Ira Tabas, MD, PhD, Columbia University

The cellular and molecular biology of macrophages during atherogenesis The laboratory utilizes cell-culture models and induced mutant mice (i.e., transgenics and knockouts) to explore areas of macrophage cellular and molecular biology that are pertinent to the development of atherosclerosis. During atherogenesis, macrophages become loaded with cholesterol ("foam cell" formation), a process that plays a critical role in this disease process. In this context, the laboratory studies basic cellular processes involved in the uptake, intracellular trafficking, metabolism, and cell biological effects of cholesterol.

A major focus of the laboratory are the molecular and cellular consequences resulting from the accumulation of unesterified, or "free", cholesterol (FC) by macrophages, which is an important event in atherosclerosis. A relatively early phase of FC loading involves the induction of phosphatidylcholine biosynthesis. We have used a macrophage-specific knockout mouse (Cre-lox manipulation of a critical gene in phosphatidylcholine biosynthesis) to show that the up-regulation of PC biosynthesis in cholesterol-loaded macrophages (above) is an adaptive response that protects the cells from cholesterol-mediated death. Eventually this adaptive response fails, and the cells undergo a series of apoptotic events. In this context, the laboratory has a major project exploring cell death pathways in cholesterol-loaded macrophages. We recently found that both activation of Fas ligand and induction of Bax, leading to activation of the mitochondrial apoptosis pathway, are involved. More detailed cell biological studies, as well as the creation of genetically altered mouse models, are underway to further explore these ideas. For example, we have developed macrophage-targeted bcl-2 knockout mouse to explore the role of macrophage apoptosis in vivo.

Most interestingly, our recent work has revealed the proximal signal transduction pathways that link FC accumulation to the induction of apoptosis. While the paradigm had been that FC loading of the plasma membrane cause dysfunction of enzymes and transporters in that site, we have found that the critical signaling events take place in the endoplasmic reticulum (ER). In particular, cholesterol loading of the ER membrane induces a classic "ER stress" pathway known as the Unfolded Protein Response (UPR). Through a series of upstream kinases and downstream transcription factors, the UPR induces a large number of genes that attempt to relieve ER stress and, if unsuccessful, to trigger apoptosis. We have utilized mutant mice to show that cholesterol trafficking to the ER is important in macrophage death in atherosclerotic lesions in vivo and that a critical UPR-induced transcription factor is expressed in atherosclerotic lesions. Moreover, using cultured macrophages from mice with null mutations in key UPR genes, we have shown that the UPR plays an important role in FC-induced macrophage apoptosis. In current studies, we are exploring the mechanism in which FC loading of the ER membrane induces the UPR. Moreover, we have conducted a cDNA microarray comparison of gene expression in control and FC-loaded macrophages, which has revealed a set of fascinating genes that are altered by FC accumulation in macrophages. Similar studies using proteomics are planned for the future. Finally, we are using induced mutant mice to study the effect of the UPR on atherosclerosis in vivo.

Macrophage cholesterol loading triggers another cell signaling pathway that leads to the accelerated degradation of an important cell-surface molecule-ABCA1-involved in cholesterol efflux from cells. Current studies are directed at elucidating the protein degradation pathway involved in this event and how this pathway is induced by cholesterol loading. There is evidence to suggest that this intracellular protein degradation pathway is somehow related to the UPR (see above).

Other projects in the laboratory study mechanisms of cholesterol uptake and trafficking by macrophages. The laboratory discovered that a novel arterial-wall enzyme, called secretory sphingomyelinase (S-SMase), modifies cholesterol-carrying lipoproteins in such a way to promote internalization by macrophages. Most interestingly, S-SMase may also play important roles cytokine signaling leading to apoptosis and in host defense. Furthermore, the gene that gives rise to S-SMase also encodes a lysosomal SMase (L-SMase), and we have recently shown that this enzyme has profound effects on vesicular trafficking in macrophages. Current work using confocal and multiphoton fluorescence microscopy is directed at further characterizing lipid and protein trafficking in mutant macrophages defective in L-SMase. In addition, we have developed both cell-culture and murine transgenic and knockout models to explore these proposed functions of S-SMase and L-SMase.

The internalization of cholesterol-containing lipoproteins is mediated by a unique phagocytic-like process that requires specific cytoskeletal signaling processes involving both tyrosine- and PI3-kinases. Current projects are exploring the roles of the cytosolic domains and adaptor molecules of two lipoprotein receptors in phagocytosis by transfecting macrophages with a variety of chimeric receptors.