No abstract

Tumor necrosis factor (TNF)-alpha and platelet-activating factor (PAF) are important mediators of inflammatory reactions, and their release is controlled by a positive feedback network. However, the regulatory mechanisms underlying the interaction of these two molecules are unknown. Within 10 min of the injection of lipopolysaccharide (LPS) into C57BL/6 mice, effects inducible by PAF such as anaphylactic shock-like symptoms, disseminated intravascular coagulation, and hemorrhage in renal medullae were observed, and all these pathological changes were prevented by the PAF antagonist, BN 50739. The plasma level of PAF after LPS injection reached a peak at 5 min. TNF-alpha gene expression was evident 20 min after LPS injection and was maximal at 40 min, and the level of serum TNF-alpha reached a peak at 1 h. Pretreatment with BN 50739 inhibited LPS-induced TNF-alpha gene expression and protein synthesis in a dose-dependent manner. Injection of PAF or treatment of the macrophage cell line, J774A.1, with PAF activated the transcription factor, nuclear factor (NF)-kappa B, which is essential for inducible TNF-alpha transcription. The activation of NF-kappa B by PAF preceded the LPS-mediated TNF-alpha gene expression. Pretreatment with BN 50739 inhibited LPS-induced mobilization of NF-kappa B in a dose-dependent manner in vivo as well as in vitro. These data suggest that PAF, which is released immediately or shortly after LPS injection, induces the expression of TNF-alpha through the activation of NF-kappa B.

In this study, we have investigated the mechanisms underlying organ susceptibility to candida infection. Infection of BALB/c mice with Candida albicans led to both an early (1–8 h) and late (24–48 h) activation of NF-κB in the organs resistant to C. albicans, including the lung and spleen. In susceptible organs such as the kidneys, early activation of NF-κB was not observed. The kinetics of TNF-α mRNA expression paralleled those of NF-κB activation in all organs examined. Blocking the effects of endogenous platelet-activating factor (PAF) by pretreatment with the PAF antagonist BN50739 or antioxidants significantly reduced the early activity of NF-κB and TNF-α mRNA expression, and increased the recovery of C. albicans in the lung and spleen. Importantly, administration of PAF 5 min prior to the infection resulted in the appearance of early activities of NF-κB and TNF-α mRNA expression, followed by a nearly complete clearance of the organisms in the kidneys. Pretreatment with anti-TNF-α Ab resulted in an enhanced susceptibility to C. albicans, and the PAF-mediated resistance was abrogated by anti-TNF-α in all organs examined. These data indicated that endogenously produced PAF in response to C. albicans is a key molecule involved in the early activation of NF-κB, which, in turn, renders the organ resistant to the fungus by promoting the production of anti-candidal proinflammatory cytokines such as TNF-α. Susceptible organs, including the kidneys, lack the capacity to generate a sufficient PAF-induced early NF-κB response.

Lipopolysaccharide (LPS) is a known inducer of numerous pro-inflammatory events including the production of platelet-activating factor (PAF). PAF released in response to LPS is a major contributor to the pathological events associated with endotoxemia. The present study demonstrates that dexmethasone (DEX) inhibited the LPS-induced early plasma PAF raise in a dose- and time-dependent manner. In addition, DEX prevented the subsequent PAF-mediated pathological phenomena such as anaphylactic shock-like symptoms, symptoms of disseminated intravascular coagulation and hemorrhage in renal medullae. DEX or the PAF antagonist BN 50739 significantly inhibited LPS-induced NF-kappaB activation. The inhibition of NF-kappaB activation by DEX was overcome by the injection of exogenous PAF. Administration of PAF or LPS resulted in a rapid loss of IkappaBalpha protein. The LPS-induced degradation of IkappaBalpha was prevented by pretreatment with BN 50739, suggesting that PAF is a critical intermediate in the LPS-triggered degradation of IkappaBalpha protein. DEX prevented the LPS-induced IkappaBalpha degradation, which was also reversed by exogenous PAF. Administration of DEX or BN 50739 caused an increase in cytoplasmic IkappaBalpha level. Our results indicate that DEX inhibits IkappaBalpha degradation and subsequent NF-kappaB activation through blocking the initial release of PAF.

We have investigated the role of TNF‐α in mast cell‐mediated late airway hyperresponsiveness (AHR) using mast cell‐deficient WBB6F1‐W/Wv (W/Wv) mice in a murine model of asthma, which exhibits a biphasic increase in AHR. TNF‐α levels in the airway and magnitude of late AHR in response to airway allergen challenge were severely impaired in W/Wv mice compared to their littermates. In addition to TNF‐α, cytosolic phospholipase A2 (cPLA2) phosphorylation and enzymatic activity in the lungs were also impaired in W/Wv mice. Either anti‐TNF‐α antibody or an inhibitor of cPLA2 abolished late AHR in congeneic +/+ mice. Intratracheal administration of TNF‐α resulted in increases in late AHR, cPLA2 phosphorylation, cPLA2 activity, and phosphorylation of mitogen‐activated protein kinases. Mast cell replacement restored airway TNF‐α level, cPLA2 phosphorylation and enzymatic activity in the lungs as well as late AHR in W/Wv mice. These data indicate that mast cells play a key role in the development of late AHR through liberation of TNF‐α.