ObjectivesSystemic sclerosis (SSc) is characterised by extensive tissue fibrosis maintained by mechanotranductive/proadhesive signalling. Drugs targeting this pathway are therefore of likely therapeutic benefit. The mechanosensitive transcriptional co-activator, yes activated protein-1 (YAP1), is activated in SSc fibroblasts. The terpenoid celastrol is a YAP1 inhibitor; however, if celastrol can alleviate SSc fibrosis is unknown. Moreover, the cell niches required for skin fibrosis are unknown.MethodsHuman dermal fibroblasts from healthy individuals and patients with diffuse cutaneous SSc were treated with or without transforming growth factor β1 (TGFβ1), with or without celastrol. Mice were subjected to the bleomycin-induced model of skin SSc, in the presence or absence of celastrol. Fibrosis was assessed using RNA Sequencing, real-time PCR, spatial transcriptomic analyses, Western blot, ELISA and histological analyses.ResultsIn dermal fibroblasts, celastrol impaired the ability of TGFβ1 to induce an SSc-like pattern of gene expression, including that of cellular communication network factor 2, collagen I and TGFβ1. Celastrol alleviated the persistent fibrotic phenotype of dermal fibroblasts cultured from lesions of SSc patients. In the bleomycin-induced model of skin SSc, increased expression of genes associated with reticular fibroblast and hippo/YAP clusters was observed; conversely, celastrol inhibited these bleomycin-induced changes and blocked nuclear localisation of YAP.ConclusionsOur data clarify niches within the skin activated in fibrosis and suggest that compounds, such as celastrol, that antagonise the YAP pathway may be potential treatments for SSc skin fibrosis.

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Background/Aims Scleroderma (systemic sclerosis, SSc) is an autoimmune inflammatory-fibrosis syndrome that damages the skin and internal organs and is considered a prototypic complex fibrotic disease. In SSc, persistently activated myofibroblasts are maintained by an excessive, autocrine mechanotranductive/pro-adhesive signaling loop. Drugs targeting this pathway are therefore of likely therapeutic benefit in SSc. The mechanosensitive transcriptional co-activator, yes activated protein-1 (YAP1), is activated in SSc fibroblasts. The terpenoid plant-derivative celastrol has recently been identified as a YAP inhibitor; however, if celastrol can alleviate SSc fibrosis, and its underlying mechanism of action, is as yet unclear. Methods We cultured age-, sex-, and site-matched human dermal fibroblasts sampled from healthy individuals and patients with early onset (<18 month after first diagnosis) diffuse cutaneous scleroderma (systemic sclerosis, dcSSc) and treated with or without transforming growth factor β1 (TGFβ1, 4ng/ml) in the presence or absence of celastrol (500nM). We also subjected C57BL6J mice to the inflammatory-driven bleomycin-induced model of skin SSc, in the presence or absence of celastrol (28days, every day, 0.1U bleomycin, 1mg/kg celastrol). RNA expression was assessed using RNAseq and real-time polymerase chain reaction analysis. Protein expression was determined using Western blot and enzyme-linked immunosorbent assay. Fibrosis was monitored by hematoxylin and eosin and trichrome staining of tissue sections, and by using indirect immunofluorescence analysis with anti-alpha-smooth muscle actin (SMA) antibodies to detect myofibroblasts. Results In dermal fibroblasts, celastrol impaired the ability of TGFβ1 to induce an SSc-like pattern of gene expression, including the induction of cellular communication network factor 2 (CCN2), collagen I and TGFβ1 protein (N = 6, all p < 0.01). Celastrol also alleviated the persistent fibrotic phenotype of dermal fibroblasts cultured from lesions of SSc patients, including the overproduction of CCN2 and collagen (N = 3, p < 0.05). Finally, in the bleomycin-induced model of SSc dermatopathology, celastrol inhibited skin thickness/collagen deposition in vivo and blocked nuclear localization of YAP in SMA-positive myofibroblasts (all N = 6, p < 0.01). Conclusion Our data are consistent with the hypothesis that compounds, such as celastrol, that antagonize the YAP pathway warrant further consideration as potential treatments for SSc skin fibrosis. Disclosure A. Leask: Grants/research support; CIHR, NSERC, Arthritis Society of Canada. R.J. Stratton: None. S. Xu: None. P. Chitturi: None.