Contributions of alveolar epithelial cell quality control to pulmonary fibrosis

Abstract

Epithelial cell dysfunction has emerged as a central component of the pathophysiology of diffuse parenchymal diseases including idiopathic pulmonary fibrosis (IPF). Alveolar type 2 (AT2) cells represent a metabolically active lung cell population important for surfactant biosynthesis and alveolar homeostasis. AT2 cells and other distal lung epithelia, like all eukaryotic cells, contain an elegant quality control network to respond to intrinsic metabolic and biosynthetic challenges imparted by mutant protein conformers, dysfunctional subcellular organelles, and dysregulated telomeres. Failed AT2 quality control components (the ubiquitin-proteasome system, unfolded protein response, macroautophagy, mitophagy, and telomere maintenance) result in diverse cellular endophenotypes and molecular signatures including ER stress, defective autophagy, mitochondrial dysfunction, apoptosis, inflammatory cell recruitment, profibrotic signaling, and altered progenitor function that ultimately converge to drive downstream fibrotic remodeling in the IPF lung. As this complex network becomes increasingly better understood, opportunities will emerge to identify targets and therapeutic strategies for IPF.

Figure 1. IPF pathogenesis driven by epithelial dysfunction occurs in three phases.

Initiation: Intrinsic (e.g., genetic) and extrinsic (e.g., infection, air pollution) factors acting through various pathways converge to produce a vulnerable alveolar type 2 epithelial cell (AT2) population (blue rectangular cells). Vulnerable AT2s subjected to continued intrinsic proteostatic/cell quality control challenges or additional secondary injurious stimuli (often recurrent) develop profound functional defects marked by aberrant activation of developmental programs, enhanced cell stress responses, impaired progenitor function, and/or apoptosis. Amplification: Dysfunctional AT2 cells (red rectangular cells) can initiate crosstalk with immune populations such as Ly6C^hi monocytes, alveolar macrophages, neutrophils, or lymphocytes, which can both amplify the initial injury events and promote mesenchymal expansion further complemented by commensurate AT2/mesenchymal crosstalk. Fibrogenesis: The dysfunctional alveolar niche exhibits further feed-forward mechanisms to promote ongoing AT2 dysfunction marked by increased proliferation (AT2 hyperplasia), impaired transdifferentiation to AT1 cells, and upregulation of senescence programs. Coupled with enhanced myofibroblast activation and matrix deposition, the disrupted injury/repair response leads to scar formation and progressive loss of lung architectural complexity culminating in progressive fibrotic remodeling, physiological derangements in gas exchange, and a clinically evident IPF phenotype.

Figure 2. AT2 cell quality control pathways: homeostasis for proteins, organelles, and DNA.

Proteostasis (blue): Competitive binding of misfolded conformers to the molecular chaperone BiP activates one or more ER UPR sensors (ATF6, IRE1, PERK), initiating signaling to upregulate chaperones via three pathways: (i) ATF6p90 cleavage to ATF6p50 in the Golgi; (ii) IRE1 endoribonuclease activity for splicing XBP1; and (iii) PERK phosphorylation of eIF2α, repressing translation and upregulating ATF4. Proteins refractory to refolding are retrotranslocated from the ER and targeted to the 26S proteasome for degradation by the UPS, via the ERAD process. UPS inhibition can promote the accumulation of cytosolic protein macroaggregates in the aggresome via a microtubule-dependent manner. Autophagy and mitophagy (purple): The autophagosome-lysosome system targets cytosolic protein aggregates (macroautophagy) and dysfunctional organelles, such as mitochondria (mitophagy), for degradation. Ubiquitin-binding receptors, such as p62/SQSTM1, recognize K-48–linked polyubiquitinated protein aggregates or K-63–linked polyubiquitin-tagged mitochondrial outer membrane proteins (initiated by PINK1 recruitment of the E3 ligase parkin). LC3 binding envelopes ubiquitinated cargo and leads to elongation of isolation membranes (phagophores) and maturation into autophagosomes. Fusion with LAMP1⁺ lysosomes results in an acidified and functional autophagolysosome (autolysosome) that degrades the internalized content. Telomere maintenance (orange): Telomere length relies on the interaction between the multiprotein shelterin complex (end protection), the telomerase holoenzyme (elongation), and the DNA helicase RTEL1. Shelterin is composed of telomeric repeat binding factors 1 and 2 (TRF1 and -2), Tin2, TPP1, Rap1, and POT1. Key telomerase components include a reverse transcriptase subunit (TERT), an RNA template (TERC), and dyskerin. PARN promotes TERC RNA maturation. Not depicted is the CST complex. This figure was adapted with permission from Mulugeta et al. (46).

Figure 3. Cell quality control dysfunction and resulting endophenotypes that contribute to fibrotic remodeling.

AT2 cell quality control defects: Effective AT2 cell quality control relies on management of malformed or misfolded proteins (proteostasis), degradation of dysfunctional organelles, and maintenance of telomere length. AT2 response: The loss of quality control involves AT2 cell adaptive compensations, such as activation of the UPR/UPS to regain proteostasis. However, sustained quality control defects lead to cellular disruption and injury from ER stress, persistent mitochondrial dysfunction, metabolic reprograming, and DNA damage responses. AT2 endophenotypes: Modeling AT2 cell quality control defects and interrogation of the IPF epithelia have identified a number of AT2 cell endophenotypes in the fibrotic lung. These include the production of profibrotic mediators including TGF-β, loss of AT2 cells through apoptosis, challenges to progenitor cell function resulting in the loss of progenitor capacity from senescence or emergence of hyperproliferative AT2 cells, and recruitment of immune cell populations. Common IPF phenotype: The resultant loss of alveolar architecture in IPF is defined by fibrotic remodeling and myofibroblast expansion into fibroblastic foci and hyperplasia of lung epithelial cells.