Bcl-2 overexpression in PhIP-induced colon tumors:: cloning of the rat Bcl-2 promoter and characterization of a pathway involving β-catenin, c-Myc and E2F1

beta-Catenin/T-cell factor (Tcf) signaling is constitutively active in the majority of human colorectal cancers, and there are accompanying changes in Bcl-2 expression. Similarly, 2-amino-1-methyl-6-phenylimidazo(4,5-b)pyridine(PhIP)-induced colon tumors in the rat have increased beta-catenin and elevated Bcl-2. To examine the possible direct transcriptional regulation of rat Bcl-2 by beta-catenin/Tcf, we cloned and characterized the corresponding promoter region and found 70.1% similarity with its human counterpart, BCL2. Bcl- 2 promoter activity was increased in response to LiCl and exogenous b-catenin, including oncogenic mutants of beta-catenin found in PhIP-induced colon tumors. Protein/DNA arrays identified E2F1, but not beta-catenin/Tcf, as interactingmost strongly with the rat Bcl- 2 promoter. Exogenous E2F1 increased the promoter activity of rat Bcl- 2, except in mutants lacking the E2F1 sites. As expected, b-catenin induced its downstream target c-Myc, as well as E2F1 and Bcl- 2, and this was blocked by siRNA to c-Myc or E2F1. These findings suggest an indirect pathway for Bcl- 2 overexpression in PhIP-induced colon tumors involving beta-catenin, c-Myc and E2F1.