Dietary modulation of plasma angiopoietin-like protein 4 concentrations in healthy volunteers and in patients with type 2 diabetes

Knockdown of ANGPTL4 inhibits adipogenesis of preadipocyte via autophagy

It has been demonstrated that angiopoietin-like protein 4 (ANGPTL4) plays an important regulatory role in lipid metabolism and backfat deposition appears to vary in different pig breeds. However, the correlation between ANGPTL4 and backfat deposition have not been well characterized and the role of ANGPTL4 in regulating adipogenesis remains unclear. Therefore, this study aimed to investigate correlation between ANGPTL4 and backfat deposition and to explore the effects of ANGPTL4 on preadipocyte differentiation and the underlying mechanism. Our results showed that the backfat thickness and the ANGPTL4 gene expression of Laiwu pigs were significantly higher than those in DLY pigs and the ANGPTL4 gene expression was positively correlated with backfat thickness both in DLY pigs and Laiwu pigs. Moreover, an increase in ANGPTL4 expression and activation of autophagy were observed during the differentiation of stromal vascular fraction cells. In addition, knockdown of ANGPTL4 inhibited the differentiation of 3T3-L1 cells with decreased expression of LC3-II and ATG5 and increased expression of SQSTM1, suggesting the involvement of autophagy in ANGPTL4-mediated adipogenesis. In conclusion, these results suggested that ANGPTL4 is positively correlated with backfat deposition in pigs and knockdown of ANGPTL4 inhibits adipogenesis of preadipocyte via autophagy, providing new insights into the regulation of fat deposition and to improve the carcass quality and meat quality of porcine.

The serum levels of angiopoietin-like protein 3 and 4 in type 2 diabetic patients with and without metabolic syndrome compared to the control group

INTRODUCTION

ANGPTLs (Angiopoietin-like proteins) 3 and 4 play an important role in the development of type 2 diabetes. These glycoproteins affect the modulation of glucose and lipid metabolism. They inhibit lipoprotein lipase (LPL) activity and provoke lipolysis. This study was aimed to investigate the protein levels of ANGPTL3 and 4 in the serum of type 2 diabetic patients with metabolic syndrome in comparison to the type 2 diabetic patients without metabolic syndrome and the control group.

METHODS

Three groups of individuals were included in this study; Group I: 47 patients with type 2 diabetes and metabolic syndrome; Group II: 25 patients with type 2 diabetes without metabolic syndrome; Group III: 40 non-diabetic healthy people without metabolic syndrome as a control group. After collection of 5 mL fasting blood samples, serum concentrations of fasting blood sugar (FBS), cholesterol (Chol), triglyceride (TG), HDL-C (High-density lipoprotein-Cholesterol) and LDL-C (Low-density lipoprotein-Cholesterol) were measured by the enzymatic method; blood pressure (BP), height and weight with stadiometers; and ANGPTL3 and 4 by the enzyme-linked immunosorbent assay (ELISA).

RESULTS

The serum levels of ANGPTL3 was significantly different among our three groups (p = .000). In patients with type 2 diabetes and metabolic syndrome (Group I), ANGPTL3 and 4 levels were lower than the control group. The serum levels of the parameters evaluated in this study (except HDL-C) was lower in the group II in comparison with the group I, and this difference was significant for TG, Chol, BP and BMI between these two groups. Also, our results revealed that there was a negative correlation between FBS, TG, Chol, LDL-C and BMI with ANGPTL3 and 4. While, there was a significant positive correlation between ANGPTL4 and ANGPTL3.

CONCLUSION

Altogether, our findings suggest that the decreased levels of ANGPTL3 and 4 may be a causative factor for type 2 diabetes.

Clinical improvement may not reflect metabolic homeostasis normalization in subjects with and without Roux-En-Y bariatric surgery after 12 years: comparison of surgical subjects to a lean cohort

Background

A 12-year study comparing clinical outcomes following Roux-en-Y bariatric surgery showed long-term weight loss with remission/prevention of type-2-diabetes (T2D), hypertension and dyslipidemia. However, it is unknown whether the underlying homeostatic metabolic processes involving hepatokines, adipokines and myokines also normalize. Using this 12-year study, we determined whether metabolic indices improved in post-surgical (BMI:34.4kg/m2) versus non-surgical comparator-subjects-with-obesity (BMI:43.8kg/m2) at 12-year follow-up (both cohorts with baseline diabetes), and if post-surgical subjects normalized their metabolic processes to those of a normal-weight cohort without diabetes.

Methods

Cross-sectional design. Plasma from a cohort of Roux-en-Y bariatric surgery (n=50) and non-surgery (n=76) comparator-subjects-with-obesity (both cohorts at 12-year follow-up) plus a normal-weight cohort (n=39) was assayed by Luminex immunoassay or ELISA for hepatokines [angiopoietin-like proteins-(ANGPTL3; ANGPTL4; ANGPTL6); fibroblast growth factors-(FGF19; FGF21; FGF23)]; adipokines [adipsin; adiponectin; FGF19] and myonectin.

Results

After age and gender adjustment, surgery versus comparator-subjects-with-obesity had lower BMI (34.4 ± 1.0 vs 43.8 ± 0.9kg/m2; p<0.0001), HbA1c (6.2 ± 0.3 vs 7.7 ± 0.2%; p<0.0001), insulin resistance (HOMA-IR, 2.0 ± 1.5 vs 10.8 ± 1.4; p<0.0001) fat mass (45.6 ± 2.2 vs 60.0 ± 2.0; p<0.0001), HDL-C (55.4 ± 2.6 vs 42.6 ± 2.3mg/dL; p<0.0001), triglycerides (130 ± 14 vs 187 ± 12mg/dL; p<0.0001) and higher adiponectin (25.9 ± 2.3 vs 15.7 ± 2.0µg/ml; p<0.001); Adipsin, ANGPTL3, ANGPTL4, ANGPTL6, FGF19, FGF21, FGF23 and myonectin did not differ. Surgery versus normal-weight group: higher ANGPTL4 (156 ± 6 vs 119 ± 7ng/mL; p<0.0001), higher FGF23 (96.4 ± 10.1 vs 50.9 ± 11.5pg/mL; p=0.007) and lower myonectin (744 ± 55 vs 969 ± 66ng/mL; p=0.002); adiponectin, adipsin ANGPTL3, ANGPTL6, FGF19, FGF21 did not differ. Non-surgery comparator-subjects-with-obesity versus normal-weight group: higher adipsin (1859 ± 94 vs 1314 ± 133ng/mL; p=0.0001), higher FGF23 (84.6 ± 8.5 vs 50.9 ± 11.5pg/mL; p<0.0001) and higher ANGPTL4 (171 ± 5 vs 119 ± 7ng/mL; p<0.0001); adiponectin ANGPTL3, ANGPTL6, FGF19, FGF21 and myonectin did not differ.

Conclusion

Bariatric surgery markedly improved anthropometric and metabolic features versus comparator-subjects-with-obesity at 12-year follow-up, indicating benefit of weight loss. However, despite weight loss, these patients still had class-1 obesity, as reflected in the adipokine, hepatokine and myokine markers of body homeostasis that did not completely normalize to indicative values of normal-weight subjects, suggesting either that this is the new normal for these patients or that weight loss to a BMI<25kg/m2 is needed for normalization of these parameters.

Lipoprotein Lipase Activity Does Not Differ in the Serum Environment of Vegans and Omnivores

Although vegan diets have been reported to be associated with a reduced risk of cardiovascular disease, it was not known whether this might be partly due to vegan diets' effects on plasma triglyceride metabolism. This study aimed to investigate if there are differences in the activity of lipoprotein lipase (LPL), an enzyme that functions at the vascular endothelium and is responsible for triglyceride breakdown, in sera obtained from vegans and omnivores. LPL activity was assessed using isothermal titration calorimetry, which allows measurements in undiluted serum samples, mimicking physiological conditions. Fasted sera from 31 healthy participants (12F 2M vegans, 11F 6M omnivores) were analyzed. The results indicated no significant differences in average LPL activity between the vegan and omnivore groups. Interestingly, despite similar triglyceride levels, there were considerable variations in LPL activity and total very-low-density lipoprotein triglyceride breakdowns between individuals within both groups. Biomarker analysis showed that vegans had lower total cholesterol and LDL-C levels compared to omnivores. These findings suggest that the lipid-related benefits of a vegan diet, in terms of atherogenic risk, may primarily stem from cholesterol reduction rather than affecting serum as a medium for LPL-mediated triglyceride breakdown. In healthy individuals, lipid-related changes in serum composition in response to a vegan diet are likely overshadowed by genetic or other lifestyle factors.

ANGPTL4 attenuates palmitic acid-induced endothelial cell injury by increasing autophagy

ANGPTL4, a member of the angiopoietin-like protein family, is reported to be involved in angiogenesis regulation, lipid metabolism, glucose metabolism and redox reactions, among others. Our previous study showed that the plasma ANGPTL4 level was lower in coronary atherosclerotic heart disease (CAHD) and could be a useful predictor of coronary atherosclerosis. However, the molecular mechanism underlying the function of ANGPTL4 in atherosclerosis is poorly understood. In this study, we found that overexpression of ANGPTL4 in HUVECs enhanced cell proliferation and clone-forming ability in vitro, whereas knockdown of ANGPTL4 resulted in the opposite. The expression of ANGPTL4 was upregulated in palmitic acid (PA)-treated HUVECs. Overexpression of ANGPTL4 protected against PA-induced endothelial injury. Knockdown of ANGPTL4 exacerbated the effects of PA on HUVECs. Mechanistically, we demonstrated that ANGPTL4 promoted endothelial cell proliferation through the regulation of autophagy. Knockdown of ATG7 or 3-MA (an autophagy inhibitor) attenuated the effects of ANGPTL4 on endothelial cells. The serum level of ANGPTL4 was downregulated in atherosclerosis mice. Furthermore, the expression of ANGPTL4 was correlated with autophagy-related proteins in aortic tissues of atherosclerotic mice. ANGPTL4 promotes endothelial cell proliferation and suppresses PA-induced endothelial cell injury by increasing autophagy, which may protect against the development of atherosclerosis.

Role and mechanism of the action of angiopoietin-like protein ANGPTL4 in plasma lipid metabolism

Triglycerides are carried in the bloodstream as the components of very low-density lipoproteins and chylomicrons. These circulating triglycerides are primarily hydrolyzed in muscle and adipose tissue by the enzyme lipoprotein lipase (LPL). The activity of LPL is regulated by numerous mechanisms, including by three members of the angiopoietin-like protein family: ANGPTL3, ANGPTL4, and ANGPTL8. In this review, we discuss the recent literature concerning the role and mechanism of action of ANGPTL4 in lipid metabolism. ANGPTL4 is a fasting- and lipid-induced factor secreted by numerous cells, including adipocytes, hepatocytes, (cardio)myocytes, and macrophages. In adipocytes, ANGPTL4 mediates the fasting-induced repression of LPL activity by promoting the unfolding of LPL, leading to the cleavage and subsequent degradation of LPL. The inhibition of LPL by ANGPTL4 is opposed by ANGPTL8, which keeps the LPL active after feeding. In macrophages and (cardio)myocytes, ANGPTL4 functions as a lipid-inducible feedback regulator of LPL-mediated lipid uptake. In comparison, in hepatocytes, ANGPTL4 functions as a local inhibitor of hepatic lipase and possibly as an endocrine inhibitor of LPL in extra-hepatic tissues. At the genetic level, loss-of-function mutations in ANGPTL4 are associated with lower plasma triglycerides and higher plasma HDL-C levels, and a reduced risk of coronary artery disease, suggesting that ANGPTL4 is a viable pharmacological target for reducing cardiovascular risk. Whole-body targeting of ANGPTL4 is contraindicated because of severe pathological complications, whereas liver-specific inactivation of ANGPTL4, either as monotherapy or coupled to anti-ANGPTL3 therapies might be a suitable strategy for lowering plasma triglycerides in selected patient groups. In conclusion, the tissue-specific targeting of ANGPTL4 appears to be a viable pharmacological approach to reduce circulating triglycerides.

Circulating ANGPTL3 and ANGPTL4 levels predict coronary artery atherosclerosis severity

Background

We investigated the role of ANGPTL3 and ANGPTL4 in atherosclerosis development and determined whether plasma concentrations of ANGPTL3 and ANGPTL4 are related to the degree of coronary stenosis.

Methods

A total of 305 consecutive patients with angina who underwent diagnostic coronary angiography were enrolled in the study between August 2017 and August 2018. The levels of ANGPTL3 and ANGPTL4 were measured by using competitive ELISA kits.

Results

According to the degree of coronary artery stenosis, patients were classified into four types: coronary artery stenosis of < 10%, 10-50%, 50-75, and > 75%. The plasma ANGPTL3 level was higher (51.71 ± 52.67 vs. 24.65 ± 10.32 ng/mL, P < 0.001) and that of ANGPTL4 was lower (454.66 ± 269.05 vs. 875.49 ± 961.15 ng/mL, P < 0.001) in the coronary artery stenosis ≥ 10% group than in the < 10% group. ANGPTL3 and ANGPTL4 levels were significantly associated with the severity of coronary vascular stenosis. ROC curve analyses indicated that ANGPTL3 concentrations above 30.5 ng/mL can predict atherosclerosis with a sensitivity of 71.2% and specificity of 75.3%, and that ANGPTL4 levels below 497.5 ng/mL can predict atherosclerosis with a sensitivity of 63.9% and specificity of 74.5%. ANGPTL3 and ANGPTL4 were determined to be independent risk factors for coronary atherosclerosis with odds ratios (ORs) of 0.189 (95% CI 0.097-0.368, P < 0.001) and 3.625 (95% CI 1.873-7.016, P < 0.001), respectively.

Conclusions

Increased ANGPTL3 or decreased ANGPTL4 shows an association with coronary atherosclerosis and, may become a predictor of coronary atherosclerosis in the future.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12944-021-01580-z.

Cross-talk between ANGPTL4 gene SNP Rs1044250 and weight management is a risk factor of metabolic syndrome

Background

The prevalence of metabolic syndrome (Mets) is closely related to an increased incidence of cardiovascular events. Angiopoietin-like protein 4 (ANGPTL4) is contributory to the regulation of lipid metabolism, herein, may provide a target for gene-aimed therapy of Mets. This observational case control study was designed to elucidate the relationship between ANGPTL4 gene single nucleotide polymorphism (SNP) rs1044250 and the onset of Mets, and to explore the interaction between SNP rs1044250 and weight management on Mets.

Methods

We have recruited 1018 Mets cases and 1029 controls in this study. The SNP rs1044250 was genotyped with blood samples, base-line information and Mets-related indicators were collected. A 5-year follow-up survey was carried out to track the lifestyle interventions and changes in Mets-related indicators.

Results

ANGPTL4 gene SNP rs1044250 is an independent risk factor for increased waist circumference (OR 1.618, 95% CI [1.119–2.340]; p = 0.011), elevated blood pressure (OR 1.323, 95% CI [1.002–1.747]; p = 0.048), and Mets (OR 1.875, 95% CI [1.363–2.580]; p < 0.001). The follow-up survey shows that rs1044250 CC genotype patients with weight gain have an increased number of Mets components (M [Q1, Q3]: CC 1 (0, 1), CT + TT 0 [− 1, 1]; p = 0.021); The interaction between SNP rs1044250 and weight management is a risk factor for increased systolic blood pressure (β = 0.075, p < 0.001) and increased diastolic blood pressure (β = 0.097, p < 0.001), the synergistic effect of weight management and SNP rs1044250 is negative (S < 1).

Conclusion

ANGPTL4 gene SNP rs1044250 is an independent risk factor for increased waist circumference and elevated blood pressure, therefore, for Mets. However, patients with wild type SNP 1044250 are more likely to have Mets when the body weight is increased, mainly due to elevated blood pressure.

Fasting induces ANGPTL4 and reduces LPL activity in human adipose tissue

Objective

Studies in mice have shown that the decrease in lipoprotein lipase (LPL) activity in adipose tissue upon fasting is mediated by induction of the inhibitor ANGPTL4. Here, we aimed to validate this concept in humans by determining the effect of a prolonged fast on ANGPTL4 and LPL gene and protein expression in human subcutaneous adipose tissue.

Methods

Twenty-three volunteers ate a standardized meal at 18.00 h and fasted until 20.00 h the next day. Blood was drawn and periumbilical adipose tissue biopsies were collected 2 h and 26 h after the meal.

Results

Consistent with previous mouse data, LPL activity in human adipose tissue was significantly decreased by fasting (−60%), concurrent with increased ANGPTL4 mRNA (+90%) and decreased ANGPTL8 mRNA (−94%). ANGPTL4 protein levels in adipose tissue were also significantly increased by fasting (+46%), whereas LPL mRNA and protein levels remained unchanged. In agreement with the adipose tissue data, plasma ANGPTL4 levels increased upon fasting (+100%), whereas plasma ANGPTL8 decreased (−79%). Insulin, levels of which significantly decreased upon fasting, downregulated ANGPTL4 mRNA and protein in primary human adipocytes. By contrast, cortisol, levels of which significantly increased upon fasting, upregulated ANGPTL4 mRNA and protein in primary human adipocytes as did fatty acids.

Conclusion

ANGPTL4 levels in human adipose tissue are increased by fasting, likely via increased plasma cortisol and free fatty acids and decreased plasma insulin, resulting in decreased LPL activity.

This clinical trial was registered with identifier NCT03757767.

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24-h fast in humans reduces LPL activity in subcutaneous adipose tissue.

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24-h fast in humans increases adipose ANGPTL4 mRNA, protein, and plasma ANGPTL4 levels.

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Cortisol, fatty acids, and insulin regulate ANGPTL4 in vitro.

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ANGPTL4 mediates the reduction in adipose LPL activity during fasting.

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24-h fast in humans decreases adipose ANGPTL8 mRNA and plasma ANGPTL8 levels.

Circulating and tissue specific transcription of angiopoietin-like protein 4 in human Type 2 diabetes

AIMS

Obesity is associated with adipose tissue (AT) dysfunction marked by cellular hypertrophy, inflammation, hypoxia and fibrosis. Angiopoietin-like protein 4 (ANGPTL4) inhibits lipoprotein lipase which regulates triglyceride storage. Recently, inhibition of ANGPTL4 has been suggested as potential treatment for type 2 diabetes. Here we evaluate ANGPTL4's role in diabetes and examine ANGPTL4 in relation to markers of AT dysfunction and fatty liver disease.

MATERIALS & METHODS

We obtained a unique set of paired samples from subjects undergoing weight loss surgery including subcutaneous AT (SCAT), omental AT (OMAT), liver, thigh muscle biopsies and serum including a post-surgical SCAT biopsy after 9 months.

RESULTS

SCAT ANGPTL4 expression and circulating protein levels were higher in people with diabetes and correlated with glucose levels and HOMA-IR but not BMI. At post-surgical follow up, SCAT ANGPTL4 declined in subjects with diabetes to levels of those without diabetes. ANGPTL4 expression correlated with HIF1A and inflammation (MCP-1, IL-6).

CONCLUSIONS

We found that SCAT ANGPTL4 was closely linked with the expression of ANGPTL4 in the liver and represented a good proxy for liver steatosis. We suggest the elevation of ANGPTL4 levels in diabetes and the association with inflammation and hypoxia is due to a compensatory mechanism to limit further AT dysfunction. A reduction of ANGPTL4 for the treatment of T2DM as previously suggested is thus unlikely to be of further benefit.

The association of angiopoietin-like peptide 4 levels with obesity and hepatosteatosis in adolescents

INTRODUCTION

Angiopoietin-like peptide 4 (ANGPTL-4) is an adipocytokine that regulates plasma lipoprotein levels by inhibiting the lipoprotein lipase enzyme. Changes in lipid profile can be seen in obese adolescents. Nonalcoholic fatty liver disease may also be a complication of obesity. Based on this information, in this study we aimed to evaluate the relationship between serum ANGPTL-4 levels and obesity and hepatosteatosis in adolescents.

MATERIALS AND METHODS

A total of 85 volunteer adolescents, 55 of them were obese and 30 of them were normal weight, were included in our study. The adolescents having body mass index (BMI) 95% percentile and over according to age and sex was defined as obese. Thirty patients with grade 2-3 hepatosteatosis in abdominal ultrasound (USG) were included in 'obese adolescents with hepatosteatosis' subgroup and 25 obese cases with no hepatosteatosis in the USG were included in the 'obese adolescents without hepatosteatosis' group. Thirty patients with no hepatosteatosis in the abdominal USG and having BMI in normal percentiles according to their age and sex constituted the 'healthy control adolescents' group. Serum ANGPTL-4 levels were measured by Enzyme Linked Immunosorbent Assay. Laboratory tests, gender, age and BMI levels were compared statistically between groups. Correlations between ANGPTL-4 and other laboratory parameters were examined statistically in obese adolescent group.

RESULTS

The BMI, ANGPTL-4, HbA1c, AST, ALT, total cholesterol, triglyceride, LDL-cholesterol, HOMA-IR and insulin levels of the obese adolescent group were found to be significantly higher than the healthy control group (p < 0.05). We found no statistically significant difference in BMI, ANGPTL-4, triglyceride, insulin and HOMA-IR levels among obese adolescents with or without hepatosteatosis (p > 0.05). In all obese adolescent groups and in obese adolescent group with hepatosteatosis; there was no statistically significant relationship between ANGPTL-4 and other variables (p > 0.05).

CONCLUSIONS

We found that the levels of ANGPTL-4 increases in obesity in adolescents. However, our results make it difficult to establish a relationship between hepatosteatosis and ANGPTL-4. Targeting ANGPTL-4 may be beneficial for the pathogenesis and associated complications of obesity.

ANGPTL-4 is Associated with Obesity and Lipid Profile in Children and Adolescents

Angiopoietin-like protein 4 (ANGPTL-4) regulates lipidic metabolism and affects energy homeostasis. However, its function in children with obesity remains unknown. We investigated plasma ANGPTL-4 levels in children and its relationship with body mass index (BMI) and different lipidic parameters such as free fatty acids (FFA). Plasma ANGPTL-4 levels were analyzed in two different cohorts. In the first cohort (n = 150, age 3–17 years), which included children with normal weight or obesity, we performed a cross-sectional study. In the second cohort, which included only children with obesity (n = 20, age 5–18 years) followed up for two years after an intervention for weight loss, in which we performed a longitudinal study measuring ANGPTL-4 before and after BMI-loss. In the cross-sectional study, circulating ANGPTL-4 levels were lower in children with obesity than in those with normal weight. Moreover, ANGPTL-4 presented a negative correlation with BMI, waist circumference, weight, insulin, homeostasis model assessment of insulin resistance index (HOMA index), triglycerides, and leptin, and a positive correlation with FFA and vitamin-D. In the longitudinal study, the percent change in plasma ANGPTL-4 was correlated with the percent change in FFA, total-cholesterol and high-density lipoprotein cholesterol. This study reveals a significant association of ANGPTL-4 with pediatric obesity and plasma lipid profile.

Association of a genetic variant in the angiopoietin-like protein 4 gene with metabolic syndrome

Background

Metabolic syndrome (MetS) is characterized by a clustering of cardiovascular risk factors that include: abdominal obesity, dyslipidemia, hypertension and glucose intolerance. Angiopoietin-like protein 4 (ANGPTL4) is a circulating peptide that is an inhibitor of lipoprotein lipase, a key enzyme in lipid metabolism. The objective of this study was to investigate the association of ANGPTL4 gene variants (E40K) with fasting serum triglyceride levels and with cardiovascular risk factors, that included the presence of MetS in 817 subjects recruited from the Mashhad stroke and heart Atherosclerosis Disorders (MASHAD) cohort Study.

Method

ANGPTL4 genotypes were determined using a TaqMan genotyping based real time PCR method. The association of the genetic variant with the risk of metabolic syndrome and its relationship with lipid profile were determined.

Result

The frequency of GG, GA and AA genotypes were 96.9, 2.7 and 0.4% in individuals with MetS, and 78.8, 20.8, 0.4%, in those without MetS. The GA genotype of the rs116843064 polymorphism was associated with a lower risk for MetS (e.g., OR in Codominant genetic model: 0.14, 95% CI: (0.06–0.33), p < 0.0001). Subject with an A allele had a higher risk for MetS (OR: 6.72, 95% CI: (3.05–14.82), p < 0.0001). There was a significant difference in fasted lipid profiles across the genotypes for ANGPTL4. Carriers of the AG genotype had higher levels of serum HDL-cholesterol (HDL-C) and lower TG, compared to the GG homozygotes genotype.

Conclusion

The G allele at the rs116843064 polymorphic locus of the ANGPTL4 gene was associated with a lower prevalence of MetS.

A review of the multifunctionality of angiopoietin-like 4 in eye disease

Angiopoietin-like protein 4 (ANGPTL4) is a multifunctional cytokine regulating vascular permeability, angiogenesis, and inflammation. Dysregulations in these responses contribute to the pathogenesis of ischemic retinopathies such as diabetic retinopathy (DR), age-related macular degeneration (AMD), retinal vein occlusion, and sickle cell retinopathy (SCR). However, the role of ANGPTL4 in these diseases remains controversial. Here, we summarize the functional mechanisms of ANGPTL4 in several diseases. We highlight original studies that provide detailed data about the mechanisms of action for ANGPTL4, its applications as a diagnostic or prognostic biomarker, and its use as a potential therapeutic target. Taken together, the discussions in this review will help us gain a better understanding of the molecular mechanisms by which ANGPTL4 functions in eye disease and will provide directions for future research.

Effect of diet-induced weight loss on angiopoietin-like protein 4 and adipose tissue lipid metabolism in overweight and obese humans

Angiopoietin-like protein 4 (ANGPTL4) plays a role in lipid partitioning by inhibiting lipoprotein lipase (LPL)-dependent plasma clearance of triacylglycerol in adipose tissue. We investigated the effects of diet-induced weight loss on plasma ANGPTL4 concentrations in relation to in vivo adipose tissue LPL activity and lipolysis and adipose tissue ANGPTL4 release in overweight/obese participants. Sixteen individuals (BMI: 28-35 kg/m2 ; 10 women) were randomized to a dietary intervention composed of either a low-calorie diet (1250 kcal/day) for 12 weeks (n = 9) or a very low-calorie diet (500 kcal/day) for 5 weeks, followed by a 4-week weight stable period. Before and after the intervention, we measured arteriovenous concentration differences in combination with adipose tissue blood flow before and after intake of a high-fat mixed meal with [U-13 C]-palmitate to assess in vivo adipose tissue LPL activity and lipolysis. The intervention significantly reduced body weight (-8.6 ± 0.6 kg, P < 0.001). Plasma ANGPTL4 concentrations were unaffected. Significant postprandial adipose tissue ANGPTL4 release into the circulation was observed (P < 0.01). No association was observed between plasma ANGPTL4 and in vivo LPL activity. After intervention, fasting and postprandial plasma ANGPTL4 concentrations were positively associated with adipose tissue nonesterified FA (NEFA) and glycerol release, reflecting in vivo adipose tissue lipolysis (fasting NEFA: P = 0.039 and postprandial NEFA: P = 0.003). In conclusion, plasma ANGPTL4 is unaffected by weight loss and is secreted from human adipose tissue after a high-fat meal in overweight/obese participants. Plasma ANGPTL4 concentrations were not related to in vivo adipose tissue LPL activity, but were positively associated with in vivo adipose tissue lipolysis after weight loss.

Serum Angiopoietin-like peptide 4 levels in patients with hepatic steatosis

OBJECTIVES

Angiopoietin-like peptide 4 (ANGPTL-4) plays an important role in lipid metabolism by inhibiting the enzyme lipoprotein lipase. This effect of ANGPTL-4 results in suppression of the release of plasma triglyceride-derived fatty acids. Increase in fatty acid levels entering to the liver and abnormalities in their secretion is one of the main mechanisms in pathogenesis of hepatic steatosis. In this study, we aimed to investigate the role of ANGPTL-4 in pathogenesis of hepatic steatosis by determining its levels in patients with fatty liver disease.

METHODS

Totally 51 patients (age: 37.9 ± 9.9 years, M/F) diagnosed with grade 2-3 hepatic steatosis with ultrasound and 30 healthy volunteers (age: 34.8 ± 9.5 years, M/F) were included in the study. In both groups, routine biochemical tests including fasting blood glucose, fasting insulin levels, triglyceride, total cholesterol, LDL- cholesterol, HDL-cholesterol, AST, ALT, ALP, GGT levels were measured together with the ANGPTL-4 levels. In determination of ANGPTL-4 levels, ELISA was performed.

RESULTS

When compared with the control group, ANGPTL-4 levels were determined to be decreased in patients with hepatic steatosis (369 ± 243 vs 303 ± 286 ng/mL, p = 0.014). There was a negative weak correlation observed between ANGPTL-4 and triglyceride levels (r = -0.246, p = 0.027). Among all groups, when patients with and without insulin resistance were compared; ANGPTL-4 levels were determined to be similar. While fasting blood glucose levels were similar between 2 groups; fasting insulin and triglyceride levels were determined to be increased in hepatic steatosis group (Insulin 17.7 ± 12 vs 7.4 ± 3.3 µIU/mL, p < 0.001, triglyceride 158 ± 46.4 vs 118 ± 59.8 mg/dL p < 0.001).

CONCLUSIONS

We have determined lower serum ANGPTL-4 levels in patients with hepatic steatosis. ANGPTL-4 that is regulating LPL activity plays an important role in fatty liver disease pathogenesis via free fatty acid metabolism and peroxisome proliferator-activated receptor-delta (PPAR-δ). We believe that the results of this study would elucidate the investigations about the mechanism of fatty liver disease development and treatments targeting ANGPTL-4.

Angiopoietin-like protein 3 and 4 in obesity, type 2 diabetes mellitus, and malnutrition: the effect of weight reduction and realimentation

Background

Angiopoietin-like proteins (ANGPTLs) 3 and 4 are circulating factors that participate in the regulation of lipid and glucose metabolism.

Subjects and methods

We measured serum ANGPTL3 and 4 levels in 23 patients with obesity, 40 patients with obesity and type 2 diabetes mellitus (T2DM), 22 patients with anorexia nervosa (AN), 15 subjects undergoing 72-h fasting, and 12 patients with short bowel syndrome (SBS), and their changes after very-low-calorie diet (VLCD), bariatric surgery, partial realimentation, acute fasting, and parenteral nutrition in order to assess their possible role in metabolic regulations.

Results

Serum ANGPTL4 levels were higher in obese subjects without/with T2DM (94.50 ± 9.51 and 134.19 ± 7.69 vs. 50.34 ± 4.22 ng/ml, p < 0.001) and lower in subjects with AN relative to healthy control subjects (38.22 ± 4.48 vs. 65.80 ± 7.98 ng/ml, p = 0.002), while serum ANGPTL3 levels demonstrated inverse tendency. Nutritional status had no effect on ANGPTL3 and 4 mRNA expression in adipose tissue. Fasting decreased ANGPTL3 and increased ANGPTL4 levels, while VLCD reduced only ANGPTL3. Bariatric surgery and realimentation of AN or SBS patients had no effect on either ANGPTL. Multiple regression analysis identified BMI as an independent predictor of ANGPTL3; and BMI and HbA_1c as independent predictors of ANGPTL4, respectively.

Conclusions

Taken together, our data suggest that serum ANGPTL3 and 4 levels are influenced by nutritional status and fasting and could be involved in the metabolic disturbances present in obesity and AN.

Angiopoietin-Like Proteins in Angiogenesis, Inflammation and Cancer

Altered expression of secreted factors by tumor cells or cells of the tumor microenvironment is a key event in cancer development and progression. In the last decade, emerging evidences supported the autocrine and paracrine activity of the members of the Angiopoietin-like (ANGPTL) protein family in angiogenesis, inflammation and in the regulation of different steps of carcinogenesis and metastasis development. Thus, ANGPTL proteins become attractive either as prognostic or predictive biomarkers, or as novel target for cancer treatment. Here, we outline the current knowledge about the functions of the ANGPTL proteins in angiogenesis, cancer progression and metastasis. Moreover, we discuss the most recent evidences sustaining their role as prognostic or predictive biomarkers for cancer therapy. Although the role of ANGPTL proteins in cancer has not been fully elucidated, increasing evidence suggest their key effects in the proliferative and invasive properties of cancer cells. Moreover, given the common overexpression of ANGPTL proteins in several aggressive solid tumors, and their role in tumor cells and cells of the tumor microenvironment, the field of research about ANGPTL proteins network may highlight new potential targets for the development of future therapeutic strategies.

A Randomised, Controlled Study of Different Glycaemic Targets during Gestational Diabetes Treatment: Effect on the Level of Adipokines in Cord Blood and ANGPTL4 Expression in Human Umbilical Vein Endothelial Cells

Our aim was to study the expression of adipokine-encoding genes (leptin, adiponectin, and angiopoietin-like protein 4 (ANGPTL4)) in human umbilical vein endothelial cells (HUVECs) and adipokine concentration in cord blood from women with gestational diabetes mellitus (GDM) depending on glycaemic targets. GDM patients were randomised to 2 groups per target glycaemic levels: GDM1 (tight glycaemic targets, fasting blood glucose < 5.1 mmol/L and <7.0 mmol/L postprandial, N = 20) and GDM2 (less tight glycaemic targets, <5.3 mmol/L and < 7.8 mmol/L, respectively, N = 21). The control group included 25 women with normal glucose tolerance. ANGPTL4 expression was decreased in the HUVECs from GDM patients versus the control group (23.11 ± 5.71, 21.47 ± 5.64, and 98.33 ± 20.92, for GDM1, GDM2, and controls; p < 0.001) with no difference between GDM1 and GDM2. The level of adiponectin gene expression was low and did not differ among the groups. Leptin gene expression was undetectable in HUVECs. In cord blood, leptin/adiponectin ratio (LAR) was increased in GDM2 compared to controls and GDM1 (p = 0.038) and did not differ between GDM1 and controls. Tight glycaemic targets were associated with normalisation of increased LAR in the cord blood. ANGPTL4 expression was downregulated in HUVECs of newborns from GDM mothers and was not affected by the intensity of glycaemic control.

Plasma levels of hypoxia-regulated factors in patients with age-related macular degeneration

PURPOSE

Various hypoxia-related proteins are differentially expressed in the retina and secreted to the vitreous and/or aqueous humor of patients affected by dry or neovascular age-related macular degeneration (nAMD). To determine whether these conditions alter concentrations of cytokines also in the systemic circulation, we measured plasma levels of six hypoxia-related proteins.

METHODS

Plasma was prepared from EDTA blood that was collected from patients affected by dry AMD (n = 5), nAMD (n = 11), proliferative diabetic retinopathy (PDR; n = 9), and patients with an epiretinal membrane (ERM; n = 11). ERM samples served as negative controls, PDR samples as positive controls. Protein concentrations of vascular endothelial growth factor (VEGF), erythropoietin (EPO), angiopoietin-like 4 (ANGPTL4), placental growth factor (PlGF), tumor necrosis factor alpha (TNF-α), and pigment epithelium-derived factor (PEDF) were determined by enzyme-linked immunosorbent assay (ELISA).

RESULTS

The concentration of PlGF was significantly increased in plasma of patients affected by nAMD. Although no statistically significant differences were found for EPO, ANGPTL4, PlGF, TNF-α, and PEDF, the mean concentration of VEGF was lowest in the nAMD group. Plasma concentrations of the six factors did not correlate with gender or age of patients.

CONCLUSIONS

nAMD may increase plasma concentrations of PlGF, making it a candidate as a biomarker for the neovascular form of AMD. Other factors, however, were not differentially regulated, suggesting that their systemic concentrations are not generally increased in hypoxia-related retinal diseases.

ANGPTL-4 induces diabetic retinal inflammation by activating Profilin-1

Diabetic retinopathy (DR), the most common cause of irreversible blindness in working-age adults, results in central vision loss that is caused by microvascular damage to the inner lining of the back of the eye, the retina. The aim of this work was to assess the temporal relationships between angiopoietin-like protein-4 (ANGPTL-4), a novel adipocytokine factor, and diabetic retinal inflammation and microvascular dysfunction. The downstream pathway(s) and upstream mediator(s) of ANGPTL-4 were then determined under high glucose (HG) conditions. Diabetic rats and control animals were randomly assigned to receive hypoxia inducible factor-1 alpha (HIF-1α) blockade (doxorubicin or shRNA) or vehicle for 8 weeks. Human retinal microvascular endothelial cells (HRMECs) were incubated with normal or high glucose, with or without blockade or recombinant proteins, for ANGPTL-4, HIF-1α, and vascular endothelial growth factor (VEGF). The levels of ANGPTL-4, profilin-1, HIF-1α, VEGF, interleukin 1 beta (IL-1β), IL-6, and intercellular adherent molecule 1 (ICAM-1) in the rat retinas and HRMEC extracts were examined by Western blotting and real-time RT-PCR. The levels of ANGPTL-4, profilin-1, HIF-1α, and VEGF protein and mRNA were significantly higher in the diabetic rats and HG-exposed HRMECs. ANGPTL-4 was a potent modulator of increased inflammation, permeability, and angiogenesis via activation of the profilin-1 signaling pathway. Our results showed that ANGPTL-4 upregulation was induced by HG, which was dependent on HIF-1α activation that was also triggered by HG, both in vivo and in vitro. Our results suggest that targeting ANGPTL-4, alone or in combination with profilin-1, may be an effective therapeutic strategy and diagnostic screening biomarker for proliferative diabetic retinopathy and other vitreous-retinal inflammatory diseases.

Angiopoietin-like-4 and minimal change disease

Background

Minimal Change Disease (MCD) is the most common type of nephrotic syndrome in children. Angiopoietin-like-4 (Angplt4) has been proposed as mediator of proteinuria in MCD. The aim of this study was to evaluate the role of Angptl4 as a biomarker in MCD.

Methods

Patients with biopsy-proven primary MCD, focal segmental glomerulosclerosis, membranous nephropathy (60, 52 and 52 respectively) and 18 control subjects had urinary and serum Angptl4 measured by Elisa. Frozen kidney tissue sections were stained for Angptl4.

Results

Angptl4 was not identified in glomeruli of MCD patients in relapse. Urinary Angptl4 levels were elevated in MCD in relapse as well as in patients with massive proteinuria due to other glomerular diseases.

Conclusion

Neither serum nor urine Angptl4 appear to be good biomarkers in MCD. Elevated urinary Angptl4 n glomerular disease appears to reflect the degree of proteinuria rather than any specific disease.

Plasma and adipose tissue level of angiopoietin-like 7 (ANGPTL7) are increased in obesity and reduced after physical exercise

Objective

ANGPTL7 is a member of the Angiopoietin-like (ANGPTL) protein family that is composed of eight proteins (1–8). Increasing evidence is associating ANGPTL proteins to obesity and insulin resistance. The biological role of ANGPTL7 is yet to be understood except for a recently proposed role in the pathophysiology of glaucoma. This study was designed to shed light on the function of ANGPTL7 in obesity and its modulation by physical exercise as well as its potential association with lipid profile.

Methods

A total of 144 subjects were enrolled in this study and finished three months of physical exercise. The participants were classified based on their BMI, 82 subjects were non-obese and 62 obese. ANGPTL7 levels in plasma and adipose tissue were measured by ELISA, RT-PCR and immunohistochemistry.

Results

In this study, we showed that ANGPTL7 level was increased in the plasma of obese subjects (1249.05± 130.39 pg/mL) as compared to non-obese (930.34 ± 87.27 pg/mL) (p-Value = 0.032). ANGPTL7 Gene and protein expression levels in adipose tissue also showed over two fold increase. Physical exercise reduced circulating level of ANGPTL7 in the obese subjects to 740.98± 127.18 pg/mL, (p-Value = 0.007). ANGPTL7 expression in adipose tissue was also reduced after exercise. Finally, ANGPTL7 circulating level showed significant association with TG level in the obese subjects (R² = 0.183, p-Value = 0.03).

Conclusion

In conclusion, our data shows for the first time that obesity increases the level of ANGPTL7 in both plasma and adipose tissue. Increased expression of ANGPTL7 might play a minor role in the regulation of TG level in obese subjects either directly or through interaction with other ANGPTL protein members. Physical exercise reduced the level of ANGPTL7 highlighting the potential for targeting this protein as a therapeutic target for regulating dyslipidemia.

Potential Role of ANGPTL4 in the Cross Talk between Metabolism and Cancer through PPAR Signaling Pathway

The angiopoietin-like 4 (ANGPTL4) protein belongs to a superfamily of secreted proteins structurally related to factors modulating angiogenesis known as angiopoietins. At first, ANGPTL4 has been identified as an adipokine exclusively involved in lipid metabolism, because of its prevalent expression in liver and adipose tissue. This protein regulates lipid metabolism by inhibiting lipoprotein lipase (LPL) activity and stimulating lipolysis of white adipose tissue (WAT), resulting in increased levels of plasma triglycerides (TG) and fatty acids. Subsequently, ANGPTL4 has been shown to be involved in several nonmetabolic and metabolic conditions, both physiological and pathological, including angiogenesis and vascular permeability, cell differentiation, tumorigenesis, glucose homoeostasis, lipid metabolism, energy homeostasis, wound healing, inflammation, and redox regulation. The transcriptional regulation of ANGPTL4 can be modulated by several transcription factors, including PPARα, PPARβ/δ, PPARγ, and HIF-1α, and nutritional and hormonal conditions. Several studies showed that high levels of ANGPTL4 are associated with poor prognosis in patients with various solid tumors, suggesting an important role in cancer onset and progression, metastasis, and anoikis resistance. Here, we have discussed the potential role of ANGPTL4 in mediating the cross talk between metabolic syndromes, such as diabetes and obesity, and cancer through regulation of its expression by PPARs.

A randomised, controlled, crossover study of the effect of diet on angiopoietin-like protein 4 (ANGPTL4) through modification of the gut microbiome

Angiopoietin-like protein 4 (ANGPTL4) is a lipoprotein lipase inhibitor that is involved in lipid metabolism and angiogenesis. Animal studies have suggested that the ANGPTL4 protein is modulated by the gut microbiota, possibly through increased concentrations of SCFA, such as C4, found in whole-fat milk or as a result of fermentation of inulin. This study investigated whether a standardised diet either high in fat content or supplemented with inulin powder would increase plasma ANGPTL4 in overweight men and whether this increase was mediated through a compositional change of the gut microbiota. The study had a crossover design with three arms, where participants were given a standardised isoenergetic diet supplemented with inulin powder, whole-fat milk or water (control). Plasma and urine samples were collected before and after each intervention period. Faecal samples and adipose tissue biopsies were collected after each intervention period. The study included twenty-one participants of whom eighteen completed the study. The dietary interventions did not change ANGPTL4 plasma concentration, nor was plasma ANGPTL4 associated with plasma lipids, TAG or NEFA concentration. The relative abundance of bifidobacteria following the inulin diet was higher, compared with the control diet. However, the changes in microbiota were not associated with plasma ANGPTL4 and the overall composition of the microbiota did not change between the dietary periods. Although weight was maintained throughout the dietary periods, weight was negatively associated with plasma ANGPTL4 concentration. In the adipose tissue, ANGPTL4 expression was correlated with leptin expression, but not with hypoxia-inducible factor 1α (HIF-1α) expression.

Angiopoietin-like protein 4 and cardiovascular function in COPD

Introduction

The coexistence of chronic obstructive pulmonary disease (COPD) and cardiovascular disease (CVD) is frequent and might be inter-related through inflammation-related processes reflected by specific markers. Here, we studied angiopoietin-like protein 4 (ANGPTL4), an upcoming cardiovascular marker, in stable COPD, and its relationship to cardiovascular function with respect to well-known CVD risk factors.

Methods

In a prospective COPD cohort study, we investigated serum ANGPTL4 levels, vascular status (ankle–brachial index (ABI)) and cardiac function (N-terminal pro-B-type natriuretic peptide (NT-proBNP)) as well as airflow limitation, objectively measured physical activity, the metabolic syndrome, high-sensitive C reactive protein (hs-CRP) and other CVD risk factors at 2 time points. We initially studied 74 stable COPD patients and 18 controls. For internal validation, we additionally studied 160 COPD patients of a former visit.

Results

ANGPTL4 was significantly elevated in COPD patients compared with controls (p=0.026). After correction for traditional CVD risk factors, including hs-CRP, higher levels of ANGPTL4 were independently associated with lower ABI (p=0.023) and higher NT-proBNP (p<0.001). These findings were confirmed in the internal validation analysis, which included echocardiographic assessments.

Conclusions

Serum ANGPTL4 is independently associated with cardiovascular function in COPD and might qualify as a biomarker reflecting a pathogenic link between COPD and CVD.

Different Training-Induced Skeletal Muscle Adaptations in COPD Patients with and without Alpha-1 Antitrypsin Deficiency

BACKGROUND

Pulmonary rehabilitation (PR) improves oxidative capacity of peripheral muscles in patients with chronic obstructive pulmonary disease (COPD). The exercise-induced oxidative skeletal muscle adaptation in COPD patients with inherited alpha-1 antitrypsin deficiency (A1ATD) has not been studied.

OBJECTIVES

To compare PR effects on skeletal muscle adaptation in COPD patients with and without A1ATD.

METHODS

Nine COPD patients with A1ATD (genotype PiZZ, 6 receiving A1AT augmentation therapy), and 10 'usual' COPD patients (genotype PiMM) performed an incremental cycling test and underwent musculus vastus lateralis biopsies before and after a 3-week PR program including exercise training.

RESULTS

PiZZ and PiMM patients improved peak work rate following PR (+9 ± 11 W, p < 0.05, and +18 ± 9 W, p < 0.001, between-group difference p < 0.05). PiMM patients increased fibre type I (+8.1%), reduced fibre type IIA (-2.1%) and hybrid fibre type IIA/IIX proportion (-3.9%). Following PR, PiMM patients also raised mitochondrial signalling proteins PGC-1α (4.5-fold), and TFAM (6.4-fold). PiZZ patients had no change in fibre type I but showed a shift of type IIA/IIX (-8.8%) towards fibre type IIA distribution (+8.9%). The capillary to fibre ratio increased by 28% (p < 0.05) in PiZZ, whereas no change was observed in PiMM patients. Linear regression analysis revealed that diffusion capacity and A1AT therapy are predictor variables for myofibre type I response to PR (r2 = 0.684, p < 0.01).

CONCLUSIONS

Following a 3-week PR with comparable training modalities, PiMM but not PiZZ patients increased the oxidative myofibre type I proportion. This skeletal muscle adaptation pattern suggests better improvement of exercise capacity in PiMM than in PiZZ patients with COPD.

Regulation of lipid metabolism by angiopoietin-like proteins

PURPOSE OF REVIEW

The angiopoietin-like proteins (ANGPTLs) 3, 4 and 8 have emerged as key regulators of plasma lipid metabolism by serving as potent inhibitors of the enzyme lipoprotein lipase (LPL). In this review, we provide an integrated picture of the role of ANGPTL3, ANGPTL4 and ANGPTL8 in lipid metabolism by focusing on their impact on LPL activity and plasma triglyceride clearance during physiological conditions such as fasting, refeeding, exercise and cold exposure.

RECENT FINDINGS

Upon refeeding, circulating ANGPTL3 and ANGPTL8 promote the replenishment of white adipose tissue depots by specifically inhibiting LPL activity in oxidative tissues. During exercise and cold exposure, ANGPTL4 represses local LPL activity to assure that plasma triglycerides are specifically shuttled to exercising muscle and brown adipose tissue, respectively. Overall, ANGPTL4 is the central component of a fatty acid-driven feedback mechanism that regulates plasma triglyceride hydrolysis and subsequent tissue fatty acid uptake in response to changes in lipid availability and cellular fuel demand.

SUMMARY

ANGPTL3, ANGPTL4 and ANGPTL8 together ensure that triglycerides from triglyceride-rich lipoproteins are adequately distributed during different physiological conditions. The impact of the ANGPTLs on plasma lipid levels has led to scrutiny of ANGPTLs as therapeutic targets for dyslipidemia.

Angiopoietin-like 4 promotes intracellular degradation of lipoprotein lipase in adipocytes

LPL hydrolyzes triglycerides in triglyceride-rich lipoproteins along the capillaries of heart, skeletal muscle, and adipose tissue. The activity of LPL is repressed by angiopoietin-like 4 (ANGPTL4) but the underlying mechanisms have not been fully elucidated. Our objective was to study the cellular location and mechanism for LPL inhibition by ANGPTL4. We performed studies in transfected cells, ex vivo studies, and in vivo studies with Angptl4(-/-) mice. Cotransfection of CHO pgsA-745 cells with ANGPTL4 and LPL reduced intracellular LPL protein levels, suggesting that ANGPTL4 promotes LPL degradation. This conclusion was supported by studies of primary adipocytes and adipose tissue explants from wild-type and Angptl4(-/-) mice. Absence of ANGPTL4 resulted in accumulation of the mature-glycosylated form of LPL and increased secretion of LPL. Blocking endoplasmic reticulum (ER)-Golgi transport abolished differences in LPL abundance between wild-type and Angptl4(-/-) adipocytes, suggesting that ANGPTL4 acts upon LPL after LPL processing in the ER. Finally, physiological changes in adipose tissue ANGPTL4 expression during fasting and cold resulted in inverse changes in the amount of mature-glycosylated LPL in wild-type mice, but not Angptl4(-/-) mice. We conclude that ANGPTL4 promotes loss of intracellular LPL by stimulating LPL degradation after LPL processing in the ER.

α1-Antitrypsin Combines with Plasma Fatty Acids and Induces Angiopoietin-like Protein 4 Expression

α1-Antitrypsin (A1AT) purified from human plasma upregulates expression and release of angiopoietin-like protein 4 (Angptl4) in adherent human blood monocytes and in human lung microvascular endothelial cells, providing a mechanism for the broad immune-regulatory properties of A1AT independent of its antiprotease activity. In this study, we demonstrate that A1AT (Prolastin), a potent inducer of Angptl4, contains significant quantities of the fatty acids (FA) linoleic acid (C18:2) and oleic acid (C18:1). However, only trace amounts of FAs were present in preparations that failed to increase Angplt4 expression, for example, A1AT (Zemaira) or M-type A1AT purified by affinity chromatography. FA pull-down assays with Western blot analysis revealed a FA-binding ability of A1AT. In human blood-adherent monocytes, A1AT-FA conjugates upregulated expression of Angptl4 (54.9-fold, p < 0.001), FA-binding protein 4 (FABP4) (11.4-fold, p < 0.001), and, to a lesser degree, FA translocase (CD36) (3.1-fold, p < 0.001) relative to A1AT devoid of FA (A1AT-0). These latter effects of A1AT-FA were blocked by inhibitors of peroxisome proliferator-activated receptor (PPAR) β/δ (ST247) and PPARγ (GW9662). When compared with controls, cell pretreatment with ST247 diminished the effect of A1AT-LA on Angptl4 mRNA (11.6- versus 4.1-fold, p < 0.001) and FABP4 mRNA (5.4- versus 2.8-fold, p < 0.001). Similarly, preincubation of cells with GW9662 inhibited inducing effect of A1AT-LA on Angptl4 mRNA (by 2-fold, p < 0.001) and FABP4 mRNA (by 3-fold, p < 0.001). Thus, A1AT binds to FA, and it is this form of A1AT that induces Angptl4 and FABP4 expression via a PPAR-dependent pathway. These findings provide a mechanism for the unexplored area of A1AT biology independent of its antiprotease properties.

Targeting fatty acid metabolism to improve glucose metabolism

Disturbances in fatty acid metabolism in adipose tissue, liver, skeletal muscle, gut and pancreas play an important role in the development of insulin resistance, impaired glucose metabolism and type 2 diabetes mellitus. Alterations in diet composition may contribute to prevent and/or reverse these disturbances through modulation of fatty acid metabolism. Besides an increased fat mass, adipose tissue dysfunction, characterized by an altered capacity to store lipids and an altered secretion of adipokines, may result in lipid overflow, systemic inflammation and excessive lipid accumulation in non-adipose tissues like liver, skeletal muscle and the pancreas. These impairments together promote the development of impaired glucose metabolism, insulin resistance and type 2 diabetes mellitus. Furthermore, intrinsic functional impairments in either of these organs may contribute to lipotoxicity and insulin resistance. The present review provides an overview of fatty acid metabolism-related pathways in adipose tissue, liver, skeletal muscle, pancreas and gut, which can be targeted by diet or food components, thereby improving glucose metabolism.

Angiopoietin-like 4 Modifies the Interactions between Lipoprotein Lipase and Its Endothelial Cell Transporter GPIHBP1

The release of fatty acids from plasma triglycerides for tissue uptake is critically dependent on the enzyme lipoprotein lipase (LPL). Hydrolysis of plasma triglycerides by LPL can be disrupted by the protein angiopoietin-like 4 (ANGPTL4), and ANGPTL4 has been shown to inactivate LPL in vitro. However, in vivo LPL is often complexed to glycosylphosphatidylinositol-anchored high density lipoprotein-binding protein 1 (GPIHBP1) on the surface of capillary endothelial cells. GPIHBP1 is responsible for trafficking LPL across capillary endothelial cells and anchors LPL to the capillary wall during lipolysis. How ANGPTL4 interacts with LPL in this context is not known. In this study, we investigated the interactions of ANGPTL4 with LPL-GPIHBP1 complexes on the surface of endothelial cells. We show that ANGPTL4 was capable of binding and inactivating LPL complexed to GPIHBP1 on the surface of endothelial cells. Once inactivated, LPL dissociated from GPIHBP1. We also show that ANGPTL4-inactivated LPL was incapable of binding GPIHBP1. ANGPTL4 was capable of binding, but not inactivating, LPL at 4 °C, suggesting that binding alone was not sufficient for ANGPTL4's inhibitory activity. We observed that although the N-terminal coiled-coil domain of ANGPTL4 by itself and full-length ANGPTL4 both bound with similar affinities to LPL, the N-terminal fragment was more potent in inactivating both free and GPIHBP1-bound LPL. These results led us to conclude that ANGPTL4 can both bind and inactivate LPL complexed to GPIHBP1 and that inactivation of LPL by ANGPTL4 greatly reduces the affinity of LPL for GPIHBP1.

High-grain diets suppress ruminal tissue abundance of angiopoietin-like protein 4 in cattle

Angiopoietin-like protein 4 (ANGPTL4) is expressed in bovine ruminal epithelium, making it possible that dietary components or commensal microbes may influence gastrointestinal ANGPTL4 production via interactions at the mucosal surface. Therefore, we conducted 3 experiments to evaluate the effects of dietary concentrate level and VFA infusions on ANGPTL4 abundance in ruminal tissue. In Exp. 1, we assigned 12 nonlactating Holstein cows to either 8% or 64% concentrate diets; diets were fed 28 d before euthanasia and ruminal tissue collection. Ruminal tissue and plasma ANGPTL4 protein abundance were unaltered by treatment. In Exp. 2, we assigned 8 continental crossbred heifers to either a 45% or 90% concentrate diet; diets were fed for 75 d before euthanasia and ruminal tissue collection. Compared with the 45% concentrate diet, the 90% concentrate diet decreased (P < 0.01) ruminal tissue ANGPTL4 protein abundance. In Exp. 3, we assigned 6 ruminally cannulated lactating Holstein cows to a treatment sequence in replicated 3 × 3 Latin squares and fed a standard lactation diet. Cows were infused with 5 mol/d sodium acetate, sodium propionate, or sodium butyrate for 2 d. Infusions of VFA did not affect (P > 0.10) mRNA or protein abundance of ANGPTL4 in ruminal papillae. Ruminal papillae ANGPTL4 abundance was, however, negatively correlated with (P = 0.01) ruminal VFA concentrations. ANGPTL4 abundance in ruminal tissue decreases in response to very high dietary concentrate and is inversely correlated with ruminal total VFA concentrations.

Angiopoietin-like proteins 3, 4 and 8: regulating lipid metabolism and providing new hope for metabolic syndrome

Angiopoietin-like proteins (ANGPTLs) are a group of eight proteins that share structural similarity to the members of the angiopoietin protein family. ANGPTL3 plays a vital role in the regulation of the plasma levels of triglyceride and cholesterol, mainly via reversible inhibition of the lipoprotein lipase activity. ANGPTL4, which functions as a homo-oligomer different from ANGPTL3 and ANGPTL8, not only regulates the plasma levels of triglyceride and prevents the uptake of dietary lipids into adipose tissues but also inhibits intravascular lipolysis. ANGPTL8 (also called betatrophin) has been identified as an important factor in regulating the triglyceride levels and adipose tissue mass as well as in replenishing the adipose tissue triglyceride store. ANGPTL8 acts together with ANGPTL3 to regulate the lipid metabolism, and ANGPTL8 promotes cleavage of ANGPTL3 to augment the activity of ANGPTL3. In addition, ANGPTL8 promotes proliferation of pancreatic β-cells and enhances insulin secretion. The properties of ANGPTLs in regulating the lipid metabolism suggest their application in the target therapy for metabolic syndrome. As ANGPTLs are regulated by several factors and may be involved in certain specific pathways of lipid metabolism, designing drugs that target ANGPTLs or factors regulating ANGPTLs may be an efficient approach to treat metabolic syndrome.

Regulation of lipoprotein lipase by Angptl4

Triglyceride (TG)-rich chylomicrons and very low density lipoproteins (VLDL) distribute fatty acids (FA) to various tissues by interacting with the enzyme lipoprotein lipase (LPL). The protein angiopoietin-like 4 (Angptl4) is under sensitive transcriptional control by FA and the FA-activated peroxisome proliferator activated receptors (PPARs), and its tissue expression largely overlaps with that of LPL. Growing evidence indicates that Angptl4 mediates the physiological fluctuations in LPL activity, including the decrease in adipose tissue LPL activity during fasting. This review focuses on the major ambiguities concerning the mechanism of LPL inhibition by Angptl4, as well as on the physiological role of Angptl4 in lipid metabolism, highlighting its function in a variety of tissues, and uses this information to make suggestions for further research.

Angiopoietin-like proteins: a comprehensive look

Angiopoietin-like proteins (ANGPTLs) are a family of proteins structurally similar to the angiopoietins. To date, eight ANGPTLs have been discovered, namely ANGPTL1 to ANGPTL8. Emerging evidence implies a key role for ANGPTLs in the regulation of a plethora of physiological and pathophysiological processes. Most of the ANGPTLs exhibit multibiological properties, including established functional roles in lipid and glucose metabolism, inflammation, hematopoiesis, and cancer. This report represents a systematic and updated appraisal of this class of proteins, focusing on the main features of each ANGPTL.

Inflammation increases plasma angiopoietin-like protein 4 in patients with the metabolic syndrome and type 2 diabetes

BACKGROUND

Angiopoietin-like protein 4 (ANGPTL4) inhibits lipoprotein lipase and associates with dyslipidemia. The expression of ANGPTL4 is regulated by free fatty acids (FFA) that activate lipid-sensing peroxisome proliferator-activated receptors (PPARs), but FFA can also activate pattern recognition receptors including Toll-like receptor 4 (TLR4) in macrophages.

OBJECTIVE

To assess whether systemic low-grade inflammation is a determinant for plasma ANGPTL4 levels in patients with the metabolic syndrome (MetS) and type 2 diabetes mellitus (T2DM).

DESIGN

We studied 335 male participants: healthy controls (Controls), patients with the MetS without inflammation (MetS-I) and with low-grade inflammation (MetS+I), and patients with T2DM. All patients without diabetes included in the present study were initially matched for waist circumference. In plasma, ANGPTL4, C reactive protein (CRP) and metabolic parameters were determined. Underlying mechanisms were examined using human macrophages in vitro.

RESULTS

As compared with Controls, plasma ANGPTL4 levels were increased in patients with MetS-I, MetS+I, and T2DM. Furthermore, ANGPTL4 was increased in T2DM compared with MetS-I. In fact, plasma CRP correlated positively with plasma ANGPTL4. In vitro studies showed that TLR 3/4 activation largely increased the expression and release of ANGPTL4 by macrophages.

CONCLUSIONS

Plasma ANGPTL4 levels in humans are predicted by CRP, a marker of inflammation, and ANGPTL4 expression by macrophages is increased by inflammatory stimuli.

Lack of relationship between cord serum angiopoietin-like protein 4 (ANGPTL4) and lipolytic activity in human neonates born by spontaneous delivery

BACKGROUND

Ligands of peroxisome-proliferator activated receptors (PPARs), such as non-esterified fatty acids (NEFAs), induce expression of angiopoietin-like protein 4 (ANGPTL4). Recently ANGPTL4 has been reported to be a mediator of intracellular adipose lipolysis induced by glucocorticoids.

OBJECTIVE

To determine the concentrations of ANGPTL4 in cord serum of neonates born by spontaneous vaginal delivery (SVD) and by pre-labor cesarean section (CS) from healthy women, and to relate them to parameters of neonatal lipolytic activity at birth.

MEASUREMENTS

In 54 neonates born by SVD and in 56 neonates born by CS, arterial cord blood was drawn to determine insulin, cortisol, triacylglycerols (TAGs), glycerol, non-esterified fatty acids (NEFAs), individual fatty acids, ANGPTL4, adiponectin, retinol binding protein 4 (RBP4) and leptin.

RESULTS

Birth weight and neonatal fat mass in SVD and CS showed no difference, but the concentrations of glycerol, adiponectin, RBP4, NEFAs and most individual fatty acids were higher in cord serum of neonates born by SVD compared to CS, indicating a higher adipose tissue breakdown in the SVD group. The concentrations of TAG and cortisol were also higher and that of insulin was lower in cord serum of SVD compared to the CS group. However, the concentration in cord serum of ANGPTL4 did not differ between the two groups and no positive correlation with either NEFA or glycerol concentrations were detected.

CONCLUSION

ANGPTL4 is known to stimulate lipolysis in adults, but does not appear to mediate the increased activity in SVD, indicating the presence of different regulatory inputs.

Role of Angptl4 in vascular permeability and inflammation

BACKGROUND

Angptl4 is a secreted protein involved in the regulation of vascular permeability, angiogenesis, and inflammatory responses in different kinds of tissues. Increases of vascular permeability and abnormality changes in angiogenesis contribute to the pathogenesis of tumor metastasis, ischemic-reperfusion injury. Inflammatory response associated with Angptl4 also leads to minimal change glomerulonephritis, wound healing. However, the role of Angptl4 in vascular permeability, angiogenesis, and inflammation is controversy. Hence, an underlying mechanism of Angptl4 in different kind of tissues needs to be further clarified.

METHODS

Keywords such as angptl4, vascular permeability, angiogenesis, inflammation, and endothelial cells were used in search tool of PUBMED, and then the literatures associated with Angptl4 were founded and read.

RESULTS

Data have established Angptl4 as the key modulator of both vascular permeability and angiogenesis; furthermore, it may also be related to the progression of metastatic tumors, cardiovascular events, and inflammatory diseases. This view focuses on the recent advances in our understanding of the role of Angptl4 in vascular permeability, angiogenesis, inflammatory signaling and the link between Angptl4 and multiple diseases such as cancer, cardiovascular diseases, diabetic retinopathy, and kidney diseases.

CONCLUSIONS

Taken together, Angptl4 modulates vascular permeability, angiogenesis, inflammatory signaling, and associated diseases. The use of Angptl4-modulating agents such as certain drugs, food constituents (such as fatty acids), nuclear factor (such as PPARα), and bacteria may treat associated diseases such as tumor metastasis, ischemic-reperfusion injury, inflammation, and chronic low-grade inflammation. However, the diverse physiological functions of Angptl4 in different tissues can lead to potentially deleterious side effects when used as a therapeutic target. In this regard, a better understanding of the underlying mechanisms for Angptl4 in different tissues is necessary.