ABSTRACT
The organic solute transporter (OST)(alpha)-OST(beta) is an unusual heteromeric carrier expressed in a variety of tissues including the small intestine, colon, liver, biliary tract, kidney, and adrenal gland. In polarized epithelial cells, OSTalpha-OSTbeta protein is localized on the basolateral membrane and functions in the export or uptake of bile acids and steroids. This article reviews recent results including studies of knockout mouse models that provide new insights to the role of OSTalpha-OSTbeta in the compartmentalization and metabolism of these important lipids.
Subject(s)
Bile Acids and Salts/metabolism , Membrane Transport Proteins/metabolism , Steroids/metabolism , Animals , Humans , MiceABSTRACT
The farnesoid X receptor (FXR) is a ligand-activated transcription factor and a member of the nuclear receptor superfamily. In the past six years, remarkable inroads have been made into determining the functional importance of FXR. This receptor has been shown to have crucial roles in controlling bile acid homeostasis, lipoprotein and glucose metabolism, hepatic regeneration, intestinal bacterial growth and the response to hepatotoxins. Thus, the development of FXR agonists might prove useful for the treatment of diabetes, cholesterol gallstones, and hepatic and intestinal toxicity.
Subject(s)
DNA-Binding Proteins/physiology , Receptors, Cytoplasmic and Nuclear/physiology , Transcription Factors/physiology , Animals , Bile Acids and Salts/metabolism , DNA-Binding Proteins/genetics , Humans , Lipid Metabolism/physiology , Liver/cytology , Liver/metabolism , Models, Biological , Protein Isoforms/genetics , Protein Isoforms/physiology , Receptors, Cytoplasmic and Nuclear/genetics , Transcription Factors/geneticsABSTRACT
Activation of the farnesoid X receptor (FXRalpha) affects genes controlling many pathways, including those involved in bile acid and glucose homeostasis. Here we report that a critical gene involved in cholesterol homeostasis, Insig-2, was induced when mice or cultured cells were treated with FXRalpha agonists or infected with constitutively active FXRalpha. No such induction was observed in agonist-treated FXRalpha-/- mice. Further analysis, which included EMSAs, reporter gene activation, and chromatin immunoprecipitation, identified two functional FXRalpha response elements within intron 2 of the mouse Insig-2 gene. In addition to increasing hepatic Insig-2 protein levels in wild-type mice, FXRalpha activation also reduced lanosterol 14alpha-demethylase mRNA levels and 3-hydroxy-3-methylglutaryl-coenzyme A reductase protein levels. Together, these changes likely account for the decrease in cholesterol synthesis observed after activation of FXR in primary hepatocytes. In conclusion, the current study links hepatic FXRalpha activation to regulation of genes involved in cholesterol synthesis.
Subject(s)
Cholesterol/metabolism , DNA-Binding Proteins/metabolism , Gene Expression Regulation , Liver/metabolism , Membrane Proteins/genetics , Receptors, Cytoplasmic and Nuclear/metabolism , Transcription Factors/metabolism , Animals , Cholesterol/genetics , Chromatin Immunoprecipitation , Cytochrome P-450 Enzyme System/genetics , DNA-Binding Proteins/agonists , DNA-Binding Proteins/genetics , Electrophoretic Mobility Shift Assay , Genes, Reporter , Hydroxymethylglutaryl-CoA-Reductases, NADP-dependent/metabolism , Mice , Mice, Mutant Strains , Oxidoreductases/genetics , RNA, Messenger/analysis , RNA, Messenger/metabolism , Receptors, Cytoplasmic and Nuclear/agonists , Receptors, Cytoplasmic and Nuclear/genetics , Regulatory Elements, Transcriptional , Sterol 14-Demethylase , Transcription Factors/agonists , Transcription Factors/genetics , Transcriptional ActivationABSTRACT
Fxralpha is known to regulate a variety of metabolic processes, including bile acid, cholesterol, and carbohydrate metabolism. In this study, we show direct evidence that Fxralpha is a key player in maintaining sulfate homeostasis. We identified and characterized the sodium/sulfate co-transporter (NaS-1; Slc13a1) as an Fxralpha target gene expressed in the kidney and intestine. Electromobility shift assays, chromatin immunoprecipitation, and promoter reporter studies identified a single functional Fxralpha response element in the second intron of the mouse Slc13a1 gene. Treatment of wild-type mice with GW4064, a synthetic Fxralpha agonist, induced Slc13a1 mRNA in the intestine and kidney. Slc13a1 mRNA was also induced in the kidney and intestine of wild-type, but not Fxralpha-/- mice, after treatment with the hepatotoxin alpha-naphthylisothiocyanate, which is known to result in elevated blood bile acid levels. Finally, we observed a decrease in Slc13a1 mRNA in the kidney and intestine of Fxralpha-/- mice and a corresponding increase in urinary excretion of free sulfates as compared with wild-type mice. These results demonstrate that mouse Slc13a1 is a novel Fxralpha target gene expressed in the kidney and intestine and that in the absence of Fxralpha, mice waste sulfate into the urine. Thus, Fxralpha is necessary for normal sulfate homeostasis in vivo.