NF-kappaB activation suppresses host cell apoptosis during Rickettsia rickettsii infection via regulatory effects on intracellular localization or levels of apoptogenic and anti-apoptotic proteins.
Rickettsia rickettsii, a gram-negative and obligate intracellular bacterium, is the causative agent of Rocky Mountain spotted fever. In human infections, the primary target of R. rickettsii infection is vascular endothelium. Our laboratory has shown that activation of nuclear transcription factor-kappa B (NF-kappaB) during R. rickettsii infection of cultured human endothelial cells protects against apoptosis by preventing the activation of apical caspases-8 and -9, and the effector caspase-3. To understand upstream signaling mechanisms, we have determined the effect of NF-kappaB blockade on the status of different Bcl-2 (B-cell lymphoma 2) proteins in this study. Quantitative analysis following TUNEL and Hoechst staining confirmed that infection of endothelial cells with R. rickettsii for 6 h in the presence of a specific NF-kappaB inhibitor, MG132, resulted in induction of apoptosis. Infection-induced apoptosis of EC was associated with decreased level of Bid and accumulation of Bad, while cytosolic level of Bax remained relatively unchanged. Further, the cellular levels of apoptosis antagonist Bcl-2 were found to be down-regulated and apoptogenic mitochondrial proteins Smac and cytochrome c were released into cytoplasm. These results implicate an important regulatory role for NF-kappaB in controlling the intracellular levels and/or localization of pro- as well as anti-apoptotic proteins of Bcl-2 family, the intricate balance of which is a critical determinant of downstream signaling mechanisms governing cell fate during intracellular infection.
Joshi
Suresh G
SG
Hemostasis and Thrombosis Program, Hematology-Oncology Unit, Department of Medicine, University of Rochester School of Medicine and Dentistry, P.O. Box 610, 601 Elmwood Avenue, Rochester, NY 14642, USA.
Francis
Charles W
CW
Silverman
David J
DJ
Sahni
Sanjeev K
SK
D017209:Apoptosis / Q000502:physiology*; D002352:Carrier Proteins / Q000378:metabolism; D002478:Cells, Cultured; D004730:Endothelium, Vascular / Q000166:cytology / Q000382:microbiology* / Q000502:physiology; D006801:Humans; D016328:NF-kappa B / Q000378:metabolism*; D011518:Proto-Oncogene Proteins / Q000378:metabolism; D019253:Proto-Oncogene Proteins c-bcl-2 / Q000378:metabolism; D012284:Rickettsia rickettsii / Q000472:pathogenicity*; D012373:Rocky Mountain Spotted Fever / Q000235:genetics / Q000503:physiopathology*; D014471:Umbilical Veins; D051028:bcl-2-Associated X Protein; D051025:bcl-Associated Death Protein
C494819:BAD protein, human; C494826:BAX protein, human; D002352:Carrier Proteins; D016328:NF-kappa B; D011518:Proto-Oncogene Proteins; D019253:Proto-Oncogene Proteins c-bcl-2; D051028:bcl-2-Associated X Protein; D051025:bcl-Associated Death Protein