Platelets induce endoplasmic reticulum stress in macrophages in vitro.

BACKGROUND: Endoplasmic reticulum (ER) stress is a key feature of lipid-laden macrophages and contributes to the development of atherosclerotic plaques. Blood platelets are known to interact with macrophages and finetune effector functions such as inflammasome activation and phagocytosis. However, the effect of platelets on ER stress induction is unknown. The objective of this study is to elucidate the potential of platelets to regulate ER stress in macrophages in vitro.

METHODS: Bone marrow-derived macrophages (BMDM) and RAW 264.7 cells were incubated with isolated murine platelets, and ER stress and inflammation markers were determined by RT-qPCR, western blotting, and ELISA. ER morphology was investigated by electron microscopy. Cell viability, lipid accumulation, and activation were measured by flow cytometry. To gain mechanistic insights, co-incubation experiments were performed with platelet decoys/releasates, as well as lipopolysaccharide, blocking antibodies, and TLR4 inhibitors.

RESULTS: Co-incubation of platelets and macrophages led to elevated levels of ER stress markers (BIP, IRE1α, CHOP, XBP1 splicing) in murine and human macrophages, which led to a pronounced enlargement of the ER. Macrophage ER stress was accompanied by increased release of pro-inflammatory cytokines and intracellular lipid accumulation, but not cell death. Platelet decoys, but not platelet releasates or lysate from other cells, phenocopied the effect of platelets. Blocking TLR4 inhibited inflammatory activation of macrophages but did not affect ER stress induction by platelet co-incubation.

CONCLUSION: This study is the first to demonstrate that platelets induce ER stress and UPR in macrophages by heat-sensitive membrane proteins independent of inflammatory activation of macrophages.