HSP27 Mediates PAR1‐stimulated Pro‐inflammatory Vascular Responses

Abstract

Endothelial cells line the lumen of blood vessels and create a semi‐permeable barrier that is disrupted during vascular inflammation resulting in increased permeability, tissue edema, and subsequent organ failure. Endothelial dysfunction also results in cytokine production and leukocyte recruitment. However, the molecular mechanisms responsible for the pro‐inflammatory state remains to be elucidated. Protease‐activated receptor‐1 (PAR1) is a G protein‐coupled receptor for the coagulant protease thrombin and mediates hemostasis, thrombosis, and inflammatory response to vascular injury. However, the distinct mechanism of how endothelial PAR1 promotes vascular pro‐inflammatory responses remains poorly understood. We recently discovered that thrombin activation of PAR1 stimulates p38 activation through a non‐canonical pathway mediated by TAB1 to promote endothelial permeability. PAR1‐induced p38 activation has been linked to vascular barrier disruption; however, the mechanism by which p38 signaling contributes to pro‐inflammatory responses is not known. We found through a phosphoproteomics screen that thrombin‐stimulated p38 signaling relies on phosphorylation of heat shock protein 27 (HSP27). Moreover, we found that this pathway is physiologically relevant by showing PAR1‐activated HSP27 modulates the closing of the endothelial barrier in vitro and vascular leakage in vivo. We show HSP27 activity is both p38‐ and PAR1‐dependent. Finally, we show PAR1‐induced p38 signals to HSP27 through two p38 downstream kinases, MAPKAPK2 (MK2) and MAPKAPK3 (MK3), to directly phosphorylate HSP27 on specific serines. Together these studies reveal a novel pathway in which thrombin‐induced p38 signals through MK2 and MK3 to regulate HSP27 activity leading to recovery of the endothelial barrier and promoting resolution of pro‐inflammatory responses.Support or Funding InformationT32 Pharmacological Sciences Training Grant (NIGMS T32 GM007752) Tobacco Related Disease Research Program (TRDRP) Pre‐doctoral fellowship (27DT‐0009)