Abstract
Idiopathic pulmonary fibrosis (IPF) is a chronic, lethal interstitial lung disease in which the aberrant PTEN/Akt axis plays a major role in conferring a survival phenotype in response to the cell death inducing properties of type I collagen matrix. The underlying mechanism by which IPF fibroblasts become desensitized to polymerized collagen, thereby eluding collagen matrix-induced cell death has not been fully elucidated. We hypothesized that the pathologically altered PTEN/Akt axis suppresses autophagy via high mTOR kinase activity, which subsequently desensitizes IPF fibroblasts to collagen matrix induced cell death. We found that the autophagosome marker LC3-2 expression is suppressed, while mTOR activity remains high when IPF fibroblasts are cultured on collagen. However, LC3-2 expression increased in response to IPF fibroblast attachment to collagen in the presence of rapamycin. In addition, PTEN over-expression or Akt inhibition suppressed mTOR activity, thereby increasing LC3-2 expression in IPF fibroblasts. Furthermore, the treatment of IPF fibroblasts over-expressing PTEN or dominant negative Akt with autophagy inhibitors increased IPF fibroblast cell death. Enhanced p-mTOR expression along with low LC3-2 expression was also found in myofibroblasts within the fibroblastic foci from IPF patients. Our data show that the aberrant PTEN/Akt/mTOR axis desensitizes IPF fibroblasts from polymerized collagen driven stress by suppressing autophagic activity, which produces a viable IPF fibroblast phenotype on collagen. This suggests that the aberrantly regulated autophagic pathway may play an important role in maintaining a pathological IPF fibroblast phenotype in response to collagen rich environment.
Highlights
Cell homeostasis is carefully regulated by two major systems in eukaryotic cells, the ubiquitin proteasome system and the lysosome
We found that Idiopathic pulmonary fibrosis (IPF) fibroblasts have abnormally low autophagic activity on polymerized collagen
We previously found that when IPF fibroblasts attach to collagen rich matrix, Akt activity is up-regulated by PTEN suppression, which subsequently increases cell proliferation [22,23]
Summary
Cell homeostasis is carefully regulated by two major systems in eukaryotic cells, the ubiquitin proteasome system and the lysosome. Prior studies have shown that stress inducing conditions such as ER stress, oxidative stress, or elevated ROS formation can promote the activation of autophagy [9], and that lung injury caused by hyperoxia, cigarette smoke, or toxins is thought to be the initial step toward autophagy in the pathogenesis of lung fibrosis [11,12]. These studies further suggest that autophagy may be an important mechanism that represents an inducible response to stress in lung cells [13,14]. The current concept of the role of autophagy is that when cells are exposed to a stressful environment, autophagy is initially activated to protect the cells from these conditions until it reaches its threshold
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