Multiple liver-enriched trans-acting factors interact with the glucocorticoid- (GRU) and cAMP-(CRU) responsive units within the h-IGFBP-1 promoter

In response to hormonal control, serum concentrations of insulin-growth factor-binding protein-1 (IGFBP-1) may vary as much as 10-fold, owing to strict control of its gene's expression in hepatocytes. IGFBP-1 gene transcription is increased by gluco-corticoids and cAMP and inhibited by insulin. The effect of insulin is dominant since it suppresses constitutive and both glucocorticoid- and cAMP-stimulated transcription. Close examination of the human (h) IGFBP-1 promoter sequences showed that the glucocorticoid (GRE, nt −88 to −102) and cAMP (CRE, nt −259 to −264) response elements are 5′-flanked by an A/T-rich imperfect palindrome (nt −102 to −117 and −265 to −285, respectively). These A/T-rich motifs are putative cis-elements for liver-enriched trans-acting factors.

Competition experiments in electrophoretic mobility shift assay were carried out using rat liver nuclear extracts and a set of synthetic oligonucleotides designed from hIGFBP-1 Glucorticoid and cAMP Response Units (GRU and CRU), the rat transthyretin HNF3 cis-element and the ‘D-site’ of the mouse albumin promoter.

The nucleotide motifs located between nt −108 and −121 of the GRU, interacted with the HNF3 family of rans-acting factors (α,β,γ), whereas those encompassing nt −181 to −104 bound DBP and/or nuclear proteins sharing similar sequence specificity (i.e. from the C/EBP family of bZIP proteins). We have also shown that the hIGFBP-1-GRE binds glucocorticoid receptor homodimers.

In the case of the CRU, the cis-elements located between nt −249 and −285 bound DBP and/or nuclear proteins sharing similar sequence specificities. In addition, the nucleotide stretch lying between nt −256 and −275 was able to interact with the HNF3 family of trans-acting factors.