Regulation of the expression of human organic anion transporter 3 by hepatocyte nuclear factor 1 α/β and DNA methylation

Human organic anion transporter 3 (hOAT3/SLC22A8) is predominantly expressed in the proximal tubules of the kidney and plays a major role in the urinary excretion of a variety of organic anions. The promoter region of hOAT3 was characterized to elucidate the mechanism underlying the tissue-specific expression of hOAT3. The minimal promoter of hOAT3 was identified to be located approximately 300 base pairs upstream of the transcriptional start site, where there are canonical TATA and hepatocyte nuclear factor (HNF1) binding motifs, which are conserved in the rodent Oat3 genes. Transactivation assays revealed that HNF1 alpha and HNF1 beta markedly increased hOAT3 promoter activity, where the transactivation potency of HNF1 beta was lower than that of HNF1 alpha. Mutations in the HNF1 binding motif prevented the transactivation. Electrophoretic mobility shift assays demonstrated binding of the HNF1 alpha/HNF1 alpha homodimer or HNF1 alpha/HNF1 beta heterodimer to the hOAT3 promoter. It was also demonstrated that the promoter activity of hOAT3 is repressed by DNA methylation. Moreover, the expression of hOAT3 was activated de novo by forced expression of HNF1 alpha alone or both HNF1 alpha and HNF1 beta together with the concomitant DNA demethylation in human embryonic kidney 293 cells that lack expression of endogenous HNF1 alpha and HNF1 beta, whereas forced expression of HNF1 beta alone could not activate the expression of hOAT3. This suggests a synergistic action of the HNF1 alpha/HNF1 beta homodimer or HNF1 alpha/HNF1 beta heterodimer and DNA demethylation for the constitutive expression of hOAT3. These results indicate that the tissue-specific expression of hOAT3 might be regulated by the concerted effect of genetic (HNF1 alpha and HNF1 beta) and epigenetic (DNA methylation) factors.