We have shown that a glycoprotein(s) with a molecular weight of ∼180,000 exists on the surface of cultured lymphoblasts selected for resistance to vinblastine (VLB) (CEM/VLB100). The amount of this glycoprotein, which is barely detectable on the VLB-sensitive cells (CEM), appears to be related in part to the degree of resistance, up to ∼270-fold. Exposure of cells to Pronase for 45 to 60 min or growth of the cells for 2 days in tunicamycin, an inhibitor of glycoprotein synthesis, resulted in the absence of the resistance-associated glycoproteins, as determined by polyacrylamide gel electrophoresis of these treated cells after labeling the cells either with sodium [3H]-borohydride or with [14C]- or [3H]glucosamine. Uptake studies with [3H]VLB revealed that CEM cells normally accumulated and retained more drug than did the CEM/VLB100 cells. While the tunicamycin or Pronase treatments slightly increased the uptake of drug by CEM cells, there was no enhanced uptake of [3H]VLB by the tunicamycin- or pronase-treated CEM/VLB100 cells, when compared with untreated controls, indicating that the loss of external surface glycoproteins did not render the resistant cells more “leaky” to drug influx. Additionally, diminished drug retention by the CEM/VLB100 cells was unaffected by these treatments. Moreover, when CEM/VLB100 cells were grown for 2 days in the presence of tunicamycin and several concentrations of VLB, no enhanced toxicity of VLB was noted. Other experiments with [14C]glucosamine and [3H]leucine revealed that treatment with tunicamycin did not affect the distribution of proteins in these cells. Taken together, these results permit the suggestion that the carbohydrate moiety of the cell surface resistance-associated glycoproteins does not mediate resistance to the alkaloid per se; however, a role for plasma membrane proteins cannot be ruled out at this time.
This work was supported in part by CORE Grant CA-21765, Program Grant CA-23099, and Grant R01 CA-30103 from the National Cancer Institute. Additional support was from American lebanese Syrian Associated Charities.