Peptidoglycan enhances secretion of monocyte chemoattractants via multiple signaling pathways

Peptidoglycan (PG) is detected in a high proportion in inflammatory cell-rich regions of human atheromatous plaques. In the present study, we determined the cellular factors involved in PG-mediated chemokine expression in mononuclear cells in order to understand the molecular mechanisms of inflammatory responses to bacterial pathogen-associated molecular patterns in the diseased artery. Exposure of human monocytic leukemia THP-1 cells to PG resulted in not only enhanced secretion of CCL2 and CCL4 but also profound induction of their gene transcripts, which were abrogated by oxidized 1-palmitoyl-2-arachidonosyl-sn-phosphatidylcholine, an inhibitor of Toll-like receptors (TLRs)-2/4, but not by polymyxin B. PG enhanced phosphorylation of Akt and mitogen-activated protein kinases and activated protein kinase C. Pharmacological inhibitors such as SB202190, SP6001250, U0126, Akt inhibitor IV, rapamycin, and RO318220 significantly attenuated PG-mediated up-regulation of CCL2 and CCL4. We propose that PG contributes to vascular inflammation in atherosclerotic plaques by upregulating expression of mononuclear cell chemoattractants via TLR-2, protein kinase C, Akt, mTOR, and mitogen-activated protein kinases.