Insulin-like growth factor-I (IGF-I) and IGF-II hormonal action in cultured rat granulosa cells: mediation via type I but not type II IGF receptors.

Both insulin-like growth factor-I (IGF-I) and IGF-II have been shown to promote granulosa cell differentiation and proliferation. While both type I and type II IGF receptors have been observed in rat granulosa cells, the identity of the IGF receptor type(s) mediating IGF hormonal action remains uncertain. Whereas the role of the rat type I IGF receptor cannot be completely evaluated at this time due to the lack of specific reagents, the availability of antibodies specific for the rat type II IGF receptor (R-II-PAB1) has made studies of this receptor type possible. To validate the utility of the R-II-PAB1 antiserum at the level of the rat granulosa cell, its ability to immunoneutralize the granulosa cell type II IGF receptor was examined. Significantly, R-II-PAB1 (10-100 micrograms/ml) proved a potent inhibitor of [125I]IGF-II (but not [125I]IGF-I) binding to granulosa cell membrane preparations. Substantial, albeit finite, R-II-PAB1-mediated inhibition of the cross-linking of [125I]IGF-II was also observed. Moreover, R-II-PAB1 proved highly potent in immunoprecipitating the rat granulosa cell type II IGF receptor. In light of these observations, we have proceeded to use R-II-PAB1 to assess the functional role of the rat granulosa cell type II IGF receptor in IGF-I and IGF-II hormonal action. To this end, FSH (20 ng/ml)-primed granulosa cells were cultured for 72 h in the absence or presence of IGF-I or IGF-II (50 ng/ml) with or without increasing (receptor-active) concentrations of R-II-PAB1 (10-100 micrograms/ml). Control incubations were carried out with an ammonium sulfate precipitate of nonimmune rabbit serum dialyzed against PBS. Significantly, both R-II-PAB1 and nonimmune rabbit serum were without effect on the cytodifferentiative action of either IGF-I or IGF-II. Subject to limitations inherent to the immunoneutralizing potency of R-II-PAB1, these findings are in keeping with the notion that (inasmuch as the conventional cytodifferentiative process is concerned) the granulosa cell type II IGF receptor does not appear to participate in transmembrane IGF signalling. By inference, these findings also suggest that IGF-I and IGF-II hormonal action at the level of the granulosa cell may be exerted largely, if not exclusively, via the type I IGF receptor. Thus, the potential relevance and the functional role(s), if any, of the granulosa cell type II IGF receptor remain to be determined.