EFFECTS OF CURCUMIN ON P-GLYCOPROTEIN IN PRIMARY CULTURES OF RAT HEPATOCYTES

Curcumin is a natural phenolic compound found in the rhizomes of Curcu ma longa and endowed with beneficial biological activities including a ntioxidant, anticarcinogenic and hepatoprotective effects. In this stu dy curcumin was tested for its potential ability to interact in vitro with hepatic P-glycoprotein (Pgp), in a model system represented by pr imary cultures of rat hepatocytes, in which spontaneous overexpression of multidrug resistance (mdr) genes occurs. In both freshly-plated he patocytes, containing low levels of Pgp, and 72 hour-cultured hepatocy tes, containing high levels of Pgp, the Rhodamine-123 (R-123) efflux, which represents a specific functional test for Pgp-mediated transport , was inhibited by curcumin in a dose-dependent manner. Western blot a nalysis showed that 25 mu M curcumin, when included in the culture med ium throughout the experimental observation (72 hours), was able to si gnificantly lower the increase of mAb C219-immunoreactive protein spon taneously occurring in the cells during culture. Curcumin, at doses ra nging from 50 to 150 mu M was cytotoxic for freshly-plated hepatocytes , as shown by the strong decrease in the cell ability to exclude trypa n blue 24 hours later, but it was significantly less cytotoxic when ad ded to 24 or 48 hour-cultured cells. The resistance to curcumin, progr essively acquired by cells during culture, was significantly reduced b y high concentrations of dexamethasone (DEX) or dimethyl-sulfoxide (DM SO), culture conditions known to inhibit the spontaneous overexpressio n of Pgp. Zn addition, in a concentration-dependent manner, verapamil reverted curcumin resistance in Pgp overexpressing hepatocytes. In pho toaffinity labeling studies, curcumin competed with azidopine for bind ing to Pgp, suggesting a direct interaction with glycoprotein. These r esults suggest that curcumin is able to modulate in vitro both express ion and function of hepatic Pgp and support the hypothesis that curcum in, a chemopreventive phytochemical, could reveal itself also as a com pound endowed with chemosensitizing properties on mdr phenotype.