Substrate stiffness engineered to replicate disease conditions influence senescence and fibrotic responses in primary lung fibroblasts

In idiopathic pulmonary fibrosis (IPF) there is excessive ECM deposition, increased stiffness and ultimately destruction of lung parenchyma. IPF presents mainly in the elderly, implying that senescence, a hallmark of ageing, contributes to disease progression. Several studies have reported that IPF is characterised by increased senescence and accumulating evidence suggests that structural changes, such as increased stiffness may contribute to senescence. This study therefore investigated if increased tissue stiffness could modulate markers of senescence and/or fibrosis in primary lung fibroblasts. Using hydrogels representing healthy and fibrotic stiffnesses, we cultured primary fibroblasts from non-diseased lung tissue on top of these hydrogels for up to seven days before assessing senescence and fibrosis markers. Fibroblasts cultured on stiff (±15kPa) hydrogels showed higher Yes-associated protein-1 (YAP) nuclear translocation compared to soft hydrogels. When looking at senescence-associated proteins we also found higher secretion of receptor activator of nuclear factor kappa-B ligand (RANKL) but no change in transforming growth factor-β1 (TGF-β1) or connective tissue growth factor (CTGF) expression and higher decorin protein deposition on stiff matrices. With respect to genes associated with fibrosis, fibroblasts on stiff hydrogels compared to soft had higher expression of smooth muscle alpha (α)-2 actin ( ACTA2), collagen (COL) 1A1 and fibulin-1 (Fbln1) and higher Fbln1 protein deposition after seven days. Our results show that exposure of lung fibroblasts to fibrotic stiffness activates genes and secreted factors that are part of fibrotic responses and part of the senescence-associated secretory profile (SASP). This overlap may contribute to the creation of a feedback loop whereby fibroblasts create a perpetuating cycle reinforcing disease progression in IPF. ### Competing Interest Statement M.N, B.N.M., and J.K.B have unrestricted research funding from Boehringer Ingelheim. The other authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.