Macrophage lipid accumulation is regulated by substrate stiffness

Abstract

Abstract Atherosclerosis is characterized by the accumulation of lipid-laden macrophage foam cells within the vessel wall, formation of a plaque, and changes in tissue stiffness from soft near the necrotic core to stiff near the fibrous cap. The mechanisms governing foam cell formation are not well understood, but recent studies demonstrate that cells probe and respond to the physical properties of their surroundings. Stiffness regulates differentiation and function of mesenchymal stem cells and endothelial cells, yet until recently, its role in immune cell function has been largely unexplored. Our lab, and others, has found that substrate stiffness regulates murine macrophage cytokine secretion and phagocytic activity. Because macrophages accumulate near the soft necrotic core where there is abundant lipid, we hypothesized that macrophages on softer surfaces would exhibit enhanced lipid accumulation. To investigate this, we cultured murine macrophages on functionalized tunable polyacrylamide gels that model physiologically relevant stiffnesses. We then used a combination of flow cytometry and immunofluorescence microscopy to show that softer substrate stiffness results in more accumulation of the model lipid, oleic acid, as well as acetylated low-density lipoprotein in macrophages. Transcriptomic analysis identified up- or down-regulated genes in macrophages in response to increased substrate stiffness, including genes implicated in atherosclerosis and lipid metabolism. Together, these data demonstrate that lipid accumulation by macrophages is regulated in part by substrate stiffness and mechanotransduction signaling and reveal a potential link between changes in tissue stiffness and macrophage lipid accumulation.