M. Pirity et al., OVEREXPRESSION OF P-GLYCOPROTEIN IN HEAT-RESISTANT AND OR DRUG-RESISTANT HEPATOMA VARIANTS/, Cytotechnology, 19(3), 1996, pp. 207-214
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.