Decay of follicle-stimulating hormone-β messenger RNA in the presence of transcriptional inhibitors and/or inhibin, activin, or follistatin

Abstract

In primary cultures of rat pituitary cells, inhibin and follistatin reduce steady state levels of FSHβ mRNA to less than 10% of control within 4-6 h, while activin increases this mRNA 2- to 3-fold after 2-4 h of treatment. The effects of these three gonadal polypeptide hormones on the LHβ and common α-subunit mRNAs are more gradual and of lesser magnitude. The present study was designed to determine whether inhibin, activin, and/or follistatin act at the posttranscriptional level by altering the stability of the gonadotropin subunit mRNAs. To determine the decay rates of FSHβ, LHβ, and α-subunit mRNAs, primary pituitary cell cultures were treated for 1-24 h with either of two transcriptional inhibitors, actinomycin-D or 5,6-dichloro-1-β-ribofuranosyl benzimidazole (DRB), in the presence or absence of recombinant human inhibin-A, recombinant human activin-A, or purified bovine follistatin. The decay of preexisting gonadotropin subunit mRNAs was followed by Northern blot analysis. Levels of LHβ and α-subunit mRNAs remained constant or increased during the 24-h exposure to transcriptional inhibitors; therefore, it was not possible to calculate their half-lives. The stability of these mRNAs was not altered by inhibin, activin, or follistatin. In contrast, FSHβ mRNA turned over rapidly: the estimated half-life was 2.6 ± 0.19 h (mean ± SEM of eight determinations) after actinomycin-D treatment and 1.9 ± 0.14 h (mean ± SEM of 12 determinations) after DRB treatment. When new RNA synthesis was blocked by either actinomycin-D or DRB, there were no significant effects of inhibin, activin, or follistatin on the stability of FSHβ mRNA (n = 2-4 for each hormone). The decay of FSHβ mRNA in the presence of inhibin or follistatin alone, however, was even more rapid than that determined after the administration of transcriptional inhibitors (P < 0.005). After an initial lag of 1-2 h, the half-life of FSHβ mRNA was 0.88 ± 0.15 h (n = 4) or 0.62 ± 0.11 h (n = 3), in the presence of inhibin or follistatin, respectively. The most likely interpretation of these results is that inhibin/follistatin reduces steady state levels of FSHβ mRNA by inducing a labile protein that accelerates the degradation of this mRNA species, and the synthesis of this protein is blocked by actinomycin-D or DRB treatment. It is not clear at present whether inhibin, follistatin, and activin have additional effects on transcription of the gonadotropin subunit genes.