Extracellular phosphatidylcholine functions as a noncanonical TLR4 activator to drive adipocyte lipolysis and apoptosis.

This study investigated whether phosphatidylcholine (PC), a key active component in injection lipolysis, functions as a damage-associated molecular pattern (DAMP) via toll-like receptor 4 (TLR4) signaling to induce lipolysis and apoptosis in 3T3-L1 adipocytes. While PC is widely used for localized fat reduction, its specific molecular mechanism-beyond detergent-like necrosis-remains a subject of debate. We hypothesized that high-dose extracellular PC triggers a specific receptor-mediated inflammatory response. Our results demonstrated that PC treatment in a range of 0-25 mg/mL reduced adipocyte viability and lipid accumulation in a clear concentration-dependent manner. These catabolic effects, including increased glycerol release, were significantly and dose-dependently attenuated by pretreatment with the specific TLR4 inhibitor TAK-242 (1 or 10 nM). To definitively distinguish the mode of cell death, Annexin V/DAPI staining was performed, confirming that PC-induced cell death is driven by programmed apoptosis rather than non-specific necrosis. Mechanistically, while total protein expression of TLR4 remained unchanged, PC treatment significantly upregulated the phosphorylation of nuclear factor-kappa B (p-NF-κB) and the expression of tumor necrosis factor-alpha (TNF-α). These pro-inflammatory and apoptotic signals were effectively reversed by TLR4 inhibition, providing definitive proof that PC functions as a functional activator of the TLR4-NF-κB-TNF-α signaling axis. In conclusion, these findings demonstrate that extracellular PC acts as a noncanonical TLR4 agonist, driving adipocyte-specific lipolysis and apoptosis. This study provides a comprehensive molecular rationale for PC-based fat reduction and suggests that the modulation of TLR4 signaling could optimize the efficacy and safety of lipolytic therapies in clinical practice.