GROWTH-HORMONE MODULATES INSULIN REGULATION OF HEPATIC INSULIN-LIKE GROWTH-FACTOR FINDING PROTEIN-1 TRANSCRIPTION

Hepatic transcription of insulin-like growth factor-binding protein-1 (IGFBP-1) is enhanced in hypophysectomized (hypox) rats and can be rap idly down-regulated by GH administration. Here we examined the effect of insulin on IGFBP-1. messenger RNA abundance in hyper rats and the e ffects of insulin and GH on IGFBP-1/chloramphenicol acetyltransferase (CAT) reporter plasmids transiently transfected into isolated hepatocy tes from pituitary-intact and hypox rats. Unlike GH, administration of insulin to hyper rats in doses of 10 or 50 mu g/100 g BW had no effec t on hepatic IGFBP-1 messenger RNA abundance. Insulin at 10(-7) M resu lted in a 42.1 +/- 9.8% suppression of CAT activity in hepatocytes fro m pituitary-intact animals transfected with a CAT reporter plasmid con taining 1671 bp of the 5'-flanking region of the rat IGFBP-1 gene. In the same assay, GH at a concentration of 2.3 x 10(-8) M significantly reduced CAT activity. In contrast, insulin had no effect on CAT activi ty in hepatocytes from hyper rats, whereas GH resulted in comparable s uppression of CAT activity in hepatocytes from hyper rats and pituitar y-intact rats, 13.6 +/- 2.3% vs. 18.2 +/- 3.28. Deletional analysis an d mobility shift assays were used to identify the GH-responsive region s in the IGFBP-1 gene. GH suppression of CAT activity was lost when th e IGFBP-1 5'-flanking region was deleted down to -277 bp, whereas insu lin suppression was retained for all but the smallest fragment of the IGFBP-1 gene, Mobility shift assays were used to compare nuclear extra cts from sham-operated, hypox, and GH-treated hypox rats. When hepatic nuclear extracts from hypox rats were incubated with the -277 to -82 and the -556 to -368 bp fragments, retarded bands were apparent that w ere not present in the extracts from sham-operated rats. GH treatment of hyper rats 15 or 30 min before death completely normalized the reta rdation pattern seen with the -277 to -82 bp fragment, but did not aff ect the pattern seen with the -556 to -368 bp fragment. A 20-bp fragme nt corresponding to the previously identified insulin response element , -108 to -89 bp, was also analyzed. An additional retarded band, not seen with nuclear extracts from sham-operated rats, was apparent when nuclear extracts of hyper rats or GH-treated hyper rats were used. The se data provide the first in vitro evidence that GH directly regulates transcription of IGFBP-1 expression. In addition, our findings sugges t that GH modulates insulin regulation of IGFBP-1 transcription, possi bly by altering the milieu of trans-acting factors that interact with both the insulin response element and distinct upstream sites.