Saturated Fatty Acid-induced cytotoxicity in liver cells does not involve phosphatase and tensin homologue deleted on chromosome 10

Inhibitory Effect of Styrylpyrone Extract of Phellinus linteus on Hepatic Steatosis in HepG2 Cells

The prevalence of nonalcoholic fatty liver disease (NAFLD) is estimated to be approximately about 25.24% of the population worldwide. NAFLD is a complex syndrome and is characterized by a simple benign hepatocyte steatosis to more severe steatohepatitis in the liver pathology. Phellinus linteus (PL) is traditionally used as a hepatoprotective supplement. Styrylpyrone-enriched extract (SPEE) obtained from the PL mycelia has been shown to have potential inhibition effects on high-fat- and high-fructose-diet-induced NAFLD. In the continuous study, we aimed to explore the inhibitory effects of SPEE on free fatty acid mixture O/P [oleic acid (OA): palmitic acid (PA); 2:1, molar ratio]-induced lipid accumulation in HepG2 cells. Results showed that SPEE presented the highest free radical scavenging ability on DPPH and ABTS, and reducing power on ferric ions, better than that of partitions obtained from n-hexane, n-butanol and distilled water. In free-fatty-acid-induced lipid accumulation in HepG2 cells, SPEE showed an inhibition effect on O/P-induced lipid accumulation of 27% at a dosage of 500 μg/mL. As compared to the O/P induction group, the antioxidant activities of superoxide dismutase, glutathione peroxidase and catalase were enhanced by 73%, 67% and 35%, respectively, in the SPEE group. In addition, the inflammatory factors (TNF-α, IL-6 and IL-1β) were significantly down-regulated by the SPEE treatment. The expressions of anti-adipogenic genes involved in hepatic lipid metabolism of 5' adenosine monophosphate (AMP)-activated protein kinase (AMPK), sirtuin 1 (SIRT1) and peroxisome proliferator-activated receptor gamma coactivator 1-alpha (PGC-1α) were enhanced in the SPEE supplemented HepG2 cells. In the protein expression study, p-AMPK, SIRT1 and PGC1-α were significantly increased to 121, 72 and 62%, respectively, after the treatment of SPEE. Conclusively, the styrylpyrone-enriched extract SPEE can ameliorate lipid accumulation and decrease inflammation and oxidative stress through the activation of SIRT1/AMPK/PGC1-α pathways.

Determining the temporal, dose, and composition effects of nutritional substrates in an in vitro model of intrahepatocellular triglyceride accumulation

Pathological accumulation of intrahepatic triglyceride underpins the early stages of nonalcoholic fatty liver disease (NAFLD) and can progress to fibrosis, cirrhosis, and cancer of the liver. Studies in humans suggest that consumption of a diet enriched in saturated compared to unsaturated fatty acids (FAs), is more detrimental to liver fat accumulation and metabolism. However, the reasons for the divergence remain unclear and physiologically-relevant cellular models are required. Therefore, the aims of this study were to investigate the effect of modifying media composition, concentration, and treatment frequency of sugars, FAs and insulin on intrahepatocellular triglyceride content and intracellular glucose, FA and circadian function. Huh7 cells were treated with 2% human serum and a combination of sugars and FAs (low fat low sugar [LFLS], high fat low sugar [HFLS], or high fat high sugar [HFHS]) enriched in either unsaturated (OPLA) or saturated (POLA) FAs for 2, 4, or 7 days with a daily or alternating treatment regime. Stable isotope tracers were utilized to investigate basal and/or insulin-responsive changes in hepatocyte metabolism in response to different treatment regimes. Cell viability, media biochemistry, intracellular metabolism, and circadian biology were quantified. The FA composition of the media (OPLA vs. POLA) did not influence cell viability or intracellular triglyceride content in hepatocytes. In contrast, POLA-treated cells had lower FA oxidation and media acetate, and with higher FA concentrations, displayed lower intracellular glycogen content and diminished insulin stimulation of glycogenesis, compared to OPLA-treated cells. The addition of HFHS also had profound effects on circadian oscillation and gene expression. Cells treated daily with HFHS for at least 4 days resulted in a cellular model displaying characteristics of early stage NAFLD seen in humans. Repeated treatment for longer durations (≥7 days) may provide opportunities to investigate lipid and glucose metabolism in more severe stages of NAFLD.

Exposure to Stearate Activates the IRE1α/XBP-1 Pathway in 3T3-L1 Adipocytes

In the endoplasmic reticulum (ER), accumulation of abnormal proteins with malformed higher-order structures activates signaling pathways (inositol-requiring enzyme 1α (IRE1α)/X-box binding protein 1 (XBP-1) pathway, protein kinase RNA-activated-like endoplasmic reticulum kinase (PERK)/CCAAT/enhancer binding protein-homologous protein (CHOP) pathway and activating transcription factor 6α (ATF6α) pathway) that result in a cellular response suppressing the production of abnormal proteins or inducing apoptosis. These responses are collectively known as the unfolded protein response (UPR). Recently, it has been suggested that the UPR induced by saturated fatty acids in hepatocytes and pancreatic β cells is involved in the development of metabolic diseases such as diabetes. The effect of palmitate, a saturated fatty acid, on the UPR has also been investigated in adipocytes, which are associated with the development of metabolic disorders, but the results were inconclusive. Therefore, as the major saturated fatty acids present in the daily diet are palmitate and stearate, we examined the effects of these saturated fatty acids on UPR in adipocytes. Here, we show that saturated fatty acids caused limited activation of the UPR in adipocytes. Exposure to stearate for several hours elevated the ratio of spliced XBP-1 mRNA, and this effect was stronger than that of palmitate. Moreover, the phosphorylation level of IRE1α, upstream of XBP-1 and expression levels of its downstream targets such as DNAJB9 and Pdia6 were elevated in 3T3-L1 adipocytes exposed to stearate. On the other hand, stearate did not affect the phosphorylation of PERK, its activation of CHOP, or the cleavage of ATF6α. Thus, in adipocytes, exposure to stearate activates the UPR via the IRE1α/XBP-1 pathway, but not the PERK/CHOP and ATF6α pathway.

Monounsaturated fatty acids protect against palmitate-induced lipoapoptosis in human umbilical vein endothelial cells

Diets high in saturated fatty acids are linked to increased cardiovascular disease risk, whereas monounsaturated fatty acids have been associated with improved cardiovascular outcomes. Accordingly, cell culture studies have demonstrated that saturated fatty acids, particularly long chain saturated fatty acids such as palmitate, induce dysfunction and cell death in a variety of cell types, and monounsaturated fatty acids may confer protection against palmitate-mediated damage. The aim of the present study was to examine whether monounsaturated fatty acids could protect against palmitate-mediated cell death in endothelial cells, to determine if AMPK inactivation and activation (via compound C and AICAR, respectively) underlies both palmitate-induced damage and monounsaturated fatty acid-mediated protection, and to explore the role of ER stress in this context. Human umbilical vein endothelial cells were examined for cell viability and apoptosis following treatment for 24 hours with palmitate (0.25 and 0.5mM) alone or in combination with the monounsaturated fatty acids oleate or palmitoleate (0.25 and 0.5mM), AICAR, compound C, 4μ8C, or TUDCA. Compared to control cells, palmitate significantly decreased cell viability and increased apoptosis in a dose-dependent manner. The monounsaturated fatty acids oleate and palmitoleate completely prevented the cytotoxic effects of palmitate. Although palmitate induced markers of ER stress, chemical inhibition of ER stress did not prevent palmitate-induced lipoapoptosis. Conversely, the AMPK activator AICAR (0.1 and 0.5mM) conferred protection from palmitate mediated-alterations in viability, apoptosis and ER stress, whereas the AMPK inhibitor compound C (20 and 40μM) significantly exacerbated palmitate-mediated damage. Lastly, co-incubation with palmitate, monounsaturated fatty acids, and compound C significantly mitigated the protective effects of both oleate and palmitoleate. In conclusion, monounsaturated fatty acids confer protection against the cytotoxic effects of palmitate in vascular endothelial cells; and palmitate-mediated damage, as well as monounsaturated-mediated protection, are due in part to inactivation and activation, respectively, of the metabolic regulator AMPK. These results may have implications for understanding the deleterious effects of high saturated fat diets on cardiovascular dysfunction and disease risk.

AEBP1 expression increases with severity of fibrosis in NASH and is regulated by glucose, palmitate, and miR-372-3p

Factors governing the development of liver fibrosis in nonalcoholic steatohepatitis (NASH) are only partially understood. We recently identified adipocyte enhancer binding protein 1 (AEBP1) as a member of a core set of dysregulated fibrosis-specific genes in human NASH. Here we sought to investigate the relationship between AEBP1 and hepatic fibrosis. We confirmed that hepatic AEBP1 expression is elevated in fibrosis compared to lobular inflammation, steatosis, and normal liver, and increases with worsening fibrosis in NASH patients. AEBP1 expression was upregulated 5.8-fold in activated hepatic stellate cells and downregulated during chemical and contact induction of biological quiescence. In LX-2 and HepG2 cells treated with high glucose (25 mM), AEBP1 expression increased over 7-fold compared to normal glucose conditions. In response to treatment with either fructose or palmitate, AEBP1 expression in primary human hepatocytes increased 2.4-fold or 9.6-fold, but was upregulated 55.8-fold in the presence of fructose and palmitate together. AEBP1 knockdown resulted in decreased expression of nine genes previously identified to be part of a predicted AEBP1-associated NASH co-regulatory network and confirmed to be upregulated in fibrotic tissue. We identified binding sites for two miRNAs known to be downregulated in NASH fibrosis, miR-372-3p and miR-373-3p in the AEBP1 3' untranslated region. Both miRNAs functionally interacted with AEBP1 to regulate its expression. These findings indicate a novel AEBP1-mediated pathway in the pathogenesis of hepatic fibrosis in NASH.

Exposure to bis(maltolato)oxovanadium(IV) increases levels of hepcidin mRNA and impairs the homeostasis of iron but not that of manganese

The aim of this study was to examine whether alterations in iron homeostasis, caused by exposure to vanadium, are related to changes in the gene expression of hepatic hepcidin. Two groups of rats were examined: control and vanadium-exposed. Vanadium, as bis(maltolato)oxovanadium(IV) was supplied in the drinking water. The experiment had a duration of five weeks. Iron and manganese were measured in excreta, serum and tissues. Leptin, ferritin, IL-1β, IL-6, TNF-α, red blood cells, haemoglobin and haematocrit were determined. Protein carbonyl group levels and hepcidin gene expression were determined in the liver. In the vanadium-exposed rats, iron absorption, serum iron and leptin and all haematological parameters decreased. Levels of IL-6, TNF-α and ferritin in serum and of iron in the liver, spleen and heart increased. In the liver, levels of protein carbonyl groups and hepcidin mRNA were also higher in the vanadium-exposed group. Exposure to vanadium did not modify manganese homeostasis. The results obtained from this study provide the first evidence that bis(maltolato)oxovanadium(IV) produces an increase in the gene expression of the hepcidin, possibly caused by an inflammatory process. Both factors could be the cause of alterations in Fe homeostasis and the appearance of anaemia. However, Mn homeostasis was not affected.