Endogenous and exogenous factors affecting lipoprotein lipase activity
Sammanfattning: Individuals with high levels of plasma triglycerides are at high risk to develop cardiovascular disease (CVD), currently one of the major causes of death worldwide. Recent epidemiological studies show that loss-of-function mutations in the APOC3 gene lower plasma triglyceride levels and reduce the incidence of coronary artery disease. The APOC3 gene encodes for apolipoprotein (APO) C3, known as an inhibitor of lipoprotein lipase (LPL) activity. Similarly, a common gain-of-function mutation in the LPL gene is associated with reduced risk for CVD.LPL is central for the metabolism of lipids in blood. The enzyme acts at the endothelial surface of the capillary bed where it hydrolyzes triglycerides in circulating triglyceride-rich lipoproteins (TRLs) and thereby allows uptake of fatty acids in adjacent tissues. LPL activity has to be rapidly modulated to adapt to the metabolic demands of different tissues. The current view is that LPL is constitutively expressed and that the rapid modulation of the enzymatic activity occurs by some different controller proteins. Angiopoietin-like protein 4 (ANGPTL4) is one of the main candidates for control of LPL activity. ANGPTL4 causes irreversible inactivation through dissociation of the active LPL dimer to inactive monomers. Other proteins that have effects on LPL activity are the APOCs which are surface components of the substrate TRLs. APOC2 is a well-known LPL co-factor, whereas APOC1 and APOC3 independently inhibit LPL activity.Given the important role of LPL for triglyceride homeostasis in blood, the aim of this thesis was to find small molecules that could increase LPL activity and serve as lead compounds in future drug discovery efforts. Another aim was to investigate the molecular mechanisms for how APOC1 and APOC3 inhibit LPL activity.Using a small molecule screening library we have identified small molecules that can protect LPL from inactivation by ANGPTL4 during incubations in vitro. Following a structure-activity relationship study we have synthesized lead compounds that more efficiently protect LPL from inactivation by ANGPTL4 in vitro and also have dramatic triglyceride-lowering properties in vivo. In a separate study we show that low concentrations of fatty acids possess the ability to prevent inactivation of LPL by ANGPTL4 under in vitro conditions.With regard to APOC1 and APOC3 we demonstrate that when bound to TRLs, these apolipoproteins prevent binding of LPL to the lipid/water interface. This results in decreased lipolysis and in an increased susceptibility of LPL to inactivation by ANGPTL4. We demonstrate that hydrophobic amino acid residues that are centrally located in the APOC3 molecule are critical for attachment of this protein to lipid emulsion particles and consequently for inhibition of LPL activity.In summary, this work has identified a lead compound that protects LPL from inactivation by ANGPTL4 in vitro and lowers triglycerides in vivo. In addition, we propose a molecular mechanism for inhibition of LPL activity by APOC1 and APOC3.
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