Acetylated α-Tubulin Regulated by N-Acetyl-Seryl-Aspartyl-Lysyl-Proline(Ac-SDKP) Exerts the Anti-fibrotic Effect in Rat Lung Fibrosis Induced by Silica

Modifications of lipid pathways restrict SARS-CoV-2 propagation in human induced pluripotent stem cell-derived 3D airway organoids

BACKGROUND

Modifications of lipid metabolism were closely associated with the manifestations and prognosis of coronavirus disease of 2019 (COVID-19). Pre-existing metabolic conditions exacerbated the severity of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection while modulations of aberrant lipid metabolisms alleviated the manifestations. To elucidate the underlying mechanisms, an experimental platform that reproduces human respiratory physiology is required.

METHODS

Here we generated induced pluripotent stem cell-derived airway organoids (iPSC-AOs) that resemble the human native airway. Single-cell sequencing (ScRNAseq) and microscopic examination verified the cellular heterogeneity and microstructures of iPSC-AOs, respectively. We subjected iPSC-AOs to SARS-CoV-2 infection and investigated the treatment effect of lipid modifiers statin drugs on viral pathogenesis, gene expression, and the intracellular trafficking of the SARS-CoV-2 entry receptor angiotensin-converting enzyme-2 (ACE-2).

RESULTS

In SARS-CoV-2-infected iPSC-AOs, immunofluorescence staining detected the SARS-CoV-2 spike (S) and nucleocapsid (N) proteins and bioinformatics analysis further showed the aberrant enrichment of lipid-associated pathways. In addition, SARS-CoV-2 hijacked the host RNA replication machinery and generated the new isoforms of a high-density lipoprotein constituent apolipoprotein A1 (APOA1) and the virus-scavenging protein deleted in malignant brain tumors 1 (DMBT1). Manipulating lipid homeostasis using cholesterol-lowering drugs (e.g. Statins) relocated the viral entry receptor angiotensin-converting enzyme-2 (ACE-2) and decreased N protein expression, leading to the reduction of SARS-CoV-2 entry and replication. The same lipid modifications suppressed the entry of luciferase-expressing SARS-CoV-2 pseudoviruses containing the S proteins derived from different SARS-CoV-2 variants, i.e. wild-type, alpha, delta, and omicron.

CONCLUSIONS

Together, our data demonstrated that modifications of lipid pathways restrict SARS-CoV-2 propagation in the iPSC-AOs, which the inhibition is speculated through the translocation of ACE2 from the cell membrane to the cytosol. Considering the highly frequent mutation and generation of SARS-CoV-2 variants, targeting host metabolisms of cholesterol or other lipids may represent an alternative approach against SARS-CoV-2 infection.

The α-tubulin acetyltransferase ATAT1: structure, cellular functions, and its emerging role in human diseases

The acetylation of α-tubulin on lysine 40 is a well-studied post-translational modification which has been associated with the presence of long-lived stable microtubules that are more resistant to mechanical breakdown. The discovery of α-tubulin acetyltransferase 1 (ATAT1), the enzyme responsible for lysine 40 acetylation on α-tubulin in a wide range of species, including protists, nematodes, and mammals, dates to about a decade ago. However, the role of ATAT1 in different cellular activities and molecular pathways has been only recently disclosed. This review comprehensively summarizes the most recent knowledge on ATAT1 structure and substrate binding and analyses the involvement of ATAT1 in a variety of cellular processes such as cell motility, mitosis, cytoskeletal organization, and intracellular trafficking. Finally, the review highlights ATAT1 emerging roles in human diseases and discusses ATAT1 potential enzymatic and non-enzymatic roles and the current efforts in developing ATAT1 inhibitors.

Results from omic approaches in rat or mouse models exposed to inhaled crystalline silica: a systematic review

BACKGROUND

Crystalline silica (cSiO2) is a mineral found in rocks; workers from the construction or denim industries are particularly exposed to cSiO2 through inhalation. cSiO2 inhalation increases the risk of silicosis and systemic autoimmune diseases. Inhaled cSiO2 microparticles can reach the alveoli where they induce inflammation, cell death, auto-immunity and fibrosis but the specific molecular pathways involved in these cSiO2 effects remain unclear. This systematic review aims to provide a comprehensive state of the art on omic approaches and exposure models used to study the effects of inhaled cSiO2 in mice and rats and to highlight key results from omic data in rodents also validated in human.

METHODS

The protocol of systematic review follows PRISMA (Preferred Reporting Items for Systematic Reviews and Meta-Analyses) guidelines. Eligible articles were identified in PubMed, Embase and Web of Science. The search strategy included original articles published after 1990 and written in English which included mouse or rat models exposed to cSiO2 and utilized omic approaches to identify pathways modulated by cSiO2. Data were extracted and quality assessment was based on the SYRCLE's Risk of Bias tool for animal studies.

RESULTS

Rats and male rodents were the more used models while female rodents and autoimmune prone models were less studied. Exposure of animals were both acute and chronic and the timing of outcome measurement through omics approaches were homogeneously distributed. Transcriptomic techniques were more commonly performed while proteomic, metabolomic and single-cell omic methods were less utilized. Immunity and inflammation were the main domains modified by cSiO2 exposure in lungs of mice and rats. Less than 20% of the results obtained in rodents were finally verified in humans.

CONCLUSION

Omic technics offer new insights on the effects of cSiO2 exposure in mice and rats although the majority of data still need to be validated in humans. Autoimmune prone model should be better characterised and systemic effects of cSiO2 need to be further studied to better understand cSiO2-induced autoimmunity. Single-cell omics should be performed to inform on pathological processes induced by cSiO2 exposure.

A Systematic Review of Artificial Intelligence Applications in the Management of Lung Disorders

This systematic review examines the transformative impact of artificial intelligence (AI) in managing lung disorders through a comprehensive analysis of articles spanning 2014 to 2023. Evaluating AI's multifaceted roles in radiological imaging, disease burden prediction, detection, diagnosis, and molecular mechanisms, this review presents a critical synthesis of key insights from select articles. The findings underscore AI's significant strides in bolstering diagnostic accuracy, interpreting radiological imaging, predicting disease burdens, and deepening the understanding of tuberculosis (TB), chronic obstructive pulmonary disease (COPD), silicosis, pneumoconiosis, and lung fibrosis. The synthesis positions AI as a revolutionary tool within the healthcare system, offering vital implications for healthcare workers, policymakers, and researchers in comprehending and leveraging AI's pivotal role in lung disease management.

Knockout of the BRAP homolog in mice leads to abnormal tracheal cilia

Both bombesin receptor-activated protein (BRAP) and its mouse homolog have been found to be expressed in bronchial epithelia but with unclear functions. Using electron microscopy combined with histological assays, we found that BRAP homolog deficiency in mice led to abnormal tracheal cilia. Rab-3A-interacting protein (Rabin8), a protein that might play a role in cilia development, was screened by yeast two-hybrid and further verified to have interaction with human BRAP by co-immunoprecipitation and pulldown assays. The expression levels of Rabin8, together with acetylated α-tubulin, a marker of cilia, were either downregulated by knockdown of BRAP or upregulated by overexpression of BRAP in cultured immortalized human bronchial epithelial cells. These results reveal a role for BRAP in airway cilia formation.

Ac-SDKP promotes KIF3A-mediated β-catenin suppression through a ciliary mechanism to constrain silica-induced epithelial-myofibroblast transition

Kinesin family member 3 A (KIF3A) decrease have been reported in silicotic patients and rats. However, the detailed mechanisms of KIF3A in silicosis remain unknown. In this study, we demonstrated that KIF3A effectively blocked the expression of β-catenin and downstream myocardin-related transcription factor (MRTF)-A/serum response factor (SRF) signaling, thus inhibiting silica-induced epithelial-myofibroblast transition (EMyT). Moreover, KIF3A was identified as a downstream mediator of an antifibrotic tetrapeptide N-acetyl-seryl-aspartyl-lysyl-proline (Ac-SDKP). Knockdown of KIF3A expression reactivated β-catenin/myocardin-related transcription factor (MRTF)-A/serum response factor (SRF) signaling that was attenuated by Ac-SDKP in vitro. Collectively, our findings suggest that Ac-SDKP plays its anti-fibrosis role via KIF3A-mediated β-catenin suppression, at least in part, in both in vivo model of silicosis and in vitro model of EMyT.

Fenbendazole Attenuates Bleomycin-Induced Pulmonary Fibrosis in Mice via Suppression of Fibroblast-to-Myofibroblast Differentiation

Idiopathic pulmonary fibrosis (IPF) is a chronic, progressive, and fatal interstitial lung disease with unknown etiology. Despite substantial progress in understanding the pathogenesis of pulmonary fibrosis and drug development, there is still no cure for this devastating disease. Fenbendazole (FBZ) is a benzimidazole compound that is widely used as an anthelmintic agent and recent studies have expanded the scope of its pharmacological effects and application prospect. This study demonstrated that FBZ treatment blunted bleomycin-induced lung fibrosis in mice. In vitro studies showed that FBZ inhibited the proliferation and migration of human embryo lung fibroblasts. Further studies showed that FBZ significantly inhibited glucose consumption, moderated glycolytic metabolism in fibroblasts, thus activated adenosine monophosphate-activated protein kinase (AMPK), and reduced the activation of the mammalian target of rapamycin (mTOR) pathway, thereby inhibiting transforming growth factor-β (TGF-β1)-induced fibroblast-to-myofibroblast differentiation and collagen synthesis. In summary, our data suggested that FBZ has potential as a novel treatment for pulmonary fibrosis.

The Role of Tβ4-POP-Ac-SDKP Axis in Organ Fibrosis

Fibrosis is a pathological process in which parenchymal cells are necrotic and excess extracellular matrix (ECM) is accumulated due to dysregulation of tissue injury repair. Thymosin β4 (Tβ4) is a 43 amino acid multifunctional polypeptide that is involved in wound healing. Prolyl oligopeptidase (POP) is the main enzyme that hydrolyzes Tβ4 to produce its derivative N-acetyl-seryl-aspartyl-lysyl-proline (Ac-SDKP) which is found to play a role in the regulation of fibrosis. Accumulating evidence suggests that the Tβ4-POP-Ac-SDKP axis widely exists in various tissues and organs including the liver, kidney, heart, and lung, and participates in the process of fibrogenesis. Herein, we aim to elucidate the role of Tβ4-POP-Ac-SDKP axis in hepatic fibrosis, renal fibrosis, cardiac fibrosis, and pulmonary fibrosis, as well as the underlying mechanisms. Based on this, we attempted to provide novel therapeutic strategies for the regulation of tissue damage repair and anti-fibrosis therapy. The Tβ4-POP-Ac-SDKP axis exerts protective effects against organ fibrosis. It is promising that appropriate dosing regimens that rely on this axis could serve as a new therapeutic strategy for alleviating organ fibrosis in the early and late stages.

Exposure Time of Silica Dust and the Incidence of Oxidative Stress, Inflammation, and Fibrosis in Rat Lungs

BACKGROUND: Until now, exposure to silica dust is still a health problem worldwide. Silica exposure in the lungs will cause pulmonary fibrosis which is initiated by inflammation. However, the results of several studies regarding the duration of inflammation and fibrosis are still inconsistent. There was a role of oxidative stress in silicosis, but there were also inconsistencies in terms of when oxidative stress occurs in silica exposure. AIM: This study aimed to study the toxic effects of silica dust exposure by looking at the picture of inflammation and fibrosis and malondialdehyde (MDA) levels in lung tissue during the observation period of 7 days, 14 days, 21 days, and 28 days. METHODS: This study used a randomized post-test only control group design. The research sample was male Wistar rat (Rattus norvegicus), aged 6–10 weeks, body weight 150–200 g (divided into 5 groups: Control group, day 7 group, group day 14, group day 21, and group day 28). We administered silica suspension through intratracheal injection of 30 mg/rat on 0.5 mL of volume. Examination of MDA level was using the ELISA technique; histopathological examination of the liver used hematoxylin-eosin (HE) staining to determine inflammation and fibrosis. Statistical test using one-way ANOVA or Kruskal–Wallis followed by post hoc test. RESULTS: The results of our study found that intratracheal silica exposure increased MDA levels on the 7th day, increased the accumulation of collagen from the 14th day, and increased the pulmonary inflammation score on the 14th day (p < 0.05). CONCLUSIONS: It was concluded that silica exposure caused significant oxidative stress on day 7 as well as significant inflammation and pulmonary fibrosis on day 14.

Recent advances in human respiratory epithelium models for drug discovery

The respiratory epithelium is intimately associated with the pathophysiologies of highly infectious viral contagions and chronic illnesses such as chronic obstructive pulmonary disorder, presently the third leading cause of death worldwide with a projected economic burden of £1.7 trillion by 2030. Preclinical studies of respiratory physiology have almost exclusively utilised non-humanised animal models, alongside reductionistic cell line-based models, and primary epithelial cell models cultured at an air-liquid interface (ALI). Despite their utility, these model systems have been limited by their poor correlation to the human condition. This has undermined the ability to identify novel therapeutics, evidenced by a 15% chance of success for medicinal respiratory compounds entering clinical trials in 2018. Consequently, preclinical studies require new translational efficacy models to address the problem of respiratory drug attrition. This review describes the utility of the current in vivo (rodent), ex vivo (isolated perfused lungs and precision cut lung slices), two-dimensional in vitro cell-line (A549, BEAS-2B, Calu-3) and three-dimensional in vitro ALI (gold-standard and co-culture) and organoid respiratory epithelium models. The limitations to the application of these model systems in drug discovery research are discussed, in addition to perspectives of the future innovations required to facilitate the next generation of human-relevant respiratory models.

Investigating the antifibrotic potential of N-acetyl seryl-aspartyl-lysyl-proline sequence peptides

N-acetyl-seryl-aspartyl-lysyl-proline (Ac-SDKP) is a physiological antifibrotic peptide that is hydrolysed by angiotensin I-converting enzyme (ACE). The beneficial antifibrotic effects of ACE inhibitors have been attributed, in part, to its inhibition of Ac-SDKP cleavage. There is indirect evidence that the SDK fragment of Ac-SDKP is the main component required for its antiproliferative action. However, the exact component of the physiological peptide that is responsible for its antifibrotic effect has yet to be determined. Ac-SDKP-derived analogues that are resistant to ACE degradation may provide a new avenue for fibrosis therapy. We tested the antifibrotic potential of various Ac-SDKP peptide sequences and an analogue resistant to ACE degradation in lung fibroblasts. We investigated the contribution and molecular mechanism of action of the amino acid residues in the Ac-SDKP sequence to its antifibrotic effects, and the effects of Ac-SDKP peptides in the prevention of collagen deposition in cells. The Ac-DKP fragment moderately inhibited endothelin-1 (ET-1) mediated transforming growth factor-β (TGF- β) expression, and could be slowly cleaved by ACE, revealing a different sequence requirement for the antifibrotic action of Ac-SDKP. The Ac-SDψKP analogue (where the peptide bond between the aspartate and lysine is reduced) inhibited TGF-β/small mother against decapentaplegic (Smad)-3 signalling and collagen deposition. The Ac-SDKP peptide, in combination with ACEi, demonstrated a greater inhibition of hydroxyproline as compared to Ac-SDKP alone.

Matrix stiffness regulates α-TAT1-mediated acetylation of α-tubulin and promotes silica-induced epithelial-mesenchymal transition via DNA damage

Silicosis is characterized by silica exposure-induced lung interstitial fibrosis and formation of silicotic nodules, resulting in lung stiffening. The acetylation of microtubules mediated by α-tubulin N-acetyltransferase 1 (α-TAT1) is a posttranslational modification that promotes microtubule stability in response to mechanical stimulation. α-TAT1 and downstream acetylated α-tubulin (Ac-α-Tub) are decreased in silicosis, promoting the epithelial-mesenchymal transition (EMT); however, the underlying mechanisms are unknown. We found that silica, matrix stiffening or their combination triggered Ac-α-Tub downregulation in alveolar epithelial cells, followed by DNA damage and replication stress. α-TAT1 elevated Ac-α-Tub to limit replication stress and the EMT via trafficking of p53-binding protein 1 (53BP1, also known as TP53BP1). The results provide evidence that α-TAT1 and Ac-α-Tub inhibit the EMT and silicosis fibrosis by preventing 53BP1 mislocalization and relieving DNA damage. This study provides insight into how the cell cycle is regulated during the EMT and why the decrease in α-TAT1 and Ac-α-Tub promotes silicosis fibrosis.This article has an associated First Person interview with the first authors of the paper.

The integrated stress response in pulmonary disease

The respiratory tract and its resident immune cells face daily exposure to stress, both from without and from within. Inhaled pathogens, including severe acute respiratory syndrome coronavirus 2, and toxins from pollution trigger a cellular defence system that reduces protein synthesis to minimise viral replication or the accumulation of misfolded proteins. Simultaneously, a gene expression programme enhances antioxidant and protein folding machineries in the lung. Four kinases (PERK, PKR, GCN2 and HRI) sense a diverse range of stresses to trigger this “integrated stress response”. Here we review recent advances identifying the integrated stress response as a critical pathway in the pathogenesis of pulmonary diseases, including pneumonias, thoracic malignancy, pulmonary fibrosis and pulmonary hypertension. Understanding the integrated stress response provides novel targets for the development of therapies.

Protein Expression Profile in Rat Silicosis Model Reveals Upregulation of PTPN2 and Its Inhibitory Effect on Epithelial-Mesenchymal Transition by Dephosphorylation of STAT3

Silicosis is a chronic occupational lung disease caused by long-term inhalation of crystalline silica particulates. We created a rat model that closely approximates the exposure and development of silicosis in humans. Isobaric tags for relative and absolute quantitation (iTRAQ) technologies we used to identify proteins differentially expressed in activated rat lung tissue. We constructed three lentiviral knockdown vectors and an overexpression vector for the protein tyrosine phosphatase non-receptor type 2 (PTPN2) gene to achieve stable long-term expression. A total of 471 proteins were differentially expressed in the silicosis group compared with controls. Twenty upregulated, and eight downregulated proteins exhibited a ≥1.5-fold change relative to controls. We next found that the PTPN2, Factor B, and VRK1 concentrations in silicotic rats silicosis and SiO₂-stimulated MLE-12 cells were significantly higher than control groups. More importantly, we found that overexpression of PTPN2 simultaneously decreased the expression of phospho–signal transducer and activator of transcription 3 (p-STAT3) and Vimentin, while increasing E-cadherin expression. The opposite pattern was observed for PTPN2-gene silencing. We identified three proteins with substantially enhanced expression in silicosis. Our study also showed that PTPN2 can inhibit epithelial-mesenchymal transition by dephosphorylating STAT3 in silicosis fibrosis.

Silica Perturbs Primary Cilia and Causes Myofibroblast Differentiation during Silicosis by Reduction of the KIF3A-Repressor GLI3 Complex

The purpose of this study was to determine the effects of Kinesin family member 3A (KIF3A) on primary cilia and myofibroblast differentiation during silicosis by regulating Sonic hedgehog (SHH) signalling. Methods: Changes in primary cilia during silicosis and myofibroblast differentiation were detected in silicotic patients, experimental silicotic rats, and a myofibroblast differentiation model induced by SiO₂. We also explored the mechanisms underlying KIF3A regulation of Glioma-associated oncogene homologs (GLIs) involved in myofibroblast differentiation. Results: Primary cilia (marked by ARL13B and Ac-α-Tub) and ciliary-related proteins (IFT 88 and KIF3A) were increased initially and then decreased as silicosis progressed. Loss and shedding of primary cilia were also found during silicosis. Treatment of MRC-5 fibroblasts with silica and then transfection of KIF3A-siRNA blocked activation of SHH signalling, but increased GLI2^FL as a transcriptional activator of SRF, and reduced the inhibitory effect of GLI3^R on ACTA2. Conclusion: Our findings indicate that primary cilia are markedly altered during silicosis and the loss of KIF3A may promote myofibroblast differentiation induced by SiO₂.

Microtubule destabilization caused by silicate via HDAC6 activation contributes to autophagic dysfunction in bone mesenchymal stem cells

BACKGROUND

Silicon-modified biomaterials have been extensively studied in bone tissue engineering. In recent years, the toxicity of silicon-doped biomaterials has gradually attracted attention but requires further elucidation. This study was designed to explore whether high-dose silicate can induce a cytotoxicity effect in bone mesenchymal stem cells (BMSCs) and the role of autophagy in its cytotoxicity and mechanism.

METHODS

Morphologic changes and cell viability of BMSCs were detected after different doses of silicate exposure. Autophagic proteins (LC3, p62), LC3 turnover assay, and RFP-GFP-LC3 assay were applied to detect the changes of autophagic flux following silicate treatment. Furthermore, to identify the potential mechanism of autophagic dysfunction, we tested the acetyl-α-tubulin protein level and histone deacetylase 6 (HDAC6) activity after high-dose silicate exposure as well as the changes in microtubule and autophagic activity after HDAC6 siRNA was applied.

RESULTS

It was found that a high dose of silicate could induce a decrease in cell viability; LC3-II and p62 simultaneously increased after high-dose silicate exposure. A high concentration of silicate could induce autophagic dysfunction and cause autophagosomes to accumulate via microtubule destabilization. Results showed that acetyl-α-tubulin decreased significantly with high-dose silicate treatment, and inhibition of HDAC6 activity can restore microtubule structure and autophagic flux.

CONCLUSIONS

Microtubule destabilization caused by a high concentration of silicate via HDAC6 activation contributed to autophagic dysfunction in BMSCs, and inhibition of HDAC6 exerted a cytoprotection effect through restoration of the microtubule structure and autophagic flux.

Interaction of N-acetyl-seryl-aspartyl-lysyl-proline with the angiotensin-converting enzyme 2-angiotensin-(1-7)-Mas axis attenuates pulmonary fibrosis in silicotic rats

NEW FINDINGS

What is the central question of this study? What are the effects of the antifibrotic peptide acetyl-seryl-aspartyl-lysyl-proline (Ac-SDKP) on the angiotensin-converting enzyme 2 (ACE2)-angiotensin-(1-7)-Mas axis during the occurrence and progression of silicosis? What is the main finding and its importance? Ac-SDKP inhibited lung fibrosis in rats exposed to silica by activation of the ACE2-angiotensin-(1-7)-Mas axis. Angiotensin-(1-7) potentially promotes Ac-SDKP by increasing the level of meprin α, the major synthetase of Ac-SDKP. Thus, the interaction Ac-SDKP and angiotesin-(1-7) in silicosis could provide a new therapeutic strategy.

ABSTRACT

The central role of angiotensin-converting enzyme (ACE) in the occurrence and progression of silicosis has been established. The antifibrotic peptide acetyl-seryl-aspartyl-lysyl-proline (Ac-SDKP) can be degraded by ACE. The ACE2-angiotensin-(1-7)-Mas axis is protective and acts to counterbalance the detrimental effects of ACE-angiotensin II (Ang II)-Ang II type 1 receptor and exerts antifibrotic effects. Here, we demonstrate an interaction between Ac-SDKP and Ang-(1-7) in the inhibition of collagen deposition and myofibroblast differentiation in rats exposed to silica. Treatment with Ac-SDKP increased the level of ACE2-Ang-(1-7)-Mas in rats or in cultured fibroblasts and decreased the levels of collagen type I and α-smooth muscle actin. Furthermore, exogenous Ang-(1-7) had similar antifibrotic effects and increased the level of meprin α, a major Ac-SDKP synthetase, both in vivo and in vitro. Compared with non-silicotic patients exposed to silica, the level of serum ACE was increased in patients with silicosis phase III; the levels of Ang II and Ang-(1-7) were high in patients with silicosis phase II; and the level of Ac-SDKP was high in the silicosis phase III group. These data imply that Ac-SDKP and Ang-(1-7) have an interactive effect as regulatory peptides of the renin-angiotensin system and exert antifibrotic effects.

Ac-SDKP increases α-TAT 1 and promotes the apoptosis in lung fibroblasts and epithelial cells double-stimulated with TGF-β1 and silica

Crystalline silica (SiO₂) particles have very strong toxicity to the lungs, and silicosis is an excessive pulmonary interstitial remodeling disease that follows persistent SiO₂ injury. We showed here that DNA double strand breaks (DSBs) and apoptosis were aggravated during rat silicosis induced by SiO₂ exposure. Ac-SDKP attenuates lung parenchymal distortion and collagen deposition, and decreases the expression of γH2AX, p21, and cleaved caspase-3, as well as improves the reduction of pulmonary function caused by silicosis. In vitro, we found an evolution of smooth muscle actin α (α-SMA), collagen type I (Col I) in both A549 and MRC-5 cells in response to transforming growth factor-beta 1 (TGF-β1) + SiO₂. Only A549 cells showed any reduction in the rate of apoptosis induced by the double stimulation, because of the anti-apoptotic effects of TGF-β1. N-acetyl-seryl-aspartyl-lysyl-proline (Ac-SDKP) is an anti-fibrotic tetrapeptide. It also has the ability to promote the apoptosis of leukemia cells. However its role in promoting cell apoptosis in silicosis is still unknown. We here found that Ac-SDKP could induce cell apoptosis and inhibit fibrotic response in A549 and MRC-5 cells treated with TGF-β1 + SiO₂, and these effects depended on regulation of α-tubulin acetyltransferase 1 (α-TAT1). These findings suggest that Ac-SDKP may have therapeutic value in the treatment of silicotic fibrosis.

Proteomic profile of TGF-β1 treated lung fibroblasts identifies novel markers of activated fibroblasts in the silica exposed rat lung

We performed liquid chromatography-tandem mass spectrometry (LC-MS/MS) on control and TGF-β1-exposed rat lung fibroblasts to identify proteins differentially expressed between cell populations. A total of 196 proteins were found to be differentially expressed in response to TGF-β1 treatment. Guided by these results, we next determined whether similar changes in protein expression were detectable in the rat lung after chronic exposure to silica dust. Of the five proteins selected for further analysis, we found that levels of all proteins were markedly increased in the silica-exposed rat lung, including the proteins for the very low density lipoprotein receptor (VLDLR) and the transmembrane (type I) heparin sulfate proteoglycan called syndecan 2 (SDC2). Because VLDLR and SDC2 have not, to our knowledge, been previously linked to the pathobiology of silicosis, we next examined whether knockdown of either gene altered responses to TGF-β1 in MRC-5 lung fibroblasts. Interestingly, we found knockdown of either VLDLR or SDC2 dramatically reduced collagen production to TGF-β1, suggesting that both proteins might play a novel role in myofibroblast biology and pathogenesis of silica-induced pulmonary fibrosis. In summary, our findings suggest that performing LC-MS/MS on TGF-β1 stimulated lung fibroblasts can uncover novel molecular targets of activated myofibroblasts in silica-exposed lung.

N-acetyl-seryl-aspartyl-lysyl-proline (Ac-SDKP) attenuates silicotic fibrosis by suppressing apoptosis of alveolar type II epithelial cells via mediation of endoplasmic reticulum stress

Damage to alveolar epithelial cells (AECs) caused by long-term inhalation of large amounts of silica dust plays a significant role in the pathology of silicosis. The present study was undertaken to investigate the regulatory mechanism(s) involved in type II AEC damage from silicon dioxide (SiO₂) as well as the mechanism(s) related to the prevention of silicosis by the antifibrotic tetra peptide, N-acetyl-seryl-aspartyl-lysyl-proline (Ac-SDKP). The 2-DE results showed that SiO₂ induced endoplasmic reticulum (ER) stress in A549 cells. In addition, typical apoptotic characteristics were observed using a transmission electron microscope (TEM) in A549 cells stimulated by SiO₂ and in type II AECs from silicotic rats. Mechanistic study showed that both Ac-SDKP and 4-phenylbutyrate (4-PBA), an inhibiter of ER stress, attenuated GRP78, phosphor-PERK, phosphor-eIF2α, CHOP and Caspase-12 protein expression in A549 cells stimulated by SiO₂ and in type II AECs from silicotic rats. Treatment with Ac-SDKP and 4-PBA in vivo effectively inhibited collagen deposition in the lungs of silicotic rats. In summary, ER stress is involved in the apoptosis of type II AECs both in vitro and in vivo. Ac-SDKP effectively suppresses SiO₂-induced apoptosis in type II AECs by attenuating the Caspase-12 and PERK/eIF2α/CHOP pathway activation caused by ER stress, thus preventing silicotic fibrosis.