Protein-DNA interactions at a drug-responsive element of the human apolipoprotein A-I gene

Previously, we demonstrated that when two human hepatoma cell lines, Hep3B and HepG2, were exposed to gemfibrozil, a hypolipidemic drug, a 2-fold induction in apolipoprotein A-I (apoA-I) mRNA levels resulted. To determine if mRNA stabilization was responsible for the changes in apoA-I mRNA levels, the half-lives for apoA-I mRNA were measured in the presence of actinomycin D with and without gemfibrozil. These experiments revealed no differences in stability. However, nuclear run-off assays indicated that the transcription rate of the apoA-I gene was increased 2-fold in gemfibrozil-treated cells. Transient transfection experiments also indicated that the induction of apoA-I mRNA level in response to gemfibrozil is mediated at the transcriptional level. We have identified two copies of the "drug-responsive element" (DRE) in the apoA-I promoter region that may be responsible for the increase in apoA-I transcriptional activity by gemfibrozil. Using gel mobility shift assays with a synthetic DRE oligonucleotide, we have demonstrated that exposure of Hep3B and HepG2 cells to gemfibrozil resulted in strong induction of a protein-DNA complex. The formation of this complex is highly sequence-specific as indicated by the DNA competition experiments. The drug-inducible nuclear proteins bind to the DRE of the human apoA-I gene with an apparent Kd of 4.1 nM. Methylation interference experiments have localized the contact sites of nuclear factors to the DRE region. Southwestern blot analyses have identified two groups of drug-inducible nuclear proteins with molecular masses of approximately 30 and 15 kDa. When a copy of synthetic DRE oligonucleotide was inserted upstream of the thymidine kinase promoter and luciferase reporter construct, a significant 2-fold induction in luciferase activity was observed in the presence of gemfibrozil following transient transfection of two human hepatoma cell lines, HepG2 and Hep3B. However, a plasmid containing one copy of mutated apoA-I-DRE oligomer did not confer responsiveness to gemfibrozil treatment. Furthermore, pGL2 (apoA-I -250 mutant DRE), which carried an internal mutation of the DRE in the human apoA-I proximal promoter region, showed no increase in luciferase activity in response to gemfibrozil. These results implicate protein-DNA interactions at the DRE region in the transcriptional induction of human apoA-I gene expression by gemfibrozil.