Rickettsia rickettsii infection protects human microvascular endothelial cells against staurosporine-induced apoptosis by a cIAP2-independent mechanism

Background. Manipulation of host cell death is an important determinant of the outcome of an infection. Here, we investigate whether Rickettsia rickettsii-infected host endothelial cells resist the effects of staurosporine, a potent inducer of apoptosis, and we explore the mechanisms underlying the anti-apoptotic effect of infection. Methods. Human microvascular endothelial cells infected with R. rickettsii for 24 or 48 h were challenged with staurosporine. The extent of apoptosis was evaluated with flow cytometry. mRNA and protein expression levels were determined by use of microarray or polymerase chain reaction and immunoblotting, respectively. Results. Staurosporine-induced apoptosis in endothelial cells infected for 24 and 48 h was significantly reduced, compared with simultaneously treated uninfected cells. A microarray of human genes involved in apoptosis and polymerase chain reaction analyses revealed increased steady-state mRNA expression of cIAP2 (a member of the inhibitor-of-apoptosis family of proteins) at 24 h after infection. The levels of cIAP2 protein (±SD) in infected cells were 3.5 ± 1.7-fold and 2.3 ± 1.2-fold higher than that in uninfected control cells at 24 and 48 h after infection. Nucleofection of human-specific cIAP2-targeted siRNA resulted in inhibition of protein expression by≥50% but had no effect on infection-induced protection against apoptosis. Conclusions. R. rickettsii-induced expression of cIAP2 in host endothelial cells is likely not a major contributor to protection against staurosporine-induced cell death.