RAT GROWTH-HORMONE RECEPTOR GROWTH HORMONE-BINDING PROTEIN MESSENGER-RNAS WITH DIVERGENT 5'-UNTRANSLATED REGIONS ARE EXPRESSED IN A TISSUE-SPECIFIC MANNER

In the rat, the growth hormone receptor (GH-R) gene generates two tran scripts, one encoding the transmembrane GH-R, and a shorter one encodi ng the GH-binding protein (GH-BP). These transcripts exhibit a high de gree of heterogeneity in their 5'-untranslated regions (5'-UTRs). Some of the exons encoding these 5'-UTR variants may be flanked by distinc t promoter regions whose activity would result in the tissue-specific expression of the GH-R gene. To assess this possibility, we used singl e-sided polymerase chain reaction (PCR) amplification to characterize 5'-UTR variants in rat GH-R cDNAs, and by using 5'-UTR-specific probes , we determined their pattern of expression in several tissues. Beside s two previously described variants (V1 and V2), three new 5'-UTR vari ants were identified, extending 56 nucleotides (V3), 135 nucleotides ( V4), and 209 nucleotides (V5) upstream of the ATG translation initiati on codon. The expression of GH-R and GH-BP transcripts was clearly tis sue specific. In the liver, GH-BP mRNA was the predominant transcript, whereas in other tissues, there was equivalent expression of both tra nscripts or predominant expression of GH-R mRNA. With respect to the t issue distribution of the 5'-UTR variants in particular, variants V1 a nd V5 exhibited a pattern of expression closely resembling that seen w ith an exon 2 probe, with the overall expression of variant V1 being m uch higher than that of variant V5. The V2 species was exclusively exp ressed in liver. Variant V3 was expressed at low levels in liver, musc le, heart, and kidney; in muscle and heart, it was preferentially asso ciated with GH-BP transcripts. Variant V4, although present in liver, was more abundant in extrahepatic tissues and predominantly found in G H-R mRNA transcripts. Southern blot analyses were consistent with exon 2 and the exons encoding the V1 and V2 sequences being in proximity, with the other 5'-UTR sequences being encoded by exons located further upstream of exon 2. These findings support the concept that different 5'-UTR variants are the result of the different promoters acting in a tissue-specific manner. The association of specific 5'-UTR variants w ith either GH-R or GH-BP transcripts raises the possibility that the a lternative splicing process that generates GH-BP mRNA in the rat might be controlled by the 5'-flanking region regulating the expression of specific leader exons.