The small heat‐shock protein αB‐crystallin is essential for the nuclear localization of Smad4: impact on pulmonary fibrosis

Abstract

Idiopathic pulmonary fibrosis (IPF) is a devastating disease characterized by the proliferation of myofibroblasts and the accumulation of extracellular matrix (ECM) in the lungs. TGF-β1 is the major profibrotic cytokine involved in IPF and is responsible for myofibroblast proliferation and differentiation and ECM synthesis. αB-crystallin is constitutively expressed in the lungs and is inducible by stress, acts as a chaperone and is known to play a role in cell cytoskeleton architecture homeostasis. The role of αB-crystallin in fibrogenesis remains unknown. The principal signalling pathway involved in this process is the Smad-dependent pathway. We demonstrate here that αB-crystallin is strongly expressed in fibrotic lung tissue from IPF patients and in vivo rodent models of pulmonary fibrosis. We also show that αB-crystallin-deficient mice are protected from bleomycin-induced fibrosis. Similar protection from fibrosis was observed in αB-crystallin KO mice after transient adenoviral-mediated over-expression of IL-1β or TGF-β1. We show in vitro in primary epithelial cells and fibroblasts that αB-crystallin increases the nuclear localization of Smad4, thereby enhancing the TGF-β1-Smad pathway and the consequent activation of TGF-β1 downstream genes. αB-crystallin over-expression disrupts Smad4 mono-ubiquitination by interacting with its E3-ubiquitin ligase, TIF1γ, thus limiting its nuclear export. Conversely, in the absence of αB-crystallin, TIF1γ can freely interact with Smad4. Consequently, Smad4 mono-ubiquitination and nuclear export are favoured and thus TGF-β1-Smad4 pro-fibrotic activity is inhibited. This study demonstrates that αB-crystallin may be a key target for the development of specific drugs in the treatment of IPF or other fibrotic diseases.

αB-Crystallin Regulates Subretinal Fibrosis by Modulation of Epithelial-Mesenchymal Transition

“…The authors elucidated the causal role of aB-crystallin in EMT by demonstrating that overexpression of aB-crystallin disrupted monoubiquitination of SMAD4 by interacting with transcriptional intermediary factor 1g, E3eubiquitin ligase and thus limiting its nuclear export. 12,46 Our study in RPE showed that knockdown of aB-crystallin increased the monotetraubiquitination of SMAD4 by threefold and decreased its nuclear localization in both TGF-b2 treated and untreated cells. Interestingly, in RPE, EMT is induced by both TGF-b2, the major TGF-b isoform in the RPE, and BMP4.…”

“…11, 12 Huang et al 11 showed that aB-crystallin protects 14-3-3z from its degradation by the proteasome leading to the activation of extracellular signaleregulated kinase signaling pathway, which can induce EMT of hepatocellular carcinoma cells. Bellaye et al 12,46 found the overexpression of aB-crystallin in human and rodent fibrotic lung tissue.…”

“…11, 12 Huang et al 11 showed that aB-crystallin protects 14-3-3z from its degradation by the proteasome leading to the activation of extracellular signaleregulated kinase signaling pathway, which can induce EMT of hepatocellular carcinoma cells. Bellaye et al 12,46 found the overexpression of aB-crystallin in human and rodent fibrotic lung tissue. The authors elucidated the causal role of aB-crystallin in EMT by demonstrating that overexpression of aB-crystallin disrupted monoubiquitination of SMAD4 by interacting with transcriptional intermediary factor 1g, E3eubiquitin ligase and thus limiting its nuclear export.…”

“…36 Recently, a regulatory role of aB-crystallin in SMAD4 monoubiquitination was reported. 12 We first isolated monotetraubiquitinated or polyubiquitinated protein from RPE cells treated with control or aB-crystallin siRNA. Western blot analysis of ubiquitin in the whole gels shows a similar pattern of monotetraubiquitinated proteins in RPE cells transfected with aB-crystallin siRNA compared with control ( Figure 5C) and showed equal loading of b-actin ( Figure 5C).…”

“…10 The role of aB-crystallin in EMT in liver and lung fibrosis has been recently reported. 11,12 Our previous work has suggested an important role for aB-crystallin in both the early and late stages of AMD. The early stage of AMD is characterized by the accumulation of drusen between the RPE and Bruch's membrane, accompanied by RPE cell death and synaptic dysfunction.…”