Macropinocytosis is the endocytic pathway that mediates macrophage foam cell formation with native low density lipoprotein.
Previously, we reported that fluid-phase endocytosis of native LDL by PMA-activated human monocytederived macrophages converted these macrophages into cholesterol-enriched foam cells (Kruth, H. S., Huang, W., Ishii, I., and Zhang, W. Y. (2002) J. Biol. Chem. 277, 34573-34580). Uptake of fluid by cells can occur either by micropinocytosis within vesicles (<0.1 microm diameter) or by macropinocytosis within vacuoles ( approximately 0.5-5.0 microm) named macropinosomes. The current investigation has identified macropinocytosis as the pathway for fluid-phase LDL endocytosis and determined signaling and cytoskeletal components involved in this LDL endocytosis. The phosphatidylinositol 3-kinase inhibitor, LY294002, which inhibits macropinocytosis but does not inhibit micropinocytosis, completely blocked PMA-activated macrophage uptake of fluid and LDL. Also, nystatin and filipin, inhibitors of micropinocytosis from lipid-raft plasma membrane domains, both failed to inhibit PMA-stimulated macrophage cholesterol accumulation. Time-lapse video phase-contrast microscopy and time-lapse digital confocal-fluorescence microscopy with fluorescent DiI-LDL showed that PMA-activated macrophages took up LDL in the fluid phase by macropinocytosis. Macropinocytosis of LDL depended on Rho GTPase signaling, actin, and microtubules. Bafilomycin A1, the vacuolar H+-ATPase inhibitor, inhibited degradation of LDL and caused accumulation of undegraded LDL within macropinosomes and multivesicular body endosomes. LDL in multivesicular body endosomes was concentrated >40-fold over its concentration in the culture medium consistent with macropinosome shrinkage by maturation into multivesicular body endosomes. Macropinocytosis of LDL taken up in the fluid phase without receptor-mediated binding of LDL is a novel endocytic pathway that generates macrophage foam cells. Macropinocytosis in macrophages and possibly other vascular cells is a new pathway to target for modulating foam cell formation in atherosclerosis.
Kruth
Howard S
HS
Section of Experimental Atherosclerosis, NHLBI, National Institutes of Health, Bethesda, Maryland 20892-1422, USA. kruthh@nhlbi.nih.gov
Jones
Nancy L
NL
Huang
Wei
W
Zhao
Bin
B
Ishii
Itsuko
I
Chang
Janet
J
Combs
Christian A
CA
Malide
Daniela
D
Zhang
Wei-Yang
WY
D002478:Cells, Cultured; D002867:Chromones / Q000494:pharmacology; D004705:Endocytosis; D004791:Enzyme Inhibitors / Q000494:pharmacology; D005487:Foam Cells / Q000187:drug effects* / Q000378:metabolism / Q000648:ultrastructure; D006801:Humans; D007150:Immunohistochemistry; D008077:Lipoproteins, LDL / Q000378:metabolism; D008262:Macrophage Activation / Q000187:drug effects; D008854:Microscopy, Electron; D009025:Morpholines / Q000494:pharmacology; D000081082:Phosphoinositide-3 Kinase Inhibitors; D010873:Pinocytosis; D006180:Proton-Translocating ATPases / Q000378:metabolism; D013755:Tetradecanoylphorbol Acetate / Q000494:pharmacology; D014617:Vacuoles / Q000201:enzymology
D002867:Chromones; D004791:Enzyme Inhibitors; D008077:Lipoproteins, LDL; D009025:Morpholines; D000081082:Phosphoinositide-3 Kinase Inhibitors; C085911:2-(4-morpholinyl)-8-phenyl-4H-1-benzopyran-4-one; D006180:Proton-Translocating ATPases; D013755:Tetradecanoylphorbol Acetate