LIGAND-INDUCED DOWN-REGULATION OF TESTICULAR AND OVARIAN LUTEINIZING-HORMONE (LH) RECEPTORS IS PRECEDED BY TISSUE-SPECIFIC INHIBITION OF ALTERNATIVELY PROCESSED LH RECEPTOR TRANSCRIPTS

Down-regulation of plasma membrane receptors by homologous hormones has been found in diverse cell types. In testicular Leydig and ovarian luteal cells, treatment with LH/hCG decreases LH receptor content. Although suppression of LH-binding sites may result from ligand-induced receptor internalization, sequestration, and/or phosphorylation, the gonadotropins may also regulate receptor mRNA levels. We examined the regulation of testis LH receptor mRNAs in adult rats that received 10 or 200 IU hCG, using cRNA probes derived from the 5' extracellular domain (EC) or the 3' transmembrane domain (TM) of the rat receptor cDNA. Probe EC hybridized to predominant signals of 7 and 1.8 kilobases (kb) and weaker signals of 4.2 and 2.5 kb. However, probe TM hybridized to the three larger forms of the LH receptor mRNA, but not to the 1.8-kb species, suggesting that the latter form lacks the transmembrane domain. After 6 and 12 h of treatment with 200 or 10 IU hCG, respectively, hybridization to the larger mRNA species decreased by more than 60%, preceding decreases in testicular [I-125]hCG binding. These transcripts were further inhibited (> 93%) between 24-72 h after hCG treatment and returned to 40% and 100% of control levels by days 6 and 9, respectively. In contrast, the truncated 1.8-kb LH receptor transcript was not affected by hCG treatment, indicating a differential suppressive effect of the ligand on its receptor mRNA levels. In the ovary, hybridization to probe EC revealed four transcripts with similar sizes as those found in the testes, with a predominant 7-kb species. In contrast to the male, administration of 20 IU hCG to pseudo-pregnant females decreased hybridization of all four LH receptor mRNAs by 90% 12 h posttreatment, compared to a 50% decrease in LH receptor content. LH receptor content and mRNA levels remained suppressed (> 70%) on day 3 of treatment, and by 7 days post-hCG, LH receptor mRNAs and binding partially returned toward control levels. Thus, ligand-induced LH receptor down-regulation is preceded by decreased LH receptor mRNA transcripts of 7, 4.2, and 2.5 kb in both gonads. However, in the testes, the truncated 1.8-kb species, lacking a portion of the transmembrane domain, is not hormonally regulated. Thus, gonadotropins may down-regulate their receptors by decreasing the levels of receptor transcripts. The lack of suppression of a truncated transcript in the testis further suggests tissue-specific differential regulation of LH receptor mRNA processing.