OVEREXPRESSION OF P-GLYCOPROTEIN IN HEAT-RESISTANT AND OR DRUG-RESISTANT HEPATOMA VARIANTS/

We have earlier isolated a glucocorticoid-resistant, dedifferentiated rat hepatoma variant, the clone 2, which exhibited deficient stress ac tivation of the major stress-inducible heat-shock protein hsp68. Multi drug-resistant variants were isolated from clone 2 cells using increas ing concentrations of colchicine. The induction deficiency of hsp68 wa s maintained in the colchicine-resistant clone 2 cells grown for sever al months in the presence of 1 mu g/ml colchicine (termed as highly mu ltidrug-resistant variant) indicating that this heat-shock protein is not involved in the multidrug resistance. No alteration of the protein synthesis pattern was observed except the strong increase of the P-gl ycoprotein, which correlated with high level of corresponding mRNA. St able heat-resistant variants of clone 2 were also isolated, which show ed increased drug resistance to several drugs, i.e. they became modera tely multidrug-resistant. This moderate multidrug resistance of the he at-resistant variants was further increased by stepwise selection with colchicine (highly multidrug-resistant heat-resistant variants). The levels of P-glycoprotein mRNA and protein were elevated both in the he at-resistant, non drug selected, moderately drug-resistant and in heat -resistant, colchicine selected, highly drug-resistant variants. Decre ased retention of antitumor drugs was observed in all multidrug-resist ant variants indicating that P-glycoprotein was functional. Verapamil increased doxorubicin retention and cytotoxicity significantly. Our re sults showing that severely stressed hepatoma cells overexpressed the multidrug resistance gene(s) raise the possibility that the P-glycopro tein may participate in protection against environmental stress such a s heat.