A ROLE FOR INCREASED LUTROPIN CHORIOGONADOTROPIN RECEPTOR (LHR) GENE-TRANSCRIPTION IN THE FOLLITROPIN-STIMULATED INDUCTION OF THE LHR IN GRANULOSA-CELLS

Follitropin (FSH) has been shown in previous studies to stimulate the induction of the LH/CG receptor (LHR) and LHR mRNA in the granulosa cells of diethylstilbesterol-primed immature rats. The present studies were undertaken to identify the mechanisms underlying the hormone-dependent induction of the LHR in rat granulosa cells. The effect of FSH on LHR mRNA stability was determined by measuring the decay of LHR mRNA after removal of FSH under conditions where transcription was inhibited. Under these conditions, readdition of FSH had little effect on mRNA stability. However, inhibitors of transcription themselves were found to have a marked effect on stabilizing the LHR mRNA, thus potentially masking an effect of FSH. These results suggest that there is a labile destabilizing factor that constitutively degrades LHR mRNA. At present, it cannot be ascertained whether FSH has any effect on this destabilizing factor. Transcriptional activity of the LHR gene was examined using nuclear run-on assays. It was found that 1) in the absence of FSH, LHR-binding activity and mRNA levels were negligible, but the LHR gene was transcriptionally active in granulosa cells of immature rats; 2) incubations of granulosa cells with FSH or 8-bromo-cAMP significantly increased endogenous LHR gene transcription (similar to 10-fold) under conditions where increases in LHR mRNA were observed; 3) the continuous presence of FSH or 8-bromo-cAMP was required to maintain elevated levels of LHR gene transcription and LHR mRNA; and 4) exogenous estradiol alone had no effect on transcription of the LHR gene although it was able to synergistically enhance FSH-induced LHR expression. These experiments suggest that while the effects of estradiol on LHR induction do not appear to be mediated by an increase in LHR gene transcription, the effects of FSH (or cAMP) on LHR induction are clearly mediated, at least in part, by significant increases in the rate of LHR gene transcription.