Regulation of Gonadotropin Subunit Transcription after Ovariectomy in the Rat: Measurement of Subunit Primary Transcripts Reveals Differential Roles of GnRH and Inhibin

Abstract

The aim of this study was to determine if the changes in gonadotropin subunit gene expression following ovariectomy reflect transcriptional and/or posttranscriptional regulation by GnRH or inhibin. Subunit transcription rates were determined by recently developed quantitative RT-PCR for subunit primary transcripts (as an indicator of gene transcription), which allow us to measure both mRNA and PT from RNA extracted from a single pituitary. Following ovariectomy, LHβ PT concentrations increased 2- to 3-fold between 72 h and 7 d, paralleling changes in serum LH and LHβ mRNA. In contrast, serum FSH, FSHβ mRNA, and FSHβ PT concentrations were 6- to 9-fold greater 12–24 h after ovariectomy followed by an additional 2.5-fold increase at 72 h. Although α RNA was elevated at 72 h after ovariectomy,α -primary transcript did not change. GnRH antagonist prevented the increase in LHβ-PT at 72 h, but had no effect on the increase in FSHβPT at 12 h and was only partially effective at 72 h. The acute GnRH-independent increase in FSHβ-primary transcript after ovariectomy could be duplicated by the administration of inhibin antiserum to intact rats; inhibin-α antiserum did not affect LHβ-primary transcript, but increased FSHβ-primary transcript concentrations 8- to 11-fold. The half-disappearance rates of LHβ and FSHβ primary transcripts were measured after GnRH blockade or administration of recombinant human inhibin A. The half-disappearance times for LHβ and FSHβ primary transcripts following GnRH blockade were 13 and 17 min, respectively; the mRNAs did not change. The effects of inhibin were specific for FSHβ; 60 min after inhibin FSHβ-primary transcript was undetectable with a half-disappearance time of 19 min, additionally FSHβ mRNA levels also fell with a half-life of 94 min. In conclusion, these data support previous evidence that GnRH regulates gonadotropin gene expression primarily at the level of transcription. However, the acute increase in FSHβ-primary transcript after ovariectomy or immunoneutralization of inhibin-α, and the rapid fall in FSHβ-primary transcript following rh inhibin, provide novel evidence that inhibin suppresses FSHβ gene transcription in addition to its action in regulating FSHβ mRNA stability.