Abstract
Proteasome inhibitors are important tools for studying the roles of the proteasome in cellular processes. In this study we observed that the proteasome inhibitors MG132, epoxomicin and lactacystin were ineffective, and bortezomib was completely effective, in inhibiting cytokine-stimulated nitric oxide production in primary cultures of human hepatocytes that had been treated with the cytochrome P450 (P450) inducer phenobarbital. The inefficacy of MG132 was due to its metabolism by CYP3A enzymes, as deduced from its rapid, ketoconazole-sensitive clearance by pooled human liver microsomes and cultured hepatocytes. The efficacy of MG132 was increased by inclusion of ketoconazole in the hepatocyte incubations, and decreased by prior treatment of the cultures with the CYP3A inducers PB or rifampicin. Epoxomicin was also rapidly metabolized by CYP3A, whereas bortezomib and lactacystin were much more stable metabolically in human liver microsomes or hepatocyte cultures. Thus, bortezomib is a better choice than MG132, epoxomicin or lactacystin in cells with high activities of CYP3A enzymes. The reason for the lack of efficacy of lactacystin in human hepatocytes has yet to be determined, but it too should not be used for studies of proteasome function in human hepatocytes.
- Received July 16, 2010.
- Accepted September 13, 2010.
- The American Society for Pharmacology and Experimental Therapeutics