Apoptosis and liver diseases

Regulation of homeostasic balance between cell proliferation and cell death, called apoptosis, is essential for development and maintenance of multicellular organisms. Recent research into the molecular mechanisms of apoptosis has revealed that apoptosis is a genetically and evolutionarily conserved process that can become deranged when the components of the cellular apoptotic machinery are mutated, perturbated by viral gene products or present in inappropriated quantities. Analysis of the regulatory apoptotic pathways has led to a better understanding of the etiology and pathogenesis of many human diseases, notably cancers, infectious diseases or autoimmune diseases. Our understanding of the regulation of apoptosis in health and disease is far from complete and the use of understanding into new therapeutic modalities has only begun to be approached.     

Systems biology of yeast cell death

Programmed cell death (PCD) (including apoptosis) is an essential process, and many human diseases of high prevalence such as neurodegenerative diseases and cancer are associated with deregulations in the cell death pathways. Yeast Saccharomyces cerevisiae, a unicellular eukaryotic organism, shares with multicellular organisms (including humans) key components and regulators of the PCD machinery. In this article, we review the current state of knowledge about cell death networks, including the modeling approaches and experimental strategies commonly used to study yeast cell death. We argue that the systems biology approach will bring valuable contributions to our understanding of regulations and mechanisms of the complex cell death pathways.     

Regulation of anti‐apoptotic BCL2‐proteins by non‐canonical interactions: The next step forward or two steps back?

All aspects of cellular biology affect the process of regulated cell death, or apoptosis, and disruption of this process is a causative event in many diseases. Therefore, a comprehensive understanding of all pathways that regulate apoptosis would increase our knowledge of basic cellular functions, as well as the etiologies of many diseases. In turn, we may be able to use this knowledge to better treat patients with diseases, including cancer. Although the basic signaling pathway that regulates apoptosis has been known for over 10 years, we still have much to learn about the upstream signaling components that can directly regulate the core apoptosis machinery. The focus of this review will be to direct attention to non-canonical regulators of the BCL2-family of proteins, especially our void of understanding of such interactions, and the controversy that surrounds some such interactions.     

Apoptosis-regulating proteins as targets for drug discovery.

Defects in the regulation of apoptosis (programmed cell death) contribute to many diseases, including pathologies associated with cell loss (e.g. stroke, heart failure, neurodegeneration and AIDS), and disorders characterized by a failure to eliminate harmful cells (e.g. cancer, autoimmunity). Apoptosis is caused by activation of intracellular proteases, known as caspases, which are responsible directly or indirectly for the morphological and biochemical events that characterize the apoptotic cell. Numerous caspase regulators have been discovered, which respond to environmental stimuli and influence the decision of cell death and survival. Knowledge of the molecular details of apoptosis regulation, and the three-dimensional structures of proteins constituting the apoptosis core machinery has revealed new strategies for identifying small-molecule drugs that could one day yield more effective treatments for a wide variety of illnesses.     

Conversion of Bcl-2 from protector to killer by interaction with nuclear orphan receptor Nur77/TR3

The Bcl-2 family proteins are key regulators of apoptosis in human diseases and cancers. Though known to block apoptosis, Bcl-2 promotes cell death through an undefined mechanism. Here, we show that Bcl-2 interacts with orphan nuclear receptor Nur77 (also known as TR3), which is required for cancer cell apoptosis induced by many antineoplastic agents. The interaction is mediated by the N-terminal loop region of Bcl-2 and is required for Nur77 mitochondrial localization and apoptosis. Nur77 binding induces a Bcl-2 conformational change that exposes its BH3 domain, resulting in conversion of Bcl-2 from a protector to a killer. These findings establish the coupling of Nur77 nuclear receptor with the Bcl-2 apoptotic machinery and demonstrate that Bcl-2 can manifest opposing phenotypes, induced by interactions with proteins such as Nur77, suggesting novel strategies for regulating apoptosis in cancer and other diseases.     

Drug discovery opportunities from apoptosis research

Cell suicide is a normal process that participates in a wide variety of physiological processes, including tissue homeostasis, immune regulation, and fertility. Physiological cell death typically occurs by apoptosis, as opposed to necrosis. Defects in apoptotic cell-death regulation contribute to many diseases, including disorders associated with cell accumulation (e.g. cancer, autoimmunity, inflammation and restenosis) or where cell loss occurs (e.g. stroke, heart failure, neurodegeneration, AIDS and osteoporosis). At the center of the apoptosis machinery is a family of intracellular proteases, known as 'caspases', that are responsible directly or indirectly for the morphological and biochemical events that characterize apoptosis. Multiple positive and negative regulators of these cell-death proteases have been discovered in the genomes of mammals, amphibians, insects, nematodes, and other animal species, as well as a variety of animal viruses. Inputs from signal-transduction pathways into the core of the cell-death machinery have also been identified, demonstrating ways of linking environmental stimuli to cell-death responses or cell-survival maintenance. Knowledge of the molecular mechanisms of apoptosis has provided important insights into the causes of multiple diseases where aberrant cell-death regulation occurs and has revealed new approaches for identifying small-molecule drugs for more effectively treating these illnesses.     

Autophagy and apoptosis: what is the connection?

The therapeutic potential of autophagy for the treatment cancer and other diseases is beset by paradoxes stemming from the complexity of the interactions between the apoptotic and autophagic machinery. The simplest question of how autophagy acts as both a protector and executioner of cell death remains the subject of substantial controversy. Elucidating the molecular interactions between the processes will help us understand how autophagy can modulate cell death, whether autophagy is truly a cell death mechanism, and how these functions are regulated. We suggest that, despite many connections between autophagy and apoptosis, a strong causal relationship wherein one process controls the other, has not been demonstrated adequately. Knowing when and how to modulate autophagy therapeutically depends on understanding these connections.     

Splicing DNA-damage responses to tumour cell death

The ability of a tumour cell to evade programmed cell death (apoptosis) is crucial in the development of cancer. The process of apoptosis is complex and involves the careful interplay of a host of signalling molecules. Cellular stresses, such as DNA-damage, can initiate apoptosis through multiple pathways, all of which eventually lead to eradication of damaged cells that may otherwise go on to form a tumour. Moreover, the relevance of this to combating cancer is very strong since several therapeutic agents used to treat malignant disease utilize the cells' apoptotic machinery. The purpose of this review is to provide an insight into what we know about how apoptosis is initiated by DNA-damaging agents, how pro- and anti-apoptotic signals converge in the execution of cell death, and how such mechanisms can be perturbed in cancer.     

Sphingolipids: regulators of crosstalk between apoptosis and autophagy

Apoptosis and autophagy are two evolutionarily conserved processes that maintain homeostasis during stress. Although the two pathways utilize fundamentally distinct machinery, apoptosis and autophagy are highly interconnected and share many key regulators. The crosstalk between apoptosis and autophagy is complex, as autophagy can function to promote cell survival or cell death under various cellular conditions. The molecular mechanisms of crosstalk are beginning to be elucidated and have critical implications for the treatment of various diseases, such as cancer. Sphingolipids are a class of bioactive lipids that mediate many key cellular processes, including apoptosis and autophagy. By targeting several of the shared regulators, sphingolipid metabolites differentially regulate the induction of apoptosis and autophagy. Importantly, individual sphingolipid species appear to “switch” autophagy toward cell survival (e.g., sphingosine-1-phosphate) or cell death (e.g., ceramide, gangliosides). This review assesses the current understanding of sphingolipid-induced apoptosis and autophagy to address how sphingolipids mediate the “switch” between the cell survival and cell death. As sphingolipid metabolism is frequently dysregulated in cancer, sphingolipid-modulating agents, or sphingomimetics, have emerged as a novel chemotherapeutic strategy. Ultimately, a greater understanding of sphingolipid-mediated crosstalk between apoptosis and autophagy may be critical for enhancing the chemotherapeutic efficacy of these agents.     

Cell death and human intestinal protozoa: a brief overview

Protozoan programmed cell death or apoptosis is an important factor in the survival of the parasite and its pathogenicity. The most amazing aspect of protozoan cell death is in its molecular architecture. To date, protozoa lack most of the components of the highly complex cell death machinery studied in multicellular organisms. Hence the unique apoptotic machinery in protozoa can be exploited for the development of therapeutic drugs and diagnostic markers. This review focuses on human intestinal protozoa undergoing cell death and inducing or inhibiting host cell apoptosis. The first part of this review focuses on intestinal protozoa that undergo PCD under various stress conditions. The second part focuses on protozoa that induce or inhibit PCD in their host cell. Although these intestinal parasites differ in their mechanism of infection and intracellular localization, they may activate conserved cell death pathways within themselves and in the host cell. Understanding conserved cell death pathways in the intestinal protozoa and their host-parasite PCD relationship may lead to drug targets which can be used for a broad range of parasitic diseases.     

Molecular mechanisms of cell death and phagocytosis in Drosophila

The genetic tools available in Drosophila have facilitated our understanding of how apoptosis is regulated and executed in the context of the developing organism. All embryonic apoptosis is initiated by the activity of three genes, rpr, grim and hid. Each of these genes is independently regulated, allowing developmental apoptosis to be finely controlled. These initiators in turn activate the core apoptotic machinery, including the caspases. Drosophila counterparts to other conserved components of the apoptotic machinery have been recently identified, and we discuss how these may be integrated into the process of normal developmentally regulated cell death. We also outline the role that phagocytosis plays in the final stages of apoptosis and consider the molecular mechanisms guiding the elimination of apoptotic corpses.     

Scission,spores, and apoptosis: a proposal for the evolutionary origin of mitochondria in cell death induction

Mitochondria fragment prior to caspase activation during many pathways of apoptosis. Inhibition of the machinery that normally regulates mitochondrial morphology in healthy cells inhibits the fission that occurs during apoptosis and actually delays the process of cell death. Interestingly, there are certain parallels between mitochondrial fission and bacterial sporulation. As bacterial sporulation can be considered a stress response we suggest that a primordial stress response of endosymbiont mitochondrial progenitors may have been adopted for the stress response of early eukaryotes. Thus, the mitochondrial fission process may represent an early stress response of primitive mitochondria that could have integrated the stress signals and acted as an initial sensor for the eukaryotic response system. The fact that mitochondria fragment during apoptosis using the machinery descended from or that superceded the bacterial stress response of sporulation is consistent with this hypothesis. This hypothesis would explain why what is generally considered the "power house" of the cell came to integrate the cell death response and regulate apoptosis.     

pRb and the cdks in apoptosis and the cell cycle

Apoptosis is a fundamental biological process present in metazoan cells. Linking apoptosis to the cell cycle machinery provides a mechanism to maintain proper control of cell proliferation in a multicellular organism. pRb and the cyclin-dependent kinases may have dual roles as integral components of the cell cycle and regulators of apoptosis. In many instances manipulation of the cell cycle through these molecules can induce or inhibit apoptosis. Recent studies also identify pRb as a substrate for an apoptotic protease; however, other cell cycle components are not known substrates. While it is clear that many common molecules can affect cell proliferation and cell death, the universality of any one cell cycle molecule in apoptosis has yet to be determined.     

A New View of the Lethal Apoptotic Pore

Cell death by apoptosis is indispensable for proper development and tissue homeostasis in all multicellular organisms, and its deregulation plays a key role in cancer and many other diseases. A crucial event in apoptosis is the formation of protein-permeable pores in the outer mitochondrial membrane that release cytochrome c and other apoptosis-promoting factors into the cytosol. Research efforts over the past two decades have established that apoptotic pores require BCL-2 family proteins, with the proapoptotic BAX-type proteins being direct effectors of pore formation. Accumulating evidence indicates that other cellular components also cooperate with BCL-2 family members to regulate the apoptotic pore. Despite this knowledge, the molecular pathway leading to apoptotic pore formation at the outer mitochondrial membrane and the precise nature of this outer membrane pore remain enigmatic. In this issue of PLOS Biology, Kushnareva and colleagues describe a novel kinetic analysis of the dynamics of BAX-dependent apoptotic pore formation recapitulated in native mitochondrial outer membranes. Their study reveals the existence of a hitherto unknown outer mitochondrial membrane factor that is critical for BAX-mediated apoptotic pore formation, and challenges the currently popular view that the apoptotic pore is a purely proteinaceous multimeric assembly of BAX proteins. It also supports the notion that membrane remodeling events are implicated in the formation of a lipid-containing apoptotic pore.Apoptosis is the orderly sequence of events that leads to the death of a cell without releasing harmful substances into the surrounding tissue; it is indispensable for normal embryonic development and maintenance of healthy tissues in all multicellular organisms and important in many pathologies. The death of neurons and lymphocytes by apoptosis, for example, contributes to neurodegeneration and AIDS, respectively. By ensuring the death of damaged cells, apoptosis also plays key roles in cancer prevention and in successful cancer treatment. Over 25 years of apoptosis research have led to the broadly accepted notion that mitochondria, traditionally viewed as the “powerhouses” of the cell, are also intimately linked to cell death.Apoptosis can be initiated either by the activation of cell-surface-expressed death receptors or by diverse intracellular signals that impinge on the mitochondria. In vertebrates, the commitment step in the mitochondrial pathway of apoptosis is the assembly of a supramolecular structure called the apoptotic pore in the outer mitochondrial membrane [1]. This outer membrane pore allows for rapid diffusion out of the mitochondria of cytochrome c and other proteins that promote the irreversible dismantling of the cell. Despite intense research efforts, our understanding of the molecular machinery and mechanisms implicated in this crucial aspect of apoptosis is still incomplete.     

Mitochondria: regulating the inevitable

Apoptosis is a form of programmed cell death important in the development and tissue homeostasis of multicellular organisms. Abnormalities in cell death control can lead to a variety of diseases, including cancer and degenerative disorders. Hence, the process of apoptosis is tightly regulated through multiple independent signalling pathways that are initiated either from triggering events within the cell or at the cell surface. In recent years, mitochondria have emerged as the central components of such apoptotic signalling pathways and are now known to control apoptosis through the release of apoptogenic proteins. In this review we aim to give an overview of the role of the mitochondria during apoptosis and the molecular mechanisms involved.     

Regulation of caspase activation in apoptosis: implications for transformation and drug resistance

Recent developments in the apoptosis field have uncovered a family of cysteine proteases, the Caspases, that act as signalling components as well as effectors of the cell death machinery. Caspases are constitutively present as inactive precursors within most cells and undergo proteolytic processing in response to diverse death-inducing stimuli to initiate the death programme. Active caspases can process other caspases of the same type as well as process caspases further downstream in the pathway that ultimately leads to collapse of the cell. This cellular collapse is thought to occur as a consequence of caspase-mediated cleavage of a diverse array of cellular substrates. Regulation of entry into the death programme is controlled at a number of levels by members of the Bcl-2 family, as well as by other cell death regulatory proteins. Recent data has shed light upon the mechanism of action of these regulatory molecules and suggests that the point of caspase activation is a major checkpoint in the cell death programme. Because many transformed cell populations possess derangements in cell death-regulatory genes, such as bcl-2, such cells frequently exhibit elevated resistance to cytotoxic chemotherapy. Thus, a deeper understanding of how apoptosis is normally regulated has therapeutic implications for disease states where the normal controls on the cell death machinery have been subverted. This revised version was published online in August 2006 with corrections to the Cover Date.     

Why do Neurons Enter the Cell Cycle?

Increasing evidence indicates that postmitotic, terminally differentiated neurons activate the cell cycle before death. The purpose of this cell cycle activation, however, remains elusive. In proliferating cells, cell cycle machinery is a major contributor to the DNA damage response, which is comprised of growth arrest. In quiescent cells such as terminally differentiated neurons, cell cycle-associated events may also be part of the DNA damage response. A link between DNA damage and repair, cell cycle regulation and cell death is becoming increasingly recognized for cycling cells but remains elusive for quiescent cells. Neurons are particularly susceptible to oxidative stress due to the high rate of oxidative metabolism in the brain and the low level of antioxidant enzymes compared to other somatic tissues. This is supported by fact that the intracellular end point of many neurotoxic stimuli is oxidative stress, which also represents a major cause of the neuropathology underlying a variety of neurodegenerative diseases. DNA is perhaps the major target of oxyradicals. Thus, oxidative stress may cause DNA damage, which is countered by a complex defense mechanism, the DNA damage response, which involves not only the elimination of DNA damage, but its coordination with other cellular processes such as cell-cycle progression, together directing to preserve genomic integrity. The function of such response is the removal of DNA damage by DNA repair pathways, or the elimination of damaged cells via apoptosis. The present review discusses the idea that the cell cycle machinery is a critical element of the DNA damage response not only in cycling, but also quiescent cells, and may bear the same function: to repair the damage or initiate apoptosis if the damage is too extensive to be repaired.     

The viral nucleocapsid protein of transmissible gastroenteritis coronavirus (TGEV) is cleaved by caspase-6 and -7 during TGEV-induced apoptosis

The transmissible gastroenteritis coronavirus (TGEV), like many other viruses, exerts much of its cytopathic effect through the induction of apoptosis of its host cell. Apoptosis is coordinated by a family of cysteine proteases, called caspases, that are activated during apoptosis and participate in dismantling the cell by cleaving key structural and regulatory proteins. We have explored the caspase activation events that are initiated upon infection of the human rectal tumor cell line HRT18 with TGEV. We show that TGEV infection results in the activation of caspase-3, -6, -7, -8, and -9 and cleavage of the caspase substrates eIF4GI, gelsolin, and alpha-fodrin. Surprisingly, the TGEV nucleoprotein (N) underwent proteolysis in parallel with the activation of caspases within the host cell. Cleavage of the N protein was inhibited by cell-permeative caspase inhibitors, suggesting that this viral structural protein is a target for host cell caspases. We show that the TGEV nucleoprotein is a substrate for both caspase-6 and -7, and using site-directed mutagenesis, we have mapped the cleavage site to VVPD(359) downward arrow. These data demonstrate that viral proteins can be targeted for destruction by the host cell death machinery.     

Interaction of viral proteins with host cell death machinery

In recent years, intense research has been directed towards understanding molecular mechanisms involved in viral pathogenesis. It is now known that many viruses manipulate host defense mechanisms to prevent apoptosis in order to maximize viral replication. Towards the end of their replication cycle, certain viruses direct the synthesis of proteins that induce apoptosis or cell lysis thereby facilitating viral release from the cell. The present review summarizes the current understanding of interactions between viral proteins and the host cell death machinery.