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 named P-glycoproteins, that act as an ATP-dependent drug efflux pump and decrease the cytosolic concentration of chemotherapeutic agents. It has been hypothesized that in rat liver, this protein may have a physiological role as a biliary transporter of xenobiotics and endobiotics. Some human tumor cell lines turn on the human multidrug resistance gene in response to high temperature and after exposure to toxic chemicals. Accordingly, it has been proposed that the human multidrug resistance gene is a heat shock gene. We have assessed whether two environmental stresses, heat shock or acute exposure to cytotoxic drugs (colchicine, vincristine, vinblastine and daunomycin), induce changes in the expression of multidrug resistance genes in the rat. Total cellular RNA extracted from rat liver was hybridized to a labeled human multidrug resistance gene cDNA probe. Temperature upshift did not increase the steady-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 relative content of mdr mRNA were not detected in kidney, adrenal gland and small bowel, suggesting that the in vivo induction of the mdr gene in the liver is a tissue-specific response. The other cytotoxic drugs that 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 mouse. The mdr gene induction was followed at 48-72 h by a stronger immunostaining in rat liver of its encoded product, suggesting that the newly synthesized protein was incorporated into the canalicular domain of hepatocytes. This is the first evidence of modulation of mdr expression gene in rodent liver in response to colchicine, a substrate of P-glycoprotein in multidrug resistant cells. (C) Journal of Hepatology.