Dyslipidemia often accompanies metabolic diseases such as obesity and type II diabetes mellitus and represents a risk factor for cardiovascular disease. Clearance of triglycerides from the plasma is mediated by lipoprotein lipase (LPL), which hydrolyzes the triglycerides in chylomicrons and VLDL, liberating fatty acids for tissue uptake. LPL functions in the capillaries of the heart, adipose tissue, and skeletal muscle where LPL is anchored to the capillary wall by its endothelial cell transporter GPIHBP1. LPL activity is regulated by several factors including three members of the angiopoietin-like (ANGPTL) family–ANGPTL3, ANGPTL4, and ANGPTL8. How these proteins interact with LPL, especially in the physiological context of LPL anchored to endothelial cells by GPIHBP1, has not been well characterized. In my studies of ANGPTL4, I found when LPL is bound to GPIHBP1, it is partially, but not completely, protected from inactivation by ANGPTL4. Inactivation of LPL by ANGPTL4 leads to the dissociation of active LPL dimers into inactive monomers and I found that these monomers have a greatly reduced affinity for GPIHBP1. ANGPTL4 can be cleaved in vivo, separating the N-terminal coiled-coil domain from the C-terminal fibrinogen like-domain. I found the N-terminal domain alone is a much more potent LPL inhibitor than the full-length protein, even though both appear to have similar binding affinities for LPL-GPIHBP1 complexes. When I investigated ANGPTL3, I found ANGPTL3 itself is not a potent inhibitor of LPL at physiological concentrations, and unlike ANGPTL4, cleavage of ANGPTL3 does not improve its ability to inhibit LPL. Instead I found that ANGPTL3 forms a complex with ANGPTL8, a complex that only forms efficiently when the two proteins are co-expressed, and that this complex allows ANGPTL3 to bind and inhibit LPL. My data provide new insights into how ANGPTL proteins regulate LPL activity and the delivery of fat to tissues.

Lipoprotein lipase (LPL) is the main enzyme hydrolyzing triglyceride-rich lipoproteins in plasma. Proteoglycan-bound LPL on the vascular endothelium represent the functional pool of active enzyme. LPL is regulated in a tissue specific manner according to metabolic demands. Rapid regulation of LPL activity is necessary to provide free fatty acids for storage or energy production. This regulatory mechanism appears to be post-translational and requires synthesis of other protein/proteins. Recently it was demonstrated that angiopoietin-like protein 4 (ANGPTL4) is involved in the metabolism of plasma triglycerides and that it is able to inhibit LPL activity in vitro. These properties were linked to the N-terminal coiled-coil domain of ANGPTL4 (ccd-ANGPTL4), but the mechanism for the inhibition was not known. The aim of this thesis was to investigate the molecular mechanism for inhibition of LPL by ccd-ANGPTL4, to characterize regions in ccd-ANGPTL4 that are important for inactivation of LPL and to study the role of ANGPTL4 for regulation of LPL in vivo. Binding of ccd-ANGPTL4 to LPL was demonstrated by several methods, including surface plasmon resonance. The interaction was transient and resulted in conversion of the enzyme from catalytically active dimers to inactive monomers with decreased affinity for heparin. We have shown that ANGPTL4 mRNA in rat adipose tissue turns over rapidly and that changes in the ANGPTL4 mRNA abundance were inversely correlated to LPL activity, both during the fed to fasted and the fasted to fed transitions. We conclude that ANGPTL4 is a fasting-induced controller of LPL in adipose tissue, acting extracellularly on the native conformation of LPL in an unusual fashion, like an unfolding molecular chaperone. Site directed mutagenesis was used to explore regions in ccd-ANGPTL4 important for inactivation of LPL, and for binding of ANGPTL4 to heparin. Others had shown that ccd-ANGPTL4 forms higher oligomers. Structure prediction analyses demonstrated that the coiled-coil domain of ccd-ANGPTL4 probably forms three consecutive α-helices with strong hydrophobic faces, and that there are clusters of positively charged residues both on the helices and in intervening sequences. We made replacements of hydrophobic residues, positively charged residues, cysteine residues and negatively charged residues in ccd-ANGPTL4. In addition, helix-breaking proline residues were introduced in all three helices. We found that hydrophobic residues are important for oligomer formation. The higher oligomers appeared to be stabilized by disulfide bonds, but cysteines are not crucial for oligomerization. Introduction of Pro-residues in the first and second helix prevented formation of higher oligomers and reduced the ability of ccd-ANGPTL4 to inactivate LPL. We found that negatively charged residues in ccd-ANGPTL4 are important for inactivation of LPL. A heparin binding site was localized in the C-terminal end of ccd-ANGPTL4 (amino acid residues 114-140). To investigate whether LPL is differently processed in different depots of adipose tissue we measured the levels of LPL mRNA, protein and activity in omental and subcutaneous adipose tissue in human subjects undergoing elective surgery. Our results show that, although the expression level of LPL was higher in subcutaneous adipose tissue, the specific LPL activity (ratio of activity over the LPL protein mass) was higher in omental adipose tissue. Interestingly, the levels of ANGPTL4 mRNA were lower in omental compared to subcutaneous adipose tissue in most of the studied subjects. This difference can possibly explain the higher specific activity of LPL in omental adipose tissue and indicated that ANGPTL4 is involved in regulation of LPL activity also in humans. LPL produced by macrophages in the artery wall promotes local accumulation of lipids in these cells, and thereby plays an important role in development of atherosclerosis. The known association between type 2 diabetes and atherosclerosis forwarded us to study production of LPL by THP-1 macrophages under hyperglycemic conditions and under treatment with a peroxisome proliferator-activated receptor delta (PPARδ) agonist (GW501516). We found that LPL activity (but not LPL mass) produced by macrophages was decreased by GW501516. The loss of LPL activity coincided with increased level of ANGPTL4 mRNA, indicating that the agonist regulates LPL activity through expression of ANGPTL4. This effect was even more pronounced in cells grown under hyperglycemic conditions. Our data suggest that a suitable PPARδ agonist, like GW501516, may have protective effects against development of atherosclerosis in subjects with diabetes type 2.

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