Insulin-like growth factor (IGF)-1 stimulates IGF-1 and type 1 IGF receptor expression in cultured rat granulosa cells - Autocrine regulation of the intrafollicular IGF-1 system

A growing body of information documents the existence of a complete rat int rafollicular insulin-like growth factor (IGF)-I system replete with a ligan d (IGF-I), a receptor (type 1 IGF receptor) IGF binding proteins (4 and 5), and IGFBP-directed endopeptidases (4 and 5). Previous studies have establi shed the ability of IGF-I to promote the elaboration of granulosa cell-deri ved IGFBP-5 and to suppress the activity of granulosa cell-derived IGFBP-5- directed endopeptidase. It was the purpose of this article to examine the e ffects of treatment with IGF-I on the other components of the intrafollicul ar IGF system, i.e., IGF-I itself and the type 1 IGF-receptor. Granulosa ce lls, obtained by follicular puncture from 25-d-old estrogen-primed rats wer e cultured in polystyrene tubes for 72 h under serum-free conditions, in th e absence or presence of the indicated agents. At the conclusion of each ex periment, media were discarded, and RNA was extracted and subjected to an R Nase protection assay. Treatment of cultured rat granulosa cells with IGF-I resulted in a significant 1.8-fold increase in the steady-state levels of IGF-I mRNA. No effect was noted on the total cellular DNA content thereby a rguing against the possibility that the relative increase in IGF-I transcri pts can be ascribed to a possible treatment-induced increase in cell number in culture. The IGF-I effect was apparent (p < 0.05) at IGF-I doses as low as 1 ng/ml, minimal additional increments being noted thereafter. Treatmen t with insulin and des (1-3) IGF-I proved equally effective, producing 2.0- and 2.6-fold increases, respectively, thereby suggesting that the IGF-1 ef fect may be mediated via the type 1 IGF receptor. Treatment with IGF-I also resulted in a significant (p < 0.005) increase in type 1 IGF receptor expr ession (2.3-fold increase), the first significant effect being noted at the 30 ng/mL dose level. Similar results obtained for insulin and des (1-3) IG F-I thereby suggest that the ability of IGF-I to upregulate the expression of its own receptor is probably type 1 IGF receptor-mediated. Taken togethe r, these findings indicate that treatment of estrogen-primed granulosa cell s with IGF-I will result in upregulation of the steady-state levels of tran scripts corresponding to IGF-I itself and to its type 1 IGF receptor. These observations emphasize the importance of positive auto regulatory phenomen a as determinants of the intrafollicular content of IGF-I and its receptor.