Activation of Acid Sphingomyelinase and Its Inhibition by the Nitric Oxide/Cyclic Guanosine 3′,5′-Monophosphate Pathway: Key Events in Escherichia coli-Elicited Apoptosis of Dendritic Cells

Abstract

Depletion of dendritic cells (DCs) via apoptosis contributes to sepsis-induced immune suppression. The mechanisms leading to DC apoptosis during sepsis are not known. In this study we report that immature DCs undergo apoptosis when treated with high numbers of Escherichia coli. This effect was mimicked by high concentrations of LPS. Apoptosis was accompanied by generation of ceramide through activation of acid sphingomyelinase (A-SMase), was prevented by inhibitors of this enzyme, and was restored by exogenous ceramide. Compared with immature DCs, mature DCs expressed significantly reduced levels of A-SMase, did not generate ceramide in response to E. coli or LPS, and were insensitive to E. coli- and LPS-triggered apoptosis. However, sensitivity to apoptosis was restored by addition of exogenous A-SMase or ceramide. Furthermore, inhibition of A-SMase activation and ceramide generation was found to be the mechanism through which the immune-modulating messenger NO protects immature DCs from the apoptogenic effects of E. coli and LPS. NO acted through formation of cGMP and stimulation of the cGMP-dependent protein kinase. The relevance of A-SMase and its inhibition by NO/cGMP were confirmed in a mouse model of LPS-induced sepsis. DC apoptosis was significantly higher in inducible NO synthase-deficient mice than in wild-type animals and was significantly reduced by treatment ex vivo with NO, cGMP, or the A-SMase inhibitor imipramine. Thus, A-SMase plays a central role in E. coli/LPS-induced DC apoptosis and its inhibition by NO, and it might be a target of new therapeutic approaches to sepsis.

Ceramide and sphingosine in pulmonary infections

“…The use of genetically altered cells or pharmacological inhibitors proved the importance of the acid sphingomyelinase/ ceramide system in the infection of mammalian cells by these pathogens. Consistent with the general function of ceramide in the signaling processes described above, the acid sphingomyelinase/ceramide system has been shown to be involved in several effects induced by the pathogens, including uptake/invasion of the pathogen, induction of apoptosis in the infected mammalian host cell, and regulation of the cellular and humoral immune responses (Grassmé et al, 1997(Grassmé et al, , 2003a(Grassmé et al, , 2005Hauck et al, 2000;Esen et al, 2001;Pfeiffer et al, 2001;Utermöhlen et al, 2003;Falcone et al, 2004;McCollister et al, 2007;Utermöhlen et al, 2008;Simonis et al, 2014).…”

“…Additional studies showed that infecting mammalian cells with S. aureus, L. monocytogenes, S. typhimurium, E. coli, or pathogenic mycobacteria also activates the acid sphingomyelinase/ceramide system (Esen et al, 2001;Pfeiffer et al, 2001;Utermöhlen et al, 2003;Falcone et al, 2004;McCollister et al, 2007;Utermöhlen et al, 2008). The use of genetically altered cells or pharmacological inhibitors proved the importance of the acid sphingomyelinase/ ceramide system in the infection of mammalian cells by these pathogens.…”

“…Ceramide in particular has been shown to be involved in the infection of mammalian cells with a variety of bacterial and viral pathogens, including Neisseria gonorrhoeae, Neisseria meningitidis, Staphylococcus aureus, Pseudomonas aeruginosa, Listeria monocytogenes, Salmonella typhimurium, Escherichia coli, mycobacteria, measles virus, rhinovirus, and sindbis virus (Grassmé et al, 1997(Grassmé et al, , 2003a(Grassmé et al, , 2005Hauck et al, 2000;Jan et al, 2000;Esen et al, 2001;Pfeiffer et al, 2001;Utermöhlen et al, 2003;Falcone et al, 2004;McCollister et al, 2007;Utermöhlen et al, 2008;Gassert et al, 2009;Avota et al, 2011;Simonis et al, 2014). The important role of ceramide in the structure of such a variety of infections is explained by its biophysics: sphingolipids, cholesterol, and glycerolipids are the main components of mammalian cell membranes.…”

Types A and B Niemann-Pick disease

“…Notably, when cells are subjected to stress, ASM rapidly translocates from lysosomes to the outer leaflet of the plasma membrane, where it also can hydrolyze sphingomyelin into ceramide [39,40]. This causes reorganization of membrane lipid microdomains, or "raft" structures, and stimulates downstream signaling events.…”

The Extra Domain A from Fibronectin Targets Antigens to TLR4-Expressing Cells and Induces Cytotoxic T Cell Responses In Vivo

“…1B, after incubation with EDA-SIINFEKL, HEK TLR4 cells presented a higher intensity of fluorescence than HEK LacZ, suggesting that EDA-SIINFEKL is indeed able to bind to TLR4. We also studied the capacity of the EDA-SIINFEKL protein to inhibit the binding of an anti-hTLR4 Ab to HEK-hTLR4 that has been shown to block activation of monocytes with LPS (28). As shown in Fig.…”