A novel bile acid-activated vitamin D receptor signaling in human hepatocytes

Mol Endocrinol. 2010 Jun;24(6):1151-64. doi: 10.1210/me.2009-0482. Epub 2010 Apr 6.

Abstract

Vitamin D receptor (VDR) is activated by natural ligands, 1alpha, 25-dihydroxy-vitamin D(3) [1alpha,25(OH)(2)-D(3)] and lithocholic acid (LCA). Our previous study shows that VDR is expressed in human hepatocytes, and VDR ligands inhibit bile acid synthesis and transcription of the gene encoding cholesterol 7alpha-hydroxylase (CYP7A1). Primary human hepatocytes were used to study LCA and 1alpha,25(OH)(2)-D(3) activation of VDR signaling. Confocal immunofluorescent microscopy imaging and immunoblot analysis showed that LCA and 1alpha, 25(OH)(2)-D(3) induced intracellular translocation of VDR from the cytosol to the nucleus and also plasma membrane where VDR colocalized with caveolin-1. VDR ligands induced tyrosine phosphorylation of c-Src and VDR and their interaction. Inhibition of c-Src abrogated VDR ligand-dependent inhibition of CYP7A1 mRNA expression. Kinase assays showed that VDR ligands specifically activated the c-Raf/MEK1/2/extracellular signal-regulated kinase (ERK) 1/2 pathway, which stimulates serine phosphorylation of VDR and hepatocyte nuclear factor-4alpha, and their interaction. Mammalian two-hybrid assays showed a VDR ligand-dependent interaction of nuclear receptor corepressor-1 and silencing mediator of retinoid and thyroid with VDR/retinoid X receptor-alpha (RXRalpha). Chromatin immunoprecipitation assays revealed that an ERK1/2 inhibitor reversed VDR ligand-induced recruitment of VDR, RXRalpha, and corepressors to human CYP7A1 promoter. In conclusion, VDR ligands activate membrane VDR signaling to activate the MEK1/2/ERK1/2 pathway, which stimulates nuclear VDR/RXRalpha recruitment of corepressors to inhibit CYP7A1 gene transcription in human hepatocytes. This membrane VDR-signaling pathway may be activated by bile acids to inhibit bile acid synthesis as a rapid response to protect hepatocytes from cholestatic liver injury.

Publication types

  • Research Support, N.I.H., Extramural

MeSH terms

  • Calcitriol / pharmacology
  • Cell Membrane / drug effects
  • Cell Membrane / metabolism
  • Cell Nucleus / drug effects
  • Cell Nucleus / metabolism
  • Cholesterol 7-alpha-Hydroxylase / antagonists & inhibitors
  • Cholesterol 7-alpha-Hydroxylase / genetics
  • Enzyme Activation / drug effects
  • Extracellular Signal-Regulated MAP Kinases / antagonists & inhibitors
  • Hep G2 Cells
  • Hepatocyte Nuclear Factor 4 / metabolism
  • Hepatocytes / drug effects*
  • Hepatocytes / enzymology
  • Hepatocytes / metabolism*
  • Humans
  • Intracellular Space / drug effects
  • Intracellular Space / metabolism
  • Ligands
  • Lithocholic Acid / pharmacology*
  • Mitogen-Activated Protein Kinase Kinases / metabolism
  • Phosphorylation / drug effects
  • Phosphotyrosine / metabolism
  • Promoter Regions, Genetic / genetics
  • Protein Kinase Inhibitors / pharmacology
  • Protein Transport / drug effects
  • Proto-Oncogene Proteins c-raf / metabolism
  • Receptors, Calcitriol / metabolism*
  • Retinoid X Receptor alpha / metabolism
  • Signal Transduction / drug effects*
  • Steroid Hydroxylases / genetics
  • Steroid Hydroxylases / metabolism
  • Vitamin D3 24-Hydroxylase
  • src-Family Kinases / metabolism

Substances

  • HNF4A protein, human
  • Hepatocyte Nuclear Factor 4
  • Ligands
  • Protein Kinase Inhibitors
  • Receptors, Calcitriol
  • Retinoid X Receptor alpha
  • Phosphotyrosine
  • Lithocholic Acid
  • Steroid Hydroxylases
  • Cholesterol 7-alpha-Hydroxylase
  • Vitamin D3 24-Hydroxylase
  • src-Family Kinases
  • Proto-Oncogene Proteins c-raf
  • Extracellular Signal-Regulated MAP Kinases
  • Mitogen-Activated Protein Kinase Kinases
  • Calcitriol