Aims: Macropinocytosis has been implicated in cardiovascular and other disorders, yet physiological factors that initiate fluid-phase internalization and the signaling mechanisms involved remain poorly identified. The present study was designed to examine whether matrix protein thrombospondin-1 (TSP1) stimulates macrophage macropinocytosis and, if so, to investigate the potential signaling mechanism involved.
Results: TSP1 treatment of human and murine macrophages stimulated membrane ruffle formation and pericellular solute internalization by macropinocytosis. Blockade of TSP1 cognate receptor CD47 and NADPH oxidase 1 (Nox1) signaling, inhibition of phosphoinositide 3-kinase, and transcriptional knockdown of myotubularin-related protein 6 abolished TSP1-induced macropinocytosis. Our results demonstrate that Nox1 signaling leads to dephosphorylation of actin-binding protein cofilin at Ser-3, actin remodeling, and macropinocytotic uptake of unmodified native low-density lipoprotein (nLDL), leading to foam cell formation. Finally, peritoneal chimera studies suggest the role of CD47 in macrophage lipid macropinocytosis in hypercholesterolemic ApoE-/- mice in vivo.
Innovation: Activation of a previously unidentified TSP1-CD47 signaling pathway in macrophages stimulates direct receptor-independent internalization of nLDL, leading to significant lipid accumulation and foam cell formation. These findings reveal a new paradigm in which delimited Nox1-mediated redox signaling, independent of classical lipid oxidation, contributes to early propagation of vascular inflammatory disease.
Conclusions: The findings of the present study demonstrate a new mechanism of solute uptake with implications for a wide array of cell types, including macrophages, dendritic cells, and cancer cells, and multiple pathological conditions in which matrix proteins are upregulated. Antioxid. Redox Signal. 26, 886-901.
Keywords: CD47; NADPH oxidase; macrophages; macropinocytosis; thrombospondin 1.