Insulin-like growth factor-binding protein-2 and -3: their biological effects in bovine thecal cells

This study was aimed at testing the hypothesis that the insulin-like growth factor-binding proteins (IGFBP)-2 and -3 can modulate the hormone-dependent differentiation of thecal cells in vitro. Thecal cells from large (> or = 8 mm) follicles were collected from cattle, cultured for 2 days in medium containing 10% fetal calf serum, washed, and then treated for an additional 2 days in serum-free medium with bovine LH (100 ng/ml), recombinant human insulin-like growth factor (IGF)-I (0 or 30 ng/ml), recombinant human IGFBP-2 (0, 200, or 400 ng/ml; i.e., 0, 6.5, or 12.9 nM), or recombinant human IGFBP-3 (0, 200, or 400 ng/ml; i.e., 0, 4.3, or 8.5 nM). IGFBP-2 (200 and 400 ng/ml) inhibited (p < 0.05) IGF-I-induced androstenedione production by 18-30% but did not influence (p > 0.10) progesterone production or thecal cell proliferation in the presence of LH and/or IGF-I. In contrast, IGFBP-3 (200 ng/ml) inhibited the IGF-I-induced increase in thecal cell numbers by 76%, and thecal cell progesterone and androstenedione production by 52% and 89%, respectively. A higher dose of IGF-I (i.e., 100 ng/ml) overcame the inhibitory effects of IGFBP-3 on IGF-I-induced cell proliferation and on progesterone and androstenedione production by thecal cells. As with IGFBP-2, IGFBP-3 had no effect (p > 0.10) on LH-induced progesterone or androstenedione production by thecal cells in the absence of IGF-I. Both IGFBP-2 and IGFBP-3 directly inhibited [125I]IGF-I and -II binding to thecal cells; IGFBP-2 was a weaker inhibitor of thecal [125I]IGF-I and -II binding than IGFBP-3. These results indicate that IGFBP-3 has a more pronounced inhibitory effect than IGFBP-2 on IGF-I action in cultured bovine thecal cells. Thus, IGFBP-3 may play a more significant role than IGFBP-2 in regulating thecal cell proliferation and steroidogenesis during follicular development in cattle.