Transport function and transcriptional regulation of a liver-enriched human organic anion transporting polypeptide 2B1 transcriptional start site variant.

Human organic anion transporting polypeptide 2B1 (OATP2B1) is a membrane transporter that facilitates the cellular uptake of a number of endogenous compounds and drugs. OATP2B1 is widely expressed in tissues including the small intestine, liver, kidney, placenta, heart, skeletal muscle, and platelets. It was recently shown that differential promoter usage in tissues results in expression of five OATP2B1 transcriptional start site variants that use distinct first exons but share common subsequent exons. These variants are expected to encode either a full-length (OATP2B1-FL) or shortened protein lacking 22 N terminus amino acids (OATP2B1-Short). Little is known regarding the transport activity and regulation of OATP2B1 variants with N terminus truncation. Here, using absolute quantitative polymerase chain reaction, we find the full-length variant is the major form expressed in duodenum but the short variant predominates in liver. Using a transient heterologous cell expression system, we find that the transport activities of the short OATP2B1 variant toward substrates estrone sulfate and rosuvastatin are similar to the well characterized full-length variant. Transcriptional activity screening of the liver-enriched OATP2B1 variant promoter identified hepatocyte nuclear factor 4 α (HNF4α) as a novel transacting factor. With a combination of in silico screening, promoter mutation in cell-based reporter assays, small interfering RNA knockdown, and chromatin immunoprecipitation (ChIP) studies, we identified a functional HNF4α binding site close to the transcription start site (-17 to -4 bp). We conclude that the major OATP2B1 protein form in liver is transport competent and its hepatic expression is regulated by HNF4α.