ACSL4 Drives C5a/C5aR1–Calcium-Induced Fibroblast-to-Myofibroblast Transition in a Bleomycin-Induced Mouse Model of Pulmonary Fibrosis

Idiopathic pulmonary fibrosis (IPF) is characterized by excessive extracellular matrix (ECM) deposition driven by aberrant fibroblast-to-myofibroblast transition (FMT). However, the upstream regulators and downstream effectors of this process remain incompletely understood. Here, we identify acyl-CoA synthetase long-chain family member 4 (ACSL4), a lipid metabolic enzyme, as a critical mediator linking complement component 5a (C5a)/C5a receptor 1 (C5aR1) signaling to FMT via calcium signaling. In bleomycin (BLM)-induced pulmonary fibrosis of C57BL/6JGpt mice, and in C5a-stimulated primary lung fibroblasts, the expression of ACSL4 was markedly upregulated. Pharmacological inhibition of ACSL4 (PRGL493) or C5aR1 (PMX53) attenuated the deposition of ECM and suppressed the expression of fibrotic markers in vivo and in vitro. Mechanistically, the activation of C5a/C5aR1 signaling increased intracellular calcium levels and promoted the expression of ACSL4, while inhibition of calcium signaling (FK506) reversed the upregulation of ACSL4 and FMT-related changes, including the expression of α-smooth muscle actin (αSMA) and the migration of fibroblasts. Notably, inhibition of ACSL4 did not affect the proliferation of fibroblasts, suggesting its specific role in phenotypic transition. These findings demonstrate that ACSL4 functions downstream of C5a/C5aR1-induced calcium signaling to promote FMT and the progression of pulmonary fibrosis. Targeting ACSL4 may therefore offer a novel therapeutic strategy for IPF.