A malfunction in triglyceride transfer from the intracellular lipid pool to apoB in enterocytes of SOD1-deficient mice. FEBS Lett. 2012;586:4289-4295. [PMID: 23098755 DOI: 10.1016/j.febslet.2012.09.047]

Defective biosynthesis of ascorbic acid in Sod1-deficient mice results in lethal damage to lung tissue

Superoxide dismutase 1 (Sod1) plays pivotal roles in antioxidation via accelerating the conversion of superoxide anion radicals into hydrogen peroxide, thus inhibiting the subsequent radical chain reactions. While Sod1 deficient cells inevitably undergo death in culture conditions, Sod1-knockout (KO) mice show relatively mild phenotypes and live approximately two years. We hypothesized that the presence of abundant levels of ascorbic acid (AsA), which is naturally produced in mice, contributes to the elimination of reactive oxygen species (ROS) in Sod1-KO mice. To verify this hypothesis, we employed mice with a genetic ablation of aldehyde reductase (Akr1a), an enzyme that is involved in the biosynthesis of AsA, and established double knockout (DKO) mice that lack both Sod1 and Akr1a. Supplementation of AsA (1.5 mg/ml in drinking water) was required for the DKO mice to breed, and, upon terminating the AsA supplementation, they died within approximately two weeks regardless of age or gender. We explored the etiology of the death from pathophysiological standpoints in principal organs of the mice. Marked changes were observed in the lungs in the form of macroscopic damage after the AsA withdrawal. Histological and immunological analyses of the lungs indicated oxidative damage of tissue and activated immune responses. Thus, preferential oxidative injury that occurred in pulmonary tissues appeared to be primary cause of the death in the mice. These collective results suggest that the pivotal function of AsA in coping with ROS in vivo, is largely in pulmonary tissues that are exposed to a hyperoxygenic microenvironment.

Research status and prgoress of nonalcoholic fatty pancreatic disease

Nonalcoholic fatty pancreatic disease (NAFPD) is a disease characterized by an increase in pancreatic fat accumulation. It is a component of the metabolic syndrome and often coexists with nonalcoholic fatty liver disease. Once the diagnosis is established, it is closely related to acute and chronic pancreatitis, type 2 diabetes mellitus, pancreatic fibrosis, and pancreatic cancer. In recent years, it has been confirmed that NAFPD is closely related to cardiovascular disease, liver fibrosis, and liver cancer. The prevalence of NAFPD ranges between 11% and 69%, and increases with age. It is worth noting that the prevalence in obese children is twice as high as that in non-obese children. The high prevalence rate and complexity of the disease have aroused people's high attention. Therefore, to improve the understanding of NAFPD, fully understand the clinical significance of NAFPD, and further study its pathogenesis, diagnosis, and treatment require the collaboration and joint efforts of multiple disciplines, including hepatopathy, gastroenterology, endocrine metabolism, cardiovascular disease, imaging, pathology, and others. In this paper, we review the clinical significance, pathogenesis, and imaging diagnosis of NAFPD and propose our personal understanding of the key points in future research.

Effects of Vitamin E and Selenium on Growth Performance, Antioxidant Capacity, and Metabolic Parameters in Growing Furring Blue Foxes (Alopex lagopus)

The objective of this study was to determine whether different dietary vitamin E (VE) and selenium (Se) levels affect the nutrient digestibility, production performance, and antioxidant abilities of growing furring blue foxes. A 4 × 2 factorial arrangement that included 4 levels of VE (0, 100, 200, or 400 mg/kg diet from α-tocopherol acetate) and 2 levels of Se (0 or 0.2 mg/kg diet from glycine selenium) was performed from mid-September to pelting. A metabolism study was conducted for four days starting at the 30th day of the trial. Serum samples were collected at the last day of the study. The results showed that supplementation of growing furring blue fox diets with VE and Se significantly affected the average daily gain (ADG), average daily feed intake, and feed conversion ratio (F:G) (P < 0.05). Dietary Se supplementation enhanced protein and fat digestibility of male blue foxes. There were significant effects of different VE and Se levels in diets on serum antioxidant parameters and metabolic parameters of blue foxes (P < 0.05). In conclusion, this research indicated that dietary supplementation with VE improved ADG and F:G of blue foxes. Addition of VE and Se to blue fox diets increased the antioxidant capacity of blue foxes. The diet with high VE and Se supplementation reduced glucose and triglycerides concentrations in serum. The present study found that growing furring blue foxes had increased growth performance and antioxidant abilities when fed diets with 200 mg VE/kg and nearly 0.1 mg Se/kg.

Mutual interaction between oxidative stress and endoplasmic reticulum stress in the pathogenesis of diseases specifically focusing on non-alcoholic fatty liver disease

Reactive oxygen species (ROS) are produced during normal physiologic processes with the consumption of oxygen. While ROS play signaling roles, when they are produced in excess beyond normal antioxidative capacity this can cause pathogenic damage to cells. The majority of such oxidation occurs in polyunsaturated fatty acids and sulfhydryl group in proteins, resulting in lipid peroxidation and protein misfolding, respectively. The accumulation of misfolded proteins in the endoplasmic reticulum (ER) is enhanced under conditions of oxidative stress and results in ER stress, which, together, leads to the malfunction of cellular homeostasis. Multiple types of defensive machinery are activated in unfolded protein response under ER stress to resolve this unfavorable situation. ER stress triggers the malfunction of protein secretion and is associated with a variety of pathogenic conditions including defective insulin secretion from pancreatic β-cells and accelerated lipid droplet formation in hepatocytes. Herein we use nonalcoholic fatty liver disease (NAFLD) as an illustration of such pathological liver conditions that result from ER stress in association with oxidative stress. Protecting the ER by eliminating excessive ROS via the administration of antioxidants or by enhancing lipid-metabolizing capacity via the activation of peroxisome proliferator-activated receptors represent promising therapeutics for NAFLD.

Unveiling systemic organ disorders associated with impaired lipid catabolism in fasted SOD1-deficient mice

Oxidative stress triggers the formation of lipid droplets in the liver by stimulating lipogenesis and simultaneously suppresses lipoprotein secretion under hypernutritional conditions. Herein we report on the observation of systemic organ failure that is associated with lipid droplet accumulation in fasting, SOD1-knockout (KO) mice. Upon a three-day fasting period, the KO mice were observed to be vulnerable, could not be rescued by refeeding and had largely died, while wild-type mice were totally recovered. Visceral fat was rapidly consumed during fasting, which resulted in energy shortage and increased fatality in the KO mice. Lipid droplets had accumulated and continued to remain in KO mouse organs that routinely catalyze fatty acids via β-oxidation, even though the levels of free fatty acids and β-hydroxybutyrate, a ketone body, in blood plasma were less in KO mice compared to WT mice during the fasting period. The fasting-triggered organ failure in the KO mice was effectively mitigated by feeding a high calorie-diet for 2 weeks prior to fasting, even though the mice had an excessive accumulation of lipid droplets in the liver. These collective data suggest that the lipid-catabolizing system is the sensitive target of oxidative stress triggered by fasting conditions in the KO mice.

Double Knockout of Peroxiredoxin 4 (Prdx4) and Superoxide Dismutase 1 (Sod1) in Mice Results in Severe Liver Failure

Mice that are deficient in superoxide dismutase 1 (Sod1), an antioxidative enzyme, are susceptible to developing liver steatosis. Peroxiredoxin 4 (Prdx4) catalyzes disulfide bond formation in proteins via the action of hydrogen peroxide and hence decreases oxidative stress and supports oxidative protein folding for the secretion of lipoproteins. Because elevated reactive oxygen species induce endoplasmic reticulum stress, this negative chain reaction is likely involved in the development of nonalcoholic fatty liver diseases and more advanced steatohepatitis (NASH). In the current study, we generated Prdx4 and Sod1 double knockout (DKO; Prdx4^-/ySod1^-/-) mice and examined whether the combined deletion of Prdx4 and Sod1 aggravated liver pathology compared to single knockout and wild-type mice. The secretion of triglyceride-rich lipoprotein was strikingly impaired in the DKO mice, leading to aggravated liver steatosis. Simultaneously, the activation of caspase-3 in the liver was observed. The hyperoxidation of Prdxs, a hallmark of oxidative stress, occurred in different isoforms that are uniquely associated with Sod1^-/- and Prdx4^-/y mice, and the effect was additive in DKO mouse livers. Because DKO mice spontaneously develop severe liver failure at a relatively young stage, they have the potential for use as a model for hepatic disorders and for testing other potential treatments.

Gastrointestinal factors regulating lipid droplet formation in the intestine

Cytoplasmic lipid droplets (CLD) are considered as neutral lipid reservoirs, which protect cells from lipotoxicity. It became clear that these fascinating dynamic organelles play a role not only in energy storage and metabolism, but also in cellular lipid and protein handling, inter-organelle communication, and signaling among diverse functions. Their dysregulation is associated with multiple disorders, including obesity, liver steatosis and cardiovascular diseases. The central aim of this review is to highlight the link between intra-enterocyte CLD dynamics and the formation of chylomicrons, the main intestinal dietary lipid vehicle, after overviewing the morphology, molecular composition, biogenesis and functions of CLD.

A high-fat diet temporarily renders Sod1-deficient mice resistant to an oxidative insult

Patients with nonalcoholic fatty liver disease may subsequently develop nonalcoholic steatohepatitis after suffering from a second insult, such as oxidative stress. Aim of this study was to investigate the pathogenesis of the liver injury caused when lipids accumulate under conditions of intrinsic oxidative stress using mice that are deficient in superoxide dismutase 1 (SOD1) and the leptin receptor (Lepr). We established Sod1^-/-::Lepr^db/db mice and carried out analyses of four groups of genetically modified mice, namely, wild type, Sod1^-/-, Lepr^db/db and Sod1^-/-::Lepr^db/db mice. Mice with defects in the SOD1 or Lepr gene are vulnerable to developing fatty livers, even when fed a normal diet. Feeding a high-fat diet (HFD) caused an increase in the number of lipid droplets in the liver to different extents in each genotypic mouse. an HFD caused the accelerated death of db/db mice, but contradictory to our expectations, the death rates for the Sod1-deficient mice were decreased by feeding HFD. Consistent with the improved probability of survival, liver damage was significantly ameliorated by feeding an HFD compared to a normal diet in the mice with an Sod1-deficient background. Oxidative stress markers, hyperoxidized peroxiredoxin and lipid peroxidation products, were decreased somewhat in Sod1^-/- mice by feeding HFD. We conclude that lipids reacted with reactive oxygen species and eliminated them in the livers of the young mice, which resulted in the alleviation of oxidative stress, but in advanced age oxidized products accumulated, leading to the aggravation of the liver injury and an increase in fatality rate.

Oxidative stress caused by a SOD1 deficiency ameliorates thioacetamide-triggered cell death via CYP2E1 inhibition but stimulates liver steatosis

We investigated the responses of mice that are defective in the superoxide-scavenging enzyme SOD1 to thioacetamide (TAA)-induced hepatotoxicity. When a lethal dose of TAA (500 mg/kg) was intraperitoneally injected, the wild-type (WT) mice all died within 36 h, but all of the SOD1-knockout (KO) mice survived. Treatment with an SOD1 inhibitor rendered the WT mice resistant to TAA toxicity. To elucidate the mechanism responsible for this, we examined the acute effects of a sublethal dose of TAA (200 mg/kg) on the livers of WT and KO mice. The extent of TAA-induced liver damage was less in the KO mice, but, instead, lipogenesis was further advanced in the SOD1-KO livers. The levels of proteins modified with acetyllysine, a marker for TAA-mediated injury, were lower in the KO mice than the WT mice upon the TAA treatment. The KO mice, which were under oxidative stress per se, exhibited a lower CYP2E1 activity, and this appeared to result in a decrease in the production of reactive oxygen species (ROS) during TAA metabolism. Both cleaved ATF6, a transcriptional regulator that is activated by endoplasmic reticulum (ER) stress, and CHOP, a death signal mediator, were highly elevated in the WT mice as the result of the TAA treatment and consistent with the liver damage. We conclude that elevated TAA metabolites and reactive oxygen species that are produced by CYP-mediated drug metabolism trigger lipogenesis as well as liver damage via ER stress and determine the fate of the mice.

The SOD1 transgene expressed in erythroid cells alleviates fatal phenotype in congenic NZB/NZW-F1 mice

Oxidative stress due to a superoxide dismutase 1 (SOD1) deficiency causes anemia and autoimmune responses, which are phenotypically similar to autoimmune hemolytic anemia (AIHA) and systemic lupus erythematosus (SLE) in C57BL/6 mice and aggravates AIHA pathogenesis in New Zealand black (NZB) mice. We report herein on an evaluation of the role of reactive oxygen species (ROS) in a model mouse with inherited SLE, that is, F1 mice of the NZB × New Zealand white (NZW) strain. The ROS levels within red blood cells (RBCs) of the F1 mice were similar to the NZW mice but lower compared to the NZB mice throughout adult period. Regarding SLE pathogenesis, we examined the effects of an SOD1 deficiency or the overexpression of human SOD1 in erythroid cells by establishing corresponding congenic F1 mice. A SOD1 deficiency caused an elevation in ROS production, methemoglobin content, and hyperoxidation of peroxiredoxin in RBC of the F1 mice, which were all consistent with elevated oxidative stress. However, while the overexpression of human SOD1 in erythroid cells extended the life span of the congenic F1 mice, the SOD1 deficiency had no effect on life span compared to wild-type F1 mice. It is generally recognized that NZW mice possess a larval defect in the immune system and that NZB mice trigger an autoimmune reaction in the F1 mice. Our results suggest that the oxidative insult originated from the NZB mouse background has a functional role in triggering an aberrant immune reaction, leading to fatal responses in F1 mice.

Recent discoveries on absorption of dietary fat: Presence, synthesis, and metabolism of cytoplasmic lipid droplets within enterocytes

Dietary fat provides essential nutrients, contributes to energy balance, and regulates blood lipid concentrations. These functions are important to health, but can also become dysregulated and contribute to diseases such as obesity, diabetes, cardiovascular disease, and cancer. Within enterocytes, the digestive products of dietary fat are re-synthesized into triacylglycerol, which is either secreted on chylomicrons or stored within cytoplasmic lipid droplets (CLDs). CLDs were originally thought to be inert stores of neutral lipids, but are now recognized as dynamic organelles that function in multiple cellular processes in addition to lipid metabolism. This review will highlight recent discoveries related to dietary fat absorption with an emphasis on the presence, synthesis, and metabolism of CLDs within this process.

Overexpression of Peroxiredoxin 4 Affects Intestinal Function in a Dietary Mouse Model of Nonalcoholic Fatty Liver Disease

Background

Accumulating evidence has shown that methionine- and choline-deficient high fat (MCD+HF) diet induces the development of nonalcoholic fatty liver disease (NAFLD), in which elevated reactive oxygen species play a crucial role. We have reported that peroxiredoxin 4 (PRDX4), a unique secretory member of the PRDX antioxidant family, protects against NAFLD progression. However, the detailed mechanism and potential effects on the intestinal function still remain unclear.

Methods & Results

Two weeks after feeding mice a MCD+HF diet, the livers of human PRDX4 transgenic (Tg) mice exhibited significant suppression in the development of NAFLD compared with wild-type (WT) mice. The serum thiobarbituric acid reactive substances levels were significantly lower in Tg mice. In contrast, the Tg small intestine with PRDX4 overexpression showed more suppressed shortening of total length and villi height, and more accumulation of lipid in the jejunum, along with lower levels of dihydroethidium binding. The enterocytes exhibited fewer apoptotic but more proliferating cells, and inflammation was reduced in the mucosa. Furthermore, the small intestine of Tg mice had significantly higher expression of cholesterol absorption-regulatory factors, including liver X receptor-α, but lower expression of microsomal triglyceride-transfer protein.

Conclusion

Our present data provide the first evidence of the beneficial effects of PRDX4 on intestinal function in the reduction of the severity of NAFLD, by ameliorating oxidative stress-induced local and systemic injury. We can suggest that both liver and intestine are spared, to some degree, by the antioxidant properties of PRDX4.

An SOD1 deficiency enhances lipid droplet accumulation in the fasted mouse liver by aborting lipophagy

Under normal feeding conditions, oxidative stress stimulates lipid droplets accumulation in hepatocytes. We found that, despite the low visceral fat in Sod1-knockout (KO) mouse, lipid droplets accumulate in the liver to a greater extent than for the wild-type mouse upon fasting. Liver damage became evident in the KO mice. While fasting caused substantial endoplasmic reticulum stress in KO mice, the expression of genes involved in fatty acid production was suppressed. LC3-II, which is essential for the dynamic process of autophagosome formation, was activated in the wild-type mouse and enhanced in the KO mouse. However, the p62, an adapter protein with the ubiquitin- and LC3-binding activity, accumulated abnormally in the livers of KO mice, implying an abortive lipophagic process as the cause for the impaired lipid metabolism and the hepatic damage that occurs upon fasting.

Oxidative stress triggers lipid droplet accumulation in primary cultured hepatocytes by activating fatty acid synthesis

Despite the impaired intestinal lipid absorption and low level of visceral fat, the Sod1-deficient mouse is susceptible to developing liver steatosis. To gain insights into the mechanism responsible for this abnormal lipid metabolism, we analyzed primary cultured hepatocytes obtained from Sod1-deficient and wild-type mice. Lipid droplets began to accumulate in the cultured hepatocytes and was further increased by a Sod1 deficiency. Levels of enzymes involved in lipogenesis were elevated. It thus appears that lipogenesis is activated by oxidative stress, which is more prominent in the case of Sod1 deficiency, and appears to participate in liver steatosis.

Mining the genome for lipid genes

Mining of the genome for lipid genes has since the early 1970s helped to shape our understanding of how triglycerides are packaged (in chylomicrons), repackaged (in very low density lipoproteins; VLDL), and hydrolyzed, and also how remnant and low-density lipoproteins (LDL) are cleared from the circulation. Gene discoveries have also provided insights into high-density lipoprotein (HDL) biogenesis and remodeling. Interestingly, at least half of these key molecular genetic studies were initiated with the benefit of prior knowledge of relevant proteins. In addition, multiple important findings originated from studies in mouse, and from other types of non-genetic approaches. Although it appears by now that the main lipid pathways have been uncovered, and that only modulators or adaptor proteins such as those encoded by LDLRAP1, APOA5, ANGPLT3/4, and PCSK9 are currently being discovered, genome wide association studies (GWAS) in particular have implicated many new loci based on statistical analyses; these may prove to have equally large impacts on lipoprotein traits as gene products that are already known. On the other hand, since 2004 - and particularly since 2010 when massively parallel sequencing has become de rigeur - no major new insights into genes governing lipid metabolism have been reported. This is probably because the etiologies of true Mendelian lipid disorders with overt clinical complications have been largely resolved. In the meantime, it has become clear that proving the importance of new candidate genes is challenging. This could be due to very low frequencies of large impact variants in the population. It must further be emphasized that functional genetic studies, while necessary, are often difficult to accomplish, making it hazardous to upgrade a variant that is simply associated to being definitively causative. Also, it is clear that applying a monogenic approach to dissect complex lipid traits that are mostly of polygenic origin is the wrong way to proceed. The hope is that large-scale data acquisition combined with sophisticated computerized analyses will help to prioritize and select the most promising candidate genes for future research. We suggest that at this point in time, investment in sequence technology driven candidate gene discovery could be recalibrated by refocusing efforts on direct functional analysis of the genes that have already been discovered. This article is part of a Special Issue entitled: From Genome to Function.

Genetics of oxidative stress in obesity

Obesity is a multifactorial disease characterized by the excessive accumulation of fat in adipose tissue and peripheral organs. Its derived metabolic complications are mediated by the associated oxidative stress, inflammation and hypoxia. Oxidative stress is due to the excessive production of reactive oxygen species or diminished antioxidant defenses. Genetic variants, such as single nucleotide polymorphisms in antioxidant defense system genes, could alter the efficacy of these enzymes and, ultimately, the risk of obesity; thus, studies investigating the role of genetic variations in genes related to oxidative stress could be useful for better understanding the etiology of obesity and its metabolic complications. The lack of existing literature reviews in this field encouraged us to gather the findings from studies focusing on the impact of single nucleotide polymorphisms in antioxidant enzymes, oxidative stress-producing systems and transcription factor genes concerning their association with obesity risk and its phenotypes. In the future, the characterization of these single nucleotide polymorphisms (SNPs) in obese patients could contribute to the development of controlled antioxidant therapies potentially beneficial for the treatment of obesity-derived metabolic complications.