Autophagy in Lung Fibrosis: Exploring the Mitophagy Pathways

Idiopathic pulmonary fibrosis (IPF) is a life-threatening interstitial lung disease for which no cure exists so far. It may be triggered due to repeated alveolar epithelial type II cell (AECII) injury, ultimately leading to the death of this cell type. As an attempt to understand the molecular mechanisms behind lung fibrosis, we recently established an amiodarone (AD) induced mouse model of lung fibrosis. AD is an excellent anti-arrhythmic drug but is limited by its use in the clinic because of severe interstitial fibrosis with AECII injury being its potential side effect. A study from our group revealed pathologic induction of autophagy in this mouse model and that AECII apoptosis in this model is autophagy-dependent. Autophagy is a lysosomal quality control mechanism that aims at cell survival, but severe autophagic stress may lead to apoptosis. Defective autophagy is implicated in the pathology of idiopathic pulmonary fibrosis (IPF) and in other forms of lung fibrosis. While autophagy is differently regulated in IPF and in the AD model of lung fibrosis, lysosomal stress resulting in increased AECII apoptosis stands as a common mechanism. Here, we aim to dissect the role of mitochondrial autophagy (mitophagy) proteins in IPF and in the AD model of lung fibrosis. Differential expression of the mitophagy protein PINK1 and the novel mitophagy markers FUNDC1 and BCL2L13 were observed in IPF lungs and in AD-treated mouse lung homogenates. In mouse lung epithelial cells (MLE12 cells) treated with AD, BCL2L13 that was localized to mitochondria, also colocalized with the autophagosomal marker LC3B and the lysosomal marker Lamp1, indicating the activation of BCL2L13-dependent mitophagy. Similarly, the selective autophagy adaptor protein, sequestosome 1 (p62) was significantly increased the mitochondrial fraction of IPF lungs and in response to AD treatment. Of note, in AD-treated MLE12 cells, mitophagy was also activated via binding of p62 to dysfunctional mitochondria. In addition, interaction of p62 with Keap1 was increased and localization of Keap1 to p62-positive vacuolar structures was observed. However, Nrf2, a competitive binding partner of Keap1 and a major transcriptional factor regulating antioxidant proteins, was significantly reduced in epithelial cells following AD treatment. Hence, stimulation of Nrf2 may regulate the p62-Keap1 binding and subsequent mitophagy pathways in AD-treated cells. Our study suggests that targeting the mitochondrial-lysosomal axis may have therapeutic importance in the context of lung fibrosis.