Emerging insights into the roles of ANGPTL8 beyond glucose and lipid metabolism

Angiopoietin-like protein 8 (ANGPTL8) is a secreted protein predominantly expressed in liver and adipose tissue. ANGPTL8 modulates the clearance of triglycerides (TGs) by suppressing the activity of lipoprotein lipase (LPL) within the plasma. Previous studies found that circulating ANGPTL8 levels were significantly increased in metabolic disorder-related diseases, such as type 2 diabetes mellitus (T2DM), obesity, metabolic syndrome and nonalcoholic fatty liver disease (NAFLD). Whether ANGPTL8 has a direct pathogenic role in these diseases remains to be determined. In this review, we summarize the emerging roles of ANGPTL8 in the regulation of inflammation, tumours, circulatory system-related diseases, and ectopic lipid deposition, which may provide new insights into the diverse functions of ANGPTL8 in various diseases beyond its well-established functions in glucose and lipid metabolism.

Plasma IGFBP-2 levels reveal heterogeneity in hepatic fat content in adults with excess visceral adiposity

Lay summary

Obesity is frequently accompanied by a fatty liver. However, some individuals with high abdominal fat levels nevertheless have low levels of liver fat. Reasons for such discordant phenotypes are unclear. In this paper, we report that among asymptomatic individuals with high levels of visceral fat, low concentrations of IGFBP-2 in the circulation were associated with significantly higher hepatic fat content compared to those with high IGFBP-2 levels. We conclude that quantification of plasma IGFBP-2 concentrations may be useful to identify the early risk for liver fat accumulation in apparently healthy individuals without cardiovascular symptoms.

Aim/hypothesis

Although excess visceral adiposity (VAT) is generally associated with increased liver fat (LF), recent evidence has revealed heterogeneity in LF content among adults with visceral obesity, potentially contributing to specific differences in cardiometabolic outcomes. Reasons for such discordant VAT-LF phenotypes are largely unknown. The present study aimed at assessing whether circulating levels of insulin growth-factor binding protein-2 (IGFBP-2) could be a useful biomarker in the identification of heterogenous and discordant VAT-LF phenotypes.

Methods

A sample of 308 middle-aged Caucasian apparently healthy men and women without cardiovascular symptoms were studied for the present cross-sectional analyses. Fasting plasma glucose and lipid levels were assessed and an oral glucose tolerance test was performed. Hepatic fat fraction (HFF) was measured using magnetic resonance spectroscopy whereas VAT was assessed by magnetic resonance imaging. Plasma IGFBP-2 levels were quantified by ELISA. Participants were then classified on the basis of median VAT (81 mL) and IGFBP-2 levels (233 ng/mL).

Results

Individuals with high levels of VAT were characterized by higher waist circumference, lower insulin sensitivity, as well as by higher plasma triglyceride and lower HDL-cholesterol levels. Plasma IGFBP-2 levels were inversely correlated with HFF (r = -0.39, p < 0.0001). Among men and women with high levels of VAT, those with low levels of IGFBP-2 had significantly higher HFF (7.5 ± 0.7%), compared to participants with high IGFBP-2 concentrations (3.2 ± 0.5%, p < 0.0001).

Conclusion

In the presence of excess VAT, high IGFBP-2 concentrations are associated with low levels of LF. Although additional studies will be necessary to establish causality and further clarify the clinical implications of these observations, these findings are concordant with a novel function of IGFBP-2 in modulating susceptibility to non-alcoholic fatty liver disease (NAFLD) in the presence of visceral obesity.

Ectopic lipid deposition and insulin resistance in patients with GH disorders before and after treatment

OBJECTIVES

Insulin resistance is associated with ectopic lipid deposition. Growth hormone (GH) status also modulates ectopic lipid accumulation, but how this associates with insulin resistance in patients with GH disorders is not well established.

DESIGN AND METHODS

Twenty-one patients diagnosed with acromegaly and 12 patients with adult GH deficiency (GHD) were studied at diagnosis and after treatment. A reference group of 12 subjects was included. Each study day comprised assessment of body composition with dual-energy X-ray absorptiometry, ectopic lipid deposition in the liver by MR spectroscopy, and Homeostatic Model Assessment for Insulin Resistance (HOMA-IR).

RESULTS

Disease control of acromegaly decreased lean body mass (LBM) (P < .000) and increased the percentage of total body fat (TBF) (P < .000). GH replacement increased LBM in the GHD patients (P = .007) and decreased the percentage of TBF (P = .010). The intrahepatic lipid (IHL) content increased after disease control in acromegaly (P = .004), whereas IHL did not change significantly after GH replacement in GHD (P = .34). Insulin resistance (HOMA-IR) improved after disease control of acromegaly (P < .000) and remained unaltered after GH replacement in the GHD patients (P = .829).

CONCLUSIONS

GH status is a significant modulator of body composition and insulin sensitivity.GH excess reduces total fat mass and intrahepatic lipid content together with induction of insulin resistance.The data support the notion that GH-induced insulin resistance is unassociated with hepatic lipid accumulation.

Adipose Tissue Dysfunction as Determinant of Obesity-Associated Metabolic Complications

Obesity is a critical risk factor for the development of type 2 diabetes (T2D), and its prevalence is rising worldwide. White adipose tissue (WAT) has a crucial role in regulating systemic energy homeostasis. Adipose tissue expands by a combination of an increase in adipocyte size (hypertrophy) and number (hyperplasia). The recruitment and differentiation of adipose precursor cells in the subcutaneous adipose tissue (SAT), rather than merely inflating the cells, would be protective from the obesity-associated metabolic complications. In metabolically unhealthy obesity, the storage capacity of SAT, the largest WAT depot, is limited, and further caloric overload leads to the fat accumulation in ectopic tissues (e.g., liver, skeletal muscle, and heart) and in the visceral adipose depots, an event commonly defined as “lipotoxicity.” Excessive ectopic lipid accumulation leads to local inflammation and insulin resistance (IR). Indeed, overnutrition triggers uncontrolled inflammatory responses in WAT, leading to chronic low-grade inflammation, therefore fostering the progression of IR. This review summarizes the current knowledge on WAT dysfunction in obesity and its associated metabolic abnormalities, such as IR. A better understanding of the mechanisms regulating adipose tissue expansion in obesity is required for the development of future therapeutic approaches in obesity-associated metabolic complications.

Inflammatory stress promotes the development of obesity-related chronic kidney disease via CD36 in mice

Ectopic fat located in the kidney has emerged as a novel cause of obesity-related chronic kidney disease (CKD). In this study, we aimed to investigate whether inflammatory stress promotes ectopic lipid deposition in the kidney and causes renal injury in obese mice and whether the pathological process is mediated by the fatty acid translocase, CD36. High-fat diet (HFD) feeding alone resulted in obesity, hyperlipidemia, and slight renal lipid accumulation in mice, which nevertheless had normal kidney function. HFD-fed mice with chronic inflammation had severe renal steatosis and obvious glomerular and tubular damage, which was accompanied by increased CD36 expression. Interestingly, CD36 deficiency in HFD-fed mice eliminated renal lipid accumulation and pathological changes induced by chronic inflammation. In both human mesangial cells (HMCs) and human kidney 2 (HK2) cells, inflammatory stress increased the efficiency of CD36 protein incorporation into membrane lipid rafts, promoting FFA uptake and intracellular lipid accumulation. Silencing of CD36 in vitro markedly attenuated FFA uptake, lipid accumulation, and cellular stress induced by inflammatory stress. We conclude that inflammatory stress aggravates renal injury by activation of the CD36 pathway, suggesting that this mechanism may operate in obese individuals with chronic inflammation, making them prone to CKD.

Perilipin 2 and Age-Related Metabolic Diseases: A New Perspective

Perilipin 2 (Plin2), a protein associated with the metabolism of intracellular lipid droplets (LDs), has long been considered only for its role in lipid storage. However, the manipulation of its expression affects the severity of a variety of metabolic and age-related diseases, such as fatty liver, insulin resistance and type 2 diabetes (T2D), cardiovascular disease, atherosclerosis, sarcopenia, and cancer, suggesting that this protein may play a role in these pathological conditions. In particular, its downregulation in mice prevents or mitigates some of the above mentioned diseases. Conversely, in humans high levels of Plin2 are present in sarcopenia, hepatic steatosis, atherosclerosis, and some types of cancer. We propose that inhibition of Plin2 might be a strategy to counteract several metabolic and age-related diseases.

Polychlorinated biphenyl-77 induces adipocyte differentiation and proinflammatory adipokines and promotes obesity and atherosclerosis

BACKGROUND

Obesity, an inflammatory condition linked to cardiovascular disease, is associated with expansion of adipose tissue. Highly prevalent coplanar polychlorinated biphenyls (PCBs) such as 3,3',4,4'-tetrachlorobiphenyl (PCB-77) accumulate in adipose tissue because of their lipophilicity and increase with obesity. However, the effects of PCBs on adipocytes, obesity, and obesity-associated cardiovascular disease are unknown.

OBJECTIVES

In this study we examined in vitro and in vivo effects of PCB-77 on adipocyte differentiation, proinflammatory adipokines, adipocyte morphology, body weight, serum lipids, and atherosclerosis.

METHODS

PCB-77 or 2,2',4,4,5,5'-hexachlorobiphenyl (PCB-153) was incubated with 3T3-L1 adipocytes either during differentiation or in mature adipocytes. Concentration-dependent effects of PCB-77 were contrasted with those of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). For in vivo studies, we treated C57BL/6 wild-type (WT) or aryl hydrocarbon receptor (AhR)(-/-) mice with vehicle or PCB-77 (49 mg/kg, by intraperitoneal injection) and examined body weight gain. In separate studies, we injected ApoE(-/-) mice with vehicle or PCB-77 over a 6-week period and examined body weight, adipocyte size, serum lipids, and atherosclerosis.

RESULTS

Low concentrations of PCB-77 or TCDD increased adipocyte differentiation, glycerol-3-phosphate dehydrogenase activity, and expression of peroxisome proliferator-activated receptor gamma, whereas higher concentrations inhibited adipocyte differentiation. Effects of PCB-77 were abolished by the AhR antagonist alpha-naphthoflavone. PCB-77 promoted the expression and release of various proinflammatory cytokines from 3T3-L1 adipocytes. Administration of PCB-77 increased body weight gain in WT but not AhR(-/-) mice. ApoE(-/-) mice injected with PCB-77 exhibited greater body weight, adipocyte hypertrophy, serum dyslipidemia, and augmented atherosclerosis.

CONCLUSIONS

Our findings suggest that PCB-77 may contribute to the development of obesity and obesity-associated atherosclerosis.