Increased sensitivity to Vinca alkaloids in cells overexpressing calmodulin by gene transfection

Mouse C127 cells, transfected with the chicken calmodulin (CaM) gene and overexpressing CaM protein, were used to evaluate the effect of elevated levels of CaM on the sensitivity of these cells to various anticancer drugs. Clones C2 and C3 overexpress CaM mRNA by 40- and 80-fold, respectively, and CaM protein 3- and 8-fold, respectively. These cell lines were tested for their sensitivity to vincristine, vinblastine, bleomycin, and Adriamycin by comparing the 50% inhibitory concentration in a 72-h growth inhibition assay. The 50% inhibitory concentration values for vincristine with C2 and C3 cells were 6.27 +/- 0.56 nM and 6.60 +/- 0.96 nM, respectively. These values were significantly lower than 13.9 +/- 0.79 nM for the parental C127 cells and 14.0 +/- 1.55 nM for clone 6.8 (the control cell line for transfection without the chicken CaM gene) at P less than or equal to 0.005. The proliferation of C2 and C3 cells was inhibited at lower concentrations of vinblastine as well. The 50% inhibitory concentration values for the C2 and C3 cell lines were approximately one-half those required for C127 or clone 6.8 cells. However, no significant difference in the sensitivity to the DNA-binding drugs, bleomycin and Adriamycin, was observed between the different cell lines. The uptake of [3H]vinblastine was evaluated and found to be increased 1.6- and 2.8-fold in C2 and C3 cells, respectively, as compared with that value obtained for C127 cells. Moreover, the efflux of [3H]vinblastine from vinblastine-loaded cells was also observed to be decreased in the C2 and C3 cell lines. These data suggest that the increase in CaM expression in the C2 and C3 cell lines might be related to the higher sensitivity of these cells to Vinca alkaloids. This increased sensitivity appears to be due to the increase in intracellular concentration of the Vinca alkaloids as a result of an increase in drug uptake and a decrease in efflux. Moreover, the increased sensitivity of clones C2 and C3 to Vinca alkaloids appears to be specific to this class of drugs in that no collateral effects were observed for the DNA-damaging drugs, Adriamycin and bleomycin.