GONADOTROPIN-RELEASING-HORMONE STIMULATES GLYCOPROTEIN HORMONE ALPHA-SUBUNIT MESSENGER-RIBONUCLEIC-ACID (MESSENGER-RNA) LEVELS IN ALPHA-T3 CELLS BY INCREASING TRANSCRIPTION AND MESSENGER-RNA STABILITY

Pulsatile GnRH stimulates gonadotropin secretion, whereas continuous e xposure to GnRH causes pituitary desensitization and suppressed levels of LH and FSH. At the level of gene expression, continuous GnRH also causes partial or complete suppression of the LH beta and FSH beta gen es, but expression of the alpha-subunit gene is stimulate without evid ence of desensitization. In this report, we examined the transcription al and posttranscriptional mechanisms by which GnRH controls alpha-gen e expression using the gonadotrope-derived alpha T3 cell line. Continu ous GnRH caused a 4- to 5-fold accumulation of alpha mRNA over 72 h, w ithout evidence of a decline. In contrast, measurements of alpha-gene transcription, either by nuclear run-on assays or using a stably integ rated alpha LUC reporter gene, revealed that GnRH caused a transient i ncrease in a-promoter activity, followed by a decline after 4-6 h. The prolonged accumulation of alpha mRNA at a time when transcriptional a ctivity had abated was accounted for by independent effects of GnRH on alpha mRNA stability. After prior treatment with GnRH, its removal ei ther by washout or using a GnRH receptor antagonist caused an abrupt d ecline in steady state alpha mRNA levels (t(1/2), <2 h). Readdition of GnRH prevented the decay in alpha mRNA, and experiments using the tra nscriptional inhibitor actinomycin-D confirmed that this effect of GnR H did not require transcription. Consistent with these results, pulse- chase analyses of mRNA stability demonstrated that GnRH increased the alpha mRNA half-life 6.7-fold, from 1.2 h in the absence of GnRH to 8. 0 h in the presence of GnRH. We conclude that GnRH induces a transient burst of alpha-gene transcription that is accompanied by marked induc tion of mRNA stability.