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.