Effects of insulin-like growth factor I and insulin on proliferation and on basal and luteinizing hormone-induced steroidogenesis of bovine thecal cells: Involvement of glucose and receptors for insulin-like growth Factor I and luteinizing hormone

The objective of the present study was to determine the effects of IGF-I and insulin on cell proliferation, LH receptors, and basal and LH-induced progesterone and androstenedione production by bovine thecal cells. Cells from large (greater than or equal to 8 mm) bovine follicles were cultured for 1 or 2 d in medium containing 10% fetal calf serum (FCS) and treated for 1 or 2 d in serum-free medium with IGF-I, insulin, and(or) LH. Treatment with 30 and 100 ng/mL of IGF-I for 1 or 2 d increased thecal cell numbers in the absence of LH regardless of whether treatments were initiated after 1 or 2 d of exposure to 10% FCS. Cotreatment with LH reduced the stimulatory effect of IGF-I on thecal cell numbers. Insulin at 10 and 100 ng/mL increased cell numbers in the presence of LH. Both IGF-I and insulin were ineffective at stimulating thecal cell progesterone or androstenedione production in the absence of LH. However, IGF-I and insulin increased(P < .05) androstenedione and progesterone production in the presence of LH. Alone, LH had little or no effect on androstenedione and progesterone production, whereas in the presence of 30 and 100 ngl mt IGF-I or 1 to 100 ng/mL insulin, LH stimulated(P < .05) androstenedione production. The stimulatory effects of IGF-I on cell proliferation and progesterone production were not detected in the presence of 100 ng/mL insulin. However, co-treatment with various doses of IGF-I and 100 ng/mL insulin further increased androstenedione production above that seen with insulin alone. In glucose-deficient medium, 25 to 75 mg/dL of glucose increased (P < .05) thecal cell proliferation, progesterone production, and androstenedione production. In the absence or presence of glucose, insulin (100 ng/mL) increased (P < .05) thecal cell proliferation, progesterone production, and androstenedione production. Treatment with 3, 10, or 100 ng/mL LH had no effect(P > .10) on the numbers of IGF-I binding sites on thecal cells but increased (P < .05) androstenedione production. Treatment with 10 and 100 ng/mL IGF-I increased (P < .01) numbers of LH/hCG binding sites. These results indicate that IGF-I and insulin may each play a significant role in thecal cell mitogenesis and LH-induced thecal cell steroidogenesis during follicular development in cattle and that glucose enhances these effects. Furthermore, the synergism between IGF-I and LH on increasing steroidogenesis does not seem to be mediated through increased binding sites for IGF-I in bovine thecal cells but rather, in part, through increased binding sites for LH.