Uncovering the truth of fibrosis

Despite its essential role in the normal wound healing process, fibrosis in humans is most often discussed in its pathological context – an irreversible, progressive and, ultimately, fatal disease. Although it can occur at any age, pathological fibrosis involving diverse organ systems increases with age due to mechanisms that are only just being elucidated. The body’s response to injury is one of nature’s most complex processes. Multiple biological pathways immediately become activated and synchronised to repair the damage. In the animal kingdom, there are many species that simply regenerate. Even complex body parts can be regenerated with full function and form following amputation or injury¹. Invertebrates such as the flatworm can regenerate the head from a piece of tail and the tail from the head. Among vertebrates, fish can regenerate parts of the brain, eye, kidney, heart and fins. Frogs can regenerate limbs, tail, brain and eye tissue as tadpoles but not as adults; salamanders can regenerate limbs, heart, tail, brain, eye tissues, kidney, brain and spinal cord throughout life. This varying regenerative capacity is largely limited by each species’ propensity to fibrosis following tissue injury or wounding. In human adults, the wound repair process commonly leads to a non-functioning mass of fibrotic tissue (a scar). The inability to terminate this reparative response may underlie the progressive nature of fibrotic reactions in injured tissues². In contrast, early in human gestation, injured foetal tissues can be completely recreated, without fibrosis, in a process resembling regeneration. Fibrosis research is beginning to discover some of the pathways and elements involved in the repair mechanisms, comparing young and old, to one day hopefully have better treatments for these incurable set of diseases. Fibrosis places a huge burden on public health, in particular dysfunctional fibrotic healing can often cause lifelong disability. Human fibrotic disorders are estimated to contribute to 45% of all-cause mortality in the United States³. One of the most commonly known fibrotic disorders is idiopathic pulmonary fibrosis (IPF), a progressive and fatal lung disease with no effective treatment or cure. Aging is a risk factor for fibrotic disease, including IPF⁴. The incidence and prevalence of IPF both increase with age, with patients having a mean age greater than 65 years at the time of diagnosis. In addition, risk of death resulting from IPF increases with age⁵. Oxidative stress is also associated with age-associated diseases such as IPF⁶. Despite the strong association between aging and oxidative stress with IPF, few studies have investigated the cellular/molecular mechanisms associated with this age related predilection. The myofibroblast is a key effector cell in a wide range of fibrotic disorders, and is primarily responsible for extracellular matrix (ECM) synthesis and tissue re-modelling in progressive fibrosis⁷. IPF is characterised by accumulating clusters of myofibroblasts in the lungs. A higher density of these clusters in lung tissue predicts a lower chance of survival⁸. A recent study published in Science Translational Medicine⁹ from my lab at the University of Alabama at Birmingham evaluated the reparative response to lung injury in young and aged mice as a preclinical model of IPF. Previous studies in similar murine models had shown an association between age and severity of fibrosis but they did not assess dynamic changes over time or the capacity to resolve established fibrosis. We found that aged mice, in comparison to young mice, develop a more persistent fibrotic response. This apparent diminished capacity for fibrosis resolution in aged mice is associated with the emergence of senescent myofibroblast cells that are resistant to apoptosis and mediated by a redox imbalance. This imbalance was triggered by sustained, elevated expression of NOX4 [NADPH (reduced form of nicotinamide adenine dinucleotide phosphate) oxidase-4], a reactive oxygen species (ROS)–generating enzyme, and a malfunction in the ability of the cells to induce the Nrf2 (NFE2-related factor 2) antioxidant response, a master regulator of antioxidant genes. In lung tissues from human subjects with IPF, we confirmed high expression of NOX4 in myofibroblasts that persisted in areas of fibrosis, while Nrf2 expression was reduced. The role of NOX4 in regulating IPF fibroblast phenotypes was then evaluated using a first-in-class small-molecule inhibitor of NOX1 and 4, GKT137831, developed by Genkyotex. GKT137831 significantly attenuated senescence-associated beta-galactosidase activity in IPF lung fibroblasts, suggesting that NOX4 contributes to their cellular senescence. GKT137831 was able to restore IPF fibroblast susceptibility to apoptosis. Mice administered GKT137831 returned to their baseline body weight, whereas vehicle-treated controls failed to fully recover their body weight.  Furthermore, collagen levels in the lungs of treated mice were markedly diminished, indicating a reversal of age-associated persistent fibrosis. Importantly, treatment with GKT137831 markedly improved survival (95% as compared with about 75% survival in vehicle treated mice). These studies support the concept that loss of redox homeostasis in aging promotes the emergence/persistence of a senescent and apoptosis-resistant myofibroblast phenotype that sustains persistent/progressive fibrotic disorders. Previous attempts to develop new therapeutics for fibrotic diseases have focused on antioxidants that neutralise the source products of oxidative stress, the reactive oxygen species (ROS). However these attempts have mostly failed in clinical trials¹⁰. We believe strategies that more directly target the source(s) of oxidative stress generation, for example with Genkyotex’s GKT137831, may prove to be more specific and effective in comparison. Additional studies are required to determine whether the therapeutic potential of NOX4 inhibition in IPF is primarily related to effects on fibroblasts or whether other cell types may be involved. Earlier studies involving models of liver fibrosis also support the potential therapeutic use of this selective NOX inhibitor to treat a broad range of fibrotic diseases¹¹. Our recently published results not only provide preclinical evidence supporting targeting NOX4 as a viable strategy for therapeutic intervention in age-associated fibrotic disorders but also provide new insights into redox mechanisms that control profibrotic effects of fibroblast senescence. References

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Author Victor J. Thannickal, M.D. Professor & Director, Division of Pulmonary, Allergy and Critical Care Medicine, Ben Vaughan Branscomb Chair of Medicine in Respiratory Disease, Department of Medicine, University of Alabama at Birmingham