Mitochondrial 8-oxoguanine DNA glycosylase mitigates alveolar epithelial cell PINK1 deficiency, mitochondrial DNA damage, apoptosis, and lung fibrosis

Alveolar epithelial cell (AEC) apoptosis, arising from mitochondrial dysfunction and mitophagy defects, is important in mediating idiopathic pulmonary fibrosis (IPF). Our group established a role for the mitochondrial (mt) DNA base excision repair enzyme, 8-oxoguanine-DNA glycosylase 1 (mtOGG1), in preventing oxidant-induced AEC mtDNA damage and apoptosis and showed that OGG1-deficient mice have increased lung fibrosis. Herein, we determined whether mice overexpressing the mtOGG1 transgene (mtOgg1^tg) are protected against lung fibrosis and whether AEC mtOGG1 preservation of mtDNA integrity mitigates phosphatase and tensin homolog-induced putative kinase 1 (PINK1) deficiency and apoptosis. Compared with wild type (WT), mtOgg1^tg mice have diminished asbestos- and bleomycin-induced pulmonary fibrosis that was accompanied by reduced lung and AEC mtDNA damage and apoptosis. Asbestos and H₂O₂ promote the MLE-12 cell PINK1 deficiency, as assessed by reductions in the expression of PINK1 mRNA and mitochondrial protein expression. Compared with WT, Pink1-knockout (Pink1-KO) mice are more susceptible to asbestos-induced lung fibrosis and have increased lung and alveolar type II (AT2) cell mtDNA damage and apoptosis. AT2 cells from Pink1-KO mice and PINK1-silenced (siRNA) MLE-12 cells have increased mtDNA damage that is augmented by oxidative stress. Interestingly, mtOGG1 overexpression attenuates oxidant-induced MLE-12 cell mtDNA damage and apoptosis despite PINK1 silencing. mtDNA damage is increased in the lungs of patients with IPF as compared with controls. Collectively, these findings suggest that mtOGG1 maintenance of AEC mtDNA is crucial for preventing PINK1 deficiency that promotes apoptosis and lung fibrosis. Given the key role of AEC apoptosis in pulmonary fibrosis, strategies aimed at preserving AT2 cell mtDNA integrity may be an innovative target.

A spatially restricted fibrotic niche in pulmonary fibrosis is sustained by M-CSF/M-CSFR signalling in monocyte-derived alveolar macrophages

Ontologically distinct populations of macrophages differentially contribute to organ fibrosis through unknown mechanisms.We applied lineage tracing, single-cell RNA sequencing and single-molecule fluorescence in situ hybridisation to a spatially restricted model of asbestos-induced pulmonary fibrosis.We demonstrate that tissue-resident alveolar macrophages, tissue-resident peribronchial and perivascular interstitial macrophages, and monocyte-derived alveolar macrophages are present in the fibrotic niche. Deletion of monocyte-derived alveolar macrophages but not tissue-resident alveolar macrophages ameliorated asbestos-induced lung fibrosis. Monocyte-derived alveolar macrophages were specifically localised to fibrotic regions in the proximity of fibroblasts where they expressed molecules known to drive fibroblast proliferation, including platelet-derived growth factor subunit A. Using single-cell RNA sequencing and spatial transcriptomics in both humans and mice, we identified macrophage colony-stimulating factor receptor (M-CSFR) signalling as one of the novel druggable targets controlling self-maintenance and persistence of these pathogenic monocyte-derived alveolar macrophages. Pharmacological blockade of M-CSFR signalling led to the disappearance of monocyte-derived alveolar macrophages and ameliorated fibrosis.Our findings suggest that inhibition of M-CSFR signalling during fibrosis disrupts an essential fibrotic niche that includes monocyte-derived alveolar macrophages and fibroblasts during asbestos-induced fibrosis.

Klotho, an antiaging molecule, attenuates oxidant-induced alveolar epithelial cell mtDNA damage and apoptosis

Alveolar epithelial cell (AEC) apoptosis and inadequate repair resulting from "exaggerated" lung aging and mitochondrial dysfunction are critical determinants promoting lung fibrosis. α-Klotho, which is an antiaging molecule that is expressed predominantly in the kidney and secreted in the blood, can protect lung epithelial cells against hyperoxia-induced apoptosis. We reasoned that Klotho protects AEC exposed to oxidative stress in part by maintaining mitochondrial DNA (mtDNA) integrity and mitigating apoptosis. We find that Klotho levels are decreased in both serum and alveolar type II (AT2) cells from asbestos-exposed mice. We show that oxidative stress reduces AEC Klotho mRNA and protein expression, whereas Klotho overexpression is protective while Klotho silencing augments AEC mtDNA damage. Compared with wild-type, Klotho heterozygous hypomorphic allele (kl/+) mice have increased asbestos-induced lung fibrosis due in part to increased AT2 cell mtDNA damage. Notably, we demonstrate that serum Klotho levels are reduced in wild-type but not mitochondrial catalase overexpressing (MCAT) mice 3 wk following exposure to asbestos and that EUK-134, a MnSOD/catalase mimetic, mitigates oxidant-induced reductions in AEC Klotho expression. Using pharmacologic and genetic silencing studies, we show that Klotho attenuates oxidant-induced AEC mtDNA damage and apoptosis via mechanisms dependent on AKT activation arising from upstream fibroblast growth factor receptor 1 activation. Our findings suggest that Klotho preserves AEC mtDNA integrity in the setting of oxidative stress necessary for preventing apoptosis and asbestos-induced lung fibrosis. We reason that strategies aimed at augmenting AEC Klotho levels may be an innovative approach for mitigating age-related lung diseases.

SIRT3 deficiency promotes lung fibrosis by augmenting alveolar epithelial cell mitochondrial DNA damage and apoptosis

Alveolar epithelial cell (AEC) mitochondrial dysfunction and apoptosis are important in idiopathic pulmonary fibrosis and asbestosis. Sirtuin 3 (SIRT3) detoxifies mitochondrial reactive oxygen species, in part, by deacetylating manganese superoxide dismutase (MnSOD) and mitochondrial 8-oxoguanine DNA glycosylase. We reasoned that SIRT3 deficiency occurs in fibrotic lungs and thereby augments AEC mtDNA damage and apoptosis. Human lungs were assessed by using immunohistochemistry for SIRT3 activity via acetylated MnSOD^K68 Murine AEC SIRT3 and cleaved caspase-9 (CC-9) expression were assayed by immunoblotting with or without SIRT3 enforced expression or silencing. mtDNA damage was measured by using quantitative PCR and apoptosis via ELISA. Pulmonary fibrosis after asbestos or bleomycin exposure was evaluated in 129SJ/wild-type and SIRT3-knockout mice (Sirt3^-/- ) by using fibrosis scoring and lung collagen levels. Idiopathic pulmonary fibrosis lung alveolar type II cells have increased MnSOD^K68 acetylation compared with controls. Asbestos and H₂O₂ diminished AEC SIRT3 protein expression and increased mitochondrial protein acetylation, including MnSOD^K68 SIRT3 enforced expression reduced oxidant-induced AEC OGG1^K338/341 acetylation, mtDNA damage, and apoptosis, whereas SIRT3 silencing promoted these effects. Asbestos- or bleomycin-induced lung fibrosis, AEC mtDNA damage, and apoptosis in wild-type mice were amplified in Sirt3^-/- animals. These data suggest a novel role for SIRT3 deficiency in mediating AEC mtDNA damage, apoptosis, and lung fibrosis.-Jablonski, R. P., Kim, S.-J., Cheresh, P., Williams, D. B., Morales-Nebreda, L., Cheng, Y., Yeldandi, A., Bhorade, S., Pardo, A., Selman, M., Ridge, K., Gius, D., Budinger, G. R. S., Kamp, D. W. SIRT3 deficiency promotes lung fibrosis by augmenting alveolar epithelial cell mitochondrial DNA damage and apoptosis.

Mitochondrial catalase overexpressed transgenic mice are protected against lung fibrosis in part via preventing alveolar epithelial cell mitochondrial DNA damage

RATIONALE

Alveolar epithelial cell (AEC) injury and mitochondrial dysfunction are important in the development of lung fibrosis. Our group has shown that in the asbestos exposed lung, the generation of mitochondrial reactive oxygen species (ROS) in AEC mediate mitochondrial DNA (mtDNA) damage and apoptosis which are necessary for lung fibrosis. These data suggest that mitochondrial-targeted antioxidants should ameliorate asbestos-induced lung.

OBJECTIVE

To determine whether transgenic mice that express mitochondrial-targeted catalase (MCAT) have reduced lung fibrosis following exposure to asbestos or bleomycin and, if so, whether this occurs in association with reduced AEC mtDNA damage and apoptosis.

METHODS

Crocidolite asbestos (100µg/50µL), TiO₂ (negative control), bleomycin (0.025 units/50µL), or PBS was instilled intratracheally in 8-10 week-old wild-type (WT - C57Bl/6J) or MCAT mice. The lungs were harvested at 21d. Lung fibrosis was quantified by collagen levels (Sircol) and lung fibrosis scores. AEC apoptosis was assessed by cleaved caspase-3 (CC-3)/Surfactant protein C (SFTPC) immunohistochemistry (IHC) and semi-quantitative analysis. AEC (primary AT2 cells from WT and MCAT mice and MLE-12 cells) mtDNA damage was assessed by a quantitative PCR-based assay, apoptosis was assessed by DNA fragmentation, and ROS production was assessed by a Mito-Sox assay.

RESULTS

Compared to WT, crocidolite-exposed MCAT mice exhibit reduced pulmonary fibrosis as measured by lung collagen levels and lung fibrosis score. The protective effects in MCAT mice were accompanied by reduced AEC mtDNA damage and apoptosis. Similar findings were noted following bleomycin exposure. Euk-134, a mitochondrial SOD/catalase mimetic, attenuated MLE-12 cell DNA damage and apoptosis. Finally, compared to WT, asbestos-induced MCAT AT2 cell ROS production was reduced.

CONCLUSIONS

Our finding that MCAT mice have reduced pulmonary fibrosis, AEC mtDNA damage and apoptosis following exposure to asbestos or bleomycin suggests an important role for AEC mitochondrial H₂O₂-induced mtDNA damage in promoting lung fibrosis. We reason that strategies aimed at limiting AEC mtDNA damage arising from excess mitochondrial H₂O₂ production may be a novel therapeutic target for mitigating pulmonary fibrosis.

ID: 140: KLOTHO, AN ANTI-AGING MOLECULE, REGULATES ALVEOLAR EPITHELIAL CELL MTDNA DAMAGE AND APOPTOSIS

Rationale

Convincing evidence has emerged that impaired alveolar epithelial cell (AEC) injury and repair resulting from ‘exaggerated’ lung aging and mitochondrial dysfunction are critical determinants of the lung fibrogenic potential of toxic agents, including asbestos fibers, but the mechanisms underlying these findings is unknown. We showed that the extent of AEC mitochondrial DNA (mtDNA) damage and apoptosis are critical determinants of asbestos-induced pulmonary fibrosis (Cheresh et al AJRCMB 2014, Kim et al JBC 2014). Klotho is an age-inhibiting gene and Klotho-deficient mice demonstrate a premature aging phenotype that includes a reduced lifespan, arteriosclerosis, and lung oxidative DNA damage, and that Klotho attenuates hyperoxic-induced AEC DNA damage and apoptosis (Ravikumar et al AJP-Lung 2014). We reason that Klotho has an important role in limiting pulmonary fibrosis by protecting the AECs from oxidative stress.

Methods

Quantitative PCR-based measurement of mtDNA damage was assessed following transient transfection with wild-type Klotho, Klotho siRNA or AKT siRNA in A549 and/or MLE-12 cells for 48 hrs followed by exposure to either amosite asbestos (25 µg/cm2) or H2O2 (200 µM) for 24 hrs. Apoptosis was assessed by cleaved caspase-9/3 levels and DNA fragmentation assay. Murine pulmonary fibrosis was analyzed in male 8–10 week old WT (C3H/C57B6J) mice or Klotho heterozygous knockout (Kl+/−) mice following intratracheal instillation of a single dose of 100 µg crocidolite asbestos or titanium dioxide (negative control) using histology (fibrosis score by Masson's trichrome staining) and lung collagen (Sircoll assay).

Results

Compared to control, amosite asbestos or H2O2 reduces Klotho mRNA/protein expression. Notably, silencing of Klotho promotes oxidative stress-induced AEC mtDNA damage and apoptosis whereas Klotho-enforced expression (EE) and Euk-134, a mitochondrial ROS scavenger, are protective. Interestingly, Kl+/− mice have increased asbestos-induced lung fibrosis. Also, we find that inhibition or silencing of AKT augments oxidant-induced AEC mtDNA damage and apoptosis.

Conclusions

Our data demonstrate a crucial role for AEC AKT signaling in mediating the mtDNA damage protective effects of Klotho. Given the importance of AEC aging and apoptosis in pulmonary fibrosis, we reason that Klotho/AKT axis is an innovative therapeutic target for preventing common lung diseases of aging (i.e. IPF, COPD, lung cancer, etc.) for which more effective management regimens are clearly needed.

Funding

NIH-RO1 ES020357-01A1 (DK) and VA Merit (DK).

ID: 117: SIRT3, THE MAJOR ANTI-AGING MITOCHONDRIAL DEACETYLASE, IS IMPORTANT FOR PREVENTING PULMONARY FIBROSIS

Objectives

Alveolar epithelial cell (AEC) injury from ‘exaggerated’ lung aging and mitochondrial dysfunction play vital roles in the development of lung fibrosis. Our group, using the asbestos lung fibrosis paradigm, has shown that AEC mitochondrial reactive oxygen species (ROS) mediate asbestos-induced AEC mitochondrial DNA (mtDNA) damage and apoptosis by a mitochondria-regulated (intrinsic) death pathway. Sirtuin 3 (SIRT3), the anti-aging major mitochondrial deacetylase governing mitochondrial function, mitigates oxidative stress and fibrosis in non-lung models though deacetylation of diverse substrates including MnSOD (Chen et al EMBO Rep 2011) and OGG1 (Cheng et al Cell Death Dis 2013). We previously reported that SIRT3 deficient (Sirt3−/−) mice have increased lung fibrosis following asbestos exposure associated with exaggerated AEC mtDNA damage and apoptosis. Herein, we determined whether SIRT3 deficiency augments bleomycin-induced lung fibrosis and whether AEC acetylation is increased in lung biopsy samples from patients with idiopathic pulmonary fibrosis (IPF).

Methods

Male 8- to 10- week-old 129SJ (Sirt3+/+) and Sirt3−/− mice were treated with a single intratracheal instillation of saline or bleomycin (0.01U). At 3 weeks, the lungs were harvested for various endpoints including Sircol collagen assay, fibrosis scoring and measurement of lung compliance. Specimens from explanted lungs of patients with IPF were subject to immunohistochemistry with antibodies to MnSODK68, Ac-OGG1 and IgG (negative control) to assess acetylation.

Results

Compared to wild type, Sirt3−/− mice developed increased pulmonary fibrosis following bleomycin exposure as measured by fibrosis score (7.8 vs. 11.25, p<0.05), Sircol assay (0.90 vs. 1.35, p<0.05) and lung compliance (0.0688 mL/cmH2O vs. 0.0449 mL/cm H2O, p<0.05). Notably, increased expression of MnSODK68 and Ac-OGG1 was evident in the lungs of patients with IPF. Co-localization studies evaluating MnSODK68 and SFPTC are ongoing.

Conclusions

SIRT3 deficiency enhances bleomycin-induced pulmonary fibrosis in a manner similar to asbestos fibers. An important role for augmented human IPF lung parenchymal cell mitochondrial acetylation is suggested by our pilot studies. Taken together, this suggests that SIRT3 plays a key role in the pathogenesis of IPF in part by preserving AEC mitochondrial function and mtDNA through modulation of the SIRT3/ACO-2/OGG1/apoptosis axis. Given the crucial role for aging in IPF as well as changes in SIRT3 expression with aging, our findings suggest a novel therapeutic target for modulating lung fibrosis.

Funding

VA Merit and NIH R01 ES02037-01A1 (DK), NIH/NHLBI T32 HL076139-11A1 (RJ).