Tobacco smoke-induced accelerated cell senescence has been implicated in the pathogenesis of chronic obstructive pulmonary disease (COPD). Cell senescence is accompanied by the accumulation of damaged cellular components suggesting that in COPD, inhibition of autophagy may contribute to cell senescence. Here we look at whether autophagy contributes to cigarette smoke extract (CSE) - induced cell senescence of primary human bronchial epithelial cells (HBEC), and further evaluate p62 and ubiquitinated protein levels in lung homogenates from COPD patients. We demonstrate that CSE transiently induces activation of autophagy in HBEC, followed by accelerated cell senescence and concomitant accumulation of p62 and ubiquitinated proteins. Autophagy inhibition further enhanced accumulations of p62 and ubiquitinated proteins, resulting in increased senescence and senescence-associated secretory phenotype (SASP) with interleukin (IL)-8 secretion. Conversely, autophagy activation by Torin1, a mammalian target of rapamycin (mTOR inhibitor), suppressed accumulations of p62 and ubiquitinated proteins and inhibits cell senescence. Despite increased baseline activity, autophagy induction in response to CSE was significantly decreased in HBEC from COPD patients. Increased accumulations of p62 and ubiquitinated proteins were detected in lung homogenates from COPD patients. Insufficient autophagic clearance of damaged proteins, including ubiquitinated proteins, is involved in accelerated cell senescence in COPD, suggesting a novel protective role for autophagy in the tobacco smoke-induced senescence-associated lung disease, COPD.

Background

Sarcopenia is characterized by the loss of skeletal muscle mass and strength and is associated with poor prognosis in patients with chronic obstructive pulmonary disease (COPD). Cigarette smoke (CS) exposure, a major cause for COPD, induces mitochondrial damage, which has been implicated in sarcopenia pathogenesis. The current study sought to examine the involvement of insufficient Parkin-mediated mitophagy, a mitochondrion-selective autophagy, in the mechanisms by which dysfunctional mitochondria accumulate with excessive reactive oxygen species (ROS) production in the development of COPD-related sarcopenia.

Methods

The involvement of Parkin-mediated mitophagy was examined using in vitro models of myotube formation, in vivo CS-exposure model using Parkin^-/- mice, and human muscle samples from patients with COPD-related sarcopenia.

Results

Cigarette smoke extract (CSE) induced myotube atrophy with concomitant 30% reduction in Parkin expression levels (P < 0.05). Parkin-mediated mitophagy regulated myotube atrophy by modulating mitochondrial damage and mitochondrial ROS production. Increased mitochondrial ROS was responsible for myotube atrophy by activating Muscle Ring Finger 1 (MuRF-1)-mediated myosin heavy chain (MHC) degradation. Parkin^-/- mice with prolonged CS exposure showed enhanced limb muscle atrophy with a 31.7% reduction in limb muscle weights (P < 0.01) and 2.3 times greater MuRF-1 expression (P < 0.01) compared with wild-type mice with concomitant accumulation of damaged mitochondria and oxidative modifications in 4HNE expression. Patients with COPD-related sarcopenia exhibited significantly reduced Parkin but increased MuRF-1 protein levels (35% lower and 2.5 times greater protein levels compared with control patients, P < 0.01 and P < 0.05, respectively) and damaged mitochondria accumulation demonstrated in muscles. Electric pulse stimulation-induced muscle contraction prevented CSE-induced MHC reduction by maintaining Parkin levels in myotubes.

Conclusions

Taken together, COPD-related sarcopenia can be attributed to insufficient Parkin-mediated mitophagy and increased mitochondrial ROS causing enhanced muscle atrophy through MuRF-1 activation, which may be at least partly preventable through optimal physical exercise.

Cigarette smoke (CS)-induced accumulation of mitochondrial damage has been widely implicated in chronic obstructive pulmonary disease (COPD) pathogenesis. Mitophagy plays a crucial role in eliminating damaged mitochondria, and is governed by the PINK1 (PTEN induced putative protein kinase 1)-PRKN (parkin RBR E3 ubiquitin protein ligase) pathway. Although both increased PINK1 and reduced PRKN have been implicated in COPD pathogenesis in association with mitophagy, there are conflicting reports for the role of mitophagy in COPD progression. To clarify the involvement of PRKN-regulated mitophagy in COPD pathogenesis, prkn knockout (KO) mouse models were used. To illuminate how PINK1 and PRKN regulate mitophagy in relation to CS-induced mitochondrial damage and cellular senescence, overexpression and knockdown experiments were performed in airway epithelial cells (AEC). In comparison to wild-type mice, prkn KO mice demonstrated enhanced airway wall thickening with emphysematous changes following CS exposure. AEC in CS-exposed prkn KO mice showed accumulation of damaged mitochondria and increased oxidative modifications accompanied by accelerated cellular senescence. In vitro experiments showed PRKN overexpression was sufficient to induce mitophagy during CSE exposure even in the setting of reduced PINK1 protein levels, resulting in attenuation of mitochondrial ROS production and cellular senescence. Conversely PINK1 overexpression failed to recover impaired mitophagy caused by PRKN knockdown, indicating that PRKN protein levels can be the rate-limiting factor in PINK1-PRKN-mediated mitophagy during CSE exposure. These results suggest that PRKN levels may play a pivotal role in COPD pathogenesis by regulating mitophagy, suggesting that PRKN induction could mitigate the progression of COPD. Abbreviations: AD: Alzheimer disease; AEC: airway epithelial cells; BALF: bronchoalveolar lavage fluid; AKT: AKT serine/threonine kinase; CALCOCO2/NDP52: calcium binding and coiled-coil domain 2; CDKN1A: cyclin dependent kinase inhibitor 1A; CDKN2A: cyclin dependent kinase inhibitor 2A; COPD: chronic obstructive pulmonary disease; CS: cigarette smoke; CSE: CS extract; CXCL1: C-X-C motif chemokine ligand 1; CXCL8: C-X-C motif chemokine ligand 8; HBEC: human bronchial epithelial cells; 4-HNE: 4-hydroxynonenal; IL: interleukin; KO: knockout; LF: lung fibroblasts; LPS: lipopolysaccharide; MAP1LC3/LC3: microtubule associated protein 1 light chain 3; MTOR: mechanistic target of rapamycin kinase; 8-OHdG: 8-hydroxy-2'-deoxyguanosine; OPTN: optineurin; PRKN: parkin RBR E3 ubiquitin protein ligase; PCD: programmed cell death; PFD: pirfenidone; PIK3C: phosphatidylinositol-4:5-bisphosphate 3-kinase catalytic subunit; PINK1: PTEN induced putative kinase 1; PTEN: phosphatase and tensin homolog; RA: rheumatoid arthritis; ROS: reactive oxygen species; SA-GLB1/β-Gal: senescence-associated-galactosidase, beta 1; SASP: senescence-associated secretory phenotype; SNP: single nucleotide polymorphism; TNF: tumor necrosis factor.