GROWTH OR DIFFERENTIATION - DETERMINATION BY FSH OF THE ACTION OF INSULIN-LIKE GROWTH-FACTOR-I IN CULTURED RAT GRANULOSA-CELLS

Insulin-like growth factor-I (IGF-I) is a potent mitogen in many cell systems. In cultured rat granulosa cells, however, IGF-I is known to b e an inducer of differentiation. The present study was conducted to id entify the factor which determines the direction of IGF-I action: eith er DNA synthesis or LH receptor expression. When granulosa cells were incubated with IGF-I in the presence of various concentrations of foll icle-stimulating hormone (FSH), DNA synthesis as assessed by [H-3]thym idine incorporation was increased only in the presence of low doses of FSH. The stimulatory effect of FSH on DNA synthesis was observed in a very narrow range of FSH concentration between 2 and 10 ng/ml. At hig her concentrations, FSH had little effect on DNA synthesis but instead induced expression of receptors for luteinizing hormone (LH), a marke r of granulosa cell differentiation. At 5 ng/ml, FSH elicited maximal stimulation of DNA synthesis and simultaneously induced LH receptor ex pression to some extent. In these cells, DNA synthesis peaked at 36 h but expression of LH receptor occurred later than 36 h, peaking at 60 h. The ability of IGF-I to stimulate DNA synthesis was enhanced by the long term pretreatment with FSH: when FSH was added from the beginnin g and IGF-I was added after 36 h or later, IGF-I-mediated DNA synthesi s was approximately twice as great, and was accompanied by a two-fold increase in the number of bromodeoxyuridine-labeled nuclei. Under thes e conditions, LH receptor expression was reduced to approximately 50%. Finally when cells were incubated for 12 h with or without FSH, washe d extensively with the medium and then IGF-I was added, DNA synthesis was augmented only in FSH-primed cells. Forskolin, an activator of ade nylate cyclase, reproduced the effect of FSH. These results indicate t hat, in the presence of FSH, IGF-I has the ability to induce both DNA synthesis and differentiation and that FSH determines the action of IG F-I on promotion of either growth or differentiation. Furthermore, pri ming with FSH renders granulosa cells responsive to IGF-I in terms of DNA synthesis.