Inactivation of the proteasome by 4-hydroxy-2-nonenal is site specific and dependant on 20S proteasome subtypes

Commentary: Aldehyde dehydrogenase, redox balance and exercise physiology: What is missing?

Aldehyde dehydrogenase 2 (ALDH2) is a mitochondrial enzyme involved in reactive aldehyde detoxification. Approximately 560 million people (about 8% of the world's population) carry a point mutation in the aldehyde dehydrogenase 2 gene (ALDH2), identified as ALDH2*2, which leads to decreased ALDH2 catalytic activity. ALDH2*2 variant is associated with an accumulation of toxic reactive aldehydes and consequent disruption of cellular metabolism, which contributes to the establishment and progression of several degenerative diseases. Consequences of aldehyde accumulation include impaired mitochondrial functional, hindered anabolic signaling in the skeletal muscle, impaired cardiovascular and pulmonary function, and reduced osteoblastogenesis. Considering that aldehydes are endogenously produced through redox processes, it is expected that conditions that have a high energy demand, such as exercise, might be affected by impaired aldehyde clearance in ALDH2*2 individuals. Despite the large body of evidence supporting the importance of ALDH2 to ethanol metabolism, redox homeostasis and overall health, specific research investigating the impact of ALDH2*2 on phenotypes relevant to exercise performance are notoriously scarce. In this commentary, we highlight the consolidated knowledge on the impact of ALDH2*2 on physiological processes that are relevant to exercise.

Neuroprotective effects of the PPARβ/δ antagonist GSK0660 in in vitro and in vivo Parkinson's disease models

BACKGROUND

The underlying mechanism of Parkinson's disease are still unidentified, but excitotoxicity, oxidative stress, and neuroinflammation are considered key actors. Proliferator activated receptors (PPARs) are transcription factors involved in the control of numerous pathways. Specifically, PPARβ/δ is recognized as an oxidative stress sensor, and we have previously reported that it plays a detrimental role in neurodegeneration.

METHODS

Basing on this concept, in this work, we tested the potential effects of a specific PPARβ/δ antagonist (GSK0660) in an in vitro model of Parkinson's disease. Specifically, live-cell imaging, gene expression, Western blot, proteasome analyses, mitochondrial and bioenergetic studies were performed. Since we obtained promising results, we tested this antagonist in a 6-hydroxydopamine hemilesioned mouse model. In the animal model, behavioral tests, histological analysis, immunofluorescence and western blot of substantia nigra and striatum upon GSK0660 were assayed.

RESULTS

Our findings suggested that PPARβ/δ antagonist has neuroprotective potential due to neurotrophic support, anti-apoptotic and anti-oxidative effects paralleled to an amelioration of mitochondria and proteasome activity. These findings are strongly supported also by the siRNA results demonstrating that by silencing PPARβ/δ a significative rescue of the dopaminergic neurons was obtained, thus indicating an involvement of PPARβ/δ in PD's pathogenesis. Interestingly, in the animal model, GSK0660 treatment confirmed neuroprotective effects observed in the in vitro studies. Neuroprotective effects were highlighted by the behavioural performance and apomorphine rotation tests amelioration and the reduction of dopaminergic neuronal loss. These data were also confirmed by imaging and western blotting, indeed, the tested compound decreased astrogliosis and activated microglia, concomitant with an upregulation of neuroprotective pathways.

CONCLUSIONS

In summary, PPARβ/δ antagonist displayed neuroprotective activities against 6-hydroxydopamine detrimental effects both in vitro and in vivo models of Parkinson's disease, suggesting that it may represent a novel therapeutic approach for this disorder.

Effects of intracardiac delivery of aldehyde dehydrogenase 2 gene in myocardial salvage

Intrinsic activity of aldehyde dehydrogenase (ALDH)2, a cardiac mitochondrial enzyme, is vital in detoxifying 4-hydroxy-2-nonenal (4HNE) like cellular reactive carbonyl species (RCS) and thereby conferring cardiac protection against pathological stress. It was also known that a single point mutation (E487K) in ALDH2 (prevalent in East Asians) known as ALDH2*2 reduces its activity intrinsically and was associated with increased cardiovascular diseases. We and others have shown that ALDH2 activity is reduced in several pathologies in WT animals as well. Thus, exogenous augmentation of ALDH2 activity is a good strategy to protect the myocardium from pathologies. In this study, we will test the efficacy of intracardiac injections of the ALDH2 gene in mice. We injected both wild type (WT) and ALDH2*2 knock-in mutant mice with ALDH2 constructs, AAv9-cTNT-hALDH2-HA tag-P2A-eGFP or their control constructs, AAv9-cTNT-eGFP. We found that intracardiac ALDH2 gene transfer increased myocardial levels of ALDH2 compared to GFP alone after 1 and 3 weeks. When we subjected the hearts of these mice to 30 min global ischemia and 90 min reperfusion (I-R) using the Langendorff perfusion system, we found reduced infarct size in the hearts of mice with ALDH2 gene vs GFP alone. A single time injection has shown increased myocardial ALDH2 activity for at least 3 weeks and reduced myocardial 4HNE adducts and infarct size along with increased contractile function of the hearts while subjected to I-R. Thus, ALDH2 overexpression protected the myocardium from I-R injury by reducing 4HNE protein adducts implicating increased 4HNE detoxification by ALDH2. In conclusion, intracardiac ALDH2 gene transfer is an effective strategy to protect the myocardium from pathological insults.

The Nrf1 transcription factor is induced by patulin and protects against patulin cytotoxicity

Patulin is a mycotoxin produced by a variety of molds that is found in various food products. The adverse health effects associated with exposure to patulin has led to many investigations into the biological basis driving the toxicity of patulin. Nevertheless, the mechanisms through which mammalian cells resists patulin-mediated toxicity is poorly understood. Here, we show that loss of the Nrf1 transcription factor renders cells sensitive to the acute cytotoxic effects of patulin. Nrf1 deficiency leads to accumulation of ubiquitinated proteins and protein aggregates in response to patulin exposure. Nrf1 expression is induced by patulin, and activation of proteasome genes by patulin is Nrf1-dependent. These findings suggest the Nrf1 transcription factor plays a crucial role in modulating cellular stress response against patulin cytotoxicity.

Diabetic Aldehyde Dehydrogenase 2 Mutant (ALDH2*2) Mice Are More Susceptible to Cardiac Ischemic-Reperfusion Injury Due to 4-Hydroxy-2-Nonenal Induced Coronary Endothelial Cell Damage

Background

Aldehyde dehydrogenase‐2 (ALDH2), a mitochondrial enzyme, detoxifies reactive aldehydes such as 4‐hydroxy‐2‐nonenal (4HNE). A highly prevalent E487K mutation in ALDH2 (ALDH2*2) in East Asian people with intrinsic low ALDH2 activity is implicated in diabetic complications. 4HNE‐induced cardiomyocyte dysfunction was studied in diabetic cardiac damage; however, coronary endothelial cell (CEC) injury in myocardial ischemia‐reperfusion injury (IRI) in diabetic mice has not been studied. Therefore, we hypothesize that the lack of ALDH2 activity exacerbates 4HNE‐induced CEC dysfunction which leads to cardiac damage in ALDH2*2 mutant diabetic mice subjected to myocardial IRI.

Methods and Results

Three weeks after diabetes mellitus (DM) induction, hearts were subjected to IRI either in vivo via left anterior descending artery occlusion and release or ex vivo IRI by using the Langendorff system. The cardiac performance was assessed by conscious echocardiography in mice or by inserting a balloon catheter in the left ventricle in the ex vivo model. Just 3 weeks of DM led to an increase in cardiac 4HNE protein adducts and, cardiac dysfunction, and a decrease in the number of CECs along with reduced myocardial ALDH2 activity in ALDH2*2 mutant diabetic mice compared with their wild‐type counterparts. Systemic pretreatment with Alda‐1 (10 mg/kg per day), an activator of both ALDH2 and ALDH2*2, led to a reduction in myocardial infarct size and dysfunction, and coronary perfusion pressure upon cardiac IRI by increasing CEC population and coronary arteriole opening.

Conclusions

Low ALDH2 activity exacerbates 4HNE‐mediated CEC injury and thereby cardiac dysfunction in diabetic mouse hearts subjected to IRI, which can be reversed by ALDH2 activation.

Proteostasis in the Male and Female Germline: A New Outlook on the Maintenance of Reproductive Health

For fully differentiated, long lived cells the maintenance of protein homeostasis (proteostasis) becomes a crucial determinant of cellular function and viability. Neurons are the most well-known example of this phenomenon where the majority of these cells must survive the entire course of life. However, male and female germ cells are also uniquely dependent on the maintenance of proteostasis to achieve successful fertilization. Oocytes, also long-lived cells, are subjected to prolonged periods of arrest and are largely reliant on the translation of stored mRNAs, accumulated during the growth period, to support meiotic maturation and subsequent embryogenesis. Conversely, sperm cells, while relatively ephemeral, are completely reliant on proteostasis due to the absence of both transcription and translation. Despite these remarkable, cell-specific features there has been little focus on understanding protein homeostasis in reproductive cells and how/whether proteostasis is "reset" during embryogenesis. Here, we seek to capture the momentum of this growing field by highlighting novel findings regarding germline proteostasis and how this knowledge can be used to promote reproductive health. In this review we capture proteostasis in the context of both somatic cell and germline aging and discuss the influence of oxidative stress on protein function. In particular, we highlight the contributions of proteostasis changes to oocyte aging and encourage a focus in this area that may complement the extensive analyses of DNA damage and aneuploidy that have long occupied the oocyte aging field. Moreover, we discuss the influence of common non-enzymatic protein modifications on the stability of proteins in the male germline, how these changes affect sperm function, and how they may be prevented to preserve fertility. Through this review we aim to bring to light a new trajectory for our field and highlight the potential to harness the germ cell's natural proteostasis mechanisms to improve reproductive health. This manuscript will be of interest to those in the fields of proteostasis, aging, male and female gamete reproductive biology, embryogenesis, and life course health.

Elevated post-ischemic ubiquitination results from suppression of deubiquitinase activity and not proteasome inhibition

Cerebral ischemia-reperfusion increases intraneuronal levels of ubiquitinated proteins, but the factors driving ubiquitination and whether it results from altered proteostasis remain unclear. To address these questions, we used in vivo and in vitro models of cerebral ischemia-reperfusion, in which hippocampal slices were transiently deprived of oxygen and glucose to simulate ischemia followed by reperfusion, or the middle cerebral artery was temporarily occluded in mice. We found that post-ischemic ubiquitination results from two key steps: restoration of ATP at reperfusion, which allows initiation of protein ubiquitination, and free radical production, which, in the presence of sufficient ATP, increases ubiquitination above pre-ischemic levels. Surprisingly, free radicals did not augment ubiquitination through inhibition of the proteasome as previously believed. Although reduced proteasomal activity was detected after ischemia, this was neither caused by free radicals nor sufficient in magnitude to induce appreciable accumulation of proteasomal target proteins or ubiquitin-proteasome reporters. Instead, we found that ischemia-derived free radicals inhibit deubiquitinases, a class of proteases that cleaves ubiquitin chains from proteins, which was sufficient to elevate ubiquitination after ischemia. Our data provide evidence that free radical-dependent deubiquitinase inactivation rather than proteasomal inhibition drives ubiquitination following ischemia-reperfusion, and as such call for a reevaluation of the mechanisms of post-ischemic ubiquitination, previously attributed to altered proteostasis. Since deubiquitinase inhibition is considered an endogenous neuroprotective mechanism to shield proteins from oxidative damage, modulation of deubiquitinase activity may be of therapeutic value to maintain protein integrity after an ischemic insult.

Proteostasis of Islet Amyloid Polypeptide: A Molecular Perspective of Risk Factors and Protective Strategies for Type II Diabetes

The possible link between hIAPP accumulation and β-cell death in diabetic patients has inspired numerous studies focusing on amyloid structures and aggregation pathways of this hormone. Recent studies have reported on the importance of early oligomeric intermediates, the many roles of their interactions with lipid membrane, pH, insulin, and zinc on the mechanism of aggregation of hIAPP. The challenges posed by the transient nature of amyloid oligomers, their structural heterogeneity, and the complex nature of their interaction with lipid membranes have resulted in the development of a wide range of biophysical and chemical approaches to characterize the aggregation process. While the cellular processes and factors activating hIAPP-mediated cytotoxicity are still not clear, it has recently been suggested that its impaired turnover and cellular processing by proteasome and autophagy may contribute significantly toward toxic hIAPP accumulation and, eventually, β-cell death. Therefore, studies focusing on the restoration of hIAPP proteostasis may represent a promising arena for the design of effective therapies. In this review we discuss the current knowledge of the structures and pathology associated with hIAPP self-assembly and point out the opportunities for therapy that a detailed biochemical, biophysical, and cellular understanding of its aggregation may unveil.

Regulation of Proteasome Activity by (Post-)transcriptional Mechanisms

Intracellular protein synthesis, folding, and degradation are tightly controlled processes to ensure proper protein homeostasis. The proteasome is responsible for the degradation of the majority of intracellular proteins, which are often targeted for degradation via polyubiquitination. However, the degradation rate of proteins is also affected by the capacity of proteasomes to recognize and degrade these substrate proteins. This capacity is regulated by a variety of proteasome modulations including (1) changes in complex composition, (2) post-translational modifications, and (3) altered transcription of proteasomal subunits and activators. Various diseases are linked to proteasome modulation and altered proteasome function. A better understanding of these modulations may offer new perspectives for therapeutic intervention. Here we present an overview of these three proteasome modulating mechanisms to give better insight into the diversity of proteasomes.

The Emerging Role of Electrophiles as a Key Regulator for Endoplasmic Reticulum (ER) Stress

The unfolded protein response (UPR) is activated by the accumulation of misfolded proteins in the endoplasmic reticulum (ER), which is called ER stress. ER stress sensors PERK, IRE1, and ATF6 play a central role in the initiation and regulation of the UPR; they inhibit novel protein synthesis and upregulate ER chaperones, such as protein disulfide isomerase, to remove unfolded proteins. However, when recovery from ER stress is difficult, the UPR pathway is activated to eliminate unhealthy cells. This signaling transition is the key event of many human diseases. However, the precise mechanisms are largely unknown. Intriguingly, reactive electrophilic species (RES), which exist in the environment or are produced through cellular metabolism, have been identified as a key player of this transition. In this review, we focused on the function of representative RES: nitric oxide (NO) as a gaseous RES, 4-hydroxynonenal (HNE) as a lipid RES, and methylmercury (MeHg) as an environmental organic compound RES, to outline the relationship between ER stress and RES. Modulation by RES might be a target for the development of next-generation therapy for ER stress-associated diseases.

Aldehyde Dehydrogenase (ALDH) 2 in Diabetic Heart Diseases

A major pathophysiological mechanism behind the development of diabetic heart diseases is oxidative stress mediated by toxic reactive aldehydes such as 4-hydroxynonenal (4HNE). Aldehyde dehydrogenase (ALDH) 2 is a mitochondrial enzyme that has been found to detoxify these deleterious aldehydes and thereby mitigate cardiac damage. Furthermore, its protective role in cellular signaling reverses aberrations caused by hyperglycemia, thereby protecting cardiac function. This chapter assesses the role of ALDH2 in diabetic heart diseases by examining preclinical studies where ALDH2 activity is perturbed in both decreased and increased directions. In doing so, issues in improving ALDH2 activity in select human populations are elucidated, and further research directions are discussed.

Mitochondrial Aldehyde Dehydrogenase in Myocardial Ischemic and Ischemia-Reperfusion Injury

Myocardial ischemia-reperfusion (IR) injury during acute myocardial infarction or open-heart surgery would promote oxidative stress, leading to the accumulation of reactive aldehydes that cause cardiac damage. It has been well demonstrated that aldehyde dehydrogenase (ALDH)-2 is an important cardioprotective enzyme for its central role in the detoxification of reactive aldehydes. ALDH2 activation by small molecule activators is a promising approach for cardioprotection for myocardial IR injury.

Oxidative damage in naturally aged mouse oocytes is exacerbated by dysregulation of proteasomal activity

An increase in oxidative protein damage is a leading contributor to the age-associated decline in oocyte quality. By removing such damaged proteins, the proteasome plays an essential role in maintaining the fidelity of oocyte meiosis. In this study, we established that decreased proteasome activity in naturally aged, germinal vesicle (GV) mouse oocytes positively correlates with increased protein modification by the lipid aldehyde 4-hydroxynonenal (4-HNE). Furthermore, attenuation of proteasome activity in GV oocytes of young animals was accompanied by an increase in 4-HNE-modified proteins, including α-tubulin, thereby contributing to a reduction in tubulin polymerization, microtubule stability, and integrity of oocyte meiosis. A decrease in proteasome activity was also recapitulated in the GV oocytes of young animals following exposure to oxidative insults in the form of either hydrogen peroxide (H₂O₂) or 4-HNE. We also observed that upon oxidative insult, 4-HNE exhibits elevated adduction to multiple proteasomal subunits. Notably, the inclusion of the antioxidant penicillamine, to limit propagation of oxidative stress cascades, led to a complete recovery of proteasome activity and enhanced clearance of 4-HNE-adducted α-tubulin during a 6-h post-treatment recovery period. This strategy also proved effective in reducing the incidence of oxidative stress-induced aneuploidy following in vitro oocyte maturation, but was ineffective for naturally aged oocytes. Taken together, our results implicate proteasome dysfunction as an important factor in the accumulation of oxidatively induced protein damage in the female germline. This discovery holds promise for the design of therapeutic interventions to address the age-dependent decline in oocyte quality.

Proteolytic degradation of heat shock protein A2 occurs in response to oxidative stress in male germ cells of the mouse

STUDY QUESTION

Does oxidative stress compromise the protein expression of heat shock protein A2 (HSPA2) in the developing germ cells of the mouse testis?

SUMMARY ANSWER

Oxidative stress leads to the modification of HSPA2 by the lipid aldehyde 4-hydroxynonenal (4HNE) and initiates its degradation via the ubiquitin-proteasome system.

WHAT IS KNOWN ALREADY

Previous work has revealed a deficiency in HSPA2 protein expression within the spermatozoa of infertile men that have failed fertilization in a clinical setting. While the biological basis of this reduction in HSPA2 remains to be established, we have recently shown that the HSPA2 expressed in the spermatozoa of normozoospermic individuals is highly susceptible to adduction, a form of post-translational modification, by the lipid aldehyde 4HNE that has been causally linked to the degradation of its substrates. This modification of HSPA2 by 4HNE adduction dramatically reduced human sperm-egg interaction in vitro. Moreover, studies in a mouse model offer compelling evidence that the co-chaperone BCL2-associated athanogene 6 (BAG6) plays a key role in regulating the stability of HSPA2 in the testis, by preventing its ubiquitination and subsequent proteolytic degradation.

STUDY DESIGN, SIZE, DURATION

Dose-dependent studies were used to establish a 4HNE-treatment regime for primary culture(s) of male mouse germ cells. The influence of 4HNE on HSPA2 protein stability was subsequently assessed in treated germ cells. Additionally, sperm lysates from infertile patients with established zona pellucida recognition defects were examined for the presence of 4HNE and ubiquitin adducts. A minimum of three biological replicates were performed to test statistical significance.

PARTICIPANTS/MATERIALS, SETTING, METHODS

Oxidative stress was induced in pachytene spermatocytes and round spermatids isolated from the mouse testis, as well as a GC-2 cell line, using 50-200 µM 4HNE or hydrogen peroxide (H2O2), and the expression of HSPA2 was monitored via immunocytochemistry and immunoblotting approaches. Using the GC-2 cell line as a model, the ubiquitination and degradation of HSPA2 was assessed using immunoprecipitation techniques and pharmacological inhibition of proteasomal and lysosomal degradation pathways. Finally, the interaction between BAG6 and HSPA2 was examined in response to 4HNE exposure via proximity ligation assays.

MAIN RESULTS AND THE ROLE OF CHANCE

HSPA2 protein levels were significantly reduced compared with controls after 4HNE treatment of round spermatids (P < 0.01) and GC-2 cells (P < 0.001) but not pachytene spermatocytes. Using GC-2 cells as a model, HSPA2 was shown to be both adducted by 4HNE and targeted for ubiquitination in response to cellular oxidative stress. Inhibition of the proteasome with MG132 prevented HSPA2 degradation after 4HNE treatment indicating that the degradation of HSPA2 is likely to occur via a proteasomal pathway. Moreover, our assessment of proteasome activity provided evidence that 4HNE treatment can significantly increase the proteasome activity of GC-2 cells (P < 0.05 versus control). Finally, 4HNE exposure to GC-2 cells resulted in the dissociation of HSPA2 from its regulatory co-chaperone BAG6, a key mediator of HSPA2 stability in male germ cells.

LIMITATIONS, REASONS FOR CAUTION

While these experiments were performed using a mouse germ cell-model system, our analyses of patient sperm lysate imply that these mechanisms are conserved between mouse and human germ cells.

WIDER IMPLICATIONS OF THE FINDINGS

This study suggests a causative link between non-enzymatic post-translational modifications and the relative levels of HSPA2 in the spermatozoa of a specific sub-class of infertile males. In doing so, this work enhances our understanding of failed sperm-egg recognition and may assist in the development of targeted antioxidant-based approaches for ameliorating the production of cytotoxic lipid aldehydes in the testis in an attempt to prevent this form of infertility.

LARGE SCALE DATA

Not applicable.

STUDY FUNDING/COMPETING INTEREST(S)

This work was supported by the National Health and Medical Research Council of Australia (APP1101953). The authors have no competing interests to declare.

An adverse outcome pathway for parkinsonian motor deficits associated with mitochondrial complex I inhibition

Epidemiological studies have observed an association between pesticide exposure and the development of Parkinson's disease, but have not established causality. The concept of an adverse outcome pathway (AOP) has been developed as a framework for the organization of available information linking the modulation of a molecular target [molecular initiating event (MIE)], via a sequence of essential biological key events (KEs), with an adverse outcome (AO). Here, we present an AOP covering the toxicological pathways that link the binding of an inhibitor to mitochondrial complex I (i.e., the MIE) with the onset of parkinsonian motor deficits (i.e., the AO). This AOP was developed according to the Organisation for Economic Co-operation and Development guidelines and uploaded to the AOP database. The KEs linking complex I inhibition to parkinsonian motor deficits are mitochondrial dysfunction, impaired proteostasis, neuroinflammation, and the degeneration of dopaminergic neurons of the substantia nigra. These KEs, by convention, were linearly organized. However, there was also evidence of additional feed-forward connections and shortcuts between the KEs, possibly depending on the intensity of the insult and the model system applied. The present AOP demonstrates mechanistic plausibility for epidemiological observations on a relationship between pesticide exposure and an elevated risk for Parkinson's disease development.

Precise assembly and regulation of 26S proteasome and correlation between proteasome dysfunction and neurodegenerative diseases

Neurodegenerative diseases (NDs) often involve the formation of abnormal and toxic protein aggregates, which are thought to be the primary factor in ND occurrence and progression. Aged neurons exhibit marked increases in aggregated protein levels, which can lead to increased cell death in specific brain regions. As no specific drugs/therapies for treating the symptoms or/and progression of NDs are available, obtaining a complete understanding of the mechanism underlying the formation of protein aggregates is needed for designing a novel and efficient removal strategy. Intracellular proteolysis generally involves either the lysosomal or ubiquitin-proteasome system. In this review, we focus on the structure and assembly of the proteasome, proteasome-mediated protein degradation, and the multiple dynamic regulatory mechanisms governing proteasome activity. We also discuss the plausibility of the correlation between changes in proteasome activity and the occurrence of NDs. [BMB Reports 2016; 49(9): 459-473]

Heat Shock Protein A2 (HSPA2): Regulatory Roles in Germ Cell Development and Sperm Function

Among the numerous families of heat shock protein (HSP) that have been implicated in the regulation of reproductive system development and function, those belonging to the 70 kDa HSP family have emerged as being indispensable for male fertility. In particular, the testis-enriched heat shock 70 kDa protein 2 (HSPA2) has been shown to be critical for the progression of germ cell differentiation during spermatogenesis in the mouse model. Beyond this developmentally important window, mounting evidence has also implicated HSPA2 in the functional transformation of the human sperm cell during their ascent of the female reproductive tract. Specifically, HSPA2 appears to coordinate the remodelling of specialised sperm domains overlying the anterior region of the sperm head compatible with their principle role in oocyte recognition. The fact that levels of the HSPA2 protein in mature spermatozoa tightly correlate with the efficacy of oocyte binding highlight its utility as a powerful prognostic biomarker of male fertility. In this chapter, we consider the unique structural and biochemical characteristics of HSPA2 that enable this heat shock protein to fulfil its prominent roles in orchestrating the morphological differentiation of male germ cells during spermatogenesis as well as their functional transformation during post-testicular sperm maturation.

Happily (n)ever after: Aging in the context of oxidative stress, proteostasis loss and cellular senescence

Aging is a complex phenomenon and its impact is becoming more relevant due to the rising life expectancy and because aging itself is the basis for the development of age-related diseases such as cancer, neurodegenerative diseases and type 2 diabetes. Recent years of scientific research have brought up different theories that attempt to explain the aging process. So far, there is no single theory that fully explains all facets of aging. The damage accumulation theory is one of the most accepted theories due to the large body of evidence found over the years. Damage accumulation is thought to be driven, among others, by oxidative stress. This condition results in an excess attack of oxidants on biomolecules, which lead to damage accumulation over time and contribute to the functional involution of cells, tissues and organisms. If oxidative stress persists, cellular senescence is a likely outcome and an important hallmark of aging. Therefore, it becomes crucial to understand how senescent cells function and how they contribute to the aging process. This review will cover cellular senescence features related to the protein pool such as morphological and molecular hallmarks, how oxidative stress promotes protein modifications, how senescent cells cope with them by proteostasis mechanisms, including antioxidant enzymes and proteolytic systems. We will also highlight the nutritional status of senescent cells and aged organisms (including human clinical studies) by exploring trace elements and micronutrients and on their importance to develop strategies that might increase both, life and health span and postpone aging onset.

Increased 4-hydroxy-2-nonenal-induced proteasome dysfunction is correlated with cardiac damage in streptozotocin-injected rats with isoproterenol infusion

Increase in 4-hydroxy-2-nonenal (4HNE) due to oxidative stress has been observed in a variety of cardiac diseases such as diabetic cardiomyopathy. 4HNE exerts a damaging effect in the myocardium by interfering with subcellular organelles like mitochondria by forming adducts. Therefore, we hypothesized that increased 4HNE adduct formation in the heart results in proteasome inactivation in isoproterenol (ISO)-infused type 1 diabetes mellitus (DM) rats. Eight-week-old male Sprague Dawley rats were injected with streptozotocin (STZ, 65 mg kg(-1) ). The rats were infused with ISO (5 mg kg(-1) ) for 2 weeks by mini pumps, after 8 weeks of STZ injection. We studied normal control (n = 8) and DM + ISO (n = 10) groups. Cardiac performance was assessed by echocardiography and Millar catheter at the end of the protocol at 20 weeks. Initially, we found an increase in 4HNE adducts in the hearts of the DM + ISO group. There was also a decrease in myocardial proteasomal peptidase (chymotrypsin and trypsin-like) activity. Increases in cardiomyocyte area (446 ± 32·7 vs 221 ± 10·83) (µm(2) ), per cent area of cardiac fibrosis (7·4 ± 0·7 vs 2·7 ± 0·5) and cardiac dysfunction were also found in DM + ISO (P < 0·05) relative to controls. We also found increased 4HNE adduct formation on proteasomal subunits. Furthermore, reduced aldehyde dehydrogenase 2 activity was observed in the myocardium of the DM + ISO group. Treatment with 4HNE (100 μM) for 4 h on cultured H9c2 cardiomyocytes attenuated proteasome activity. Therefore, we conclude that the 4HNE-induced decrease in proteasome activity may be involved in the cardiac pathology in STZ-injected rats infused with ISO. Copyright © 2016 John Wiley & Sons, Ltd.

Alcohol Toxicity in Diabetes and Its Complications: A Double Trouble?

BACKGROUND

Eight percent of the U.S. population has been diagnosed with diabetes mellitus (DM), while another large percentage has gone undiagnosed. As the epidemiology of this disease constitutes a larger percentage of the American population, another factor presents a dangerous dilemma that can exacerbate the hazardous effects imposed by DM. Excessive alcohol consumption concerns the health of more than 50% of all adults. When this heavy-alcohol-drinking population overlaps with DM and its complications, the effects can be dangerous. In this review, we term it as "double trouble."

METHODS

We provide evidence of alcohol-induced exacerbation of organ damage in diabetic conditions. In certain cases, we have explained how diabetes and alcohol induce similar pathological effects.

RESULTS

Known exacerbated complications include those related to heart diseases, liver damage, kidney dysfunction, as well as retinal and neurological impairment. Often, pathophysiological damage concludes with end-stage disorders and even mortality. The metabolic, cell signaling, and pathophysiological changes associated with "double trouble" would lead to the identification of novel therapeutic targets.

CONCLUSIONS

This review summarizes the epidemiology, diagnosis, pathophysiology, metabolic, and cell signaling alterations and finally brushes upon issues and strategies to manage the "double trouble."

Oxidative stress and lipotoxicity

The α,β polyunsaturated lipid aldehydes are potent lipid electrophiles that covalently modify lipids, proteins, and nucleic acids. Recent work highlights the critical role these lipids play under both physiological and pathological conditions. Protein carbonylation resulting from nucleophilic attack of lysine, histidine, and cysteine residues is a major outcome of oxidative stress and functions as a redox-sensitive signaling mechanism with roles in autophagy, cell proliferation, transcriptional control, and apoptosis. In addition, protein carbonylation is implicated as an initiating factor in mitochondrial dysfunction and endoplasmic reticulum stress, providing a mechanistic connection between oxidative stress and metabolic disease. In this review, we discuss the generation and metabolism of reactive lipid aldehydes, as well as their signaling roles.

UPS Activation in the Battle Against Aging and Aggregation-Related Diseases: An Extended Review

Aging is a biological process accompanied by gradual increase of damage in all cellular macromolecules, i.e., nucleic acids, lipids, and proteins. When the proteostasis network (chaperones and proteolytic systems) cannot reverse the damage load due to its excess as compared to cellular repair/regeneration capacity, failure of homeostasis is established. This failure is a major hallmark of aging and/or aggregation-related diseases. Dysfunction of the major cellular proteolytic machineries, namely the proteasome and the lysosome, has been reported during the progression of aging and aggregation-prone diseases. Therefore, activation of these pathways is considered as a possible preventive or therapeutic approach against the progression of these processes. This chapter focuses on UPS activation studies in cellular and organismal models and the effects of such activation on aging, longevity and disease prevention or reversal.

Identification of an unstable 4-hydroxynoneal modification on the 20S proteasome subunit α7 by recombinant antibody technology

Numerous cellular functions rely on an active proteasome allowing degradation of damaged or misfolded proteins. Therefore changes in the proteasomal activity have important physiological consequences. During oxidative stress the production of free radicals can result in the formation of 4-hydroxynonenal (HNE) following lipid peroxidiation. The HNE moiety is highly reactive and via a nucleophilic attack readily forms covalent links to cysteine, histidine and lysine side chains. However, as the chemical properties of these amino acids differ, so does the kinetics of the reactions. While covalent linkage through Michael addition is well established, reversible and unstable associations have only been indicated in a few cases. In the present study we have identified an unstable HNE adduct on the α7 subunit of the 20S proteasome using phage display of recombinant antibodies. This recombinant antibody fragment recognized HNE modified proteasomes in vitro and showed that this epitope was easily HNE modified, yet unstable, and influenced by experimental procedures. Hence unstable HNE-adducts could be overlooked as a regulatory mechanism of proteasomal activity and a participating factor in the decreased proteasomal activity associated with oxidative stress.

Hydrogen Peroxide-Induced Oxidative Stress Activates Proteasomal Trypsin-Like Activity in Human U373 Glioma Cells

Degradation of oxidized or oxidatively modified proteins is an essential part of the cellular antioxidant defense system. 4-Hydroxy-2-nonenal, a major reactive aldehyde formed by lipid peroxidation, causes many types of cellular damage. The major proteolytic system for modified protein degradation is the ubiquitin-proteasome pathway. However, our previous studies using U937 human leukemic cells showed that 4-hydroxy-2-nonenal-modified glyceraldehyde-3-phosphate dehydrogenase (GAPDH) is degraded by cathepsin G. In the present study, U373 human glioma cells were cultured in the presence of hydrogen peroxide (H2O2) to investigate the relationships of proteasome and/or cathepsin G activities and H2O2-induced GAPDH degradation. Treatment of cells with H2O2 for 5 h in culture decreased GAPDH activity as well as its protein concentration in a concentration-dependent manner. Two proteasomal activities (peptidylglutamyl-peptide hydrolase and chymotrypsin-like hydrolase activities) and cathepsin G activity were decreased by H2O2 treatment in a concentration-dependent manner, but proteasomal trypsin-like hydrolase activity increased with cell exposure to high H2O2 concentrations. Among the protease inhibitors examined here, H2O2-induced activation of trypsin-like activity and GAPDH degradation were inhibited by the proteasome inhibitor lactacystin. Furthermore, H2O2-induced activation of trypsin-like activity was also inhibited by another proteasome inhibitor MG-132. These results suggested that proteasomal trypsin-like activity played an important role in eliminating oxidatively modified GAPDH formed in these cells during H2O2 exposure.

Metabolic methanol: molecular pathways and physiological roles

Methanol has been historically considered an exogenous product that leads only to pathological changes in the human body when consumed. However, in normal, healthy individuals, methanol and its short-lived oxidized product, formaldehyde, are naturally occurring compounds whose functions and origins have received limited attention. There are several sources of human physiological methanol. Fruits, vegetables, and alcoholic beverages are likely the main sources of exogenous methanol in the healthy human body. Metabolic methanol may occur as a result of fermentation by gut bacteria and metabolic processes involving S-adenosyl methionine. Regardless of its source, low levels of methanol in the body are maintained by physiological and metabolic clearance mechanisms. Although human blood contains small amounts of methanol and formaldehyde, the content of these molecules increases sharply after receiving even methanol-free ethanol, indicating an endogenous source of the metabolic methanol present at low levels in the blood regulated by a cluster of genes. Recent studies of the pathogenesis of neurological disorders indicate metabolic formaldehyde as a putative causative agent. The detection of increased formaldehyde content in the blood of both neurological patients and the elderly indicates the important role of genetic and biochemical mechanisms of maintaining low levels of methanol and formaldehyde.

Increased clearance of reactive aldehydes and damaged proteins in hypertension-induced compensated cardiac hypertrophy: impact of exercise training

Background. We previously reported that exercise training (ET) facilitates the clearance of damaged proteins in heart failure. Here, we characterized the impact of ET on cardiac protein quality control during compensated ventricular hypertrophy in spontaneously hypertensive rats (SHR). Methods and Results. SHR were randomly assigned into sedentary and swimming-trained groups. Sedentary SHR displayed cardiac hypertrophy with preserved ventricular function compared to normotensive rats, characterizing a compensated cardiac hypertrophy. Hypertensive rats presented signs of cardiac oxidative stress, depicted by increased lipid peroxidation. However, these changes were not followed by accumulation of lipid peroxidation-generated reactive aldehydes and damaged proteins. This scenario was explained, at least in part, by the increased catalytic activity of both aldehyde dehydrogenase 2 (ALDH2) and proteasome. Of interest, ET exacerbated cardiac hypertrophy, improved ventricular function, induced resting bradycardia, and decreased blood pressure in SHR. These changes were accompanied by reduced cardiac oxidative stress and a consequent decrease in ALDH2 and proteasome activities, without affecting small chaperones levels and apoptosis in SHR. Conclusion. Increased cardiac ALDH2 and proteasomal activities counteract the deleterious effect of excessive oxidative stress in hypertension-induced compensated cardiac hypertrophy in rats. ET has a positive effect in reducing cardiac oxidative stress without affecting protein quality control.

Targeting the ubiquitin-proteasome system in heart disease: the basis for new therapeutic strategies

SIGNIFICANCE

Novel therapeutic strategies to treat heart failure are greatly needed. The ubiquitin-proteasome system (UPS) affects the structure and function of cardiac cells through targeted degradation of signaling and structural proteins. This review discusses both beneficial and detrimental consequences of modulating the UPS in the heart.

RECENT ADVANCES

Proteasome inhibitors were first used to test the role of the UPS in cardiac disease phenotypes, indicating therapeutic potential. In early cardiac remodeling and pathological hypertrophy with increased proteasome activities, proteasome inhibition prevented or restricted disease progression and contractile dysfunction. Conversely, enhancing proteasome activities by genetic manipulation, pharmacological intervention, or ischemic preconditioning also improved the outcome of cardiomyopathies and infarcted hearts with impaired cardiac and UPS function, which is, at least in part, caused by oxidative damage.

CRITICAL ISSUES

An understanding of the UPS status and the underlying mechanisms for its potential deregulation in cardiac disease is critical for targeted interventions. Several studies indicate that type and stage of cardiac disease influence the dynamics of UPS regulation in a nonlinear and multifactorial manner. Proteasome inhibitors targeting all proteasome complexes are associated with cardiotoxicity in humans. Furthermore, the type and dosage of proteasome inhibitor impact the pathogenesis in nonuniform ways.

FUTURE DIRECTIONS

Systematic analysis and targeting of individual UPS components with established and innovative tools will unravel and discriminate regulatory mechanisms that contribute to and protect against the progression of cardiac disease. Integrating this knowledge in drug design may reduce adverse effects on the heart as observed in patients treated with proteasome inhibitors against noncardiac diseases, especially cancer.

The amazing ubiquitin-proteasome system: structural components and implication in aging

Proteome quality control (PQC) is critical for the maintenance of cellular functionality and it is assured by the curating activity of the proteostasis network (PN). PN is constituted of several complex protein machines that under conditions of proteome instability aim to, firstly identify, and then, either rescue or degrade nonnative polypeptides. Central to the PN functionality is the ubiquitin-proteasome system (UPS) which is composed from the ubiquitin-conjugating enzymes and the proteasome; the latter is a sophisticated multi-subunit molecular machine that functions in a bimodal way as it degrades both short-lived ubiquitinated normal proteins and nonfunctional polypeptides. UPS is also involved in PQC of the nucleus, the endoplasmic reticulum and the mitochondria and it also interacts with the other main cellular degradation axis, namely the autophagy-lysosome system. UPS functionality is optimum in the young organism but it is gradually compromised during aging resulting in increasing proteotoxic stress; these effects correlate not only with aging but also with most age-related diseases. Herein, we present a synopsis of the UPS components and of their functional alterations during cellular senescence or in vivo aging. We propose that mild UPS activation in the young organism will, likely, promote antiaging effects and/or suppress age-related diseases.

The interplay between autophagy and the ubiquitin-proteasome system in cardiac proteotoxicity

Proteotoxicity refers to the detrimental effects of damaged/misfolded proteins on the cell. Cardiac muscle is particularly susceptible to proteotoxicity because sustained and severe proteotoxic stress leads to cell death and the cardiac muscle has very limited self-renewal capacity. The ubiquitin-proteasome system (UPS) and the autophagic-lysosomal pathway (ALP) are two major pathways responsible for degradation of most cellular proteins. Alterations of UPS and ALP functions are associated with the accumulation of proteotoxic species in the heart, a key pathological feature of common forms of heart disease including idiopathic, ischemic, and pressure-overloaded cardiomyopathies and a large subset of congestive heart failure. Emerging evidence suggests that proteasome inhibition or impairment activates autophagy and conversely, acute ALP inhibition may sometimes increase intrinsic proteasome peptidase activities but chronic ALP inhibition hinders UPS performance in ubiquitinated protein degradation. The exact molecular basis on which the two degradative pathways interact remains largely undefined. Here we review current understanding of the roles of the UPS and autophagy in the control of cardiac proteotoxicity, with a specific focus on the crosstalk between the two pathways. This article is part of a Special Issue entitled: Autophagy and protein quality control in cardiometabolic diseases.

Regulation of cardiac proteasomes by ubiquitination, SUMOylation, and beyond

The ubiquitin-proteasome system (UPS) is the major intracellular degradation system, and its proper function is critical to the health and function of cardiac cells. Alterations in cardiac proteasomes have been linked to several pathological phenotypes, including cardiomyopathies, ischemia-reperfusion injury, heart failure, and hypertrophy. Defects in proteasome-dependent cellular protein homeostasis can be causal for the initiation and progression of certain cardiovascular diseases. Emerging evidence suggests that the UPS can specifically target proteins that govern pathological signaling pathways for degradation, thus altering downstream effectors and disease outcomes. Alterations in UPS-substrate interactions in disease occur, in part, due to direct modifications of 19S, 11S or 20S proteasome subunits. Post-translational modifications (PTMs) are one facet of this proteasomal regulation, with over 400 known phosphorylation sites, over 500 ubiquitination sites and 83 internal lysine acetylation sites, as well as multiple sites for caspase cleavage, glycosylation (such as O-GlcNAc modification), methylation, nitrosylation, oxidation, and SUMOylation. Changes in cardiac proteasome PTMs, which occur in ischemia and cardiomyopathies, are associated with changes in proteasome activity and proteasome assembly; however several features of this regulation remain to be explored. In this review, we focus on how some of the less common PTMs affect proteasome function and alter cellular protein homeostasis. This article is part of a Special Issue entitled "Protein Quality Control, the Ubiquitin Proteasome System, and Autophagy".

Targeting aldehyde dehydrogenase 2: new therapeutic opportunities

A family of detoxifying enzymes called aldehyde dehydrogenases (ALDHs) has been a subject of recent interest, as its role in detoxifying aldehydes that accumulate through metabolism and to which we are exposed from the environment has been elucidated. Although the human genome has 19 ALDH genes, one ALDH emerges as a particularly important enzyme in a variety of human pathologies. This ALDH, ALDH2, is located in the mitochondrial matrix with much known about its role in ethanol metabolism. Less known is a new body of research to be discussed in this review, suggesting that ALDH2 dysfunction may contribute to a variety of human diseases including cardiovascular diseases, diabetes, neurodegenerative diseases, stroke, and cancer. Recent studies suggest that ALDH2 dysfunction is also associated with Fanconi anemia, pain, osteoporosis, and the process of aging. Furthermore, an ALDH2 inactivating mutation (termed ALDH2*2) is the most common single point mutation in humans, and epidemiological studies suggest a correlation between this inactivating mutation and increased propensity for common human pathologies. These data together with studies in animal models and the use of new pharmacological tools that activate ALDH2 depict a new picture related to ALDH2 as a critical health-promoting enzyme.

The proteasome and the degradation of oxidized proteins: part III-Redox regulation of the proteasomal system

Here, we review shortly the current knowledge on the regulation of the proteasomal system during and after oxidative stress. After addressing the components of the proteasomal system and the degradation of oxidatively damaged proteins in part I and II of this series, we address here which changes in activity undergo the proteasome and the ubiquitin-proteasomal system itself under oxidative conditions. While several components of the proteasomal system undergo direct oxidative modification, a number of redox-regulated events are modulating the proteasomal activity in a way it can address the major tasks in an oxidative stress situation: the removal of oxidized proteins and the adaptation of the cellular metabolism to the stress situation.

Regulation and therapeutic strategies of 4-hydroxy-2-nonenal metabolism in heart disease

4-Hydroxy-2-nonenal (4-HNE), a reactive aldehyde, is generated from polyunsaturated fatty acids (PUFAs) in biological membranes. Reactive oxygen species (ROS) generated during oxidative stress react with PUFAs to form aldehydes like 4-HNE, which inactivates proteins and DNA by forming hybrid covalent chemical addition compounds called adducts. The ensuing chain reaction results in cellular dysfunction and tissue damage. It includes a wide spectrum of events ranging from electron transport chain dysfunction to apoptosis. In addition, 4-HNE directly depresses contractile function, enhances ROS formation, modulates cell signaling pathways, and can contribute to many cardiovascular diseases, including atherosclerosis, myocardial ischemia-reperfusion injury, heart failure, and cardiomyopathy. Therefore, targeting 4-HNE could help reverse these pathologies. This review will focus on 4-HNE generation, the role of 4-HNE in cardiovascular diseases, cellular targets (especially mitochondria), processes and mechanisms for 4-HNE-induced toxicity, regulation of 4-HNE metabolism, and finally strategies for developing potential therapies for cardiovascular disease by attenuating 4-HNEinduced toxicity.

HIV protease inhibitors and onset of cardiovascular diseases: a central role for oxidative stress and dysregulation of the ubiquitin-proteasome system

The successful roll-out of highly active antiretroviral therapy (HAART) has extended life expectancy and enhanced the overall well-being of HIV-positive individuals. There are, however, increased concerns regarding HAART-mediated metabolic derangements and its potential risk for cardiovascular diseases (CVD) in the long-term. Here certain classes of antiretroviral drugs such as the HIV protease inhibitors (PIs) are strongly implicated in this process. This article largely focuses on the direct PI-linked development of cardio-metabolic complications, and reviews the inter-linked roles of oxidative stress and the ubiquitin-proteasome system (UPS) as key mediators driving this process. It is proposed that PIs trigger reactive oxygen species (ROS) production that leads to serious downstream consequences such as cell death, impaired mitochondrial function, and UPS dysregulation. Moreover, we advocate that HIV PIs may also directly lower myocardial UPS function. The attenuation of cardiac UPS can initiate transcriptional changes that contribute to perturbed lipid metabolism, thereby fueling a pro-atherogenic milieu. It may also directly alter ionic channels and interfere with electrical signaling in the myocardium. Therefore HIV PI-induced ROS together with a dysfunctional UPS elicit detrimental effects on the cardiovascular system that will eventually result in the onset of heart diseases. Thus while HIV PIs substantially improve life expectancy and quality of life in HIV-positive patients, its longer-term side-effects on the cardiovascular system should lead to a) greater clinical awareness regarding its benefit-harm paradigm, and b) the development and evaluation of novel co-treatment strategies.

Oxidative stress, neurodegeneration, and the balance of protein degradation and protein synthesis

Oxidative stress occurs in a variety of disease settings and is strongly linked to the development of neuron death and neuronal dysfunction. Cells are equipped with numerous pathways to prevent the genesis, as well as the consequences, of oxidative stress in the brain. In this review we discuss the various forms and sources of oxidative stress in the brain and briefly discuss some of the complexities in detecting the presence of oxidative stress. We then focus the review on the interplay between the diverse cellular proteolytic pathways and their roles in regulating oxidative stress in the brain. Additionally, we discuss the involvement of protein synthesis in regulating the downstream effects of oxidative stress. Together, these components of the review demonstrate that the removal of damaged proteins by effective proteolysis and the synthesis of new and protective proteins are vital in the preservation of brain homeostasis during periods of increased levels of reactive oxygen species. Last, studies from our laboratory and others have demonstrated that protein synthesis is intricately linked to the rates of protein degradation, with impairment of protein degradation sufficient to decrease the rates of protein synthesis, which has important implications for successfully responding to periods of oxidative stress. Specific neurodegenerative diseases, including Alzheimer disease, Parkinson disease, amyotrophic lateral sclerosis, and stroke, are discussed in this context. Taken together, these findings add to our understanding of how oxidative stress is effectively managed in the healthy brain and help elucidate how impairments in proteolysis and/or protein synthesis contribute to the development of neurodegeneration and neuronal dysfunction in a variety of clinical settings.

Aldehyde dehydrogenase-2 activation during cardioplegic arrest enhances the cardioprotection against myocardial ischemia-reperfusion injury

Ischemia/reperfusion damage is common during open-heart surgery. Activation of aldehyde dehydrogenase-2 can significantly reduce ischemia/reperfusion damage. We hypothesized that adding aldehyde dehydrogenase-2 agonist to regular cardioplegia solution would further ameliorate ischemia/reperfusion damage. Alda-1 was used as an aldehyde dehydrogenase-2 agonist. Cardioprotection by histidine-tryptophan-ketoglutarate solution with and without Alda-1 was compared using an ex vivo perfused rat heart model of ischemia/reperfusion. Three groups of ex vivo rat hearts endured different treatments with variant ischemia or an ischemia/reperfusion time course: sham, no ischemia/reperfusion; histidine-tryptophan-ketoglutarate; and histidine-tryptophan-ketoglutarate plus Alda-1. Aldehyde dehydrogenase-2 expressions and activities, oxidative parameters (including 4-hydroxy-2-nonenal-His adducts, malondialdehyde levels, and glutathione/oxidized glutathione ratios), myocardial protein carbonyl levels, coronary effluents creatine kinase isoenzyme MB levels, and heart function parameters were measured and compared. Alda-1 significantly elevated myocardium aldehyde dehydrogenase-2 activity (P < .01). Increased aldehyde dehydrogenase-2 activity in turn attenuated ischemia/reperfusion-induced elevation in cardiac aldehydes, creatine kinase isoenzyme MB leakage, and protein carbonyl formation (P < .01). The Alda-1 group also obtained higher glutathione/oxidized glutathione ratios (P < .01). Aldehyde dehydrogenase-2 activation alleviated ischemia/reperfusion-induced cardiomyocyte contractile function impairment as evidenced by improved maximal velocity of pressure development and decline, left ventricular developed pressure, and heart rate (P < .01). Alda-1 supplementation can significantly improve the cardioprotection effect of cardioplegia solution, possibly through activation of aldehyde dehydrogenase-2, to remove toxic aldehydes. This may aid in the identification of novel cardioplegia solutions.

Regulation of the proteasome by ATP: implications for ischemic myocardial injury and donor heart preservation

Several lines of evidence suggest that proteasomes are involved in multiple aspects of myocardial physiology and pathology, including myocardial ischemia-reperfusion injury. It is well established that the 26S proteasome is an ATP-dependent enzyme and that ischemic heart disease is associated with changes in the ATP content of the cardiomyocyte. A functional link between the 26S proteasome, myocardial ATP concentrations, and ischemic cardiac injury, however, has been suggested only recently. This review discusses the currently available data on the pathophysiological role of the cardiac proteasome during ischemia and reperfusion in the context of the cellular ATP content. Depletion of the myocardial ATP content during ischemia appears to activate the 26S proteasome via direct regulatory effects of ATP on 26S proteasome stability and activity. This implies pathological degradation of target proteins by the proteasome and could provide a pathophysiological basis for beneficial effects of proteasome inhibitors in various models of myocardial ischemia. In contrast to that in the ischemic heart, reduced and impaired proteasome activity is detectable in the postischemic heart. The paradoxical findings that proteasome inhibitors showed beneficial effects when administered during reperfusion in some studies could be explained by their anti-inflammatory and immune suppressive actions, leading to reduction of leukocyte-mediated myocardial reperfusion injury. The direct regulatory effects of ATP on the 26S proteasome have implications for the understanding of the contribution of the 26S proteasome to the pathophysiology of the ischemic heart and its possible role as a therapeutic target.

Effects of 4-hydroxynonenal on vascular endothelial and smooth muscle cell redox signaling and function in health and disease

4-hydroxynonenal (HNE) is a lipid hydroperoxide end product formed from the oxidation of n-6 polyunsaturated fatty acids. The relative abundance of HNE within the vasculature is dependent not only on the rate of lipid peroxidation and HNE synthesis but also on the removal of HNE adducts by phase II metabolic pathways such as glutathione-S-transferases. Depending on its relative concentration, HNE can induce a range of hormetic effects in vascular endothelial and smooth muscle cells, including kinase activation, proliferation, induction of phase II enzymes and in high doses inactivation of enzymatic processes and apoptosis. HNE also plays an important role in the pathogenesis of vascular diseases such as atherosclerosis, diabetes, neurodegenerative disorders and in utero diseases such as pre-eclampsia. This review examines the known production, metabolism and consequences of HNE synthesis within vascular endothelial and smooth muscle cells, highlighting alterations in mitochondrial and endoplasmic reticulum function and their association with various vascular pathologies.

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HNE is a lipid peroxidation endproduct regulating vascular redox signaling.

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HNE detoxification is tightly regulated in vascular and other cell types.

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Elevated HNE levels are associated with various vascular diseases.

Oxidized lipids activate autophagy in a JNK-dependent manner by stimulating the endoplasmic reticulum stress response

Excessive production of unsaturated aldehydes from oxidized lipoproteins and membrane lipids is a characteristic feature of cardiovascular disease. Our previous studies show that unsaturated lipid peroxidation-derived aldehydes such as 4-hydroxy-trans-2-nonenal (HNE) promote autophagy in rat aortic smooth muscle cells (RASMC). In this study, we examined the mechanism by which HNE induces autophagy. Exposure of RASMC to HNE led to the modification of several proteins, most of which were identified by mass spectrometry and confocal microscopy to be localized to the endoplasmic reticulum (ER). HNE stimulated the phosphorylation of PKR-like ER kinase and eukaryotic initiation factor 2α and increased heme oxygenase-1 (HO-1) abundance. HNE treatment also increased LC3-II formation and the phosphorylation of JNK and p38. Pharmacological inhibition of JNK, but not p38, prevented HNE-induced HO-1 expression and LC3-II formation. Inhibition of JNK increased cell death in HNE-treated cells. Pretreatment with the chemical chaperone phenylbutryic acid prevented LC3-II formation as well as JNK phosphorylation and HO-1 induction. Taken together, these data suggest that autophagic responses triggered by unsaturated aldehydes could be attributed, in part, to ER stress, which stimulates autophagy by a JNK-dependent mechanism and promotes cell survival during oxidative stress.

Exercise training restores cardiac protein quality control in heart failure

Exercise training is a well-known coadjuvant in heart failure treatment; however, the molecular mechanisms underlying its beneficial effects remain elusive. Despite the primary cause, heart failure is often preceded by two distinct phenomena: mitochondria dysfunction and cytosolic protein quality control disruption. The objective of the study was to determine the contribution of exercise training in regulating cardiac mitochondria metabolism and cytosolic protein quality control in a post-myocardial infarction-induced heart failure (MI-HF) animal model. Our data demonstrated that isolated cardiac mitochondria from MI-HF rats displayed decreased oxygen consumption, reduced maximum calcium uptake and elevated H₂O₂ release. These changes were accompanied by exacerbated cardiac oxidative stress and proteasomal insufficiency. Declined proteasomal activity contributes to cardiac protein quality control disruption in our MI-HF model. Using cultured neonatal cardiomyocytes, we showed that either antimycin A or H₂O₂ resulted in inactivation of proteasomal peptidase activity, accumulation of oxidized proteins and cell death, recapitulating our in vivo model. Of interest, eight weeks of exercise training improved cardiac function, peak oxygen uptake and exercise tolerance in MI-HF rats. Moreover, exercise training restored mitochondrial oxygen consumption, increased Ca²⁺-induced permeability transition and reduced H₂O₂ release in MI-HF rats. These changes were followed by reduced oxidative stress and better cardiac protein quality control. Taken together, our findings uncover the potential contribution of mitochondrial dysfunction and cytosolic protein quality control disruption to heart failure and highlight the positive effects of exercise training in re-establishing cardiac mitochondrial physiology and protein quality control, reinforcing the importance of this intervention as a non-pharmacological tool for heart failure therapy.

The ubiquitin proteasome system and myocardial ischemia

The ubiquitin proteasome system (UPS) has been the subject of intensive research over the past 20 years to define its role in normal physiology and in pathophysiology. Many of these studies have focused in on the cardiovascular system and have determined that the UPS becomes dysfunctional in several pathologies such as familial and idiopathic cardiomyopathies, atherosclerosis, and myocardial ischemia. This review presents a synopsis of the literature as it relates to the role of the UPS in myocardial ischemia. Studies have shown that the UPS is dysfunctional during myocardial ischemia, and recent studies have shed some light on possible mechanisms. Other studies have defined a role for the UPS in ischemic preconditioning which is best associated with myocardial ischemia and is thus presented here. Very recent studies have started to define roles for specific proteasome subunits and components of the ubiquitination machinery in various aspects of myocardial ischemia. Lastly, despite the evidence linking myocardial ischemia and proteasome dysfunction, there are continuing suggestions that proteasome inhibitors may be useful to mitigate ischemic injury. This review presents the rationale behind this and discusses both supportive and nonsupportive studies and presents possible future directions that may help in clarifying this controversy.

The electrophile responsive proteome: integrating proteomics and lipidomics with cellular function

SIGNIFICANCE

The process of lipid peroxidation is emerging as an important mechanism that mediates the post-translational modification of proteins. Through advanced analytical techniques, lipidomics is now emerging as a critical factor in our understanding of the pathology of a broad range of diseases.

RECENT ADVANCES

During enzymatic or nonenzymatic lipid peroxidation, the simple structure of an unsaturated fatty acid is converted to an oxylipidome, many members of which are electrophilic and form the reactive lipid species (RLS). This aspect of lipid biology is particularly important, as it directly connects lipidomics with proteomics through the post-translational modification of a sub-proteome in the cell. This arises, because the electrophilic members of the oxylipidome react with proteins at nucleophilic amino-acid residues and so change their structure and function to form electrophile-responsive proteomes (ERP).

CRITICAL ISSUES

Biological systems have relatively few but well-defined and mechanistically distinct pro-oxidant pathways generating RLS. Defining the ERPs and the mechanisms underlying their formation and action has been a major focus for the field of lipidomics and redox signaling.

FUTURE DIRECTIONS

We propose that a unique oxylipidome can be defined for specific oxidants and will predict the biological responses through the reaction with proteins to form a specific ERP. In this review, we will describe the ERPs that modulate antioxidant and anti-inflammatory protective pathways, including the activation of Keap1/Nrf2 and the promotion of cell death through interactions with mitochondria.