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.Angiopoietin-like protein 4 (ANGPTL4) is also known as hepatic fibrinogen/angiopoietin-related protein, fasting-induced adipose factor, peroxisome proliferator-activated receptors, and ␥-angiopoietin-related protein. It is one of the seven members of the ANGPTL family (ANGPTL1-7). Mainly produced in hepatocytes in humans and adipocytes in mice, ANGPTL4 exerts its biological effects by means of autocrine/ paracrine and endocrine processes. ANGPTL4 has been implicated in a variety of diseases, including cardiovascular disease (1, 2), cancer metastasis (3), obesity (4), diabetes (5), wound repair (6, 7), inflammation (8), and arthritis (9).ANGPTL4 is a fusion protein consisting of an N-terminal coiled-coil domain and a C-terminal fibrinogen-like domain.These two domains have been shown to have distinct biological functions (10). The N-terminal domain is responsible for the inhibitory effects on LPL, 4 converting the active form of LPL into an inactive form (11), and the C terminus mediates its antiangiogenic functions (12). Interestingly, these two domains are separated by a short linker that can be cleaved after secretion. The cleavage phenomenon has been shown to occur in humans as well as in rodents (13,14). Cleavage of ANGPTL4 appears to be tissue-dependent in humans; liver secretes cleaved ANGPTL4, whereas adipose tissue secretes the fulllength form (14).The physiological relevance of the proteolytic processing of ANGPTL4 is largely unknown. In a human study, treatment with fenofibrate, a potent peroxisome proliferator-activated receptor-␣ agonist, markedly increased plasma levels of cleaved ANGPTL4. On this basis, it was proposed that the cleaved form of ANGPTL4 may have specific functions (14). There is evidence that the antiangiogenic activity of ANGPTL4 is regulated by an interaction of its coiled-coil domain with heparan sulfate proteoglygans (HSPGs) (18). This may be due to an ability of the N-terminal domain to facilitate the interaction between t...

Lipoprotein lipase (LPL)-mediated lipolysis of triglycerides is the first and rate-limiting step in chylomicron/very low density lipoprotein clearance at the luminal surface of the capillaries. Angiopoietin-like protein 3 (ANGPTL3) is shown to inhibit LPL activity and plays important roles in modulating lipoprotein metabolism in vivo. However, the mechanism by which it inhibits LPL activity remains poorly understood. Using cell-based analysis of the interaction between ANGPTL3, furin, proprotein convertase subtilisin/kexin type 5 (PCSK5), paired amino acid converting enzyme-4 (PACE4), and LPL, we demonstrated that the cleavage of LPL by proprotein convertases is an inactivation process, similar to that seen for endothelial lipase cleavage. At physiological concentrations and in the presence of cells, ANGPTL3 is a potent inhibitor of LPL. This action is due to the fact that ANGPTL3 can enhance LPL cleavage by endogenous furin and PACE4 but not by PCSK5. This effect is specific to LPL but not endothelial lipase. Both N-and C-terminal domains of LPL are required for ANGPTL3-enhanced cleavage, and the N-terminal domain of ANGPTL3 is sufficient to exert its effect on LPL cleavage. Moreover, ANGPTL3 enhances LPL cleavage in the presence of either heparan sulfate proteoglycans or glycosylphosphatidylinositol-anchored high density lipoprotein-binding protein 1 (GPIHBP1). By enhancing LPL cleavage, ANGPTL3 dissociates LPL from the cell surface, inhibiting both the catalytic and noncatalytic functions of LPL. Taken together, our data provide a molecular connection between ANGPTL3, LPL, and proprotein convertases, which may represent a rapid signal communication among different metabolically active tissues to maintain energy homeostasis. These novel findings provide a new paradigm of specific protease-substrate interaction and further improve our knowledge of LPL biology.ANGPTL3 is a newly discovered modulator of plasma triglyceride levels, an independent risk factor of atherosclerosis. Genome-wide association studies in humans link single nucleotide polymorphisms near ANGPTL3 to plasma triglyceride levels (1-3). Several loss of function mutations of ANGPTL3 contribute to variation in plasma triglyceride levels in the Dallas Heart Study population as well as in subjects in the Atherosclerosis Risk in Communities study (4). However, two small studies suggested that plasma ANGPTL3 protein concentration does not correlate with plasma triglyceride levels (5, 6). Data from ANGPTL3 knock-out mice, the mutant KK/san mice, neutralization using antibody, and overexpression studies demonstrate that ANGPTL3 raises plasma triglyceride levels by inhibiting LPL, 2 a rate-limiting enzyme in plasma triglyceride metabolism (7). ANGPTL3 is mainly made in liver, whereas most active LPL protein resides in adipose and muscle, yet the nature of its interaction with LPL is not fully understood. Based on several studies from cell-free systems (8), ANGPTL3 is proposed to directly bind and inhibit the LPL catalytic activity, and the inhibitory cons...

We showed previously that type I interferon causes a down-regulation of mitochondrial gene expression. We show here that IFN treatment leads to functional impairment of mitochondria. Western blot analysis indicated that interferon treatment reduces the steady-state level of cytochrome b in murine L-929 cells. Interferon produced a reduction in cytochrome c oxidase and NADH-cytochrome c reductase activities of isolated mitochondria as well as inhibiting electron transport in isolated mitochondria and in intact cells. Several mitochondrial mRNAs are affected by interferon treatment in human Daudi lymphoblastoid cells, which are highly sensitive to the antiproliferative effects of interferon. Electron transport in Daudi cells was also inhibited by interferon both in intact cells and isolated mitochondria with a dose response identical to that for the antiproliferative response. In contrast, a Daudi strain resistant to the antiproliferative effects of interferon showed no down-regulation of mRNA expression and no inhibition of electron transport. Possibly as a consequence of the inhibitory effect on mitochondrial gene expression, treatment with interferon causes a reduction in cellular ATP levels. The inhibition of cellular growth by interferon may thus be partly a consequence of a reduction in cellular ATP levels.