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ANGPTL 3 & 4

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 (betatrophin) , 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 (betatrophin) 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.

Quagliarini F, Wang Y, Kozlitina J, et al., Proc Natl Acad Sci U S A. 2012 Nov 27;109(48):19751-6. doi: 10.1073/pnas.1217552109. Epub 2012 Nov 12.

Plasma levels of angiopoietin-like protein 4 (ANGPTL4) are significantly lower preoperatively in colorectal cancer patients than in cancer-free patients and are further decreased during the first month after minimally invasive colorectal resection.

BACKGROUND:
Surgery has been associated with proangiogenic plasma protein changes that may promote tumor growth. Angiopoietin-like protein 4 (ANGPTL4) is expressed by endothelial cells and other tissues in response to hypoxia. Both intact ANGPTL4 and its partly degraded C-terminal fragment may promote tumor angiogenesis. This study had two purposes: to measure and compare preoperative plasma ANGPTL4 levels in patients with colorectal cancer (CRC) and benign colorectal disease (BCD) and to determine plasma levels after minimally invasive colorectal resection (MICR) for CRC.

METHODS:
Plasma was obtained from an IRB-approved plasma/data bank. Preoperative plasma ANGPTL4 levels were measured for CRC and BCD patients, but postoperative levels were determined only for CRC patients for whom a preoperative, a postoperative day (POD) 3, and at least one late postoperative sample (POD 7-55) were available. Late samples were bundled into four time blocks and considered as single time points. ANGPTL4 levels (mean ± SD) were measured via ELISA and compared (significance, p < 0.01 after Bonferroni correction).

RESULTS:
Eighty CRC (71 % colon, 29 % rectal) and 60 BCD (62 % diverticulitis, 38 % adenoma) patients were studied. The mean preoperative plasma ANGPTL4 level in CRC patients (247.2 ± 230.7 ng/ml) was lower than the BCD group result (330.8 ± 239.0 ng/ml, p = 0.01). There was an inverse relationship between plasma levels and advanced CRC as judged by three criteria. In regard to the postoperative CRC analysis, the "n" for each time point varied: lower plasma levels (p < 0.001) were noted on POD 3 (161.4 ± 140.4 ng/ml, n = 80), POD 7-13 (144.6 ± 134.5 ng/ml, n = 46), POD 14-20 (139.0 ± 117.8 ng/ml, n = 27), POD 21-27 (138.9 ± 202.4, n = 20), and POD 28-55 (160.1 ± 179.0, n = 42) when compared to preoperative results.

CONCLUSION:
CRC is associated with lower preoperative plasma ANGPTL4 levels compared with BCD, and the levels may vary inversely with disease severity. After MICR for CRC, levels are significantly lower for over a month compared with the preoperative level; the cause for this persistent decrease is unclear. The implications of both the lower preoperative level and the persistently decreased postoperative levels are unclear. Further studies are needed.

Kumara HM, Kirchoff D, Herath SA, et al., Surg Endosc. 2012 Oct;26(10):2751-7. Epub 2012 May 2.

Angiopoietin-like 4 (Angptl4) protein is a physiological mediator of intracellular lipolysis in murine adipocytes.

Intracellular triacylglycerol (TG) hydrolysis and fatty acid release by the white adipose tissue (WAT) during a fast is stimulated by counter-regulatory factors acting in concert, although how adipocytes integrate these lipolytic inputs is unknown. We tested the role of angiopoietin-like 4 (Angptl4), a secreted protein induced by fasting or glucocorticoid treatment, in modulating intracellular adipocyte lipolysis. Glucocorticoid receptor blockade prevented fasting-induced tissue Angptl4 expression and WAT TG hydrolysis in mice, and TG hydrolysis induced by fasts of 6 or 24 h was greatly reduced in mice lacking Angptl4 (Angptl4(-/-)). Glucocorticoid treatment mimicked the lipolytic effects of fasting, although with slower kinetics, and this too required Angptl4. Thus, fasting-induced WAT TG hydrolysis requires glucocorticoid action and Angptl4. Both fasting and glucocorticoid treatment also increased WAT cAMP levels and downstream phosphorylation of lipolytic enzymes. Angptl4 deficiency markedly reduced these effects, suggesting that Angptl4 may stimulate lipolysis by modulating cAMP-dependent signaling. In support of this, cAMP levels and TG hydrolysis were reduced in primary Angptl4(-/-) murine adipocytes treated with catecholamines, which stimulate cAMP-dependent signaling to promote lipolysis, and was restored by treatment with purified human ANGPTL4. Remarkably, human ANGPTL4 treatment alone increased cAMP levels and induced lipolysis in these cells. Pharmacologic agents revealed that Angptl4 modulation of cAMP-dependent signaling occurs upstream of adenylate cyclase and downstream of receptor activation. We show that Angptl4 is a glucocorticoid-responsive mediator of fasting-induced intracellular lipolysis and stimulates cAMP signaling in adipocytes. Such a role is relevant to diseases of aberrant lipolysis, such as insulin resistance.

Gray NE, Lam LN, Yang K et al., J Biol Chem. 2012 Mar 9;287(11):8444-56. doi: 10.1074/jbc.M111.294124. Epub 2012 Jan 19.

Proteolytic processing of angiopoietin-like protein 4 by proprotein convertases modulates its inhibitory effects on lipoprotein lipase activity.

Angiopoietin-like protein 4 (ANGPTL4) has been associated with a variety of diseases. It is known as an endogenous inhibitor of lipoprotein lipase (LPL), and it modulates lipid deposition and energy homeostasis. ANGPTL4 is cleaved by unidentified protease(s), and the biological importance of this cleavage event is not fully understood with respect to its inhibitory effect on LPL activity. Here, we show that ANGPTL4 appears on the cell surface as the full-length form, where it can be released by heparin treatment in culture and in vivo. ANGPTL4 protein is then proteolytically cleaved into several forms by proprotein convertases (PCs). Several PCs, including furin, PC5/6, paired basic amino acid-cleaving enzyme 4, and PC7, are able to cleave human ANGPTL4 at a consensus site. PC-specific inhibitors block the processing of ANGPTL4. Blockage of ANGPTL4 cleavage reduces its inhibitory effects on LPL activity and decreases its ability to raise plasma triglyceride levels. In summary, the cleavage of ANGPTL4 by these PCs modulates its inhibitory effect on LPL activity.

Lei X, Shi F, Basu D, et al., J Biol Chem. 2011 May 6;286(18):15747-56. doi: 10.1074/jbc.M110.217638. Epub 2011 Mar 12.

 

betatrophin

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