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Keywords:

  • 1α,25-dihydroxyvitamin D3;
  • nuclear receptors or NRs;
  • the vitamin D receptor or VDR;
  • farnesoid X receptor or FXR;
  • short heterodimer partner or SHP;
  • fibroblast growth factor 15 or FGF15;
  • Western blotting;
  • qRT-PCR;
  • liver;
  • intestine;
  • transporters;
  • enzymes;
  • bile acids;
  • Cyp3a;
  • Cyp7a1;
  • Asbt;
  • Bsep;
  • P-gp;
  • Mrp2;
  • Mrp3;
  • Mrp4

Abstract

1α,25-Dihydroxyvitamin D3 (1,25(OH)2D3), a natural ligand of the vitamin D receptor (VDR), was found to increase the rat ileal Asbt and bile acid absorption. The effects of VDR, whose expression is low in liver, on hepatic transporters and enzymes are unknown. Protein and mRNA levels of target genes in the small intestine, colon and liver after intraperitoneal dosing of 1,25(OH)2D3 (0–2.56 nmol/kg/day for 4 days) to the rat were determined by Western blotting and qPCR, respectively. The 1,25(OH)2D3 treatment increased total Cyp3a protein and Cyp3a1 mRNA expressions in the proximal small intestine, and the short heterodimer partner (SHP), the fibroblast growth factor 15 (FGF15), organic solute transporter (Ostα and Ostβ) mRNA and Asbt protein expressions in the ileum. About 50% higher portal bile acid concentration (65.1±14.9 vs 41.9±7.8 µm, p<0.05) and elevated expressions of the hepatic farnesoid X receptor (FXR) and SHP mRNA resulted with 1,25(OH)2D3 treatment. Increased Bsep and Ostα mRNA expressions in liver and a>50% reduction in the Cyp7a1 protein level (p<0.05) and cholesterol metabolism in rat liver microsomes (p=0.002), likely consequences of the bile acid-FXR-SHP cascade and activation of the signaling pathway for Cyp7a1 inhibition by FGF15, were found. Increased hepatic multidrug resistance-associated protein (Mrp3) and multidrug resistance protein 1a (Mdr1a) mRNA and P-gp protein were also observed. It was concluded that the changes in hepatic transporters and enzymes in the rat were indirect, secondary effects of the liver FXR-SHP cascade due to increased intestinal absorption of bile acids and elevated levels of FGF15, events that led to the activation of FXR. Copyright © 2009 John Wiley & Sons, Ltd.