capacity of 5α-dihydrocorticosterone (5α-DHB) to the testicular GR show that 5α-DHB is able to displace the binding of [(3)H]dexamethasone to testicular cytosol with a similar potency than corticosterone. The inhibition constant (Ki) values for corticosterone and 5α-DHB were similar, 31.33±2.9 nM and 35.24±2.3 nM, respectively. In vitro experiments suggest that 5α-DHB is an agonist of toad testicular GR, decreasing the activity of the key enzyme for androgen synthesis, the cytochrome P450 17-hydroxylase, C17,20-lyase.

Hexachlorobenzene as hormonal disruptor--studies about glucocorticoids: their hepatic receptors, adrenal synthesis and plasma levels in relation to impaired gluconeogenesis

In Wistar rats, hexachlorobenzene (HCB) depresses the gluconeogenic enzyme phosphoenolpyruvate-carboxykinase (PEPCK). In the liver, glucocorticoids (GC) normally regulate the glucose synthesis by acting on PEPCK. Thus, the aim of this work was to investigate, in a time-course study, the effects of HCB on plasma GC, its adrenal synthesis and stimulation, and the kinetic parameters of its hepatic receptors (GR) in relation to the gluconeogenic blockage produced by HCB. Plasma corticosterone (CORT) concentration, urinary porphyrins and hepatic PEPCK were determined after 2, 4, 6 and 8 weeks of HCB-treatment. The effect of HCB on kinetic parameters of GR was studied in adrenalectomized porphyric rats after 2, 4 and 8 weeks of treatment. Additionally, adrenal CORT synthesis in the same weeks was measured with or without ACTH. Results show that plasma CORT in intoxicated animals dropped significantly after 2 and 4 weeks of treatment (23% and 58%, respectively), and then remained constant until the 8th week. HCB also promoted a reduction in the number of hepatic GR (50-55%) without modifying affinity. After 8 weeks, when porphyria was well established (40-50-fold increase in urinary porphyrins), a reduction (52%) in hepatic GR number, as well as a decrease in PEPCK activity (56%) were observed. Moreover, CORT biosynthesis in adrenals from intoxicated animals significantly decreased (60%) without changes in ACTH effect. Briefly, this paper shows that HCB causes a disruption in GC and GR. This disturbance could contribute to the negative effect on glucose synthesis through PEPCK regulation, thus modulating porphyria. These results enhance the knowledge about the hormonal disruption produced by chlorinated xenobiotics.

Cytosolic glucocorticoid receptor in the testis of Bufo arenarum: seasonal changes in its binding parameters

Glucocorticoids (GC) are the hormonal mediators of stress. In mammals, high levels of GC have negative effects on reproductive physiology. For instance, GC can inhibit testicular testosterone synthesis by acting via glucocorticoid receptors (GR), the extent of the inhibition being dependent on GC levels. However, the effect of GC on testicular function and even the presence of GR in amphibians are still unclear. The purpose of this work was to characterise testicular cytosolic GR in Bufo arenarum, determining the seasonal changes in its binding parameters as well as the intratesticular localisation. The binding assays were performed in testis cytosol with [3H]dexamethasone (DEX) and [3H]corticosterone (CORT). Binding kinetics of DEX and CORT fitted to a one-site model. Results were expressed as means +/- standard error. Apparent number of binding sites (Bapp) was similar for both steroids (Bapp DEX = 352.53 +/- 72.08 fmol/mg protein; Bapp CORT = 454.24 +/- 134.97 fmol/mg protein) suggesting that both hormones bind to the same site. Competition studies with different steroids showed that the order of displacement of [3H]DEX and [3H]CORT specific binding is: DEX approximately RU486 approximately deoxycorticosterone (DOC) > CORT > aldosterone > RU28362 > progesterone >>> 11-dehydroCORT. The affinity of GR for DEX (Kd = 11.2 +/- 1.5 nM) remained constant throughout the year while circulating CORT clearly increased during the reproductive season. Therefore, testis sensitivity to GC action would depend mainly on inactivating mechanisms (11beta-hydroxysteroid dehydrogenase type 2) and CORT plasma levels. Since total and free CORT are higher in the reproductive than in the non-reproductive period, the magnitude of GC actions could be higher during the breeding season. The intratesticular localisation of the GR was determined after separation of cells by a Percoll density gradient followed by binding assays in each fraction. DEX binds to two different fractions corresponding to Leydig and Sertoli cells. In conclusion, in the testis of B. arenarum GC could regulate the function of both cellular types particularly during breeding when CORT reaches the highest plasma concentration.

11beta-hydroxysteroid dehydrogenase in the testis of Bufo arenarum: changes in its seasonal activity

In rat Leydig cells, glucocorticoids (GC) inhibit testosterone (T) synthesis via glucocorticoid receptor (GR). However, GC access to GR is regulated by the local expression of 11beta-hydroxysteroid dehydrogenase (11beta-HSD). Two isoforms were identified in mammals: type 1, a NADP+-preferring enzyme with K(m) in the muM range for GC and type 2, NAD+-dependent, with K(m) in the nM range for GC. In amphibians, a seasonal rhythm in baseline GC levels was described. However, a shift in the amount of deactivating 11beta-HSD activity could alter GC effects. The purpose of this work is to describe seasonal changes in testicular activity of 11beta-HSD in Bufo arenarum as well as the annual and seasonal patterns of plasma corticosterone (B) and T. The activity of 11beta-HSD was assayed in homogenate and subcellular fractions in pre-reproductive (Pre-R), reproductive (R) and post-reproductive (Post-R) periods, using [3H]B. Plasma B and T were determined by RIA. Testicular 11beta-HSD is a microsomal NAD+-dependent enzyme with a K(m) in the nM order, its activity being strongly reduced by glycyrrhetinic acid. These results indicate that toad testes express an 11beta-HSD similar to mammalian type 2. Although 11beta-HSD activity is higher in the Post-R than in the R and Pre-R seasons (V(max): Pre-R: 0.26+/-0.10, R: 0.14+/-0.01, Post-R: 1.37+/-0.45, pmol/minmg protein), K(m) value remains constant throughout the year. A seasonal rhythm in baseline GC concentrations inversely correlated with plasma T was also described. T concentration is lower in the R season than in the other periods (Pre-R: 90+/-6; R: 12+/-1; Post-R: 56+/-3, nM) while total B concentration is higher in the breeding than in the other seasons (Pre-R: 62+/-10; R: 145+/-18; Post-R: 96+/-10, nM). Furthermore, free B (Pre-R: 51+/-8; R: 94+/-12; Post-R: 70+/-7, nM) was always below K(m) values. In conclusion, this work shows that the activity of 11beta-HSD in toad testes could modulate GC action by transforming active hormones in the corresponding inactive steroid.

Seasonal Changes in the Activity of 11β-Hydroxysteroid Dehydrogenase in Toad Testes

In mammals, glucocorticoids (GC) are inactivated by the oxidative activity of 11beta-hydroxysteroid dehydrogenase (11beta-HSD). To study that mechanism in the testes of Bufo arenarum, 11beta-HSD activity and plasma corticosterone (B) were determined in toads from pre- (PR), post- (P), and reproductive (R) periods. Toad 11beta-HSD is NAD(+)-dependent and strongly inhibited by glycyrrhetinic acid. V(max) is higher in the P period (1.37 vs. R: 0.13 and PR: 0.26 pmol/min-mg protein), whereas K(m) values (around 200 nM) remain constant. Plasma B concentrations (R: 178; PR: 61; P: 114 nM) are close to K(m) values, suggesting that, under baseline conditions, 11beta-HSD could protect testes against GC action.

Steroid production in toads

In Bufo arenarum, androgen biosynthesis occurs through a complete 5-ene pathway, including 5-androstane-3beta,17beta-diol as the immediate precursor of testosterone. Besides, steroidogenesis changes during the breeding period, turning from androgens to C(21)-steroids such as 5alpha-pregnan-3alpha,20alpha-diol, 3alpha-hydroxy-5alpha-pregnan-20-one and 5alpha-pregnan-3,20-dione. In B. arenarum, steroid hormones are not involved in hCG-induced spermiation, suggesting that the steroidogenic shift to C(21)-steroids during the breeding be not related to spermiation. The activity of 17-hydroxylase-C(17-20) lyase (CypP450(c17)) decreases during the reproductive season, suggesting that this enzyme would represent a key enzyme in the regulation of seasonal changes. However, the increase in the affinity for pregnenolone of 3beta-hydroxysteroid dehydrogenase (3alphaHSD)/isomerase could also be involved. Moreover, the reduction in CypP450(c17) leading to a reduction in C(19)-steroids, among them dehydroepiandrosterone (DHE), would contribute to the conversion of pregnenolone into progesterone, avoiding the non-competitive inhibition exerted by DHE on this transformation. Additionally, CypP450(c17) possesses a higher affinity for pregnenolone than for progesterone, explaining the predominance of the 5-ene pathway for testosterone biosynthesis. Animals in reproductive condition showed a significant reduction in circulating androgens, enhancing the physiological relevance of all the in vitro results. The in vitro effects of mGnRH and hrFSH on testicular steroidogenesis revealed that both hormones inhibited CypP450(c17) activity. In summary, these results demonstrate that, in B. arenarum, the change in testicular steroidogenesis during the reproductive period could be partially due to an FSH and GnRH-induced decrease in CypP450(c17) activity.

Effects of mGnRH on testicular steroidogenesis in the toad Bufo arenarum

GnRH controls vertebrate reproduction in several ways. This hormone not only affects the secretion of gonadotropins from the pituitary gland but also has a direct influence on several gonadal functions such as steroidogenesis, spermatogenesis, and spermiation. In the present paper we have studied the in vitro effects of GnRH on the testicular steroidogenesis of Bufo arenarum to ascertain the role of this peptide in the control of the steroidogenic pathway previously described in this species. It was found that GnRH is able to reduce basal as well as hCG-stimulated testosterone release, having an inhibitory effect on P450(c17) activity. Thus, GnRH could be involved in the mechanism that regulates the metabolic change in the testicular steroidogenesis. Additionally, testicular GnRH binding site has been characterised, showing a K(d) of 34 nM and a maximum binding of 4.7 pmol/mg protein.

In vitro hCG and human recombinant FSH actions on testicular steroidogenesis in the toad Bufo arenarum

In order to study the regulation of testicular steroidogenesis in the toad Bufo arenarum, the effect of gonadotropins (hCG and hrFSH) on steroidogenic enzymes was determined using an in vitro system. 3beta-Hydroxysteroid dehydrogenase/isomerase activity was not affected by any of the gonadotropins, at any of the concentrations used. In contrast, 5alpha-reductase activity was strongly reduced by both hCG and hrFSH. Human chorionic gonadotropin inhibited the activity of cytochrome P450 17alpha-hydroxylase-C(17-20) lyase (P450(c17)), only at the highest concentration used, while hrFSH strongly reduced P450(c17) activity at all the doses assayed. In conclusion, these data suggest that LH (hCG) and FSH regulate steroidogenic enzymes such as 5alphaRed and P450(c17). The results also suggest that FSH could be involved in the regulation of the change in steroidogenesis undergone by the testis during the breeding season. In turn, the inhibition of P450(c17) activity could result in a reduction of androgen production and an increment of C21 steroids.

Effect of salt acclimatization on 3 beta-hydroxysteroid dehydrogenase/isomerase activity in the interrenal of Bufo arenarum

In amphibians, aldosterone (Aldo) is particularly important in the regulation of Na(+) exchange by skin and urinary bladder. In previous works we studied a key enzyme in Aldo biosynthesis, the 3 beta-hydroxysteroid dehydrogenase/isomerase (3 beta HSD/I), in the interrenals of Bufo arenarum. In those works a dual localization of the 3 beta HSD/I in both microsomes and mitochondria was described. The mitochondrial, but not the microsomal, enzyme prefers the immediate Aldo precursor, 3 beta-analogue of aldosterone, as substrate. In this order, the enzyme 3 beta HSD/I would be not only a key enzyme for the synthesis of Aldo but additionally, due to its microsomal and mitochondrial localization, a possible target for the regulation of Aldo biosynthesis. With this rationale in mind, we have used in vivo and in vitro approaches to study Aldo regulation. In the present investigation the levels of Aldo were determined in plasma of winter (W) and summer (S) toads subjected to different saline concentrations (0.125 and 0.15 M) or kept on wet land. Saline hyperosmotically treated toads had significantly lower levels than isoosmotically treated toads. These results are consistent with the response in mammals, in which salt loading provokes a reduction in Aldo secretion. In W toads, plasmatic corticosterone (B) concentration was higher than Aldo concentration, whereas in S toads, B/Aldo ratio approached unity. The reduction of Aldo levels after saline dehydration was due to a decline in its biosynthesis. K(m) and V(max) values for 3 beta HSD/I were calculated for mitochondrial and microsomal fractions obtained from animals acclimated to 0.15 M NaCl or kept on land. As previously described, V(max) differs between W and S toads. However, only mitochondrial V(max) changed as a consequence of saline adaptation, suggesting that the mitochondrial enzyme could be involved in the regulation of Aldo biosynthesis.

Human chorionic gonadotropin-induced spermiation in Bufo arenarum is not mediated by steroid biosynthesis

This study employed an in vitro system to identify potential steroidal mediators of spermiation in Bufo arenarum. Testicular fragments were incubated for 2 h at 28 degrees. Spermiation was induced by 10 IU human chorionic gonadotropin (hCG) and the effect of different inhibitors of steroid biosynthesis was analyzed. Cyanoketone (10(-5)-10(-6) M), an inhibitor of 3-oxo-4-ene steroid biosynthesis, did not block hCG-inducing activity even when biosynthesis of 3-oxo-4-ene steroids and its reduced metabolites was inhibited by 95%. Aminogluthetimide at a concentration that inhibited testosterone biosynthesis (10(-4) and 10(-5) M) did not alter hCG actions. Similar results were obtained with spironolactone, an inhibitor of 17alpha-hydroxylase/17-20 lyase activity. No spermiation-inducing activity was found with different steroids (progesterone, 17-hydroxypregnenolone, 17, 20alpha/beta-dihydroxy-4-pregnene-3-one, estradiol, testosterone, etc.). It is concluded that spermiation induced by hCG is not steroid mediated in B. arenarum.

Kinetic properties of microsomal 3beta hydroxysteroid dehydrogenase-isomerase from the testis of Bufo arenarum H

3beta-hydroxysteroid dehydrogenase 5-ene isomerase (3betaHSD/I) activity is necessary for the biosynthesis of hormonally active steroids. A dual distribution of the enzyme was described in toad testes. The present study demonstrates that in testicular tissue of Bufo arenarum H., microsomal 3betaHSD/I has more affinity for dehydroepiandrosterone (DHEA) than for pregnenolone (K(m)=0.17+/-0. 03 and 1.02 microM, respectively). The Hill coefficient for the conversion of DHEA and pregnenolone were 1.04 and 1.01, respectively. The inclusion of DHEA in the kinetic analysis of pregnenolone conversion affected V(max) while K(m) was not modified, suggesting a non-competitive inhibition of the conversion of pregnenolone. K(i) was calculated from replot of Dixon's slope for each substrate concentration. K(i) from the intercept and the slope of this replot were similar (0.276+/-0.01 and 0.263+/-0.02 microM) and higher than the K(m) for DHEA. The K(m) and K(i) values suggest the presence of two different binding sites. When pregnenolone was present in the assays with DHEA as substrate, no effect was observed on the V(max) while K(m) values slightly increased with pregnenolone concentration. Consequently, pregnenolone inhibited the transformation of DHEA in a competitive fashion. These studies suggest that, in this species, the microsomal biosyntheses of androgens and progesterone are catalysed by different active sites.