In the mouse, GH-binding protein (GHBP) and GH receptor (GHR) are encoded by a single gene via alternative splicing. We previously demonstrated that the steady-state levels of the GHR and GHBP mRNAs are significantly elevated in mouse liver during pregnancy. Hepatic GHR and GHBP mRNAs are associated primarily with one of two different 5' untranslated regions (5' UTRs), designated 5' UTR Liver1 (L1) and Liver2 (L2). Distinct promoters associated with each of these 5' UTRs have recently been characterized. In the present study, we have investigated the role of transcriptional activation in the pregnancy-induced upregulation of GHR and GHBP mRNAs in liver. We also report on the relative contribution of the 5' UTR L1 and 5' UTR L2 promoters to the hepatic expression of the GHR/GHBP gene in the liver. Our approach was to compare, by ribonuclease protection assay (RPA), GHR/GHBP transcript levels in hepatic nuclear and total cellular RNA samples from virgin and late-pregnant mice. In these RPAs we utilized riboprobes that were complementary to the coding region of GHR/GHBP transcripts, as well as to the two noncoding, alternative first exons 5' UTR L1 and L2. When employing the coding region probe, RPAs revealed that the gestational increase in the levels of nuclear GHR/GHBP transcripts were statistically comparable with the increase in GHR/GHBP transcript levels in total cellular RNA. This finding suggests that enhanced transcriptional activity, rather than increased cytoplasmic half-life, is responsible for the upregulation of GHR/GHBP RNA in the pregnant liver. In RPAs utilizing the noncoding region probes, both nuclear and total cellular GHR/GHBP transcripts associated with 5' UTR L1 were significantly upregulated in late-pregnant as compared with virgin mice. In contrast, the levels of both nuclear and total GHR/GHBP transcripts associated with 5' UTR L2 were comparable between nonpregnant and pregnant animals. Moreover, 5' UTR L2-containing transcripts were present at levels that were only 3-5% of the 5' UTR L1-associated transcripts in the late-pregnant liver. Thus, we conclude that the gestational upregulation of the GHR/GHBP gene in the mouse liver can be ascribed to the significantly enhanced transcriptional activity of the 5' UTR L1 promoter.

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