Porphyromonas gingivalisinfection sequesters pro-apoptotic Bad through Akt in primary gingival epithelial cells

Abstract

SUMMARY Porphyromonas gingivalis, a self-limiting oral pathogen, can colonize and replicate in gingival epithelial cells (GECs). P. gingivalis-infected GECs are protected from mitochondrion-dependent apoptosis, partially through activation of phosphatidyl inositol 3-kinase/Akt signaling. Biochemical events associated with P. gingivalisinduced inhibition of apoptosis include the blocking of mitochondrial membrane permeability and cytochrome-c release. We studied functional importance of Akt and the status of associated key mitochondrial molecules, pro-apoptotic Bad and caspase-9, during infection of GECs. We found that P. gingivalis infection caused significant phosphorylation of Bad progressively, while messenger RNA levels for Bad slowly decreased. Fluorescence microscopy showed translocation of the mitochondrial Bad to the cytosol post-infection. Conversely, P. gingivalis lost the ability to promote phosphorylation and translocation of Bad in Akt-deficient GECs. Caspase-9 activation induced by a chemical inducer of apoptosis was significantly inhibited by infection over time. However, Akt depletion by small interfering RNA did not reverse inhibition of caspase-9 activation by infection. Hence, P. gingivalis inactivates pro-apoptotic Bad through Akt. The inhibition of caspase-9 activation appears to be independent of Akt. Overall, our findings suggest that Akt is a key component of anti-apoptotic pathways stimulated by P. gingivalis. The P. gingivalis uses other mitochondrial pathways to protect host cells from cell-death and to ensure its survival in gingival epithelium.

Attenuation of the Phosphatidylinositol 3-Kinase/Akt Signaling Pathway by Porphyromonas gingivalis Gingipains RgpA, RgpB, and Kgp

“…However, other groups have reported the activation of PI3K and Akt by P. gingivalis infection (30,31,43,48). These apparently contradictory results may be due to differences in cell type and experimental conditions such as infection time, strains, and culture conditions of P. gingivalis.…”

“…Interestingly, bacteria and their virulence factors have been shown to regulate this pathway positively or negatively during infection (26 -29). P. gingivalis activates the PI3K/Akt signaling pathway, which is linked to cell survival (30,31) and immune responses (32). Among several known virulence factors of P. gingivalis, both LPS and fimbriae regulate immune responses through PI3K/Akt activation in monocytes/macrophages (5,(33)(34)(35) and human gingival fibroblasts (36).…”

Vitamin D-Mediated Induction of Innate Immunity in Gingival Epithelial Cells

“…Three-dimensional (3D) Transwell cultures were grown as previously described (15,16). Primary cultures of normal human gingival epithelial cells were grown as previously described (31,55).…”

Noncanonical Activation of β-Catenin by Porphyromonas gingivalis

“…Processes that could be relevant to the development of cancer include the ability of P. gingivalis to suppress apoptosis in gingival epithelial cells (20,21) and accelerate progression through the S phase of the cell cycle (22). Indeed, P. gingivalis alters the expression and activity of a number of proteins involved in cell cycle regulation, including several cyclins and cyclin-dependent kinases (22).…”