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Molecular Pathogenesis of Progressive Lung Diseases

Author: Larry H. Bernstein, MD, FCAP

 

Abstract

The lung and its airways are constantly exposed to the air we breath, its contaminants, microparticulates (asbestose), and incidental microorganisms, such as viruses. These are sources of acute and chronic pulmonary diseases. Just as the lung remodels in normal growth and development, the lung remodels following acute injuries, but in the case of chronic conditions, the remodeling capacity is stressed. The lung is potentially stressed even without exposure to external contaminants or viruses. This stress is related to its normal function of gas exchange between oxygen and carbon dioxide across the alveolar wall. This involves a mechanism for tissue repair initiated by signaling pathways that are triggered in response to oxidative stress that result in a process called the unfolded protein response (UPR). The UPR does not necessarily lead to tissue damage. Damage only occurs when there is sustained stress that exceeds the ability of the tissue to repair the cellular framework. Here, we shall visit the underlying repair process that may be undermined in different lung diseases, all of which involve the inflammatory response, but not necessarily under the same course and conditions.

 

Introduction

The lung develops as an outpouching of the foregut and consists of the trachea and bronchi, and the alveoli. Air exchange occurs in the alveoli. In utero, the lungs are filled with fluid, and breathing occurs at the time of birth. When birth is premature, the surfactant produced by the alveolar lining cells that is necessary for passage of air into and expand the lungs may be insufficient, leading to alveolar collapse. Another problem may be neonatal hypertension. The discussion that follows will only deal with a common metabolic condition that underlies the conditions that underlie the development of chronic pulmonary diseases in the neonate and the adult.

The main feature of the alveoli is that they consist of a single layer of epithelium lining the airspaces beneath which lies a capillary, ideally suited for the exchange of O₂ and CO₂. There is a basement membrane between the epithelial cells and the capillaries. Two types of alveolar epithelial cells cover 90% of the airway surface. The alveolar type I epithelial cells (ATI), whose main function is gas interchange, are the larger flattened phenotype. Alveolar type II epithelial cells (ATII) are the most abundant epithelial cell type functioning to maintain the alveolar space by secretion of several types of surfactant proteins and other ECM components. There is also a basement membrane beneath the epithelium to be considered. Secretory Clara and goblet cells, ciliated, basal and neuroendocrine cells are also found in the tracheo-bronchial pseudostratified epithelium. Ciliated and secretory cells are involved in clearing the airway passages from microorganisms, air pollutants and other inhaled pathogens. Mucous and goblet cells secrete mucous into the apical surface of the epithelium, which traps foreign particles. These are then cleared out by the action of ciliated cells.

In cellular senescence there are secretory phenotypes that produce pro-inflammatory and pro-fibrotic factors. In the case of subepithelial fibrosis immune cells, like macrophages and neutrophils as well as activated myofibroblasts populate the subcellular matrix and release of pro-fibrotic transforming growth factor beta and continuous deposition of ECM stiffens the basement membrane. This is accompanied by interstitial fibrosis (1).

The remainder of this review will consider how the lung reacts to stresses that may be functionally inherent, genetic mediated, environmental, or virus. This requires an understanding of the UPR, a common mechanism for cellular repair in response to oxidative and nitrosative stress, which is the common mechanism for protecting the alveolar cell, but becomes pathogenic when the stress exceeds the clearance mechanism.

 

The unfolded protein response (UPR)

The mitochondria (mi) and the endoplasmic reticulum (ER) play key roles in the response to stress, and the mitochondria are also involved by way of signaling mechanisms. We shall begin by considering the ER role (^ERUPR). The ER are tubular structures that have smooth and rough portions. The rough ER are essential for translation of the genetic code into an amino acid sequence. The smooth ER is involved in lipid synthesis, and other processes. Just as tRNAs are important building blocks for protein, microRNAs come into the picture as well. The microRNAs have a regulatory role in that they are noncoding, but they repress gene expression and thereby, protein homeostasis (protostasis) under the influence of ^ERUPR signaling. They have their expression under the influence of UPR signaling when there is oxidative/nitrosative stress (2).

The ER-induced ^ERUPR is mediated by three major ER-resident transmembrane sensors named PKR-like endoplasmic reticulum kinase (PERK), activating transcription factor 6 (ATF6a and isoforms), and inositol requiring enzyme 1 (IRE1a and isoforms)(2-4). BiP is an abundant ER chaperone that dissociates from these three sensors, leading to their activation of the ER stress response.

The first step is activation of IRE1a, which dimerizes, forms oligomers, and autophosphorylates. This results in a conformational change that activates RNAse. IRE1a RNAse excises a 26-nucleotide intron of the mRNA encoding the transcription factor X-box binding protein-1 (XBP1). This in turn is ligated resulting in a coding reading phase frame shift in the mRNA and leads to the expression of a more stable and active transcription factor, termed XBP1 spliced (XBP1s). XBP1s trans-activates target genes, which depends on the context of tissue and the stress stimuli. The targets of XBP1s are genes involved in protein folding, endoplasmic reticulum-associated degradation (ERAD), protein translocation to the ER, and protein secretion. IRE1a also signals through the assembly of many adapter proteins and regulators, referred to as the UPRosome, as well as control gene expression through ER stress-dependent XBP1 mRNA splicing (2-5)).

The ER is the site where protein is synthesis and maturation occurs. It is also where the transportation and release of correctly folded proteins together with the Golgi apparatus. ER dysfunction has been viewed in the context of adaptation to protein processing and folding in the ER lumen (3).

Activation of the UPR results in accumulation of reactive oxygen species (ROS) in cells devoid of PERK. ATF4 and PERK knockout cells require amino acid and cysteine supplementation. This is thought to be to replenish amino acids lost during secretion and to increase glutathione levels. ATF4 is essential for regulating amino acid metabolism and oxidative stress response. (In addition, PERK knockout cells cannot activate eIF2a dependent translational up-regulation of ATF4, and ATF4-/- cells lack ATF4 protein. ATF4 induces the transcription of genes involved in amino acid import, glutathione biosynthesis and resistance to oxidative stress (3).

NFD-kB is released from its inhibitor IkB as a result of PERK-mediated attenuation of translation. A variety of different genes involved in inflammatory pathways are expressed, such as those encoding the cytokines IL-1 and TNF-a, as NF-kB moves to the nucleus and switches on. Activated IRE1a recruits tumor necrosis factor-a (TNF-a)-receptor-associated factor 2 (TRAF2) in the second branch of the UPR. TRAF2 then activates JNK and IkB kinase (IKK). These are inflammatory kinases that phosphorylate and activate downstream mediators of inflammation. The third branch of the UPR, the ATF6 pathway, also activates NF-kB. The crosstalk between the three branches is evidenced by the spliced X-box binding protein 1 (XBP1s) and ATF4 both inducing production of the cytokines IL-8, IL-6, and monocyte chemoattractant protein 1 (MCP1) by endothelial cells. XBP1s and IFN-b are both initiated in IFN-b production when ER stress is combined with activation of Toll-like receptor (TLR) signaling and in IFN-a production by dendritic cells. ER calcium stores are mediated in calcium-dependent inflammatory responses that produce IL-8. The XBP1s expand the capacity of the ER for protein folding and results in the assembly of the metainflammasome. This protein complex integrates pathogen and nutrient sensing with ER stress, inflammatory kinases, insulin action, and metabolic homeostasis. The eIF2a kinase PKR (double-stranded RNA-activated protein kinase) is a core component of the metaflammasome which interacts directly with several inflammatory kinases such as IKK and JNK, insulin receptor signaling components such as IRS1, and the translational machinery via eIF2a (4).

The conformational alteration of IRE1 via phosphorylation which exposes the RNAse that removes an intron from XBP1 mRNA generates of a protein that is a transcriptional regulator of genes involved in protein folding and degradation, both necessary mechanisms needed to restore ER homeostasis. GRP78 dissociation activates PERK, which in turn phosphorylates eIF2a, an inhibitor of new protein translation and activator of the transcription factor ATF4. The phosphorylation of PERK observed in primary aveolar epithelial cells comes with significant increase in the expression of ATF4 (5).

ER stress-dependent activation of UPR-mediated ER Ca²⁺ store expansion (via XBP-1 mRNA splicing) is induced by inflammation. This response is coupled to amplification of Ca²⁺-dependent inflammation and may be beneficial or adverse for the airways. This depends on whether airways are competent to clear or are obstructed. In normal airways (competent to clear), the airway epithelial ER Ca²⁺ store expansion provides a beneficial response, and reverses the expanded ER Ca²⁺ stores back to normal levels. However, the airway epithelial ER Ca²⁺ store expansion-mediated amplification of airway inflammation may be maladaptive for CF and COPD airways. This results in persistent airway epithelial ER Ca²⁺ store expansion that leads to chronic airway inflammation (3).

XBP1s launches a transcriptional program to produce chaperones (such as Grp78) and proteins involved in ER biogenesis, phospholipid synthesis, ER-associated protein degradation (ERAD), and secretion alone or in conjunction with ATF6a, which are key regulators of the transcriptional response programs.

There are five proteins that have sequence similarity with ATF6a and are anchored to the ER and in response to activation by specific stimuli. They undergo regulated intramembrane proteolysis in the Golgi and subsequent translocation to the nucleus. They all have been implicated in the ER stress response due to their ability to respond to traditional ER stressors. They activate known UPR targets, or show activity at UPR response elements.

Activation of the third arm of the UPR through PERK results in phosphorylation of eukaryotic translational initiation factor 2a (eIF2a) converting eIF2a to a competitor of eIF2b, which then results in reduced global protein synthesis. PERK is one of four protein kinases that can mediate eIF2a phosphorylation; the other three kinases are double stranded RNA-activated protein kinase (PKR), GCN2 general control non-derepressible kinase 2 (GCN2), and heme-regulated inhibitor kinase (HRI)(4).

 

Oxidative stress

Several oxidants give rise to reactive oxygen species (ROS) by inflammatory and epithelial cells within the lung as part of an inflammatory-immune response. These interfere with protein folding in the ER and the compensatory response is the ‘‘unfolded protein response’’ (UPR). Superoxide radicals (O2 ^•−) can either react with nitric oxide (NO) to form highly reactive peroxynitrite molecules (ONOO⁻) or are rapidly converted into hydrogen peroxide (H₂O₂) under the influence of superoxide dismutase (SOD) by activation of NADPH oxidase 2 (Nox2) on macrophages, neutrophils and epithelium. The non-enzymatic production of damaging hydroxyl radical (^•OH) from H₂O₂ occurs in the presence of Fe²⁺. Glutathione peroxidases (Gpxs) and catalase catalyze H₂O₂ to formH₂O and O₂. The ROS O2^•−, ONOO⁻, H₂O₂ and ^•OH trigger extensive inflammation, DNA damage, protein denaturation and lipid peroxidation. Lipid peroxidation products are 8-isoprostane, 4-hydroxy-2-nonenal (4-OH-2-nonenal) and malondialdehyde (MDA)], LTB4, carbon monoxide and myeloperoxidase (MPO)(6).

 

Mitochondrial phase of UPR (^mtUPR)

Mitochondria have a role in regulating alveolar epithelial cell (AEC) programmed cell death (apoptosis), and they are impaired by the generation of ROS, previously discussed. mtDNA encodes for 13 proteins, and that includes several essential for oxidative phosphorylation. The role of hemoglobin in O₂/CO₂ exchange given the redox state of iron, and the significant concentration of mitochondria in the AEC provides a suitable environment for the generation of ROS, which trigger an AEC mtDNA damage response and apoptosis (7). AEC mtDNA damage repair depends on 8-oxoguanine DNA glycosylase (OGG1) and mitochondrial aconitase (ACO-2), as they actively maintain mtDNA integrity. Reactive oxygen species (ROS)-driven mitochondrial metabolism is modulated by SIRTs. Indeed, SIRT3 is a mitochondrial deactylase linked to mitochondrial metabolism and mtDNA integrity. Moreover, it is known that there is crosstalk between mitochondrial ROS production, mtDNA damage, p53 activation, OGG1, and ACO-2 acting as a mitochondrial redox-sensor involved in mtDNA maintenance (7). Oxidative stress-induced mtROS induces mtDNA damage. It decreases the concentration of SIRT3, ACo-2 and mtOGG1 in AEC, and thereby causes a defective electron transport (ETC) that results in mitochondrial dysfunction, AEC apoptosis, and pulmonary fibrosis.

MtDNA encodes only 3% of mitochondrial proteins, and the rest are nuclear DNA proteins that are transported into the mitochondrion by transfer from the cytosol into the inner membrane. However, OGG1, ACO-2, mitochondrial transcription factor A (Tfam) are among those proteins encoded by nDNA essential for maintaining mtDNA integrity, as are those proteins involved in mtDNA repair. Nevertheless, mtDNA is ~50-fold more sensitive to oxidative damage because of proximity to the ETC, and are without histone protection, and repair mechanisms are limited. Consequently, stress-induced mtDNA damage has a mutation rate that is 10-fold greater than nDNA mtDNA damage. mtDNA mutations can then lead to mitochondrial dysfunction, including the collapse in the mitochondrial membrane potential (ΔΨm) and release of pro-apoptogenic agents (7).

The mitochondrial ETC generates hydroxyl radicals (^•HO), superoxide anions (O2^•−), and hydrogen peroxide (H₂O₂) generated from redox-active ferrous (Fe²⁺) iron or contact with asbestos fibers that impair ETC function by decreasing SIRT3, ACo-2 and mtOGG1 in AEC, causing mtDNA damage creating energy imbalance leading to apoptosis. (Not shown. from Seok-Jo Kim, P Cheresh, RP Jablonski, DB Williams and DW Kamp. Int. J. Mol. Sci. 2015; 16: 21486-21519. http://dx.doi.org:/10.3390/ijms160921486)

 

AEC apoptosis and pulmonary fibrosis

AEC apoptosis is followed by pulmonary fibrosis (PF) because of mutation –related damage to AEC Type 2 (AT2) cells (i.e., surfactant C and A2 genes, MUC5b). Oxidative stress occurs in the majority of AT2 cells, many of them having shortened telomeres, and PF occurs in the underlying matrix (7). This is evidenced with activation by various fibrotic stimuli that stimulate pro-apoptotic Bcl-2 family members action (i.e., ROS, DNA damage, asbestos, etc.). The intrinsic apoptotic death pathway acting in mitochondria results in increased permeability of the outer mitochondrial membrane, reduced ΔΨm. This is accompanied by the release of apoptotic proteins, such as cytochrome c, that activate pro-apoptotic caspase-9 and caspase-3.

Pulmonary fibrosis is driven by PINK1 expression and AEC apoptosis. Pro-apoptotic Bim activation is associated with mitochondria-regulated apoptosis and fibrosis. In addition, mitochondrial quality control pathway disruptions lead to accumulation of mtDNA mutations. These mtDNA mutations may compromise ETC function, and they also drive AEC to aerobic glycolysis, associated with the lung cancer phenotype (7).

AEC mtDNA damage is modulated by p53 in the pro-fibrotic lung response (8). In this process, plasminogen activator inhibitor (PAI-1) promotes AEC apoptosis, and at the same time reduces fibroblast proliferation and collagen production. At the same time there is crosstalk between the p53-uPA fibrinolytic system in AT2 cells. A change in phenotype in lung fibroblasts and tissue injury includes lung fibrosis. This is brought on by mtDNA damage and a DNA damage-associated molecular pattern (DAMP) that activates innate immun responses, especially toll like receptor (TLR)-9 signaling (8).
Concurrently, ACO-2 can be relocated from the TCA cycle to the nucleosome to stabilize the mtDNA with subsequent removal of oxidized Aco-2 by Lon protease (9).

Consider the role that UPR plays a role in lung diseases caused by the expression of genetically mutated, misfolded proteins. In cystic fibrosis, The UPR in airway epithelial cells is activated by mutant cystic fibrosis transmembrane conductance regulator (CFTR) delta F508, which interferes with CFTR expression and activates the innate immune response. The UPR in AT2 induces AT2 apoptosis concomitant with epithelial–mesenchymal transformation and extracellular matrix production in mutant surfactant protein C–induced interstitial pulmonary fibrosis (IPF)(10). It also is assumed to play a role in the pathogenesis of COPD. Potential mechanisms that activate the UPR in AT2 cells include direct oxidation of client proteins or chaperones, impaired function of the proteasome or autophagosomes, and decreased expression of miRNAs.

 

Disease specific UPR involvement in pulmonary fibrosis

• Idiopathic Pulmonary Fibrosis (IPF)

Idiopathic pulmonary fibrosis (IPF) is characterized by repeated injury to the alveolar epithelium with loss of lung epithelial cells and abnormal tissue repair, which results in accumulation of fibroblasts and myofibroblasts, deposit of extracellular matrix components and distorted lung architecture (11). The expression of heme oxygenase-1, a critical defender against oxidative stress, is decreased in macrophages of idiopathic pulmonary fibrosis patients, suggesting an oxidant–antioxidant imbalance in the pathogenesis of idiopathic pulmonary fibrosis (12).

Epithelial apoptosis leads to the release of growth factors and chemokines, which recruit fibroblasts to the site of injury (fibroblastic foci). Thus, myofibroblasts proliferate and extracellular matrix is deposited continues unabated in IPF. The transformation of epithelial cells into mesenchymal cells is a process known as epithelial mesenchymal transition. It allows direct communication between cells, and may explain the buildup of myofibroblasts in interstitial pulmonary fibrosis (IPF). When the distal epithelium in the lung becomes injured the basement membrane loses its integrity. It has to re-epithelialize the surface. Growth factors locally produced can potentially recruit fibroblasts or myofibroblasts (11). TGFb
-/- mice are devoid of avb6 integrin. Hence, they are unable to activate latent TGF-b1 and are protected from bleomycin-induced pulmonary fibrosis. Primary AECs were found to produce ET-1 at physiologically active levels and increased synthesis of TGF-b1 and the induction of EMT in AECs (11). The fibrosis of IPF occurs only in the lung, is the major source of surfactant proteins (SPs), such as SP-C. This protein appears vulnerable to mutations that disrupt folding and secretion. Recent studies found that IPF patients carry increased number of apoptotic cells in alveolar and bronchial epithelia. The bleomycin mouse model supports an hypothesis that inhibition of epithelial cell apoptosis prevents the development of the fibrosis (1).

• Interstitial pulmonary fibrosis (IPF)

IPF is the most common variety of lung fibrosis and carries a sobering mortality approaching 50% at 3–4 years (7). Increased oxidative DNA damage is seen in IPF, silicosis, and asbestosis patients, as well in experimental animal models. Ras-related C3 botulinum toxin substrate 1 (Rac1), is a protein encoded by the RAC1 gene found in human cells, which has a variety of alternatively spliced versions of the Rac1 protein (13). The UPR is activated in AT2 cells and induces epithelial–mesenchymal transformation, extracellular matrix production, and type II cell apoptosis In mutant surfactant protein C–induced interstitial pulmonary fibrosis (IPF) (10). The fibrotic phenotype of activated myofibroblasts show inhibition of the ER stress-induced IRE1a signaling pathway by using the inhibitor 4l8C that blocks TGFb-induced activation of myofibroblasts in vitro (13). IRE1a cleaves miR-150 releasing the suppressive effect that miR-150 exerts on aSMA expression through c-Myb. It also blocks ER expansion through an XBP-1-dependent pathway. In addition, prominent expression of UPR markers in AECs has been shown in the lungs of patients with surfactant protein C (SFTPC) mutation-associated fibrosis (14). Patients without SFTPC mutations with familial interstitial pneumonia and patients with sporadic IPF had selective UPR activation of AECs lining areas where there was fibrotic remodeling.
Activation of the UPR pathways may result from altered surfactant protein processing or chronic herpesvirus infection.

Fibroblasts in fibroblastic foci of IPF showed immunoreactivity for GRP78. In addition, TGF-b1 increased expression of GRP78, XBP-1, and ATF6a, which was accompanied by increases in a-SMA and collagen type I expression in mouse and human fibroblasts (15). TGF-b1–induced UPR and a-SMA and collagen type I induction were suppressed by the 4-PBA chaperone. Therefore, UPR is involved in myofibroblastic differentiation during fibrosis.

Initial observations linking ER stress and IPF were made in cases of familial interstitial pneumonia (FIP), the familial form of IPF, in a family with a mutation in surfactant protein C (SFTPC). ER stress markers are highly expressed in the alveolar epithelium in IPF and FIP (15). ER stress is induced in the alveolar epithelium predisposed to enhanced lung fibrosis after treatment with bleomycin, which is mediated at least in part by increased alveolar epithelial cell (AEC) apoptosis. In another study, aged mice developed greater ER stress in the AEC population linked to MHV68 infection as a result of increased BiP expression and increased XBP1 splicing, as well as increased AEC apoptosis, compared with young mice (16).

 

Chronic Obstructive Lung Disease (COPD)

Inflammatory and infectious factors are present in diseased airways that interact with G-protein coupled receptors (GPCRs), such as purinergic receptors and bradykinin (BK) receptors, to stimulate phospholipase C [PLC]. This is followed by the activation of inositol 1,4,5-trisphosphate (IP3)-dependent activation of IP3 channel receptors in the ER, which results in channel opening and release of stored Ca²⁺ into the cytoplasm. When ER Ca2+ stores are depleted a pathway for Ca²⁺ influx across the plasma membrane is activated. This has been referred to as “capacitative Ca²⁺ entry”, and “store-operated calcium entry” (3). In the next step PLC mediated Ca²⁺ i is mobilized as a result of GPCR activation by inflammatory mediators, which triggers cytokine production by Ca²⁺ i-dependent activation of the transcription factor nuclear factor kB (NF-kB) in airway epithelia. Ca²⁺ binding proteins including calmodulin, protein kinases C (PKCs) and the phosphatidylinositol 3-kinase (PI3K) can link Ca²⁺ i mobilization to NF-kB activation. Ca²⁺ i from ER Ca²⁺ release and/or a Ca²⁺ influx through the plasma membrane can be sensed by Ca²⁺ binding proteins (3). Chronically infected/inflamed native human bronchial epithelia exhibit UPR activation-dependent XBP-1 mRNA splicing and ER Ca²⁺ store expansion.

Protein secretion can constitute an irreversible loss of amino acids into the extracellular environment and produce net loss of equivalents from the cell. The greater the secretory burden, the greater the loss of amino acids and reducing equivalents from the cell. Activation of the UPR results in accumulation of ROS in PERK knockout cells. ATF4(-/-) and PERK (-/-) knockout cells require amino acid and cysteine supplementation to replenish amino acids lost during secretion. ATF4 would be required to induce the transcription of genes involved in amino acid import, glutathione biosynthesis and resistance to oxidative stress (3). Oxidative stress is a hallmark of CF airways disease and ATF4-induced amino acid transport is necessary for a protective role in inflamed CF airway epithelia.

Airway epithelial infection/inflammation induces ER stress-dependent activation of UPR-mediated ER Ca²⁺ store expansion (via XBP-1 mRNA splicing). The airway epithelial ER Ca²⁺ store is beneficial to the clearing of infection in normal airways. Epithelial ER Ca²⁺ store expansion-mediated amplification of airway inflammation may not be adequate for cystic fibrosis (CF) and COPD airways (3).

Changes in phosphor-eIF2a and CHOP expression correlate directly with the severity of airflow obstruction in COPD (10). An increase in CHOP in COPD was associated with increases in caspase 3 and 7, suggesting that the PERK pathway was contributing to heightened apoptosis in COPD. Mucous hypersecretion contributes to symptomatology and morbidity in COPD. IRE1b expression in airway epithelial cells promotes mucus cell development and mucin production.

 

Cigarette Smoke and COPD

Cigarette smoking is the major cause of COPD and accounts for more than 95% of cases in industrialized countries. It is the third largest cause of death in the world. It is now well established that cardiovascular -related comorbidities such as stroke contribute to morbidity and mortality in COPD (18). COPD involves chronic obstructive bronchiolitis with fibrosis, obstruction of small airways, emphysema with enlargement of airspaces, destruction of lung parenchyma, and loss of lung elasticity and closure of small airways. Chronic obstructive pulmonary disease (COPD) is characterized by progressive airflow limitation and loss of lung function (18). Chronic obstructive bronchiolitis, emphysema and mucus plugging are all characteristic features.

Proteomes of lung samples were taken from chronic cigarette smokers. There were 26 differentially expressed proteins (20 were up-regulated, 5 were down-regulated, and 1 was detected only in the smoking group) compared with nonsmokers. Several UPR proteins were up-regulated in smokers compared with nonsmokers and ex-smokers, including the chaperones, glucose-regulated protein 78 (GRP78) and calreticulin; a foldase, protein disulfide isomerase (PDI), and enzymes involved in antioxidant defense (18). Indeed, a UPR response in the human lung occurs in cigarette smoking that is rapid in onset, concentration dependent, and may be partially reversible with smoking cessation.
Of the proteins reported in chronic smokers, four are involved in translation and ribosome formation (60S acidic ribosomal protein P2, heat shock protein 27, and elongation factors-1b and -1d). Heat shock protein 27 inhibits formation of the large and small ribosomal complex, and 60S acidic ribosomal protein P2 associates with elongation factor-2 to form the large and small ribosomal complex. Glyceraldehyde-3-phosphate dehydrogenase, malate dehydrogenase, and ATP synthase subunit beta were up-regulated, and the inflammatory protein S100-A9/calgranulin C, an EF hand calcium-binding protein, was down-regulated (19).

Conclusion

The current status of a consolidated view of chronic pulmonary fibrotic diseases could not have been envisioned in a 19^th century scientific framework. There was no scientific guideline for constructing such a perspective. I have written this perspective on lung diseases keeping in memory the contributions of my mentor, Averill A. Liebow. I have not included pulmonary carcinoma in this discussion, although it too has a place. It was in 1927 that Otto Warburg conducted his historic work with rediscovery of the observation of Louis Pasteur more than a half century earlier in his observation of aerobic glycolysis in cancer cells. The mitochondrion was not known then, which he referred to as “grana”. There was no clear mechanism for such a phenomenon. This discussion based on a growing body of work brings greater clarity to the relationship between lung development, the aging of pulmonary tissue, and the process of tissue remodeling, with a more unified view of pulmonary degeneration that even applies to pulmonary hypertension.

 

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