Abstract Body: In human atherosclerosis, a positive-feedback cycle in lesional macrophages (MΦs) involving efferocytosis and inflammation resolution is impaired, leading to unstable plaques with thin fibrous caps. Lowering plasma LDL cholesterol reawakens this effero-resolution cycle in lesional MΦs and promotes plaque stabilization ("atherosclerosis regression"). However, the MΦ-intrinsic mechanisms that restore fibrous cap integrity during regression remain poorly understood. We recently demonstrated (Nature Metabolism, 2024) that efferocytosis activates a tryptophan (Trp) metabolism pathway in mouse and human MΦs essential for resolution and fibrous cap thickening during atherosclerosis regression. In this pathway, Trp derived from the phagolysosomal degradation of apoptotic cells (ACs) is transported into the MΦ cytoplasm by the Trp transporter SLC36A4, fueling indoleamine 2,3-dioxygenase 1 (IDO1)–dependent kynurenine (Kyn) production, aryl hydrocarbon receptor (AhR) activation, and AhR-mediated induction of pro-resolving and athero-protective IL-10 and TGF-β1. Silencing SLC36A4, IDO1, or AhR in effero-MΦs blocks this pro-resolving pathway, and genetic disruption of MΦ IDO1 or AhR during athero-regression lowers or impairs resolution in lesional MΦs and prevents fibrous cap thickening.
Here, we define how efferocytosis regulates SLC36A4, the upstream gatekeeper of this athero-protective pro-resolving pathway. SLC36A4 protein abundance, but not mRNA expression, was increased soon after mouse or human MΦs were exposed to ACs, indicating rapid post-translational regulation. We discovered that proteins released from ACs blocked the normal, rapid proteasomal turnover of SLC36A4, leading to its upregulation. Using biochemical, proteomic, and genetic approaches, we found that AC releasate inhibited the E3 ubiquitin ligase MARCH1, leading to decreased ubiquitination and proteasomal degradation of SLC36A4. Accordingly, silencing or knockout of MARCH1 upregulated SLC36A4 in the absence of ACs. Mechanistically, AC releasate induced the expression of CD83, an endogenous inhibitor of MARCH1. Thus, silencing CD83 in MΦs exposed to ACs reduced SLC36A4 abundance and impaired downstream Trp-dependent pro-resolving signaling.
Thus, a CD83–MARCH1–SLC36A4 axis defines a novel macrophage-intrinsic mechanism by which efferocytosis sustains resolution signaling and enables fibrous cap thickening during LDL-lowering–induced atherosclerosis regression.