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In Vivo Investigations on Anti-Fibrotic Potential of Proteasome Inhibition in Lung and Skin Fibrosis

¹ Immunology and Allergy, Department of Internal Medicine, School of Medicine and University Hospital; ² Department of Genetics and Laboratory Medicine, University Hospital; ³ Department of Pediatrics and Pathology-Immunology, School of Medicine and University Hospital, Geneva, Switzerland; and ⁴ Department of Experimental Biomedical Sciences, Padua University, Padua, Italy

Correspondence and requests for reprints should be addressed to Carlo Chizzolini, M.D., Immunology and Allergy, Geneva University Hospital, 1211 Geneva 14, Switzerland. E-mail: chizzolini{at}medecine.unige.ch

In systemic sclerosis (SSc), a disease characterized by fibrosis of the skin and internal organs, the occurrence of interstitial lung disease is responsible for high morbidity and mortality. We previously demonstrated that proteasome inhibitors (PI) show anti-fibrotic properties in vitro by reducing collagen production and favoring collagen degradation in a c-jun N-terminal kinase (JNK)-dependent manner in human fibroblasts. Therefore, we tested whether PI could control fibrosis development in bleomycin-induced lung injury, which is preceded by massive inflammation. We extended the study to test PI in TSK-1/+ mice, where skin fibrosis develops in the absence of overt inflammation. C57Bl/6 mice received bleomycin intratracheally and were treated or not with PI. Lung inflammation and fibrosis were assessed by histology and quantification of hydroxyproline content, type I collagen mRNA, and TGF-β at Days 7, 15, and 21, respectively. Histology was used to detect skin fibrosis in TSK-1/+mice. The chymotryptic activity of 20S proteasome was assessed in mice blood. JNK and Smad2 phosphorylation were evaluated by Western blot on lung protein extracts. PI reduced collagen mRNA levels in murine lung fibroblasts, without affecting their viability in vitro. In addition, PI inhibited the chymotryptic activity of proteasome and enhanced JNK and TGF-β signaling in vivo. PI failed to prevent bleomycin-induced lung inflammation and fibrosis and to attenuate skin fibrosis in TSK-1/+mice. In conclusion, our results provide direct evidence that, despite promising in vitro results, proteasome blockade may not be a strategy easily applicable to control fibrosis development in diseases such as lung fibrosis and scleroderma.

Key Words: proteasome inhibitors • bleomycin-induced pulmonary fibrosis • TSK-1/+mice • TGF-β

CLINICAL RELEVANCE In the quest for novel anti-fibrotic strategies, we explored the potential of proteasome inhibition in models of lung and skin fibrosis driven by distinct pathogenetic mechanisms. In both, proteasome inhibition failed to control fibrosis development.