Apoptosis and carcinogenesis.

This article attempts to summarize the rapidly advancing field of apoptosis and its regulation, with particular reference to cancer. The long-recognized stereotyped morphology of apoptosis is seen to be the result of convergence of biochemical pathways on common effector mechanisms in which a major element is activation of cysteine proteases with a preference for cleavage at aspartate residues (caspases). The substrates of this reaction are widely dispersed in the nucleus, cytoplasm and cytoskeleton. Caspase activation is the end result of protean stimuli, physiological and pathological. Pathological stimuli include damage to cell membranes, mitochondrial function, DNA and possibly other critical intracellular organelles. Several, distinct agents are known that may be part of the signaling pathways that couple injury to these cellular components to apoptosis: ceramide, collapse of mitochondrial transmembrane potential, p53 activation. Other stimuli are signaled through cytokine receptors (such as fas/APO-1/CD 95 and TNFRI and II) or transcription factors (such as p53, IRF-1 and rb). The transduction of these stimuli into caspase activation is regulated by a large family of proteins (the bcl-2 family). Cancer and apoptosis are related in many ways. In particular, this article explores the possibility that defective apoptosis may permit the persistence of damaged, mutated cells that would otherwise have been deleted. The conditions that lead to this scenario appear to be tissue-specific.