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Background and Aims: Increased receptor tyrosine kinase (RTK) activity has been historically linked to atherosclerosis. Paradoxically, we recently found that global deficiency in c-Kit function increased atherosclerosis in hyperlipidemic mice. This study aimed to investigate if such unusual atheroprotective phenotype depends upon c-Kit's function in smooth muscle cells (SMC)., Methods: We studied atherosclerosis in a SMC-specific conditional knockout mice (Kit SMC ) and control littermate. Tamoxifen (TAM) and vehicle treated mice were fed high fat diet for 16 weeks before atherosclerosis assessment in the whole aorta using oil red staining. Smooth muscle cells were traced within the aortic sinus of conditional c-Kit tracing mice (Kit SMC eYFP) and their control littermates (Kit WT eYFP) by immunofluorescent confocal microscopy. We then performed RNA sequencing on primary SMC from c-Kit deficient and control mice, and identified significantly altered genes and pathways as a result of c-Kit deficiency in SMC., Results: Atherosclerosis significantly increased in Kit SMC mice with respect to control groups. In addition, the loss of c-Kit in SMC increased plaque size and necrotic core area in the aortic sinus of hyperlipidemic mice. Smooth muscle cells from Kit SMC eYFP mice were more prone to migrate and express foam cell markers (e.g., Mac2 and MCAM) than those from control littermate animals. RNAseq analysis showed a significant upregulation in genes associated with cell proliferation, migration, lipid metabolism, and inflammation secondary to the loss of Kit function in primary SMCs., Conclusions: Loss of c-Kit increases SMC migration, proliferation, and expression of foam cell markers in atherosclerotic plaques from hyperlipidemic mice., (Published by Elsevier B.V.)

Atherosclerosis is the most common underlying cause of cardiovascular morbidity and mortality worldwide. c-Kit (CD117) is a member of the receptor tyrosine kinase family, which regulates differentiation, proliferation, and survival of multiple cell types. Recent studies have shown that c-Kit and its ligand stem cell factor (SCF) are present in arterial endothelial cells and smooth muscle cells (SMCs). The role of c-Kit in cardiovascular disease remains unclear. The aim of the current study is to determine the role of c-Kit in atherogenesis. For this purpose, atherosclerotic plaques were quantified in c-Kit-deficient mice (Kit Mut ) after they were fed a high-fat diet (HFD) for 16 wk. Kit Mut mice demonstrated substantially greater atherosclerosis compared with control (Kit WT ) littermates ( P < 0.01). Transplantation of c-Kit-positive bone marrow cells into Kit Mut mice failed to rescue the atherogenic phenotype, an indication that increased atherosclerosis was associated with reduced arterial c-Kit. To investigate the mechanism, SMC organization and morphology were analyzed in the aorta by histopathology and electron microscopy. SMCs were more abundant, disorganized, and vacuolated in aortas of c-Kit mutant mice compared with controls ( P < 0.05). Markers of the "contractile" SMC phenotype (calponin, SM22α) were downregulated with pharmacological and genetic c-Kit inhibition ( P < 0.05). The absence of c-Kit increased lipid accumulation and significantly reduced the expression of the ATP-binding cassette transporter G1 (ABCG1) necessary for lipid efflux in SMCs. Reconstitution of c-Kit in cultured Kit Mut SMCs resulted in increased spindle-shaped morphology, reduced proliferation, and elevated levels of contractile markers, all indicators of their restored contractile phenotype ( P < 0.05). NEW & NOTEWORTHY This study describes the novel vasculoprotective role of c-Kit against atherosclerosis and its function in the preservation of the SMC contractile phenotype.

The primary purpose of this investigation was to determine whether ApoE(-/-) mice, when subjected to chronic stress, exhibit lesions characteristic of human vulnerable plaque and, if so, to determine the time course of such changes. We found that the lesions were remarkably similar to human vulnerable plaque, and that the time course of lesion progression raised interesting insights into the process of plaque development. Lard-fed mixed-background ApoE(-/-) mice exposed to chronic stress develop lesions with large necrotic core, thin fibrous cap and a high degree of inflammation. Neovascularization and intraplaque hemorrhage are observed in over 80% of stressed animals at 20 weeks of age. Previously described models report a prevalence of only 13% for neovascularization observed at a much later time point, between 36 and 60 weeks of age. Thus, our new stress-induced model of advanced atherosclerotic plaque provides an improvement over what is currently available. This model offers a tool to further investigate progression of plaque phenotype to a more vulnerable phenotype in humans. Our findings also suggest a possible use of this stress-induced model to determine whether therapeutic interventions have effects not only on plaque burden, but also, and importantly, on plaque vulnerability.