Lipasin, a novel nutritionally-regulated liver-enriched factor that regulates serum triglyceride levels

Correlation of circulating betatrophin concentrations with insulin secretion capacity, evaluated by glucagon stimulation tests

AIM

To investigate the relationship between plasma betatrophin concentrations and insulin secretion capacity in people with Type 2 diabetes.

METHODS

Glucagon stimulation tests (1 mg) were performed in 70 people with Type 2 diabetes after an overnight fast. Plasma betatrophin concentrations were measured using an enzyme-linked immunosorbent assay. Insulin secretion capacity was evaluated by measuring increments of C-peptide concentration in response to glucagon stimulation, and creatinine clearance was determined by comparing creatinine concentrations in serum and 24-h urine samples.

RESULTS

Plasma betatrophin concentrations were positively correlated with duration of Type 2 diabetes (r = 0.34, P = 0.003), and negatively correlated with increments of C-peptide concentration (r = 0.37, P = 0.001) and creatinine clearance (r = 0.37, P = 0.001). The correlation with increments of C-peptide concentration remained significant after adjustment for age and duration of Type 2 diabetes (r = 0.25, P = 0.037). Multivariate analysis identified age and increments of C-peptide concentration as independent factors associated with plasma betatrophin levels.

CONCLUSION

Plasma betatrophin levels inversely correlate with insulin secretion capacity, suggesting that betatrophin levels are regulated by insulin secretion capacity in humans.

Serum betatrophin concentrations are significantly increased in overweight but not in obese or type 2 diabetic individuals

OBJECTIVE

In this study, circulating serum betatrophin levels were quantitated and their relationships with insulin resistance (IR) and other metabolic parameters in Chinese subjects with varying degrees of obesity and glucose tolerance were examined.

METHODS

Serum betatrophin levels were determined using ELISA in 60 subjects with normal glucose tolerance (NGT: 17 lean, 23 overweight, and 20 obese subjects) and 56 subjects with type 2 diabetes mellitus (T2DM: 14 lean, 23 overweight, and 19 obese subjects). The associations of serum betatrophin levels with adiposity, glucose, lipid profile, and hepatic enzyme parameters were studied.

RESULTS

Serum betatrophin concentrations were significantly higher in overweight subjects in both the NGT and T2DM groups; however, no significant difference between lean and obese participants was observed. No significant difference was found between males and females or between NGT and T2DM subjects. Serum betatrophin concentrations correlated positively with fasting insulin, homeostasis model assessment-estimated insulin resistance (HOMA-IR), γ-glutamyl transpeptidase (γ-GT), and alanine aminotransferase (ALT) in all subjects. Serum betatrophin concentrations showed an independent association with γ-GT and HOMA-IR.

CONCLUSIONS

Serum betatrophin levels were significantly increased in overweight individuals but not in individuals with obesity or T2DM. Serum betatrophin concentrations were significantly associated with IR, but not with lipid profiles, glucose homeostasis, or diabetes.

Betatrophin Acts as a Diagnostic Biomarker in Type 2 Diabetes Mellitus and Is Negatively Associated with HDL-Cholesterol

Objective. By assessing its circulating concentrations in type 2 diabetes mellitus (T2DM) patients, we aimed to explore the associations of betatrophin with various metabolic parameters and evaluate its diagnostic value in T2DM. Methods. A total of 58 non-diabetes-mellitus (NDM) subjects and 73 age- and sex-matched newly diagnosed T2DM patients were enrolled. Correlation analyses between circulating betatrophin levels and multiple metabolic parameters were performed. Receiver operating characteristic (ROC) curve analysis was used to assess the diagnostic value of betatrophin concentration in T2DM. Results. Circulating betatrophin levels were approximately 1.8 times higher in T2DM patients than in NDM individuals (median 747.12 versus 407.41 pg/mL, P < 0.001). Correlation analysis showed that betatrophin was negatively associated with high-density lipoprotein cholesterol (HDL-C) levels in all subjects. ROC curve analysis identified betatrophin as a potent diagnostic biomarker for T2DM. The optimal cut-off point of betatrophin concentration for predicting T2DM was 501.23 pg/mL. Conclusions. Serum betatrophin levels were markedly increased in newly diagnosed T2DM patients and further elevated in obese T2DM subjects. Betatrophin was negatively correlated with HDL-C levels. Our findings indicate that betatrophin could be a potent diagnostic biomarker for T2DM.

Circulating betatrophin is elevated in patients with type 1 and type 2 diabetes

There is evidence that betatrophin, a hormone derived from adipose tissue and liver, affects the proliferation of pancreatic beta cells in mice. The aim of this study was to examine circulating betatrophin concentrations in Japanese healthy controls and patients with type 1 and type 2 diabetes. A total of 76 subjects (12 healthy controls, 34 type 1 diabetes, 30 type 2 diabetes) were enrolled in the study. Circulating betatrophin was measured with an ELISA kit and clinical parameters related to betatrophin were analyzed statistically. Circulating betatrophin (Log transformed) was significantly increased in patients with diabetes compared with healthy subjects (healthy controls, 2.29 ± 0.51; type 1 diabetes, 2.94 ± 0.44; type 2 diabetes, 3.17 ± 0.18; p<0.001, 4.1 to 5.4 times in pg/mL order). Age, HbA1c, fasting plasma glucose and Log triglyceride were strongly associated with Log betatrophin in all subjects (n=76) in correlation analysis. In type 1 diabetes, there was a correlation between Log betatrophin and Log CPR. These results provide the first evidence that circulating betatrophin is significantly elevated in Japanese patients with diabetes. The findings of this pilot study also suggest a possibility of association between the level of betatrophin and the levels of glucose and triglycerides.

Evaluation of circulating betatrophin levels in gestational diabetes mellitus

Experimental data indicate that betatrophin plays a significant role in the regulation of lipid metabolism and glucose homeostasis. In recent years, considerable attention has focused on the relationship between betatrophin and diabetes mellitus in humans. This case-control study included 45 women diagnosed with gestational diabetes mellitus (GDM) and 45 pregnant healthy controls. The groups were matched for maternal and gestational age and body mass index. Serum betatrophin levels were significantly higher in women with GDM (median = 635.8 ng/L; range: 290-1841.6 ng/L) compared to control subjects (median = 320.1 ng/L; range: 94.6-936.8 ng/L; p = 0.001). No significant correlations were observed between serum betatrophin levels and clinical or biochemical parameters in the control group. However, in the GDM group, serum betatrophin levels were positively correlated with weight gain during pregnancy (r = 0.304, p = 0.042), systolic blood pressure (r = 0.394, p = 0.007), fasting insulin level (r = 0.348, p = 0.019), and homeostatic model assessment insulin resistance (HOMA-IR; r = 0.311, p = 0.038). Multivariate stepwise linear regression analysis revealed that fasting insulin levels (β = 0.342, p = 0.022) and HOMA-IR (β = 0.312, p = 0.037) were independently associated with serum betatrophin levels.

Associations of betatrophin levels with irisin in Chinese women with normal glucose tolerance

BACKGROUND

Betatrophin may increase islet β cell proliferation in insulin resistance and irisin may improve glucose tolerance in mice. To examine the relationship between betatrophin and irisin, we investigated it in middle-aged Chinese subjects with normal glucose tolerance (NGT) and type 2 diabetes mellitus (T2DM).

METHODS

A total of 460 permanent residents of Fengxian District, aged 40-60 years and without T2DM, were enrolled. Anthropometric parameters, oral glucose tolerance test (OGTT) results, glycosylated haemoglobin levels, blood lipid levels, insulin sensitivity (homeostasis model assessment of insulin resistance, HOMA-IR), β cell function (homeostasis model assessment-β, HOMA-β), estimated glomerular filtration rate (eGFR) and body fat composition were determined. Matched for age, gender and body mass index (BMI, 18-28 kg/m2), newly diagnosed T2DM (n = 50, male/female = 23/27) and NGT (n = 50, male/female = 21/29) subjects were selected based on the results of an OGTT. Serum betatrophin and irisin levels were determined by enzyme linked immune sorbent assay (ELISA).

RESULTS

Males had higher levels of betatrophin compared with females in both the NGT and T2DM groups. Compared with NGT subjects, the level of betatrophin in the T2DM group was higher, and males in the T2DM group had higher betatrophin levels than males in the NGT group, but there was no significant difference in betatrophin levels in females between the T2DM and NGT groups. Spearman's correlation analysis revealed that serum betatrophin levels in females with NGT were positively correlated with irisin and negatively correlated with FINS (fasting insulin) levels ( p < 0.05), but no correlation was found between betatrophin and irisin levels in males with NGT or in males or females with T2DM. In females with T2DM, circulating betatrophin levels were positively correlated with weight, BMI and hip circumference (p < 0.05) but negatively correlated with FPG (fasting plasma glucose) and HOMA-IR (p < 0.05).

CONCLUSIONS

Gender differences in the relationship between betatrophin and irisin indicate that there might be cytokine-mediated crosstalk among the liver, adipose tissue and skeletal muscle.

Emerging regulation and function of betatrophin

Betatrophin, also known as TD26/RIFL/lipasin/ANGPTL8/C19orf80, is a novel protein predominantly expressed in human liver. To date, several betatrophin orthologs have been identified in mammals. Increasing evidence has revealed an association between betatrophin expression and serum lipid profiles, particularly in patients with obesity or diabetes. Stimulators of betatrophin, such as insulin, thyroid hormone, irisin and caloric intake, are usually relevant to energy expenditure or thermogenesis. In murine models, serum triglyceride levels as well as pancreatic cell proliferation are potently enhanced by betatrophin. Intriguingly, conflicting phenomena have also been reported that betatrophin suppresses hepatic triglyceride levels, suggesting that betatrophin function is mediated by complex regulatory processes. However, its precise physiological role remains unclear at present. In this review, we have summarized the current findings on betatrophin and their implications.

Increased circulating levels of betatrophin in newly diagnosed type 2 diabetic patients

OBJECTIVE

Betatrophin, a newly identified hormone, has been recently characterized as a potent stimulator that increases the production and expansion of insulin-secreting β-cells in mice, but the physiological role of betatrophin remains poorly understood. This study measured for the first time serum betatrophin levels in newly diagnosed patients with type 2 diabetes (T2DM) and explored the correlations between its serum levels and various metabolic parameters in T2DM.

RESEARCH DESIGN AND METHODS

We analyzed the concentrations of betatrophin by ELISA in blood samples of 166 well-characterized individuals in whom anthropometric parameters, oral glucose tolerance test (OGTT), glycosylated hemoglobin, blood lipids, insulin sensitivity (1/homeostasis model assesment of insulin resistance [1/HOMA-IR] and Matsuda index [ISIM]), and insulin secretion were measured. The participants were divided into newly diagnosed T2DM patients (n = 83) and age-, sex- and BMI-matched healthy control subjects (n = 83).

RESULTS

Serum betatrophin levels were significantly higher in T2DM patients than in healthy control subjects (613.08 [422.19-813.08] vs. 296.57 [196.53-509.46] pg/mL; P < 0.01). Serum betatrophin positively correlated with age, 2-h post-OGTT glucose (2hPG), and postprandial serum insulin (PSI), but negatively with 1/HOMA-IR and ISIM in T2DM patients. In the control group, betatrophin was only positively associated with age. In T2DM subjects, multivariate regression analyses showed that age, 2hPG, and PSI were independent factors influencing serum betatrophin levels.

CONCLUSIONS

Circulating concentrations of betatrophin are significantly increased in T2DM patients. Our results suggest that betatrophin may play a role in the pathogenesis of T2DM.

A dual role of lipasin (betatrophin) in lipid metabolism and glucose homeostasis: consensus and controversy

Metabolic syndrome includes glucose intolerance and dyslipidemia, both of which are strong risk factors for developing diabetes and atherosclerotic cardiovascular diseases. Recently, multiple groups independently studied a previously uncharacterized gene, officially named C19orf80 (human) and Gm6484 (mouse), but more commonly known as RIFL, Angptl8, betatrophin and lipasin. Both exciting and conflicting results have been obtained, and significant controversy is ongoing. Accumulating evidence from genome wide association studies and mouse genetic studies convincingly shows that lipasin is involved in lipid regulation. However, the mechanism of action, the identity of transcription factors mediating its nutritional regulation, circulating levels, and relationship among lipasin, Angptl3 and Angptl4, remain elusive. Betatrophin represents a promising drug target for replenishing β-cell mass, but current results have not been conclusive regarding its potency and specificity. Here, we summarize the consensus and controversy regarding functions of lipasin/betatrophin based on currently available evidence.

Role of growth factors in control of pancreatic beta cell mass: focus on betatrophin

PURPOSE OF REVIEW

Betatrophin is a newly described hormone, which potently stimulates beta cell replication in mice. This discovery has engendered great hope that it could prove clinically important in the treatment of type 1 and type 2 diabetes.

RECENT FINDINGS

Betatrophin, a 198-amino acid protein secreted by liver and adipose tissue, stimulates growth of pancreatic beta cell mass in insulin-resistant mice. Betatrophin has previously been named RIFL, lipasin, and ANGPLT8, and its salutory effects on lipid metabolism have been described in mouse and human studies. Serum betatrophin levels in humans correlate with improved adipose tissue lipid storage and lower serum triglyceride levels in the fed state, but do not correlate with insulin resistance or carbohydrate tolerance in humans. Betatrophin has not yet been shown to have an effect on beta cell replication in human pancreatic islets.

SUMMARY

Many endocrine and paracrine factors, of which betatrophin is the newest described, increase beta cell mass in murine models. None of these factors, including betatrophin, have displayed the same activity in clinical studies. This may reflect a profound species difference in beta cell regeneration pathways in mice and humans.

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.

Circulating betatrophin correlates with atherogenic lipid profiles but not with glucose and insulin levels in insulin-resistant individuals

AIMS/HYPOTHESIS

The newly identified liver- and fat-derived hormone, betatrophin, has recently been linked to insulin resistance and pancreatic beta cell growth in mice. These preclinical findings have suggested betatrophin as a potential candidate for novel glucose-lowering treatment concepts involving beta cell regeneration. However, the role of betatrophin in human insulin resistance and type 2 diabetes is currently unknown. Hence, the aim of this study was to investigate circulating betatrophin concentrations in two distinct cohorts with insulin resistance.

METHODS

Betatrophin concentrations were analysed in (1) age- and sex-matched lean (n = 20) and morbidly obese individuals (n = 19), and (2) age-, sex- and BMI-matched non-diabetic (n = 19) and type 2 diabetic individuals (n = 18).

RESULTS

Betatrophin concentrations did not differ between lean and morbidly obese or between non-diabetic and type 2 diabetic participants. No association was found with variables of beta cell function and glucose homeostasis. However, betatrophin did correlate significantly with plasma atherogenic lipids including total cholesterol, LDL-cholesterol and apolipoprotein B in morbidly obese and type 2 diabetic patients but not in controls. Insulin-resistant individuals with hypercholesterolaemia (≥5.2 mmol/l) had significantly higher betatrophin concentrations than those with normal cholesterol (<5.2 mmol/l).

CONCLUSIONS/INTERPRETATION

Betatrophin is a recently identified hormone, the circulating concentrations of which are unaltered in human insulin resistance but correlate significantly with atherogenic lipid profiles in high-risk cohorts with morbid obesity or type 2 diabetes. Betatrophin could therefore be a novel pathomechanistic player in dysfunctional lipid metabolism associated with high cardiovascular risk.

Elevated circulating lipasin/betatrophin in human type 2 diabetes and obesity

Lipasin (also known as C19ORF80, RIFL, ANGPTL8 and betatrophin) is a newly discovered circulating factor that regulates lipid metabolism and promotes pancreatic β-cell proliferation. Whether circulating levels of lipasin in humans are altered in a) type 2 diabetes; b) obesity and c) the postprandial state, however, is unknown. The current study aimed to compare serum lipasin levels in those who were a) non-diabetic (N=15) or diabetic (BMI- and age-matched; N=14); b) lean or obese (N=53 totally) and c) fasting and 2 hours following a defined meal (N=12). Serum lipasin levels were determined by the enzyme-linked immunosorbent assay. Lipasin levels [mean±SEM] were increased by more than two fold (P<0.001) in the diabetic patients (5.56±0.73 ng/mL) as compared to the control subjects (2.19±0.24 ng/mL). Serum lipasin levels were positively correlated with BMI (rho=0.49, P<0.001), and showed a 35% increase 2 hours following a defined meal (P=0.009). Therefore, lipasin/betatrophin is nutritionally-regulated hepatokine that is increased in human type 2 diabetes and obesity.

Betatrophin provides a new insight into diabetes treatment and lipid metabolism (Review)

Replenishing the insulin-producing β-cell mass is considered to be a potential cure for diabetes. A recent study identified a secreted protein, known as betatrophin, which potently induces pancreatic β-cell proliferation. Notably, a number of studies reportedly identified betatrophin, which is also known as lipasin, atypical angiopoietin-like 8 and refeeding-induced fat and liver protein, and considered to be a novel regulator in lipid metabolism according to the studies. The identification of betatrophin was considered to create novel opportunities for potential diabetes therapy. In the present study, the current knowledge of betatrophin is reviewed, with regards to its character and function in lipid homeostasis and pancreatic β-cell proliferation.

Physiological regulation of lipoprotein lipase

The enzyme lipoprotein lipase (LPL), originally identified as the clearing factor lipase, hydrolyzes triglycerides present in the triglyceride-rich lipoproteins VLDL and chylomicrons. LPL is primarily expressed in tissues that oxidize or store fatty acids in large quantities such as the heart, skeletal muscle, brown adipose tissue and white adipose tissue. Upon production by the underlying parenchymal cells, LPL is transported and attached to the capillary endothelium by the protein GPIHBP1. Because LPL is rate limiting for plasma triglyceride clearance and tissue uptake of fatty acids, the activity of LPL is carefully controlled to adjust fatty acid uptake to the requirements of the underlying tissue via multiple mechanisms at the transcriptional and post-translational level. Although various stimuli influence LPL gene transcription, it is now evident that most of the physiological variation in LPL activity, such as during fasting and exercise, appears to be driven via post-translational mechanisms by extracellular proteins. These proteins can be divided into two main groups: the liver-derived apolipoproteins APOC1, APOC2, APOC3, APOA5, and APOE, and the angiopoietin-like proteins ANGPTL3, ANGPTL4 and ANGPTL8, which have a broader expression profile. This review will summarize the available literature on the regulation of LPL activity in various tissues, with an emphasis on the response to diverse physiological stimuli.

Elevated mouse hepatic betatrophin expression does not increase human β-cell replication in the transplant setting

The recent discovery of betatrophin, a protein secreted by the liver and white adipose tissue in conditions of insulin resistance and shown to dramatically stimulate replication of mouse insulin-producing β-cells, has raised high hopes for the rapid development of a novel therapeutic approach for the treatment of diabetes. At present, however, the effects of betatrophin on human β-cells are not known. Here we use administration of the insulin receptor antagonist S961, shown to increase betatrophin gene expression and stimulate β-cell replication in mice, to test its effect on human β-cells. Although mouse β-cells, in their normal location in the pancreas or when transplanted under the kidney capsule, respond with a dramatic increase in β-cell DNA replication, human β-cells are completely unresponsive. These results put into question whether betatrophin can be developed as a therapeutic approach for treating human diabetes.

Role of Ink4a/Arf locus in beta cell mass expansion under physiological and pathological conditions

The ARF/INK4A (Cdkn2a) locus includes the linked tumour suppressor genes p16INK4a and p14ARF (p19ARF in mice) that trigger the antiproliferative activities of both RB and p53. With beta cell self-replication being the primary source for new beta cell generation in adult animals, the network by which beta cell replication could be increased to enhance beta cell mass and function is one of the approaches in diabetes research. In this review, we show a general view of the regulation points at transcriptional and posttranslational levels of Cdkn2a locus. We describe the molecular pathways and functions of Cdkn2a in beta cell cycle regulation. Given that aging reveals increased p16Ink4a levels in the pancreas that inhibit the proliferation of beta cells and decrease their ability to respond to injury, we show the state of the art about the role of this locus in beta cell senescence and diabetes development. Additionally, we focus on two approaches in beta cell regeneration strategies that rely on Cdkn2a locus negative regulation: long noncoding RNAs and betatrophin.

Increased circulating levels of betatrophin in individuals with long-standing type 1 diabetes

AIMS/HYPOTHESIS

The hormone betatrophin was recently described as a potent stimulator of beta cell proliferation in mice. Insulin resistance, but not insulin deficiency, caused upregulation of betatrophin expression. If these findings were found to be fully applicable in humans, this would open up the possibility of future betatrophin treatment in type 1 diabetes. The present study measured for the first time betatrophin concentrations in humans and tested the hypothesis that there would be no difference in circulating betatrophin concentrations between patients with type 1 diabetes and healthy individuals.

METHODS

Betatrophin concentrations in plasma of 33 patients with type 1 diabetes and 24 age-matched healthy controls were measured by ELISA. The study participants were characterised for blood lipids, BMI, plasma glucose and HbA1c, and, for the diabetic patients, their insulin requirements and any residual C-peptide concentrations.

RESULTS

Plasma betatrophin concentrations were normally ~300 pg/ml, but were approximately doubled in patients with type 1 diabetes. In the patients, there were no correlations between betatrophin and age, blood lipids, BMI, glucose control or insulin requirement, whereas in controls betatrophin levels increased with age. BMI, blood pressure and triacylglycerol, LDL-cholesterol and HDL-cholesterol levels were similar in patients and healthy controls.

CONCLUSIONS/INTERPRETATION

Circulating concentrations of betatrophin are increased in type 1 diabetes in contrast with what was recently described in an insulin-deficient mouse model. However, increased betatrophin concentrations do not protect against loss of C-peptide. Betatrophin treatment in type 1 diabetes would therefore probably not be successful without the use of supraphysiological doses or a combination with immune regulatory treatment.

Betatrophin

Regenerative therapy in diabetes with the capacity to reconstitute a functional β-cell mass sufficient for glycemic control holds the promise to effectively prevent the development of devastating late complications due to the unique ability of the β-cell to sense and regulate blood-glucose levels. An ability that cannot be mimicked by insulin replacement therapy or any other means of current treatment regiments for very large patient populations. Recently, Douglas A. Melton's group from Harvard University reported the identification of a circulating protein secreted from the liver under insulin resistant states which is sufficient to dramatically and specifically increase the replication rate of β-cells in the mouse resulting in an increased functional β-cell mass over time. They re-named the factor betatrophin and described a number of exciting features of this molecule which suggested that it could be a potential candidate for development as a regenerative medicine in diabetes. The official name of the gene encoding mouse betatrophin is Gm6484, but it has been annotated a number of times under different names: EG624219, RIFL, Lipasin and ANGPTL8. The official human gene name is C19orf80, but it has also been annotated as TD26, LOC55908, as well as RIFL, Lipasin, ANGPTL8 and betatrophin.

Increased circulating betatrophin concentrations in patients with type 2 diabetes

Betatrophin has recently been described as a key hormone to stimulate beta-cell mass expansion in response to insulin resistance and obesity in mice. The finding has generated an interest in the development of antidiabetic drugs with betatrophin as the active component. However, the circulating levels of betatrophin in patients with type 2 diabetes are not well known. Betatrophin concentrations in plasma of 27 type 2 diabetes patients and 18 gender-, age-, and BMI-matched controls were measured. Study participants were characterized with regard to BMI, waist and hip circumference, blood pressure, and fasting plasma blood lipids, creatinine, glucose, HbA1c, and C-peptide. HOMA2 indices were calculated. Betatrophin was 40% higher in patients with type 2 diabetes (893 ± 80 versus 639 ± 66 pg/mL). Betatrophin positively correlated with age in the controls and with HbA1c in the type 2 diabetes patients. All study participants were insulin resistant with mean HOMA2B IR in both groups exceeding 2 and HOMA2%S < 50%. Control individuals had impaired fasting glucose concentrations. In this report on betatrophin concentrations in type 2 diabetes and insulin resistance, elevated betatrophin levels were measured in the patients with type 2 diabetes. Future studies are clearly needed to delineate the exact role, if any, of betatrophin in regulating human beta-cell mass.

Angptl4 serves as an endogenous inhibitor of intestinal lipid digestion

Dietary triglycerides are hydrolyzed in the small intestine principally by pancreatic lipase. Following uptake by enterocytes and secretion as chylomicrons, dietary lipids are cleared from the bloodstream via lipoprotein lipase. Whereas lipoprotein lipase is inhibited by several proteins including Angiopoietin-like 4 (Angptl4), no endogenous regulator of pancreatic lipase has yet been identified. Here we present evidence that Angptl4 is an endogenous inhibitor of dietary lipid digestion. Angptl4-/- mice were heavier compared to their wild-type counterparts without any difference in food intake, energy expenditure or locomotor activity. However, Angptl4-/- mice showed decreased lipid content in the stools and increased accumulation of dietary triglycerides in the small intestine, which coincided with elevated luminal lipase activity in Angptl4-/- mice. Furthermore, recombinant Angptl4 reduced the activity of pancreatic lipase as well as the lipase activity in human ileostomy output. In conclusion, our data suggest that Angptl4 is an endogenous inhibitor of intestinal lipase activity.

Mice lacking ANGPTL8 (Betatrophin) manifest disrupted triglyceride metabolism without impaired glucose homeostasis

Angiopoietin-like protein (ANGPTL)8 (alternatively called TD26, RIFL, Lipasin, and Betatrophin) is a newly recognized ANGPTL family member that has been implicated in both triglyceride (TG) and glucose metabolism. Hepatic overexpression of ANGPTL8 causes hypertriglyceridemia and increased insulin secretion. Here we examined the effects of inactivating Angptl8 on TG and glucose metabolism in mice. Angptl8 knockout (Angptl8(-/-)) mice gained weight more slowly than wild-type littermates due to a selective reduction in adipose tissue accretion. Plasma levels of TGs of the Angptl8(-/-) mice were similar to wild-type animals in the fasted state but paradoxically decreased after refeeding. The lower TG levels were associated with both a reduction in very low density lipoprotein secretion and an increase in lipoprotein lipase (LPL) activity. Despite the increase in LPL activity, the uptake of very low density lipoprotein-TG is markedly reduced in adipose tissue but preserved in hearts of fed Angptl8(-/-) mice. Taken together, these data indicate that ANGPTL8 plays a key role in the metabolic transition between fasting and refeeding; it is required to direct fatty acids to adipose tissue for storage in the fed state. Finally, glucose and insulin tolerance testing revealed no alterations in glucose homeostasis in mice fed either a chow or high fat diet. Thus, although absence of ANGPTL8 profoundly disrupts TG metabolism, we found no evidence that it is required for maintenance of glucose homeostasis.

The angiopoietin-like protein 3: a hepatokine with expanding role in metabolism

PURPOSE OF REVIEW

Cumulating evidence are revealing roles of angiopoietin-like proteins (ANGPTLs) in lipid, glucose, and energy metabolism. In this review, we discuss the recent developments in understanding the specific role in metabolic processes of the liver-derived ANGPTL3.

RECENT FINDINGS

Several groups have reported clinical and metabolic characterization of individuals with loss-of-function variants in ANGPTL3 showing familial combined hypolipidemia, a syndrome characterized by marked reduction of all plasma lipoproteins. Their findings indicate that in humans, ANGPTL3 has a broader action on apoB and apoA-I-containing lipoproteins, as well as on free fatty acid and adipose tissue metabolism.

SUMMARY

The identification of loss-of-function ANGPTL3 mutation is shedding light on a possible role of ANGPTL3 at the crossroads of lipoproteins, fatty acids, and glucose metabolism, thus making ANGPTL3 an attractive protein to target the cardio-metabolic risk.

Betatrophin: a hormone that controls pancreatic β cell proliferation

Replenishing insulin-producing pancreatic β cell mass will benefit both type I and type II diabetics. In adults, pancreatic β cells are generated primarily by self-duplication. We report on a mouse model of insulin resistance that induces dramatic pancreatic β cell proliferation and β cell mass expansion. Using this model, we identify a hormone, betatrophin, that is primarily expressed in liver and fat. Expression of betatrophin correlates with β cell proliferation in other mouse models of insulin resistance and during gestation. Transient expression of betatrophin in mouse liver significantly and specifically promotes pancreatic β cell proliferation, expands β cell mass, and improves glucose tolerance. Thus, betatrophin treatment could augment or replace insulin injections by increasing the number of endogenous insulin-producing cells in diabetics.

Betatrophin: a hormone that controls pancreatic β cell proliferation

Replenishing insulin-producing pancreatic β cell mass will benefit both type I and type II diabetics. In adults, pancreatic β cells are generated primarily by self-duplication. We report on a mouse model of insulin resistance that induces dramatic pancreatic β cell proliferation and β cell mass expansion. Using this model, we identify a hormone, betatrophin, that is primarily expressed in liver and fat. Expression of betatrophin correlates with β cell proliferation in other mouse models of insulin resistance and during gestation. Transient expression of betatrophin in mouse liver significantly and specifically promotes pancreatic β cell proliferation, expands β cell mass, and improves glucose tolerance. Thus, betatrophin treatment could augment or replace insulin injections by increasing the number of endogenous insulin-producing cells in diabetics.

Lipasin, thermoregulated in brown fat, is a novel but atypical member of the angiopoietin-like protein family

Hyperlipidemia is a major contributor to cardiovascular diseases. Members of the angiopoietin-like protein family (ANGPTLs) are important determinants of blood lipid levels. Lipasin, a newly identified gene that regulates serum triglycerides, is homologous to ANGPTL3's N-terminal domain, which is sufficient and necessary for blood lipid regulation. Brown fat is critical in mediating energy homeostasis. Thermogenesis is the primary function of brown fat, in which Lipasin and some ANGPTLs are abundant; it is unknown, however, whether these genes are thermoregulated. We therefore comprehensively examined the thermoregulation of Lipasin and ANGPTLs in brown fat. Here we show that Lipasin is a novel but atypical member of the ANGPTL family because it is within the same branch as ANGPTL3 and 4 by phylogenetic analysis. The mRNA levels of Lipasin are dramatically increased in the cold environment (4 °C for 4 h) whereas those of ANGPTL4 and ANGPTL2 are suppressed. Fasting dramatically suppresses Lipasin but increases ANGPTL4. High-fat diet treatment increases Lipasin, but reduces ANGPTL2. The distinct transcriptional regulations of Lipasin, ANGPTL2 and ANGPTL4 in brown fat in response to cold exposure and nutritional stimulation suggest distinct physiological roles for ANGPTL family members in mediating thermogenesis and energy homeostasis.

Atypical angiopoietin-like protein that regulates ANGPTL3

Angiopoietin-like proteins (ANGPTLs) play major roles in the trafficking and metabolism of lipids. Inactivation of ANGPTL3, a gene located in an intron of DOCK7, results in very low levels of LDL-cholesterol (C), HDL-C and triglyceride (TAG). We identified another ANGPTL family member, ANGPTL8, which is located in the corresponding intron of DOCK6. A variant in this family member (rs2278426, R59W) was associated with lower plasma LDL-C and HDL-C levels in three populations. ANGPTL8 is expressed in liver and adipose tissue, and circulates in plasma of humans. Expression of ANGPTL8 was reduced by fasting and increased by refeeding in both mice and humans. To examine the functional relationship between the two ANGPTL family members, we expressed ANGPTL3 at physiological levels alone or together with ANGPTL8 in livers of mice. Plasma TAG level did not change in mice expressing ANGPTL3 alone, whereas coexpression with ANGPTL8 resulted in hypertriglyceridemia, despite a reduction in circulating ANGPTL3. ANGPTL8 coimmunoprecipitated with the N-terminal domain of ANGPTL3 in plasma of these mice. In cultured hepatocytes, ANGPTL8 expression increased the appearance of N-terminal ANGPTL3 in the medium, suggesting ANGPTL8 may activate ANGPTL3. Consistent with this scenario, expression of ANGPTL8 in Angptl3(-/-) mice failed to promote hypertriglyceridemia. Thus, ANGPTL8, a paralog of ANGPTL3 that arose through duplication of an ancestral DOCK gene, regulates postprandial TAG and fatty acid metabolism by controlling activation of its progenitor, and perhaps other ANGPTLs. Inhibition of ANGPTL8 provides a new therapeutic strategy for reducing plasma lipoprotein levels.