The Role of Defective Macrophage Efferocytosis During Early Atherosclerosis in Humans
Abstract
Atherosclerosis is a chronic inflammatory disease characterized by thickening of the arterial wall from accumulation of lipoproteins and immune cells within the vessel intima, forming a lipid-rich plaque. Macrophages play a central role in atherosclerosis progression through phagocytic removal of apoptotic cells in a process termed efferocytosis. With disease progression however, macrophage efferocytosis becomes defective and uncleared apoptotic cells undergo secondary necrosis and form a necrotic core that increases the risk of plaque rupture. Despite the importance of defective efferocytosis in atherosclerosis, little is known about how efferocytosis becomes impaired. The aim of this thesis was to characterize mechanisms regulating efferocytic clearance of apoptotic cells and to identify potential mechanisms that contribute to impaired efferocytosis in atherosclerosis. Using a combination of mass spectrometry and microscopy-based efferocytosis uptake assays, we identified Rab17 as a regulator of apoptotic cargo trafficking to the recycling endosomes. Disruption of Rab17 function using a dominant-negative mutant resulted in mis-trafficking of apoptotic cargo away from recycling endosomes and into the MHC class II loading compartment. We then examined the gene expression profile of macrophages from the aortas of patients undergoing open heart surgery to identify potential mechanisms driving defective efferocytosis. We found over 3,000 differentially-expressed protein-coding genes in these macrophages, with particular enrichment in pathways involved in cholesterol handling, efferocytosis and efferosome maturation. Interestingly, we observed upregulation of the hematopoietic transcription factor GATA2, which has been shown by genetic linkage studies to be associated with coronary artery disease. By perturbing GATA2 expression in oxLDL-treated THP-1 human macrophages, we found that oxLDL-induced impairment in efferocytosis and efferosome maturation is dependent on GATA2. Our findings indicate that exposure to pro-atherogenic conditions induces upregulation of GATA2 expression, which specifically impairs efferocytic uptake of apoptotic cells and efferosome maturation. Our data shed new light on the process of macrophage efferosome maturation and how this process is impaired in atherosclerosis. We are also the first to describe a role for GATA2 in mediating oxLDL-induced impairment in efferocytosis. Therefore, GATA2 and other processes involved in dysregulation of efferosome maturation may be suitable therapeutic targets in human atherosclerotic disease.