EFFECTS OF THE ANTIOXIDANT ALPHA-LIPOIC ACID ON HUMAN UMBILICAL VEIN ENDOTHELIAL-CELLS INFECTED WITH RICKETTSIA-RICKETTSII

Rickettsia rickettsii infection of endothelial cells is manifested in very distinctive changes in cell morphology, consisting of extensive d ilatation of the membranes of the endoplasmic reticulum and outer nucl ear envelope and blebbing of the plasma membrane, as seen by transmiss ion electron microscopy (D. J. Silverman, Infect. Immun. 44:545-553, 1 984), These changes in cellular architecture are thought to be due to oxidant-mediated cell injury, since their occurrence correlates with d ramatic alterations in cellular metabolism, particularly with regard t o antioxidant systems, In this study, it was shown that R. rickettsii infection of human umbilical vein endothelial cells resulted in a sign ificant depletion of intracellular reduced glutathione (thiol) content at 72 and 96 h and decreased glutathione peroxidase activity at 72 h postinfection. Infected cells displayed a dramatic increase ire the co ncentration of intracellular peroxides by 72 h, Supplementation of the cell culture medium with 100, 200, or 500 mu M alpha-lipoic acid, a m etabolic antioxidant, after inoculation with R. rickettsii restored th e intracellular levels of thiols and glutathione peroxidase and reduce d the intracellular peroxide levels in infected cells. These effects w ere dose dependent, Treated infected monolayers maintained better viab ility at 96 h after inoculation with X. rickettsii than did untreated infected cells. Moreover, supplementation of the cell culture medium w ith 100 mu M alpha-lipoic acid for 72 h after infection prevented the occurrence of morphological changes in the infected cells. The presenc e of 100 or 200 mu M alpha-lipoic acid did not influence rickettsial g rowth in endothelial cells, nor did it affect the ability of R. ricket tsii to form lytic plaques in Vero cells. Treatment with 500 mu M alph a-lipoic acid decreased by 50% both the number and size of lytic plaqu es in Vero cells, and it also decreased the recovery of viable rickett siae from endothelial cells. However, under all treatment conditions, a significant number of rickettsiae could be detected microscopically. Furthermore, the rickettsiae apparently retained their capacity for i ntracellular movement, since they possessed long polymerized actin tai ls after 72 and 96 h of treatment regardless of the concentration of a lpha-lipoic acid used. Since alpha-lipoic acid does not seem to exhibi t direct antirickettsial activity except with long-term exposure at ve ry high concentrations, the mechanism of its protective activity for e ndothelial cells infected with rickettsiae may involve complex changes in cellular metabolism that only indirectly affect rickettsiae.