RESISTANCE TO RADIATION-INDUCED APOPTOSIS IN BCL-2-EXPRESSING CELLS IS REVERSED BY DEPLETING CELLULAR THIOLS

The mechanism by which Bcl-2 oncogene expression inhibits radiation-in duced apoptosis has been investigated in two mouse lymphoma cell lines : line LY-as is radiation sensitive, displays substantial radiaton-ind uced apoptosis. and expresses low levels of Bcl-2; line LY-ar is radia tion-resistant, displays a low apoptosis propensity, and expresses 30- fold higher amount of Bcl-2 protein than does the sensitive line. We o bserved that upon incubation in cystine/methionine-free (C/M-) medium, radiation-induced apoptosis in the LY-ar cells was restored to levels comparable to that seen in the LY-as cells. Intracellular glutathione (GSH) concentrations in LY-ar cells incubated in C/M- medium plummete d to 50% of control values within 2 h. LY-ar cells treated with diethy l maleate (DEM) or diamide, agents that deplete cellular thiols, had i ncreased susceptibility to radiation-induced apoptosis in a manner sim ilar to C/M- medium. These results are consistent with the general ide a that Bcl-2 expression blocks apoptosis through an antioxidant pathwa y that involves cellular thiols. That Bcl-2-expressing tumor cells can be sensitized by exogeneous agents that modify cellular thiols offers strategies for overcoming such resistance.