Caspase-3 mediated feedback activation of apical caspases in doxorubicin and TNF-α induced apoptosis

Aberrant apoptosis has been associated with the development and therapeutic resistance of cancer. Recent studies suggest that caspase deficiency/downregulation is frequently detected in different cancers. We have previously shown that caspase-3 reconstitution significantly sensitized MCF-7 cells to doxorubicin and etoposide. In contrast to the well established role of caspase-3 as an effector caspase, the focus of this study is to delineate caspase-3 induced feedback activation of the apical caspases-2, -8, -9 and -10A in doxorubicin and TNF-alpha induced apoptosis. Using cell-free systems we show that caspases-9 and 2 are the most sensitive, caspase-8 is less sensitive and caspase-10A is the least sensitive to caspase-3 mediated-cleavage. When apoptosis is induced by doxorubicin or TNF-alpha in an intact cell model, cleavage of caspases-8 and -9, but not caspase-2, was markedly enhanced by caspase-3. Caspase-3 mediated-feedback and activation of caspase-8 and -9 in MCF-7/C3 cells is further supported by an increase in the cleavage of caspase-8 and 9 substrates and cytochrome c release. These data indicate that, in addition to its function as an effector caspase, caspase-3 plays an important role in maximizing the activation of apical caspases and crosstalk between the two major apoptotic pathways. The significant impact of caspase-3 on both effector and apical caspases suggests that modulation of caspase-3 activity would be a useful approach to overcome drug resistance in clinical oncology.