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 mRNA 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′ regions are spliced. Transcription occurs from four dispersed start sites (ss) ≈ 382 (ss 1), ≈ 343 (ss 2), ≈ 245 (ss ≈ 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 regulation. Alternate splicing of exon 5 produces E-peptide coding domain variants (Ea and Eb mRNAs), with the Eb form found predominantly in the liver. The regulation of IGF-I mRNA expression by GH and IGF-I in the GH-deficient dwarf (dw /dw) rat was investigated using antisense RNA probes in a solution hybridization RNase protection assay to detect leader exon and E domain variant mRNAs. GH treatment of dw /dw and normal 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), although the increase observed in Eb transcripts was significantly higher in the dw /dw compared to the normal rat (p < 0.05). In kidney, GH treatment significantly increased exon 1 ss 3 and ss 4 transcripts by approximately 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 significant increase in IGF-I mRNA expression in skeletal muscle (p < 0.05) but not cardiac muscle or spleen. IGF-I treatment was associated with minor (≈ 20%) but significant (p < 0.05) reductions in IGF-I mRNA expression in the liver and kidney of dw /dw rats, suggesting 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 only, and was not affected by either GH or IGF-I treatment. In summary, start site-specific regulation by GH was observed in kidney. GH increased IGF-I mRNA expression in muscle, kidney and liver, but had no effect 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 GH-deficient state.