A cysteine proteinase inhibitor ALLN alleviates bleomycin-induced skin and lung fibrosis

Arthritis Res Ther. 2023 Aug 25;25(1):156. doi: 10.1186/s13075-023-03130-7.


Background: Systemic sclerosis (SSc) is a connective tissue disease that is characterized by fibrosis in the skin and internal organs, such as the lungs. Activated differentiation of progenitor cells, which are mainly resident fibroblasts, into myofibroblasts is considered a key mechanism underlying the overproduction of extracellular matrix and the resultant tissue fibrosis in SSc. Calpains are members of the Ca2+-dependent cysteine protease family, whose enzymatic activities participate in signal transduction and tissue remodeling, potentially contributing to fibrosis in various organs. However, the roles of calpain in the pathogenesis of SSc remain unknown. This study aimed to examine the anti-fibrotic properties of N-acetyl-Leu-Leu-norleucinal (ALLN), one of the cysteine proteinase inhibitors that primarily inhibit calpain, in vitro and in vivo, to optimally translate into the therapeutic utility in human SSc.

Methods: Normal human dermal and lung fibroblasts pretreated with ALLN were stimulated with recombinant transforming growth factor beta 1 (TGF-β1), followed by assessment of TGF-β1/Smad signaling and fibrogenic molecules.

Results: ALLN treatment significantly inhibited TGF-β1-induced phosphorylation and nuclear transport of Smad2/3 in skin and lung fibroblasts. TGF-β1-dependent increases in α-smooth muscle actin (αSMA), collagen type I, fibronectin 1, and some mesenchymal transcription markers were attenuated by ALLN. Moreover, our findings suggest that ALLN inhibits TGF-β1-induced mesenchymal transition in human lung epithelial cells. Consistent with these in vitro findings, administering ALLN (3 mg/kg/day) three times a week intraperitoneally remarkably suppressed the development of skin and lung fibrosis in a SSc mouse model induced by daily subcutaneous bleomycin injection. The number of skin- and lung-infiltrating CD3+ T cells decreased in ALLN-treated mice compared with that in control-treated mice. Phosphorylation of Smad3 and/or an increase in αSMA-positive myofibroblasts was significantly inhibited by ALLN treatment on the skin and lungs. However, no adverse effects were observed.

Conclusions: Our results prove that calpains can be a novel therapeutic target for skin and lung fibrosis in SSc, considering its inhibitor ALLN.

Keywords: Calpain; Cysteine proteinase inhibitor; Endothelial-mesenchymal transition; Fibrosis; Lung; Skin; Systemic sclerosis; TGF-β.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Animals
  • Bleomycin / toxicity
  • Calpain
  • Humans
  • Mice
  • Pulmonary Fibrosis* / chemically induced
  • Pulmonary Fibrosis* / drug therapy
  • Scleroderma, Systemic* / chemically induced
  • Scleroderma, Systemic* / drug therapy
  • Transforming Growth Factor beta1


  • leucylleucine
  • Transforming Growth Factor beta1
  • Calpain
  • Bleomycin