Apoptosis: molecular mechanisms and implications for cancer chemotherapy

Apoptosis, or programmed cell death, is an orderly and genetically controlled form of cell death. In a morphological sense, it differs from necrosis in that cellular shrinkage and chromatin condensation occurs, followed by fragmentation of nuclear components within membrane-bound vesicles which are cleared by phagocytosis without damage to adjacent tissue. The molecular pathway includes an initiating phase, which starts after signalling by external triggers, such as ligation to distinct receptors or by endogenous mechanisms related to aging or to exogenous irreversible cellular or nuclear damage. The initiation phase is followed by a decision phase. During this phase transduction occurs of the apoptotic signal to nuclear and cytoplasmatic target enzymes, which includes activation of endonucleases and enzymatic alterations of the cytoskeleton. There are numerous proteins and lipid-derived moieties which modulate the apoptotic mechanism in positive or negative direction. The execution phase is started when the cell has arrived at a stage of no return. The nuclear DNA is cleaved into multiples of 180-200 basepairs, the plasma membrane integrity and the mitochondria remain initially intact, the cell splits up into apoptotic bodies, small vesicles which enclose the nuclear and cellular remnants. Finally, the clearing phase is arrived, when the apoptotic bodies are phagocytosed by adjacent cells and macrophages. It is thought that the pharmacodynamics of anticancer drugs consists of two distinct steps. The first step includes the interaction with its cellular target; which is not lethal per se. The commitment of the cell to undergo apoptosis forms the second step. The efficacy of anticancer drugs is determined by the ability to selectively sensitize tumor cells to apoptosis, which depends to a large extent from the expression of various oncogenes, such as bcl-2, p53, bax, ras, c-myc and others, and from endogenous factors. It is a challenge in pharmacological research to explore apoptosis by modulating the extrinsic and intrinsic regulators in a positive or negative direction in order to improve the efficacy of anticancer treatment.