Angiopoietin-like protein 4 converts lipoprotein lipase to inactive monomers and modulates lipase activity in adipose tissue

Acquisition of anoikis resistance reveals a synoikis-like survival style in BEL7402 hepatoma cells

Resistance to anoikis is a hallmark of human malignancies. Our results showed that hepatoma cells resisted anoikis by non-proliferation, non-apoptosis and cell cycle arrest which were termed synoikis-like. These synoikis-like cells are more resistant to extracellular stimuli and could spontaneously attach and proliferate again under suitable conditions, which indicate a reversible property of these cells. Microarray expression profile reveals the change of molecules involved in the synoikis-like hepatoma cells and our data indicated that ANGPTL4 contributed to anoikis resistance of hepatoma cells. These results demonstrated that hepatoma cells might resist anoikis through a synoikis-like survival style, which may facilitate tumor metastasis.

Angiopoietin-like 7 secretion is induced by glaucoma stimuli and its concentration is elevated in glaucomatous aqueous humor

PURPOSE

To investigate the possibility that Angiopoietin-like 7 (ANGPTL7) protein is involved in the pathogenesis of glaucoma.

METHODS

Primary human trabecular meshwork (TM) cells and corneoscleral explants were stimulated with either dexamethasone (DEX) or transforming growth factor beta (TGFbeta), and ANGPTL7 protein secreted into culture medium was determined by Western blot analysis. The effect of stable overexpression of ANGPTL7 in transfected immortalized TM cell lines on collagen expression was investigated by immunocytochemistry. Localization of ANGPTL7 protein in human eyes was determined by immunohistochemistry. The concentration of ANGPTL7 protein in aqueous humor (AH) from patients with glaucoma and control patients was compared by Western blot analysis. The beagle model of primary open-angle glaucoma (POAG) was used to correlate ANGPTL7 protein levels in canine AH with disease progression.

RESULTS

TGFbeta and DEX stimulated secretion of ANGPTL7 protein by TM cells and corneoscleral explants. Overexpression of ANGPTL7 by immortalized TM cell lines increased expression of type I collagen. Expression of ANGPTL7 protein was located in the corneal stroma, near the limbus, and throughout the sclera, with lower expression in the TM. In the lamina cribrosa, ANGPTL7 expression was associated with the cribriform plates. The concentration of ANGPTL7 protein was elevated in AH from patients with glaucoma and increased as disease progressed in POAG beagle dogs.

CONCLUSIONS

Induction of ANGPTL7 secretion by glaucoma stimuli and increased concentration of ANGPTL7 in glaucomatous AH suggest that ANGPTL7 is overexpressed in glaucoma. Since overexpression of ANGPTL7 increases collagen expression, a potential disease mechanism, ANGPTL7 could have a pathogenic role in glaucoma, and may serve as a potential therapeutic target.

The role of angiopoietin-like proteins in angiogenesis and metabolism

Recently, a family of proteins structurally similar to the angiogenic regulating factors angiopoietins was identified and designated "angiopoietin-like proteins" (Angptls). Encoded by seven genes, Angptls 1 to 7 all possess an N-terminal coiled-coil domain and a C-terminal fibrinogen-like domain, both characteristic of angiopoietins. However, Angptls do not bind to either the angiopoietin receptor Tie2 or the related protein Tie1 and remain orphan ligands. Nonetheless, Angptls 1, 2, 3, 4, and Angptl6/angiopoietin-related growth factor function to regulate angiogenesis. Angptls 3, 4, and Angptl6/angiopoietin-related growth factor also appear to directly regulate lipid, glucose, and energy metabolism independently of angiogenic effects. Recently, several lines of evidence reveal differential roles of Angptl structural domains in both angiogenesis and metabolism. Here, we briefly review what is currently known about Angptls function.

Suppression of the Raf/MEK/ERK signaling cascade and inhibition of angiogenesis by the carboxyl terminus of angiopoietin-like protein 4

OBJECTIVE

Angiopoietin-like protein 4 (Angptl4) is a secreted glycoprotein that has recently been implicated in the regulation of angiogenesis and metastasis. This study aimed to investigate the structural and cellular basis underlying the biological actions of Angptl4.

METHODS AND RESULTS

Circulating Angptl4 was proteolytically cleaved into NH2-terminal coiled-coil domain (N-Angptl4) and COOH-terminal fibrinogen-like domain (C-Angptl4). Using amino acid sequencing analysis, we identified a major cleavage site between Lys(168) and Leu(169) and a minor cleavage site between Lys(170) and Met(171) in mouse Angptl4. C-Angptl4, but not N-Angptl4, potently inhibited both bFGF- and VEGF-induced cell proliferation, migration, and tubule formation in endothelial cells, and prevented neovascularization in mice. Treatment of C-Angptl4 with PNGase F (an N-glycosidase) ablated its N-linked glycosylation, and also significantly attenuated its antiangiogenic activities. C-Angptl4 blocked bFGF-induced activation of ERK1/2 MAP kinase, but had no obvious effect on Akt and P38 MAP kinase. Furthermore, C-Angptl4 abrogated bFGF-induced phosphorylation of Raf-1 and MEK1/2, whereas neither auto-phosphorylation of FGF receptor-1 nor activation of Ras was affected, suggesting that the blockage occurs at the level of Raf-1 activation.

CONCLUSIONS

The carboxyl terminus of Angptl4 alone is sufficient to suppress angiogenesis, possibly through inhibiting the Raf/MEK/ERK1/2 MAP kinase pathway in endothelial cells.

A transcription-dependent mechanism, akin to that in adipose tissue, modulates lipoprotein lipase activity in rat heart

The enzyme lipoprotein lipase (LPL) releases fatty acids from lipoprotein triglycerides for use in cell metabolism. LPL activity is rapidly modulated in a tissue-specific manner. Recent studies have shown that in rat adipose tissue this occurs by a shift of extracellular LPL toward an inactive form catalyzed by an LPL-controlling protein whose expression changes in response to the nutritional state. To explore whether a similar mechanism operates in other tissues we injected actinomycin D to block transcription of the putative LPL controlling protein(s). When actinomycin was given to fed rats, heparin-releasable LPL activity increased by 160% in heart and by 150% in a skeletal muscle (soleus) in 6 h. Postheparin LPL activity in blood increased by about 200%. To assess the state of extracellular LPL we subjected the spontaneously released LPL in heart perfusates to chromatography on heparin-agarose, which separates the active and inactive forms of the lipase. The amount of lipase protein released remained relatively constant on changes in the nutritional state and/or blockade of transcription, but the distribution between the active and inactive forms changed. Less of the LPL protein was in the active form in perfusates from hearts from fed compared with fasted rats. When glucose was given to fasted rats the proportion of LPL protein in the active form decreased. Actinomycin D increased the proportion that was active, in accord with the hypothesis that the message for a rapidly turning over LPL-controlling protein was being removed.

Angptl4 upregulates cholesterol synthesis in liver via inhibition of LPL- and HL-dependent hepatic cholesterol uptake

BACKGROUND

Dysregulation of plasma lipoprotein levels may increase the risk for atherosclerosis. Recently, angiopoietin-like protein 4, also known as fasting-induced adipose factor Fiaf, was uncovered as a novel modulator of plasma lipoprotein metabolism. Here we take advantage of the fasting-dependent phenotype of Angptl4-transgenic (Angptl4-Tg) mice to better characterize the metabolic function of Angptl4.

METHODS AND RESULTS

In 24-hour fasted mice, Angptl4 overexpression increased plasma triglycerides (TG) by 24-fold, which was attributable to elevated VLDL-, IDL/LDL- and HDL-TG content. Angptl4 overexpression decreased post-heparin LPL activity by stimulating conversion of endothelial-bound LPL dimers to circulating LPL monomers. In fasted but not fed state, Angptl4 overexpression severely impaired LPL-dependent plasma TG and cholesteryl ester clearance and subsequent uptake of fatty acids and cholesterol into tissues. Consequently, hepatic cholesterol content was significantly decreased, leading to universal upregulation of cholesterol and fatty acid synthesis pathways and increased rate of cholesterol synthesis.

CONCLUSIONS

The hypertriglyceridemic effect of Angptl4 is attributable to inhibition of LPL-dependent VLDL lipolysis by converting LPL dimers to monomers, and Angptl4 upregulates cholesterol synthesis in liver secondary to inhibition of LPL- and HL-dependent hepatic cholesterol uptake.

Removal of triacylglycerols from chylomicrons and VLDL by capillary beds: the basis of lipoprotein remnant formation

The triacylglycerol content of chylomicrons and VLDL (very-low-density lipoprotein) compete for the same lipolytic pathway in the capillary beds. Although chylomicron triacylglycerols appear to be the favoured substrate for lipoprotein lipase, VLDL particles compete in numbers. Methods to quantify the specific triacylglycerol removal from VLDL and chylomicrons may involve endogenous labelling of the triacylglycerol substrate with stable isotopes in combination with arteriovenous blood sampling in humans. Arteriovenous quantification of remnant lipoproteins suggests that adipose tissue with its high lipoprotein lipase activity is a principal site for generation of remnant lipoproteins. Under circumstances of reduced efficiency in the removal of triacylglycerols from lipoproteins, there is accumulation of remnant lipoproteins, which are potentially atherogenic.

GPIHBP1: an endothelial cell molecule important for the lipolytic processing of chylomicrons

PURPOSE OF REVIEW

To summarize recent data indicating that glycosylphosphatidylinositol-anchored high density lipoprotein-binding protein 1 (GPIHBP1) plays a key role in the lipolytic processing of chylomicrons.

RECENT FINDINGS

Lipoprotein lipase hydrolyses triglycerides in chylomicrons at the luminal surface of the capillaries in heart, adipose tissue, and skeletal muscle. The endothelial cell molecule that facilitates the lipolytic processing of chylomicrons has never been clearly defined. Mice lacking GPIHBP1 manifest chylomicronemia, with plasma triglyceride levels as high as 5000 mg/dl. In wild-type mice, GPIHBP1 is expressed on the luminal surface of capillaries in heart, adipose tissue, and skeletal muscle. Cells transfected with GPIHBP1 bind both chylomicrons and lipoprotein lipase avidly.

SUMMARY

The chylomicronemia in Gpihbp1-deficient mice, the fact that GPIHBP1 is located within the lumen of capillaries, and the fact that GPIHBP1 binds lipoprotein lipase and chylomicrons suggest that GPIHBP1 is a key platform for the lipolytic processing of triglyceride-rich lipoproteins.

Lipids in the heart: a source of fuel and a source of toxins

PURPOSE OF REVIEW

How do lipids arrive in the heart and other tissues? This review focuses on new information on pathways of lipid uptake into the heart.

RECENT FINDINGS

Fatty acids, the major cardiac fuel, are obtained from either lipoproteins or free fatty acids associated with albumin. The heart is the tissue with the most robust expression of lipoprotein lipase, and recent data attest to the importance of this enzyme in supplying optimal amounts of fatty acids for the heart. Genetic deletion of CD36 also shows that this transporter is important for cardiac uptake of lipids. Retinoid acquisition by the heart involves pathways parallel to those used for fatty acid uptake: a pathway for acquisition of core lipoprotein retinyl ester and another for nonlipoprotein retinol. Dilated lipotoxic cardiomyopathy is the consequence of excess lipid uptake.

SUMMARY

Genetic modifications that affect lipid uptake, oxidation, and storage are being exploited to elucidate the pathophysiology of cardiomyopathies and to discover how lipids relate to heart failure in humans with obesity and diabetes mellitus. This information is likely to lead to new diagnostic categories of cardiomyopathy and more pathophysiologically appropriate treatments.

In vivo imaging and genetic analysis link bacterial motility and symbiosis in the zebrafish gut

Complex microbial communities reside within the intestines of humans and other vertebrates. Remarkably little is known about how these microbial consortia are established in various locations within the gut, how members of these consortia behave within their dynamic ecosystems, or what microbial factors mediate mutually beneficial host-microbial interactions. Using a gnotobiotic zebrafish-Pseudomonas aeruginosa model, we show that the transparency of this vertebrate species, coupled with methods for raising these animals under germ-free conditions can be used to monitor microbial movement and localization within the intestine in vivo and in real time. Germ-free zebrafish colonized with isogenic P. aeruginosa strains containing deletions of genes related to motility and pathogenesis revealed that loss of flagellar function results in attenuation of evolutionarily conserved host innate immune responses but not conserved nutrient responses. These results demonstrate the utility of gnotobiotic zebrafish in defining the behavior and localization of bacteria within the living vertebrate gut, identifying bacterial genes that affect these processes, and assessing the impact of these genes on host-microbial interactions.

Postnatal lymphatic partitioning from the blood vasculature in the small intestine requires fasting-induced adipose factor

Lymphatic vessels develop from specialized venous endothelial cells. Using knockout mice, we found that fasting-induced adipose factor (Fiaf) is required for functional partitioning of postnatal intestinal lymphatic and blood vessels. In wild-type animals, levels of intestinal Fiaf expression rise during the first postnatal day and peak at day 2, which coincides with the onset of the lymphatico-venous partitioning abnormality in Fiaf-/- mutants on a mixed 129/SvJ:C57BL/6 genetic background. Fiaf deficiency is not associated with disruption of the blood vasculature or with lymphatic endothelial recruitment of smooth muscle cells. We identified Prox1, a critical regulator of lymphangiogenesis, as a downstream target for Fiaf signaling in the intestinal lymphatic endothelium. This organ-specific lymphovascular abnormality can be rescued by allowing embryonic Fiaf-/- intestinal isografts to develop in Fiaf+/+ recipients.