A novel study discovered a critical pathway to break off atherosclerotic inflammation through a specific receptor called GPR32. Chronic inflammation remains a dominant element in the changing landscape of atherosclerosis, which was elegantly outlined in a recent newsletter from our Working Group (Libby P, 2021). In the atherosclerotic terrain, a radar also captures a failure in the resolution of inflammation, obstructing the active process to end inflammation and promote healing repair of the vascular wall (Bäck et al., 2019). Lipid stop-signals called resolvins are enzymatically synthesized from omega-3 fatty acids to ensure protective immunological responses for a normal termination through a functional resolution of inflammation (Serhan CN, 2014).
Resolvin D1 (RvD1) signals through a G-protein coupled receptor (GPR) termed GPR32. We found that this receptor is dysregulated in human atherosclerotic lesions, indicating a disruption of lesional healing processes. Such failed resolution of inflammation may take part in the mining of the atherosclerotic ground to become vulnerable and eventually rupture as a cause of cardiovascular events. Transcriptional associations and immunohistochemical co-localizations identified resident pro-resolving macrophages as the main host of the atherosclerotic GPR32 expression and the effector cell that transduced RvD1 signaling through GPR32 towards resolution.
The lack of murine GPR32 homologues has hampered the mechanistic insights and pathophysiological exploration of this receptor transduction of the resolution of inflammation. This is further complicated by its ligand RvD1 also signaling through the FPR2/ALX receptor. To be able decipher the pro-resolving role of GPR32 in atherosclerosis, a novel mouse model was created by introducing the human GPR32 receptor to atherosclerotic apolipoprotein knock-out mice with an additional genetic deletion of the FPR2/ALX receptor (upper left corner of the Figure). This work generated a novel functional model with similar GPR32 signaling to humans and permitted to reveal that the RvD1 – GPR32 pathway resolved inflammation and reduced atherosclerosis as illustrated in the Figure.
Exudates from an early time-point after induction of peritonitis contained lower neutrophil counts and lower inflammatory chemokine levels in mice expressing compared with those lacking human GPR32. During the resolution phase, which occurs 24-48h after peritonitis-induction, the macrophage to neutrophil ratio was increased in mice transgenically expressing the human GPR32 receptor. Importantly, the GPR32-expressing macrophages contained engulfed apoptotic neutrophils, illustrating the importance of this efferocytosis process to actively resolve and limit an inflammatory reaction. These results showed that GPR32 participates to both limit the acute phase and to enhance the resolution phase to enable a functional self-limited immune response.
Enabling an endogenous resolution of inflammation would potentially transfer the chronic atherosclerotic inflammation into a functional self-limited immune response. Indeed, mice expressing the human GPR32 exhibited decreased atherosclerosis. The importance of the resolution phase was also illustrated by the decreased necrotic core in GPR32-expressing atherosclerotic lesions, pointing to enhanced cleaning mechanisms towards vascular healing and less vulnerable atherosclerotic lesions.
The macrophage uptake mechanisms are perfect tools in a healing response but must be tuned to perform the appropriate tasks. In isolated macrophages, the GPR32 pro-resolving ligand AT-RvD1 enhanced zymosan phagocytosis but reduced the uptake of oxidized low density lipoprotein LDL (oxLDL). In contrast AT-RvD1 did not alter these uptake mechanisms in macrophages derived from mice not expressing the human GPR32 receptor. As illustrated in the Figure, these macrophage uptake mechanisms may be decisive for the crossroad turning point leading to either the resolving inflammation or the chronic inflammatory and vulnerable atherosclerotic terrain.
The results depicted in the Figure underline the importance to restore the failed resolution of atherosclerotic inflammation and provide novel evidence for GPR32 in the beneficial effects of omega-3-derived lipid mediators. These findings may pave the way for a better understanding how to turn-on the resolution processes to reshape the inflammatory atherosclerosis landscape.
Figure
Abbrevations: hGPR32mycTg×Fpr2-/-×Apoe-/-: Expression of the human G-protein coupled receptor 32 (hGPR32) transgenically (Tg) in mice with a deletion of the Fpr2 receptor and apolipoprotein E (apoe). oxLDL: oxidized low density lipoprotein, ERK: extracellular signal-regulated kinase; P: phosphorylation. Illustration created with BioRender.com.
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