Sodium taurocholate cotransporting polypeptide is a serine, threonine phosphoprotein and is dephosphorylated by cyclic adenosine monophosphate

Hepatology. 1998 Dec;28(6):1629-36. doi: 10.1002/hep.510280624.

Abstract

Na+/taurocholate (Na+/TC) cotransport in hepatocytes is mediated primarily by Na+/TC cotransporting polypeptide (Ntcp), and cyclic adenosine monophosphate (cAMP) stimulates Na+/TC cotransport by inducing translocation of Ntcp to the plasma membrane. The aim of the present study was to determine if Ntcp is a phosphoprotein and if cAMP alters Ntcp phosphorylation. Freshly prepared hepatocytes from rat livers were incubated with carrier-free 32PO4 for 2 hours, followed by incubation with 10 micromol/L 8-chlorophenylthio adenosin 3':5'-cyclic monophosphate (CPT-cAMP) for 15 minutes. Subcellular fractions isolated from 32P-labeled hepatocytes were subjected to immunoprecipitation using Ntcp antibody, followed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) and autoradiography to determine if Ntcp is phosphorylated. Ntcp immunoprecipitated from plasma membranes isolated from nonlabeled hepatocytes was subjected to immunoblot analysis using anti-phosphoserine, anti-phosphothreonine, or anti-phosphotyrosine antibody to determine whether Ntcp is a serine, threonine, or tyrosine phosphoprotein. Hepatocytes were loaded with bis-(2-amino-5-methylphenoxy)-ethane-N,N,N',N'-tetraacetic acid (MAPTA), a Ca2+ buffering agent, and the effect of CPT-cAMP on TC uptake, cytosolic [Ca2+], and ntcp phosphorylation and translocation was determined. In addition, the effect of cAMP on protein phosphatases 1 and 2A (PP1/2A) was determined in homogenates and plasma membranes obtained from CPT-cAMP-treated hepatocytes. Phosphorylation study showed that phosphorylated Ntcp is detectable in plasma membranes, and cAMP treatment resulted in dephosphorylation of Ntcp. Immunoblot analysis with phosphoamino antibodies revealed that Ntcp is a serine/threonine, and not a tyrosine, phosphoprotein, and cAMP inhibited both serine and threonine phosphorylation. In MAPTA-loaded hepatocytes, CPT-cAMP failed to stimulate TC uptake, failed to increase cytosolic [Ca2+], and failed to induce translocation and dephosphorylation of Ntcp. cAMP did not alter the activity of PP1/2A in either homogenates or in plasma membranes. Taken together, these results suggest that Ntcp is a serine/threonine phosphoprotein and is dephosphorylated by cAMP treatment. Activation of PP1/2A is not involved in cAMP-mediated dephosphorylation of Ntcp. Both translocation and dephosphorylation of Ntcp may be involved in the regulation of hepatic Na+/TC cotransport.

Publication types

  • Research Support, Non-U.S. Gov't
  • Research Support, U.S. Gov't, P.H.S.

MeSH terms

  • Amino Acids / analysis
  • Animals
  • Carrier Proteins / chemistry
  • Carrier Proteins / metabolism*
  • Cyclic AMP / metabolism*
  • Cyclic AMP / pharmacology
  • Liver / cytology
  • Liver / metabolism
  • Organic Anion Transporters, Sodium-Dependent*
  • Phosphoprotein Phosphatases / metabolism
  • Phosphoproteins / metabolism*
  • Phosphorylation / drug effects
  • Rats
  • Serine / metabolism*
  • Subcellular Fractions / metabolism
  • Symporters*
  • Threonine / metabolism*
  • Tissue Distribution

Substances

  • Amino Acids
  • Carrier Proteins
  • Organic Anion Transporters, Sodium-Dependent
  • Phosphoproteins
  • Symporters
  • sodium-bile acid cotransporter
  • Threonine
  • Serine
  • Cyclic AMP
  • Phosphoprotein Phosphatases