Suppression of cytochrome P450 3A protein levels by proteasome inhibitors

We have previously reported that CYP3A cross-links with polyubiquitinated proteins in microsomes from nicardipine-treated rats in a process that is distinct from classical polyubiquitination. To further examine the role of the proteasome in CYP3A degradation, we investigated the effects of proteasome inhibitors lactacystin, MG132, proteasome inhibitor 1, and hemin in primary cultures of rat and human hepatocytes. With the exception of hemin, these agents increased the total pool of ubiquitinated proteins in microsomes isolated from rat hepatocytes, indicating that lactacystin, MG132, and proteasome inhibitor 1 effectively inhibited the proteasome in these cells. All four agents caused a reduction in the amount of the major approximately 55-kDa CYP3A band, opposite to what would be expected if the ubiquitin-proteasome pathway degraded CYP3A. Only hemin treatment caused an increase in high molecular mass (HMM) CYP3A bands. Because hemin treatment did not alter levels of ubiquitin in CYP3A immunoprecipitates, the HMM CYP3A bands formed in response to hemin treatment clearly were not due to proteasome inhibition. Rather, because hemin treatment also caused an increase in HMM CYP3A in the detergent-insoluble fraction of the 10,000g pellet, the HMM CYP3A seems to represent a large protein complex that is unlikely to primarily represent ubiquitination.