Inhibition of UDP‐glucuronosyltransferase (UGT) enzymes by protein kinase C inhibitors

Abstract

UGTs are involved in the clearance of ∼10% of the top drugs on the market. Evidence for a novel mechanism of UGT inhibition involving protein kinase C (PKC) is accumulating. Therefore, we screened a panel of ten PKC inhibitors, including PKC‐α, β, δ, and η specific inhibitors, to determine their effect on acetaminophen (APAP) glucuronidation in human LS180 colon cells. Of these, only calphostin‐C (selective PKC inhibitor), curcumin (non‐specific PKC), hypericin (non‐specific PKC), and rottlerin (PKC‐δ selective) decreased glucuronidation by more than 50%. To ensure that this inhibition was not due to direct effects, these compounds were counterscreened in human liver microsomes (HLM), where these inhibitors were 5 to 15 times less potent. Curcumin and calphostin‐C were also investigated for their effects on serotonin glucuronidation in human UGT1A6‐infected Sf9 insect cells and cell lysates. Calphostin‐C was at least 3 times more active in the whole cells (IC50 = 0.7 μM) compared to cell lysates (IC50 > 2 μM), while curcumin was a potent direct inhibitor of UGT1A6 and so effects via PKC could not be differentiated. These data further support the role of PKC in phosphorylating and activating UGTs and indicates that PKC‐δ, but perhaps not α, β, or η, is involved in phosphorylating the UGTs required for APAP glucuronidation. This work was made possible by grants F31AT003973, R01GM061834, and R21GM074369 from the NIH.