Fibroblast activation protein (FAP) promotes cardiac fibroblast activation in association with impaired ER-phagy.

Dilated cardiomyopathy (DCM) is characterized by progressive myocardial fibrosis and extracellular matrix remodeling, which severely impair cardiac function. The endoplasmic reticulum (ER) is essential for collagen synthesis, but the role of ER-phagy, a selective autophagy pathway that maintains ER proteostasis, in pathological cardiac fibroblast activation remains unclear. We integrated single-cell RNA-seq, bulk RNA-seq, and ATAC-seq datasets from human cardiac tissue and cultured human and rat cardiac fibroblasts. Key findings were validated in TGF-β-treated primary neonatal rat cardiac fibroblasts using siRNA-mediated FAP silencing and autophagy-associated assays. Fibroblast activation was accompanied by reduced expression and promoter accessibility of selected ER-phagy-related genes. An FAP-high activated fibroblast population showed a lower ER-phagy-related transcriptional signature. Integrated regulon and promoter-motif analyses linked FAP induction to a SMAD3-centered, BET-sensitive transcriptional program associated with repression of a subset of ER-phagy genes. FAP silencing attenuated fibroblast activation and ER stress and was accompanied by restored expression of selected ER-phagy-related transcripts and changes in autophagy-associated markers. These findings support a role for FAP in cardiac fibroblast activation and link its induction to altered ER-phagy-related proteostasis and ER stress.