Intracapillary LPL levels in brown adipose tissue, visualized with an antibody-based approach, are regulated by ANGPTL4 at thermoneutral temperatures

ANGPTL4-A protein involved in glucose metabolism, lipid metabolism, and tumor development

Since ANGPTL4 was discovered to be involved in lipid metabolism in 2000 for the first time, Angptl4 has attracted the attention of researchers. With the further research, it was found that angptl4 was also involved in many biological activities (glucose metabolism, angiogenesis, wound healing, tumor growth, etc.) in vivo. In this review, we provide an overview of the fundamental role of ANGPTL4 in metabolic regulation and its impact on tumor growth. These insights may provide a way for exploring ANGPTL4 as a potential therapeutic target for future disease treatments.

Distinct strategies for intravascular triglyceride metabolism in hearts of mammals and lower vertebrate species

Lipoprotein lipase (LPL) and multiple regulators of LPL activity (e.g., APOC2 and ANGPTL4) are present in all vertebrates, but GPIHBP1-the endothelial cell (EC) protein that captures LPL within the subendothelial spaces and transports it to its site of action in the capillary lumen-is present in mammals but in not chickens or other lower vertebrates. In mammals, GPIHBP1 deficiency causes severe hypertriglyceridemia, but chickens maintain low triglyceride levels despite the absence of GPIHBP1. To understand intravascular lipolysis in lower vertebrates, we examined LPL expression in mouse and chicken hearts. In both species, LPL was abundant on capillaries, but the distribution of Lpl transcripts was strikingly different. In mouse hearts, Lpl transcripts were extremely abundant in cardiomyocytes but were barely detectable in capillary ECs. In chicken hearts, Lpl transcripts were absent in cardiomyocytes but abundant in capillary ECs. In zebrafish hearts, lpl transcripts were also in capillary ECs but not cardiomyocytes. In both mouse and chicken hearts, LPL was present, as judged by immunogold electron microscopy, in the glycocalyx of capillary ECs. Thus, mammals produce LPL in cardiomyocytes and rely on GPIHBP1 to transport the LPL into capillaries, whereas lower vertebrates produce LPL directly in capillary ECs, rendering an LPL transporter unnecessary.

APOA5 deficiency causes hypertriglyceridemia by reducing amounts of lipoprotein lipase in capillaries

Apolipoprotein AV (APOA5) deficiency causes hypertriglyceridemia in mice and humans. For years, the cause remained a mystery, but the mechanisms have now come into focus. Here, we review progress in defining APOA5's function in plasma triglyceride metabolism. Biochemical studies revealed that APOA5 binds to the angiopoietin-like protein 3/8 complex (ANGPTL3/8) and suppresses its ability to inhibit the activity of lipoprotein lipase (LPL). Thus, APOA5 deficiency is accompanied by increased ANGPTL3/8 activity and lower levels of LPL activity. APOA5 deficiency also reduces amounts of LPL in capillaries of oxidative tissues (e.g., heart, brown adipose tissue). Cell culture experiments revealed the likely explanation: ANGPTL3/8 detaches LPL from its binding sites on the surface of cells, and that effect is blocked by APOA5. Both the low intracapillary LPL levels and the high plasma triglyceride levels in Apoa5-/- mice are normalized by recombinant APOA5. Carboxyl-terminal sequences in APOA5 are crucial for its function; a mutant APOA5 lacking 40-carboxyl-terminal residues cannot bind to ANGPTL3/8 and lacks the ability to change intracapillary LPL levels or plasma triglyceride levels in Apoa5-/- mice. Also, an antibody against the last 26 amino acids of APOA5 reduces intracapillary LPL levels and increases plasma triglyceride levels in wild-type mice. An inhibitory ANGPTL3/8-specific antibody functions as an APOA5-mimetic reagent, increasing intracapillary LPL levels and lowering plasma triglyceride levels in both Apoa5-/- and wild-type mice. That antibody is a potentially attractive strategy for treating elevated plasma lipid levels in human patients.

Effects of myonectin on porcine intramuscular adipocyte differentiation and exogenous free fatty acid utilization

As an important factor secreted by skeletal muscle, myonectin can regulate lipid metabolism and energy metabolism, but its role in the utilization of peripheral free fatty acids (FFAs) by porcine intramuscular fat cells remains to be further investigated. In this study, porcine intramuscular adipocytes were treated with recombinant myonectin and palmitic acid (PA), either alone or in combination, and then were examined for their uptake of exogenous FFAs, intracellular lipid synthesis and catabolism, and mitochondrial oxidation of fatty acids. The results showed that myonectin decreased the area of lipid droplets in intramuscular adipocytes (p < 0.05) and significantly increased (p < 0.05) the expression levels of hormone-sensitive lipase (HSL) and lipoprotein lipase (LPL). Moreover, myonectin can up-regulate the expression of p38 mitogen-activated protein kinase (p38 MAPK). Myonectin significantly promoted the uptake of peripheral FFAs (p < 0.01), improved (p < 0.05) the expression of fatty transport protein 1 (FATP1) and fatty acid binding protein 4 (FABP4) in intramuscular adipocytes. Myonectin also significantly increased (p < 0.05) the expression levels of fatty acid oxidation markers: transcription factor (TFAM), uncoupling protein-2 (UCP2) and oxidative respiratory chain marker protein complex I (NADH-CoQ) in mitochondria of intramuscular adipocytes. In summary, myonectin promoted the absorption, transport, and oxidative metabolism of exogenous FFAs in mitochondria, thereby inhibiting lipid deposition in porcine intramuscular adipocytes.

Hypertriglyceridemia in Apoa5-/- mice results from reduced amounts of lipoprotein lipase in the capillary lumen

Why apolipoprotein AV (APOA5) deficiency causes hypertriglyceridemia has remained unclear, but we have suspected that the underlying cause is reduced amounts of lipoprotein lipase (LPL) in capillaries. By routine immunohistochemistry, we observed reduced LPL staining of heart and brown adipose tissue (BAT) capillaries in Apoa5-/- mice. Also, after an intravenous injection of LPL-, CD31-, and GPIHBP1-specific mAbs, the binding of LPL Abs to heart and BAT capillaries (relative to CD31 or GPIHBP1 Abs) was reduced in Apoa5-/- mice. LPL levels in the postheparin plasma were also lower in Apoa5-/- mice. We suspected that a recent biochemical observation - that APOA5 binds to the ANGPTL3/8 complex and suppresses its capacity to inhibit LPL catalytic activity - could be related to the low intracapillary LPL levels in Apoa5-/- mice. We showed that an ANGPTL3/8-specific mAb (IBA490) and APOA5 normalized plasma triglyceride (TG) levels and intracapillary LPL levels in Apoa5-/- mice. We also showed that ANGPTL3/8 detached LPL from heparan sulfate proteoglycans and GPIHBP1 on the surface of cells and that the LPL detachment was blocked by IBA490 and APOA5. Our studies explain the hypertriglyceridemia in Apoa5-/- mice and further illuminate the molecular mechanisms that regulate plasma TG metabolism.

The lipoprotein lipase that is shuttled into capillaries by GPIHBP1 enters the glycocalyx where it mediates lipoprotein processing

Lipoprotein lipase (LPL), the enzyme that carries out the lipolytic processing of triglyceride-rich lipoproteins (TRLs), is synthesized by adipocytes and myocytes and secreted into the interstitial spaces. The LPL is then bound by GPIHBP1, a GPI-anchored protein of endothelial cells (ECs), and transported across ECs to the capillary lumen. The assumption has been that the LPL that is moved into capillaries remains attached to GPIHBP1 and that GPIHBP1 serves as a platform for TRL processing. In the current studies, we examined the validity of that assumption. We found that an LPL-specific monoclonal antibody (mAb), 88B8, which lacks the ability to detect GPIHBP1-bound LPL, binds avidly to LPL within capillaries. We further demonstrated, by confocal microscopy, immunogold electron microscopy, and nanoscale secondary ion mass spectrometry analyses, that the LPL detected by mAb 88B8 is located within the EC glycocalyx, distant from the GPIHBP1 on the EC plasma membrane. The LPL within the glycocalyx mediates the margination of TRLs along capillaries and is active in TRL processing, resulting in the delivery of lipoprotein-derived lipids to immediately adjacent parenchymal cells. Thus, the LPL that GPIHBP1 transports into capillaries can detach and move into the EC glycocalyx, where it functions in the intravascular processing of TRLs.

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.