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Loss of proteostasis and cellular senescence are key hallmarks of aging. Recent studies suggest that lung fibroblasts from idiopathic pulmonary fibrosis (IPF) show features of cellular senescence, decline in heat shock proteins (HSPs) expression and impaired protein homeostasis (proteostasis). However, direct cause-effect relationships are still mostly unknown. In this study, we sought to investigate whether the heat shock factor 1 (HSF1), a major transcription factor that regulates the cellular HSPs network and cytoplasmic proteostasis, contributes to cellular senescence in lung fibroblasts. We found that IPF lung fibroblasts showed an upregulation in the expression of various cellular senescence markers, including β -galactosidase activity (SA-β-gal) staining, the DNA damage marker γ H2Ax, the cell cycle inhibitor protein p21, and multiple senescence-associated secretory proteins (SASP), as well as upregulation of collagen 1a1, fibronectin and alpha-smooth muscle actin (α-SMA) gene expression compared with age-matched controls. These changes were associated with impaired proteostasis, as judged by an increase in levels of p-HSF1 ser307 and HSF1 K298 sumo , downregulation of HSPs expression, and increased cellular protein aggregation. Similarly, lung fibroblasts isolated from a mouse model of bleomycin-induced lung fibrosis and mouse lung fibroblast chronically treated with H 2 O 2 showed downregulation in HSPs and increased in cellular senescence and SASP markers. Moreover, sustained pharmacologic activation of HSF1 increased the expression of HSPs, reduced cellular senescence markers and effectively reduced the expression of pro-fibrotic genes in IPF fibroblast. Our data provide evidence that the HSF1-mediated proteostasis is important for driving lung fibroblasts toward cellular senescence and a myofibroblast phenotype. We postulate that enhancing HSF1 activity could be effective in the treatment of lung fibrosis. METHODSAnimals. C57B/6J mice (8-10 weeks old) were purchased from the Jackson Laboratory (Bar Harbor, ME) and housed in a pathogen-free animal facility at Thomas Jefferson University. Throughout the study period, mice were maintained on a standard chow diet (13.5% calories from fat, 58% from carbohydrates, and 28.5% from protein) and permitted to feed ad libitum. ). They were isolated from the uninvolved periphery of the organ of six lung cancer patients. Fresh tissues were transported immediately to the laboratory for isolating fibroblasts according to procedures described before [35].Bleomycin-induced lung injury: Bleomycin sulphate (Enzo life science, BML-AP302-0050) was dissolved in phosphate buffered saline. Lung injury was induced by instilling 0.04U of bleomycin into the posterior oropharynx of anesthetized mice every other week for five doses [36]. The mice were observed following intubation to ensure complete recovery from anesthesia. Mice were euthanized 2 weeks after the last dose by exposure to carbon dioxide, and lungs were harvested for fibroblast isolation and histological preparation...