MDR1 activation is the predominant resistance mechanism selected by vinblastine in MES-SA cells

Single-step selection with vinblastine was performed in populations of the human sarcoma cell line MES-SA, to assess cellular mechanisms of resistance to the drug and mutation rates via fluctuation analysis. At a stringent se lection with 20 nM vinblastine, resulting in 5-6 logs of cell killing, the mutation rate was 7 x 10(-7) per cell generation. Analysis of variance supp orted the hypothesis of spontaneous mutations conferring vinblastine resist ance, rather than induction of adaptive response elements. Surviving clones displayed a stable multidrug resistance phenotype over a 3-month period. A ll propagated clones demonstrated high levels of resistance to vinblastine and paclitaxel, and lower cross-resistance to doxorubicin and etoposide. Ac tivation of MDR1 gene expression and P-glycoprotein function was demonstrab le in all clones. No elevation was found in the expression of the mrp gene, the LRP-56 major vault protein and B-tubulin isotypes (M40, beta 4, 5 beta , and beta 9) in these mutants. We conclude that initial-step resistant mec hanism in these vinblastine-selected mutants commonly arises from a stochas tic mutation event with activation of the MDR1 gene. (C) 2000 Cancer Resear ch Campaign.