Characterization of Growth Hormone Receptor Messenger Ribonucleic Acid Variants in Human Adipocytes

Abstract

Human GH exerts profound effects on adiposity through its specific receptor, hGHR. Eight hGHR mRNAs are produced by the hGHR gene due to splicing from alternate 5'-untranslated region first exons into a common acceptor site upstream of the start codon in exon 2. Four transcripts (V2, V3, V5, V9) are ubiquitously expressed, whereas the other four (V1, V4, V7, V8) are expressed only in normal postnatal liver, suggesting that different promoter usage is a mechanism for developmental- and tissue-specific regulation of the hGHR gene. Because it is unknown whether this occurs in adipocytes, we screened human adipocyte cDNA for hGHR mRNAs using 5'-rapid amplification of cDNA ends. Eighty-nine percent of the clones were V2-like, 3% were V3-like, and 8% were five new mRNA variants (VA-VE). All new 5'-untranslated region sequences mapped within the hGHR 5'flanking region. RT-PCR assays showed expression in multiple fetal and adult tissues, and, thus, they are not adipocyte specific. We compared expression of hGHR mRNAs in adult liver, adult fat, and the human preadipocyte SGBS cell line, using duplex RT-PCR. In liver, V1 and V2 are the major hGHR mRNAs, whereas in adipose, V2 predominates; VA and VC are expressed at similar lower levels in both. In SGBS preadipocytes, approximately 70% of hGHR mRNA is V2. During differentiation, total hGHR and V2 transcripts are markedly up-regulated [hGHR: 2.3 +/- 0.2-fold (mean +/- se), P < 0.01; V2: 3.0 +/- 0.8, P < 0.03], whereas other variants also increased but remained relatively minor transcripts. We have identified five new hGHR mRNA variants. Because the V2 transcript is predominant in adipocytes at all developmental stages, the mechanisms regulating its expression should be examined.