Enteric commensal bacteria induce extracellular signal-regulated kinase pathway signaling via formyl peptide receptor-dependent redox modulation of dual specific phosphatase 3.

The normal microbial occupants of the mammalian intestine are crucial for maintaining gut homeostasis, yet the mechanisms by which intestinal cells perceive and respond to the microbiota are largely unknown. Intestinal epithelial contact with commensal bacteria and/or their products has been shown to activate noninflammatory signaling pathways, such as extracellular signal-related kinase (ERK), thus influencing homeostatic processes. We previously demonstrated that commensal bacteria stimulate ERK pathway activity via interaction with formyl peptide receptors (FPRs). In the current study, we expand on these findings and show that commensal bacteria initiate ERK signaling through rapid FPR-dependent reactive oxygen species (ROS) generation and subsequent modulation of MAP kinase phosphatase redox status. ROS generation induced by the commensal bacteria Lactobacillus rhamnosus GG and the FPR peptide ligand, N-formyl-Met-Leu-Phe, was abolished in the presence of selective inhibitors for G protein-coupled signaling and FPR ligand interaction. In addition, pretreatment of cells with inhibitors of ROS generation attenuated commensal bacteria-induced ERK signaling, indicating that ROS generation is required for ERK pathway activation. Bacterial colonization also led to oxidative inactivation of the redox-sensitive and ERK-specific phosphatase, DUSP3/VHR, and consequent stimulation of ERK pathway signaling. Together, these data demonstrate that commensal bacteria and their products activate ROS signaling in an FPR-dependent manner and define a mechanism by which cellular ROS influences the ERK pathway through a redox-sensitive regulatory circuit.

Commensal-epithelial signaling mediated via formyl peptide receptors.

Commensal bacteria and/or their products engender beneficial effects to the mammalian gut, including stimulating physiological cellular turnover and enhancing wound healing, without activating overt inflammation. In the present study, we observed commensal bacteria-mediated activation of the noninflammatory extracellular signal-regulated kinase[ERK]/mitogen-activated protein kinase and Akt signaling pathways in gut epithelial cells and delineated a mechanism for this bacterially activated signaling. All tested strains of commensal bacteria induced ERK phosphorylation without stimulating pro-inflammatory phospho-IκB or pro-apoptotic phospho-c-Jun NH(2)-terminal kinase, with Lactobacillus species being most potent. This pattern of signaling activation was recapitulated using the peptide N-formyl-Met-Leu-Phe, a bacterial product known to stimulate signaling events in mammalian phagocytes. Sensing of N-formyl-Met-Leu-Phe by gut epithelial cells occurs via recently characterized formyl peptide receptors located in the plasma membrane. Both commensal bacteria and N-formyl-Met-Leu-Phe application to the apical surface of polarized gut epithelial cells resulted in specific formyl peptide receptor activation. In addition, pretreatment of model epithelia and murine colon with Boc2 (a specific peptide antagonist) or pertussis toxin (a G(i)-protein inhibitor) abolished commensal-mediated ERK phosphorylation. Taken together, these data show that commensal bacteria specifically activate the ERK/mitogen-activated protein kinase pathway in an formyl peptide receptor-dependent manner, delineating a mechanism by which commensal bacteria contribute to cellular signaling in gut epithelia.