ENERGY SENSING PATHWAYS IN AGING AND CHRONIC LUNG DISEASE

Abstract

Aging is associated with an increased risk of a number of clinical syndromes, including chronic lung disease. There have been significant advances in our understanding of the biology of aging leading to the elucidation of the so-called “hallmarks of aging.” The cause-effect relationships between various hallmarks such as dysregulated nutrient sensing, mitochondrial dysfunction, and cellular senescence are not well understood. Here, I discuss the evidence for alterations in energy/metabolic sensing pathways in the degenerative chronic lung disease called idiopathic pulmonary fibrosis (IPF). The pathobiological mechanisms by which this defect may contribute to age-related susceptibility to IPF and potentially other diseases of the elderly are also discussed.

INTRODUCTION

The global population aged 60 and older has just surpassed 1 billion and will reach 2 billion by 2048 (U.S. Census Bureau, 2018; www.census.gov). For the first time in U.S. history, by the year 2035, individuals over the age of 65 are projected to outnumber those under the age of 18. This aging epidemic will result in a disproportionate increase in age-related chronic diseases with widespread socioeconomic impact. Thus, a deeper understanding of the pathobiology of age-related pathologies and the discovery and development of interventions to alleviate and treat these diseases should be a priority (1,2). In this article for the Transactions of the American Clinical and Climatological Association, I will summarize recent evidence for dysregulated energy sensing and redox imbalance in IPF.

IPF: A DISEASE OF AGING

IPF infrequently manifests itself as a clinical syndrome in patients under the age of 50, with the mean age at diagnosis being over 65 years of age. The incidence and prevalence of IPF increases, almost exponentially, after the age of 60 (3). Due to the rapid growth of the aging population, enhanced recognition of the disease, and improved diagnostic accuracy, the reported incidence of this disease will continue to increase in future years. Diagnosis of IPF should be suspected in middle-aged and elderly subjects who present with progressive exertional dyspnea, often accompanied by a nonproductive cough. Chest radiographs usually show peripheral interstitial opacities that are most prominent in the bases of the lung, and pulmonary function testing reveals a restrictive pattern with reductions in gas exchange capacity. Known causes of interstitial lung disease must be excluded, most notably connective tissue disease-associated lung disorders, drug-induced lung disease, and disorders associated with occupational and environmental exposures to organic or inorganic dust. In most cases, a typical pattern of usual interstitial pneumonia (UIP) on high-resolution computed tomography (HRCT), characterized by peripheral and basilar-predominant reticulation and septal thickening with honeycomb change and traction bronchiectasis in the absence of atypical features, may be sufficient to make the diagnosis. Atypical features on HRCT that make UIP less probable/indeterminate, or should suggest alternative diagnoses, include upper/mid-lung predominance, peri-bronchovascular distribution, subpleural sparing, extensive ground glass opacification, mosaic attenuation, diffuse nodules/cysts, and consolidation and/or moderate-severe mediastinal adenopathy. In most specialized centers, a multidisciplinary approach to diagnosis improves diagnostic accuracy and may obviate the need for invasive testing such as a surgical lung biopsy (4,5). In one study, increasing age and average total HRCT interstitial score in comparison to multiple other clinical parameters were found to be predictive of biopsy-confirmed IPF (6).

In addition to epidemiologic data that support aging as a risk factor for IPF, genetic evidence also links biological aging to disease susceptibility. Of all the clinical syndromes associated with inherited “telomeropathies” caused by germline mutations in telomerase or related genes that maintain telomere length, IPF is the most common human phenotype (7). Telomere shortening, long recognized as one of the hallmarks of aging, is found not only in familial IPF but also in sporadic cases of IPF (8), suggesting a common pathophysiology. Although the precise mechanisms have yet to be defined, loss of regenerative functions of lung epithelial cells, which results in apoptosis and/or senescence, is thought to predispose to fibrotic pathology (7,9).

MYOFIBROBLASTS IN RESOLVING VERSUS PATHOLOGICAL FIBROSIS

Fibrosis involving diverse organs, including the lung, is characterized by the persistence of apoptosis-resistant myofibroblasts at sites of active tissue remodeling (10-12). While the recruitment, differentiation, and activation of myofibroblasts are stereotypical responses to wound repair, normal wound healing requires the apoptotic clearance of these cells (13,14). Myofibroblast evasion of apoptosis is a cardinal feature of progressive fibrotic disorders, including IPF (15,16). There is growing interest in determining the ultimate/proximate causes of the emergence of this apoptosis-resistant population of myofibroblasts and the biological factors that determine the fate of myofibroblasts during the evolution of tissue repair and regeneration (12).

ENERGY SENSING IN MYOFIBROBLAST FATE DETERMINATION

The fate of myofibroblasts during the tissue repair process likely involves both cell autonomous (intrinsic) and non-cell autonomous (extrinsic) factors. Non-cell autonomous factors constitute tissue microenvironmental signals that include both soluble mediators and the extracellular matrix (ECM) (17,18). Genetic factors that may account for the phenotype/fate of myofibroblasts in fibrotic disorders are not well understood; however, accumulated damage to cells over their lifespan may contribute to derangements in cell autonomous behavior. Accumulating data indicate that dysregulation of cell metabolism may contribute to the altered cellular phenotypes/fates in pulmonary fibrosis (19-21).

A central hub in the control of cell metabolism is the 5' adenosine monophosphate-activated protein kinase (AMPK) pathway, which controls the “switch” from anabolic to catabolic metabolism; AMPK activation typically serves an adaptive function when energy demand exceeds nutrient supply. Our recent studies demonstrate that, despite the high bioenergetic demand of activated myofibroblasts (20), myofibroblastic foci in the lungs of IPF subjects show relatively low levels of AMPK activation (22). We found that this deficiency in AMPK activation results in reduced autophagy that controls the steady-state levels of ECM proteins, in particular collagen protein expression that can be downregulated by pharmacologic AMPK activators (22). Additionally, our studies support the concept that AMPK-dependent mitochondrial biogenesis may restore apoptosis susceptibility to resistant myofibroblasts. The AMPK activator, metformin, was able to mediate anti-fibrotic effects even when administered after established fibrosis in a lung injury model, indicating that fibrosis can be reversed by metabolic reprograming that alters the fate of myofibroblasts (22,23).

REDOX IMBALANCE IN MYOFIBROBLAST SENESCENCE

The lungs are particularly susceptible to oxidative stress due to the higher concentrations of ambient oxygen in the air we breathe (24), as well as exposure to airborne particulates/toxins. The lungs of an average adult human exchange over 8,000 liters of air per day. In addition to “oxidative stress,” which is defined as an excess in the generation of reactive oxygen species (ROS) relative to the antioxidant capacity of cells/tissues (25), ROS can also participate in physiological cell signaling—often referred to as “redox signaling” (26). Dysregulated redox signaling and/or oxidative stress has been associated with IPF (27,28), as well as with the aging biology (29).

Almost 25 years ago, we made the observation that the pro-fibrotic cytokine, transforming growth factor-β1 (TGF-β1), activates a ROS-generating flavoenzyme through a transcriptional response resulting in the extracellular release of hydrogen peroxide (H₂O₂) (30). Subsequently, the enzymatic source of TGF-β1-indcued H₂O₂ was determined to be a member of the NADPH oxidase (Nox) family, Nox4, which mediates myofibroblast differentiation and activation (31). Nox4 is unique in that it is primarily regulated at the transcriptional level and it involves tandem AP-1/Smad binding to the far upstream region of the Nox4 promoter in response to TGF-β1 signaling (32). Both genetic and pharmacologic approaches targeting this enzyme resulted in a protective effect in experimental models of lung fibrosis (31,33). Based on the age-associated predilection for fibrosis, we have studied the lung injury response, both in young mice (2 months of age) and old mice (18 months of age). In contrast to young mice, old mice have a markedly diminished capacity for fibrosis resolution (34). The non-resolving phenotype of fibrosis in aged mice was associated with a higher expression of Nox4 and impaired activation of the Nrf2-dependent antioxidant pathway (35). A pharmacologic inhibitor of Nox1/4, GKT137831, administered during the period of persistent fibrosis was capable of accelerating fibrosis resolution and decreasing the population of accumulated senescent, apoptosis-resistant myofibroblasts in the lung (34). Although this drug appears to be well tolerated in Phase II studies of other disease indications (namely, diabetic nephropathy and primary biliary cirrhosis), its specificity as a bona fide Nox4 inhibitor has been questioned in favor of its activity as a ROS scavenger and/or peroxidase inhibitor (36). The World Health Organization (WHO) recommended Setanaxib as the international nonproprietary name (INN) for GKT137831; the new stem “naxib” approved by the WHO refers to NADPH oxidase inhibitors and formally establishes a new therapeutic class under the WHO INN system. A Phase II clinical trial of Setanaxib in IPF will test a pre-specified oxidative stress biomarker as the primary endpoint in addition to a number of secondary clinical endpoints (NCT03865927; https://clinicaltrials.gov/ct2/show/NCT03865927). We believe that these advancements will mark a new era in the development of targeted therapies for age-related disorders in which there is growing evidence for dysregulated redox signaling.

CONCLUSIONS

Aging has traditionally been viewed as the inevitable fate of life on our oxygen-rich planet (24); only recently has there been an impetus to study interventions that extend human longevity (37). The biomedical sciences have largely tackled the problem of age-related diseases in an organ-specific manner, without taking into account the profound effects of aging biology on the development and/or progression of a large number of these chronic diseases (1,2). In this article, I have highlighted novel therapeutic strategies for the age-related lung disease, IPF, which have emerged as a result of addressing aging biology, specifically, dysregulated nutrient/energy sensing and cellular senescence. We anticipate that similar strategies for age-related diseases that affect the lungs and other organ systems will lead to the development of more effective therapeutics for organismal health and longevity.

DISCUSSION

Zeidel, Boston: Wonderful talk. I am reminded of recent work going on in the kidney by Samir Parikh and others who are linking failure of appropriate regulation of the NAD⁺ pathways in mitochondria with the destruction of the kidney rather than its recovery in acute and chronic settings. I think it has been found in several organs now, including the brain, that mitochondrial regulation and NAD⁺ regulation of mitochondria are very important. You seem to be touching on the same thing. Do you believe there are similar findings in other organs?

Thannickal, Birmingham: Yes, absolutely, and I think the same bioenergetic pathways involving NAD⁺ as you mentioned are important in regulation of sirtuin (SIRT) family enzymes. As you know, NAD⁺ is a cofactor for activation of SIRT enzymes, which have been implicated in aging and age-related diseases. I think it's really exciting that there are close biological similarities across tissues such as kidney, heart, lung, and even liver, although there are going to be some differences. Either there are alterations in bioenergetics in terms of the ability of healthy mitochondria to respond to stress to induce apoptosis when necessary or the accumulation of dysfunctional mitochondria may contribute to the emergence of an alternative phenotype such as cellular senescence.

Zeidel, Boston: If the nicotinamide recovery pathway is stimulated in a mouse, it would be interesting to know if that mouse is protected from bleomycin injury. Patients who are about to undergo cardiac operations have been given nicotinamide, and I believe this has gotten rid of the acute renal failure that occurs in such patients. So it's kind of interesting … I wonder if there may be some connection here. It seems tantalizing.

Thannickal, Birmingham: I am not sure if those specific studies have used lung injury models, but that's a great idea and I think there's much work that still needs to be done.