Lipopolysaccharide Activates Toll-like Receptor 4 (TLR4)-mediated NF-κB Signaling Pathway and Proinflammatory Response in Human PericytesThis work was supported by Fondo de Investigaciones Sanitarias/Instituto de Salud Carlos III Grants PI08/90856 and PS09/00227 and Fundación Investigación Biomédica Hospital Puerta de Hierro (to L. S.) and by Ministerio de Economía y Competitividad Grant BIO2011-22738 and Comunidad de Madrid Grant S2010/BMD-2312 (to L. A.-V.)

Pericytes and mesenchymal stem cells (MSCs) are ontogenically related, and in fact, no significant phenotypic differences could be observed by flow cytometry. Transcriptome analysis of human pericytes and MSCs revealed that 43 genes were up-regulated more than 10-fold in pericytes compared with MSCs. Identification of Toll-like receptor 4 (TLR4) as one of the most abundant RNA species in pericytes with respect to MSCs and confirmation of TLR4 expression on the cell surface led us to obtain a comprehensive overview of the expression program of lipopolysaccharide (LPS)-stimulated pericytes. Transcriptional profiling of LPS-treated cells revealed that 22 genes were up-regulated more than 5-fold. Of them, 10 genes encoded chemokines and cytokines (CXCL10, CCL20, IL8, CXCL1, IL6, CCL2, IL1B, CXCL2, IL1A, and CXCL6), and three genes encoded adhesion molecules (ICAM1, VCAM1, and SELE). LPS induced nuclear translocation of the transcription factor NF-κB in stimulated pericytes. Moreover, inhibition of NF-κB activation by SC-514 blocked LPS-induced up-regulation of a subset of chemokine genes, confirming the key role of NF-κB in LPS signaling in pericytes. At the protein level, we assessed the secretion of the proinflammatory cytokines and chemokines IL-6, IL-8, CXCL1, CXCL2, CXCL3, and CCL2 not only after LPS treatment but also in HMGB1-stimulated pericytes. Up-regulation of the adhesion molecules ICAM-1 and VCAM-1 resulted in an increased adhesion of peripheral blood leukocytes to an LPS-treated pericyte monolayer. The role of pericytes in the inflammatory context has been scarcely addressed; according to these results, pericytes should be considered as active players in the inflammatory cascade with potential physiopathological implications. Background: Endothelial cells are well known inflammatory effectors, but TLR4 expression and function in human pericytes have not been addressed. Results: Pericytes express TLR4, and stimulation with LPS or HMGB1 promotes cytokine and chemokine secretion and overexpression of adhesion molecules via NF-κB. Conclusion: TLR4 activation in pericytes triggers a proinflammatory and proangiogenic program. Significance: Pericytes are active players in inflammatory responses beyond their homeostatic role.