A74-22 Loss of Fibroblast RIAM Promotes Senescence and Impairs Resolution in Pulmonary Fibrosis

Abstract Idiopathic pulmonary fibrosis (IPF) is the most common interstitial lung diseases, characterized by its progressive, irreversible nature, often leads to fatality, exhibiting a five-year survival rate below 40%. The underlying causes of IPF are multifaceted and remain insufficiently understood. Rap1-GTP-interacting adaptor molecule (RIAM) is an effector protein for Rap1 that belongs to the MRL (Mig-10/ RIAM/Lamellipodin) family of adaptor proteins. Previous studies indicate that RIAM plays a key role in Rap1-induced adhesion and interacts with modulators of the actin cytoskeleton in immune cells. Given that fibroblasts migration and adhesion contributes to pulmonary pathogenesis, this study aims to explore the role of RIAM in lung fibroblast during PF and its potential therapeutic value. Our data demonstrated that RIAM was elevated in senescent fibroblasts, in lung fibroblasts from IPF patients compared to normal controls, and in the lungs of bleomycin-induced lung fibrosis models in mice. Moreover, in vitro studies showed that RIAM knockdown in lung fibroblasts resulted in reduced cell adhesion and decreased α-smooth muscle actin (α-SMA) expression, while paradoxically increasing the production of proinflammatory cytokines, suggesting that loss of RIAM shifts fibroblasts toward a more inflammatory phenotype. Additionally, RNA-seq data in RIAM knockdown fibroblasts revealed downregulation of mitosis-related genes, accompanied by elevated transcription of apoptosis- and senescence-related genes. Consistently, RIAM depletion in mouse or human fibroblasts resulted in reduced mitotic activity and cell proliferation. Finally, to investigate the fibroblast-specific role of RIAM in vivo, we utilized a bleomycin-induced pulmonary fibrosis model in fibroblast-specific RIAM knockout mice (RIAMf/f Col1a2-CreER). Fibroblast-specific RIAM deletion attenuated lung fibrosis and inflammation at early stages but impaired fibrosis resolution at later stages following bleomycin challenge. This biphasic phenotype may result from enhanced fibroblast senescence and upregulation of inflammatory gene expression in RIAM-deficient fibroblasts. Together, our findings identified fibroblast RIAM as an critical regulator in lung fibrosis. Funding Information This work was supported by NIH National Institute on Aging grant to T.M. (1R56AG076144-01A1), NIH National Heart, Lung, and Blood Institute grant to T.M. (1R01HL168128). This abstract is funded by: NIH National Institute on Aging grant to T.M. (1R56AG076144-01A1), NIH National Heart, Lung, and Blood Institute grant to T.M. (1R01HL168128)