Autophagy dysregulation caused by ApoM deficiency plays an important role in liver lipid metabolic disorder

Circulating Proteomics Identifies a Dynamic Profile of Hepatic Steatosis During Metabolic Intervention

BACKGROUND

Weight reduction through lifestyle, activity, and dietary interventions are the mainstay of initial therapy for metabolic dysfunction associated steatotic liver disease. Data on the relative effectiveness and metabolic pathways linking weight loss and decreased hepatic steatosis are lacking. We sought to identify coordinated changes between the circulating proteome and hepatic steatosis within a randomized clinical trial of alternate day fasting and exercise and prioritize proteins relevant to hepatic steatosis within a broader context using a community cohort.

METHODS AND RESULTS

We quantified a broad cardiometabolic proteome (>300 proteins) in 67 individuals randomized in a 2×2 factorial design to alternate day fasting and exercise before and after the 3-month intervention to identify proteomic signatures of hepatic steatosis (measured by magnetic resonance imaging proton density fat fraction). Then, we analyzed the cross-sectional relationship of overlapping proteins (≈170) with hepatic attenuation (a computed tomographic technique linked to steatosis) in 707 participants from a community cohort. Principal component analysis demonstrated a proteomic signature associated with intrahepatic triglyceride content (Spearman rho=0.55, P<0.001) and insulin resistance (homeostatic model assessment for insulin resistance, Spearman rho=0.39, P=0.001). Changes in this proteomic signature were associated with changes in intrahepatic triglyceride content over the intervention period (beta=0.12, P<0.001). Moreover, cross-sectional analysis of overlapping proteins with hepatic attenuation in the community cohort showed generally, directionally consistent associations with hepatic steatosis.

CONCLUSIONS

These findings highlight the potential for broad proteomic profiling in small nutritional interventional studies with serial phenotyping alongside confirmatory large cohort epidemiology to prioritize targets of hepatic steatosis and cardiometabolic risk for mechanistic study.

Differential levels of thyroid hormones, cortisol, and apolipoprotein M in fatty liver disease

OBJECTIVES

Non-alcoholic fatty liver disease (NAFLD), recently reclassified as metabolic dysfunction-associated fatty liver disease (MAFLD), can also manifest in patients classified as non-MAFLD who do not meet MAFLD criteria. The involvement of cortisol and thyroid hormones may play a role in the pathogenesis of FLD by modifying the metabolism of specific lipoproteins, particularly apolipoprotein M (Apo M). This study investigated cortisol and thyroid hormones levels and Apo M gene expression in white blood cells (WBCs) of individuals with MAFLD, non-MAFLD, and healthy controls.

METHODS

The serum and WBCs of the study subjects were collected from patients with FLD (n=99) including 58 MAFLD and 41 non-MAFLD and healthy individuals (n=23). To investigate the gene expression of Apo M and thyroid and cortisol hormones, qRT-PCR and ELISA methods were used, respectively.

RESULTS

The Apo M gene expression was significantly lower in FLD patients, both non-MAFLD, and MAFLD patients compared to the control group (p<0.05). Total T4 and TSH hormone levels in the MAFLD patients were significantly decreased and increased compared to the control group, respectively (p<0.05). The cortisol level was significantly elevated in the FLD and MAFLD patients compared to the control group (p<0.01).

CONCLUSIONS

Alterations in Apo M gene expression also cortisol and thyroid hormones levels in non-MAFLD patients were milder than MAFLD patients when compared to the control. Also, likely Apo M may be involved in FLD pathogenesis.

APOM Modulates the Glycolysis Process in Liver Cancer Cells by Controlling the Expression and Activity of HK2 via the Notch Pathway

The metabolic pathway of aerobic glycolysis in tumor cells has garnered significant attention in tumor research because of its high activation in cancer cells. Previous research conducted by our team has demonstrated that Apolipoprotein M (APOM) exhibits potential as a factor against liver cancer. However, further investigations are needed to elucidate the precise approach and mechanism that are involved in this process. The findings of this study demonstrated that the inhibition of APOM gene expression led to a notable increase in glucose uptake within liver cancer cells, along with increased levels of lactate dehydrogenase A (LDHA) mRNA and protein expression, as well as increased lactate and adenosine triphosphate (ATP) levels (P < 0.05). These alterations in the cellular microenvironment may be associated with a significant increase in the expression level and enzyme activity of the pivotal enzyme hexokinase 2 (HK2) (P < 0.05). Subsequent investigations revealed notable enrichment of the Notch pathway in liver cancer samples exhibiting low expression of the APOM gene. Western blot experiments demonstrated that the inhibition of APOM gene expression triggers the activation of the Notch pathway in liver cancer cells. Furthermore, the administration of a γ-secretase inhibitor (DAPT) successfully mitigated the increase in HK2 levels, glucose uptake, lactate production, and proliferation of liver cancer cells induced by the downregulation of the APOM gene (P < 0.05). In conclusion, diminished APOM expression may facilitate the progression of liver cancer by stimulating the aerobic glycolysis pathway, which is mediated by the Notch signaling pathway.

Non-alcoholic fatty liver disease-associated DNA methylation and gene expression alterations in the livers of Collaborative Cross mice fed an obesogenic high-fat and high-sucrose diet

Non-alcoholic fatty liver disease (NAFLD) is a highly prevalent chronic liver disease, and patient susceptibility to its onset and progression is influenced by several factors. In this study, we investigated whether altered hepatic DNA methylation in liver tissue correlates with the degree of severity of NAFLD-like liver injury induced by a high-fat and high-sucrose (HF/HS) diet in Collaborative Cross (CC) mice. Using genome-wide targeted bisulphite DNA methylation next-generation sequencing, we found that mice with different non-alcoholic fatty liver (NAFL) phenotypes could be distinguished by changes in hepatic DNA methylation profiles. Specifically, NAFL-prone male CC042 mice exhibited more prominent DNA methylation changes compared with male CC011 mice and female CC011 and CC042 mice that developed only a mild NAFL phenotype. Moreover, these mouse strains demonstrated different patterns of DNA methylation. While the HF/HS diet induced both DNA hypomethylation and DNA hypermethylation changes in all the mouse strains, the NAFL-prone male CC042 mice demonstrated a global predominance of DNA hypermethylation, whereas a more pronounced DNA hypomethylation pattern developed in the mild-NAFL phenotypic mice. In a targeted analysis of selected genes that contain differentially methylated regions (DMRs), we identified NAFL phenotype-associated differences in DNA methylation and gene expression of the Apoa4, Gls2, and Apom genes in severe NAFL-prone mice but not in mice with mild NAFL phenotypes. These changes in the expression of Apoa4 and Gls2 coincided with similar findings in a human in vitro cell model of diet-induced steatosis and in patients with NAFL. These results suggest that changes in the expression and DNA methylation status of these three genes may serve as a set of predictive markers for the development of NAFLD.

Exendin-4 gene modification and microscaffold encapsulation promote self-persistence and antidiabetic activity of MSCs

Mesenchymal stem cell (MSC)-based therapy to combat diabetic-associated metabolic disorders is hindered by impoverished cell survival and limited therapeutic effects under high glucose stress. Here, we genetically engineered MSCs with Exendin-4 (MSC-Ex-4), a glucagon-like peptide-1 (GLP-1) analog, and demonstrated their boosted cellular functions and antidiabetic efficacy in the type 2 diabetes mellitus (T2DM) mouse model. Mechanistically, MSC-Ex-4 achieved self-augmentation and improved survival under high glucose stress via autocrine activation of the GLP-1R-mediated AMPK signaling pathway. Meanwhile, MSC-Ex-4-secreted Exendin-4 suppressed senescence and apoptosis of pancreatic β cells through endocrine effects, while MSC-Ex-4-secreted bioactive factors (e.g., IGFBP2 and APOM) paracrinely augmented insulin sensitivity and decreased lipid accumulation in hepatocytes through PI3K-Akt activation. Furthermore, we encapsulated MSC-Ex-4 in 3D gelatin microscaffolds for single-dose administration to extend the therapeutic effect for 3 months. Together, our findings provide mechanistic insights into Exendin-4-mediated MSCs self-persistence and antidiabetic activity that offer more effective MSC-based therapy for T2DM.

ApoM is an important potential protective factor in the pathogenesis of primary liver cancer

In recent years, abnormal liver lipid metabolism has emerged as one of the important pathogenesis pathways of primary liver cancer. It is highly important to identify the mechanisms to explore potential prevention and treatment targets. Apolipoprotein M is specifically expressed in the liver and participates in liver lipid metabolism, but the evidence that ApoM affects primary liver cancer is insufficient. The Cancer Genome Atlas (TCGA) database and clinical case analysis, as well as animal level and cell level analysis suggest that the expression level of ApoM gene in cancer tissues is lower than that in paracarcinoma tissues. Further experimental research found that the deletion of ApoM significantly increased the proliferation of mouse liver cancer cells (Hepa1-6) and inhibited the level of apoptosis induced by cisplatin. In addition, mouse liver cancer cells lacking ApoM showed stronger migration and invasion capabilities in transwell experiments. In contrast, overexpression of ApoM in Hepa1-6 cells and Huh-7 cells showed an inhibition of proliferation, up-regulation apoptosis and reduced migration and invasion. In vivo, the deletion of the ApoM accelerated tumorigenesis in nude mice and allowed the mice to develop liver tumor mutations more quickly under the induction of N-nitrosodiethylamine and the survival time of mice was shorter than that control. Therefore, ApoM may be a potential protective factor to inhibit the occurrence and development of primary liver cancer.

The apoM-S1P axis in hepatic diseases

Liver dysfunction is always accompanied by lipid metabolism dysfunction. Apolipoprotein M (apoM), a member of the apolipoprotein family, is primarily expressed and secreted from the liver. apoM is the main chaperone of sphingosine-1-phosphate (S1P), a small signalling molecule associated with numerous physiologic and pathophysiologic processes. In addition to transport, apoM also influences the biologic effects of S1P. Most recently, numerous studies have investigated the potential role of the apoM-S1P axis in a variety of hepatic diseases. These include liver fibrosis, viral hepatitis B and C infection, hepatobiliary disease, non-alcoholic and alcoholic steatohepatitis, acute liver injury and hepatocellular carcinoma. In this review, the roles of apoM and S1P in the development of hepatic diseases are summarized, and novel insights into the diagnosis and treatment of hepatic diseases are discussed.

Comprehensive lipidomics in apoM-/- mice reveals an overall state of metabolic distress and attenuated hepatic lipid secretion into the circulation

Apolipoprotein M (apoM) participates in both high-density lipoprotein and cholesterol metabolism. Little is known about how apoM affects lipid composition of the liver and serum. In this study, we systemically investigated the effects of apoM on liver and plasma lipidomes and how apoM participates in lipid cycling, via apoM knockout in mice and the human SMMC-7721 cell line. We used integrated mass spectrometry-based lipidomics approaches to semiquantify more than 600 lipid species from various lipid classes, which include free fatty acids, glycerolipids, phospholipids, sphingolipids, glycosphingolipids, cholesterol, and cholesteryl esters (CEs), in apoM-/- mouse. Hepatic accumulation of neutral lipids, including CEs, triacylglycerols, and diacylglycerols, was observed in apoM-/- mice; while serum lipidomic analyses showed that, in contrast to the liver, the overall levels of CEs and saturated/monounsaturated fatty acids were markedly diminished. Furthermore, the level of ApoB-100 was dramatically increased in the liver, whereas significant reductions in both ApoB-100 and low-density lipoprotein (LDL) cholesterol were observed in the serum of apoM-/- mice, which indicated attenuated hepatic LDL secretion into the circulation. Lipid profiles and proinflammatory cytokine levels indicated that apoM-/- leads to hepatic steatosis and an overall state of metabolic distress. Taken together, these results revealed that apoM knockout leads to hepatic steatosis, impaired lipid secretion, and an overall state of metabolic distress.

Apolipoprotein M: new connections with diet, adipose tissue and metabolic syndrome

PURPOSE OF REVIEW

To focus on state-of-the-art knowledge on the apolipoprotein M (ApoM) physiology and physiopathology regarding metabolism.

RECENT FINDINGS

In humans, the ApoM was recently described as secreted by adipocytes. Obesity, metabolic syndrome and type 2 diabetes are associated with low circulating ApoM and adipose tissue APOM expression. Dieting-induced weight loss enhances adipose tissue expression and secretion, and exercise training increases plasma ApoM. The ApoM is a chaperone for the bioactive sphingolipid, sphingosine-1-phosphate (S1P), which has a specific role in inflammation. Its association with S1P in the inhibition of brown adipose tissue activity and subsequent insulin sensitivity was reported with the model of ApoM-deficient mouse.

SUMMARY

The adipose tissue is an endocrine organ responsible for obesity-related comorbidities. Obesity and dieting impact the adipose tissue secretory profile. The recent demonstration of ApoM being secreted by healthy adipocytes questions about the possible role of this adipose production in metabolic diseases. Low-circulating ApoM is associated with unhealthy metabolic phenotype. The lower circulating apoM during metabolic syndrome might be a cause of obesity-related comorbidities. Lifestyle interventions enhance ApoM production. Whether it acts in combination to S1P or other small lipidic molecules deserves further investigations.

Apolipoprotein M: Research Progress and Clinical Perspective

Apolipoprotein M (apoM) was first identified and characterized to the apolipoprotein family in 1999. Human apoM gene is located in a highly conserved segment in the major histocompatibility complex (MHC) class III locus on chromosome 6 and codes for an about 23 kDa protein that structurally belongs to the lipocalin superfamily. ApoM is selectively expressed in hepatocytes and in the tubular epithelium of kidney. In human plasma, apoM is mainly confined to the high-density lipoprotein (HDL) particles, but it may also occur in other lipoprotein classes, such as in the triglyceride-rich particles after fat intake. It has been demonstrated that apoM is critical for the formation of HDL, notably pre-beta HDL1. The antiatherogenic function of HDL is well established, and its ability to promote cholesterol efflux from foam cells in the atherosclerotic lesions is generally regarded as one of the key mechanisms behind this protective function. However, HDL could also display a variety of properties that may affect the complex atherosclerotic processes by other mechanisms, thus being involved in processes related to antioxidant defense, immune system, and systemic effects in septicemia, which may be partly contributed via its apolipoproteins and/or phospholipids. Moreover, it has been demonstrated that apoM functions as a natural carrier of sphingosin-1-phosphate (S1P) in vivo which may be related to its antiatherosclerotic and protective effects on endothelial cell barrier and anti-inflammatory properties. These may also provide a link between the diverse effects of HDL.

Apolipoprotein M/sphingosine-1-phosphate: novel effects on lipids, inflammation and kidney biology

PURPOSE OF REVIEW

In 2011, the crystal structure of apolipoprotein M (apoM) and its capacity to bind sphingosine-1-phosphate (S1P) was characterized. Since then, a variety of studies has increased our knowledge on apoM biology and functionality. From being an unknown and hardly significant player in overall metabolism, apoM has gained significant interest.

RECENT FINDINGS

Key discoveries in the last 2 years have indicated that the apoM/S1P complex has important roles in lipid metabolism (affecting triglyceride turnover), inflammation (a marker of severe sepsis and potentially providing anti-inflammatory signaling) and kidney biology (potential to protect against immunoglobulin A nephropathy).

SUMMARY

Several studies suggest a potential for apoM/S1P as biomarkers for inflammation, sepsis and nephropathy. Also, a novel chaperone is characterized and could have potential as a drug for treatment in inflammation and nephropathy.

[Crocetin promotes autophagy in injured rat hepatocytes induced by lipopolysaccharide and D-galactosamine in vitro]

OBJECTIVE

To observe the effect of crocetin on autophagy in rat hepatocytes exposed to lipopolysaccharide (LPS) and D-galactosamine (D-gal) and explore the mechanism.

METHODS

Cultured rat hepatocytes were exposed to LPS (1 mg/L) and Dgal (60 mg/L) to induce cell injury and treated with crocetin, 3MA, or crocetin+3MA. Twelve hours after the treatments, the cells were examined for levels of ALT, AST and LDH in the supernatant using ELISA. LC3 fluorescence in the cells following immunofluorescence staining was observed using fluorescence microscopy. Autophagosomes in the cells were observed by transmission electron microscopy, and the cellular expressions of LC3, p62 and SIRT1 were detected using Western blotting.

RESULTS

The levels of ALT, AST and LDH in the hepatocytes were elevated after LPS- and D-gal-induced injury, reached the highest levels after 3MA treatment, but were decreased significantly by crocetin treatment. LC3 fluorescence increased obviously in the injured hepatoctyes, and the increment was the most obvious in crocetin-treated cells; LC3 fluorescence was decreased significantly after 3MA treatment. Cell injury induced obvious increase in autophagy in the hepatocytes, and the number of autophagosomes increased significantly after crocetin treatment but was reduced significantly after 3MA treatment. The cell injury caused an obvious up-regulation of LC3 and SIRT1 expression and down-regulated p62 expression. LC3 and SIRT1 expression levels were the highest and the expression of p62 was the lowest in cells with crocetin treatment. 3MA treatment significantly reduced the expression of LC3 and SIRT1 and increased the expression of p62 in the injured cells.

CONCLUSIONS

Autophagy is increased in injured rat hepatocytes, and crocetin can promote autophagy in the injured cells to reduce further cell injury.