Abstract
Mitochondrial antiviral signalling protein (MAVS) mediates pulmonary fibrosis by regulating cGAS-STING and senescence programming https://bit.ly/35Tijjj
Idiopathic pulmonary fibrosis (IPF) is a chronic, progressive, age-related interstitial lung disease. Incidence of IPF in the USA ranges between 14 and 63 per 100 000 people [1]. The average survival rate from the time of diagnosis is 3–5 years. IPF is associated with several genetic and environmental factors. Genes associated with a risk for developing IPF generally fall into two categories: epithelial cell genes (e.g. MUC5B, DSP, SFTPA2, ABCA3) or genes involved in telomere maintenance (e.g. TERT, TERC, RTEL1) [2, 3]. Environmental factors such as smoking, air pollution [4, 5] or accumulation of chitin polysaccharides [6] may also promote lung fibrosis. Consistent with the association of genes of the epithelium and telomere maintenance to IPF, short telomeres are a common finding in epithelial cells of IPF patients. An animal model of telomere dysfunction mediated by genetic deletion of TRF1 caused fibrosis that was progressive and chronic [7] indicating that telomere dysfunction is a molecular driver of fibrosis.
Pathologic findings in IPF include lung remodelling in sub-pleural regions, excess collagen deposition, accumulation of fibroblast foci and leukocyte infiltration [8]. These changes are associated with activation of a DNA damage response, senescence reprogramming, unfolded protein response, and accumulation of dysfunctional mitochondria [9]. In IPF patients, dysfunctional mitochondria are present in type 2 alveolar epithelial cells [10], alveolar macrophages [11] and fibroblasts [12]. Why mitochondria are abnormal in IPF lungs remains to be defined. One possibility is that stress on the affected cells alters cell cycle and cell functionality associated with mitochondrial dysfunction. Another possibility is that activation of P53, mediated by telomere dysfunction, leads to impaired mitochondrial function and reduced mitochondrial DNA copy number [13]. A relationship between telomere dysfunction and mitochondrial dysfunction in IPF lungs has yet to be explored.
Mitochondria are known as the “powerhouse of the cell” because they are primarily involved in generating ATP for cellular processes. Optimal mitochondrial function is required for maintaining cellular homeostasis. Mitochondria may respond to intrinsic (genetic, ageing) or extrinsic stimuli (injury, environmental pollutants) by releasing mitochondrial DNA (mt DNA) and reactive oxygen species (ROS), which activate inflammatory pathways. Excess ROS generation is a common finding in multiple fibrotic conditions including liver fibrosis, renal fibrosis and lung fibrosis. Damaged mitochondria that release ROS are cleared by mitophagy to maintain a functional mitochondrial pool [14]. Mitochondrial DNA damage, defective mitophagy, defective mitochondrial biogenesis and clearance are all possible factors that may pathologically activate tissue repair processes [15]. These processes may in part be regulated by PTEN-induced putative kinase 1 (PINK1) because PINK1 deletion in mouse lung epithelial cells caused dysfunctional mitochondria and expression of pro-fibrotic factors [16].
In response to injury, microbial infection or stress, endogenous molecules termed damage-associated molecular patterns (DAMPs) are released, leading to a loss of cellular homeostasis. DAMPs may be released into the cytosol of a cell or the extracellular compartment. DAMPs released by mitochondria include cardiolipin, mt DNA, ROS and n-formyl peptides (n-fp). Released DAMPs may activate pattern recognition receptorsn such as C-type lectin receptors, nucleotide-binding oligomerisation domain-like receptors (NLRs), toll-like receptors, retinoic acid inducible gene-1-like receptors and cyclic GMP-AMP synthase (cGAS). Following activation, NLRs form protein complexes called inflammasomes. A mitochondrial role for NLRP3 inflammasome activation was reported by Zhou et al. [17]. Mitochondrial antiviral signalling protein (MAVS) was identified [18] as an activator of NFkB and IRF3 and is primarily involved in antiviral immunity. It was later found that MAVS also acts as an adaptor molecule for the DAMP signalling molecule NLRP3 and regulates its inflammasome activity [19]. Similarly, the DNA sensing cGAS detects DNA in subcellular compartments and triggers host defence by activating downstream effector stimulator of interferon genes (STING) in response to microbial infections. cGAS may also be activated by endogenous DNA, including DNA released from mitochondria, extracellular chromatin [20, 21], or telomere fragments [22], where it has been reported to be involved in senescence reprogramming [23], as depicted in figure 1.
In this issue of the European Respiratory Journal, Kim et al. [24] investigate whether MAVS plays a role in mediating pulmonary fibrosis by testing the effect of absence of MAVS on lung fibrogenic response to intratracheal bleomycin exposure. This study used MAVS-deficient mice to establish that, in the absence of MAVS, mice exposed to bleomycin survived longer with significantly fewer senescent cells and less collagen deposition in the lungs. Absence of MAVS was associated with lower levels of transforming growth factor-β, p-SMAD2, fibronectin and α-smooth muscle actin in bronchoalveolar lavage fluid and whole lung lysates. In pursuit of identifying the molecular mechanism by which MAVS regulates fibrosis, Kim et al. [24] examined DAMP signalling, finding that the DAMP signalling molecules cGAS, STING and Nlrp3 were activated upon bleomycin challenge. Absence of MAVS abrogated the activation of cGAS and STING, indicating that MAVS may be regulating fibrosis via activation of these DAMPs.
Accumulation of senescent cells is a characteristic feature of chronic age-related diseases, such as pulmonary fibrosis and chronic kidney disease. Senescent cells are molecularly reprogrammed to acquire a senescence-associated secretory phenotype involving release of interleukins, cytokines and chemokines. Presence of these factors has been suggested to contribute to the onset or progression of various diseases including lung fibrosis. The study of senescent cells as drivers of disease pathology was advanced by the discovery that the BH-3 mimetic ABT-263-mediated clearance of senescent cells caused restoration of aged haematopoietic stem cells [25]. ABT-263 has since been used as a tool compound to target senescent cells and study their role in disease models. For example, ABT-263 has been used to selectively clear senescent myofibroblasts from skin of mice subcutaneously injected with bleomycin, reversing established fibrosis [26].
In the study by Kim et al. [24], collagen levels and lung fibrosis markers were alleviated in mice treated with ABT-263 administered after bleomycin administration. When murine cell lines were treated with ABT-263 in vitro, after bleomycin exposure, there were fewer senescence associated-beta-galactosidase (SA-Beta gal) positive cells with decreased cell viability and concurrent increase in the apoptosis marker cleaved caspase-3. Interestingly, bleomycin mediated induction of MAVS was blocked when murine cells were treated in vitro with BH3 mimetic ABT-263. It was unclear whether this inhibitory effect on MAVS was due to senolytic effect of ABT-263 (due to reduced cell number caused by clearance) or whether it is a senolysis-independent mechanism. To test this, cells treated with ABT-263 in the absence of bleomycin also showed reduction in MAVS levels, suggesting that regulation of MAVS by ABT-263 was in part via a senolytic-independent mechanism. The relationship between MAVS and bleomycin-induced senescence was tested using MAVS-deficient mice. Bleomycin treated MAVS−/− mice showed reduced expression of senescence markers P16INK4a and P19Arf compared to MAVS+/+ controls. Mouse embryonic fibroblasts from MAVS−/− mice exposed to bleomycin demonstrated significantly fewer SA-Beta gal positive cells with no considerable change in cleaved caspase-3 levels, indicating that bleomycin-induced senescence was associated with MAVS activation, without any additive increase in apoptosis as seen with senolytics.
In conclusion, the findings presented in the article by Kim et al. [24] unravel an unexpected role for MAVS in regulating fibrosis, either by influencing senescence reprogramming and/or DAMP activation via cGAS-STING (figure 1). There are, however, unanswered questions as clear molecular connection between the findings is lacking. For example, do BH-3 mimetics clear senescent cells by inhibiting MAVS or by enabling apoptosis as previously reported? Establishing that BH-3 mimetics reverse senescence programming by inhibiting MAVS would define a new mechanism by which BH-3 mimetics influence senescence and suggest that MAVS is a proximal regulator of senescence programming. It is also unclear whether MAVS-mediated senescence and regulation of cGAS-STING are acting in concert to modulate fibrosis or whether they are independent mechanisms occurring in parallel. The relationship between STING activation and fibrosis needs to be explored in more detail. Furthermore, the link between innate immune signalling and abrogation of fibrosis in absence of MAVS remains unresolved. A possible relationship between telomere dysfunction and MAVS, both of which are dysregulated in IPF patients, should be explored. Finally, there is increasing pre-clinical data indicating that senolytics may alleviate an array of chronic diseases. Although senolytics may have beneficial activities, the findings by Kim et al. [24] suggest that some of their activity may be due to modulating MAVS rather than senolysis. It is also possible they may have detrimental effects on healthier cells by modulating MAVS. Future studies building on the findings of Kim et al. [24] may address these questions and advance our understanding of the complex relationships between MAVS, cGAS-STING, senescence programming, lung remodelling and fibrosis.
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Footnotes
Conflict of interest: R.P. Naikawadi has nothing to disclose.
Conflict of interest: P.J. Wolters reports grants and personal fees from Boehringer Ingelheim and Roche/Genentech, personal fees from Gossamer Bio, Blade Therapeutics and Pliant, outside the submitted work.
- Received December 11, 2020.
- Accepted January 11, 2021.
- Copyright ©The authors 2021. For reproduction rights and permissions contact permissions{at}ersnet.org