Post-transcriptional regulation of luteinizing hormone/human chorionic gonadotropin receptor expression.

Abstract

Luteinizing hormone (LH), a heterodimeric glycoprotein hormone secreted in the pituitary, is required for gonadal function. LH and its placental counterpart, human chorionic gonadotropin (hCG), stimulate cell function by interacting with their receptor, which belongs to the family of G$\sb{\rm s}$ protein coupled receptors. Like other G$\sb{\rm s}$ protein coupled receptors, the LH/hCG receptor undergoes ligand-induced down-regulation. During down-regulation, the number of receptors dramatically decreases with a concomitant reduction in steady state levels of mRNA. This decline was determined to be a result of increased mRNA degradation, and not decreased gene transcription. Since receptor down-regulation also occurs physiologically, post-transcriptional regulation of LH/hCG receptor mRNA is most likely an important control mechanism for receptor expression. The receptor is encoded by a single-copy gene, but is processed into multiple transcripts in all target tissues. In rat ovary, the transcript sizes are 6.7, 4.4, 2.6, and 1.8 kb, with the 6.7 kb mRNA existing as the predominant form. A novel 3.5 kb cDNA was cloned and identified as the 3$\sp\prime$ untranslated region (3$\sp\prime$ UTR) of the 6.7 kb transcript. Thus, the 6.7 kb mRNA consists of a 2.2 kb open reading frame with an unusually long 3$\sp\prime$ UTR. Sequence comparisons, northern and Southern hybridization, and oligonucleotide-directed cleavage with RNase H confirmed that the 3.5 kb cDNA corresponds to the 3$\sp\prime$ UTR of the 6.7 kb transcript. Since 3$\sp\prime$ UTRs have been implicated in determining mRNA stability and translational efficiency, the possible regulatory role of the long 3$\sp\prime$ UTR present in the 6.7 kb transcript was determined in transfected cells. Ligand binding analysis revealed that the long 3$\sp\prime$ UTR inhibited receptor expression. Furthermore, the 6.7 kb mRNA had a shorter half life and associated with significantly fewer ribosomes than the smaller receptor mRNAs. In addition, sequences in the longer 3$\sp\prime$ UTR were sufficient decrease expression of the luciferase reporter gene. This 3$\sp\prime$ UTR-mediated repression in receptor expression is due, in part, to a decrease in both mRNA stability and translation.