Objective: We reported that human monocyte cell line, THP-1, underwent apoptosis by Actinobacillus actinomycetemcomitans infection. In addition, TNF-alpha production was increased by A. actinomycetemcomitans infection. Mitogen-activated protein kinases (MAPK) signal cascade is activated in response to physiological stresses including protein synthesis inhibitors, oxidative agents and TNF-alpha. Several reports suggested that activated MAPK contributes to the secretion of cytokines from human monocytes/macrophages mediated by lipopolysaccharide and bacterial components. However, the role of MAPK is not clear in regulating apoptosis after bacterial infection. The aim of this study was to examine a role of MAPK of apoptosis in A. actinomycetemcomitans-infected monocytes. Methods: A. actinomycetemcomitanswas added to THP-1 cells suspended in microtubes, and the tubes were centrifuged at 1000 x g for 10 min. The tubes were incubated at 37 degrees for 30 min, and non-adherent bacteria were washed out by a series of centrifugation. Infected cells were transferred to 24-well culture plate, and incubated for 24 hours in presence or absence of SB203580, an inhibitor of p38 MAPK and SP600125, an inhibitor of JNK. Culture supernatant and nuclei were prepared from the infected cells to measure for LDH activity, TNF-alpha production, DNA fragmentation and MAPK activity. Results: Both p38 MAPK and JNK activities were increased in A. actinomycetemcomitans-infected cells. LDH activity and DNA fragmentation were inhibited by addition of SB203580 and SP600125. DNA ladder-like pattern was disappeared by MAPK inhibitors. Furthermore, these MAPK inhibitors suppressed TNF-alpha production from infected cells in a dose-depended manner. Conclusion: These results indicate that MAPK signal cascades, p38 MAPK and JNK, are involved in apoptotic cell death in A. actinomycetemcomitansI infected monocytes.

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