An Athero-Protective Pathway Mediated by Macrophage Tryptophan Metabolism and AhR Activation

About This Grant

Atherosclerotic cardiovascular disease (CVD) is the leading cause of death worldwide. While many athero- sclerotic plaques remain clinically silent, a subset of high-risk plaques with thin fibrous caps and necrosis can rupture or erode, leading to acute CVD events such as myocardial infarction. There is abundant evidence in humans and experimental models that one major cause of thin-capped, necrotic plaques is defective clear- ance of apoptotic cells (ACs) by macrophages (Ms), a process known as efferocytosis. Efferocytosis pre- vents dead cells from becoming necrotic and also activates pathways that promote both additional dead cell clearance (continuing efferocytosis) and tissue resolution. For these reasons, the impairment of efferocy- tosis that occurs in atherosclerosis promotes thin-capped, necrotic lesions. However, when plasma LDL is markedly lowered, the effero-resolution cycle is "re-awakened," leading to cap thickening and lower necrosis (regression) and, in humans, lower risk for CVD. Accordingly, understanding the mechanisms linking ef- ferocytosis to resolution and cap thickening, which represents a major gap in CVD research, could suggest novel treatments. I have begun to address this gap through my recent discovery of a novel pathway in effero- Mϕs: tryptophan (Trp) from the phagolysosomal degradation of AC proteins is metabolized by IDO1 to kynurenine, which activates the aryl hydrocarbon receptor (AhR) to promote tissue resolution. Most im- portantly, I have strong preliminary evidence that this pathway is crucial for enhancing plaque stability during athero-regression. In this context, the goal of my proposal is to elucidate the molecular-cellular mechanisms that link the Trp-AhR pathway to cap thickening; test causation in mouse models of athero-regression; and show relevance to human atherosclerosis. I hypothesize: (1) AhR activation in effero-Ms enhances cap formation by promoting crosstalk with smooth muscle cell (SMC)-derived cap forming cells; and (2) the pathway relies on two essential processes: upregulation of the lysosomal Trp transporter SLC36A4 by pro- teasomal regulation; and enhancement of continuing efferocytosis by two complementary pathways that ac- tivate the actin-remodeling GTPase Rac1. For hypothesis #1 (Aim1, K99 phase), I will use inducible knockout models of AhR and IDO1 in Ms and SMCs and single-cell RNA sequencing in SMC-tracer mice to explore how the Trp-AhR pathway in effero-Ms communicates with cap-forming cells in athero-regression to pro- mote cap thickening. For hypothesis #2 (Aim2, R00 phase), I will examine how efferocytosis enhances SLC36A4 stability by regulating the ubiquitin-proteasome system; and explore both genomic and non-ge- nomic mechanisms linking the Trp-AhR pathway to Rac1 activation and continuing efferocytosis. This re- search will provide new insights into the mechanisms of plaque stabilization and thereby suggest novel ther- apeutic ideas to promote stable plaques. Importantly, the program will provide me with outstanding mentor- ship and career guidance to enable my transition to an independent career in cardiovascular research.