A lipase fusion feasts on fat

The enzyme lipoprotein lipase (LPL) is responsible for breaking down triglycerides in the blood. Mutations in LPL cause a rare but debilitating disorder characterized by excessive plasma triglyceride levels for which treatment options are limited. Nimonkar et al. now present a fusion protein between LPL and its physiological transporter GBIHBP1 that is highly active and largely resistant to physiological inhibitors of LPL. Injecting this fusion protein effectively lowers plasma triglycerides in mice and represents a promising new approach for lowering triglycerides in patients with familial chylomicronemia syndrome.</p

Angiopoietin-like 4 promotes the intracellular cleavage of lipoprotein lipase by PCSK3/furin in adipocytes

Lipoprotein lipase (LPL) catalyzes the breakdown of circulating triglycerides in muscle and fat. LPL is inhibited by several proteins, including angiopoietin-like 4 (ANGPTL4), and may be cleaved by members of the proprotein convertase subtilisin/ kexin (PCSK) family. Here, we aimed to investigate the cleavage of LPL in adipocytes by PCSKs and study the potential involvement of ANGPTL4. A substantial portion of LPL in mouse and human adipose tissue was cleaved into N- and C-terminal fragments. Treatment of different adipocytes with the PCSK inhibitor decanoyl-RVKR-chloromethyl ketone markedly decreased LPL cleavage, indicating that LPL is cleaved by PCSKs. Silencing of Pcsk3/furin significantly decreased LPL cleavage in cell culture medium and lysates of 3T3-L1 adipocytes. Remarkably, PCSK-mediated cleavage of LPL in adipocytes was diminished by Angptl4 silencing and was decreased in adipocytes and adipose tissue of Angptl4/ mice. Differences in LPL cleavage between Angptl4/ and WT mice were abrogated by treatment with decanoyl-RVKR-chloromethyl ketone. Induction of ANGPTL4 in adipose tissue during fasting enhanced PCSK-mediated LPL cleavage, concurrent with decreased LPL activity, in WT but not Angptl4/ mice. In adipocytes, after removal of cell surface LPL by heparin, levels of N-terminal LPL were still markedly higher in WT compared with Angptl4/ adipocytes, suggesting that stimulation of PCSK-mediated LPL cleavage by ANGPTL4 occurs intracellularly. Finally, treating adipocytes with insulin increased full-length LPL and decreased N-terminal LPL in an ANGPTL4-dependent manner. In conclusion, ANGPTL4 promotes PCSK-mediated intracellular cleavage of LPL in adipocytes, likely contributing to regulation of LPL in adipose tissue. Our data provide further support for an intracellular action of ANGPTL4 in adipocytes.</p

Transcriptional profiling of PPARα−/− and CREB3L3−/− livers reveals disparate regulation of hepatoproliferative and metabolic functions of PPARα

Background: Peroxisome Proliferator-Activated receptor α (PPARα) and cAMP-Responsive Element Binding Protein 3-Like 3 (CREB3L3) are transcription factors involved in the regulation of lipid metabolism in the liver. The aim of the present study was to characterize the interrelationship between PPARα and CREB3L3 in regulating hepatic gene expression. Male wild-type, PPARα−/−, CREB3L3−/− and combined PPARα/CREB3L3−/− mice were subjected to a 16-h fast or 4 days of ketogenic diet. Whole genome expression analysis was performed on liver samples. Results: Under conditions of overnight fasting, the effects of PPARα ablation and CREB3L3 ablation on plasma triglyceride, plasma β-hydroxybutyrate, and hepatic gene expression were largely disparate, and showed only limited interdependence. Gene and pathway analysis underscored the importance of CREB3L3 in regulating (apo)lipoprotein metabolism, and of PPARα as master regulator of intracellular lipid metabolism. A small number of genes, including Fgf21 and Mfsd2a, were under dual control of PPARα and CREB3L3. By contrast, a strong interaction between PPARα and CREB3L3 ablation was observed during ketogenic diet feeding. Specifically, the pronounced effects of CREB3L3 ablation on liver damage and hepatic gene expression during ketogenic diet were almost completely abolished by the simultaneous ablation of PPARα. Loss of CREB3L3 influenced PPARα signalling in two major ways. Firstly, it reduced expression of PPARα and its target genes involved in fatty acid oxidation and ketogenesis. In stark contrast, the hepatoproliferative function of PPARα was markedly activated by loss of CREB3L3. Conclusions: These data indicate that CREB3L3 ablation uncouples the hepatoproliferative and lipid metabolic effects of PPARα. Overall, except for the shared regulation of a very limited number of genes, the roles of PPARα and CREB3L3 in hepatic lipid metabolism are clearly distinct and are highly dependent on dietary status

Characterization of ANGPTL4 function in macrophages and adipocytes using Angptl4-knockout and Angptl4-hypomorphic mice

ANGPTL4 regulates plasma lipids, making it an attractive target for correcting dyslipidemia. However, ANGPTL4 inactivation in mice fed a high fat diet causes chylous ascites, an acute-phase response, and mesenteric lymphadenopathy. Here, we studied the role of ANGPTL4 in lipid uptake in macrophages and in the above-mentioned pathologies using Angptl4-hypomorphic and Angptl4-/- mice. Angptl4 expression in peritoneal and bone marrow-derived macrophages was highly induced by lipids. Recombinant ANGPTL4 decreased lipid uptake in macrophages, whereas deficiency of ANGPTL4 increased lipid uptake, upregulated lipid-induced genes, and increased respiration. ANGPTL4 deficiency did not alter LPL protein levels in macrophages. Angptl4-hypomorphic mice with partial expression of a truncated N-terminal ANGPTL4 exhibited reduced fasting plasma triglyceride, cholesterol, and non-esterified fatty acid levels, strongly resembling Angptl4-/- mice. However, during high fat feeding, Angptl4-hypomorphic mice showed markedly delayed and attenuated elevation in plasma serum amyloid A and much milder chylous ascites than Angptl4-/- mice, despite similar abundance of lipid-laden giant cells in mesenteric lymph nodes. In conclusion, ANGPTL4 deficiency increases lipid uptake and respiration in macrophages without affecting LPL protein levels. Compared with the absence of ANGPTL4, low levels of N-terminal ANGPTL4 mitigate the development of chylous ascites and an acute-phase response in mice