Heme oxygenase 1 protects ethanol-administered liver tissue in Aldh2 knockout mice

Research Progress on the Correlation between Acetaldehyde Dehydrogenase 2 and Hepatocellular Carcinoma Development

Hepatocellular carcinoma (HCC) is the predominant pathologic type of primary liver cancer. It is a malignant tumor of liver epithelial cells. There are many ways to treat HCC, but the survival rate for HCC patients remains low. Therefore, understanding the underlying mechanisms by which HCC occurs and develops is critical to explore new therapeutic targets. Aldehyde dehydrogenase 2 (ALDH2) is an important player in the redox reaction of ethanol with endogenous aldehyde products released by lipid peroxidation. Increasing evidence suggests that ALDH2 is a crucial regulator of human tumor development, including HCC. Therefore, clarifying the relationship between ALDH2 and HCC is helpful for formulating rational treatment strategies. This review highlights the regulatory roles of ALDH2 in the development of HCC, elucidates the multiple potential mechanisms by which ALDH2 regulates the development of HCC, and summarizes the progress of research on ALDH2 gene polymorphisms and HCC susceptibility. Meanwhile, we envision viable strategies for targeting ALDH2 in the treatment of HCC SIGNIFICANCE STATEMENT: Numerous studies have aimed to explore novel therapeutic targets for HCC, and ALDH2 has been reported to be a critical regulator of HCC progression. This review discusses the functions, molecular mechanisms, and clinical significance of ALDH2 in the development of HCC and examines the prospects of ALDH2-based therapy for HCC.

Loss of ERdj5 exacerbates oxidative stress in mice with alcoholic liver disease via suppressing Nrf2

Alcoholic liver disease is the major cause of chronic liver diseases. Excessive alcohol intake results in endoplasmic reticulum (ER) stress. ERdj5, a member of DNAJ family, is an ER-resident chaperone protein, whose role in alcoholic liver disease remains to be investigated. In this study, we aim to address the effect of ERdj5 on alcoholic liver disease and the underlying mechanism. Hepatic Dnajc10 (ERdj5) mRNA expression was elevated in both human and mouse alcoholic hepatitis. In mice subjected to chronic and binge ethanol feeding, ERdj5 levels were also markedly increased. Hepatic Dnajc10 correlated with Xbp1s mRNA. Tunicamycin, an ER stress inducer, increased ERdj5 levels. Dnajc10 knockout mice exhibited exacerbated alcohol-induced liver injury and hepatic steatosis. However, the macrophage numbers and chemokine levels were similar to those in wild-type mice. Depletion of Dnajc10 promoted oxidative stress. Ethanol feeding increased hepatic H₂O₂ levels, and these were further increased in Dnajc10 knockout mice. Additionally, Dnajc10-deficient hepatocytes produced large amounts of reactive oxygen species. Notably, Nrf2, a central regulator of oxidative stress, was decreased by depletion of Dnajc10 in the nuclear fraction of ethanol-treated mouse liver. Consistently, liver tissues from ethanol-fed Dnajc10 knockout mice had reduced expression of downstream antioxidant genes. Furthermore, hepatic glutathione content in the liver of knockout mice declined compared to wild-type mice. In conclusion, our results demonstrate that ethanol-induced ERdj5 may regulate the Nrf2 pathway and glutathione contents, and have protective effects on liver damage and alcohol-mediated oxidative stress in mice. These suggest that ERdj5 has the potential to protect against alcoholic liver disease.

Engineered Animal Models Designed for Investigating Ethanol Metabolism, Toxicity and Cancer

Excessive consumption of alcohol is a leading cause of lifestyle-induced morbidity and mortality worldwide. Although long-term alcohol abuse has been shown to be detrimental to the liver, brain and many other organs, our understanding of the exact molecular mechanisms by which this occurs is still limited. In tissues, ethanol is metabolized to acetaldehyde (mainly by alcohol dehydrogenase and cytochrome p450 2E1) and subsequently to acetic acid by aldehyde dehydrogenases. Intracellular generation of free radicals and depletion of the antioxidant glutathione (GSH) are believed to be key steps involved in the cellular pathogenic events caused by ethanol. With continued excessive alcohol consumption, further tissue damage can result from the production of cellular protein and DNA adducts caused by accumulating ethanol-derived aldehydes. Much of our understanding about the pathophysiological consequences of ethanol metabolism comes from genetically-engineered mouse models of ethanol-induced tissue injury. In this review, we provide an update on the current understanding of important mouse models in which ethanol-metabolizing and GSH-synthesizing enzymes have been manipulated to investigate alcohol-induced disease.

Ethanol induces skin hyperpigmentation in mice with aldehyde dehydrogenase 2 deficiency

Alcohol induces various cutaneous changes, such as palmar erythema and jaundice. However, alcohol-induced skin hyperpigmentation due to melanin deposition has not been reported. Aldehyde dehydrogenase 2 (ALDH2), one of 19 human ALDH isozymes, metabolizes endogenous and exogenous aldehydes to their respective carboxylic acids. Reduced ALDH2 greatly affects acetaldehyde metabolism, leading to its accumulation in the body after the consumption of alcohol and the consequent development of a wide range of phenotypes. In the present study, we report a novel phenotype manifesting in a mouse model with the altered expression of ALDH2. Aldh2 knockout (Aldh2+/- and Aldh2-/-) and wild-type (Aldh2+/+) mice were fed a standard solid rodent chow and a bottle of ethanol solution at concentrations of 0%, 3%, 10%, or 20% (v/v) for more than 10 weeks. The intensity of their skin pigmentation was evaluated by macroscopic observation. Ethanol-exposed Aldh2+/- and Aldh2-/- mice exhibited dose-dependent skin pigmentation in areas of hairless skin, including the soles of the paws and tail; no such changes were observed in wild-type mice. The intensity of skin pigmentation correlated with the number of Aldh2 alleles that were altered in the mice (i.e., 0, 1 and 2 for Aldh2+/+, Aldh2+/-, Aldh2-/-, respectively). Interestingly, the skin pigmentation changes reversed upon the discontinuation of ethanol. The histological examination of the pigmented skin demonstrated the presence of melanin-like deposits, mainly in the epidermis. In conclusion, we report a novel finding that the intake of ethanol induces skin hyperpigmentation in an ALDH2 activity-dependent manner.

The Bidirectional Effect of Defective ALDH2 Polymorphism and Disease Prevention

Despite the role of aldehyde dehydrogenase 2 (ALDH2) in the detoxification of endogenous aldehydes, the defective polymorphism (rs671), which is highly prevalent among East Asians, does not show a serious phenotype, such as congenital abnormality. However, unfavorable and favorable impacts of the variant allele, ALDH2*2, on various disease risks have been reported. The underlying mechanisms are often complicated due to the compensatory aldehyde detoxification systems. As the phenotypes emerge due to overlapping environmental factors (e.g., alcohol intake and tobacco smoke) or individual vulnerabilities (e.g., aging and apolipoprotein E ε4 allele), polymorphism is therefore considered to be important in the field of preventative medicine. For example, it is important to recognize that ALDH2*2 carriers are at a high risk of alcohol drinking-related cancers; however, their drinking habit has less adverse effects on physiological indices, such as blood pressure, body mass index, levels of lipids, and hepatic deviation enzymes in the blood, than in non-ALDH2*2 carriers. Therefore, opportunities to reconsider their excessive drinking habit before adverse events occur can be missed. To perform effective disease prevention, the effects of ALDH2*2 on various diseases and the biological mechanisms should be clarified.

Aldehyde dehydrogenase 2 activation ameliorates CCl4 -induced chronic liver fibrosis in mice by up-regulating Nrf2/HO-1 antioxidant pathway

Mitochondrial aldehyde dehydrogenase 2 (ALDH2) is critical in the pathogenesis of alcoholic liver cirrhosis. However, the effect of ALHD2 on liver fibrosis remains to be further elucidated. This study aimed to demonstrate whether ALDH2 regulates carbon tetrachloride (CCl₄)‐induced liver fibrosis and to investigate the efficacy of Alda‐1, a specific activator of ALDH2, on attenuating liver fibrosis. ALDH2 expression was increased after chronic CCl₄ exposure. ALDH2 deficiency accentuated CCl₄‐induced liver fibrosis in mice, accompanied by increased expression of collagen 1α1, α‐SMA and TIMP‐1. Moreover, ALDH2 knockout triggered more ROS generation, hepatocyte apoptosis and impaired mitophagy after CCl₄ treatment. In cultured HSC‐T6 cells, ALDH2 knockdown by transfecting with lentivirus vector increased ROS generation and α‐SMA expression in an in vitro hepatocyte fibrosis model using TGF‐β1. ALDH2 overexpression by lentivirus or activation by Alda‐1 administration partly reversed the effect of TGF‐β1, whereas ALDH2 knockdown totally blocked the protective effect of Alda‐1. Furthermore, Alda‐1 administration protected against liver fibrosis in vivo, which might be mediated through up‐regulation of Nrf2/HO‐1 cascade and activation of Parkin‐related mitophagy. These findings indicate that ALDH2 deficiency aggravated CCl₄‐induced hepatic fibrosis through ROS overproduction, increased apoptosis and mitochondrial damage, whereas ALDH2 activation through Alda‐1 administration alleviated hepatic fibrosis partly through activation of the Nrf2/HO‐1 antioxidant pathway and Parkin‐related mitophagy, which indicate ALDH2 as a promising anti‐fibrotic target and Alda‐1 as a potential therapeutic agent in treating CCl₄‐induced liver fibrosis.

[Importance of an Aldehyde Dehydrogenase 2 Polymorphism in Preventive Medicine]

Unlike genetic alterations in other aldehyde dehydrogenase (ALDH) isozymes, a defective ALDH2 polymorphism (rs671), which is carried by almost half of East Asians, does not show a clear phenotype such as a shortened life span. However, impacts of a defective ALDH2 allele, ALDH2*2, on various disease risks have been reported. As ALDH2 is responsible for the detoxification of endogenous aldehydes, a negative effect of this polymorphism is predicted, but bidirectional effects have been actually observed and the mechanisms underlying such influences are often complex. One reason for this complexity may be the existence of compensatory aldehyde detoxification systems and the secondary effects of these systems. There are many issues to be addressed with regard to the ALDH2 polymorphism in the field of preventive medicine, including the following concerns. First, ALDH2 in the fetal stage plays a role in aldehyde detoxification; therefore, prenatal health effects of environmental aldehyde exposure are of concern for ALDH2*2-carrying fetuses. Second, ALDH2*2 carriers are at high risk of drinking-related cancers. However, their drinking habits result in less worsening of physiological findings, such as energy metabolism index and liver functions, compared with non-ALDH2*2 carriers, and therefore opportunities to detect excessive drinking can be lost. Third, personalized medicine such as personalized prescriptions for ALDH2*2 carriers will be required in the clinical setting, and accumulation of evidence is awaited. Lastly, since the ALDH2 polymorphism is not considered in workers' limits of exposure to aldehydes and their precursors, efforts to lower exposure levels beyond legal standards are required.

Amelioration of diet-induced steatohepatitis in mice following combined therapy with ASO-Fsp27 and fenofibrate

Nonalcoholic fatty liver disease (NAFLD) is the leading cause of chronic liver disease. NAFLD progresses from benign steatosis to steatohepatitis (NASH) to cirrhosis and is linked to hepatocellular carcinoma. No targeted treatment is currently approved for NAFLD/NASH. We previously showed that fat-specific protein 27 (FSP27), a lipid droplet-associated protein that controls triglyceride turnover in the hepatocyte, is required for fasting- and diet-induced triglyceride accumulation in the liver. However, silencing Fsp27 with antisense oligonucleotides (ASOs) did not improve hepatosteatosis in genetic nor nutritional mouse models of obesity. Herein, we tested the therapeutic potential of ASO-Fsp27 when used in combination with the PPARα agonist fenofibrate. C57BL/6 mice were fed a high-trans-fat, high-cholesterol, high-fructose diet for eight weeks to establish NASH, then kept on diet for six additional weeks while dosed with ASOs and fenofibrate, alone or in combination. Data show that ASO-Fsp27 and fenofibrate synergize to promote resistance to diet-induced obesity and hypertriglyceridemia and to reverse hepatic steatosis, inflammation, oxidative stress, and fibrosis. This multifactorial improvement of liver disease noted when combining both drugs suggests that a course of treatment that includes both reduced FSP27 activity and activation of PPARα could provide therapeutic benefit to patients with NAFLD/NASH.

Roles of defective ALDH2 polymorphism on liver protection and cancer development

Because serum transaminases elevate alcohol dose dependently as a consequence of liver injury, they serve as useful biological markers of excessive drinking. However, these markers are inadequate in individuals with a defective allele of the aldehyde dehydrogenase 2 gene, ALDH2*2, because they show a different correlation with the amount of ethanol. For example, the serum alanine aminotransferase (ALT) level could become even lower than the baseline after alcohol intake in ALDH2*2 carriers. In fact, multiple studies suggest that ALDH2*2 is a hepato-protective factor in healthy individuals. Importantly, excessive drinking is particularly dangerous in carriers of ALDH2*2 because the risk of alcohol-related cancer is much higher than that for ALDH2*1/*1 carriers. Without recognizing the genotype interaction on serum transaminase, the opportunity to warn people about potential cancer risks is missed owing to incorrect interpretation. This is particularly important in East Asian countries where approximately half of the population carries the ALDH2*2 allele. To date, the mechanism of liver protection from ethanol load in individuals with ALDH2*2 has not been fully elucidated. However, some reasonable mechanisms have been suggested by experimental studies, including remodelling of detoxifying systems. Further studies to uncover the whole mechanism are anticipated.