The coordinated neutralization of Bcl-X_L and Mcl-1 by ABT-737 in combination with radiation treatment leads to enhanced cytotoxicity in NSCLC cells

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The Bcl-2 family proteins are critical regulators of apoptosis that have been associated with tumor survival and drug resistance. In particular, overexpression of Bcl-2 and/or Bcl-X_L has been shown to increase resistance to therapies like irradiation. Recently, we disclosed ABT-737, a highly selective Bcl-2 family protein inhibitor with sub-nM affinity towards Bcl-2/Bcl-X_L/Bcl-w but not Mcl-1 or A1. Here we examine the ability of ABT-737 to enhance radiation-induced apoptosis in non-small cell lung cancer cell lines, in vitro, and the role of Bcl-2 family proteins in this process. Irradiation alone at 5 Gy induced only a mild cellular toxicity that was associated with activation of the BH3-only proteins Bim and Noxa, a consequent decrease in Mcl-1, and no observed changes in Bcl-xL, Bax or Bak . In these cell lines, Noxa neutralization of Mcl-1 was not sufficient to elicit significant cellular toxicity. Immunoprecipitation studies indicated that radiation-induced toxicity was attenuated by the direct sequestration of activated Bim by Bcl-X_L. Combination treatment of radiation and ABT-737, on the other hand, resulted in robust cell killing. ABT-737-induced displacement of Bim from Bcl-X_L was demonstrated by immunoprecipitation. Therefore, the observed synergism can be attributed to the coordinated neutralization of Bcl-X_L and Mcl-1 by ABT-737 and radiation-induced activation of Noxa. This leads to the liberation of Bim, Bax activation, and subsequent initiation of the apoptotic cascade. Taken together, these results provide a molecular basis by which Bcl-2 family inhibitors like ABT-737 can enhance radiation therapy in non-small lung cancer.