Execution of Apoptosis: Converging or Diverging Pathways?

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Abstract
There is increasing evidence that apoptosis and necrosis represent only two of several possible ways for cells to die. These two types of demise can occur simultaneously in tissues or cell cultures exposed to the same stimulus, and often local metabolic conditions and the intensity of the same initial insult decide the prevalence of either apoptosis or necrosis. Recent work has shown that execution of the apoptotic programme involves a relatively limited number of pathways. According to a general view, these would converge to activate the caspase family of proteases. However, there is increasing evidence that apoptotic-like features can be observed also in cells where caspases are inhibited by cell-permeable tripeptides, such as z-VaD-Ala-Asp-fluoromethyl ketone (z-VAD-fmk), or analogous compounds. This has posed the question as to whether apoptosis may or may not occur in a caspase independent way, and whether caspase inhibitors may be effective in the treatment of disease. Also relevant is the understanding that low intracellular energy levels during apoptosis can preclude caspase activation, and consequently decide the occurrence and mode of demise in damaged cells. In vivo, incomplete execution of damaged cells by apoptosis may have profound implications, as their persistence within a tissue, followed by delayed lysis, may elicit delayed pro-inflammatory reactions. In this minireview, we discuss some recent findings suggesting that cells may use diverging execution pathways, with different implications in pathology and therapy.