Activin-attenuated expression of transcripts encoding granulosa cell- derived insulin-like growth factor binding proteins 4 and 5 in the rat: A putative antiatretic effect

Given the suggestion that intraovarian insulin-like growth factor (IGF)- binding proteins (IGFBPs) may constitute markers of follicular atresia, we investigated the possibility that activin, a putative antiatretic principle, may modulate granulosa cell-derived IGFBPs. Untreated granulosa cells cultured for 72 h exhibited a progressive increase in the steady-state levels of transcripts corresponding to IGFBP-4 and IGFBP-5 (1.5-fold and 12-fold, respectively). Transcript levels corresponding to IGFBP-5 were consistently higher than their IGFBP-4 counterparts. Treatment with activin-A (50 ng/ml) for 72 h produced significant (p < 0.05) decrements in the steady-state levels of IGFBP-4 and IGFBP-5 transcripts (46% and 79%, respectively) as compared to controls. Thus, treatment with activin-A appears to be capable of blocking the spontaneous increase in IGFBP-4 and IGFBP-5 transcripts exhibited by untreated cultured granulosa cells. Consistent activin-A- induced decrements were also observed in the accumulation of the IGFBP-5 (but not the IGFBP-4) protein. Dose-response analysis revealed monophasic dose dependence (half maximal inhibitory doses of 16.2 and 7.8 ng/ml for IGFBP-5 and IGFBP-4 transcripts, respectively). The addition of increasing concentrations of the putative activin-binding protein, follistatin, produced dose-dependent reversal of the activin-A effect on IGFBP transcripts (IGFBP- 5 > IGFBP-4). Activin-B was as effective as activin-A in reducing IGFBP-4 transcripts (31% decrement, p < 0.05) whereas it had little or no effect on IGFBP-5 transcripts (21% decrement, p > 0.1). No apparent effect was observed for the corresponding proteins. Activin-A action was specific in that treatment with transforming growth factor (TGF)-β₁, inhibin-A, or Mullerian-inhibiting substance (MIS)-all related peptides-failed to produce statistically significant alterations in the steady-state levels of IGFBP-4 and IGFBP-5 transcripts. Taken together, these observations reveal that activin-A exerts a substantial, relatively rapid, follistatin-neutralizable, dose- and time-dependent inhibitory effect on granulosa cell-derived IGFBP transcripts (IGFBP-5 > IGFBP-4). Other members of the TGFβ superfamily (e.g., inhibin-A, TGFβ₁, and MIS) were without significant effect on the expression of IGFBP-4 and IGFBP-5. To the extent that the inhibition of IGFBP-4 and IGFBP-5 expression is associated with, and possibly causally related to, the promotion of follicular health, the activin may play an antiatretic role in the dynamic process of follicular selection.