Transcriptional regulation of human organic anion transporter 1 by B-cell CLL/lymphoma 6

The human organic anion transporter 1 (OAT1) is crucial for the excretion of organic anions in renal proximal tubular cells and has been classified as a clinically relevant transporter in the kidneys. Our previous study indicated that renal male-predominant expression of rat Oat1 and Oat3 appears to be regulated by transcription factor B-cell CLL/lymphoma 6 (BCL6). The aim of this study was to characterize the effect of BCL6 on human OAT1 promoter and on the transcription of OAT1 mediated by hepatocyte nuclear factor-1 alpha (HNF-1 alpha). Luciferase assays were carried out in opossum kidney (OK) cells transiently transfected with promoter constructs of OAT1, expression vectors for BCL6 and HNF-1 alpha, and the empty control vectors. BCL6 and HNF-1 alpha binding on OAT1 promoter was analyzed using electrophoretic mobility shift assay (EMSA). Protein expression of HNF-1 alpha was investigated by Western blot analysis. Site-directed mutagenesis was used to introduce mutations into BCL6 and HNF-1 alpha binding sites within the OAT1 promoter. BCL6 enhanced the promoter activity of OAT1 independently of predicted BCL6 binding sites but was dependent on HNF-1 alpha response element and HNF-1 alpha protein. Coexpression of BCL6 and HNF-1 alpha induced an additive effect on OAT1 promoter activation compared with BCL6 or HNF-1 alpha alone. BCL6 does not bind directly or indirectly to OAT1 promoter but increases the protein expression of HNF-1 alpha and thereby indirectly enhances OAT1 gene transcription. BCL6 constitutes a promising candidate gene for the regulation of human OAT1 transcription and other renal and/or hepatic drug transporters that have been already shown to be activated by HNF-1 alpha.