Role for protein kinase C delta in the functional activity of human UGT1A6: implications for drug–drug interactions between PKC inhibitors and UGT1A6

Abstract

1. Many UDP-glucuronosyltransferases (UGTs) require phosphorylation by protein kinase C (PKC) for glucuronidation activity. Inhibition of UGT phosphorylation by PKC inhibitor drugs may represent a novel mechanism for drug–drug interactions.

2. The potential for PKC-mediated inhibition of human UGT1A6, an isoform involved in the glucuronidation of drugs such as acetaminophen (paracetamol) and endogenous substrates including serotonin, was evaluated using various cell model systems.

3. Of ten different PKC inhibitors screened for their effects on acetaminophen glucuronidation by human LS180 colon cells, only rottlerin (PKC δ selective inhibitor; IC₅₀ = 9.0 ± 1.2 μM) and the non-selective PKC inhibitors (calphostin-C, curcumin and hypericin) decreased glucuronidation by more than 50%.

4. Using UGT1A6-infected Sf9 insect cells, calphostin-C and hypericin showed three times more potent inhibition of serotonin glucuronidation in treated whole cells versus cell lysates. However, both curcumin and rottlerin showed significant direct inhibition and so (indirect) PKC effects could not be differentiated in this model system.

5. Of nine PKC isoforms co-expressed with UGT1A6 in human embryonic kidney 293T cells only PKC δ increased protein-normalized UGT1A6-mediated serotonin glucuronidation significantly (by 63% ± 4%).

6. These results identify an important role for PKC δ in UGT1A6-mediated glucuronidation and suggest that PKC δ inhibitors could interfere with glucuronidation of UGT1A6 substrates.

Keywords::

• UDP-glucuronosyltransferase (UGT)
• UGT1A6
• glucuronidation
• protein kinase C (PKC)
• drug–drug interaction

Acknowledgements

The authors would like to thank Dr Jae-Won Soh, Inha University (Incheon, Korea) for providing the pHACE and pHANB vectors containing the catalytic domain of the nine different PKC isoforms used in this study. This paper was made possible by a predoctoral training grant (Grant Number F31AT003973) from the National Center for Complementary and Alternative Medicine (NCCAM), National Institutes of Health (Bethesda, MD, USA) to L. P. V. Its contents are solely the responsibility of the authors and do not necessarily represent the official views of the NCCAM, or the National Institutes of Health. Other support was also provided by Grant Number R01GM061834 from the National Institute of General Medical Sciences (NIGMS), National Institutes of Health (Bethesda, MD) to M. H. C.

Declaration of interest

The authors report no conflicts of interest. The authors alone are responsible for the content and writing of the paper.