Steroidogenic activities in MA-10 Leydig cells are differentially altered by cAMP and Müllerian inhibiting substance.

In addition to causing Müllerian duct regression in fetal males, Müllerian inhibiting substance (MIS) inhibits the expression of the bifunctional cytochrome P450, C17 hydroxylase/C(17-20) lyase (Cyp17), the enzyme that catalyzes the committed step in sex steroid synthesis. To investigate the paracrine effects of MIS on steroidogenic activity, we have performed assays with microsomes from mouse MA-10 Leydig cells. With microsomes from untreated MA-10 cells, progesterone was largely metabolized by 5alpha-reductase and subsequently converted by 3-keto steroid reductases to allopregnanolone and epiallopregnanolone. Addition of cAMP to the cells shifted microsomal steroid production to the Cyp17 product androstenedione and its 5alpha,3beta-reduced form, epiandrosterone. Microsomes from MIS-treated cells were less active with the progesterone substrate than those of untreated cells but co-treatment of the cells with both MIS and cAMP mitigated the cAMP-induced shift of the microsomes to androstenedione production. Quantitative analyses of steroid production by Cyp17 showed that cAMP decreased the amount of 17-hydroxyprogesterone produced relative to the androstenedione, suggesting that cAMP signaling lowers the efficiency of the Cyp17 hydroxylase activity or else increases the efficiency of its lyase activity. Addition of MIS to the cAMP-treated cells partially reversed this effect, as well. These results indicate that cAMP induces MA-10 cells to switch from producing 5alpha-reduced progesterone metabolites to producing androstenedione and its metabolites by increasing Cyp17 expression and its relative lyase activity, both of which are inhibited by MIS.

Feedback inhibition of steroidogenic acute regulatory protein expression in vitro and in vivo by androgens.

Müllerian-inhibiting substance (MIS) reduces testosterone synthesis in Leydig cells by inhibiting cytochrome P450C17 hydroxylase/C17-20 lyase expression. However, in mouse Leydig MA-10 cells, MIS also enhances the cAMP-induced expression of mRNA for steroidogenic acute regulatory protein (StAR), which transports cholesterol to the inner mitochondrial membrane for conversion to pregnenolone. We hypothesized that the MIS-induced StAR expression is the indirect result of reduced testosterone synthesis in Leydig cells caused by MIS. We show that, in addition to MIS, flutamide, an androgen receptor antagonist, enhanced StAR mRNA expression when added to cAMP-treated MA-10 cells, whereas dihydrotestosterone, a potent androgen receptor agonist, attenuated these responses. Progesterone, dexamethasone, and estradiol also inhibited StAR mRNA expression. Addition of MIS to cAMP-treated MA-10 cells transfected with a StAR-promoter luciferase reporter resulted in increased StAR promoter activity over cAMP alone; this effect was inhibited by dihydrotestosterone, suggesting that androgens inhibit StAR mRNA expression at the transcriptional level. Androgen-mediated inhibition of StAR expression was also observed in primary Leydig cell culture and in vivo using both hypophysectomized mice and mice treated with the GnRH antagonist, acyline. These results suggest that the induction of StAR expression by MIS occurs secondary to the MIS-mediated reduction in testosterone synthesis by relieving a hitherto uncharacterized androgen-dependent feedback inhibition on StAR expression. These findings may impact future treatment strategies aimed at reducing androgen; for example, in the treatment of prostatic cancer, antiandrogen treatment might benefit from adjuvant therapy to inhibit StAR expression.

EGF stimulates rat spermatogonial DNA synthesis in seminiferous tubule segments in vitro.

Epidermal growth factor (EGF) superfamily of peptide growth factors (EGF-GFs) plays a role in male germ cell development, but the precise function is yet to be defined. The present study shows that EGF-GFs stimulate spermatogonial proliferation in vitro. The EGF-GF ligands, EGF, transforming growth factor-alpha and betacellulin all stimulated DNA synthesis in microdissected stage I segments of rat testis seminiferous tubules in vitro, as revealed by 3H-thymidine incorporation and 5-bromo-2'-deoxyuridine (BrdU) labeling. A fourfold increase over control of BrdU labeled cells, identified as spermatogonia, was seen after treatment with EGF. RT-PCR analysis revealed that the EGF receptors erbB1, erbB2, erbB3 and erbB4 were expressed at all stages of the spermatogenic wave, whereas differential expression was found in isolated Leydig, Sertoli and peritubular cells. The results show that EGF-GFs is spermatogonial growth factor(s) in vitro, although we have not discriminated between a direct action and an indirect effect via somatic cells. We suggest that EGF-GFs is involved in the paracrine control of spermatogenesis in vivo.