Expression of hepatic insulin-like growth factor-I and insulin-like growth factor-binding protein-1 genes is transcriptionally regulated in streptozotocin-diabetic rats.

Abstract

Insulin-like growth factor-I (IGF-I) and IGF-binding protein-1 (BP-1) are critical cell regulators, with regulation and action in endocrine, paracrine, and autocrine modes. Although IGF-I and BP-1 are thought to be modulated mainly at the level of synthesis, underlying molecular mechanisms are poorly understood. To examine regulation by insulin, we used run-on assays to measure IGF-I and BP-1 gene transcription rates in nuclei isolated from the livers of normal and diabetic rats. Streptozotocin (STZ)-treated rats exhibited 20-25% weight loss, a 2.5- to 3-fold increase in serum glucose, and a 50-60% fall in circulating IGF-I levels (all P less than 0.001). Diabetic animals also had a 45% reduction in hepatic IGF-I mRNA and over 400% increases in BP-1 mRNA (both P less than 0.005); all parameters were restored toward normal after treatment with insulin. Metabolically responsive IGF-I gene transcription was evaluated effectively with a 3.2-kilobase BglII/EcoRI genomic probe located down-stream from all initiation sites in exon 1, while BP-1 gene transcription was studied with a cDNA probe. Animals treated with 144 mg/kg STZ exhibited 50-97% decreases in IGF-I gene transcription (P less than 0.05), while insulin treatment raised IGF-I gene transcription to control levels (P less than 0.02). IGF-I gene transcription appeared to be more sensitive to metabolic status than IGF-I mRNA levels, resulting in a modest correlation between transcription rates and mRNA levels (r = 0.68; P less than 0.001). In contrast, changes in BP-1 mRNA and gene transcription appeared to be exquisitely sensitive to metabolic status.(ABSTRACT TRUNCATED AT 250 WORDS)