The Glucocorticoid Response Element II Is Functionally Homologous in Rat and Human Insulin-like Growth Factor-binding Protein-1 Promoters

In vivo, insulin-like growth factor-binding protein-1 (IGFBP-1) modulates the IGFs’ bioavailability and may contribute to their delivery to peripheral tissues. In rat and human hepatocytes, glucocorticoids stimulate IGFBP-1 gene transcription through homologous glucocorticoid response units (GRU). Transfection experiments have shown that one of these, GRU2 (nucleotide (nt) −121 to −85 and nt −111 to −74 in human and rat promoters, respectively), was on its own able to mediate the glucocorticoid response in rat but not in human species (Suwanichkul, A., Allander, S., Morris, S. L. & Powell, D. R. (1994) J. Biol. Chem. 269, 30835–30841, Goswami, R., Lacson, R., Yang, E., Sam, R. & Unterman, T. (1994) Endocrinology 134, 736–743, and Suh, D. S., Ooi, G. T. & Rechler, M. M. (1994) Mol. Endocrinol. 8, 794–805). A close comparison of GRU2 sequences has pointed out a C to A transition in the underlying GREII, which creates a GATC tetranucleotide in rat species. This tetranucleotide is submitted to adenosyl methylation ( dam methylation) in most Escherichia coli bacterial strains, but not in eucaryotic cells. We showed (i) that on its own, the unmethylated rat GRU2 (propagated in dam E. coli strains) was inactive, as is the case for its human counterpart (nonsignificant glucocorticoid inductions, 1.48 ± 0.23 and 1.7 ± 0.35-fold in Chinese hamster ovary cells, respectively) and (ii) that its adenosyl methylation in standard dam + bacterial strains yielded a functional GRU (6.5 ± 1.1 and 13.1 ± 3.9-fold glucocorticoid inductions in Chinese hamster ovary and HepG2 cells, respectively). Transient transfection in HepG2 hepatoma cells clearly showed that the interaction of liver-enriched trans- acting factor(s) with the 5′-overlapping insulin response element does not enable the unmethylated rat GRU2 or the human GRU2 to become responsive to glucocorticoids (nonsignificant 2.21 ± 0.48 and 1.20 ± 0.06-fold induction, respectively). Furthermore, we have correlated these functional data with in vitro DNA-protein interaction studies: the dam methylated rat GREII displayed a 2.8-fold higher affinity for the glucocorticoid receptor than its unmethylated counterpart.