Pro-Angiogenic Activity of TLRs and NLRs: A Novel Link Between Gut Microbiota and Intestinal Angiogenesis

Abstract

Background & Aims In intestinal inflammation the gut microbiota induces an innate immune response by activating epithelial and immune cells that initiate or maintain inflammation. We investigated whether the microbiota can also activate local microvascular cells and induce angiogenesis. Methods Human intestinal microvascular endothelial cells (HIMEC) and intestinal fibroblasts (HIF) were exposed to bacterial ligands specific for TLR2/6 and 4, and NOD1 and NOD2, and cell proliferation, migration, transmigration, tube formation and production of pro-angiogenic factors were measured. The ability of the ligands to induce ex vivo vessel sprouting in an aortic ring assay and in vivo angiogenesis using a collagen gel assay were also assessed. Results Bacterial ligands induced proliferation, migration, transmigration, tube formation of HIMEC, vessel sprouting and in vivo angiogenesis; they also stimulated production of angiogenic factors from HIMEC and HIF, and HIF-derived angiogenic factors promoted HIMEC proliferation. To various degrees, all ligands induced angiogenic responses, but these were ligand- and cell type-dependent. Responses were mediated through RIP2-and TRAF6-dependent signaling, involved the MAPK and NF-κB pathways and the upregulation of VEGF-R2 and FAK. Knockdown of RIP2 and TRAF6 by RNA interference and neutralization of IL-8, bFGF and VEGF inhibited TLR/NLR-induced HIMEC angiogenesis. Conclusions The gut microbiota can selectively activate mucosal endothelial and mesenchymal cells to promote specific angiogenic responses in a TLR- and NLR-dependent fashion. This innate immunity-mediated response may expand the mucosal microvascular network, foster immune cell recruitment, and contribute to chronic intestinal inflammation.

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