Apoptotic cell death is executed by the caspase-mediated cleavage of various vital proteins. Elucidating the consequences of this endoproteolytic cleavage is crucial for our understanding of cell death and other biological processes. Many caspase substrates are just cleaved as bystanders, because they happen to contain a caspase cleavage site in their sequence. Several targets, however, have a discrete function in propagation of the cell death process. Many structural and regulatory proteins are inactivated by caspases, while other substrates can be activated. In most cases, the consequences of this gain-of-function are poorly understood. Caspase substrates can regulate the key morphological changes in apoptosis. Several caspase substrates also act as transducers and amplifiers that determine the apoptotic threshold and cell fate. This review summarizes the known caspase substrates comprising a bewildering list of more than 280 different proteins. We highlight some recent aspects inferred by the cleavage of certain proteins in apoptosis. We also discuss emerging themes of caspase cleavage in other forms of cell death and, in particular, in apparently unrelated processes, such as cell cycle regulation and cellular differentiation.

Efficient engulfment of the intact cell corpse is a critical end point of apoptosis, required to prevent secondary necrosis and inflammation. The presentation of "eat-me" signals on the dying cell is an important part of this process of recognition and engulfment by professional phagocytes. Here, we present evidence that apoptotic cells secrete chemotactic factor(s) that stimulate the attraction of monocytic cells and primary macrophages. The activation of caspase-3 in the apoptotic cell was found to be required for the release of this chemotactic factor(s). The putative chemoattractant was identified as the phospholipid, lysophosphatidylcholine. Further analysis showed that lysophosphatidylcholine was released from apoptotic cells due to the caspase-3 mediated activation of the calcium-independent phospholipase A(2). These data suggest that in addition to eat-me signals, apoptotic cells display attraction signals to ensure the efficient removal of apoptotic cells and prevent postapoptotic necrosis.

Death receptors have been recently identified as a subgroup of the TNF-receptor superfamily with a predominant function in induction of apoptosis. The receptors are characterized by an intracellular region, called the death domain, which is required for the transmission of the cytotoxic signal. Currently, five different such death receptors are known including tumor necrosis factor (TNF) receptor-1, CD95 (Fas/ APO-1), TNF-receptor-related apoptosis-mediated protein (TRAMP) and TNF-related apoptosis-inducing ligand (TRAIL) receptor-1 and -2. The signaling pathways by which these receptors induce apoptosis are rather similar. Ligand binding induces receptor oligomerization, followed by the recruitment of an adaptor protein to the death domain through homophilic interaction. The adaptor protein then binds a proximal caspase, thereby connecting receptor signaling to the apoptotic effector machinery. In addition, further pathways have been linked to death receptor-mediated apoptosis, such as sphingomyelinases, JNK kinases and oxidative stress. These pro-apoptotic signals are counteracted by several mechanisms which inhibit apoptosis at different levels. This review summarizes the current and rapidly expanding knowledge about the biological functions of death receptors and the mechanisms to trigger or to counteract cell death.Keywords : apoptosis; Bcl-2; caspase; CD95 (APO-1/Fas) ; death receptor; inhibitor of apoptosis protein; nuclear factor-κB ; tumor-necrosis factor; tumor-necrosis-factor-related apoptosis-inducing ligand; tumornecrosis-factor-receptor-related apoptosis-mediating protein.Apoptosis or programmed cell death is the innate mechanism modelling, immune regulation and tumor regression. Cells undergoing apoptosis show a sequence of cardinal morphological by which the organism eliminates unwanted cells. In contrast to necrosis, apoptosis is the most common physiological form of features including membrane blebbing, cellular shrinkage and condensation of chromatin. Biochemically, these alterations are cell death and occurs during embryonic development, tissue reassociated with the translocation of phosphatidylserine to the duced following traumatic injury or exposure to high concentra-

Cancer control by adaptive immunity involves a number of defined death and clearance mechanisms. However, efficient inhibition of exponential cancer growth by T cells and interferon-γ (IFN-γ) requires additional undefined mechanisms that arrest cancer cell proliferation. Here we show that the combined action of the T-helper-1-cell cytokines IFN-γ and tumour necrosis factor (TNF) directly induces permanent growth arrest in cancers. To safely separate senescence induced by tumour immunity from oncogene-induced senescence, we used a mouse model in which the Simian virus 40 large T antigen (Tag) expressed under the control of the rat insulin promoter creates tumours by attenuating p53- and Rb-mediated cell cycle control. When combined, IFN-γ and TNF drive Tag-expressing cancers into senescence by inducing permanent growth arrest in G1/G0, activation of p16INK4a (also known as CDKN2A), and downstream Rb hypophosphorylation at serine 795. This cytokine-induced senescence strictly requires STAT1 and TNFR1 (also known as TNFRSF1A) signalling in addition to p16INK4a. In vivo, Tag-specific T-helper 1 cells permanently arrest Tag-expressing cancers by inducing IFN-γ- and TNFR1-dependent senescence. Conversely, Tnfr1(-/-)Tag-expressing cancers resist cytokine-induced senescence and grow aggressively, even in TNFR1-expressing hosts. Finally, as IFN-γ and TNF induce senescence in numerous murine and human cancers, this may be a general mechanism for arresting cancer progression.

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Fumarates suppress Th1 responses by blocking IL-12 and IL-23 production by dendritic cells via distinct pathways.

Mice lacking the gene encoding poly(ADP-ribosyl) transferase (PARP or ADPRT) display no phenotypic abnormalities, although aged mice are susceptible to epidermal hyperplasia and obesity in a mixed genetic background. Whereas embryonic fibroblasts lacking PARP exhibit normal DNA excision repair, they grow more slowly in vitro. Here we investigated the putative roles of PARP in cell proliferation, cell death, radiosensitivity, and DNA recombination, as well as chromosomal stability. We show that the proliferation deficiency in vitro and in vivo is most likely caused by a hypersensitive response to environmental stress. Although PARP is specifically cleaved during apoptosis, cells lacking this molecule apoptosed normally in response to treatment with anti-Fas, tumor neurosis factor ␣, ␥-irradiation, and dexamethasone, indicating that PARP is dispensable in apoptosis and that PARP−/− thymocytes are not hypersensitive to ionizing radiation. Furthermore, the capacity of mutant cells to carry out immunoglobulin class switching and V(D)J recombination is normal. Finally, primary PARP mutant fibroblasts and splenocytes exhibited an elevated frequency of spontaneous sister chromatid exchanges and elevated micronuclei formation after treatment with genotoxic agents, establishing an important role for PARP in the maintenance of genomic integrity.

BelgiumTHE Fas/ APO-l receptor is one of the major regulators of apoptosisl-7. We report here that Fas/ APO-l-mediated apoptosis requires the activation of a new class of cysteine proteases, including interleukin-lJl-converting enzyme (ICE)s--lo, which are homologous to the product of the Caenorhabditis elegans cell-death gene ced-3 (refs 11, 12). Triggering of Fas/ APO-l rapidly stimulated the proteolytic activity of ICE. Overexpression of ICE, achieved by electroporation and microinjection, strongly potentiated Fas/ APO-l-mediated cell death. In addition, inhibition of ICE activity by protease inhibitors, as well as by transient expression of the pox virus-derived serpin inhibitor, CrmA or an antisense ICE construct, substantially suppressed Fas/ APO-l-triggered cell death. We conclude that activation of ICE or an ICE-related protease is a critical event in Fas/ APO-l-mediated cell death.The signal transduction pathway elicited by Fas/ APO-l is almost completely unknown. Initiation of apoptosis may involve a new class of cysteine proteases, including the product of the C. elegans cell-death gene eed-3, mammalian interleukin-lfJ-converting enzyme (ICE) and the related proteases Nedd-2/Ich-l, prICE and CPP-32 (refs 11-17). Overexpression ofCED-3, ICE or Nedd-2/Ich-1 in Rat-l fibroblasts has been shown to result in apoptotic cell death 12.15. We therefore investigated whether Fas/ APO-I-mediated apoptosis involved an ICE-related proteolytic activity. In L929-APO-l cells 18 or B-lymphoblastoid SKW 6.4 cells, apoptosis triggered by the agonistic monoclonal antibody anti-APO-I was strongly inhibited by the ICE inhibitor YVAD-CHO, a tetrapeptide aldehyde (Ki = 0.76 nM)8 (Fig. la).