Caspase-2 regulates S-phase cell cycle events to protect from DNA damage accumulation independent of apoptosis.

In addition to its classical role in apoptosis, accumulating evidence suggests that caspase-2 has non-apoptotic functions, including regulation of cell division. Loss of caspase-2 is known to increase proliferation rates but how caspase-2 is regulating this process is currently unclear. We show that caspase-2 is activated in dividing cells in G1-phase of the cell cycle. In the absence of caspase-2, cells exhibit numerous S-phase defects including delayed exit from S-phase, defects in repair of chromosomal aberrations during S-phase, and increased DNA damage following S-phase arrest. In addition, caspase-2-deficient cells have a higher frequency of stalled replication forks, decreased DNA fiber length, and impeded progression of DNA replication tracts. This indicates that caspase-2 protects from replication stress and promotes replication fork protection to maintain genomic stability. These functions are independent of the pro-apoptotic function of caspase-2 because blocking caspase-2-induced cell death had no effect on cell division, DNA damage-induced cell cycle arrest, or DNA damage. Thus, our data supports a model where caspase-2 regulates cell cycle and DNA repair events to protect from the accumulation of DNA damage independently of its pro-apoptotic function.

NPM1 directs PIDDosome-dependent caspase-2 activation in the nucleolus.

The PIDDosome (PIDD-RAIDD-caspase-2 complex) is considered to be the primary signaling platform for caspase-2 activation in response to genotoxic stress. Yet studies of PIDD-deficient mice show that caspase-2 activation can proceed in the absence of PIDD. Here we show that DNA damage induces the assembly of at least two distinct activation platforms for caspase-2: a cytoplasmic platform that is RAIDD dependent but PIDD independent, and a nucleolar platform that requires both PIDD and RAIDD. Furthermore, the nucleolar phosphoprotein nucleophosmin (NPM1) acts as a scaffold for PIDD and is essential for PIDDosome assembly in the nucleolus after DNA damage. Inhibition of NPM1 impairs caspase-2 processing, apoptosis, and caspase-2-dependent inhibition of cell growth, demonstrating that the NPM1-dependent nucleolar PIDDosome is a key initiator of the caspase-2 activation cascade. Thus we have identified the nucleolus as a novel site for caspase-2 activation and function.

BOK Is a Non-canonical BCL-2 Family Effector of Apoptosis Regulated by ER-Associated Degradation.

The mitochondrial pathway of apoptosis is initiated by mitochondrial outer membrane permeabilization (MOMP). The BCL-2 family effectors BAX and BAK are thought to be absolutely required for this process. Here, we report that BCL-2 ovarian killer (BOK) is a bona fide yet unconventional effector of MOMP that can trigger apoptosis in the absence of both BAX and BAK. However, unlike the canonical effectors, BOK appears to be constitutively active and unresponsive to antagonistic effects of the antiapoptotic BCL-2 proteins. Rather, BOK is controlled at the level of protein stability by components of the endoplasmic reticulum (ER)-associated degradation pathway. BOK is ubiquitylated by the AMFR/gp78 E3 ubiquitin ligase complex and targeted for proteasomal degradation in a VCP/p97-dependent manner, which allows survival of the cell. When proteasome function, VCP, or gp78 activity is compromised, BOK is stabilized to induce MOMP and apoptosis independently of other BCL-2 proteins.

Limited mitochondrial permeabilization causes DNA damage and genomic instability in the absence of cell death.

During apoptosis, the mitochondrial outer membrane is permeabilized, leading to the release of cytochrome c that activates downstream caspases. Mitochondrial outer membrane permeabilization (MOMP) has historically been thought to occur synchronously and completely throughout a cell, leading to rapid caspase activation and apoptosis. Using a new imaging approach, we demonstrate that MOMP is not an all-or-nothing event. Rather, we find that a minority of mitochondria can undergo MOMP in a stress-regulated manner, a phenomenon we term "minority MOMP." Crucially, minority MOMP leads to limited caspase activation, which is insufficient to trigger cell death. Instead, this caspase activity leads to DNA damage that, in turn, promotes genomic instability, cellular transformation, and tumorigenesis. Our data demonstrate that, in contrast to its well-established tumor suppressor function, apoptosis also has oncogenic potential that is regulated by the extent of MOMP. These findings have important implications for oncogenesis following either physiological or therapeutic engagement of apoptosis.

Imaging-based methods for assessing caspase activity in single cells.

Caspases, a family of proteases that are essential mediators of apoptosis, are divided into two groups: initiator caspases and executioner caspases. Each initiator caspase is activated at the apex of its respective pathway, which generally leads to the cleavage and activation of executioner caspases. Executioner caspases in turn cleave numerous substrates in the cell, leading to its demise. Initiator caspases are activated when inactive monomers undergo induced proximity to form an active caspase. In contrast, executioner caspases are activated by cleavage. Based on this key difference, different imaging techniques have been developed to measure caspase activation and activity on a single-cell basis. Bimolecular fluorescence complementation (BiFC) is used to measure induced proximity of initiator caspases, whereas Förster resonance energy transfer (FRET) permits the investigation of caspase-mediated substrate cleavage in real time. Because many of the events in apoptosis, including caspase activation, are asynchronous in nature, these single-cell imaging techniques have proven to be immensely powerful in ordering and dissecting caspase pathways. When coupled with parallel detection of additional hallmark events of apoptosis, they provide detailed and quantitative kinetic and positional insights into the signal transduction pathways that regulate cell death. Here we provide a brief introduction into BiFC- and FRET-based imaging of caspase activation and activity in single cells.

Mitochondria in cell death.

Apoptosis can be thought of as a signalling cascade that results in the death of the cell. Properly executed apoptosis is critically important for both development and homoeostasis of most animals. Accordingly, defects in apoptosis can contribute to the development of autoimmune disorders, neurological diseases and cancer. Broadly speaking, there are two main pathways by which a cell can engage apoptosis: the extrinsic apoptotic pathway and the intrinsic apoptotic pathway. At the centre of the intrinsic apoptotic signalling pathway lies the mitochondrion, which, in addition to its role as the bioenergetic centre of the cell, is also the cell's reservoir of pro-death factors which reside in the mitochondrial IMS (intermembrane space). During intrinsic apoptosis, pores are formed in the OMM (outer mitochondrial membrane) of the mitochondria in a process termed MOMP (mitochondrial outer membrane permeabilization). This allows for the release of IMS proteins; once released during MOMP, some IMS proteins, notably cytochrome c and Smac/DIABLO (Second mitochondria-derived activator of caspase/direct inhibitor of apoptosis-binding protein with low pI), promote caspase activation and subsequent cleavage of structural and regulatory proteins in the cytoplasm and the nucleus, leading to the demise of the cell. MOMP is achieved through the co-ordinated actions of pro-apoptotic members and inhibited by anti-apoptotic members of the Bcl-2 family of proteins. Other aspects of mitochondrial physiology, such as mitochondrial bioenergetics and dynamics, are also involved in processes of cell death that proceed through the mitochondria. Proper regulation of these mitochondrial functions is vitally important for the life and death of the cell and for the organism as a whole.

The BCL-2 family reunion.

B cell CLL/lymphoma-2 (BCL-2) and its relatives comprise the BCL-2 family of proteins, which were originally characterized with respect to their roles in controlling outer mitochondrial membrane integrity and apoptosis. Current observations expand BCL-2 family function to include numerous cellular pathways. Here we will discuss the mechanisms and functions of the BCL-2 family in the context of these pathways, highlighting the complex integration and regulation of the BCL-2 family in cell fate decisions.

Silencing of TMS1/ASC promotes resistance to anoikis in breast epithelial cells.

Ductal carcinoma in situ (DCIS) is characterized by ductal epithelial cells that have filled the luminal space of the breast duct and survive despite loss of extracellular matrix contact. In normal epithelial cells, the loss of such contact triggers a form of apoptosis known as detachment-induced apoptosis or "anoikis." TMS1/ASC is a bipartite adaptor molecule that participates in inflammatory and apoptotic signaling pathways. Epigenetic silencing of TMS1 has been observed in a significant proportion of human breast and other cancers, but the mechanism by which TMS1 silencing contributes to carcinogenesis is unknown. Here, we examined the role of TMS1 in anoikis. We found that TMS1 expression is induced in response to loss of substratum interactions in breast epithelial cells. siRNA-mediated knockdown of TMS1 leads to anoikis resistance, due in part to the persistent activation of extracellular signal-regulated kinase and an impaired ability to up-regulate the BH3-only protein Bim. We further show that the detachment-induced cleavage of procaspase-8, a newly described mediator of cellular adhesion, is significantly inhibited in the absence of TMS1. These data show a novel upstream role for TMS1 in the promotion of anoikis, and suggest that silencing of TMS1 may contribute to the pathogenesis of breast cancer by allowing epithelial cells to bypass cell death in the early stages of breast cancer development. This conclusion is supported by in vivo data showing that TMS1 is selectively down-regulated in the aberrant epithelial cells filling the lumen of the breast duct in a subset of primary DCIS lesions.