REGULATION OF LUTEINIZING-HORMONE RECEPTOR GENE-EXPRESSION BY INSULIN-LIKE GROWTH-FACTOR-I IN AN IMMORTALIZED MURINE LEYDIG TUMOR-CELL LINE(BLT-1)

It is postulated that insulin-like growth factor-I (IGF-I), a 70-amino acid mitogenic polypeptide, regulates Leydig cell steroidogenesis. In the present study, we assessed the effect of ICF-I on LH receptor (LH R) gene expression in an immortalized murine Leydig tumor cell line (B LT-1). Culture of BLT-1 cells in the presence of IGF-I (0.1-100 ng/ml) for 24 or 48 h increased their [I-125]iodo-hCG binding in a dose-depe ndent manner up to 275% of the control level. Northern hybridization a nalysis revealed four major transcripts of LHR mRNA in BLT-1 cells (6. 9, 2.6, 1.7, and 1.2 kilobases), and treatment at 10-100 ng/ml of IGF- I increased steady-state levels of LHR mRNAs in coordinate fashion up to 2.2-fold. IGF-I (30 ng/ml) induced a time-dependent increase in [I- 125]hCG binding after a lag period of 2-6 h when studied up to 48 h, w ith a subsequent decrease. A similar response with steady increase up to 72 h was observed in total LHR mRNA. To elucidate the molecular mec hanism of ICF-I action on LHR mRNA expression, we measured the transcr iption rate of the LHR gene by nuclear run-off assay and assessed tran script stability by the actinomycin D blocking method. The results sho wed that IGF-I treatment had no effect on the transcription rate of th e LHR gene, whereas the half-life (t(1/2)) of LHR mRNA was significant ly prolonged (IGF-I-treated cells, 30 +/- 3.8 h; controls, 17 +/- 2.5 h). Furthermore, IGF-I at 30 ng/ml and 100 ng/ml increased the express ion of LHR promoter-driven luciferase and cytomegalovirus-promoter dri ven beta-galactosidase activities in BLT-1 cells; however, the former increased only marginally more than the latter. This suggests that the increase of LHR mRNA by IGF-I in Leydig cells is mainly due to increa sed mRNA stability.