GROWTH-HORMONE (GH) AND INSULIN-LIKE GROWTH-FACTOR-I (IGF-I) TREATMENT OF THE GH-DEFICIENT DWARF RAT - DIFFERENTIAL-EFFECTS ON IGF-I TRANSCRIPTION START SITE EXPRESSION IN HEPATIC AND EXTRAHEPATIC TISSUES AND LACK OF EFFECT ON TYPE-I IGF RECEPTOR MESSENGER-RNA EXPRESSION

The rat IGF-I gene consists of six exons, with exons 3 and 4 forming a 'core' mature IGF-I coding region to which alternate 5' and 3' region s are spliced. Transcription occurs from four dispersed start sites (s s) approximate to 382 (ss 1), approximate to 343 (ss 2), approximate t o 245 (ss 3) and approximate to 30-40 (ss 4) basepairs (bp) from the 3 ' end of exon 1, and from a region 50-70 bp from the 3' end of exon 2. The expression of ss mRNAs displays tissue-specific and ontogenic reg ulation. Alternate splicing of exon 5 produces E-peptide coding domain variants (Ea and Eb mRNAs), with the Eb form found predominantly in t he liver. The regulation of IGF-I mRNA expression by GH and IGF-I in t he GH-deficient dwarf (dw/dw) rat was investigated using antisense RNA probes in a solution hybridization RNase protection assay to detect l eader exon and E domain variant mRNAs. GH treatment of dw/dw and norma l Lewis rats increased the expression of all liver leader exon ss and E domain variants coordinately (1.6-1.9-fold increase, p < 0.01), alth ough the increase observed in Eb transcripts was significantly higher in the dw/dw compared to the normal rat (p < 0.05). In kidney, GH trea tment significantly increased exon 1 ss 3 and ss 4 transcripts by appr oximately 40% (p < 0.05). The expression of the other start sites was not affected by GH, suggesting that transcription factors may regulate start site usage independently. GH treatment was associated with a si gnificant increase in IGF-I mRNA expression in skeletal muscle (p < 0. 05) but not cardiac muscle or spleen. IGF-I treatment was associated w ith minor (approximate to 20%) but significant (p < 0.05) reductions i n IGF-I mRNA expression in the liver and kidney of dw/dw rats, suggest ing that IGF-I can suppress IGF-I mRNA expression. IGF-I treatment did not affect IGF-I mRNA expression in cardiac and skeletal muscle of dw /dw rats. IGF-I receptor mRNA was detected in extrahepatic tissues onl y, and was not affected by either GH or IGF-I treatment. In summary, s tart site-specific regulation by GH was observed in kidney. GH increas ed IGF-I mRNA expression in muscle, kidney and liver, but had no effec t in heart or spleen in the dw/dw rat. Our data suggest that systemic IGF-I can feedback on hepatic and renal IGF-I mRNA expression in the G H-deficient state.