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Inhibin and activin are protein hormones with diverse physiological roles including the regulation of pituitary FSH secretion. Like other members of the transforming growth factor-beta gene family, they undergo processing from larger precursor molecules as well as assembly into functional dimers. Isolation of inhibin and activin from natural sources can only produce limited quantities of bioactive protein. To purify large-scale quantities of recombinant human inhibin and activin, we have utilized stably transfected cell lines in self-contained bioreactors to produce protein. These cells produce approximately 200 microg/ml per day total recombinant human inhibin. Conditioned cell media can be purified through column chromatography resulting in dimeric mature 32-34 kDa inhibin A and 28 kDa activin A. The purified recombinant proteins maintain their biological activity as measured by traditional in vitro assays including the regulation of FSH in rat anterior pituitary cultures and the regulation of promoter activity of the activin-responsive promoter p3TP-luc in tissue culture cells. These proteins will be valuable for future analysis of inhibin and activin function and have been distributed to the US National Hormone and Peptide Program.

The presence of activin A and its effects have previously been documented in the adrenal gland, particularly in the human fetal adrenal gland and the rat adrenal gland. The primary signaling pathway of activin involves interactions between receptor and intracellular (Smad) proteins that have not been completely described in the adrenal gland. In this study, we demonstrate that the components of the activin signaling cascade are present in two complementary models, the fetal rat adrenal gland and the human adrenocortical cell line, H295R, by means of RT-PCR, western analysis, and immunoprecipitation techniques. Using the cell line, activin signaling was analyzed using an activin-responsive reporter gene, p3TP-luc, and luciferase assays to assess transcriptional activity with co-expression of the different activin receptors and Smads to demonstrate the functionality of the signaling cascade. In the fetal rat adrenal gland, the relative amounts of mRNA of the type II receptors, RII and RIIB, were regulated by gestational age, such that the RIIB levels increased after birth while RII levels fell. Using immunodetection techniques, the activin receptors and the different Smad proteins were detected in the rat fetal adrenal glands. Notably, the presence of Smad4 protein is significantly increased after birth in the rat adrenal gland. RT-PCR established a similar profile in the H295R cells. Using p3TP-luc, the H295R cells show transcriptional activation of this activin-responsive reporter in the presence of activin A. Co-expression of type I and type II receptors as well as Smads, results in ligand-independent transcriptional activity in addition to an activin-stimulated response. In determining activin's effects on adrenal function, adrenal steroid production was evaluated by incubation of the H295R cells with increasing doses of activin A and inhibin A, resulting in a detectable increase in P450c17 expression. Co-incubation of activin A with follistatin diminishes this response. These results are consistent with a role for activin A in the adrenal gland by demonstrating that the elements of the activin signaling pathway are present, intact, and functional. This suggests that in the adrenal gland the components of the activin receptor/Smad pathway are dynamically changing in the transition from fetal to neonatal life, and are important to the function of this organ.

Follicle-stimulating hormone (FSH) stimulates ovarian follicle development and the production of protein hormones including inhibin A and inhibin B. The inhibins are dimeric proteins (alpha-beta(A) or alpha-beta(B)) secreted by growing follicles that suppress FSH in a classical endocrine negative feedback loop. Siberian hamsters, Phodopus sungorus, exhibit seasonal variation in FSH levels. Given the role of inhibin in FSH regulation, we hypothesized that ovarian inhibin expression differs between animals reared in long (16 h light:8 h darkness) and short (6 h light:18 h darkness) photoperiods. To examine inhibin expression in animals housed under long or short photoperiods, hamster inhibin alpha-, beta(A)-, and beta(B)-subunits were cloned and used to detect and localize inhibin subunit mRNA in developing follicles. Ovarian inhibin alpha-subunit mRNA levels were significantly higher in long day-exposed (LD) than in short day-exposed (SD) hamsters. In addition, dimeric inhibin, as well as inhibin alpha-, beta(A)-, and beta(B)-subunit protein levels were higher in the LD than in the SD hamster ovaries.

Inhibin production differs in ovaries of Siberian hamsters (Phodopus sungorus) exposed to long days (LD) or short days (SD). We believe that seasonal differences in serum follicle-stimulating hormone contribute to this difference. However, given the profound photoperiodic differences in follicle maturation, serum gonadotropins alone may not account for all of the observed differences in inhibin processing. To test this hypothesis, we challenged LD and SD female hamsters with exogenous gonadotropins. While both groups responded with increased inhibin expression, the effects were muted in ovaries of SD females and there was no evidence of ovulation in these animals. These data indicate that the ovaries of SD females are not immediately equipped to respond to gonadotropin stimulation. More generally, these data suggest that photoperiodic history affects ovarian inhibin production and secretion in response to gonadotropins.