Organic anion transporter 1 (OAT1/SLC22A6) is predominantly expressed in the proximal tubules of the kidney. Cumulative studies have shown its critical role in the tubular secretion of a variety of organic anions, including several clinically important drugs. In addition, OAT1 is also involved in the pharmacological effect of diuretics and the nephrotoxicity of antiviral drugs. In contrast to these functional characterizations, the regulatory mechanism of OAT1 expression is poorly understood. It was recently demonstrated that the expression of Oat1 was markedly reduced in the kidneys of hepatocyte nuclear factor 1␣ (Hnf1␣)-null mice. However, in vitro evidence for the involvement of HNF1␣ and further analyses are required to illustrate the transcriptional regulation of OAT1 genes in more detail. Computational analysis of the potential transcription factor binding sites revealed that the HNF1-motif was conserved in the proximal-promoter region of human and mouse OAT1 genes. The mRNA expression of mouse organic anion transporter 1 was drastically reduced in Hnf1␣-null mice compared with that in wild-type mice, which was consistent with a previous report (Maher et al., 2006). Forced expression of HNF1␣ alone or both HNF1␣ and HNF1␤ enhanced the activity of human and mouse OAT1 promoters in the transactivation assays, whereas HNF1␤ alone was not active. Mutations in the HNF1-motif significantly reduced this transactivation. Direct binding of HNF1␣/HNF1␣ homodimer and HNF1␣/HNF1␤ heterodimer to the HNF1-motif found in the human OAT1 promoter was demonstrated by electrophoretic mobility shift assays. These results provide convincing evidence for the involvement of HNF1␣/␤ in the constitutive expression of human and mouse OAT1 in the kidney.

We have reported previously that the kidney-and liver-specific expression of transporters in mice involves the coordinated regulation by hepatocyte nuclear factor 1 (HNF1) and DNA methylation. The present study was aimed at investigating the role of this cascade in the transcriptional regulation of renal organic anion transporters (OATs) yet to be characterized in human and mouse. Luciferase assays and electrophoretic mobility-shift assays demonstrated that HNF1␣/␤ enhances the promoter activity of OAT4/SLC22A11 via binding to the HNF1 motif located near the transcriptional start site (TSS). DNA methylation profiles of human OAT1, OAT3, OAT4, and urate transporter 1 (URAT1) were determined in human liver and kidney cortex by bisulfite sequencing. Most of the CpG dinucleotides around the TSSs of OAT1 and OAT3 were highly methylated in the liver compared with kidney cortex, being consistent with their tissue specificity, whereas the difference in the DNA methylation status was less remarkable between the two tissues for OAT4 and URAT1. Mouse Oat1 gene also contained CpG dinucleotides hypomethylated in the kidney and hypermethylated in the liver downstream its TSS, whereas two of the seven CpG dinucleotides around the TSS of mouse Oat3 were significantly methylated in the liver compared with the kidney. Taken together, these findings underscored the central role of HNF1␣/␤ in the transcriptional regulation of OATs and highlighted DNA methylation-dependent gene silencing as one of the mechanisms underlying the tissue-specific transactivation by this master regulator.

Plasma vildagliptin levels were decreased when voglibose was co-administered, although DPP- 4 inhibition remained unchanged. Co-administration led to significantly better pharmacodynamic response compared with each treatment alone, including higher active GLP-1 and lower glucose levels. The results indicate that this coadministration may be beneficial in the clinical situation. Vildagliptin and voglibose treatments, alone or when co-administered, were well tolerated in Japanese patients with Type 2 diabetes.

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