PT - JOURNAL ARTICLE AU - Richard C. Zangar AU - Thomas A. Kocarek AU - Shang Shen AU - Nikki Bollinger AU - Michael S. Dahn AU - Donna W. Lee TI - Suppression of Cytochrome P450 3A Protein Levels by Proteasome Inhibitors AID - 10.1124/jpet.102.044628 DP - 2003 Jun 01 TA - Journal of Pharmacology and Experimental Therapeutics PG - 872--879 VI - 305 IP - 3 4099 - http://jpet.aspetjournals.org/content/305/3/872.short 4100 - http://jpet.aspetjournals.org/content/305/3/872.full SO - J Pharmacol Exp Ther2003 Jun 01; 305 AB - 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 ∼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. The American Society for Pharmacology and Experimental Therapeutics