Interpretation of the effects of protein kinase C inhibitors on human UDP-glucuronosyltransferase 1A (UGT1A) proteins in cellulo

Drug Metab Pharmacokinet. 2011 Jun;26(3):256-65. doi: 10.2133/dmpk.DMPK-10-RG-121. Epub 2011 Feb 8.


UDP-glucuronosyltransferases (UGTs) catalyze the glucuronidation of a wide variety of xeno/endobiotics. Previous studies have reported that human UGT enzymes are phosphorylated and that treatment of cells with protein kinase C (PKC) inhibitors results in decreased UGT activities without affecting the UGT protein levels. In this study, we investigated the effects of PKC inhibitors on human UGT1A protein levels and activities in detail. When UGT1A-expressing HEK293 cells and LS180 cells were treated with curcumin or calphostin C, the exogenous and endogenous UGT1A protein levels in homogenates prepared with Tris-buffered saline were significantly decreased. Enzyme activity levels mirrored the changes in UGT protein levels. When the curcumin- or calphostin C-treated cells were lysed with buffer containing a detergent, the UGT protein levels did not decrease. We found that curcumin or calphostin C treatment facilitated the degradation of UGT protein after the cells were collected in the absence of a detergent. Finally, by in cellulo evaluation, we found that curcumin decreased UGT activity by the direct inhibitory effect, but calphostin C did not affect UGT activity. Thus, this study suggests that we should evaluate the data carefully when interpreting the effects of PKC inhibitors on UGT activity.

MeSH terms

  • Autophagy / drug effects
  • Biocatalysis / drug effects
  • Cell Line, Tumor
  • Curcumin / metabolism
  • Curcumin / pharmacology
  • Gene Expression / drug effects
  • Gene Expression / genetics
  • Glucuronosyltransferase / antagonists & inhibitors*
  • Glucuronosyltransferase / metabolism*
  • HEK293 Cells
  • Humans
  • Isoenzymes / metabolism
  • Naphthalenes / pharmacology
  • Phosphorylation / drug effects
  • Phosphorylation / physiology
  • Protease Inhibitors / pharmacology
  • Proteasome Endopeptidase Complex / metabolism
  • Proteasome Inhibitors
  • Protein Kinase C / antagonists & inhibitors*
  • Protein Kinase Inhibitors / pharmacology*
  • Protein-Tyrosine Kinases / antagonists & inhibitors
  • Proteolysis / drug effects
  • Recombinant Fusion Proteins / genetics
  • Recombinant Fusion Proteins / metabolism
  • Sodium Dodecyl Sulfate / pharmacology
  • Time Factors
  • Transfection
  • UDP-Glucuronosyltransferase 1A9


  • Isoenzymes
  • Naphthalenes
  • Protease Inhibitors
  • Proteasome Inhibitors
  • Protein Kinase Inhibitors
  • Recombinant Fusion Proteins
  • Sodium Dodecyl Sulfate
  • UGT1A1 enzyme
  • Glucuronosyltransferase
  • UDP-Glucuronosyltransferase 1A9
  • Protein-Tyrosine Kinases
  • Protein Kinase C
  • Proteasome Endopeptidase Complex
  • calphostin C
  • Curcumin