The Role of Extracellular Amino Acids in the Regulation of Macropinocytosis

Abstract

Macropinocytosis is an endocytic pathway by which cells internalize extracellular solutes from the environment in vesicles known as macropinosomes. It is involved in a wide array of functions ranging from antigen presentation to cancer cell development. In bone marrow-derived macrophages, macropinocytosis can be induced by growth factors such as colony-stimulating factor-1 (CSF1). While much is known about the molecular mechanisms involved in the formation and internalization of macropinosomes, relatively little is known about its regulation. As macropinocytosis functions as a mechanism for internalizing nutrients from the environment, we tested whether nutrients modulate solute uptake by macropinocytosis. One category of nutrient that was relatively understudied is amino acids, and as such are the focus of this thesis. We show that nine amino acids, when present individually or together, can suppress macropinocytosis. Furthermore, we show that suppression only occurs when macrophages are stimulated with ligands of the CSF1 receptor (CSF1R), which include CSF1 and the cytokine IL-34. Suppressive amino acids had no effect when macropinocytosis was induced by lipopolysaccharide or phorbol myristate acetate. Mechanistically, suppressive amino acids activate the metalloproteinase ADAM17 which cleaves CSF1R, resulting in the release of CSF1R from the cell surface. This in turn leads to the formation of smaller macropinosomes and consequently less total accumulation by macropinocytosis. Our findings may have implications for macrophage polarization, especially in nutrient-poor environments such as the tumor microenvironment.