Age related decline in cytokine induced nitric oxide synthase activation and apoptosis in cultured endothelial cells: minimal involvement of nitric oxide in the apoptosis

Caspase-3 is activated and rapidly released from human umbilical vein endothelial cells in response to lipopolysaccharide

Endothelial cell injury/dysfunction is considered to play a critical role in the pathogenesis of severe sepsis and septic shock. Although it is considered that endothelial cell apoptosis is involved in endothelial injury/dysfunction, physiological involvement remains ambiguous since the induction of apoptosis requires the inhibition of endogenous apoptosis inhibitors. Here we show that caspase-3 activation, a biological indicator of apoptosis, is observed in response to lipopolysaccharide (LPS) stimulation even under the influence of endogenous apoptosis inhibitors, and that activated caspase-3 is rapidly released from human umbilical vein endothelial cells (HUVEC). In the presence of cycloheximide (CHX), an increase in intracellular caspase-3/7 activity in response to LPS was not detected in HUVEC up to 24 h following stimulation even in the presence of LPS-binding protein (LBP), soluble CD14 and soluble MD-2, whereas the decrease in cell viability and increase in release of the cellular enzyme lactate dehydrogenase (LDH) were observed in a soluble CD14/LBP-dependent manner. On the other hand, even in the absence of CHX, a significant increase in caspase-3/7 activity and a cleaved caspase-3 fragment with a slight increase in LDH release was observed in culture supernatants in response to LPS. This increase in caspase-3/7 activity was observed even when LDH release was undetected. These results indicate that caspase-3 is activated by LPS under physiological conditions and suggest that HUVEC escape from cell death by rapidly releasing activated caspase-3 into extracellular space. Failure of this escape mechanism may result in endothelial injury/dysfunction.

Lipopolysaccharide initiates a TRAF6-mediated endothelial survival signal

Similar to tumor necrosis factor (TNF), bacterial lipopolysaccharide (LPS) elicits parallel apoptotic and antiapoptotic pathways in endothelial cells. The overall result is that there is minimal endothelial cell death in response to LPS without inhibition of the cytoprotective pathway. While the TNF-induced death and survival pathways have been relatively well elucidated, much remains to be learned about LPS signaling events in this regard. It is known that the transcription factor nuclear factor-κB (NF-κB) provides a critical cell survival signal in response to TNF, but is not an essential component of the LPS-induced survival pathway. The TNF receptor-associated factor 6 (TRAF6) is a major effector of multiple LPS-induced signals, including a c-Jun N-terminal kinase (JNK)-mediated apoptotic response. In this report we demonstrate that following LPS stimulation, TRAF6 also transmits an important endothelial cell survival signal in a situation of complete NF-κB blockade. In response to LPS, TRAF6 activates the phosphatidylinositol 3′-kinase (PI3K)/Akt pathway, but not ERK1/2 mitogen-activated protein kinases (MAPKs) in endothelial cells. Activation of PI3K signals a critical antiapoptotic pathway in response to LPS in endothelial cells, whereas ERK1/2 does not. Thus TRAF6 acts as a bifurcation point of the LPS-initiated death and survival signals in endothelial cells. (Blood. 2004;103:4520-4526)

Nafamostat mesilate suppresses NF-kappaB activation and NO overproduction in LPS-treated macrophages

Nafamostat mesilate (NM), a clinically used serine protease inhibitor, suppressed the overproduction of nitric oxide (NO) and the expression of inducible nitric oxide synthase (iNOS) in RAW264.7 murine macrophages treated with lipopolysaccharide (LPS, 100 ng/ml); however, it had little effect on endothelial NOS (eNOS) in human umbilical vein endothelial cells (HUVEC). Electrophoretic mobility shift assay (EMSA) revealed that LPS activated nuclear factor-kappaB (NF-kappaB) in RAW264.7 cells and that this activation was suppressed by nafamostat mesilate. Western blotting showed that nafamostat mesilate suppressed the phosphorylation and degradation of inhibitor kappaB-alpha (IkappaB-alpha), which holds NF-kappaB in the cytoplasm in an inactivated state. Our observations suggest that nafamostat mesilate is a candidate agent for various diseases such as ischemia-reperfusion, graft rejection, inflammatory diseases, and autoimmune diseases, in which iNOS and/or NF-kappaB are upregulated.

Mechanisms of bacterial lipopolysaccharide-induced endothelial apoptosis

Gram-negative bacterial sepsis remains a common, life-threatening event. The prognosis for patients who develop sepsis-related complications, including the development of acute respiratory distress syndrome (ARDS), remains poor. A common finding among patients and experimental animals with sepsis and ARDS is endothelial injury and/or dysfunction. A component of the outer membrane of gram-negative bacteria, lipopolysaccharide (LPS) or endotoxin, has been implicated in the pathogenesis of much of the endothelial cell injury and/or dysfunction associated with these disease states. LPS is a highly proinflammatory molecule that elicits a wide array of endothelial responses, including the upregulation of cytokines, adhesion molecules, and tissue factor. In addition to activation, LPS induces endothelial cell death that is apoptotic in nature. This review summarizes the evidence for LPS-induced vascular endothelial injury and examines the molecular signaling pathways that activate and inhibit LPS-induced endothelial apoptosis. Furthermore, the role of apoptotic signaling molecules in mediating LPS-induced activation of endothelial cells will be considered.

Lipopolysaccharide signals an endothelial apoptosis pathway through TNF receptor-associated factor 6-mediated activation of c-Jun NH2-terminal kinase

Inflammatory mediators such as TNF and bacterial LPS do not cause significant apoptosis of endothelial cells unless the expression of cytoprotective genes is blocked. In the case of TNF, the transcription factor NF-kappaB conveys an important survival signal. In contrast, even though LPS can also activate NF-kappaB, this signal is dispensable for LPS-inducible cytoprotective activity. LPS intracellular signals are transmitted through a member of the Toll-like receptor family, TLR4. This family of receptors transduces signals through a downstream molecule, TNFR-associated factor 6 (TRAF6). In this study, we demonstrate that the C-terminal fragment of TRAF6 (TRAF6-C) inhibits LPS-induced NF-kappaB nuclear translocation and c-Jun NH(2)-terminal kinase (JNK) activation in endothelial cells. In contrast, LPS activation of p38 kinase is not inhibited by TRAF6-C. TRAF6-C also inhibits LPS-initiated endothelial apoptosis, but potentiates TNF-induced apoptosis. LPS-induced loss of mitochondrial transmembrane potential, cytochrome c release, and caspase activation are all blocked by TRAF6-C. We demonstrate that TRAF6 signals apoptosis via JNK activation, since inhibition of JNK activation using a dominant-negative mutant also inhibits apoptosis. JNK inhibition blocks caspase activation, but the reverse is not true. Hence, JNK activation lies upstream of caspase activation in response to LPS stimulation.

Progression of atheroma: a struggle between death and procreation

Traditional thinking accorded a major role to deranged cell proliferation as a determinant of the abnormal cellularity of atheroma. However, studies conducted in several laboratories have documented the occurrence of disordered apoptosis during atherogenesis, leading to the death of lipid-rich foam cells (promoting lipid-core formation) and depletion of vascular smooth muscle cells (fostering fragility of the fibrous cap). A complex interplay of environmental factors and endogenous proteins regulates apoptosis and contributes to the struggle between cell death and procreation in atherosclerosis. In addition to a variety of growth factors, chemically modified lipids, reactive oxygen species, proinflammatory cytokines, and Fas ligand produced by activated immune cells may influence cell viability through a diversity of pathways, including the caspase cascade, the Bcl-2 protein family, and the oncogene/antioncogene system. A clarification of the molecular mechanisms responsible for vascular cell death may aid in the development of novel therapeutic strategies to treat atherosclerosis and its complications, including the acute coronary syndromes.

Cytotoxicity of IFN-gamma and TNF-alpha for vascular endothelial cell is mediated by nitric oxide

Endothelial cell injury is a critical event in tissue damage accompanying inflammation, in which both inflammatory cytokines and reactive oxygen species may play pivotal roles, although the exact mechanism has not yet been clarified. We found that combined stimulation with interferon-gamma (IFN-gamma) and tumor necrosis factor-alpha (TNF-alpha) induced both cytotoxicity to murine vascular endothelial cell line F-2 and an increase in nitric oxide (NO). Therefore, in the present study, the implication of NO in cytotoxicity was examined. A potent iNOS-specific inhibitor ONO-1714 completely blocked both cytokine-induced cytotoxicity and NO production. NO scavengers such as carboxy-PTIO and hemoglobin blocked cytotoxicity. Moreover, exogenous NO from NOC 18 also caused cytotoxicity. These results together demonstrated that cytotoxicity of IFN-gamma and TNF-alpha for endothelial cell F-2 was mediated by NO, suggesting a pathogenic role of cytokine-induced NO production in endothelial damage under inflammatory conditions.

Nitric oxide levels in the aqueous humor in cataract patients

PURPOSE

To determine the role of nitric oxide (NO) in the clinical manifestations of cataract in different age and etiological groups.

SETTING

Multicenter study, Taipei, Taiwan, Republic of China.

METHODS

Aqueous humor samples were obtained from 114 cataract patients in different etiological and age groups (eg, juvenile, traumatic, and age-related cataract). Using chemiluminescence assay, NO was detected through the light generated by ozone and NO interaction. The amount of NO production was calculated and compared among groups. The results were correlated with patient age.

RESULTS

The NO levels in aqueous humor were highest in the 12 patients with traumatic cataract and lowest in the 15 patients with juvenile cataract (47.59 micromol/L +/- 12.81 [SD] and 7.66 +/- 2.62 micromol/L, respectively; P <.001). Aqueous humor NO levels were correlated with age and were highest in patients older than 80 years (mean 38.78 +/- 6.29 micromol/L) and in those with mature cataract (mean 40.15 +/- 6.15 micromol/L) (P <.05).

CONCLUSIONS

The NO levels in the aqueous humor increased with age and traumatic events. The results suggest that NO could be a risk factor in cataract formation.

Apoptosis in atherosclerosis: pathological and pharmacological implications

Normal embryonic development, tissue differentiation and repair in the eukaryote requires a tightly regulated apoptosis, or programmed cell death. Apoptosis also plays an essential role in different pathological processes including atherosclerosis, in which it affects all cell types in the atherosclerotic lesion, including endothelial cells, vascular smooth muscle cells, and macrophages. During atherosclerosis progression, pro- and anti-apoptotic signals abound in the evolving lesion. Apoptosis limits the number of a particular cell type that accumulates in the lesion and slows down the overall progression of the lesion. On the other hand, it contributes to the production of unstable plaques. Many pharmacological agents used to treat cardiovascular and lipid disorders have pro- or/and anti-apoptotic effects. Pharmaceuticals that modulate apoptosis in specific types of cell can potentially serve as anti-atherogenic agents. However, to develop agents for clinical use requires a thorough knowledge of the pathophysiology of apoptosis in atheromatous lesions, a highly cell-specific process. Here we review our current understanding of the process to provide a background for future pharmacological research in the area.

Stable expression of varied levels of inducible nitric oxide synthase in primary cultures of endothelial cells

Nitric oxide (NO*), generated by nitric oxide synthase (NOS II) from immunostimulated cells during infection, plays an important role in host immune defense against microbial invasion. The impact of different rates of NO* production on host cell function has not been defined. Herein, we describe the development of a method to express varied levels of murine NOS II in bovine pulmonary artery endothelial cells. A retroviral vector (pMFGSNOS) encoding NOS II was used to transduce primary cultures of endothelial cells. Bovine endothelial cells were susceptible to this transduction and up to 18% of the cells expressed immunodetectable murine NOS II. The NOS II-transduced endothelial cells were cultured on the three-dimensional matrix, Gelfoam, for 8-10 days. Stable expression of NOS II was assessed by measuring nitrite accumulation in media every 2 days. By day 10, endothelial cells on Gelfoam were found to secrete NO* at a rate exceeding 1.0 microM/h/10(6) cells, concomitant with an enhanced level of NOS II activity. Argininosuccinate synthetase, a key enzyme in the metabolism of l-citrulline to l-arginine, increased as well, perhaps in response to dimunition of the intracellular arginine pool corresponding to the observed high output of NO*. In spite of the continuous flux of NO*, endothelial cell viability was not effected. This system provides the opportunity to assess the impact of different levels of sustained NO* production on endothelial cell physiology.

Lipopolysaccharide Induces the Antiapoptotic Molecules, A1 and A20, in Microvascular Endothelial Cells

The effect of lipopolysaccharide (LPS) on endothelial cells is a key component of the inflammatory response seen in Gram-negative sepsis. LPS does not cause death of cultured human endothelial cells. However, when the expression of new proteins is inhibited by cycloheximide, microvascular endothelial cells in culture undergo apoptosis. This finding suggests that LPS induces apoptotic and antiapoptotic pathways, with the antiapoptotic response being dependent on the synthesis of new proteins. Concurrent activation of apoptotic and antiapoptotic pathways has previously been documented for tumor necrosis factor (TNF). In the case of TNF, the antiapoptotic signal has been attributed to at least two cytoprotective proteins: the Bcl-2 homologue, A1, and the zinc-finger protein, A20. In this study, we demonstrate that both these molecules are induced in microvascular endothelial cells by LPS. Enforced overexpression of either A1 or A20 inhibits LPS and cycloheximide-initiated apoptosis. Induction of A1 and A20 does not require synthesis of intermediary proteins, but is dependent on the presence of soluble CD14. In addition, we show that inhibition of signaling by the transcription factor, NF-κB, blocks accumulation of A1 and A20 mRNA. Taken together, our findings suggest that LPS directly induces expression of the cytoprotective proteins, A1 and A20, via a CD14-dependent pathway requiring activation of NF-κB.

© 1998 by The American Society of Hematology.

Lipopolysaccharide Induces the Antiapoptotic Molecules, A1 and A20, in Microvascular Endothelial Cells

The effect of lipopolysaccharide (LPS) on endothelial cells is a key component of the inflammatory response seen in Gram-negative sepsis. LPS does not cause death of cultured human endothelial cells. However, when the expression of new proteins is inhibited by cycloheximide, microvascular endothelial cells in culture undergo apoptosis. This finding suggests that LPS induces apoptotic and antiapoptotic pathways, with the antiapoptotic response being dependent on the synthesis of new proteins. Concurrent activation of apoptotic and antiapoptotic pathways has previously been documented for tumor necrosis factor (TNF). In the case of TNF, the antiapoptotic signal has been attributed to at least two cytoprotective proteins: the Bcl-2 homologue, A1, and the zinc-finger protein, A20. In this study, we demonstrate that both these molecules are induced in microvascular endothelial cells by LPS. Enforced overexpression of either A1 or A20 inhibits LPS and cycloheximide-initiated apoptosis. Induction of A1 and A20 does not require synthesis of intermediary proteins, but is dependent on the presence of soluble CD14. In addition, we show that inhibition of signaling by the transcription factor, NF-κB, blocks accumulation of A1 and A20 mRNA. Taken together, our findings suggest that LPS directly induces expression of the cytoprotective proteins, A1 and A20, via a CD14-dependent pathway requiring activation of NF-κB.

© 1998 by The American Society of Hematology.

Transfection of inducible nitric oxide synthase gene causes apoptosis in vascular smooth muscle cells

BACKGROUND

Excess production of nitric oxide (NO) by inducible NO synthase (iNOS) has been implicated in a variety of physiological processes including vascular remodeling. To elucidate whether endogenous NO generated by iNOS is involved in the programmed cell death (apoptosis) of the vasculature, iNOS cDNA- expressing construct was transfected into rat and human vascular smooth muscle cells (VSMCs) by lipofection.

METHODS AND RESULTS

VSMCs transiently transfected with iNOS cDNA functionally expressed 130 kd iNOS protein with full catalytic activity to generate massive NO in proportion to the doses of cDNA used; its enzymatic activity as well as NO production was completely blocked by an NOS inhibitor, NG-monomethyl-L-arginine (LNMMA). Overexpression of iNOS led to a marked inhibition of DNA synthesis as well as induction of apoptosis in VSMCs. Evidence for apoptotic cell death was provided by internucleosomal DNA fragmentation by agarose gel electrophoresis, positive staining for TdT-mediated dUTP biotin nick end-labeling, and appearance of hypodiploid cells by flow cytometry analysis. Apoptosis after transfection with iNOS cDNA was abrogated by LNMMA. Transfection of iNOS cDNA caused accumulation of the tumor suppressor gene p53 but not of bcl-2, which was also blocked by LNMMA.

CONCLUSIONS

These results demonstrate that massive generation of endogenous NO derived from iNOS overexpression leads to a marked apoptosis in VSMCs, thus suggesting an important role of NO as a proapoptotic factor for VSMCs in the process of vascular remodeling.

Lipopolysaccharide mediates endothelial apoptosis by a FADD-dependent pathway

Endothelial cells play a pivotal role in the inflammatory process by coordinating the recruitment of inflammatory cells to sites of tissue injury. Lipopolysaccharide (LPS) activates many of the proinflammatory and procoagulant responses of endothelial cells, and endothelial injury is thought to play a crucial role in the pathogenesis of septic shock due to Gram-negative bacteria. The receptor used by LPS to signal endothelial responses has not been identified. It is also not known how LPS induces endothelial injury/death. In this study, we demonstrate that LPS mediates endothelial apoptosis by a FADD-dependent pathway. FADD is a death domain-containing protein that binds to certain members of the tumor necrosis factor receptor family, namely TNFR1, Fas, and DR3. However, none of these receptors appear to be involved in LPS-mediated death, suggesting that LPS may utilize a novel death domain-containing protein to transduce a death signal.

Lipopolysaccharide-induced nitric oxide synthase activity in cultured cerebellar granule neurons

We examined the inducible form of nitric oxide synthase (iNOS) activity in cerebellar primary cultures enriched with granule neurons. Treatment with lipopolysaccharide (LPS) for 24 h was performed in two different cultures in which the survival of neurons was controlled by the levels of extracellular K+. Treatment of the granule neurons with the contaminating non-neuronal cells (< 5%) in a high K+ (25 mM) -containing culture medium with LPS induced four-fold NOS activity compared to that of the control, and the induced NOS activity was calcium-independent. LPS did not induce the NOS activity at all in the contaminating non-neuronal cells alone which were obtained by eliminating all the granule neurons from the culture by lowering the K+ concentration (5 mM). We conclude that the LPS-activated granule neurons can express iNOS activity and that this induction is not attributable to the contaminating nonneuronal cells.