Growth or Differentiation

Determination by FSH of the Action of Insulin-Like Growth Factor-I in Cultured Rat Granulosa Cells

Abstract

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 be an inducer of differentiation. The present study was conducted to identify the factor which determines the direction of IGF-I action: either DNA synthesis or LH receptor expression. When granulosa cells were incubated with IGF-I in the presence of various concentrations of follicle-stimulating hormone (FSH), DNA synthesis as assessed by [³H]thymidine 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 10ng/ml. At higher concentrations, FSH had little effect on DNA synthesis but instead induced expression of receptors for luteinizing hormone (LH), a marker of granulosa cell differentiation. At 5ng/ml, FSH elicited maximal stimulation of DNA synthesis and simultaneously induced LH receptor expression to some extent. In these cells, DNA synthesis peaked at 36h but expression of LH receptor occurred later than 36h, peaking at 60h. The ability of IGF-I to stimulate DNA synthesis was enhanced by the long term pretreatment with FSH: when FSH was added from the beginning and IGF-I was added after 36h or later, ICE-I-mediated DNA synthesis was approximately twice as great, and was accompanied by a two-fold increase in the number of bromodeoxyuridine-labeled nuclei. Under these conditions, LH receptor expression was reduced to approximately 50%. Finally when cells were incubated for 12h with or without FSH, washed extensively with the medium and then IGF-I was added, DNA synthesis was augmented only in FSH-primed cells. Forskolin, an activator of adenylate cyclase, reproduced the effect of FSH. These results indicate that, in the presence of FSH, IGF-I has the ability to induce both DNA synthesis and differentiation and that FSH determines the action of IGF-I on promotion of either growth or differentiation. Furthermore, priming with FSH renders granulosa cells responsive to IGF-I in terms of DNA synthesis.