Sp1 and Egr-1 have opposing effects on the regulation of the rat pgp2/mdr1b gene

The promoter of the rat pgp2/mdr1b gene has a GC-rich region (pgp2GC) that is highly conserved in mdr genes and contains an consensus Sp1 site. Sp1's role in transactivation of the pgp2/mdr1b promoter was tested in Drosophila Schneider cells. The pgp2/mdr1b promoter was strongly activated by co-tran sfected wild type Sp1 but not mutant Sp1 and mutation of the Sp1 site abrog ated Sp1-dependent transactivation, In gel shift assays, the same mutations abolished Sp1-DNA complex formation. Moreover, basal activity of the pgp2/ mdr1b Sp1 mutant promoter was dramatically lower. Enforced ectopic overexpr ession of Sp1 in H35 rat hepatoma cells revealed that cell lines overexpres sing Sp1 had increased endogenous pgp2/mdr1b mRNA, demonstrating that Spl a ctivates the endogenous pgp2/mdr1b gene. Pgp2GC oligonucleotide also bound Egr-1 in gel shift assays and Egr-1 competitively displaced bound Sp1. In t ransient transfections of H35 cells land human LS180 and HepG2 cells) Egr-1 potently and specifically suppressed pgp2/mdr1b promoter activity and muta tions in the Egr-1 site decreased Egr-1 binding and correlated with pgp2/md r1b up-regulation. Ectopic overexpression of Egr-1 in H35 cells decreased P gp expression and selectively increased vinblastine sensitivity. In conclus ion, Sp1 positively regulates while Egr-1 negatively regulates the rat pgp2 /mdr1b gene, Moreover, competitive interactions between Sp1 and Egr-1 in al l likelihood determine the constitutive expression of the pgp2/mdr1b gene i n H35 cells.