Apoptosis: one of the mechanisms that maintains unresponsiveness of the intestinal mucosal immune system

Intestinal mucosa is constantly exposed to environmental AGS: Activation of lamina propria (LP) T cells by luminal Ags may lead to the production of inflammatory cytokines and subsequent mucosal inflammation and tissue damage. However, in normal circumstances, LP T cells do not respond to antigenic stimulation. The mechanisms of this unresponsiveness in healthy subjects are not fully understood. In this study, we found by in vivo analysis that, except for T cells in lymph nodules of the mucosa, 15% of LP T cells underwent apoptosis in normal individuals. In contrast, there was a marked reduction in apoptosis of LP T cells in patients with inflammatory bowel disease (Crohn's disease and ulcerative colitis) and those with specific colitis. Our findings suggest that apoptosis might be a mechanism that turns off mucosal T cell responses to environmental Ags in healthy subjects, and resistance to apoptosis could be an important cause of mucosal immune dysregulation and tissue inflammation in colitis.     

Amelioration of experimental acute pancreatitis with Dachengqi Decoction via regulation of necrosis-apoptosis switch in the pancreatic acinar cell

Severity of acute pancreatitis contributes to the modality of cell death. Pervious studies have demonstrated that the herb medicine formula "Dachengqi Decoction" (DCQD) could ameliorate the severity of acute pancreatitis. However, the biological mechanisms governing its action of most remain unclear. The role of apoptosis/necrosis switch within acute pancreatitis has attracted much interest, because the induction of apoptosis within injured cells might suppress inflammation and ameliorate the disease. In this study, we used cerulein (10(-8) M)-stimulated AR42J cells as an in vitro model of acute pancreatitis and retrograde perfusion into the biliopancreatic duct of 3.5% sodium taurocholate as an in vivo rat model. After the treatment of DCQD, cell viability, levels of apoptosis and necrosis, reactive oxygen species positive cells, serum amylase, concentration of nitric oxide and inducible nitric oxide syntheses, pancreatic tissue pathological score and inflammatory cell infiltration were tested. Pretreatment with DCQD increased cell viability, induced apoptosis, decreased necrosis and reduced the severity of pancreatitis tissue. Moreover, treatment with DCQD reduced the generation of reactive oxygen species in AR42J cells but increased the concentration of nitric oxide of pancreatitis tissues. Therefore, the regulation of apoptosis/necrosis switch by DCQD might contribute to ameliorating the pancreatic inflammation and pathological damage. Further, the different effect on reactive oxygen species and nitric oxide may play an important role in DCQD-regulated apoptosis/necrosis switch in acute pancreatitis.     

Interactions between CD47 and thrombospondin reduce inflammation

CD47 on the surface of T cells was shown in vitro to mediate either T cell activation or, in the presence of high amounts of thrombospondin (TSP), T cell apoptosis. We report here that CD47-deficient mice, as well as TSP-1 or TSP-2-deficient mice, sustain oxazolone-induced inflammation for more than four days, whereas wild-type mice reduce the inflammation within 48 h. We observe that prolonged inflammation in CD47-, TSP-1-, or TSP-2-deficient mice is accompanied by a local deficiency of T cell apoptosis. Finally, we show that upon activation normal T cells increase the expression of the proapoptotic Bcl-2 family member BNIP3 (Bcl-2/adenovirus E1B 19-kDa interacting protein) and undergo CD47-mediated apoptosis. This finding is consistent with our previous demonstration of a physical interaction between BNIP3 and CD47 that inhibits BNIP3 degradation by the proteasome, sensitizing T cells to CD47-induced apoptosis. Overall, these results reveal an important role in vivo for this new CD47/BNIP3 pathway in limiting inflammation by controlling the number of activated T cells.     

Epithelial cell apoptosis and neutrophil recruitment in acute lung injury-a unifying hypothesis? What we have learned from small interfering RNAs

In spite of protective ventilatory strategies, Acute Lung Injury (ALI) remains associated with high morbidity and mortality. One reason for the lack of therapeutic options might be that ALI is a co-morbid event associated with a diverse family of diseases and, thus, may be the result of distinct pathological processes. Among them, activated neutrophil- (PMN-) induced tissue injury and epithelial cell apoptosis mediated lung damage represent two potentially important candidate pathomechanisms that have been put forward. Several approaches have been undertaken to test these hypotheses, with substantial success in the treatment of experimental forms of ALI. With this in mind, we will summarize these two current hypotheses of ALI briefly, emphasizing the role of apoptosis in regulating PMN and/or lung epithelial cell responses. In addition, the contribution that Fas-mediated inflammation may play as a potential biological link between lung cell apoptosis and PMN recruitment will be considered, as well as the in vivo application of small interfering RNA (siRNA) as a novel approach to the inhibition of ALI and its therapeutic implications.     

The induction by human interleukin-6 of apoptosis in the promonocytic cell line U937 and human neutrophils.

Apoptosis of neutrophils at sites of inflammation in vivo is thought to lead to their recognition and safe elimination by macrophages. Little is known, however, about the regulation of apoptosis in myeloid cells. We report here that the human promonocytic leukemic cell line, U937, and mature human neutrophils can be induced to become apoptotic when cultured with interleukin-6. Apoptosis of U937 cells, assessed morphologically and by the presence of DNA fragmentation, was increased significantly in a dose-dependent fashion by concentrations of 0.5-100 ng/ml interleukin-6. Apoptosis of U937 cells was evident after 48 h of incubation with 20 ng/ml interleukin-6, and the effect was eliminated by adsorption of interleukin-6 with a specific monoclonal antibody. Apoptosis was not evident in the presence of the differentiating agent phorbol 12-myristate 13 acetate; the induction of apoptosis in U937 cells was not therefore a consequence of differentiation. Apoptosis of mature neutrophils was enhanced after 24 h in culture with interleukin-6. Interleukin-6 might be an important factor in the normal resolution of inflammation through the induction of apoptosis of neutrophils.     

Simultaneous activation of apoptosis and inflammation in pathogenesis of septic shock: a hypothesis

Sepsis, a widely prevalent disease with increasing morbidity and mortality, is thought to result from uncontrolled inflammatory responses to microbial infection and/or components. However, failure of several experimental anti-inflammatory therapies has necessitated re-evaluation of the paradigm underlying the pathogenesis of this complex disorder. Apoptotic cell death forms a second dominant feature of septic shock in patients and animal models. Anti-apoptotic strategies may protect animals from septic death. However, simultaneous occurrence of apoptosis and inflammation is necessary for septic death. At the cellular level, apoptosis plays a central role in the development of the lymphoid system and regulation of immune responses. Immune activation renders cells refractory to apoptosis while apoptosis of activated lymphocytes is an important immunoregulatory mechanism. Factors such as complement factor 5a, caspase-1 and mitogen-activated protein kinase, which participate in apoptosis as well as pro-inflammatory pathways, may be responsible for simultaneous activation of apoptosis and inflammation in sepsis. Further identification of other similar biochemical events capable of co-activating inflammation and apoptosis may provide new targets for therapy of this hitherto untreatable disease.     

Differentiation of the Gastric Mucosa IV. Role of trefoil peptides and IL-6 cytokine family signaling in gastric homeostasis

Gastric trefoil peptides mediate mucosal repair by stimulating cell migration, inhibiting apoptosis and inflammation, and likely augmenting the barrier function of mucus. One of these, tff1, is a gastric-specific tumor suppressor gene, which when repressed is associated with gastric cancer progression. IL-6 family cytokines play an important role in maintaining gastric homeostasis by regulating tff1 and other mediators of mucosal proliferation, inflammation, angiogenesis, and apoptosis. In this review the signaling cascades downstream of the common IL-6 cytokine family coreceptor gp130 that contribute to control of this homeostasis are described, as are the pathological outcomes of imbalancing these pathways.     

Sirtuin 6 protects human retinal pigment epithelium cells from LPS-induced inflammation and apoptosis partly by regulating autophagy

ABSTRACT

Lipopolysaccharides (LPS)-induced retinal inflammation is an important factor in retinal diseases. This study was aimed to investigate the effect of Sirt6 on LPS-induced retinal injury. ARPE-19 cells were incubated with LPS to induce inflammation. The cell viability was determined using CCK-8 assay. The mRNA level and protein expression of corresponding genes was detected using qRT-PCR and western blot, respectively. The production of inflammatory cytokines was measured using ELISA kit. The levels of oxidative stress-related factors were measured using their detection kits. Cell apoptosis was observed using TUNEL assay. The results showed that Sirt6 was downregulated after LPS treatment. Sirt6 strengthened LPS-induced autophagy by promoting the expression of LC3II/I, beclin1 and ATG5. Sirt6 treatment significantly inhibited LPS-induced inflammation, oxidative stress and cell apoptosis, which was then partly abolished by 3 MA. These results suggest Sirt6 to be an important regulator for LPS-induced inflammation, oxidative stress, and apoptosis partly by regulating cell autophagy.     

Molecular mechanisms underlying delayed apoptosis in neutrophils from multiple trauma patients with and without sepsis

Delayed neutrophil apoptosis and overshooting neutrophil activity contribute to organ dysfunction and subsequent organ failure in sepsis. Here, we investigated apoptotic signaling pathways that are involved in the inhibition of spontaneous apoptosis in neutrophils isolated from major trauma patients with uneventful outcome as well as in those with sepsis development. DNA fragmentation in peripheral blood neutrophils showed an inverse correlation with the organ dysfunction at d 10 after trauma in all patients, supporting the important role of neutrophil apoptosis regulation for patient's outcome. The expression of the antiapoptotic Bcl-2 protein members A1 and Mcl-1 were found to be diminished in the septic patients at d 5 and d 10 after trauma. This decrease was also linked to an impaired intrinsic apoptosis resistance, which has been previously shown to occur in neutrophils during systemic inflammation. In patients with sepsis development, delayed neutrophil apoptosis was found to be associated with a disturbed extrinsic pathway, as demonstrated by reduced caspase-8 activity and Bid truncation. Notably, the expression of Dad1 protein, which is involved in protein N-glycosylation, was significantly increased in septic patients at d 10 after trauma. Taken together, our data demonstrate that neutrophil apoptosis is regulated by both the intrinsic and extrinsic pathway, depending on patient's outcome. These findings might provide a molecular basis for new strategies targeting cell death pathways in apoptosis-resistant neutrophils during systemic inflammation.     

Apoptosis of vascular smooth muscle cells induces features of plaque vulnerability in atherosclerosis

Vascular smooth muscle cell (VSMC) apoptosis occurs in many arterial diseases, including aneurysm formation, angioplasty restenosis and atherosclerosis. Although VSMC apoptosis promotes vessel remodeling, coagulation and inflammation, its precise contribution to these diseases is unknown, given that apoptosis frequently accompanies vessel injury or alterations to flow. To study the direct consequences of VSMC apoptosis, we generated transgenic mice expressing the human diphtheria toxin receptor (hDTR, encoded by HBEGF) from a minimal Tagln (also known as SM22alpha) promoter. Despite apoptosis inducing loss of 50-70% of VSMCs, normal arteries showed no inflammation, reactive proliferation, thrombosis, remodeling or aneurysm formation. In contrast, VSMC apoptosis in atherosclerotic plaques of SM22alpha-hDTR Apoe-/- mice induced marked thinning of fibrous cap, loss of collagen and matrix, accumulation of cell debris and intense intimal inflammation. We conclude that VSMC apoptosis is 'silent' in normal arteries, which have a large capacity to withstand cell loss. In contrast, VSMC apoptosis alone is sufficient to induce features of plaque vulnerability in atherosclerosis. SM22alpha-hDTR Apoe-/- mice may represent an important new model to test agents proposed to stabilize atherosclerotic plaques.     

Apoptosis in atherosclerosis: focus on oxidized lipids and inflammation

An increasing body of evidence from both animal models and human specimens suggests that apoptosis or programmed cell death is a major event in the pathophysiology of atherosclerosis. Although the significance of apoptosis in atherosclerosis remains unclear, it has been proposed that apoptotic cell death contributes to plaque instability, rupture and thrombus formation. Biochemical and genetic analyses of apoptosis provide an increasingly detailed picture of the intracellular signaling pathways involved. Nevertheless, it remains to be determined whether apoptosis can become a clinically important approach to modulate plaque progression. In this review, we have outlined some of the most recent results concerning apoptosis in atherosclerosis with a special focus on oxidized lipids, inflammation and therapeutic regulation of the apoptotic cell death process.     

Galectin-4 controls intestinal inflammation by selective regulation of peripheral and mucosal T cell apoptosis and cell cycle

Galectin-4 is a carbohydrate-binding protein belonging to the galectin family. Here we provide novel evidence that galectin-4 is selectively expressed and secreted by intestinal epithelial cells and binds potently to activated peripheral and mucosal lamina propria T-cells at the CD3 epitope. The carbohydrate-dependent binding of galectin-4 at the CD3 epitope is fully functional and inhibited T cell activation, cycling and expansion. Galectin-4 induced apoptosis of activated peripheral and mucosal lamina propria T cells via calpain-, but not caspase-dependent, pathways. Providing further evidence for its important role in regulating T cell function, galectin-4 blockade by antisense oligonucleotides reduced TNF-alpha inhibitor induced T cell death. Furthermore, in T cells, galectin-4 reduced pro-inflammatory cytokine secretion including IL-17. In a model of experimental colitis, galectin-4 ameliorated mucosal inflammation, induced apoptosis of mucosal T-cells and decreased the secretion of pro-inflammatory cytokines. Our results show that galectin-4 plays a unique role in the intestine and assign a novel role of this protein in controlling intestinal inflammation by a selective induction of T cell apoptosis and cell cycle restriction. Conclusively, after defining its biological role, we propose Galectin-4 is a novel anti-inflammatory agent that could be therapeutically effective in diseases with a disturbed T cell expansion and apoptosis such as inflammatory bowel disease.     

Voices from beyond the grave: The impact of apoptosis on the microenvironment

Programmed cell death, in particular apoptosis, has vital functions in every healthy organism. In a highly regulated manner cells which are no longer needed or are harmful to the organism undergo suicide. More than just the mere elimination of a cell, apoptosis is increasingly being recognized performing important roles in cellular communication with the microenvironment. These interactions with surrounding cells can have various, and sometimes competing outcomes. Apoptotic cells can promote survival, proliferation and inflammation, but depending on the context also prevent survival and inflammation. In this review, we will summarize the emerging literature on how dying cells can transfer information to their neighbours, and which outcomes this communication has for the whole tissue.     

Dendritic cell co-transfected with FasL and allergen genes induces T cell tolerance and decreases airway inflammation in allergen induced murine model

Dendritic cell (DC) displays tremendous functional plasticity in response to antigens and plays important roles in inducing immune tolerance. In this study, we investigated the effects of immature DC (imDC) co-transfected with FasL and allergen Der p2 genes (FasL-Der p2-DC) on inducing immune tolerance and modulating airway inflammation of Der p2-induced allergic mice. Moreover, we compared the effects of FasL-Der p2-DC with FasL-transfected imDC (FasL-DC) and Der p2-transfected imDC (Der p2-DC) respectively. Results showed that FasL-Der p2-DC and Der p2-pulsed FasL-DC induced T cell unresponsiveness to Der p2 via apoptosis. Der p2-DC could induce T cell hyporesponsiveness to Der p2. FasL-Der p2-DC, FasL-DC and Der p2-DC could inhibit Th2 response and reduce allergic airway inflammation. FasL-Der p2-DC was more effective than FasL-DC and Der p2-DC, respectively. These results demonstrate that FasL and allergen genes co-expressing DC might be a promising approach to allergy therapy.     

Saikosaponin C inhibits lipopolysaccharide-induced apoptosis by suppressing caspase-3 activation and subsequent degradation of focal adhesion kinase in human umbilical vein endothelial cells

Bacterial lipopolysaccharide (LPS) is an important mediator of inflammation and a potent inducer of endothelial cell damage and apoptosis. In this study, we investigated the protective effects of saikosaponin C (SSc), one of the active ingredients produced by the traditional Chinese herb, Radix Bupleuri, against LPS-induced apoptosis in human umbilical endothelial cells (HUVECs). LPS triggered caspase-3 activation, which was found to be important in LPS-induced HUVEC apoptosis. Inhibition of caspase-3 also inhibited LPS-induced degradation of focal adhesion kinase (FAK), indicating that caspase-3 is important in LPS-mediated FAK degradation as well as in apoptosis in HUVECs. SSc significantly inhibited LPS-induced apoptotic cell death in HUVECs through the selective suppression of caspase-3. SSc was also shown to rescue LPS-induced FAK degradation and other cell adhesion signals. Furthermore, the protective effects of SSc against LPS-induced apoptosis were abolished upon pretreatment with a FAK inhibitor, highlighting the importance of FAK in SSc activity. Taken together, these results show that SSc efficiently inhibited LPS-induced apoptotic cell death via inhibition of caspase-3 activation and caspase-3-mediated-FAK degradation. Therefore, SSc represents a promising therapeutic candidate for the treatment of vascular endothelial cell injury and cellular dysfunction.     

One in vitro model for visceral adipose-derived fibroblasts in chronic inflammation

One pathogenesis of the obesity-associated complications is that consistent with increased body fat mass, the elevation of adipose tissue-derived cytokines inflicts a low-grade chronic inflammation, which ultimately leads to metabolic disorders. Adipocytes and macrophages in visceral adipose (VA) have been confirmed to contribute to the chronic inflammation; however, the role of the resident fibroblasts is still unknown. We established one VA fibroblast cell line, termed VAFC. Morphological analysis indicated that there were large numbers of pits at the cell plasma membrane. In vitro VAFC cells promoted bone marrow cells to differentiate into macrophages and protected them from apoptosis in the serum-free conditions. Additionally, they also interfered in lymphocytes proliferation. On the basis of these results, this cell line might be an in vitro model for understanding the role of adipose-derived fibroblasts in obesity-associated chronic inflammation.     

Sirtuin 1 alleviates neuroinflammation-induced apoptosis after traumatic brain injury

Sirtuin 1 (SIRT1) plays a very important role in a wide range of biological responses, such as metabolism, inflammation and cell apoptosis. Changes in the levels of SIRT1 have been detected in the brain after traumatic brain injury (TBI). Further, SIRT1 has shown a neuroprotective effect in some models of neuronal death; however, its role and working mechanisms are not well understood in the model of TBI. This study aimed to address this issue. SIRT1-specific inhibitor (sirtinol) and activator (A3) were introduced to explore the role of SIRT1 in cell apoptosis. Results of the study suggest that SIRT1 plays an important role in neuronal apoptosis after TBI by inhibiting NF-κB, IL-6 and TNF-α deacetylation and the apoptotic pathway sequentially, possibly by alleviating neuroinflammation.     

Cell death in allergic diseases

Apoptosis, the most common form of cell death, is a key mechanism in the build up and maintenance of both innate and adaptive immunity. Central to the apoptotic process is a family of intracellular cysteine proteases with aspartate-specificity, called caspases. Caspases are counter-regulated by multiple anti-apoptotic molecules, and the expression of the latter in leukocytes is largely dependent on survival factors. Therefore, the physiologic rates of apoptosis change under pathologic conditions. For instance, in inflammation, the expression of survival factors is usually elevated, resulting in increased cell survival and consequently in the accumulation of the involved immune cells. In many allergic diseases, eosinophil apoptosis is delayed contributing to both blood and tissue eosinophilia. Besides eosinophils, apoptosis of other leukocytes is also frequently prevented or delayed during allergic inflammatory processes. In contrast to inflammatory cells, accelerated cell death is often observed in epithelial cells, a mechanism, which amplifies or at least maintains allergic inflammation. In conclusion, deregulated cell death is a common phenomenon of allergic diseases that likely plays an important role in their pathogenesis. Whether the apoptosis is too little or too much depends on the cell type. In this review, we discuss the regulation of the lifespan of the participating leukocytes in allergic inflammatory responses.