Macropinocytosis facilitates amino acid acquisition from extracellular fluid to support cell proliferation in macrophages

Although many cancer cells proliferate by metabolizing extracellular proteins internalized by macropinocytosis and degraded in lysosomes, the extent to which macropinocytosis contributes to the growth of other metazoan cells remains undefined. This study analyzed macropinocytosis in proliferating murine macrophages as a mechanism for extracting amino acids from growth media. Macrophages internalized the fluid-phase probe Lucifer Yellow by macropinocytosis and recycled much of it from their lysosomes by a first-order process. Inhibitors of pinocytosis inhibited cell growth. Removal of the essential amino acid leucine from growth medium reduced proliferation, and allowed analysis of pinocytosis and the higher growth rates achieved by supplementation with either free leucine or bovine serum albumin (BSA) as a source of leucine. Macrophages could proliferate by utilizing macropinocytosis and digestion of BSA. In contrast, growth on free leucine exceeded the capacity of macropinocytosis to extract leucine from the medium. Dye molecules released from proteins by hydrolysis in lysosomes were recycled from cells efficiently. We propose that macropinocytosis concentrates large solutes, such as proteins, into lysosomes but allows amino acids and other products of lysosomal hydrolases to redistribute into macropinosomes and outside of the cell.