Superoxide dismutase-dependent, catalase-sensitive peroxides in human endothelial cells infected by Rickettsia rickettsii.

The generation and intracellular accumulation of reactive oxygen species have been shown to be associated with the infection of human umbilical vein endothelial cells (HUVEC) by Rickettsia rickettsii. In response to the oxidant superoxide, the activity of the enzyme superoxide dismutase (SOD) increases following infection by this obligate intracellular bacterium. Other oxidants which are capable of oxidizing the fluorescent probe 2',7'-dichlorofluorescin (DCFH) also accumulate intracellularly within infected cells. In the study reported here, we show that (i) an inhibitor of SOD, diethyldithiocarbamic acid, reduces the observed rise in SOD activity in infected cells by 40 to 60% and at the same time reduces the degree of intracellular oxidation of DCFH; (ii) catalase-sensitive peroxides can be detected in supernatants of R. rickettsii-infected cells shortly after rickettsial exposure; and (iii) fluorescence-activated cell sorter analysis demonstrates significant intracellular oxidant activity in infected cells within 5 h after exposure to R. rickettsii. The results of these experiments indicate that hydrogen peroxide is a major oxidant associated with infection of HUVEC by R. rickettsii and that intracellular oxidant activity sensitive to SOD inhibition is detectable early and prior to significant rickettsial multiplication and much earlier than the ultrastructural manifestations of cell injury seen by electron microscopy.

Infection of human endothelial cells by Rickettsia rickettsii causes a significant reduction in the levels of key enzymes involved in protection against oxidative injury.

The activities of glucose-6-phosphate dehydrogenase, catalase, and glutathione peroxidase were significantly decreased in human endothelial cells infected with the obligate intracellular bacterium Rickettsia rickettsii, the causative agent of Rocky Mountain spotted fever. This observation lends additional support to our hypothesis implicating oxidative damage in endothelial cell injury caused by this microorganism.

Rickettsia rickettsii induces superoxide radical and superoxide dismutase in human endothelial cells.

Human endothelial cells infected with Rickettsia rickettsii, the etiological agent of Rocky Mountain spotted fever, undergo striking morphological changes to the endoplasmic reticulum-outer nuclear envelope complex. These changes are accompanied by concurrent accumulation of intracellular peroxides. Both of these findings are consistent with the notion that cells undergo some form of oxidative stress. Since oxidant injury is often initiated or mediated through oxygen radicals, we examined superoxide radical generation when endothelial cells were exposed to R. rickettsii. We also examined the levels of superoxide dismutase, an enzyme induced in response to increased superoxide formation. The levels of both superoxide and superoxide dismutase increased when endothelial cells were exposed to R. rickettsii. These results, together with our previous findings, support our hypothesis that cells infected by this intracellular bacterium experience oxidant-mediated injury that may eventually contribute to cell death.

Penetration of host cells by Rickettsia rickettsii appears to be mediated by a phospholipase of rickettsial origin.

Internalization of obligate intracellular bacteria belonging to the genus Rickettsia by eukaryotic cells requires participation of both the parasitized host and the microorganism. The term "induced phagocytosis" has been used specifically to describe the entry of Rickettsia prowazekii, although a similar mechanism is likely for R. rickettsii. A role for a phospholipase in the internalization process has been proposed for both of these organisms, with the strongest supporting evidence provided for R. prowazekii. Despite general acceptance of the notion that phospholipase activity is involved in the internalization process of these bacteria, the origin of the enzyme is not known. The results of the study presented here, which used R. rickettsii and Vero cells, suggest that a rickettsial phospholipase, rather than a host cell phospholipase, mediates internalization of the organism. This conclusion is based upon results which show that pretreatment of R. rickettsii, but not of host cells, with a specific chemical inhibitor of phospholipase, and also antiserum to this enzyme, significantly reduces uptake of the organism and its ability to cause plaque formation.

Heparin protects human endothelial cells infected by Rickettsia rickettsii.

Routine culture of endothelial cells currently includes the use of heparin, which significantly reduces cell doubling time and increases cell population size. Heparin protects cultured arterial endothelial cells from damage by toxic oxygen metabolites produced by the action of xanthine and xanthine oxidase. Because of our hypothesis implicating free radicals in cell injury caused by Rickettsia rickettsii, we have carried out a series of experiments to examine the effects of heparin on injury to endothelial cells infected by this microorganism. These studies showed that heparin does not inhibit replication of R. rickettsii in the cytoplasm of endothelial cells. Furthermore, heparin appears to exhibit a protective effect on the infected host cell as measured by (i) reduced plaque size, (ii) increased longevity of the cell monolayer, (iii) reduction in the amount of lactic dehydrogenase released from infected cells, and (iv) reduction in the levels of intracellular peroxides formed in infected cells. Electron microscopic studies also show a significant reduction in dilatation of the rough-surfaced endoplasmic reticulum of the infected cells in the presence of heparin. These observations appear to lend additional support to involvement of an oxidative mechanism in human endothelial cell injury caused by R. rickettsii.

Potential for free radical-induced lipid peroxidation as a cause of endothelial cell injury in Rocky Mountain spotted fever.

Cells infected by Rickettsia rickettsii, the causative agent of Rocky Mountain spotted fever, display unusual intracellular morphological changes characterized by dilatation of the membranes of the endoplasmic reticulum and outer nuclear envelope. These changes are consistent with those that might be expected to occur following peroxidation of membrane lipids initiated by oxygen radical species, such as the hydroxyl radical or a variety of organic radicals. Using a fluorescent probe, we have found significantly increased levels of peroxides in human endothelial cells infected by R. rickettsii. Studies with desferrioxamine, an iron chelator effective in preventing formation of the hydroxyl radical from hydrogen peroxide and the superoxide free radical, reduced peroxide levels in infected cells to those found in uninfected cells. This observation suggests that the increased peroxides in infected cells may be lipid peroxides, degradation products of free radical attack on polyenoic fatty acids. The potential for lipid peroxidation as an important mechanism in endothelial cell injury caused by R. rickettsii is discussed.