Balance between proliferation and apoptosis in leukemic cell lines resistant to cytostatics

Resistance to apoptosis may contribute to tumorigenesis and, in part, explains treatment failures in neoplastic diseases. We evaluated in vitro drug-induced apoptosis in leukemic cells using TdT-dependent labeling of DNA breaks with digoxigenine-dUTP and PI DNA staining in multiparameter flowcytometry. In cell lines developing drug resistance, a significant inhibition of proliferation and increased cell clearance via apoptosis was shown. Moreover, in drug resistant sub-lines and in blasts from AML patients, a variable apoptotic response to in vitro exposure to cytostatics was seen. Half of the studied AML cases were completely resistant to Novantrone-induced apoptosis with no correlation between sensitivity to Novantrone and bcl-2 expression. One case showed intraclonal heterogeneity with two coexisting populations: an immature blast population resistant to Novantrone and a differentiating blast population showing apoptotic response. Another case showed complete resistance to various cytostatics, but incubation with anti-CD95 monoclonal antibody resulted in a considerable apoptotic response. This case demonstrates that a lack of apoptotic response to cytostatics does not preclude sensitivity to other apoptotic stimuli. Our results confirm the role apoptosis plays in selection of drug-resistant clones and suggest different signaling pathways for apoptosis operating in various leukemic blasts.