a1-Antitrypsin Combines with Plasma Fatty Acids and Induces Angiopoietin-like Protein 4 Expression

a1-Antitrypsin (A1AT) purified from human plasma upregulates expression and release of angiopoietin-like protein 4 (Angptl4) in adherent human blood monocytes and in human lung microvascular endothelial cells, providing a mechanism for the broad immune-regulatory properties of A1AT independent of its antiprotease activity. In this study, we demonstrate that A1AT (Pro-lastin), a potent inducer of Angptl4, contains significant quantities of the fatty acids (FA) linoleic acid (C18:2) and oleic acid (C18:1). However, only trace amounts of FAs were present in preparations that failed to increase Angplt4 expression, for example, A1AT (Zemaira) or M-type A1AT purified by affinity chromatography. FA pull-down assays with Western blot analysis revealed a FA-binding ability of A1AT. In human blood-adherent monocytes, A1AT-FA conjugates upregulated expression of Angptl4 (54.9-fold, p < 0.001), FA-binding protein 4 (FABP4) (11.4-fold, p < 0.001), and, to a lesser degree, FA translocase (CD36) (3.1-fold, p < 0.001) relative to A1AT devoid of FA (A1AT-0). These latter effects of A1AT-FA were blocked by inhibitors of peroxisome proliferator-activated receptor (PPAR) b/d (ST247) and PPARg (GW9662). When compared with controls, cell pretreatment with ST247 diminished the effect of A1AT-LA on Angptl4 mRNA (11.6-versus 4.1-fold, p < 0.001) and FABP4 mRNA (5.4-versus 2.8-fold, p < 0.001). Similarly, preincubation of cells with GW9662 inhibited inducing effect of A1AT-LA on Angptl4 mRNA (by 2-fold, p < 0.001) and FABP4 mRNA (by 3-fold, p < 0.001). Thus, A1AT binds to FA, and it is this form of A1AT that induces Angptl4 and FABP4 expression via a PPAR-dependent pathway. These findings provide a mechanism for the unexplored area of A1AT biology independent of its antiprotease properties. A ngiopoietin-like protein 4 (Angptl4), also named PPARg angiopoietin-related, fasting-induced adipose factor, was originally discovered as one of the target genes of per-oxisome proliferator-activated receptor (PPAR) g (1). The most well-characterized function of Angptl4 is the regulation of lipid metabolism through the inhibition of lipoprotein lipase, an enzyme that hydrolyzes triglycerides from the apolipoprotein B– containing lipoproteins chylomicrons (2, 3). Studies using Angptl4-knockout mice suggest that Angptl4 plays a role in inflammation, atherosclerosis, and wound healing (4–8), whereas data from Angptl4-overexpressing models imply that Angptl4 is involved in the development of cancer, nephrotic syndrome, and car-diovascular diseases (9–15). Angptl4 also seems to play a role in type 2 diabetes mellitus and metabolic syndrome, both of which are associated with dyslipidemia (5, 16–18). Angptl4 is therefore a multifunctional protein, and there is a considerable interest in …