Integration of cell death responses. Cell death through an apoptotic mechanism requires active participation of the cell. In response to interruption of growth factor (GF) or propagation of certain cytokine death signals (., tumor necrosis factor receptor, TNF-R), there is activation of "upstream" cysteine aspartyl proteases (caspases), which then directly digest cytoplasmic and nuclear proteins, resulting in activation of "downstream" caspases; these cause activation of nucleases, resulting in the characteristic DNA fragmentation that is a hallmark of apoptosis. . | Chapter 081. Principles of Cancer Treatment Part 10 Integration of cell death responses. Cell death through an apoptotic mechanism requires active participation of the cell. In response to interruption of growth factor GF or propagation of certain cytokine death signals . tumor necrosis factor receptor TNF-R there is activation of upstream cysteine aspartyl proteases caspases which then directly digest cytoplasmic and nuclear proteins resulting in activation of downstream caspases these cause activation of nucleases resulting in the characteristic DNA fragmentation that is a hallmark of apoptosis. Chemotherapy agents that create lesions in DNA or alter mitotic spindle function seem to activate aspects of this process by damage ultimately conveyed to the mitochondria perhaps by activating the transcription of genes whose products can produce or modulate the toxicity of free radicals. In addition membrane damage with activation of sphingomyelinases results in the production of ceramides that can have a direct action at mitochondria. The antiapoptotic protein bcl2 attenuates mitochondrial toxicity while proapoptotic gene products such as bax antagonize the action of bcl2. Damaged mitochondria release cytochrome C and apoptosis-activating factor APAF which can directly activate caspase 9 resulting in propagation of a direct signal to other downstream caspases through protease activation. Apoptosis-inducing factor AIF is also released from the mitochondrion and then can translocate to the nucleus bind to DNA and generate free radicals to further damage DNA. An additional proapoptotic stimulus is the bad protein which can heterodimerize with bcl2 gene family members to antagonize apoptosis. Importantly though bad protein function can be retarded by its sequestration as phospho-bad through the 14-3-3 adapter proteins. The phosphorylation of bad is mediated by the action of the AKT kinase in a way that defines how growth factors that activate this kinase can retard .
