Regulation of Nrf2 by X Box-Binding Protein 1 in Retinal Pigment Epithelium

Regulatory factors of Nrf2 in age-related macular degeneration pathogenesis

Age-related macular degeneration (AMD) is a complicated disease that causes irreversible visual impairment. Increasing evidences pointed retinal pigment epithelia (RPE) cells as the decisive cell involved in the progress of AMD, and the function of anti-oxidant capacity of PRE plays a fundamental physiological role. Nuclear factor erythroid 2 related factor 2 (Nrf2) is a significant transcription factor in the cellular anti-oxidant system as it regulates the expression of multiple anti-oxidative genes. Its functions of protecting RPE cells against oxidative stress (OS) and ensuing physiological changes, including inflammation, mitochondrial damage and autophagy dysregulation, have already been elucidated. Understanding the roles of upstream regulators of Nrf2 could provide further insight to the OS-mediated AMD pathogenesis. For the first time, this review summarized the reported upstream regulators of Nrf2 in AMD pathogenesis, including proteins and miRNAs, and their underlying molecular mechanisms, which may help to find potential targets via regulating the Nrf2 pathway in the future research and further discuss the existing Nrf2 regulators proved to be beneficial in preventing AMD.

The endoplasmic reticulum: Homeostasis and crosstalk in retinal health and disease

The endoplasmic reticulum (ER) is the largest intracellular organelle carrying out a broad range of important cellular functions including protein biosynthesis, folding, and trafficking, lipid and sterol biosynthesis, carbohydrate metabolism, and calcium storage and gated release. In addition, the ER makes close contact with multiple intracellular organelles such as mitochondria and the plasma membrane to actively regulate the biogenesis, remodeling, and function of these organelles. Therefore, maintaining a homeostatic and functional ER is critical for the survival and function of cells. This vital process is implemented through well-orchestrated signaling pathways of the unfolded protein response (UPR). The UPR is activated when misfolded or unfolded proteins accumulate in the ER, a condition known as ER stress, and functions to restore ER homeostasis thus promoting cell survival. However, prolonged activation or dysregulation of the UPR can lead to cell death and other detrimental events such as inflammation and oxidative stress; these processes are implicated in the pathogenesis of many human diseases including retinal disorders. In this review manuscript, we discuss the unique features of the ER and ER stress signaling in the retina and retinal neurons and describe recent advances in the research to uncover the role of ER stress signaling in neurodegenerative retinal diseases including age-related macular degeneration, inherited retinal degeneration, achromatopsia and cone diseases, and diabetic retinopathy. In some chapters, we highlight the complex interactions between the ER and other intracellular organelles focusing on mitochondria and illustrate how ER stress signaling regulates common cellular stress pathways such as autophagy. We also touch upon the integrated stress response in retinal degeneration and diabetic retinopathy. Finally, we provide an update on the current development of pharmacological agents targeting the UPR response and discuss some unresolved questions and knowledge gaps to be addressed by future research.

Endoplasmic reticulum stress: molecular mechanism and therapeutic targets

The endoplasmic reticulum (ER) functions as a quality-control organelle for protein homeostasis, or "proteostasis". The protein quality control systems involve ER-associated degradation, protein chaperons, and autophagy. ER stress is activated when proteostasis is broken with an accumulation of misfolded and unfolded proteins in the ER. ER stress activates an adaptive unfolded protein response to restore proteostasis by initiating protein kinase R-like ER kinase, activating transcription factor 6, and inositol requiring enzyme 1. ER stress is multifaceted, and acts on aspects at the epigenetic level, including transcription and protein processing. Accumulated data indicates its key role in protein homeostasis and other diverse functions involved in various ocular diseases, such as glaucoma, diabetic retinopathy, age-related macular degeneration, retinitis pigmentosa, achromatopsia, cataracts, ocular tumors, ocular surface diseases, and myopia. This review summarizes the molecular mechanisms underlying the aforementioned ocular diseases from an ER stress perspective. Drugs (chemicals, neurotrophic factors, and nanoparticles), gene therapy, and stem cell therapy are used to treat ocular diseases by alleviating ER stress. We delineate the advancement of therapy targeting ER stress to provide new treatment strategies for ocular diseases.

XBP1 as a novel molecular target to attenuate drug resistance in hepatocellular carcinoma

INTRODUCTION

Despite improvements in clinical management of hepatocellular carcinoma (HCC), prognosis remains poor with a 5-year survival rate less than 40%. Drug resistance in HCC makes it challenging to treat; therefore, it is imperative to develop new therapeutic strategies. Higher expression of X-box binding protein 1 (XBP1) in tumor cells is highly correlated with poor prognosis. In tumor cells, XBP1 modulates the unfolded protein response (UPR) to restore homeostasis in endoplasmic reticulum. Targeting XBP1 could be a promising therapeutic strategy to overcome HCC resistance and improve the survival rate of patients.

AREAS COVERED

This review provides the recent evidence that indicates XBP1 is involved in HCC drug resistance via DNA damage response, drug inactivation, and inhibition of apoptosis. In addition, the potential roles of XBP1 in inducing resistance in HCC cells were highlighted, and we showed how its inhibition could sensitize tumor cells to controlled cell death.

EXPERT OPINION

Due to the diversity in molecular mechanism of multidrug-resistance, targeting one specific pathway is inadequate. XBP1 inhibition could be a potential therapeutic target to overcome verity of resistance mechanisms. The main function of this transcription factor in HCC treatment response is an attractive area for further studies and should be discussed more.

Acute deltamethrin exposure induces oxidative stress, triggers endoplasmic reticulum stress, and impairs hypoxic resistance of crucian carp

Deltamethrin (Del) has been widely used for effectively controlling ectoparasites of crucian carp and was also strictly prohibited in a hypoxic environment. A previous study indicated that Del exposure causes gill injury in Carassius auratus, which is associated with oxidative stress and endoplasmic reticulum stress (ER stress), but the precise mechanism is not well understood. Here, crucian carp were exposed to Del (0.61, 1.22, 2.44, 4.88 μg/L) for 24 h and then subjected to acute hypoxia challenge (1.0 mg/L) for 24 h. The results revealed that acute exposure to Del notably increased MDA content but markedly decreased CAT activities. Moreover, the T-AOC and SOD activities first increased and then decreased in the 4.88 μg/L Del group. Likewise, the mRNA levels of Nrf2 signaling and its target genes (ho-1, mt, sod, cat, and gpx1) were significantly downregulated in the high concentration exposure groups, while the mRNA levels of keap1 showed the opposite change trend. Meanwhile, Del exposure evoked the PERK-ATF4-CHOP and IRE1 signaling pathways and triggered ER stress in a dose-dependent manner in crucian carp. Importantly, we found that Del exposure significantly decreased the survival rate of crucian carp after hypoxia challenge by reducing oxygen uptake, modifying energy metabolism, and promoting lactate accumulation. Additionally, Del exposure aggravated gill damage and apoptosis under hypoxic stress, which was confirmed by histological assays. Collectively, we inferred that acute exposure to deltamethrin induces oxidative stress and ER stress and impairs hypoxic resistance of crucian carp.

PERK/EIF2AK3 integrates endoplasmic reticulum stress-induced apoptosis, oxidative stress and autophagy responses in immortalised retinal pigment epithelial cells

Retinal pigment epithelium (RPE) performs essential functions for ensuring retinal homeostasis and is a key site for pathogenic changes leading to age-related macular degeneration (AMD). Compromised proteostasis in RPE results in ER stress and ER stress-dependent antioxidant, apoptosis and autophagic responses. ER stress induces the unfolded protein response (UPR) in which EIF2AK3, encoding the protein kinase RNA-like ER kinase (PERK), acts as a key regulator. Downregulated EIF2AK3 gene expression has recently been identified in AMD using human donor RPE, however the molecular mechanisms that integrate the various ER-mediated cellular pathways underpinning progressive RPE dysfunction in AMD have not been fully characterised. This study investigated the downstream effects of PERK downregulation in response to Brefeldin A (BFA)-induced ER stress in ARPE-19 cells. PERK downregulation resulted in increased ER stress and impaired apoptosis induction, antioxidant responses and autophagic flux. ARPE-19 cells were unable to efficiently induce autophagy following PERK downregulation and PERK presented a role in regulating the rate of autophagy induction. The findings support PERK downregulation as an integrative event facilitating dysregulation of RPE processes critical to cell survival known to contribute to AMD development and highlight PERK as a potential future therapeutic target for AMD.

Pak2 Regulation of Nrf2 Serves as a Novel Signaling Nexus Linking ER Stress Response and Oxidative Stress in the Heart

Endoplasmic Reticulum (ER) stress and oxidative stress have been highly implicated in the pathogenesis of cardiac hypertrophy and heart failure (HF). However, the mechanisms involved in the interplay between these processes in the heart are not fully understood. The present study sought to determine a causative link between Pak2-dependent UPR activation and oxidative stress via Nrf2 regulation under pathological ER stress. We report that sustained ER stress and Pak2 deletion in cardiomyocytes enhance Nrf2 expression. Conversely, AAV9 mediated Pak2 delivery in the heart leads to a significant decrease in Nrf2 levels. Pak2 overexpression enhances the XBP1-Hrd1 UPR axis and ameliorates tunicamycin induced cardiac apoptosis and dysfunction in mice. We found that Pak2 deletion and altered proteostasis render Nrf2 detrimental by switching from its antioxidant role to renin-angiotensin aldosterone system (RAAS) gene regulator. Mechanistically, Pak2 mediated Hrd1 expression targets Nrf2 for ubiquitination and degradation thus preventing its aberrant activation. Moreover, we find a significant increase in Nrf2 with a decrease in Pak2 in human myocardium of dilated heart disease. Using human-induced pluripotent stem cell-derived cardiomyocytes (iPSC-CMs), we find that Pak2 is able to ameliorate Nrf2 induced RAAS activation under ER stress. These findings demonstrate that Pak2 is a novel Nrf2 regulator in the stressed heart. Activation of XBP1-Hrd1 is attributed to prevent ER stress-induced Nrf2 RAAS component upregulation. This mechanism explains the functional dichotomy of Nrf2 in the stressed heart. Thus, Pak2 regulation of Nrf2 homeostasis may present as a potential therapeutic route to alleviate detrimental ER stress and heart failure.

Cellular stress signaling and the unfolded protein response in retinal degeneration: mechanisms and therapeutic implications

Background

The retina, as part of the central nervous system (CNS) with limited capacity for self-reparation and regeneration in mammals, is under cumulative environmental stress due to high-energy demands and rapid protein turnover. These stressors disrupt the cellular protein and metabolic homeostasis, which, if not alleviated, can lead to dysfunction and cell death of retinal neurons. One primary cellular stress response is the highly conserved unfolded protein response (UPR). The UPR acts through three main signaling pathways in an attempt to restore the protein homeostasis in the endoplasmic reticulum (ER) by various means, including but not limited to, reducing protein translation, increasing protein-folding capacity, and promoting misfolded protein degradation. Moreover, recent work has identified a novel function of the UPR in regulation of cellular metabolism and mitochondrial function, disturbance of which contributes to neuronal degeneration and dysfunction. The role of the UPR in retinal neurons during aging and under disease conditions in age-related macular degeneration (AMD), retinitis pigmentosa (RP), glaucoma, and diabetic retinopathy (DR) has been explored over the past two decades. Each of the disease conditions and their corresponding animal models provide distinct challenges and unique opportunities to gain a better understanding of the role of the UPR in the maintenance of retinal health and function.

Method

We performed an extensive literature search on PubMed and Google Scholar using the following keywords: unfolded protein response, metabolism, ER stress, retinal degeneration, aging, age-related macular degeneration, retinitis pigmentosa, glaucoma, diabetic retinopathy.

Results and conclusion

We summarize recent advances in understanding cellular stress response, in particular the UPR, in retinal diseases, highlighting the potential roles of UPR pathways in regulation of cellular metabolism and mitochondrial function in retinal neurons. Further, we provide perspective on the promise and challenges for targeting the UPR pathways as a new therapeutic approach in age- and disease-related retinal degeneration.

Grape Seed Proanthocyanidin Extract Moderated Retinal Pigment Epithelium Cellular Senescence Through NAMPT/SIRT1/NLRP3 Pathway

Background

Retinal pigment epithelium (RPE) cellular senescence is an important process in degenerative retinal disorders. Grape seed proanthocyanidin extract (GSPE) alleviates senescence-related degenerative disorders; however, the potential effects of GSPE intake on RPE cellular senescence through regulating NAMPT/SIRT1/NLRP3 pathway remain unclear.

Methods

The effects of GSPE on NAMPT expression and NAD+ contents were detected with Western blot and assay kit in both in-vivo and in-vitro AMD models. Senescence-related biomarkers, including p16, p21 expressions and β-gal staining, were conducted in different groups. The protective effects of GSPE treatment on the mitochondrial homeostasis and barrier function of RPE cells were detected using mtDNA lesions analyses, JC-1 staining, ZO1 staining and trans-epithelial cell resistance (TEER) detection. The expression of senescence-associated secretory phenotype (SASP) in different groups would be conducted with qPCR. To demonstrate the potential effects of NAMPT/SIRT1/NLRP3 pathway after GSPE treatment, the protein levels of relevant key regulators after applications of NAMPT inhibitor, Fk866, and SIRT1 inhibitor, EX-527.

Results

GSPE significantly improves the NAMPT expression and NAD+ content in aging mice, and thus alleviates the RPE cellular senescence. In advanced in-vitro studies, GSPE significantly up-regulated NAMPT content and thus relieved H₂O₂ induced NAD+ depression through analyzing the NAD+ contents in different groups. In advanced analyses, it was reported that GSPE could alleviate mitochondrial permeability, mtDNA damage, ZO1 expression and SASP levels in aging RPE cells. Thus, GSPE treatment significantly decreased senescence-related protein p16 and p21, as well as SASP levels in in-vitro aging model, and it was demonstrated that GSPE could illustrate a significant anti-aging effect. The Western blot data in GSPE treatment of aging RPE cells demonstrated that GSPE could significantly improve NAMPT and SIRT1 levels, and thus depressed NLRP3 expression.

Conclusion

This study indicated that GSPE alleviated RPE cellular senescence through NAMPT/SIRT1/NLRP3 pathway. This study highlighted the potential effects of GSPE on degenerative retinopathy through the crosstalk of NAD+ metabolism, SIRT1 function and NLRP3 activation.

Downregulation of XBP1 protects kidney against ischemia-reperfusion injury via suppressing HRD1-mediated NRF2 ubiquitylation

Ischemia-reperfusion (IR) injury to the renal epithelia is associated with endoplasmic reticulum stress (ERS) and mitochondria dysfunction, which lead to oxidative stress-induced acute kidney injury (AKI). X-box binding protein 1 (XBP1), an ERS response protein, could play a prominent role in IR-induced AKI. In this study, we revealed that XBP1 and its downstream target HRD1 participated in the crosstalk between ERS and mitochondrial dysfunction via regulation of NRF2/HO-1-mediated reactive oxidative stress (ROS) signaling. Mice with reduced expression of XBP1 (heterozygous Xbp1±) were resistant to IR-induced AKI due to the enhanced expression of NRF2/HO-1 and diminished ROS in the kidney. Downregulation of XBP1 in renal epithelial cells resulted in reduced HRD1 expression and increased NRF2/HO-1 function, accompanied with enhanced antioxidant response. Furthermore, HRD1 served as an E3-ligase to facilitate the downregulation of NRF2 through ubiquitination-degradation pathway, and the QSLVPDI motif on NRF2 constituted an active site for its interaction with HRD1. Thus, our findings unveil an important physiological role for XBP1/HRD1 in modulating the antioxidant function of NRF2/HO-1 in the kidney under stress conditions. Molecular therapeutic approaches that target XBP1-HRD1-NRF2 pathway may represent potential effective means to treat renal IR injury.

Quantitative and multiplexed chemical-genetic phenotyping in mammalian cells with QMAP-Seq

Chemical-genetic interaction profiling in model organisms has proven powerful in providing insights into compound mechanism of action and gene function. However, identifying chemical-genetic interactions in mammalian systems has been limited to low-throughput or computational methods. Here, we develop Quantitative and Multiplexed Analysis of Phenotype by Sequencing (QMAP-Seq), which leverages next-generation sequencing for pooled high-throughput chemical-genetic profiling. We apply QMAP-Seq to investigate how cellular stress response factors affect therapeutic response in cancer. Using minimal automation, we treat pools of 60 cell types—comprising 12 genetic perturbations in five cell lines—with 1440 compound-dose combinations, generating 86,400 chemical-genetic measurements. QMAP-Seq produces precise and accurate quantitative measures of acute drug response comparable to gold standard assays, but with increased throughput at lower cost. Moreover, QMAP-Seq reveals clinically actionable drug vulnerabilities and functional relationships involving these stress response factors, many of which are activated in cancer. Thus, QMAP-Seq provides a broadly accessible and scalable strategy for chemical-genetic profiling in mammalian cells.

Identifying chemical-genetic interactions in mammalian cells is limited to low-throughput or computational methods. Here, the authors present QMAP-Seq, a broadly accessible and scalable approach that uses NGS for pooled high-throughput chemical-genetic profiling in mammalian cells.

MITF protects against oxidative damage-induced retinal degeneration by regulating the NRF2 pathway in the retinal pigment epithelium

Oxidative damage is one of the major contributors to retinal degenerative diseases such as age-related macular degeneration (AMD), while RPE mediated antioxidant defense plays an important role in preventing retinopathies. However, the regulatory mechanisms of antioxidant signaling in RPE cells are poorly understood. Here we show that transcription factor MITF regulates the antioxidant response in RPE cells, protecting the neural retina from oxidative damage. In the oxidative stress-induced retinal degeneration mouse model, retinal degeneration in Mitf+/- mice is significantly aggravated compared to WT mice. In contrast, overexpression of Mitf in Dct-Mitf transgenic mice and AAV mediated overexpression in RPE cells protect the neural retina against oxidative damage. Mechanistically, MITF both directly regulates the transcription of NRF2, a master regulator of antioxidant signaling, and promotes its nuclear translocation. Furthermore, specific overexpression of NRF2 in Mitf+/- RPE cells activates antioxidant signaling and partially protects the retina from oxidative damage. Taken together, our findings demonstrate the regulation of NRF2 by MITF in RPE cells and provide new insights into potential therapeutic approaches for prevention of oxidative damage diseases.

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MITF haploinsufficiency exacerbates oxidative stress-induced retinal degeneration.

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Specific overexpression of MITF in RPE cells protects retinas from oxidative damage in vivo.

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MITF directly regulates the transcription and nuclear translocation of NRF2.

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Partial rescue of retinal oxidative damage in Mitf ±mice by gene transfer mediated RPE cell specific expression of NRF2.

Endoplasmic reticulum stress and the protein degradation system in ophthalmic diseases

Objective

Endoplasmic reticulum (ER) stress is involved in the pathogenesis of various ophthalmic diseases, and ER stress-mediated degradation systems play an important role in maintaining ER homeostasis during ER stress. The purpose of this review is to explore the potential relationship between them and to find their equilibrium sites.

Design

This review illustrates the important role of reasonable regulation of the protein degradation system in ER stress-mediated ophthalmic diseases. There were 128 articles chosen for review in this study, and the keywords used for article research are ER stress, autophagy, UPS, ophthalmic disease, and ocular.

Data sources

The data are from Web of Science, PubMed, with no language restrictions from inception until 2019 Jul.

Results

The ubiquitin proteasome system (UPS) and autophagy are important degradation systems in ER stress. They can restore ER homeostasis, but if ER stress cannot be relieved in time, cell death may occur. However, they are not independent of each other, and the relationship between them is complementary. Therefore, we propose that ER stability can be achieved by adjusting the balance between them.

Conclusion

The degradation system of ER stress, UPS and autophagy are interrelated. Because an imbalance between the UPS and autophagy can cause cell death, regulating that balance may suppress ER stress and protect cells against pathological stress damage.

S-allyl-l-cysteine (SAC) protects hepatocytes from alcohol-induced apoptosis

Hepatocyte apoptosis is frequently observed in alcohol‐related liver disease (ARLD), which ranks among the 30 leading causes of death worldwide. In the current study, we explored the impact of S‐allyl‐l‐cysteine (SAC), an organosulfur component of garlic, on hepatocyte apoptosis induced by alcohol. Rat liver (BRL‐3A) cells were challenged by ethanol with or without SAC treatment. Cell death/viability, reactive oxygen species (ROS) generation, mitochondrial Cytochrome C release, and caspase 3 activity were then examined. We found that ethanol remarkably induced apoptosis of hepatocytes, while SAC treatment rescued ethanol‐induced hepatocyte injury, as demonstrated by cell counting kit‐8 (CCK8) assay, TUNEL assay, and annexin V/PI staining assay. Ethanol evoked ROS generation in BRL‐3A cells, and this was abated by SAC pretreatment, as indicated by 2′,7′‐dichlorofluorescin diacetate (DCFDA) staining assay. Moreover, ethanol suppressed cellular anti‐apoptotic protein B‐cell lymphoma‐2 (Bcl‐2) expression, increased pro‐apoptotic protein Bcl‐2‐associated X protein (Bax) expression, induced mitochondrial Cytochrome C release, and activated the caspase 3‐dependent apoptosis pathway in BRL‐3A cells. SAC was sufficient to abolish all these changes induced by ethanol, thereby revealing the molecular mechanisms underlying its protective effects. In conclusion, SAC protects hepatocytes from ethanol‐induced apoptosis and may be suitable for use as a novel anti‐apoptotic agent for treating ARLD.