Aging exacerbates high-fat diet-induced steatohepatitis through alteration in hepatic lipid metabolism in mice

Recent advances on the physiological and pathophysiological roles of polyunsaturated fatty acids and their biosynthetic pathway

Polyunsaturated fatty acids (PUFAs)-fatty acids containing multiple double bonds within their carbon chain-are an indispensable component of the cell membrane. PUFAs, including the omega-6 PUFA arachidonic acid (ARA; C20:4n-6) and the omega-3 PUFAs eicosapentaenoic acid (EPA; C20:5n-3) and docosahexaenoic acid (DHA; C22:6n-3), have been implicated in various (patho)physiological events. These PUFAs are either obtained from the diet or biosynthesized from the essential fatty acids linoleic acid (LA; C18:2n-6) and α-linolenic acid (ALA; C18:3n-3) via enzymatic reactions that are catalyzed by fatty acid elongases (ELOVL2 and ELOVL5) and fatty acid desaturases (FADS1 and FADS2). In this review, we summarize the recent literature studying the role of PUFAs, placing a special emphasis on the newly discovered functions of PUFAs and their biosynthetic pathway as revealed by studies using animal models targeting the PUFA biosynthetic pathway and genetic approaches including genome-wide association studies.

Multi-omics reveals aging-related pathway in natural aging mouse liver

Aging is associated with gradual changes in liver structure, altered metabolites and other physiological/pathological functions in hepatic cells. However, its characterized phenotypes based on altered metabolites and the underlying biological mechanism are unclear. Advancements in high-throughput omics technology provide new opportunities to understand the pathological process of aging. Here, in our present study, both metabolomics and phosphoproteomics were applied to identify the altered metabolites and phosphorylated proteins in liver of young (the WTY group) and naturally aged (the WTA group) mice, to find novel biomarkers and pathways, and uncover the biological mechanism. Analysis showed that the body weights, alanine aminotransferase (ALT) and aspartate aminotransferase (AST) increased in the WTA group. The grips decreased with age, while the triglyceride (TG) and cholesterol (TC) did not change significantly. The increase of fibrosis, accumulation of inflammatory cells, hepatocytes degeneration, the deposition of lipid droplets and glycogen, the damaged mitochondria, and deduction of endoplasmic reticulum were observed in the aging liver under optical and electron microscopes. In addition, a network of metabolites and phosphorylated proteomes of the aging liver was established. Metabolomics detected 970 metabolites in the positive ion mode and 778 metabolites in the negative ion mode. A total of 150 pathways were pooled. Phosphoproteomics identified 2618 proteins which contained 16621 phosphosites. A total of 164 pathways were detected. 65 common pathways were detected in two omics. Phosphorylated protein heat shock protein HSP 90-alpha (HSP90A) and v-raf murine viral oncogene homolog B1(BRAF), related to cancer pathway, were significantly upregulated in aged mice liver. Western blot verified that protein expression of MEK and ERK, downstream of BRAF pathway were elevated in the liver of aging mice. However, the protein expression of BRAF was not a significant difference. Overall, these findings revealed a close link between aging and cancer and contributed to our understanding of the multi-omics changes in natural aging.

The Roles of Lipid Metabolism in the Pathogenesis of Chronic Diseases in the Elderly

Lipid metabolism plays crucial roles in cellular processes such as hormone synthesis, energy production, and fat storage. Older adults are at risk of the dysregulation of lipid metabolism, which is associated with progressive declines in the physiological function of various organs. With advancing age, digestion and absorption commonly change, thereby resulting in decreased nutrient uptake. However, in the elderly population, the accumulation of excess fat becomes more pronounced due to a decline in the body's capacity to utilize lipids effectively. This is characterized by enhanced adipocyte synthesis and reduced breakdown, along with diminished peripheral tissue utilization capacity. Excessive lipid accumulation in the body, which manifests as hyperlipidemia and accumulated visceral fat, is linked to several chronic lipid-related diseases, including cardiovascular disease, type 2 diabetes, obesity, and nonalcoholic fatty liver disease. This review provides a summary of the altered lipid metabolism during aging, including lipid digestion, absorption, anabolism, and catabolism, as well as their associations with age-related chronic diseases, which aids in developing nutritional interventions for older adults to prevent or alleviate age-related chronic diseases.

Combined treatment with teneligliptin and canagliflozin additively suppresses high-fat diet-induced body weight gain in mice with modulation of lipid metabolism-related gene expression

In the treatment of type 2 diabetes mellitus (T2DM), comprehensive management of multiple risk factors, such as blood glucose, body weight, and lipids, is important to prevent disease progression. Although the combination of dipeptidyl peptidase-4 (DPP-4) inhibitor and sodium-glucose co-transporter 2 (SGLT2) inhibitor is often used clinically, the effects of this combination, other than glucose metabolism, have yet to be thoroughly investigated. In this study, we evaluated the effects of combined treatment with a DPP-4 inhibitor, teneligliptin, and an SGLT2 inhibitor, canagliflozin, on the body weight and lipid metabolism in high-fat diet (HFD)-induced obese mice. We found that monotherapy with teneligliptin or canagliflozin showed suppressive effects on high-fat diet-induced body weight gain and reduced inguinal white adipose tissue (iWAT) mass, and combined treatment additively reduced body weight gain and iWAT mass. Teneligliptin significantly increased oxygen consumption during the light phase, and this effect was preserved in the combined treatment. The combined treatment did not alter the mRNA expression levels of thermogenesis-related genes in adipose tissue but showed the tendency to additively induce mRNA of fatty acid oxidation-related genes in brown adipose tissue and tended to additively decrease mRNA of fatty acid synthesis-related genes in iWAT and liver tissues. These results suggest that combined treatment with teneligliptin and canagliflozin additively suppresses HFD-induced body weight gain with increasing oxygen consumption and modulating the expression of lipid metabolism-related genes. This combination therapy may provide effective body weight management for patients with T2DM and obesity.

Skeletal muscle atrophy is exacerbated by steatotic and fibrotic liver-derived TNF-α in senescence-accelerated mice

BACKGROUND AND AIM

Although liver diseases, including non-alcoholic steatohepatitis, are associated with skeletal muscle atrophy, the mechanism behind their association has not been fully elucidated. In this study, the effects of aging and non-alcoholic steatohepatitis on the skeletal muscle, and the interaction between the liver and muscle were investigated using a diet-induced non-alcoholic steatohepatitis model in senescence-accelerated mice.

METHODS

A total of four groups of senescence-accelerated mice and the control mice were fed either a non-alcoholic steatohepatitis-inducing or control diet, and their livers and skeletal muscles were removed for examinations.

RESULTS

In the senescence-accelerated/non-alcoholic steatohepatitis group, serum level of alanine aminotransferase was markedly elevated and histopathology of non-alcoholic steatohepatitis was significant. Skeletal muscles were also markedly atrophied. The expression of the ubiquitin ligase Murf1 in the muscle was significantly increased with muscle atrophy, while that of Tnfa was not significantly different. In contrast, the hepatic Tnfa expression and serum TNF-α levels were significantly increased in the senescence-accelerated/non-alcoholic steatohepatitis group. These results suggest that liver-derived TNF-α might promote muscle atrophy associated with steatohepatitis and aging through Murf-1. The metabolomic analysis of skeletal muscle indicated higher spermidine and lower tryptophan levels in the steatohepatitis-diet group.

CONCLUSIONS

The findings of this study revealed an aspect of liver-muscle interaction, which might be important in developing treatments for sarcopenia associated with liver diseases.

The CD38 glycohydrolase and the NAD sink: implications for pathological conditions

Nicotinamide adenine dinucleotide (NAD) acts as a cofactor in several oxidation-reduction (redox) reactions and is a substrate for a number of nonredox enzymes. NAD is fundamental to a variety of cellular processes including energy metabolism, cell signaling, and epigenetics. NAD homeostasis appears to be of paramount importance to health span and longevity, and its dysregulation is associated with multiple diseases. NAD metabolism is dynamic and maintained by synthesis and degradation. The enzyme CD38, one of the main NAD-consuming enzymes, is a key component of NAD homeostasis. The majority of CD38 is localized in the plasma membrane with its catalytic domain facing the extracellular environment, likely for the purpose of controlling systemic levels of NAD. Several cell types express CD38, but its expression predominates on endothelial cells and immune cells capable of infiltrating organs and tissues. Here we review potential roles of CD38 in health and disease and postulate ways in which CD38 dysregulation causes changes in NAD homeostasis and contributes to the pathophysiology of multiple conditions. Indeed, in animal models the development of infectious diseases, autoimmune disorders, fibrosis, metabolic diseases, and age-associated diseases including cancer, heart disease, and neurodegeneration are associated with altered CD38 enzymatic activity. Many of these conditions are modified in CD38-deficient mice or by blocking CD38 NADase activity. In diseases in which CD38 appears to play a role, CD38-dependent NAD decline is often a common denominator of pathophysiology. Thus, understanding dysregulation of NAD homeostasis by CD38 may open new avenues for the treatment of human diseases.

Metabolic profiling of lysophosphatidylcholines in chlorpromazine hydrochloride- and N-acetyl-p-amino-phenoltriptolide-induced liver injured rats based on ultra-performance liquid chromatography quadrupole time-of-flight mass spectrometry

Chlorpromazine hydrochloride (CH) and N-acetyl-p-amino-phenoltriptolide (APAP) are typical acentral dopamine receptor antagonists and antipyretic analgesics in clinical applications, respectively. However, it has been reported that these 2 drugs could cause liver damage. Lysophosphatidylcholines (LPCs) have multiple physiological functions and are metabolized primarily in the liver, where it undergoes significant changes when the liver is damaged. In the study, 15 LPCs in the rat serum with CH- and APAP-induced liver injury were quantified based on ultra-performance liquid chromatography quadrupole time-of-flight mass spectrometry, and multivariate statistical analyses including principal component analysis (PCA) and orthogonal partial least squares discriminate analysis (OPLS-DA) were combined to understand CH- and APAP-induced liver injury from the perspective of LPC metabolic profiling. The quantitative results showed that there were significant changes in 10 LPCs and 5 LPCs after CH- and APAP-administration, separately. The results of PCA and OPLS-DA indicated that CH- and APAP-induced liver injury could be well distinguished by the LPC metabolic profiling, and 7 LPCs and 1 LPC biomarkers that could characterize CH- and APAP-induced liver damage in turn had been screened. This study will not only provide a new perspective for the clinical diagnosis of CH- and APAP-induced liver injury, but also offer a reference for further study of their hepatotoxicity mechanisms.

Sirtuins-Mediated System-Level Regulation of Mammalian Tissues at the Interface between Metabolism and Cell Cycle: A Systematic Review

Sirtuins are a family of highly conserved NAD+-dependent proteins and this dependency links Sirtuins directly to metabolism. Sirtuins' activity has been shown to extend the lifespan of several organisms and mainly through the post-translational modification of their many target proteins, with deacetylation being the most common modification. The seven mammalian Sirtuins, SIRT1 through SIRT7, have been implicated in regulating physiological responses to metabolism and stress by acting as nutrient sensors, linking environmental and nutrient signals to mammalian metabolic homeostasis. Furthermore, mammalian Sirtuins have been implicated in playing major roles in mammalian pathophysiological conditions such as inflammation, obesity and cancer. Mammalian Sirtuins are expressed heterogeneously among different organs and tissues, and the same holds true for their substrates. Thus, the function of mammalian Sirtuins together with their substrates is expected to vary among tissues. Any therapy depending on Sirtuins could therefore have different local as well as systemic effects. Here, an introduction to processes relevant for the actions of Sirtuins, such as metabolism and cell cycle, will be followed by reasoning on the system-level function of Sirtuins and their substrates in different mammalian tissues. Their involvement in the healthy metabolism and metabolic disorders will be reviewed and critically discussed.

"Cell Membrane Theory of Senescence" and the Role of Bioactive Lipids in Aging, and Aging Associated Diseases and Their Therapeutic Implications

Lipids are an essential constituent of the cell membrane of which polyunsaturated fatty acids (PUFAs) are the most important component. Activation of phospholipase A2 (PLA2) induces the release of PUFAs from the cell membrane that form precursors to both pro- and ant-inflammatory bioactive lipids that participate in several cellular processes. PUFAs GLA (gamma-linolenic acid), DGLA (dihomo-GLA), AA (arachidonic acid), EPA (eicosapentaenoic acid) and DHA (docosahexaenoic acid) are derived from dietary linoleic acid (LA) and alpha-linolenic acid (ALA) by the action of desaturases whose activity declines with age. Consequently, aged cells are deficient in GLA, DGLA, AA, AA, EPA and DHA and their metabolites. LA, ALA, AA, EPA and DHA can also be obtained direct from diet and their deficiency (fatty acids) may indicate malnutrition and deficiency of several minerals, trace elements and vitamins some of which are also much needed co-factors for the normal activity of desaturases. In many instances (patients) the plasma and tissue levels of GLA, DGLA, AA, EPA and DHA are low (as seen in patients with hypertension, type 2 diabetes mellitus) but they do not have deficiency of other nutrients. Hence, it is reasonable to consider that the deficiency of GLA, DGLA, AA, EPA and DHA noted in these conditions are due to the decreased activity of desaturases and elongases. PUFAs stimulate SIRT1 through protein kinase A-dependent activation of SIRT1-PGC1α complex and thus, increase rates of fatty acid oxidation and prevent lipid dysregulation associated with aging. SIRT1 activation prevents aging. Of all the SIRTs, SIRT6 is critical for intermediary metabolism and genomic stability. SIRT6-deficient mice show shortened lifespan, defects in DNA repair and have a high incidence of cancer due to oncogene activation. SIRT6 overexpression lowers LDL and triglyceride level, improves glucose tolerance, and increases lifespan of mice in addition to its anti-inflammatory effects at the transcriptional level. PUFAs and their anti-inflammatory metabolites influence the activity of SIRT6 and other SIRTs and thus, bring about their actions on metabolism, inflammation, and genome maintenance. GLA, DGLA, AA, EPA and DHA and prostaglandin E2 (PGE2), lipoxin A4 (LXA4) (pro- and anti-inflammatory metabolites of AA respectively) activate/suppress various SIRTs (SIRt1 SIRT2, SIRT3, SIRT4, SIRT5, SIRT6), PPAR-γ, PARP, p53, SREBP1, intracellular cAMP content, PKA activity and peroxisome proliferator-activated receptor γ coactivator 1-α (PGC1-α). This implies that changes in the metabolism of bioactive lipids as a result of altered activities of desaturases, COX-2 and 5-, 12-, 15-LOX (cyclo-oxygenase and lipoxygenases respectively) may have a critical role in determining cell age and development of several aging associated diseases and genomic stability and gene and oncogene activation. Thus, methods designed to maintain homeostasis of bioactive lipids (GLA, DGLA, AA, EPA, DHA, PGE2, LXA4) may arrest aging process and associated metabolic abnormalities.

Peroxisome proliferator-activated receptors in the pathogenesis and therapies of liver fibrosis

Liver fibrosis is a dynamic wound-healing process associated with the deposition of extracellular matrix produced by myofibroblasts. HSCs activation, inflammation, oxidative stress, steatosis and aging play critical roles in the progression of liver fibrosis, which is correlated with the regulation of the peroxisome proliferator-activated receptor (PPAR) pathway. As nuclear receptors, PPARs reduce inflammatory response, regulate lipid metabolism, and inhibit fibrogenesis in the liver associated with aging. Thus, PPAR ligands have been investigated as possible therapeutic agents. Mounting evidence indicated that some PPAR agonists could reverse steatohepatitis and liver fibrosis. Consequently, targeting PPARs might be a promising and novel therapeutic option against liver fibrosis. This review summarizes recent studies on the role of PPARs on the pathogenesis and treatment of liver fibrosis.

Emerging Roles of SIRT1 in Alcoholic Liver Disease

Alcoholic liver disease (ALD) is the most prevalent type of chronic liver disease worldwide with a wide spectrum of liver pathologies ranging from simple steatosis to steatohepatitis, cirrhosis, and even hepatocellular carcinoma. It has been demonstrated that ALD is mediated in whole or in part by a central signaling molecule sirtuin 1 (SIRT1), a conserved class III histone deacetylase.SIRT1 plays beneficial roles in regulating hepatic lipid metabolism, inhibiting hepatic inflammation, controlling hepatic fibrosis and mediating hepatocellular carcinoma in ALD. However, underlying molecular mechanisms are complex and remain incompletely understood. The aim of this review was to highlight the latest advances in understanding of SIRT1 regulatory mechanisms in ALD and discuss their unique potential role as novel therapeutic target for ALD treatment.

Hepatic Levels of DHA-Containing Phospholipids Instruct SREBP1-Mediated Synthesis and Systemic Delivery of Polyunsaturated Fatty Acids

Polyunsaturated fatty acids (PUFAs), such as docosahexaenoic acid (DHA) and arachidonic acid (ARA), play fundamental roles in mammalian physiology. Although PUFA imbalance causes various disorders, mechanisms of the regulation of their systemic levels are poorly understood. Here, we report that hepatic DHA-containing phospholipids (DHA-PLs) determine the systemic levels of PUFAs through the SREBP1-mediated transcriptional program. We demonstrated that liver-specific deletion of Agpat3 leads to a decrease of DHA-PLs and a compensatory increase of ARA-PLs not only in the liver but also in other tissues including the brain. Together with recent findings that plasma lysophosphatidylcholine (lysoPC) is the major source of brain DHA, our results indicate that hepatic AGPAT3 contributes to brain DHA accumulation by supplying DHA-PLs as precursors of DHA-lysoPC. Furthermore, dietary fish oil-mediated suppression of hepatic PUFA biosynthetic program was blunted in liver-specific Agpat3 deletion. Our findings highlight the central role of hepatic DHA-PLs as the molecular rheostat for systemic homeostasis of PUFAs.