Glucocorticoid receptor regulates organic cation transporter 1 (OCT1, SLC22A1) expression via HNF4a upregulation in primary human hepatocytes

Background: Organic cation transporter 1 (OCT1, SLC22A1) is a membrane transporter that is important for therapeutic effect of the antidiabetic drug metformin. Its liver-specific expression in hepatocytes is strongly controlled by hepatocyte nuclear factor-4a (HNF4a). HNF4a expression and transcriptional activity have been demonstrated to be augmented by glucocorticoid receptor (GR) in human hepatocytes and rodent livers. Methods: It was examined whether GR activation indirectly induces OCT1 gene expression via HNF4a up-regulation in primary human hepatocytes. We also examined which other transcription factors are involved in OCT1 gene expression and whether they are regulated by dexamethasone using qRT-PCR and gene reporter assays. Results: We found that dexamethasone significantly up-regulates OCT1 mRNA and protein in normal primary human hepatocytes, but not in hepatocyte-derived tumor cell lines HepG2 and MZ-Hep1. Consistently, we observed that HNF4a is induced by dexamethasone in primary human hepatocytes, but not in hepatocyte tumor-derived cell lines. Viral transduction of MZ-Hep1 cells with the expression constructs for HNF4a, CCAAT/enhancer binding proteins b (C/EBPb) and peroxisome proliferator-activated receptor-g coactivator 1a (PGC1a) demonstrated significant roles of the transcription factors in OCT1 gene regulation. We found that expression of OCT1 mRNA in human livers significantly correlates with C/EBPb and HNF4a mRNAs expression and that C/EBPb co-transfection stimulates OCT1 gene reporter construct in HepG2 cells. Nevertheless, neither C/EBPb nor PGC1a were upregulated in human hepatocytes by dexamethasone. Conclusion: We can conclude that GR-induced expression of HNF4a may contribute to indirect OCT1 gene up-regulation by dexamethasone in primary human hepatocytes, but not in hepatocyte-derived tumor cell lines.