Mucosal Immunity and the FOXO1 Transcription Factors

Abstract

FOXO1 transcription factors affect a number of cell types that are important in the host response. Cell types whose functions are modulated by FOXO1 include keratinocytes in the skin and mucosal dermis, neutrophils and macrophages, dendritic cells, Tregs and B-cells. FOXO1 is activated by bacterial or cytokine stimulation. Its translocation to the nucleus and binding to promoter regions of genes that have FOXO response elements is stimulated by the MAP kinase pathway and inhibited by the PI3 kinase/AKT pathway. Downstream gene targets of FOXO1 include pro-inflammatory signaling molecules (TLR2, TLR4, IL-1β, and TNF-α), wound healing factors (TGF-β, VEGF, and CTGF) adhesion molecules (integrins-β1, -β3, -β6, α_vβ₃, CD11b, CD18, and ICAM-1), chemokine receptors (CCR7 and CXCR2), B cell regulators (APRIL and BLYS), T-regulatory modulators (Foxp3 and CTLA-4), antioxidants (GPX-2 and cytoglobin), and DNA repair enzymes (GADD45α). Each of the above cell types are found in oral mucosa and modulated by bacteria or an inflammatory microenvironment. FOXO1 contributes to the regulation of these cells, which collectively maintain and repair the epithelial barrier, formation and activation of Tregs that are needed to resolve inflammation, mobilization, infiltration, and activation of anti-bacterial defenses in neutrophils, and the homing of dendritic cells to lymph nodes to induce T-cell and B-cell responses. The goal of the manuscript is to review how the transcription factor, FOXO1, contributes to the activation and regulation of key leukocytes needed to maintain homeostasis and respond to bacterial challenge in oral mucosal tissues. Examples are given with an emphasis on lineage specific deletion of Foxo1 to explore the impact of FOXO1 on cell behavior, inflammation and susceptibility to infection.

Keywords: bacteria; bone loss; forkhead; gingiva; immune; mucosa; periodontal disease; periodontitis.

Figure 1

FOXO1 is activated in keratinocytes to induce gene expression that modulates keratinocyte behavior. FoxO1 induces the expression of genes that affect keratinocyte function such as TGF-ß, VEGF, CTGF, integrins beta-1, -3, -6, Keratin-1, -10, -14, antioxidants GPX-2, cytoglobin, catalase and superoxide dismutase, and a DNA repair enzyme, Gadd45α. Bacteria through pattern recognition receptors or cytokine stimulation induces formation of reactive oxygen species that activate components of the MAP kinase pathway such as JNK, which stimulate FoxO1 nuclear localization where FoxO1 modulates gene transcription.

Figure 2

FOXO1 regulates activation and function of dendritic cells. (A) FoxO1 nuclear localization is stimulated by activation of the MAP kinase pathway (JNK, p38, and ERK). In the nucleus FOXO1 affects DC function by increasing expression of genes such as ICAM1, CCR7, APRIL, BLYS, IL-1-β, IL-6, IL12, IFN-γ, TNF-α, and αvβ3. The change in gene expression promotes DC homing to the lymph nodes, antigen presentation, DC activation of T-and B- cells, and inflammation. (B) Cytokine receptors and pattern recognition receptors such as toll-like receptors (TLRs) stimulate activation of FOXO1 through the MAP kinase pathway. Activated FOXO1 can bind to the promoter region of target genes and regulate transcription. AKT is a major downstream target of PI3K that functions as a negative regulator of FOXO1. Stimulation of mTOR activates AKT to inhibit FoxO1 activity, which has been proposed to prevent a hyperinflammatory response.

Figure 3

FOXO1 regulates formation of T-regulatory cells. FoxO1 induces Foxp3, CTLA-4, IL-10, and TGF-β to enhance Treg formation and function. Activation of AKT phosphatase blocks AKT activation, which functions to increase FOXO1 nuclear localization to maintain Treg function.

Figure 4

FOXO1 increases production of inflammatory mediators by monocytes/macrophages. Pattern recognition receptors and cytokine receptor stimulation induces FOXO1 activity. FoxO1 increases expression of TLRs, IL-1β, CXCL10, MIP-1α, TNF-α, IL-6, and IFN-γ and can also reduce IL-10, which combined, increase inflammation. Pattern recognition receptors such as TLR4 can induce CaMKKβ that leads to increased FOXO1 nuclear localization and activation, which has been linked to increased bacterial killing. In addition, FOXO1 can act to increase phagocytosis and reduce apoptosis. There is conflicting data on FOXO1 and macrophage polarization. In tumor-associated macrophages FOXO1 is reported to increase the M1 phenotype but in asthma to increase the M2 phenotype.

Figure 5

FOXO1 increases neutrophil mobilization and function. The formation of ROS and NOS induced by pattern recognition and cytokine receptors increase FOXO1 nuclear localization and activity. FOXO1 nuclear localization is also stimulated by deacetylation due to deacetylase activity. FOXO1 increases expression of TLR2, TLR4, TNF, and IL-1β to increase inflammation. CXCL2 and CXCR2 are induced by FOXO1 and are associated with enhanced neutrophil mobilization resulting from infection. CD11b and CD18 are integrins that are induced by FOXO1 and facilitate migration, phagocytosis, and bacterial killing.