Overexpression of the multidrug resistance gene product in adult rat hepatocytes during primary culture

Abstract

Expression of P‐glycoprotein (P‐gp), the product of multidrug resistance gene(s), was investigated in primary cultures of normal adult rat hepatocytes. Levels of P‐gp mRNAs determined by Northern blotting and of P‐gp measured by immunoblotting increased in parallel with time in culture. As in normal liver, P‐gp was found to be localized on the membrane of bile canaliculus‐like structures. This increased expression of P‐gp was associated with decreased intracellular retention of doxorubicin, which could be restored by compounds such as verapamil and cyclosporin; doxorubicin (and also vincristine) was more cytotoxic to early than to late cultures. As in preneoplastic and neoplastic liver, overexpression of P‐gp in cultured hepatocytes was associated with differential changes in drug‐metabolizing enzymes, including increased glutathione S‐transferase 7‐7. Functional P‐gp over‐expression was observed in the absence of xenobiotic exposure or cell division; it could be linked to cellular stress occurring during cell isolation and plating. Increased expression of P‐gp was blocked by actinomycin D, indicating its dependence on increased transcription, while cycloheximide led to a superinduction suggesting negative regulation by a protein factor. Copyright © 1992, Wiley Blackwell. All rights reserved