Lipopolysaccharide Induces Disseminated Endothelial Apoptosis Requiring Ceramide Generation

Abstract

The endotoxic shock syndrome is characterized by systemic inflammation, multiple organ damage, circulatory collapse and death. Systemic release of tumor necrosis factor (TNF)-α and other cytokines purportedly mediates this process. However, the primary tissue target remains unidentified. The present studies provide evidence that endotoxic shock results from disseminated endothelial apoptosis. Injection of lipopolysaccharide (LPS), and its putative effector TNF-α, into C57BL/6 mice induced apoptosis in endothelium of intestine, lung, fat and thymus after 6 h, preceding nonendothelial tissue damage. LPS or TNF-α injection was followed within 1 h by tissue generation of the pro-apoptotic lipid ceramide. TNF-binding protein, which protects against LPS-induced death, blocked LPS-induced ceramide generation and endothelial apoptosis, suggesting systemic TNF is required for both responses. Acid sphingomyelinase knockout mice displayed a normal increase in serum TNF-α in response to LPS, yet were protected against endothelial apoptosis and animal death, defining a role for ceramide in mediating the endotoxic response. Furthermore, intravenous injection of basic fibroblast growth factor, which acts as an intravascular survival factor for endothelial cells, blocked LPS-induced ceramide elevation, endothelial apoptosis and animal death, but did not affect LPS-induced elevation of serum TNF-α. These investigations demonstrate that LPS induces a disseminated form of endothelial apoptosis, mediated sequentially by TNF and ceramide generation, and suggest that this cascade is mandatory for evolution of the endotoxic syndrome.

Apoptosis in the vasculature: mechanisms and functional importance

“…This latter molecule also prevents LPS-induced ceramide generation, suggesting that systemic TNF is required for both responses (Haimovitz-Friedman et al, 1997). The indispensable role of ceramide generation in disseminated endothelial apoptosis is demonstrated by the observation that acid sphingomyelinase knockout mice are protected from endothelial cell apoptosis and from death despite the normal increase in serum TNF-a in response to LPS (Haimovitz-Friedman et al, 1997). It would be interesting to examine whether the bene®cial e ects of IL-10 in reducing mortality from septic shock in animals (Howard et al, 1993) are related to its potential anti-apoptotic properties.…”

“…Unlike studies performed in vitro, LPSinduced endothelial apoptosis in vivo is dependent on TNF-a since it is blocked by the TNF-binding protein. This latter molecule also prevents LPS-induced ceramide generation, suggesting that systemic TNF is required for both responses (Haimovitz-Friedman et al, 1997). The indispensable role of ceramide generation in disseminated endothelial apoptosis is demonstrated by the observation that acid sphingomyelinase knockout mice are protected from endothelial cell apoptosis and from death despite the normal increase in serum TNF-a in response to LPS (Haimovitz-Friedman et al, 1997).…”

Stress signals for apoptosis: ceramide and c-Jun kinase

“…Since sphingomyelin is preferentially located in the outer lea¯et of the plasma membrane, S-SMase may be the form of ASMase responsible for ceramidemediated apoptosis. Endothelium appear to be a particularly rich source of S-SMase (Marathe et al, 1998) (Figure 3), and ASMase knockout mice display a de®cit in radiation-and endotoxin/TNF-induced ceramide generation and apoptosis in endothelium, but no defect in p53-mediated death of thymocytes (Haimovitz-Friedman et al, 1997;Santana et al, 1996) (Figure 4). ASMase involvement in apoptosis is not restricted to the endothelium, however, since B cells from patients with Niemann-Pick disease (NPD), another genetic model of SMase de®ciency, fail to respond to ionizing radiation with ceramide generation or apoptosis, and retroviral transfer of the ASMase gene restores both events (Santana et al, 1996).…”

IL-12 Suppression During Experimental Endotoxin Tolerance: Dendritic Cell Loss and Macrophage Hyporesponsiveness

“…As such, endotoxins are central to the pathogenesis of Gram-negative sepsis, a state marked by pathological release of proinflammatory mediators, lymphocyte and endothelial cell apoptosis, and disseminated intravascular coagulation (1)(2)(3)(4). Despite the fact that Gramnegative sepsis is a major cause of death throughout the world, such deleterious hyperactivation of the innate immune system is an uncommon result of exposure to endotoxins.…”