Postprandial VLDL-TG metabolism in type 2 diabetes

Adipose ZFP36 protects against diet-induced obesity and insulin resistance

AIMS

Obesity, as a worldwide healthcare problem, has become more prevalent. ZFP36 is a well-known RNA-binding protein and involved in the posttranscriptional regulation of many physiological processes. Whether the adipose ZFP36 plays a role in obesity and insulin resistance remains unclear.

METHODS

The expression levels of ZFP36 were analyzed in adipose tissues of obese patients, diet-induced obese mice, ob/ob mice and db/db mice. To determine whether adipose ZFP36 protects against the diet-induced obesity, we generated adipose-specific ZFP36 knockout (ZFP36AKO) mice, which were subjected to high-fat-diet (HFD) for 16 weeks. To explore the specific molecular mechanisms of ZFP36 regulating metabolic disorders, we used gene array assay of control and ZFP36-deficient adipose tissue, and assessed the pathways in vitro and vivo.

RESULTS

Western blotting and RT-PCR were performed to determine the downregulating level of ZFP36 in adipose tissues of obese patients, diet-induced obese mice, ob/ob mice and db/db mice. Relative to control mice, ZFP36AKO mice were more susceptible to HFD-induced obesity, along with insulin resistance, glucose tolerance, and increased metabolic disorders. The obesity of ZFP36AKO mice was attributed to hypertrophy of adipocytes in white adipose tissue via decreased expression of Perilipin1 (PLIN1), adipose triglyceride lipase (ATGL), and hormone-sensitive lipase (HSL). We discovered that ZFP36 oppositely regulated RNF128 expression by repressing the mRNA stability and translation of RNF128, a negative regulator of Sirt1 expression.

CONCLUSIONS

This study suggests that ZFP36 in adipose tissue plays an important role in diet-induced obesity, and identifies a novel molecular signaling pathway of ZFP36/RNF128/Sirt1 involved in obesity.

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.

Emerging Evidence of Pathological Roles of Very-Low-Density Lipoprotein (VLDL)

Embraced with apolipoproteins (Apo) B and Apo E, triglyceride-enriched very-low-density lipoprotein (VLDL) is secreted by the liver into circulation, mainly during post-meal hours. Here, we present a brief review of the physiological role of VLDL and a systemic review of the emerging evidence supporting its pathological roles. VLDL promotes atherosclerosis in metabolic syndrome (MetS). VLDL isolated from subjects with MetS exhibits cytotoxicity to atrial myocytes, induces atrial myopathy, and promotes vulnerability to atrial fibrillation. VLDL levels are affected by a number of endocrinological disorders and can be increased by therapeutic supplementation with cortisol, growth hormone, progesterone, and estrogen. VLDL promotes aldosterone secretion, which contributes to hypertension. VLDL induces neuroinflammation, leading to cognitive dysfunction. VLDL levels are also correlated with chronic kidney disease, autoimmune disorders, and some dermatological diseases. The extra-hepatic secretion of VLDL derived from intestinal dysbiosis is suggested to be harmful. Emerging evidence suggests disturbed VLDL metabolism in sleep disorders and in cancer development and progression. In addition to VLDL, the VLDL receptor (VLDLR) may affect both VLDL metabolism and carcinogenesis. Overall, emerging evidence supports the pathological roles of VLDL in multi-organ diseases. To better understand the fundamental mechanisms of how VLDL promotes disease development, elucidation of the quality control of VLDL and of the regulation and signaling of VLDLR should be indispensable. With this, successful VLDL-targeted therapies can be discovered in the future.

Postprandial dyslipidemia after a standardized high-fat meal in BMI-matched healthy individuals, and in subjects with prediabetes or type 2 diabetes

BACKGROUND & AIMS

A relationship between postprandial hyperlipidemia and glucose homeostasis/cardiovascular diseases has been suggested. We investigated postprandial plasma lipid patterns after a standardized high-fat meal and their association with glucose homeostasis and subclinical atherosclerosis.

METHODS

Using matching by BMI, 32 healthy individuals with normal glucose tolerance (NGT), 21 subjects with impaired glucose tolerance (IGT), and 20 subjects with drug-naïve type 2 diabetes (T2D) were enrolled. Plasma concentrations of triglycerides (TGs), apolipoprotein-B (ApoB), ApoB48, ApoB100, glucose, and insulin at baseline and 1, 2, 3, 4, 5, 6, and 8 h after a standardized meal (1041.03 kcal with 70.99 g of fat) were measured. Body composition, abdominal visceral fat area, and resting energy expenditure (REE) were measured using dual energy X-ray absorptiometry, computed tomography, and indirect calorimetry, respectively. The intima-media thickness (IMT) of the carotid artery and the ankle-brachial index (ABI) were used to detect subclinical atherosclerosis.

RESULTS

Baseline data and area under the curve (AUC) of plasma concentrations of TGs, ApoB, and ApoB48 in the IGT and T2D groups were higher than in the NGT group. The peak TG concentrations after the meal was observed at 5 h in subjects with IGT and T2D, while healthy subjects showed the highest concentrations at 4 h. In multivariable analysis, high abdominal visceral fat area and low HDL-cholesterol concentrations were independently associated with the AUC_TG and AUC_ApoB after adjusting for confounders including baseline TG and the REE. High LDL-cholesterol and high HbA1c concentrations were also associated with the AUC_ApoB. Furthermore, high AUC_TG and AUC_ApoB values were independent factors for an increased carotid IMT and a low ABI after adjusting for relevant variables.

CONCLUSIONS

Abdominal visceral obesity and low HDL-cholesterol concentrations were associated with increased post load excursions of TGs and ApoB in this series. These elevated concentrations of TGs and ApoB were linked with subclinical atherosclerosis.

Cardiovascular disease in diabetes, beyond glucose

Despite the decades-old knowledge that diabetes mellitus is a major risk factor for cardiovascular disease, the reasons for this association are only partially understood. While this association is true for both type 1 and type 2 diabetes, different pathophysiological processes may be responsible. Lipids and other risk factors are indeed important, whereas the role of glucose is less clear. This lack of clarity stems from clinical trials that do not unambiguously show that intensive glycemic control reduces cardiovascular events. Animal models have provided mechanisms that link diabetes to increased atherosclerosis, and evidence consistent with the importance of factors beyond hyperglycemia has emerged. We review clinical, pathological, and animal studies exploring the pathogenesis of atherosclerosis in humans living with diabetes and in mouse models of diabetes. An increased effort to identify risk factors beyond glucose is now needed to prevent the increased cardiovascular disease risk associated with diabetes.

Body composition, but not insulin resistance, influences postprandial lipemia in patients with Turner's syndrome

OBJECTIVE

The aim of the present study was to examine the influence of body composition and insulin resistance on the magnitude of postprandial lipemia in patients with Turner's syndrome receiving oral versus transdermal estrogen replacement.

METHODS

Twenty-five patients with Turner's syndrome receiving oral or transdermal estrogen replacement were evaluated for body mass index, waist-to-hip and waist-to-height ratios, fasting glycemia, insulin, body composition (dual-energy X-ray absorptiometry), and postprandial lipid metabolism. For statistical analysis, we used parametric tests to compare numeric variables between the two subgroups.

RESULTS

We observed no difference in postprandial triglyceride levels between patients receiving oral versus transdermal hormone replacement therapy. The postprandial triglycerides increment correlated positively with the percentage of total fat mass (p=0.02) and android fat mass (p=0.02) in the transdermal group. In the oral estrogen group, a positive correlation was observed between the increment in postprandial triglycerides and waist-to-hip (p=0.15) and waist-to-height (p=0.009) ratios. No association was observed between the estrogen replacement route and insulin resistance evaluated by the homeostatic model assessment-insulin resistance (HOMA-IR) index (p=0.19 and p=0.65 for the oral and transdermal groups, respectively).

CONCLUSION

We concluded that body composition and anthropometric characteristics possibly affect the extent of postprandial lipemia independently from the route of estrogen replacement.

Postprandial triglyceride reduction following acute treatment of a selective 5-hydroxytryptamine-2c agonist and characterization using a semi-physiological model

AIM

To investigate the tolerability, pharmacokinetics (PK) and postprandial triglyceride (TG) response of single, escalating oral doses of a selective 5-hydroxytryptamine-2c (5-HT_2c ) agonist in subjects with overweight/obesity and apply mechanistic population pharmacokinetic-pharmacodynamic modelling to identify a plausible drug mechanism of action.

MATERIALS AND METHODS

This phase 1, single-centre, double-blind, randomized, placebo-controlled, four-period, two-alternating cohorts study evaluated single escalating oral doses ranging from 5 to 130 mg of LY2140112 (LY) in subjects with overweight/obesity (body mass index: 27-39 kg/m² ). Postprandial TG response (total TG, chylomicrons and very low-density lipoprotein particles [VLDL]-V6) following a high-fat meal were assessed for 11 h postmeal for each dose level. The PK profile was assessed for 96 h postdose. Drug exposure and TG concentrations in chylomicrons and VLDL-V6 were used to characterize the drug mechanism of action using non-linear mixed-effect modelling.

RESULTS

Seventeen subjects entered the study and 16 subjects received at least one dose of LY. LY2140112 was generally well tolerated up to 75 mg. The PK of LY were described by a two-compartment model with first-order elimination. The 100 and 130 mg dose levels of LY significantly reduced the postprandial TG of VLDL-V6 by approximately 50%, while total TG and chylomicrons were not significantly different from placebo. The application of a published lipokinetic model successfully described the postprandial TG response in this study and indicated that LY reduced the conversion of TGs from chylomicron to VLDL-V6.

CONCLUSIONS

LY significantly reduced the postprandial TG of VLDL-V6 following a single dose, when food consumption was controlled. The data indicate that a selective 5-HT_2c agonist alters lipid metabolism, beyond the reported reduction in satiety. The application of a semi-physiological lipokinetic model enabled identification of a plausible drug mechanism of action of LY.

100th anniversary of the discovery of insulin perspective: insulin and adipose tissue fatty acid metabolism

Insulin inhibits systemic nonesterified fatty acid (NEFA) flux to a greater degree than glucose or any other metabolite. This remarkable effect is mainly due to insulin-mediated inhibition of intracellular triglyceride (TG) lipolysis in adipose tissues and is essential to prevent diabetic ketoacidosis, but also to limit the potential lipotoxic effects of NEFA in lean tissues that contribute to the development of diabetes complications. Insulin also regulates adipose tissue fatty acid esterification, glycerol and TG synthesis, lipogenesis, and possibly oxidation, contributing to the trapping of dietary fatty acids in the postprandial state. Excess NEFA flux at a given insulin level has been used to define in vivo adipose tissue insulin resistance. Adipose tissue insulin resistance defined in this fashion has been associated with several dysmetabolic features and complications of diabetes, but the mechanistic significance of this concept is not fully understood. This review focusses on the in vivo regulation of adipose tissue fatty acid metabolism by insulin and the mechanistic significance of the current definition of adipose tissue insulin resistance. One hundred years after the discovery of insulin and despite decades of investigations, much is still to be understood about the multifaceted in vivo actions of this hormone on adipose tissue fatty acid metabolism.

ANGPLT3 in cardio-metabolic disorders

Dyslipidemia is associated with numerous health problems that include the combination of insulin resistance, hypertension and obesity, which is always grouped together asmetabolic syndrome. Given that metabolic syndrome leads to a high mortality and poses serious risks to human health worldwide, it is vital to explore the mechanisms whereby dyslipidemia modulates the risk and the severity of cardio-metabolic disorders. Recently, a specific secretory protein family, named angiopoietin-like protein (ANGPTL), is considered as one of the significant biomarkers which facilitate the development of angiogenesis. Among the eight proteins of ANGPTL family, ANGPTL3 has been demonstrated as an essential modulator of lipid catabolism within circulation by inhibiting the activity of lipoprotein lipase (LPL) and endothelial lipase (EL). Consistent with these notions, mice with ANGPTL3 gene-deficiency presented reduced circulating levels of low density lipoprotein cholesterol (LDL-C) and lower risk of atherosclerosis. On the other hand, participants carrying homozygous loss-of function (LOF) mutation in ANGPTL3 gene also displayed lower circulating LDL-C levels and atherosclerotic risk. In the current review, we summarized the recent understanding of ANGPTL3 in controlling the risk and the development of dyslipidemia and its related cardio-metabolic disorders. Moreover, we also provided the perspectives which potentially suggested that ANGPTL3 could be considered as a promising target in treating metabolic syndrome.

Efficacy and Mechanism of Polymerized Anthocyanin from Grape-Skin Extract on High-Fat-Diet-Induced Nonalcoholic Fatty Liver Disease

We investigated the therapeutic potential of polymerized anthocyanin (PA) on a nonalcoholic fatty liver disease (NAFLD) model in mice. C57BL/6 mice were fed a high-fat diet (HFD) for 8 weeks to establish the NAFLD mouse model and randomly divided into four groups: control diet (con), NAFLD mice treated with saline (NAFLD), NAFLD mice treated with PA (PA), and NAFLD mice treated with orlistat (Orlistat) for four weeks. Mice were euthanized at the end of the four weeks. Total cholesterol (TC) and triglyceride (TG) levels were estimated, and pathological changes in the liver, white adipose tissue, and signaling pathways related to lipid metabolism were evaluated. Results revealed that the body, liver, and white fat weight of the NAFLD group was significantly increased compared to that of the con group, while that of the PA group showed significant reduction. NAFLD led to an increase in blood lipids in mice (except for HDL). Conversely, PA effectively reduced TC and LDL-C. Compared to the control group, the degree of steatosis in the mice of PA group was decreased. Moreover, PA also regulated the NAFLD signaling pathway. In agreement with improved lipid deposition, PA supplementation inhibited the activation of inflammatory pathways, depressing oxidative stress through increased antioxidant levels, and increasing β-oxidation to inhibit mitochondrial dysfunction. Taken together, our results demonstrate that PA can improve the liver function of NAFLD mice, regulating blood lipids, reducing liver-fat accumulation, and regulating lipid metabolism.

The influence of gene polymorphisms on postprandial triglyceride response after oral fat tolerance test meal in patients with diabetes mellitus

AIMS

We evaluated the influence of CETP (rs5882 and rs708272), APOE (rs7412, rs429358) and LPL (rs328) gene polymorphisms on triglyceride (TG) response to oral fat tolerance test (OFTT) meal in patients with well-controlled type 2 diabetes mellitus (T2DM).

METHODS

Fifty-one men underwent OFTT and according to postprandial TG response patients were divided into two subgroups (positive [TG ≥ 220 mg/dL, 31 patients] and negative [TG < 220 mg/dL, 20 patients]). All patients were genotyped, and study variants were detected using polymerase chain reaction (PCR) and restricted fragment length polymorphism (RFLP) analysis.

RESULTS

Patients with genotype SS of LPL gene compared with genotype SX had more frequently positive response to OFTT (P = .04) and lower high-density lipoprotein cholesterol (HDL-C) concentration (P = .03). Patients with positive response to OFTT and genotype SS of LPL gene compared with genotype SX had lower AUC (area under the curve)-TG, 1744 (368) vs 1887 (807) mg/dL/h, respectively, P = .04. CETP and APOE gene polymorphisms had no influence on postprandial TG response to OFTT.

CONCLUSIONS

In patients with well-controlled T2DM, LPL but not CETP and APOE gene polymorphisms influenced TG postprandial response. Particularly, S447 allele carriers of LPL gene presented more frequently positive postprandial TG response to OFTT compared with 447X allele carriers. No differences were found between allele carriers of patients with negative response to OFTT in any other studied gene polymorphism.

Impact of decreased insulin resistance by ezetimibe on postprandial lipid profiles and endothelial functions in obese, non-diabetic-metabolic syndrome patients with coronary artery disease

The association between insulin resistance and lipid dysmetabolism after consuming a meal is unclear. We aimed at assessing the effects of ezetimibe on postprandial hyperlipidemia and hyperinsulinemia and to find out whether the medication improves endothelial function in obese metabolic syndrome (MetS) patients with coronary artery disease (CAD). We obtained oral fat loading test results (4 and 6 h after load) and brachial flow-mediated vasodilation (FMD) measurements before and 24 weeks after ezetimibe treatment initiation from 27 MetS patients with CAD and from 68 control patients with CAD alone. Serum triglyceride (TG) and insulin levels (2 h after the loading dose) were significantly higher in MetS patients than in control patients. The incremental areas under the curve (iAUCs) for these levels decreased significantly after ezetimibe treatment in MetS patients but not in control patients. Treatment with ezetimibe resulted in significant FMD changes in MetS patients (from 3.4 to 4.9%, P = 0.002), but not in control patients (from 5.1 to 5.4%, P = 0.216). When MetS patients were divided into two groups based on the median insulin iAUC reduction rate (higher group ≥ 34%, n = 14; lower group < 34%, n = 13), those in the higher group showed a significantly higher rate of change in the iAUCs of TG and FMD than those in the lower group (TG, 31.0% vs. 10.8%; P = 0.033; FMD, 39.2% vs. 9.8%; P = 0.037). These results suggest that ezetimibe may reverse insulin resistance, reducing lipid dysmetabolism after a meal and endothelial dysfunction in MetS patients with CAD.

Decreased GPIHBP1 protein levels in visceral adipose tissue partly underlie the hypertriglyceridemic phenotype in insulin resistance

GPIHBP1 is a protein localized at the endothelial cell surface that facilitates triglyceride (TG) lipolysis by binding lipoprotein lipase (LPL). Whether Glycosyl Phosphatidyl Inositol high density lipoprotein binding protein 1 (GPIHBP1) function is impaired and may underlie the hyperTG phenotype observed in type 2 diabetes is not yet established. To elucidate the mechanism underlying impaired TG homeostasis in insulin resistance state we studied the effect of insulin on GPIHBP1 protein expression in human microvascular endothelial cells (HMVEC) under flow conditions. Next, we assessed visceral adipose tissue GPIHBP1 protein expression in type 2 diabetes Lepr^db/db mouse model as well as in subjects with ranging levels of insulin resistance. We report that insulin reduces the expression of GPIHBP1 protein in HMVECs. Furthermore, GPIHBP1 protein expression in visceral adipose tissue in Lepr^db/db mice is significantly reduced as is the active monomeric form of GPIHBP1 as compared to Lepr^db/m mice. A similar decrease in GPIHBP1 protein was observed in subjects with increased body weight. GPIHBP1 protein expression was negatively associated with insulin and HOMA-IR.

In conclusion, our data suggest that decreased GPIHBP1 availability in insulin resistant state may hamper peripheral lipolysis capacity.

Unexplained reciprocal regulation of diabetes and lipoproteins

PURPOSE OF REVIEW

Type 2 diabetes is associated with a characteristic dyslipidemia that may exacerbate cardiovascular risk. The causes of, and the effects of new antihyperglycemia medications on, this dyslipidemia, are under investigation. In an unexpected reciprocal manner, lowering LDL-cholesterol with statins slightly increases the risk of diabetes. Here we review the latest findings.

RECENT FINDINGS

The inverse relationship between LDL-cholesterol and diabetes has now been confirmed by multiple lines of evidence. This includes clinical trials, genetic instruments using aggregate single nucleotide polymorphisms, as well as at least eight individual genes - HMGCR, NPC1L1, HNF4A, GCKR, APOE, PCKS9, TM6SF2, and PNPLA3 - support this inverse association. Genetic and pharmacologic evidence suggest that HDL-cholesterol may also be inversely associated with diabetes risk. Regarding the effects of diabetes on lipoproteins, new evidence suggests that insulin resistance but not diabetes per se may explain impaired secretion and clearance of VLDL-triglycerides. Weight loss, bariatric surgery, and incretin-based therapies all lower triglycerides, whereas SGLT2 inhibitors may slightly increase HDL-cholesterol and LDL-cholesterol.

SUMMARY

Diabetes and lipoproteins are highly interregulated. Further research is expected to uncover new mechanisms governing the metabolism of glucose, fat, and cholesterol. This topic has important implications for treating type 2 diabetes and cardiovascular disease.

VLDL triglyceride accumulation in skeletal muscle and adipose tissue in type 2 diabetes

PURPOSE OF REVIEW

Insulin resistance is closely linked to accumulation of lipid outside adipose tissue (ectopic fat storage). VLDL particles transport lipids from the liver to peripheral tissues. However, whether abnormalities in VLDL-triglyceride storage in muscle and adipose tissue exist in type 2 diabetes has previously been unknown, primarily because of methodological difficulties. Here, we review recent research on VLDL-triglyceride storage.

RECENT FINDINGS

In a recent study, men with type 2 diabetes had increased skeletal muscle VLDL-triglyceride storage compared to weight-matched nondiabetic men, potentially leading to intramyocellular triglyceride accumulation. In contrast, studies of adipose tissue VLDL-triglyceride storage have shown similar storage capacity in men with and without diabetes, both in the postabsorptive and the postprandial period. In the initial submission, studies have failed to show associations between lipoprotein lipase activity, considered the rate-limiting step in storage of lipids from lipoproteins, and VLDL-TG storage in both muscle and adipose tissue.

SUMMARY

Differences in muscle VLDL-triglyceride storage may lead to ectopic fat storage and contribute to the development of type 2 diabetes, whereas the ability to store VLDL-triglyceride in adipose tissue is preserved in type 2 diabetes.