TLR4 promotes smooth muscle cell-derived foam cells formation by inducing receptor-independent macropinocytosis.

Foam cells are primarily formed through scavenger receptors that mediate the uptake of various modified low-density lipoproteins (LDL) into cells. In addition to the receptor-dependent pathway, macropinocytosis is an essential nonreceptor endocytic pathway for vascular smooth muscle cells (VSMCs) to take up lipids. However, the molecular mechanisms underlying this process remain unclear. Primary cultured VSMCs were stimulated with 200 ng/mL lipopolysaccharide (LPS) and 200 µg/mL native LDL (nLDL). We observed a significant increase in Toll-like receptor 4 (TLR4) protein expression and a significant activation of macropinocytosis, which correlated with the highest uptake of nLDL and intracellular lipid deposition in WT VSMCs. However, macropinocytosis was inhibited and lipid accumulation decreased after treatment with macropinocytosis inhibitors and Syk inhibitors in WT VSMCs. Consistently, TLR4 knockout significantly suppressed macropinocytosis and lipid droplets accumulation in VSMCs. Taken together, our findings suggest a critical role of TLR4/Syk signaling in promoting receptor-independent macropinocytosis leading to VSMC-derived foam cells formation.