The acquisition of FSH receptor during preantral folliculogenesis is believed to be a key step in the subsequent development of follicles. We examined the interaction between activin and cAMP in FSH receptor induction in rat granulosa cells by measuring 125I-FSH binding and FSH receptor mRNA. In the 125I-FSH binding study, 0.2 mM 8-Br-cAMP and 1 microM forskolin were maximally effective in FSH receptor induction (169 and 220% respectively of control), while higher concentrations gave attenuated responses. It appears that cAMP has ambivalent effects on FSH receptor induction depending on the concentration and length of exposure. Activin alone dramatically increased the number of FSH receptors (314% of control). Moreover, synergistic effects of activin and 8-Br-cAMP or forskolin were observed on FSH receptor induction: a combination of activin (80 ng/ml) and low doses of 8-Br-cAMP (0.2 mM) or forskolin (1 microM) was most effective (160 or 140% of that induced by activin alone) and receptor levels reached a maximum at 24 h. These levels than markedly decreased after 72 h of incubation. Northern blot analysis revealed that the combination of activin (80 ng/ml) and 8-Br-cAMP (0.2 mM) or forskolin (1 microM) increased FSH receptor mRNA to about 140% of that induced by activin alone. These results indicate that activin and cAMP induced FSH receptor synergistically. However, activin did not enhance the production of cAMP induced by forskolin. In addition, a protein kinase A inhibitor (H89) (2 microM), which inhibited the effects of forskolin, had no effect on the action of activin.(ABSTRACT TRUNCATED AT 250 WORDS)

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The effect of FSH on the induction of FSH receptors in granulosa cells is believed to be mediated, at least in part, by the cAMP second messenger system. We examined the effect of activin and cAMP on FSH receptor expression in this culture system. Steady-state levels of FSH receptor mRNA, analyzed by Northern blot hybridization, increased 3.5-fold in response to 24-h incubation with activin and 1.7-fold with 12-h incubation with 8-bromoadenosine 3,5-cyclic monophosphate (8-Br-cAMP; 0.2 mM). We have investigated whether 8-Br-cAMP- and/or activin-induced increases in FSH receptor mRNA levels are the result of increased transcription and/or altered mRNA stability. The rates of FSH receptor mRNA gene transcription, assessed by nuclear run-on transcription assay, increased 3-fold in cells treated with activin and 1.5-fold in cells treated with 8-Br-cAMP for 2 h. To examine the degradation rates of FSH receptor mRNA transcripts, granulosa cells were preincubated with 8-Br-cAMP, activin, or medium alone for 6 h. After the preincubation period, 5 microM actinomycin-D or 200 microM 5,6-dichloro-1-beta-ribofuranosyl benzimidazole were added to arrest new RNA synthesis. The decay curves for the 2.4 kb FSH receptor mRNA transcript in granulosa cells were not significantly different in the absence or presence of 8-Br-cAMP. Activin, on the other hand, significantly altered the slope of the FSH receptor mRNA decay curve and increased the half-life of the 2.4 kb FSH receptor mRNA transcript. These data provide evidence that cAMP induces FSH receptor mRNA levels by stimulating the transcription rate and that activin increases FSH receptor mRNA levels both by stimulating transcription rates and by stabilizing the FSH receptor mRNA transcripts.