Mechano-transduction in Lung Fibrosis

 Mechano-transduction in Lung Fibrosis

 

Abstract

Mechano-transduction is the phenomenon of conversion of mechanical forces to biochemical signals in fibrosis. Knowledge on how mechano-transduction influences the behavior of cells and tissues will help to identify novel therapeutic targets for mechano-modulatory approaches. Innovative therapies based on these advances will potentially transform fibrotic healing into tissue regeneration.

Keywords: Mechanotransduction, Extracellular matrix, Cytoskeleton

Introduction

The typical response to injury anywhere in the body is scar formation, which provides early restoration of tissue integrity rather than functional regeneration. Scar development serves as a rapid ‘patch’ response, providing a survival advantage as an evolutionarily conserved repair mechanism. All phases of wound healing are influenced by mechanical forces and there is increasing evidence that mechanical influences regulate post-injury inflammation and fibrosis across multiple organ systems. The intensity of scar formation and fibrosis is different in all organs. Lung being an organ with heavy collagen network, fibrosis is a usual phenomenon after tissue injury. Although fibrosis in the setting of cutaneous injury is highly visible, there are a variety of organ systems that demonstrate pathologic fibrotic response to injury, including lung tissue in idiopathic pulmonary fibrosis and the cardiovascular system following ischemic insult.

Acute wound healing typically occurs through a complex cascade of carefully orchestrated biochemical and cellular events in overlapping phases like hemostasis, inflammation,  proliferation  and remodeling with scar formation.

Mechanism

Mechanical stress causes an infiltration of inflammatory cells and decreased apoptosis of local cells involved in the healing response, resulting in proliferative scarring¹. Mechanical force regulates fibrosis in part via an inflammatory focal adhesion kinase - extracellular signal-regulated kinase - monocyte chemotactic protein-1 (FAK-ERK-MCP-1) pathway². Impact of FAK signalling on fibrosis formation has been demonstrated in lung tissue using a bleomycin-induced pulmonary fibrosis model in mice^3,4. Bleomycin, originally developed as an anticancer agent, causes an inflammatory response resembling acute lung injury (ALI) and ultimately leads to the development of fibrosis, which can be markedly decreased by FAK inhibition. Through the process of mechano-transduction, cells are able to convert mechanical stimuli into biochemical or transcriptional changes⁵.  This signal transduction involves proteins and molecules of the extra cellular matrix (ECM), the cytoplasmic membrane, the cytoskeleton and the nuclear membrane, eventually affecting the nuclear chromatin at a genetic and epigenetic level. Specifically, the response to continuous mechanical overload is a maladaptive remodeling of myocytes and the ECM, as well as increased interstitial fibrosis. There will be ECM activation through cellular traction forces and extracellular stretch. This is followed by cell surface, trans-membrane mechanotransduction. Next stage is mechanotransduction through cytoskeleton and nuclear mechanotransduction.

Mechano-transduction pathways are also important in the myocardium, as pathological hypertrophy can result from the abnormal cardiac workloads associated with systemic hypertension, aortic stenosis, or myocardial infarction⁶.

Extra Cellular Matrix

Extra Cellular Matrix (ECM) is a dynamic and living component possessing multiple functions, including playing a pivotal role in cell adhesion, migration, differentiation, proliferation, apoptosis and mechanotransduction. Reciprocal communication of mechanical cues between the ECM and cells can even directly influence gene expression, providing at least one mechanism of how physical cues from the ECM are able to alter cell functionality and phenotype. Mechanical forces can expose hidden domains and alter spatial density of growth factors within the ECM, thereby influencing cell behavior. Finally, cytokines such as TGF-β can bind to ECM domains and be released based on mechanical cues. Micro-environmental cues can influence fibroblast proliferation and collagen production via mechanoresponsive cell surface receptors. TGF-ß superfamily have a significant influence on fibrosis and inflammation.

Mediators

Multiple interrelated signaling pathways have been shown to participate in the complex mechanism of intracellular mechano-transduction. Mediators responsible for transducing signals from the biomechanical environment include integrin-matrix interactions, growth factor receptors (for TGF-β), G protein-coupled receptors (GPCRs), mechanoresponsive ion channels (e.g., Ca2+), and cytoskeletal strain responses. Through single-cell RNA sequencing, identified up-regulation of mechano-transduction signaling pathways in the healing grafts. Applying a hydrogel containing a focal adhesion kinase (FAK) inhibitor to the grafts to disrupt mechano-transduction, improved healing and reduced contracture and scar formation, with anti-inflammatory effects in the acute setting and pro-regenerative effects at later phase.

Therapeutic targets

The above findings suggest that FAK inhibition could be beneficial for treatment of injuries. The major therapeutic strategies involving the TGF-β pathway thus include using neutralizing antibodies to TGF-βl and 2, or increasing TGF-β3. Neutralizing antibodies bind directly to the ligand and prevent receptor activation, and TGF-β1 and 2 specific antibodies have been successfully used to reduce fibrosis in a number of organs in animal models. The first clinical trial assessing an anti-TGF-β antibody (metelimumab) was used for patients with systemic sclerosis and demonstrated no significant improvement. A similar lack of benefit was seen in a Phase II clinical trial using imatinib mesylate, an inhibitor of TGF-β and platelet-derived growth factor signaling, for the treatment of scleroderma⁷.

Nintedanib, an antifibrotic agent used in idiopathic pulmonary fibrosis (IPF) is a potent small molecule inhibitor of the receptor tyrosine kinases platelet-derived growth factor (PDGF), fibroblast growth factor (FGF) and vascular endothelial growth factor receptor. Nintedanib interferes with processes active in fibrosis such as fibroblast proliferation, migration and differentiation, and the secretion of ECM⁸.

Pirfenidone, another antifibrotic agent used in IPF, attenuates the production of transforming growth factor-β1 (TGF-β1). By suppressing TGF-β1, pirfenidone inhibits TGF-β1-induced differentiation of human lung fibroblasts into myofibroblasts, thereby preventing excess collagen synthesis and extracellular matrix production⁹.

Conclusion

A thorough understanding of the various signaling pathways involved scar formation is essential to formulate strategies to combat fibrosis and scarring. While initial efforts focused primarily on the biochemical mechanisms involved in scar formation, more recent research has revealed a central role for mechanical forces in modulating these pathways. Many molecules are being tried to prevent fibrosis based on mechano-transduction such as anti TGF beta 1 and 2. More research is needed to identify different biochemical pathways leading to fibrosis which may help in targeting these molecules to prevent fibrosis. Pirfenidone and nintedanib, presently used as antifibrotic agents in IPF acts by inhibiting various stages of fibroblast proliferation implicated in mechanotransduction.

References

1.     S Aarabi, K A Bhatt, Y Shi, J Paterno, E I. Chang, S A Loh, et al. Mechanical load initiates hypertrophic scar formation through decreased cellular apoptosis. FAEB Journal 2007; 21(12):3250-3261.

2.     D Duscher, Z N.Maan, V W.Wong, R C. Rennert, M Januszyk, M Rodrigues, et al. Mechanotransduction and fibrosis. Journal of Biomechanics 2014; 47 (9): 1997-2005.

3.     D Lagares, O Busnadiego, R A García-Fernández, M Kapoor, S Liu, D E. Carter,et al. Inhibition of focal adhesion kinase prevents experimental lung fibrosis and myofibroblast formation. Arthritis and Rheumatology 2012; 64 (5): 1653-1664.

4.     K Kinoshita, YAono, M Azuma, J Kishi, A Takezaki, M Kishi, et al. Antifibrotic Effects of Focal Adhesion Kinase Inhibitor in Bleomycin-Induced Pulmonary Fibrosis in Mice. Am J Respir Crit Care Med, 2013; 49(4): 536-543.

5.     F J. Alenghat, D E. Ingber. Mechanotransduction: All Signals Point to Cytoskeleton, Matrix, and Integrins. Science's STKE 2002; 119: pe6. DOI: 10.1126/stke.2002.119.pe6

6.     Jaalouk, D., Lammerding, J. Mechanotransduction gone awry. Nat Rev Mol Cell Biol 2009;10, 63–73. https://doi.org/10.1038/nrm2597.

7.     S. Prey, K. Ezzedine, A. Doussau, A.S. Grandoulier, D. Barcat, E. Chatelus, et al. Imatinib mesylate in scleroderma-associated diffuse skin fibrosis: a phase II multicentre randomized double-blinded controlled trial. Brit J Dermat 2012;167(5): 1138-1144.

8.     L Wollin, E Wex, A Pautsch, G Schnapp, K E. Hostettler, Su Stowasser, et al. Mode of action of nintedanib in the treatment of idiopathic pulmonary fibrosis. Eur Respir J. 2015 May; 45(5): 1434–1445. doi: 10.1183/09031936.00174914

9.     Kim ES, Keating GM: Pirfenidone: a review of its use in idiopathic pulmonary fibrosis. Drugs. 2015 Feb;75(2):219-30. doi: 10.1007/s40265-015-0350-9.