Disruption of Immune Homeostasis in Human Dendritic Cells via Regulation of Autophagy and Apoptosis by Porphyromonas gingivalis

Abstract

As fundamental processes of immune homeostasis, autophagy, and apoptosis must be maintained to mitigate risk of chronic inflammation and autoimmune diseases. Periodontitis is a chronic inflammatory disease characterized by oral microbial dysbiosis, and dysregulation of dendritic cell (DC) and T cell responses. The aim of this study was to elucidate the underlying mechanisms by which the oral microbe Porphyromonas gingivalis (P. gingivalis) manipulates dendritic cell signaling to perturb both autophagy and apoptosis. Using a combination of Western blotting, flow cytometry, qRT-PCR and immunofluorescence analysis, we show a pivotal role for the minor (Mfa1) fimbriae of P. gingivalis in nuclear/cytoplasmic shuttling of Akt and FOXO1 in human monocyte-derived DCs. Mfa1-induced Akt nuclear localization and activation ultimately induced mTOR. Activation of the Akt/mTOR axis downregulated intracellular LC3II, also known as Atg8, required for autophagosome formation and maturation. Use of allosteric panAkt inhibitor MK2206 and mTOR inhibitor rapamycin confirmed the role of Akt/mTOR signaling in autophagy inhibition by P. gingivalis in DCs. Interestingly, this pathway was also linked to induction of the anti-apoptotic protein Bcl2, decreased caspase-3 cleavage and decreased expression of pro-apoptotic proteins Bax and Bim, thus promoting longevity of host DCs. Addition of ABT-199 peptide to disrupt the interaction of antiapoptotic Bcl2 and its proapoptotic partners BAK/BAX restored apoptotic death to P. gingivalis-infected DC cells. In summary, we have identified the underlying mechanism by which P. gingivalis promotes its own survival and that of its host DCs.

Porphyromonas gingivalis adopts intricate and unique molecular mechanisms to survive and persist within the host: a critical update

“…The inhibition of apoptosis contributes to the chronicity of the periodontal disease by promoting the growth of other bacterial species ( Figure 6) [17,91]. The LPS of P. gingivalis activates caspase 11 dependent noncanonical inflammasome and initiates the process of pyroptosis and lytic cell death within the macrophages [135,[184][185][186][187][188]. P. gingivalis LPS, specifically the O-antigen region and HmuY is known to affect the 'viability and apoptosis of gingival epithelial cells' [54,62,155,[187][188][189][190].…”

“…The results of their study showed that the minor (Mfa1) fimbriae of P. gingivalis induce Akt nuclear localization and activate the Akt/mTOR axis required for autophagosome formation and maturation [190]. P. gingivalis also increase the mitotic cell cycle and suppress apoptosis by inhibiting Janus A Kinase (JAK), Phosphoinositide 3 Kinase (PI3 K), Signal Transducer of Activation (STAT), alpha-serine /threonine-protein kinase (Akt) and purinoceptor (P2X7) pathways ( Figure 7) [91] Furthermore, P. gingivalis reduce the renewal capacity of cells and inhibit apoptosis by activating p38, mitogen-activated protein kinase (MAPK), and extracellular-signalregulated kinase (Erk1/2) pathways [184][185][186] The activation of p38, MAPK, and Erk1/2 pathways decrease the expression of cyclin D and inhibit cellular proliferation by arresting the cell cycle at the G1 phase [72,73]. Studies have confirmed the deregulation of apoptosis-related genes, such as Bax, Bcl2, Nlrp3, or Smad2, in the gingival tissues of patients with periodontitis [181,189] P. gingivalis has also confirmed to possess cellspecific modulation of apoptosis receptors and signaling pathways such as Apoptotic protease activating factor (APAF 1), B-cell lymphoma Associated X (Bax1) and Caspase and reduce B-cell lymphoma (BCL 2) in the epithelial cells and fibroblasts [184].…”

“…The activation of caspase 9 cleaves and activates caspase effector caspase 3 and caspase 7, leading to apoptosis [184]. The increased caspase expression following P. gingivalis infection increases the process of pyroptosis, hypoxia, and exaggerate the inflammatory burden in the periodontal tissues and systemic circulation [190][191][192][193][194][195]. P. gingivalis can even inhibit F. nucleatum induced activation of the NLR family pyrin domain containing 3 (NLRP3) inflammasome and modulates the NLRP3 inflammasome cytokine secretion from the macrophages [189].…”

Oral Microbes and Mucosal Dendritic Cells, “Spark and Flame” of Local and Distant Inflammatory Diseases

“…As a result, phosphorylated FOXO1 (p-FOXO1) translocates from the nucleus to the cytoplasm, where it undergoes proteosomal degradation, leading to inhibition of apoptosis, as FOXO1 is an important transcription factor involved in the regulation of the expression levels of proapoptotic genes such as Bim. DC infection by P. gingivalis significantly increases level of expression of phosphorylated Akt (p-Akt), increases nuclear translocation of p-Akt and cytoplasmic translocation of p-FOXO1, and ultimately decreases the levels of expression of proapoptotic proteins and extends the survival of DCs [68]. Not only does P. gingivalis disrupt homeostatic apoptosis in DCs [51], but it also disrupts DCs homing to secondary lymphoid organs via reprograming chemokine receptors on DCs, leading to transferring the inflammation to vascular sites [34].…”

“…In addition, blocking DC-SIGN by HIV glycoprotein 120 reduces P. gingivalis survival inside DCs, but the mechanism of this phenomenon was unclear [102]. A recent report revealed that inhibition of autophagy in DCs by P. gingivalis involves targeting the Akt-mTOR pathway, which is an important regulator of autophagy [68]. P. gingivalis infection increases expression of p-Akt Ser473, p-mTOR Ser2448, p-Raptor Ser792, and p-ULK1 Ser757, all of which are important elements in mTOR-dependent autophagy inhibition [68].…”