AMPK/mTOR/p70S6K axis prevents apoptosis of Porphyromonas gingivalis-infected gingival epithelial cells via BadSer136 phosphorylation

Epithelial disruption is the initiation of most infectious disease. Regulation of epithelium apoptosis may play a key role in balance the survival competition between resident bacteria and host cells. The role of the mTOR/p70S6K pathway in preventing apoptosis of human gingival epithelial cells (hGECs) infected with Porphyromonas gingivalis (Pg) was investigated in order to further understand the survival strategy of the epithelial cells in during Pg infecting. hGECs was challenged with Pg for 4, 12, and 24 h. Additionally, hGECs was pretreated with LY294002 (PI3K signaling inhibitor) or Compound C (AMPK inhibitor) for 12 h and exposed them to Pg for 24 h. Subsequently, apoptosis was detected using flow cytometry, and expression and activity of Bcl-2, Bad, Bax, PI3K, AKT, AMPK, mTOR, and p70S6K proteins were analyzed using western blotting. Pg-infecting did not increase apoptosis of hGECs; but the expression ratio of Bad to Bcl-2 was increased after infecting. In contrast, Bad^Ser136 phosphorylation was promoted, accompanied by a significant reduction of mTOR/p70S6K and PI3K/AKT signaling, along with the upregulation of AMPK^Thr172 signaling. Morrover, the PI3K inhibitor LY294002 promoted Pg-mediated reduction of mTOR/p70S6K expression, and the increase of AMPK signaling and Bad^Ser136 phosphorylation rate, eventually decreasing apoptosis. While Compound C inhibited Pg-mediated activation of AMPK and downregulation of mTOR/p70S6K signaling, significantly reduced the Bad^Ser136 phosphorylation rate, thereby increasing apoptosis. Thus, hGECs prevent apoptosis via an inherent cellular-homeostasis, pro-survival mechanism during Pg infection, the AMPK/mTOR/p70S6K pathway helps prevent apoptosis in hGECs infected with Pg by regulating Bad^Ser136 phosphorylation.