Cyclic AMP (cAMP) effects on chorionic gonadotropin gene transcription and mRNA stability: Labile proteins mediate basal expression whereas stable proteins mediate cAMP stimulation

Abstract

Cyclic AMP stimulates a marked accumulation of CGα and CGβ mRNAs that reflects, in part, increased rates of gene transcription. We find that a major component of cAMP stimulation of α and CGβ mRNAs is independent of new protein synthesis. After treatment of JEG-3 choriocarcinoma cells with cycloheximide, basal levels of α and CGβ mRNAs decreased over 12 h to 27% and 13% of control values, respectively. However, cycloheximide treatment did not affect the degree of cAMP-stimulation of a and CGβ mRNA levels which increased 20- and 26-fold, respectively. Similarly, cycloheximide did not block cAMP-stimulated transcription of the a and CGβ genes. The effect of cAMP treatment on a and CGβ mRNA stability was assessed by decay after removal of cAMP, pulse-chase analyses, and decay after inhibition of RNA synthesis by actinomycin D. The half-lives of α and CGβ mRNAs determined by decay rates after removal of cAMP were 6.0 h and 7.2 h, respectively. Consistent with these measurements of mRNA stability, α and CGβ mRNA half-lives determined by pulse-chase analyses were 8.8 h and 8.6 h, respectively. Cyclic AMP treatment increased the half-lives of a and CG0Β mRNAs 1.8- and 3.4-fold, respectively. Thus, the effects of cAMP on a and CGβ gene expression are predominantly transcriptional, but cAMP also increases mRNA levels via a posttranscriptional mechanism. Inhibition of RNA synthesis by treatment with actinomycin D caused a rapid decay of both α (t1/2 = 2.8h) amd CGβ (t1/2 = 3.1 h) mRNAs with no apparent stabilization by cAMP, providing evidence that labile proteins also stabilize basal α and β mRNA levels. We conclude that cAMP stimulates α and β transcription via modification of preexisting proteins. However, other labile proteins are required for gene transcription and mRNA stability and the effects of cAMP on α and CGβ mRNA accumulation occur in part via selective mRNA stabilization. © 1989 by The Endocrine Society.