Effect of Hypertriglyceridemia on Beta Cell Mass and Function in ApoC3 Transgenic Mice

Apolipoprotein C-III itself stimulates the Syk/cPLA2-induced inflammasome activation of macrophage to boost anti-tumor activity of CD8+ T cell

Increased prevalence of cancer in obese individuals is involved with dyslipidemia- induced chronic inflammation and immune suppression. Although apolipoprotein C-III (ApoC3)-transgenic mice (ApoC3TG mice) or poloxamer 407 (P407)-treated mice had hyperlipidemia, CD8+ T cells with upregulated antitumor activities were observed in ApoC3TG mice, and decreased CD8+ T cell activities were observed in P407-treated mice. Increased ApoC3 expression in hepatocellular carcinoma was associated with increased infiltration of CD8+ T cells and predicted survival. Recombinant ApoC3 had no direct effects on CD8+ T cells. The upregulation of CD8+ T cells in ApoC3TG mice was due to cross-talk with context cells, as indicated by metabolic changes and RNA sequencing results. In contrast to dendritic cells, the macrophages of ApoC3TG mice (macrophagesTG) displayed an activated phenotype and increased IL-1β, TNF-α, and IL-6 production. Coculture with macrophagesTG increased CD8+ T cell function, and the adoptive transfer of macrophagesTG suppressed tumor progression in vivo. Furthermore, spleen tyrosine kinase (Syk) activation induced by TLR2/TLR4 cross-linking after ApoC3 ligation promoted cellular phospholipase A2 (cPLA2) activation, which in turn activated NADPH oxidase 2 (NOX2) to promote an alternative mode of inflammasome activation. Meanwhile, mitochondrial ROS produced by increased oxidative phosphorylation of free fatty acids facilitated the classical inflammasome activation, which exerted an auxiliary effect on inflammasome activation of macrophagesTG. Collectively, the increased antitumor activity of CD8+ T cells was mediated by the ApoC3-stimulated inflammasome activation of macrophages, and the mimetic ApoC3 peptides that can bind TLR2/4 could be a future strategy to target liver cancer.

Quartet of APOCs and the Different Roles They Play in Diabetes

APOA1 and APOB are the structural proteins of high-density lipoprotein and APOB-containing lipoproteins, such as low-density lipoprotein and very low-density lipoprotein, respectively. The 4 smaller APOCs (APOC1, APOC2, APOC3, and APOC4) are exchangeable apolipoproteins; they are readily transferred among high-density lipoproteins and APOB-containing lipoproteins. The APOCs regulate plasma triglyceride and cholesterol levels by modulating substrate availability and activities of enzymes interacting with lipoproteins and by interfering with APOB-containing lipoprotein uptake through hepatic receptors. Of the 4 APOCs, APOC3 has been best studied in relation to diabetes. Elevated serum APOC3 levels predict incident cardiovascular disease and progression of kidney disease in people with type 1 diabetes. Insulin suppresses APOC3 levels, and accordingly, elevated APOC3 levels associate with insulin deficiency and insulin resistance. Mechanistic studies in a mouse model of type 1 diabetes have demonstrated that APOC3 acts in the causal pathway of diabetes-accelerated atherosclerosis. The mechanism is likely due to the ability of APOC3 to slow the clearance of triglyceride-rich lipoproteins and their remnants, thereby causing an increased accumulation of atherogenic lipoprotein remnants in lesions of atherosclerosis. Less is known about the roles of APOC1, APOC2, and APOC4 in diabetes.

Lipid-induced glucose intolerance is driven by impaired glucose kinetics and insulin metabolism in healthy individuals

AIMS

Hypertriglyceridemia is associated with an increased risk of type 2 diabetes. We aimed to comprehensively examine the effects of hypertriglyceridemia on major glucose homeostatic mechanisms involved in diabetes progression.

METHODS

In this randomized, cross-over, single-blinded study, two dual-labeled, 3-hour oral glucose tolerance tests were performed during 5-hour intravenous infusions of either 20 % Intralipid or saline in 12 healthy subjects (age 27.9 ± 2.6 years, 11 men, BMI 22.6 ± 1.4 kg/m²) to evaluate lipid-induced changes in insulin metabolism and glucose kinetics. Insulin sensitivity, β cell secretory function, and insulin clearance were assessed by modeling glucose, insulin and C-peptide data. Intestinal glucose absorption, endogenous glucose production, and glucose clearance were assessed from glucose tracers. The effect of triglycerides on β-cell secretory function was examined in perifusion experiments in murine pseudoislets and human pancreatic islets.

RESULTS

Mild acute hypertriglyceridemia impaired oral glucose tolerance (mean glucose: +0.9 [0.3, 1.5] mmol/L, p = 0.008) and whole-body insulin sensitivity (Matsuda index: -1.67 [-0.50, -2.84], p = 0.009). Post-glucose hyperinsulinemia (mean insulin: +99 [17, 182] pmol/L, p = 0.009) resulted from reduced insulin clearance (-0.16 [-0.32, -0.01] L min^-1 m^-2, p = 0.04) and enhanced hyperglycemia-induced total insulin secretion (+11.9 [1.1, 22.8] nmol/m², p = 0.02), which occurred despite a decline in model-derived β cell glucose sensitivity (-41 [-74, -7] pmol min^-1 m^-2 mmol^-1 L, p = 0.04). The analysis of tracer-derived glucose metabolic fluxes during lipid infusion revealed lower glucose clearance (-96 [-152, -41] mL/kg_FFM, p = 0.005), increased 2-hour oral glucose absorption (+380 [42, 718] μmol/kg_FFM, p = 0.04) and suppressed endogenous glucose production (-448 [-573, -123] μmol/kg_FFM, p = 0.005). High-physiologic triglyceride levels increased acute basal insulin secretion in murine pseudoislets (+11 [3, 19] pg/aliquot, p = 0.02) and human pancreatic islets (+286 [59, 512] pg/islet, p = 0.02).

CONCLUSION

Our findings support a critical role for hypertriglyceridemia in the pathogenesis of type 2 diabetes in otherwise healthy individuals and dissect the glucose homeostatic mechanisms involved, encompassing insulin sensitivity, β cell function and oral glucose absorption.

Gestational Diabetes Sensitizes Mice to Future Metabolic Syndrome That Can Be Relieved by Activating CAR

Diabetes and related metabolic syndrome are common metabolic disorders. Gestational diabetes mellitus (GDM) is rather prevalent in the clinic. Although most GDM resolves after therapeutic intervention and/or after delivery, the long-term health effect of GDM remains to be better understood. The constitutive androstane receptor (CAR), initially characterized as a xenobiotic receptor, was more recently proposed to be a therapeutic target for obesity and type 2 diabetes mellitus (T2DM). In this study, high-fat diet (HFD) feeding was used to induce GDM. Upon delivery, GDM mice were returned to chow diet until the metabolic parameters were normalized. Parous non-GDM control females or metabolically normalized GDM females were then subjected to HFD feeding to induce nongestational obesity and T2DM. Our results showed that GDM sensitized mice to metabolic abnormalities induced by a second hit of HFD. Treatment with the CAR agonist 1,4-bis [2-(3,5 dichloropyridyloxy)] benzene efficiently attenuated GDM-sensitized and HFD-induced obesity and T2DM, including decreased body weight, improved insulin sensitivity, inhibition of hyperglycemia and hepatic steatosis, increased oxygen consumption, and decreased adipocyte hypertrophy. In conclusion, our results have established GDM as a key risk factor for the future development of metabolic disease. We also propose that CAR is a therapeutic target for the management of metabolic disease sensitized by GDM.

Lipid Droplet Accumulation in Human Pancreatic Islets Is Dependent On Both Donor Age and Health

Human but not mouse islets transplanted into immunodeficient NSG mice effectively accumulate lipid droplets (LDs). Because chronic lipid exposure is associated with islet β-cell dysfunction, we investigated LD accumulation in the intact human and mouse pancreas over a range of ages and states of diabetes. Very few LDs were found in normal human juvenile pancreatic acinar and islet cells, with numbers subsequently increasing throughout adulthood. While accumulation appeared evenly distributed in postjuvenile acinar and islet cells in donors without diabetes, LDs were enriched in islet α- and β-cells from donors with type 2 diabetes (T2D). LDs were also found in the islet β-like cells produced from human embryonic cell-derived β-cell clusters. In contrast, LD accumulation was nearly undetectable in the adult rodent pancreas, even in hyperglycemic and hyperlipidemic models or 1.5-year-old mice. Taken together, there appear to be significant differences in pancreas islet cell lipid handling between species, and the human juvenile and adult cell populations. Moreover, our results suggest that LD enrichment could be impactful to T2D islet cell function.

Electrophysiological characterization of acutely isolated spiral ganglion neurons in neonatal and mature sonic hedgehog knock-in mice

Spiral ganglion neurons (SGNs) are primary afferent auditory neurons activated by inner hair cells in mammalian cochlea. Here, for the convenience of SGN studies such as patch-clamp or single cell RNA-sequence studies, a knock-in mouse (Shh^CreEGFP/+; Rosa26-Tdtomato^loxp/+) was generated for the purpose of obtaining fluorescence SGNs. Auditory brainstem response (ABR) and Tuj1 immunohistochemistry staining were performed to verify the hearing function and the morphological characteristics. The results showed that there was no significant difference between shh and wild type mice. In electrophysiological studies, we verified a series of electrophysiological characteristics including the amplitude of sodium and potassium currents and action potential characteristics of shh and wild type mice and no significant differences were found either. From the above, shh mice have the same cell function and morphology as their littermate control wild type mice and could be used as an ideal tool to study the function and characteristics of spiral ganglion neurons. Potassium channels of SGNs play an important role in resolving time accuracy. We obtained similar amplitude of I_K⁺ in neonatal and mature mice in the aging competition experiment, however, the density of I_K⁺ from mature mice were significantly different from those of neonatal mice, a phenomenon that may play a key role in the nervous system. Potassium channels have been shown to contribute to apoptosis induced by cisplatin administration in various cell lines. Here we used cisplatin administration to study the ototoxicity and found that the effects of a low dose of cisplatin (0.5 mM correspond to therapeutic doses) causes a decrease in currents and is reversible after a short administration time. Moreover, we propose the activated state of potassium channels has changed but the characteristic and number remain still after cisplatin administration. The excess potassium ions may accumulate in the cell body, which had affected the firing properties and induce cytotoxicity and apoptosis. We suggest that the electrophysiological properties of acutely isolated SGNs may support further research on the mechanics of auditory propagation and ion channel pharmacology.

Apolipoprotein CIII and diabetes. Is there a link?

Apolipoprotein CIII (ApoCIII), a small protein that resides on the surface of lipoprotein particles, is a key regulator of triglyceride metabolism. The inhibition of lipoprotein lipase (LPL), the increased assembly and secretion of very low-density lipoproteins (VLDL) and the decreased reuptake of triglyceride-rich lipoproteins (TRLs) by the liver are mechanisms associating elevated serum ApoCIII levels and hypertriglyceridemia. ApoCIII concentration is high in individuals with diabetes mellitus, indicating a possible positive correlation with impairment of glucose metabolism. The aim of this review (based on a Pubmed search until August 2018) is to present the possible mechanisms linking ApoCIII and deterioration of carbohydrate homeostasis. ApoCIII enhances pancreatic β-cells apoptosis via an increase of the cytoplasmic Ca²⁺ levels in the insulin-producing cells. In addition, overexpression of ApoCIII enhances non-alcoholic fatty liver disease and exacerbates inflammatory pathways in skeletal muscles, affecting insulin signalling and thereby inducing insulin resistance. Moreover, recent studies reveal a possible mechanism of body weight increase and glucose production through a potential ApoCIII-induced LPL inhibition in the hypothalamus. Also, the presence of ApoCIII on the surface of high-density lipoprotein particles is associated with impairment of their antiglycemic and atheroprotective properties. Modulating ApoCIII may be a potent therapeutic approach to manage hypertriglyceridemia and improve carbohydrate metabolism.

Sex- and Tissue-Specific Role of Estrogen Sulfotransferase in Energy Homeostasis and Insulin Sensitivity

Estrogen sulfotransferase catalyzes the sulfoconjugation and deactivation of estrogens. Previously, we showed that loss of Est in male ob/ob mice, but not in female ob/ob mice, exacerbated the diabetic phenotype, but the underlying mechanism was unclear. In this study, we show that transgenic reconstitution of Est in the adipose tissue, but not in the liver, attenuated diabetic phenotype in Est-deficient ob/ob mice (obe mice). Mechanistically, adipose reconstitution of Est in obe mice (oae mice) resulted in reduced local and systemic inflammation, improved insulin sensitivity, and increased energy expenditure. At the molecular level, adipose induction of lipocalin-2 (Lcn2) in oae males may have contributed to the inhibition of inflammation because the level of Lcn2 was negatively associated with tumor necrosis factor (Tnf) α expression, and treatment of differentiated adipocytes with Lcn2 antagonized Tnfα-responsive inhibition of insulin signaling. The metabolic benefit of adipose reconstitution of Est was sex specific, because adipose reconstitution of Est in obe females had little effect. Interestingly, despite their improved metabolic functions, obe male mice with reconstituted Est in their adipose tissue failed to ameliorate the impairment of the structure and function of the pancreatic islets. In summary, our study uncovers a crucial adipose- and male-specific role of Est in maintaining the whole-body energy homeostasis.

APOC3 Protein Is Not a Predisposing Factor for Fat-induced Nonalcoholic Fatty Liver Disease in Mice

Nonalcoholic fatty liver disease (NAFLD), characterized by excessive fat accumulation in liver, is prevalent in obesity. Genetic factors that link obesity to NAFLD remain obscure. Apolipoprotein C3 (APOC3) is a lipid-binding protein with a pivotal role in triglyceride metabolism. Humans with APOC3 gain-of-function mutations and mice with APOC3 overproduction are associated with hypertriglyceridemia. Nonetheless, it remains controversial whether APOC3 is culpable for diet-induced NAFLD. To address this fundamental issue, we fed APOC3-transgenic and wild-type littermates a high fructose diet or high fat diet, followed by determination of the effect of APOC3 on hepatic lipid metabolism and inflammation and the progression of NAFLD. To gain mechanistic insight into NAFLD, we determined the impact of APOC3 on hepatic triglyceride synthesis and secretion versus fatty acid oxidation. APOC3-transgenic mice were hypertriglyceridemic, culminating in marked elevation of triglycerides, cholesterols, and non-esterified fatty acids in plasma. Despite the prevailing hypertriglyceridemia, APOC3-transgenic mice, relative to wild-type littermates, had similar weight gain and hepatic lipid content without alterations in hepatic expression of key genes involved in triglyceride synthesis and secretion and fatty acid oxidation. APOC3-transgenic and wild-type mice had similar Kupffer cell content without alterations in hepatic expression of pro- and anti-inflammatory cytokines. APOC3 neither exacerbated diet-induced adiposity nor aggravated the degree of steatosis in high fructose or high fat-fed APOC3-transgenic mice. These effects ensued independently of weight gain even after 10-month high fat feeding. We concluded that APOC3, whose dysregulation is liable for hypertriglyceridemia, is not a predisposing factor for linking overnutrition to NAFLD in obesity.