Nrf2 Sequesters Keap1 Preventing Podosome Disassembly: A Quintessential Duet Moonlights in Endothelium

The lack of transcriptionally active Nrf2 triggers colon dysfunction in female mice - The role of estrogens

BACKGROUND AND AIM

The proper functioning of the gastrointestinal system relies on an intricate crosstalk between a plethora of cell types and signaling pathways. Recently we identified that the lack of NRF2 transcriptional activity (NRF2 tKO) triggers significant colon microscopical alterations, still they do not affect the general functioning of mice. Therefore, in this study, we aimed to address the gender-dependent impact of NRF2 transcriptional deficiency on colon function, and relate them to an established model of inflammatory bowel disease (IBD).

METHODS

In the study we subjected 3- and 6-month old mice deficient in IL-10 and NRF2 transcriptional activity and wild-type counterparts to tests assessing colon functionality, and histological analyses. To address the role of estrogens, we attempted to rescue the phenotype by the delivery of 17β-estradiol through subcutaneous implants.

RESULTS

In females, NRF2 transcriptional abrogation, like IL-10 deficiency, triggers a functional and microscopic phenotype, that resembles IBD. The females are significantly more affected by the dysfunctional phenotype, and the functional impairmentdecreases with age. We found that NRF2 transcriptional activity influences 17β-estradiol level and the estrogen receptors expression and location. Exogenous delivery of 17β-estradiol normalized colon motility in the NRF2 tKO mice, which is related to enhanced ERβ signaling.

CONCLUSIONS

Summing up, in this study, we underline that NRF2 transcriptional deficiency or the lack of IL-10 results in pronounced GI functional decline in young females. Mechanistically, we show that the impaired distal colon motility is dependent on ERβ signaling. Targeting estrogen signaling seems a promising therapeutic strategy to counteract colonic dysfunction.

NRF2 Regulates Viability, Proliferation, Resistance to Oxidative Stress, and Differentiation of Murine Myoblasts and Muscle Satellite Cells

Increased oxidative stress can slow down the regeneration of skeletal muscle and affect the activity of muscle satellite cells (mSCs). Therefore, we evaluated the role of the NRF2 transcription factor (encoded by the Nfe2l2 gene), the main regulator of the antioxidant response, in muscle cell biology. We used (i) an immortalized murine myoblast cell line (C2C12) with stable overexpression of NRF2 and (ii) primary mSCs isolated from wild-type and Nfe2l2 (transcriptionally)-deficient mice (Nfe2l2^tKO). NRF2 promoted myoblast proliferation and viability under oxidative stress conditions and decreased the production of reactive oxygen species. Furthermore, NRF2 overexpression inhibited C2C12 cell differentiation by down-regulating the expression of myogenic regulatory factors (MRFs) and muscle-specific microRNAs. We also showed that NRF2 is indispensable for the viability of mSCs since the lack of its transcriptional activity caused high mortality of cells cultured in vitro under normoxic conditions. Concomitantly, Nfe2l2^tKO mSCs grown and differentiated under hypoxic conditions were viable and much more differentiated compared to cells isolated from wild-type mice. Taken together, NRF2 significantly influences the properties of myoblasts and muscle satellite cells. This effect might be modulated by the muscle microenvironment.

Nrf2 Transcriptional Activity Governs Intestine Development

Our recent findings indicate that Nrf2 transcriptional activity is essential in maintaining the proper large intestinal structure in adult mice. Here, we aimed to verify whether Nrf2-related intestine abnormalities stemmed from the early weaning or gestational periods. Therefore, we analyzed 4-day-old pups and embryos devoid of Nrf2 transcriptional activity (tKO) and their wild-type counterparts. We found significant changes in the intestinal structure of 4-day-old Nrf2 tKO pups including a longer colon, altered crypt distribution, and enlargement of the goblet cells with a markedly higher level of mucin 2. Tracing back the origin of these alterations, we observed that they appeared as early as day 14.5 of embryonic development, independently of sex. Importantly, in this period, we observed a significant increase in the Nrf2 level and a distinctive, untimely pattern of expression of the proliferation factor Ki67. At the latest stage of embryonic development, we detected a premature drop in the differentiation factor Notch1. We suspect that intestine abnormalities in mice lacking Nrf2 transcriptional activity stem from sex-independent disturbed intestinal cell proliferation and could be further exacerbated by altered differentiation. Summing up, we identified Nrf2 transcriptional activity as an important regulator of intestinal formation. It influences the hindgut cell proliferation and differentiation at different stages of embryonic development.

Transcription Factor NRF2 Participates in Cell Cycle Progression at the Level of G1/S and Mitotic Checkpoints

Transcription factor NRF2 is a master regulator of the multiple cytoprotective responses that confer growth advantages on a cell. However, its participation in the mechanisms that govern the cell division cycle has not been explored in detail. In this study, we used several standard methods of synchronization of proliferating cells together with flow cytometry and monitored the participation of NRF2 along the cell cycle by the knockdown of its gene expression. We found that the NRF2 levels were highest at S phase entry, and lowest at mitosis. NRF2 depletion promoted both G1 and M arrest. Targeted transcriptomics analysis of cell cycle regulators showed that NRF2 depletion leads to changes in key cell cycle regulators, such as CDK2, TFDP1, CDK6, CDKN1A (p21), CDKN1B (p27), CCNG1, and RAD51. This study gives a new dimension to NRF2 effects, showing their implication in cell cycle progression.

Lability of the Nrf2/Keap/ARE Cell Defense System in Different Models of Cell Aging and Age-Related Pathologies

The level of oxidative stress in an organism increases with age. Accumulation of damages resulting in the disruption of genome integrity can be the cause of many age-related diseases and appearance of phenotypic and physiological signs of aging. In this regard, the Nrf2 system, which regulates expression of numerous enzymes responsible for the antioxidant defense and detoxification, is of great interest. This review summarizes and analyzes the data on the age-related changes in the Nrf2 system in vivo and in vitro in various organs and tissues. Analysis of published data suggests that the capacity for Nrf2 activation (triggered by the increased level of oxidative stress) steadily declines with age. At the same time, changes in the Nrf2 activity under the stress-free conditions do not have such unambiguous directionality; in many studies, these changes were statistically insignificant, although it is commonly accepted that the level of oxidative stress steadily increases with aging. This review examines the role of cell regulatory systems limiting the ability of Nrf2 to respond to oxidative stress. Senescent cells are extremely susceptible to the oxidative damage due to the impaired Nrf2 signaling. Activation of the Nrf2 pathway is a promising target for new pharmacological or genetic therapeutic strategies. Suppressors of the Nrf2 expression, such as Keap1, GSK3, c-Myc, and Bach1, may contribute to the age-related impairments in the induction of Nrf2-regulated antioxidant genes. Understanding the mechanisms of regulatory cascades linking the programs responsible for the maintenance of homeostasis and cell response to the oxidative stress will contribute to the elucidation of molecular mechanisms underlying aging and longevity.

The role of the Nrf2/Keap1 signaling cascade in mechanobiology and bone health

In conjunction with advancements in modern medicine, bone health is becoming an increasingly prevalent concern among a global population with an ever-growing life expectancy. Countless factors contribute to declining bone strength, and age exacerbates nearly all of them. The detrimental effects of bone loss have a profound impact on quality of life. As such, there is a great need for full exploration of potential therapeutic targets that may provide antiaging benefits and increase the life and strength of bone tissues. The Keap1-Nrf2 pathway is a promising avenue of this research. The cytoprotective and antioxidant functions of this pathway have been shown to mitigate the deleterious effects of oxidative stress on bone tissues, but the exact cellular and molecular mechanisms by which this occurs are not yet fully understood. Presently, refined animal and loading models are allowing exploration into the effect of the Keap1-Nrf2 pathway in a tissue-specific or even cell-specific manner. In addition, Nrf2 activators currently undergoing clinical trials can be utilized to investigate the particular cellular mechanisms at work in this cytoprotective cascade. Although the timing and dosing of treatment with Nrf2 activators need to be further investigated, these activators have great potential to be used clinically to prevent and treat osteoporosis.

Targeting Nrf2 for the treatment of Duchenne Muscular Dystrophy

Imbalances in redox homeostasis can result in oxidative stress, which is implicated in various pathological conditions including the fatal neuromuscular disease Duchenne Muscular Dystrophy (DMD). DMD is a complicated disease, with many druggable targets at the cellular and molecular level including calcium-mediated muscle degeneration; mitochondrial dysfunction; oxidative stress; inflammation; insufficient muscle regeneration and dysregulated protein and organelle maintenance. Previous investigative therapeutics tended to isolate and focus on just one of these targets and, consequently, therapeutic activity has been limited. Nuclear erythroid 2-related factor 2 (Nrf2) is a transcription factor that upregulates many cytoprotective gene products in response to oxidants and other toxic stressors. Unlike other strategies, targeted Nrf2 activation has the potential to simultaneously modulate separate pathological features of DMD to amplify therapeutic benefits. Here, we review the literature providing theoretical context for targeting Nrf2 as a disease modifying treatment against DMD.

The role of Nrf2 in acute and chronic muscle injury

The nuclear factor erythroid 2-related factor 2 (Nrf2) is considered as a master cytoprotective factor regulating the expression of genes encoding anti-oxidant, anti-inflammatory, and detoxifying proteins. The role of Nrf2 in the pathophysiology of skeletal muscles has been evaluated in different experimental models, however, due to inconsistent data, we aimed to investigate how Nrf2 transcriptional deficiency (Nrf2tKO) affects muscle functions both in an acute and chronic injury. The acute muscle damage was induced in mice of two genotypes-WT and Nrf2tKO mice by cardiotoxin (CTX) injection. To investigate the role of Nrf2 in chronic muscle pathology, mdx mice that share genetic, biochemical, and histopathological features with Duchenne muscular dystrophy (DMD) were crossed with mice lacking transcriptionally active Nrf2 and double knockouts (mdx/Nrf2tKO) were generated. To worsen the dystrophic phenotype, the analysis of disease pathology was also performed in aggravated conditions, by applying a long-term treadmill test. We have observed slightly increased muscle damage in Nrf2tKO mice after CTX injection. Nevertheless, transcriptional ablation of Nrf2 in mdx mice did not significantly aggravate the most deleterious, pathological hallmarks of DMD related to degeneration, inflammation, fibrotic scar formation, angiogenesis, and the number and proliferation of satellite cells in non-exercised conditions. On the other hand, upon chronic exercises, the degeneration and inflammatory infiltration of the gastrocnemius muscle, but not the diaphragm, turned to be increased in Nrf2tKOmdx in comparison to mdx mice. In conclusion, the lack of transcriptionally active Nrf2 influences moderately muscle pathology in acute CTX-induced muscle injury and chronic DMD mouse model, without affecting muscle functionality. Hence, in general, we demonstrated that the deficiency of Nrf2 transcriptional activity has no profound impact on muscle pathology in various models of muscle injury.

Beyond repression of Nrf2: An update on Keap1

Nrf2 (NFE2L2 - nuclear factor (erythroid-derived 2)-like 2) is a transcription factor, which is repressed by interaction with a redox-sensitive protein Keap1 (Kelch-like ECH-associated protein 1). Deregulation of Nrf2 transcriptional activity has been described in the pathogenesis of multiple diseases, and the Nrf2/Keap1 axis has emerged as a crucial modulator of cellular homeostasis. Whereas the significance of Nrf2 in the modulation of biological processes has been well established and broadly discussed in detail, the focus on Keap1 rarely goes beyond the regulation of Nrf2 activity and redox sensing. However, recent studies and scrutinized analysis of available data point to Keap1 as an intriguing and potent regulator of cellular function. This review aims to shed more light on Keap1 structure, interactome, regulation and non-canonical functions, thereby enhancing its significance in cell biology. We also intend to highlight the impact of balance between Keap1 and Nrf2 in the maintenance of cellular homeostasis.

Simvastatin Attenuates Abdominal Aortic Aneurysm Formation Favoured by Lack of Nrf2 Transcriptional Activity

Surgical intervention is currently the only option for an abdominal aortic aneurysm (AAA), preventing its rupture and sudden death of a patient. Therefore, it is crucial to determine the pathogenic mechanisms of this disease for the development of effective pharmacological therapies. Oxidative stress is said to be one of the pivotal factors in the pathogenesis of AAAs. Thus, we aimed to evaluate the significance of nuclear factor erythroid 2-related factor 2 (Nrf2) transcriptional activity in the development of AAA and to verify if simvastatin, administered as pre- and cotreatment, may counteract this structural malformation. Experiments were performed on mice with inhibited transcriptional activity of Nrf2 (tKO) and wild-type (WT) counterparts. We used a model of angiotensin II- (AngII-) induced AAA, combined with a fat-enriched diet. Mice were administered with AngII or saline for up to 28 days via osmotic minipumps. Simvastatin administration was started 7 days before the osmotic pump placement and then continued until the end of the experiment. We found that Nrf2 inactivation increased the risk of development and rupture of AAA. Importantly, these effects were reversed by simvastatin in tKO mice, but not in WT. The abrupt blood pressure rise induced by AngII was mitigated in simvastatin-treated animals regardless of the genotype. Simvastatin-affected parameters that differed between the healthy structure of the aorta and aneurysmal tissue included immune cell infiltration of the aortic wall, VCAM1 mRNA and protein level, extracellular matrix degradation, TGF-β1 mRNA level, and ERK phosphorylation, but neither oxidative stress nor the level of Angiotensin II Type 1 Receptor (AT1R). Taken together, the inhibition of Nrf2 transcriptional activity facilitates AAA formation in mice, which can be prevented by simvastatin. It suggests that statin treatment of patients with hypercholesterolemia might have not only a beneficial effect in terms of controlling atherosclerosis but also potential AAA prevention.

Keap1 governs ageing-induced protein aggregation in endothelial cells

The breach of proteostasis, leading to the accumulation of protein aggregates, is a hallmark of ageing and age-associated disorders, up to now well-established in neurodegeneration. Few studies have addressed the issue of dysfunctional cell response to protein deposition also for the cardiovascular system. However, the molecular basis of proteostasis decline in vascular cells, as well as its relation to ageing, are not understood. Recent studies have indicated the associations of Nrf2 transcription factor, the critical modulator of cellular stress-response, with ageing and premature senescence. In this report, we outline the significance of protein aggregation in physiological and premature ageing of murine and human endothelial cells (ECs). Our study shows that aged donor-derived and prematurely senescent Nrf2-deficient primary human ECs, but not those overexpressing dominant-negative Nrf2, exhibit increased accumulation of protein aggregates. Such phenotype is also found in the aortas of aged mice and young Nrf2 tKO mice. Ageing-related loss of proteostasis in ECs depends on Keap1, well-known repressor of Nrf2, recently perceived as a key independent regulator of EC function and protein S-nitrosation (SNO). Deposition of protein aggregates in ECs is associated with impaired autophagy. It can be counteracted by Keap1 depletion, S-nitrosothiol reductant or rapamycin treatment. Our results show that Keap1:Nrf2 protein balance and Keap1-dependent SNO predominate Nrf2 transcriptional activity-driven mechanisms in governing proteostasis in ageing ECs.

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Physiological and premature ageing facilitates aggregation of proteins in ECs.

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Loss of proteostasis depends on Keap1-driven S-nitrosation in ageing ECs.

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Keap1:Nrf2 ratio predominates Nrf2 transcriptional activity in proteostasis control.

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Keap1 or SNO depletion, or rapamycin treatment restore proteostasis in ageing ECs.

Ozone at low concentrations does not affect motility and proliferation of cancer cells in vitro

Exposure to low ozone concentrations is used in medicine as an adjuvant/complementary treatment for a variety of diseases. The therapeutic potential of low ozone concentrations relies on their capability to increase the nuclear translocation of the Nuclear factor erythroid 2-related factor 2 (Nrf2), thus inducing the transcription of Antioxidant Response Elements (ARE)-driven genes and, through a cascade of events, a general cytoprotective response. However, based on the controversial role of Nrf2 in cancer initiation, progression and resistance to therapies, possible negative effects of ozone therapy may be hypothesised in oncological patients. With the aim to elucidate the possible changes in morphology, migration capability and proliferation of cancer cells following mild ozone exposure, we performed wound healing experiments in vitro on HeLa cells treated with low ozone concentrations currently used in the clinical practice. By combining a multimodal microscopy approach (light and fluorescence microscopy, scanning electron microscopy, atomic force microscopy) with morphometric analyses, we demonstrated that, under our experimental conditions, exposure to low ozone concentrations does not alter cytomorphology, motility and proliferation features, thus supporting the notion that ozone therapy should not positively affect tumour cell growth and metastasis.

Keap1 controls protein S-nitrosation and apoptosis-senescence switch in endothelial cells

Premature senescence, a death escaping pathway for cells experiencing stress, is conducive to aging and cardiovascular diseases. The molecular switch between senescent and apoptotic fate remains, however, poorly recognized. Nrf2 is an important transcription factor orchestrating adaptive response to cellular stress. Here, we show that both human primary endothelial cells (ECs) and murine aortas lacking Nrf2 signaling are senescent but unexpectedly do not encounter damaging oxidative stress. Instead, they exhibit markedly increased S-nitrosation of proteins. A functional role of S-nitrosation is protection of ECs from death by inhibition of NOX4-mediated oxidative damage and redirection of ECs to premature senescence. S-nitrosation and senescence are mediated by Keap1, a direct binding partner of Nrf2, which colocalizes and precipitates with nitric oxide synthase (NOS) and transnitrosating protein GAPDH in ECs devoid of Nrf2. We conclude that the overabundance of this "unrestrained" Keap1 determines the fate of ECs by regulation of S-nitrosation and propose that Keap1/GAPDH/NOS complex may serve as an enzymatic machinery for S-nitrosation in mammalian cells.