Functional analysis of glucocorticoid and insulin response sequences in the rat insulin-like growth factor-binding protein-1 promoter

Abstract

Insulin-like growth factor-binding protein-1 (IGFBP-1) is produced by the liver and regulated by glucocorticoids and insulin at the level of gene transcription. To identify DNA sequences mediating the effects of glucocorticoids and insulin on IGFBP-1 promoter activity we created luciferase reporter gene constructs and performed transfection studies in H4IIE hepatoma cells. Initial studies confirmed that the IGFBP-1 promoter is functional when inserted in the correct orientation, but not in the reverse orientation. Dexamethasone (DEX) increased promoter activity 10-fold, and insulin reversed this effect of DEX by 85% at 8 h. The effects of DEX were abolished when constructs were truncated to 89 bases from the RNA cap site, and DNase footprinting with the DNA-binding domain of the human glucocorticoid receptor identified an imperfect palindrome containing two receptor-binding sites separated by three nucleotides typical of a glucocorticoid response element (GRE) at this location. Mutation of either binding site (or half-site) disrupted the effects of DEX, confirming that this sequence functions as a GRE. Two other regions of the promoter also footprinted with the glucocorticoid receptor protein and contained sequences consistent with glucocorticoid receptor-binding sites; however, neither of these footprints contained the full structure expected of a functional GRE, and neither mutation nor deletion of these other sequences altered the effects of DEX on promoter activity. To identify the DNA sequences required for the effects of insulin on glucocorticoid-stimulated promoter activity, we created internal deletions throughout the IGFBP-1 promoter region. Deletion of the 22-basepair (bp) sequence immediately 5' from the GRE disrupted the effect of insulin and appeared to increase basal promoter activity at least 2-fold in each of eight experiments (P < 0.001 vs. intact promoter). This region of the IGFBP-1 promoter contains a 19-bp palindrome (CAAAACAAACTTATTTTG) that overlaps the 5'-end of the GRE and is fully conserved in the human IGFBP-1 promoter. Each half of this palindrome resembles previously identified insulin response sequences, and deletion/mutation analysis suggests that each half may contribute to the effects of insulin on promoter activity. Gel shift studies confirmed that this palindrome binds H4IIE nuclear proteins. In summary, we have identified a GRE in the 5'-promoter region of the rat IGFBP-1 gene approximately 90 bp up-stream from the RNA cap site as well as a contiguous 22-bp region that plays a critical role in mediating the effects of insulin on glucocorticoid-stimulated promoter activity.(ABSTRACT TRUNCATED AT 400 WORDS)