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DOI: 10.1055/s-0030-1268973
Novel concept of plaque progression: TLR4 activation propagates inflammatory response in macrophages
Objective: Macrophage clearance in plaques is initiated upon cholesterol-induced apoptosis that is mediated by the unfolded protein response (UPR). Toll-like receptor 4 (TLR4) signaling has been shown to promote atherogenesis. The aim of the present study was to evaluate interactions of TLR4 with the UPR signaling cascade.
Methods: Peritoneal macrophages of wildtype or genetically modified (TLR4–/–, TRIF–/–, CHOP–/–) mice as well as macrophages of human origin were stimulated with cholesterol to undergo apoptosis with or without addition of TLR4 ligands (LPS). In addition, in vivo experiments with WT and TRIF–/– mice were conducted. Protein expresssion and activation of TLR4 and UPR signaling pathways was assessed by western blotting and qPCR.
Results: TLR4 stimulation offsets macrophage apoptosis induced by cholesterol-loading. The unfolded protein response was blocked at the level of ATF4 thereby preventing expression of proapoptotic transcription factor CHOP. This led to a significant decrease in macrophage apoptosis as measured by TUNEL and annexin V test. TLR4 signaling was mediated through the TRIF-TRAM-branch of TLR4, additionally involving regulatory factors IRF5 and IRF7. In addition, in vivo experiments could corroborate this effect in mouse macrophages.
Conclusion: Macrophage apoptosis following uptake (scavenging) of cholesterol with subsequent removal (phagocytosis) of the cells represents an important clearing step of atherosclerotic lesions. TLR4 ligands are capable of blocking macophage apoptosis thereby propagating the presence of inflammatory macrophages and enhancing the plaque progression. These novel results can provide explanation for the proatherogenic role of TLR4 and the increased cardiovascular mortality in sepsis.