Taxanes (taxol and taxotere) are highly active against human breast cancer. Taxane-induced microtubule bundling and CDK1 activation promotes prolonged mitotic arrest of cancer cells. This is associated with the pre-apoptotic mitochondrial permeability transition and release of cytochrome c, which in the cytosol binds to APAF-1 and triggers the cleavage and activity of caspase-9, followed by caspase-3, resulting in the morphologic and DNA fragmentation of apoptosis. Treatment with taxanes also causes the phosphorylation of Bcl-2 and Bcl-xL, which is associated with a rise in free Bax levels. Bcl-2 phosphorylation may negatively regulate its anti-apoptotic effect and mediate taxane-induced apoptosis. Bcl-2 or Bcl-xL blocks taxane-induced pre-apoptotic mitochondrial events and the activation of caspases, but does not affect microtubule bundling and mitotic arrest of cancer cells. Recently, apoptosis due to some chemotherapeutic drugs has been shown to involve Fas death receptor signaling. The recruitment of Fas signaling has not been examined in the context of taxane-induced apoptosis. Utilizing the relevant biologic sub-types of human breast cancer cells, this application would investigate the following molecular aspects of taxane activity: 1) the temporal and mechanistic relationship of the anti-microtubule and cell-cycle effects with taxane-induced pre-apoptotic mitochondrial perturbations, caspase activation and apoptosis; 2) the mechanistic role of Bcl-2 phosphorylation, the biologic activity of the 'loop' domain of Bcl-2 and Bcl-xL, as well as the impact of the levels of APAF-1, Bcl-2, and Bcl-xL relative to Bax, Bak, and Bad on taxane-induced apoptosis; 3) the role of Fas-signaling in taxane-induced apoptosis. Based on the observation that staurosporine (ST) enhances taxol-induced apoptosis, the mechanism of interaction as well as the optimum dosage and schedules of taxanes and UCN-01, a clinically relevant analogue of ST, would also be investigated against human breast cancer cells. These in vitro studies may define the potential molecular targets or elucidate mechanisms that could be manipulated to promote taxane-induced apoptosis of breast cancer cells.
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