Role of Bax in apoptosis of IL-3-dependent cells

IL-3 removal was reported to induce membrane association of the apoptotic e ffector Bax. This report demonstrates that IL-3-dependent cells from Bax-nu ll mice failed to activate caspases after IL-3 removal and survived in an 1 0-fold lower concentration of IL-3. As IL-3 removal also down-regulates exp ression of Bcl-X, we examined the relationship between Bcl-X decrease and B ax membrane association. IL-3 removal from BAF-3 cells, followed by sorting caspase-active and caspase-inactive populations, showed that both expresse d similar levels of Bcl-X. Inhibition of IL-3 signalling via PI-3 kinase an d MEK1/2 resulted in cells with minimal Bcl-X, which remained viable with s oluble Bax. However BAF-3-derived cells, which maintained Bcl-X expression without IL-3, also remained viable with soluble Bax on IL-3 removal. Theref ore a decrease in Bcl-X is necessary, though not sufficient, for Bax membra ne association on IL-3 removal. In contrast, treatment of BAF-3 cells with hydroxyurea induced apoptosis in the absence of a Bcl-X decrease. Furthermo re, IL-3-dependent cells from Bax-null mice activated caspases after hydrox yurea treatment and show the same sensitivity to a variety of cytotoxic dru gs. Thus, apoptosis after IL-3 removal requires a decrease in Bcl-X and Bax membrane association, whereas that induced by cytotoxic drugs does not.