EFFECT OF COLCHICINE AND HEAT-SHOCK ON MULTIDRUG-RESISTANCE GENE AND P-GLYCOPROTEIN EXPRESSION IN RAT-LIVER

The multidrug resistance genes encode plasma membrane glycoproteins na med P-glycoproteins, that act as an ATP-dependent drug efflux pump and decrease the cytosolic concentration of chemotherapeutic agents. It h as been hypothesized that in rat liver, this protein may have a physio logical role as a biliary transporter of xenobiotics and endobiotics. Some human tumor cell lines turn on the human multidrug resistance gen e in response to high temperature and after exposure to toxic chemical s. Accordingly, it has been proposed that the human multidrug resistan ce gene is a heat shock gene. We have assessed whether two environment al stresses, heat shock or acute exposure to cytotoxic drugs (colchici ne, vincristine, vinblastine and daunomycin), induce changes in the ex pression of multidrug resistance genes in the rat. Total cellular RNA extracted from rat liver was hybridized to a labeled human multidrug r esistance gene cDNA probe. Temperature upshift did not increase the st eady-state of mdf mRNA levels in the tissues studied, suggesting that the mdr genes are not activated as part of a heat shock response. The mdi mRNA levels increased in rat liver as early as 3 h after a single injection of colchicine, reached a peak (500%; p<0.05) after around 24 h and returned to constitutive levels after 48 h. Changes in the rela tive content of mdr mRNA were not detected in kidney, adrenal gland an d small bowel, suggesting that the in vivo induction of the mdr gene i n the liver is a tissue-specific response. The other cytotoxic drugs t hat were tested did not increase the steady-state of mdr mRNA levels. Using specific PCR-generated mouse mdr cDNA probes, we found that only the mdr 2 gene is overexpressed in the liver of colchicine-treated mo use. The mdr gene induction was followed at 48-72 h by a stronger immu nostaining in rat liver of its encoded product, suggesting that the ne wly synthesized protein was incorporated into the canalicular domain o f hepatocytes. This is the first evidence of modulation of mdr express ion gene in rodent liver in response to colchicine, a substrate of P-g lycoprotein in multidrug resistant cells. (C) Journal of Hepatology.