Model of how lesion stage and phagocytic efficiency might determine the consequences of macrophage apoptosis in atherosclerosis.

The laboratory utilizes cell-culture models and induced mutant mice to explore areas of macrophage cellular and molecular biology that are pertinent to the development of atherosclerosis. A major focus of the laboratory is the molecular and cellular mechanisms and consequences resulting from macrophage apoptosis and from the phagocytic clearance of apoptotic cells, particularly in advanced atherosclerotic lesions. We have elucidated a multi-hit model of macrophage apoptosis that involves an ER stress pathway known as the Unfolded Protein Response (UPR). The key distal UPR effector responsible for cholesterol-induced macrophage death is the molecule CHOP (GADD153). CHOP enables apoptosis by promoting calcium release from the ER, which in turn induces a number of downstream apoptotic pathways through activation of calcium/calmodulin-dependent protein kinase II (CaMKII), STAT1, and NADPH oxidase. However, apoptosis requires a "second hit," and we have evidence that engagement of macrophage pattern recognition receptors (PRRs), notably toll-like receptors and scavenger receptors, play an important role in this regard. PRR engagement triggers apoptosis in ER-stressed macrophages by further promoting apoptotic signaling and suppressing compensatory cell survival signaling. We have developed a number of genetically altered mouse models to test these ideas in vivo. For example, deficiencies of CHOP, STAT1, or both SRA and CD36 (scavenger receptors) decrease advanced lesional macrophage apoptosis and plaque necrosis in Apoe-/- and/or Ldlr-/- mice. Moreover, in collaboration with Drs. Domenico Accili and Alan Tall at Columbia, we have shown the relevance of this overall pathway of macrophage apoptosis to advanced atherosclerosis in insulin resistant-states. Finally, the laboratory has in-vitro mechanistic studies and in-vivo mouse studies to explore the critical role of apoptotic cell clearance ("efferocytosis"), which we feel is a major determinant of whether macrophage death leads to the beneficial consequence of decreased cellularity or to the detrimental plaque-disrupting consequence of lesional necrosis. We discovered that a receptor called Mertk plays a critical role in the efferocytosis of apoptotic macrophages both in vitro and in advanced atherosclerotic lesions. The future goals of the laboratory are to pursue each of the areas in more depth both mechanistically and physiologically and to continually pinpoint areas of therapeutic potential, particularly in preventing the conversion of benign atherosclerotic lesions into disease-causing vulnerable plaques.