Cholesterol and hematopoietic stem cells: inflammatory mediators of atherosclerosis

Atherosclerosis causing heart attack and stroke is the leading cause of death in the modern world. Therapy for end-stage atherosclerotic disease using CD34(+) hematopoietic cells has shown promise in human clinical trials, and the in vivo function of hematopoietic and progenitor cells in atherogenesis is becoming apparent. Inflammation plays a central role in the pathogenesis of atherosclerosis. Cholesterol is a modifiable risk factor in atherosclerosis, but in many patients cholesterol levels are only mildly elevated. Those with high cholesterol levels often have elevated circulating monocyte and neutrophil counts. How cholesterol affects inflammatory cell levels was not well understood. Recent findings have provided new insight into the interaction among hematopoietic stem cells, cholesterol, and atherosclerosis. In mice, high cholesterol levels or inactivation of cholesterol efflux transporters have multiple effects on hematopoietic stem cells (HSPCs), including promoting their mobilization into the bloodstream, increasing proliferation, and differentiating HSPCs to the inflammatory monocytes and neutrophils that participate in atherosclerosis. Increased levels of interleukin-23 (IL-23) stimulate IL-17 production, resulting in granulocyte colony-stimulating factor (G-CSF) secretion, which subsequently leads to HSPC release into the bloodstream. Collectively, these findings clearly link elevated cholesterol levels to increased circulating HSPC levels and differentiation to inflammatory cells that participate in atherosclerosis. Seminal questions remain to be answered to understand how cholesterol affects HSPC-mobilizing cytokines and the role they play in atherosclerosis. Translation of findings in animal models to human subjects may include HSPCs as new targets for therapy to prevent or regress atherosclerosis in patients.