Tumor necrosis factor-mediated cell death: to break or to burst,that’s the question

In this review, we discuss the signal-transduction pathways of three major cellular responses induced by tumor necrosis factor (TNF): cell survival through NF-κB activation, apoptosis, and necrosis. Recruitment and activation of caspases plays a crucial role in the initiation and execution of TNF-induced apoptosis. However, experimental inhibition of caspases reveals an alternative cell death pathway, namely necrosis, also called necroptosis, suggesting that caspases actively suppress the latter outcome. TNF-induced necrotic cell death crucially depends on the kinase activity of receptor interacting protein serine-threonine kinase 1 (RIP1) and RIP3. It was recently demonstrated that ubiquitination of RIP1 determines whether it will function as a pro-survival or pro-cell death molecule. Deeper insight into the mechanisms that control the molecular switches between cell survival and cell death will help us to understand why TNF can exert so many different biological functions in the etiology and pathogenesis of human diseases.     

Nitric oxide can inhibit apoptosis or switch it into necrosis

Nitric oxide (NO) and its related molecules are important messengers that play central roles in pathophysiology. Redox modulation of thiol groups on protein cysteine residues by S-nitrosylation can modulate protein function. NO has emerged as a potent regulator of apoptosis in many cell types, either preventing cell death or driving an apoptotic response into a necrotic one. NO protects neuroblastoma cells from retinoid- and cisplatin-induced apoptosis, without significantly increasing necrotic cell damage. Nitrosylation of thiol groups of several critical factors may be important for cell survival. Indeed, S-nitrosylation of the active-site cysteine residue of apoptotic molecules, such as caspases and tissue transglutaminase, results in the inhibition of their catalytic activities and has important implications for the regulation of apoptosis by NO. On the other hand, NO is able to shift the anti-CD95- and ceramide-triggered apoptotic response of Jurkat T cells into necrotic cell death. In these apoptotic models, NO is therefore unable to solely inhibit cell death, indicating that it may act below the point of no return elicited by CD95-ligation and ceramide stimulation.     

Mitochondrial control of caspase-dependent and -independent cell death

Mitochondria control whether a cell lives or dies. The role mitochondria play in deciding the fate of a cell was first identified in the mid-1990s, because mitochondria-enriched fractions were found to be necessary for activation of death proteases, the caspases, in a cell-free model of apoptotic cell death. Mitochondrial involvement in apoptosis was subsequently shown to be regulated by Bcl-2, a protein that was known to contribute to cancer in specific circumstances. The important role of mitochondria in promoting caspase activation has therefore been a major focus of apoptosis research; however, it is also clear that mitochondria contribute to cell death by caspase-independent mechanisms. In this review, we will highlight recent findings and discuss the mechanism underlying the mitochondrial control of apoptosis and caspase-independent cell death.     

Crossing paths: interactions between the cell death machinery and growth factor survival signals

Cytokines and growth factors play a crucial role in the maintenance of haematopoietic homeostasis. They transduce signals that regulate the competing commitments of haematopoietic stem cells, quiescence or proliferation, retention of stem cell pluripotency or differentiation, and survival or demise. When the balance between these commitments and the requirements of the organisms is disturbed, particularly when it favours survival and proliferation, cancer may result. Cell death provoked by loss of growth factor signalling is regulated by the Bcl-2 family of apoptosis regulators, and thus survival messages transduced by growth factors must regulate the activity of these proteins. Many aspects of direct interactions between cytokine signalling and regulation of apoptosis remain elusive. In this review, we explore the mechanisms by which cytokines, in particular Interleukin-3 and granulocyte–macrophage colony-stimulating factor, promote cell survival and suppress apoptosis as models of how cytokine signalling and apoptotic pathways intersect.     

Proteases in apoptosis

The interleukin-1 β-converting enzyme (ICE)-like family proteases have recently been identified as key enzymes in apoptotic cell death. Among these proteases one can identify specific activities which may be involved in cytokine production or in resident protein cleavage. Several factors influence the constitutive apoptotic mechanism and may provide insight into the role of protease(s) in apoptosis. Although it appears that ICE family members play a most important role in promoting apoptotic cell death, evidence has been advanced that other proteases are also involved in sequential or parallel steps of apoptosis. Activation of a particular protease can lead to processing molecules either of the same or different proteases, leading to an activation of a protease cascade. Here we attempt to summarize the current thinking concerning these proteases and their involvement in apoptosis.     

Tyrosine 842 in the activation loop is required for full transformation by the oncogenic mutant FLT3-ITD

The type III receptor tyrosine kinase FLT3 is frequently mutated in acute myeloid leukemia. Oncogenic FLT3 mutants display constitutive activity leading to aberrant cell proliferation and survival. Phosphorylation on several critical tyrosine residues is known to be essential for FLT3 signaling. Among these tyrosine residues, Y842 is located in the so-called activation loop. The position of this tyrosine residue is well conserved in all receptor tyrosine kinases. It has been reported that phosphorylation of the activation loop tyrosine is critical for catalytic activity for some but not all receptor tyrosine kinases. The role of Y842 residue in FLT3 signaling has not yet been studied. In this report, we show that Y842 is not important for FLT3 activation or ubiquitination but plays a critical role in regulating signaling downstream of the receptor as well as controlling receptor stability. We found that mutation of Y842 in the FLT3-ITD oncogenic mutant background reduced cell viability and increased apoptosis. Furthermore, the introduction of the Y842 mutation in the FLT3-ITD background led to a dramatic reduction in in vitro colony forming capacity. Additionally, mice injected with cells expressing FLT3-ITD/Y842F displayed a significant delay in tumor formation, compared to FLT3-ITD expressing cells. Microarray analysis comparing gene expression regulated by FLT3-ITD versus FLT3-ITD/Y842F demonstrated that mutation of Y842 causes suppression of anti-apoptotic genes. Furthermore, we showed that cells expressing FLT3-ITD/Y842F display impaired activity of the RAS/ERK pathway due to reduced interaction between FLT3 and SHP2 leading to reduced SHP2 activation. Thus, we suggest that Y842 is critical for FLT3-mediated RAS/ERK signaling and cellular transformation.     

Simvastatin overcomes the resistance to serum withdrawal-induced apoptosis of lymphocytes from Alzheimer’s disease patients

Statins may exert beneficial effects on Alzheimer’s disease (AD) patients. Based on the antineoplastic and apoptotic effects of statins in a number of cell types, we hypothesized that statins may be able to protect neurons by controlling the regulation of cell cycle and/or apoptosis. A growing body of evidence indicates that neurodegeneration involves the cell-cycle activation in postmitotic neurons. Failure of cell-cycle control is not restricted to neurons in AD patients, but occurs in peripheral cells as well. For these reasons, we studied the role of simvastatin (SIM) on cell survival/death in lymphoblasts from AD patients. We report here that SIM induces apoptosis in AD lymphoblasts deprived of serum. SIM interacts with PI3K/Akt and ERK1/2 signaling pathways thereby decreasing the serum withdrawal-enhanced levels of the CDK inhibitor p21^Cip1 (p21) and restoring the vulnerability of AD cells to trophic factor deprivation.     

Role of noncoding RNAs in regulation of cardiac cell death and cardiovascular diseases

Loss of functional cardiomyocytes is a major underlying mechanism for myocardial remodeling and heart diseases, due to the limited regenerative capacity of adult myocardium. Apoptosis, programmed necrosis, and autophagy contribute to loss of cardiac myocytes that control the balance of cardiac cell death and cell survival through multiple intricate signaling pathways. In recent years, non-coding RNAs (ncRNAs) have received much attention to uncover their roles in cell death of cardiovascular diseases, such as myocardial infarction, cardiac hypertrophy, and heart failure. In addition, based on the view that mitochondrial morphology is linked to three types of cell death, ncRNAs are able to regulate mitochondrial fission/fusion of cardiomyocytes by targeting genes involved in cell death pathways. This review focuses on recent progress regarding the complex relationship between apoptosis/necrosis/autophagy and ncRNAs in the context of myocardial cell death in response to stress. This review also provides insight into the treatment for heart diseases that will guide novel therapies in the future.     

The cellular autophagy/apoptosis checkpoint during inflammation

Cell death is a major determinant of inflammatory disease severity. Whether cells live or die during inflammation largely depends on the relative success of the pro-survival process of autophagy versus the pro-death process of apoptosis. These processes interact and influence each other during inflammation and there is a checkpoint at which cells irrevocably commit to either one pathway or another. This review will discuss the concept of the autophagy/apoptosis checkpoint and its importance during inflammation, the mechanisms of inflammation leading up to the checkpoint, and how the checkpoint is regulated. Understanding these concepts is important since manipulation of the autophagy/apoptosis checkpoint represents a novel opportunity for treatment of inflammatory diseases caused by too much or too little cell death.     

Mechanisms controlling cellular suicide: role of Bcl-2 and caspases

Apoptosis is an essential and highly conserved mode of cell death that is important for normal development, host defense and suppression of oncogenesis. Faulty regulation of apoptosis has been implicated in degenerative conditions, vascular diseases, AIDS and cancer. Among the numerous proteins and genes involved, members of the Bcl-2 family play a central role to inhibit or promote apoptosis. In this article, we present up-to-date information and recent discoveries regarding biochemical functions of Bcl-2 family proteins, positive and negative interactions between these proteins, and their modification and regulation by either proteolytic cleavage or by cytosolic kinases, such as Raf-1 and stress-activated protein kinases. We have critically reviewed the functional role of caspases and the consequences of cleaving key substrates, including lamins, poly(ADP ribose) polymerase and the Rb protein. In addition, we have presented the latest Fas-induced signalling mechanism as a model for receptor-linked caspase regulation. Finally, the structural and functional interactions of Ced-4 and its partial mam malian homologue, apoptosis protease activating factor-1 (Apaf-1), are presented in a model which includes other Apafs. This model culminates in a caspase/Apaf regulatory cascade to activate the executioners of programmed cell death following cytochrome c release from the mitochondria of mammalian cells. The importance of these pathways in the treatment of disease is highly dependent on further characterization of genes and other regulatory molecules in mammals. Received 18 February 1998; accepted February 1998     

Apoptotic and necrotic cell death induced by death domain receptors

Apoptosis and necrosis are two distinct forms of cell death. Caspases are indispensable as initiators and effectors of apoptotic cell death and are involved in many of the morphological and biochemical features of apoptosis. Major changes in mitochondrial membrane integrity and release of proapoptotic factors, such as cytochrome c from the mitochondrial intermembrane space, play an important sensor and amplifying role during apoptotic cell death. In vitro studies of cell death in cell lines have revealed that inhibition of the classical caspase-dependent apoptotic pathway leads in several cases to necrotic cell death. Thus, the same cell death stimulus can result either in apoptotic or necrotic cell death, depending on the availability of activated caspase. Therefore, death domain receptors may initiate an active caspase-independent necrotic signaling pathway. In this review, we describe what is known about the apoptotic and necrotic cell death pathways. Principal elements of necrosis include mitochondrial oxidative phosphorylation, reactive oxygen production, and non-caspase proteolytic cascades depending on serine proteases, calpains, or cathepsins.     

Cytomegalovirus infection blocks apoptosis in cancer cells

Recent pathological findings reveal a higher frequency of human cytomegalovirus (HCMV) in tumor cells from different tumors compared with surrounding tissues. Experimental investigations suggest possible supportive effects of HCMV for tumor development and progression. One HCMV effect on tumor cells is the inhibition of apoptosis, leading to the promotion of tumor cell survival. Decreased sensitivity to treatment-induced tumor cell death is a major reason for failure of anticancer chemotherapy. HCMV infection interferes with both the intrinsic and extrinsic cellular apoptosis pathways. HCMV promotes cell survival signaling influencing the tumor suppressor p53 and its relative p73, and stimulates the antiapoptotic Ras/Raf/MEK/Erk- and PI-3K-signaling pathways. Antiapoptotic effects mediated by HCMV are inhibited by antiviral treatment in cell culture. Therefore, a better understanding of the influence of HCMV infection on tumor cell apoptosis might translate into improved anti-cancer therapy.Received 10 November 2003; received after revision 22 December 2003; accepted 14 January 2004     

Polyamines in cell growth and cell death: molecular mechanisms and therapeutic applications

Polyamines are aliphatic cations with multiple functions and are essential for life. Cellular polyamine levels are regulated by multiple pathways such as synthesis from amino acid precursors, cellular uptake mechanisms that salvage polyamines from diet and intestinal microorganisms, as well as stepwise degradation and efflux. Investigations using polyamine biosynthetic inhibitors indicate that alterations in cellular polyamine levels modulate normal and cancer cell growth. Studies using transgenic mice overexpressing polyamine biosynthetic enzymes support a role of polyamines in carcinogenesis. Many, if not all, signal transduction pathways intersect with polyamine biosynthetic pathways and the regulation of intracellular polyamine levels. Direct binding of polyamines to DNA and their ability to modulate DNA-protein interactions appear to be important in the molecular mechanisms of polyamine action in cell proliferation. Consistent with the role of polyamines as facilitators of cell growth, several studies have shown their ability to protect cells from apoptosis. However, polyamines also have a role in facilitating cell death. The basis of these diverse cellular responses is currently not known. Cell death response might be partly mediated by the production of hydrogen peroxide during polyamine catabolism. In addition, the ability of polyamines to alter DNA-protein and protein-protein interactions might be disruptive to cellular functions, when abnormally high levels are accumulated due to defects in polyamine catabolic or efflux pathways. A large body of data indicates that polyamine pathway can be a molecular target for therapeutic intervention in several types cancers. Inhibitors of biosynthesis, polyamine analogues as well as oligonucleotide/polyamine analogue combinations are promising drug candidates for chemoprevention and/or treatment of cancer.