Inactivation Rap2a in Endothelial Cell Prevents Pulmonary Fibrosis by Regulating Immune Microenvironment Through MAP4K4‐VCAM1 Signaling

ABSTRACT Idiopathic pulmonary fibrosis (IPF) is characterized by progressive fibrotic remodeling accompanied by persistent endothelial activation and leukocyte infiltration. Although endothelial dysfunction is increasingly recognized as a key contributor to fibrogenesis, the intracellular signaling pathways that couple inflammatory cues to endothelial–immune interactions remain incompletely defined. Ras‐related protein Rap2a (RAP2A), a small GTPase implicated in stress and inflammatory signaling, has not been systematically investigated in pulmonary endothelial cells during fibrotic lung injury. Here, using a bleomycin‐induced experimental lung fibrosis model, we observed that RAP2A expression was markedly upregulated in pulmonary endothelial cells and correlated with disease severity. Endothelium‐enriched knockdown of Rap2a via AAV9‐Cdh5‐shRNA attenuated inflammatory cell adhesion to the pulmonary endothelium, reduced fibrotic remodeling, and improved lung function. Mechanistically, RAP2A promoted endothelial activation by enhancing MAP4K4‐dependent signaling and upregulating vascular cell adhesion molecule 1 (VCAM1) in response to pro‐inflammatory stimulation, thereby facilitating leukocyte–endothelial interactions. In vitro assays further demonstrated that RAP2A deficiency impaired tumor necrosis factor‐α–induced endothelial adhesiveness without affecting basal endothelial integrity. Collectively, our findings identify endothelial RAP2A as a regulator of inflammatory endothelial activation in experimental lung fibrosis and suggest that targeting RAP2A‐mediated signaling may represent a potential strategy to modulate endothelial–immune crosstalk during fibrotic lung injury.