Actinobacillus actinomycetemcomitans induces apoptosis in human monocytic THP-1 cells

It has previously been reported that the murine macrophage cell line J774.1 and the human oral epithelial cell line KB undergo apoptosis as a result of Actinobacillus actinomycetemcomitans infection. Recent studies have demonstrated that apoptosis regulation is modulated by multiple phosphorylation of several different protein kinases, including the major subtypes of the mitogenactivated protein kinase (MAPK) family. The MAPK family promotes cell survival and/or proliferation in response to growth factor stimulation, or apoptosis in response to various stress stimuli. The primary objective of the present investigation was to clarify whether human immune cells undergo apoptosis following A. actinomycetemcomitans infection and, if so, to establish the involvement of the MAPK family. Human monocytic THP-1 cells were infected with A. actinomycetemcomitans in microtubes. Lactate dehydrogenase release into the culture supernatant and DNA fragmentation in the cells were monitored. DNA fragmentation was also identified by agarose gel electrophoresis. Cell death following A. actinomycetemcomitans infection occurred by apoptosis, shown by an increase in the proportion of fragmented DNA and the typical ladder pattern of DNA fragmentation indicative of apoptosis. Furthermore, p38 MAPK activity and tumour necrosis factor alpha (TNF-alpha) levels increased following A. actinomycetemcomitans infection. In contrast, cell death and TNF-alpha levels in infected cells decreased upon addition of a p38 inhibitor or an anti-TNF-alpha antibody. However, exogenous TNF-alpha could not induce apoptosis in uninfected THP-1 cells. Interestingly, p38 MAPK activity diminished in the presence of anti-TNF-alpha antibody. These findings indicated that A. actinomycetemcomitans infection induces apoptosis in THP-1 cells and that p38 MAPK activity is directly involved in apoptosis. TNF-alpha may play an indirect role in apoptosis via enhanced p38 MAPK activity. A. actinomycetemcomitans-induced apoptosis of human immune cells may be important in terms of initiation and progression of periodontal diseases.