Proteomic Analysis of Mitochondria-Enriched Fraction Isolated from the Frontal Cortex and Hippocampus of Apolipoprotein E Knockout Mice Treated with Alda-1, an Activator of Mitochondrial Aldehyde Dehydrogenase (ALDH2)

Functional diversity of apolipoprotein E: from subcellular localization to mitochondrial function

Human apolipoprotein E (APOE), originally known for its role in lipid metabolism, is polymorphic with three major allele forms, namely, APOEε2, APOEε3, and APOEε4, leading to three different human APOE isoforms. The ε4 allele is a genetic risk factor for Alzheimer's disease (AD); therefore, the vast majority of APOE research focuses on its role in AD pathology. However, there is increasing evidence for other functions of APOE through the involvement in other biological processes such as transcriptional regulation, mitochondrial metabolism, immune response, and responsiveness to dietary factors. Therefore, the aim of this review is to provide an overview of the potential novel functions of APOE and their characterization. The detection of APOE in various cell organelles points to previously unrecognized roles in mitochondria and others, although it is actually considered a secretory protein. Furthermore, numerous interactions of APOE with other proteins have been detected, providing indications for new metabolic pathways involving APOE. The present review summarizes the current evidence on APOE beyond its original role in lipid metabolism, to change the perspective and encourage novel approaches to future research on APOE and its isoform-dependent role in the cellular metabolism.

Activation of TGR5 with INT-777 attenuates oxidative stress and neuronal apoptosis via cAMP/PKCε/ALDH2 pathway after subarachnoid hemorrhage in rats

BACKGROUND

Oxidative stress and neuronal apoptosis play important roles in the pathogenesis of early brain injury (EBI) after subarachnoid hemorrhage (SAH). The activation of TGR5, a novel membrane-bound bile acid receptor, possesses anti-oxidative stress and anti-apoptotic effects in hepatobiliary disease and kidney disease. The present study aimed to explore the neuroprotective effect of TGR5 activation against EBI after SAH and the potential underlying mechanisms.

METHODS

The endovascular perforation model of SAH was performed on 199 Sprague Dawley rats to investigate the beneficial effects of TGR5 activation after SAH. INT-777, a specific synthetic TGR5 agonist, was administered intranasally at 1 h after SAH induction. TGR5 CRISPR and ALDH2 CRISPR were administered intracerebroventricularly at 48 h before SAH to illuminate potential mechanisms. The SAH grade, short-term and long-term neurobehavioral tests, TUNEL staining, Fluoro-Jade C staining, Nissl staining, immunofluorescence staining, and western blots were performed at 24 h after SAH.

RESULTS

The expressions of endogenous TGR5 and ALDH2 gradually increased and peaked at 24 h after SAH. TGR5 was expressed primarily in neurons, as well as in astrocytes and microglia. The activation of TGR5 with INT-777 significantly improved the short-term and long-term neurological deficits, accompanied by reduced the oxidative stress and neuronal apoptosis at 24 h after SAH. Moreover, INT-777 treatment significantly increased the expressions of TGR5, cAMP, phosphorylated PKCε, ALDH2, HO-1, and Bcl-2, while downregulated the expressions of 4-HNE, Bax, and Cleaved Caspase-3. TGR5 CRISPR and ALDH2 CRISPR abolished the neuroprotective effects of TGR5 activation after SAH.

CONCLUSIONS

In summary, the activation of TGR5 with INT-777 attenuated oxidative stress and neuronal apoptosis via the cAMP/PKCε/ALDH2 signaling pathway after SAH in rats. Furthermore, TGR5 may serve as a novel therapeutic target to ameliorate EBI after SAH.

Role of Aldehyde Dehydrogenases in Physiopathological Processes

Many different diseases are associated with oxidative stress. One of the main consequences of oxidative stress at the cellular level is lipid peroxidation, from which toxic aldehydes may be generated. Below their toxicity thresholds, some aldehydes are involved in signaling processes, while others are intermediaries in the metabolism of lipids, amino acids, neurotransmitters, and carbohydrates. Some aldehydes ubiquitously distributed in the environment, such as acrolein or formaldehyde, are extremely toxic to the cell. On the other hand, aldehyde dehydrogenases (ALDHs) are able to detoxify a wide variety of aldehydes to their corresponding carboxylic acids, thus helping to protect from oxidative stress. ALDHs are located in different subcellular compartments such as cytosol, mitochondria, nucleus, and endoplasmic reticulum. The aim of this review is to analyze, and highlight, the role of different ALDH isoforms in the detoxification of aldehydes generated in processes that involve high levels of oxidative stress. The ALDH physiological relevance becomes evident by the observation that their expression and activity are enhanced in different pathologies that involve oxidative stress such as neurodegenerative disorders, cardiopathies, atherosclerosis, and cancer as well as inflammatory processes. Furthermore, ALDH mutations bring about several disorders in the cell. Thus, understanding the mechanisms by which these enzymes participate in diverse cellular processes may lead to better contend with the damage caused by toxic aldehydes in different pathologies by designing modulators and/or protocols to modify their activity or expression.

The potential of aldehyde dehydrogenase 2 as a therapeutic target in cardiovascular disease

INTRODUCTION

Mitochondrial aldehyde dehydrogenase (ALDH-2) plays a major role in the ethanol detoxification pathway by removing acetaldehyde. Therefore, ALDH-2 inhibitors such as disulfiram represent the first therapeutic targeting of ALDH-2 for alcoholism therapy. Areas covered: Recently, ALDH-2 was identified as an essential bioactivating enzyme of the anti-ischemic organic nitrate nitroglycerin, bringing ALDH-2 again into the focus of clinical interest. Mechanistic studies on the nitroglycerin bioactivation process revealed that during bioconversion of nitroglycerin and in the presence of reactive oxygen and nitrogen species the active site thiols of ALDH-2 are oxidized and the enzyme activity is lost. Thus, ALDH-2 activity represents a useful marker for cardiovascular oxidative stress, a concept, which has been meanwhile supported by a number of animal disease models. Mechanistic studies on the protective role of ALDH-2 in different disease processes identified the detoxification of 4-hydroxynonenal by ALDH-2 as a fundamental process of cardiovascular, cerebral and antioxidant protection. Expert opinion: The most recent therapeutic exploitation of ALDH-2 includes activators of the enzyme such as Alda-1 but also cell-based therapies (ALDH-bright cells) that deserve further clinical characterization in the future.

Pathophysiology of mitochondrial lipid oxidation: Role of 4-hydroxynonenal (4-HNE) and other bioactive lipids in mitochondria

Mitochondrial lipids are essential for maintaining the integrity of mitochondrial membranes and the proper functions of mitochondria. As the "powerhouse" of a cell, mitochondria are also the major cellular source of reactive oxygen species (ROS). Oxidative stress occurs when the antioxidant system is overwhelmed by overproduction of ROS. Polyunsaturated fatty acids in mitochondrial membranes are primary targets for ROS attack, which may lead to lipid peroxidation (LPO) and generation of reactive lipids, such as 4-hydroxynonenal. When mitochondrial lipids are oxidized, the integrity and function of mitochondria may be compromised and this may eventually lead to mitochondrial dysfunction, which has been associated with many human diseases including cancer, cardiovascular diseases, diabetes, and neurodegenerative diseases. How mitochondrial lipids are oxidized and the underlying molecular mechanisms and pathophysiological consequences associated with mitochondrial LPO remain poorly defined. Oxidation of the mitochondria-specific phospholipid cardiolipin and generation of bioactive lipids through mitochondrial LPO has been increasingly recognized as an important event orchestrating apoptosis, metabolic reprogramming of energy production, mitophagy, and immune responses. In this review, we focus on the current understanding of how mitochondrial LPO and generation of bioactive lipid mediators in mitochondria are involved in the modulation of mitochondrial functions in the context of relevant human diseases associated with oxidative stress.

The influence of AICAR - direct activator of AMP-activated protein kinase (AMPK) - on liver proteome in apoE-knockout mice

There is a growing body of evidence that altered functioning of apoE may aggravate cellular energy homeostasis and stress response, leading to oxidative stress, mitochondrial dysfunction, endoplasmic reticulum (ER) stress and inflammation, leading to hypercholesterolemia, dyslipidemia, liver steatosis and neurodegeneration. One of the key cellular responses to mitochondria and ER-stress related processes and cellular energy imbalance is AMP-activated protein kinase (AMPK), considered as a cellular master energy sensor and critical regulator of mitochondrial homeostasis. The aim of our study was to use differential proteomics and transcriptomics approach to elucidate the effect of direct AMPK activator AICAR on liver proteome in apoE^-/- mice - experimental model of atherosclerosis and moderate nonalcoholic steatosis. We applied Isobaric Tags for Relative and Absolute Quantitation (iTRAQ) labeling and two-dimensional chromatography coupled with mass spectrometry (2DLC-MS/MS) MudPIT strategy, as well as RT-PCR to investigate the changes in mitochondrial and cytosolic proteins and transcripts expression in 6-month old AICAR-treated apoE^-/-. AICAR elicited induction of proteins related to mitochondrial β-oxidation, protein degradation and energy producing pathways (i.a. tricarboxylic acid cycle members and mitochondrial adenylate kinase 2). On the other hand, AICAR repressed inflammatory and pro-apoptotic markers in the apoE^-/- mice liver, alongside reduction in several peroxisomal proteins, possibly suggesting induction of anti-oxidative pexophagy.