Oleic acid-induced hepatic steatosis is coupled with downregulation of aquaporin 3 and upregulation of aquaporin 9 via activation of p38 signaling

The Multifaceted Role of Aquaporin-9 in Health and Its Potential as a Clinical Biomarker

Aquaporins (AQPs) are transmembrane channels essential for water, energy, and redox homeostasis, with proven involvement in a variety of pathophysiological conditions such as edema, glaucoma, nephrogenic diabetes insipidus, oxidative stress, sepsis, cancer, and metabolic dysfunctions. The 13 AQPs present in humans are widely distributed in all body districts, drawing cell lineage-specific expression patterns closely related to cell native functions. Compelling evidence indicates that AQPs are proteins with great potential as biomarkers and targets for therapeutic intervention. Aquaporin-9 (AQP9) is the most expressed in the liver, with implications in general metabolic and redox balance due to its aquaglyceroporin and peroxiporin activities, facilitating glycerol and hydrogen peroxide (H₂O₂) diffusion across membranes. AQP9 is also expressed in other tissues, and their altered expression is described in several human diseases, such as liver injury, inflammation, cancer, infertility, and immune disorders. The present review compiles the current knowledge of AQP9 implication in diseases and highlights its potential as a new biomarker for diagnosis and prognosis in clinical medicine.

The Dietary Replacement of Soybean Oil by Canola Oil Does Not Prevent Liver Fatty Acid Accumulation and Liver Inflammation in Mice

A high-carbohydrate diet (HCD) is a well-established experimental model of accelerated liver fatty acid (FA) deposition and inflammation. In this study, we evaluated whether canola oil can prevent these physiopathological changes. We evaluated hepatic FA accumulation and inflammation in mice fed with a HCD (72.1% carbohydrates) and either canola oil (C group) or soybean oil (S group) as a lipid source for 0, 7, 14, 28, or 56 days. Liver FA compositions were analyzed by gas chromatography. The mRNA expression of acetyl-CoA carboxylase 1 (ACC1) was measured as an indicator of lipogenesis. The mRNA expression of F4/80, tumor necrosis factor-α (TNF-α), interleukin (IL)-1β, IL-6, and IL-10, as mediators of liver inflammation, were also measured. The C group stored less n-6 polyunsaturated FAs (n-6 PUFAs) and had more intense lipid deposition of monounsaturated FAs (MUFAs), n-3 PUFAs, and total FAs. The C group also showed higher ACC1 expression. Moreover, on day 56, the C group showed higher expressions of the inflammatory genes F4/80, TNF-α, IL-1β, and IL-6, as well as the anti-inflammatory IL-10. In conclusion, a diet containing canola oil as a lipid source does not prevent the fatty acid accumulation and inflammation induced by a HCD.

Polyunsaturated fatty acids and p38-MAPK link metabolic reprogramming to cytoprotective gene expression during dietary restriction

The metabolic state of an organism instructs gene expression modalities, leading to changes in complex life history traits, such as longevity. Dietary restriction (DR), which positively affects health and life span across species, leads to metabolic reprogramming that enhances utilisation of fatty acids for energy generation. One direct consequence of this metabolic shift is the upregulation of cytoprotective (CyTP) genes categorized in the Gene Ontology (GO) term of "Xenobiotic Detoxification Program" (XDP). How an organism senses metabolic changes during nutritional stress to alter gene expression programs is less known. Here, using a genetic model of DR, we show that the levels of polyunsaturated fatty acids (PUFAs), especially linoleic acid (LA) and eicosapentaenoic acid (EPA), are increased following DR and these PUFAs are able to activate the CyTP genes. This activation of CyTP genes is mediated by the conserved p38 mitogen-activated protein kinase (p38-MAPK) pathway. Consequently, genes of the PUFA biosynthesis and p38-MAPK pathway are required for multiple paradigms of DR-mediated longevity, suggesting conservation of mechanism. Thus, our study shows that PUFAs and p38-MAPK pathway function downstream of DR to help communicate the metabolic state of an organism to regulate expression of CyTP genes, ensuring extended life span.

Expression of Notch family is altered in non‑alcoholic fatty liver disease

The aim of the present study was to explore the dynamic relationship between Notch and non‑alcoholic fatty liver disease (NAFLD), both in vitro and in vivo. The LX2, Huh7 and MIHA hepatic cell lines were used to establish a cell steatosis model induced by palmitic acid (PA) at different concentrations (0.1, 0.25 and 0.5 mM). Cell proliferation and migration were assessed using a 5‑bromo‑2'‑deoxyuridine kit and a wound healing assay. The dosage of 0.25 mM PA for 36‑48 h treatment was chosen for subsequent experiments. Steatotic cells were identified by Oil Red O staining. Feeding mice a methionine‑choline‑deficient (MCD) diet is known induce a model of NAFLD, compared with a methionine‑choline‑sufficient (MCS) diet. Therefore, Notch family mRNA expression was evaluated in the liver of MCD‑fed mice at varying time points (days 5, 10, 21 and 70) using reverse transcription‑quantitative PCR. Notch expression levels were also assessed in cell lines at 12, 24, 36 and 48 h after PA treatment. Notch signaling molecules changed in the PA or MCD model over time. In vitro, the mRNA levels of Notch1, ‑2 and ‑4 increased in all cell lines after 12‑h PA treatment. At 24 h, these genes were upregulated only in LX2 cells, while showing a 'down‑up' pattern in MIHA cells (i.e. these genes were downregulated at 24 h but upregulated at 36 h). However, expression of Notch1, ‑2, ‑3 and ‑4 mRNA rose significantly in the early stage (day 10) of NAFLD. At week 3, the levels of Notch1 and ‑2 were higher in the MCD group than in the MCS group, while the reverse was observed for Notch3 and ‑4. Expression of these four genes increased again in the late stage (day 70) of NAFLD. Therefore, these results indicated that Notch family members Notch1‑4 were involved in the development of NAFLD and played an important role in steatosis in this model.

Fc Gamma Receptor IIb Expressed in Hepatocytes Promotes Lipid Accumulation and Gluconeogenesis

Non-alcoholic fatty liver disease (NAFLD) is characterized by ectopic lipid accumulation in the liver, usually combined with hepatic insulin resistance. Fc-gamma receptor-IIb (FcγRIIb) and its ligand are reported to be associated with obesity and type 2 diabetes mellitus (T2DM). As knowledge about FcγRIIb in the literature is mostly generated from studies on skeletal muscle tissue, the expression and function of FcγRIIb in the liver and hepatocytes are largely unknown. In this study, we identified the expression of FcγRIIb in primary cultured mouse hepatocytes: FcγRIIb was upregulated in response to oleic acid (OA) in a dose dependent manner. FcγRIIb knockdown using shRNA suppressed the lipid and triglyceride accumulation, and mRNA expression of ACC1, FASn, CD36, MTTP, and ApoB in OA-treated HepG2 cells. FcγRIIb deficiency mice fed with high fat diet (HFD) had significantly lower liver weight and liver to body weight ratio, as well as less triglyceride accumulation in the livers. In glycometabolism, FcγRIIb hindered insulin-induced phosphorylation of AKT and FOXO1, and in turn upregulated G6Pase and PEPCK mRNA expression, suggesting that FcγRIIb promotes gluconeogenesis by suppressing the AKT/FOXO1/G6Pase/PEPCK pathway in hepatocytes. This study reveals a novel role for FcγRIIb in regulating lipid metabolism and glycometabolism, and provides a new therapeutic target to improve NAFLD.

Aquaporin 9 inhibits hepatocellular carcinoma through up-regulating FOXO1 expression

Aquaporin 9 (AQP9) is the main aquaglyceroporin in the liver. Few studies have been performed regarding the role of AQP9 in liver cancer. Here we report AQP9 expression and function in liver cancer. We found that AQP9 mRNA and protein levels were reduced in human hepatocellular cancer compared to the para-tumor normal liver tissues. Human hepatoma cell line SMMC7721 expressed low basal levels of AQP9. When AQP9 was overexpressed in SMMC7721 cell line, cell proliferation was inhibited due to cell cycle arrest at G1 phase and increased apoptosis. At the molecular level, AQP9 overexpression decreased the protein levels of phosphatidylinositol-3-kinase (PI3K), leading to reduced phosphorylation of Akt. Subsequently, the protein levels of forkhead box protein O1 (FOXO1) were increased, resulting in down-regulation of proliferating cell nuclear antigen (PCNA) expression and up-regulation of caspase-3 expression. AQP9 overexpression inhibited growth of subcutaneously xenografted liver tumors in nude mice. These findings suggest that AQP9 expression is down-regulated in liver cancer compared to the normal liver tissue and restoration of AQP9 expression can inhibit development of liver cancer.

Role of bioactive fatty acids in nonalcoholic fatty liver disease

Nonalcoholic fatty liver disease (NAFLD) is characterized by fat deposition in hepatocytes, and a strong association with nutritional factors. Dietary fatty acids are classified according to their biochemical properties, which confer their bioactive roles. Monounsaturated fatty acids have a dual role in various human and murine models. In contrast, polyunsaturated fatty acids exhibit antiobesity, anti steatosic and anti-inflammatory effects. The combination of these forms of fatty acids—according to dietary type, daily intake and the proportion of n-6 to n-3 fats—can compromise hepatic lipid metabolism. A chemosensory rather than a nutritional role makes bioactive fatty acids possible biomarkers for NAFLD. Bioactive fatty acids provide health benefits through modification of fatty acid composition and modulating the activity of liver cells during liver fibrosis. More and better evidence is necessary to elucidate the role of bioactive fatty acids in nutritional and clinical treatment strategies for patients with NAFLD.

A Systematic Characterization of Aquaporin-9 Expression in Human Normal and Pathological Tissues

AQP9 is known to facilitate hepatocyte glycerol uptake. Murine AQP9 protein expression has been verified in liver, skin, epididymis, epidermis and neuronal cells using knockout mice. Further expression sites have been reported in humans. We aimed to verify AQP9 expression in a large set of human normal organs, different cancer types, rheumatoid arthritis synovial biopsies as well as in cell lines and primary cells. Combining standardized immunohistochemistry with high-throughput mRNA sequencing, we found that AQP9 expression in normal tissues was limited, with high membranous expression only in hepatocytes. In cancer tissues, AQP9 expression was mainly found in hepatocellular carcinomas, suggesting no general contribution of AQP9 to carcinogenesis. AQP9 expression in a subset of rheumatoid arthritis synovial tissue samples was affected by Humira, thereby supporting a suggested role of TNFα in AQP9 regulation in this disease. Among cell lines and primary cells, LP-1 myeloma cells expressed high levels of AQP9, whereas low expression was observed in a few other lymphoid cell lines. AQP9 mRNA and protein expression was absent in HepG2 hepatocellular carcinoma cells. Overall, AQP9 expression in human tissues appears to be more selective than in mice.

Analysis of aquaporin expression in liver with a focus on hepatocytes

A deeper understanding of aquaporins (AQPs) expression and transcriptional regulation will provide useful information for liver pathophysiology. We established a complete AQPs mRNA expression profile in human and mouse liver, as well as protein localization of expressed AQPs. Additionally, the modulation of AQPs mRNA levels in response to various agents was determined in human HuH7 cells and in primary culture of mouse hepatocytes. AQP1, AQP3, AQP7, AQP8, and AQP9 mRNA and protein expressions were detected in human liver, while only AQP6 and AQP11 mRNAs were detected. We reported for the first time the localization of AQP3 in Kupffer cells, AQP7 in hepatocytes and endothelial cells, and AQP9 in cholangiocytes. In addition, we confirmed the localization of AQP1 in endothelial cells, and of AQP8 and AQP9 in hepatocytes. On HuH7 cells, we reported the presence of AQP4 mRNA, confirmed the presence of AQP3, AQP7, and AQP11 mRNAs, but not of AQP8 mRNA. On primary culture of murine hepatocytes, AQP1 and AQP7 mRNAs were identified, while the presence of AQP3, AQP8, AQP9, and AQP11 mRNAs was confirmed. At the protein level, murine endothelial liver cells expressed AQP1 and AQP9, while hepatocytes expressed AQP3, AQP7, AQP8, and AQP9, and macrophages expressed AQP3. Dexamethasone, forskolin, AICAR, rosiglitazone, octanoylated, and non-octanoylated ghrelin regulated some AQP expression in primary culture of murine hepatocytes and human HuH7 cells. Additional studies will be required to further assess the role of AQPs expression in human and murine liver and understand the transcriptional regulation of AQPs in hepatocytes under pathophysiological conditions.

Sexual Dimorphism of Adipose and Hepatic Aquaglyceroporins in Health and Metabolic Disorders

Gender differences in the relative risk of developing metabolic complications, such as insulin resistance or non-alcoholic fatty liver disease (NAFLD), have been reported. The deregulation of glycerol metabolism partly contributes to the onset of these metabolic diseases, since glycerol constitutes a key substrate for the synthesis of triacylglycerols (TAGs) as well as for hepatic gluconeogenesis. The present mini-review covers the sex--related differences in glycerol metabolism and aquaglyceroporins (AQPs) and its impact in the control of adipose and hepatic fat accumulation as well as in whole-body glucose homeostasis. Plasma glycerol concentrations are increased in women compared to men probably due to the higher lipolytic rate and larger AQP7 amounts in visceral fat as well as the well-known sexual dimorphism in fat mass with women showing higher adiposity. AQP9 represents the primary route for glycerol uptake in hepatocytes, where glycerol is converted by the glycerol-kinase enzyme into glycerol-3-phosphate, a key substrate for de novo synthesis of glucose and TAG. In spite of showing similar hepatic AQP9 protein, women exhibit lower hepatocyte glycerol permeability than men, which might contribute to their lower prevalence of insulin resistance and NAFLD.