Date of Award

2014

Publication Type

Doctoral Thesis

Degree Name

Ph.D.

Department

Chemistry and Biochemistry

Supervisor

Koschinsky, Marlys

Rights

info:eu-repo/semantics/openAccess

Abstract

Elevated plasma concentrations of lipoprotein(a) (Lp(a)) have been identified as an independent, causal risk factor for coronary heart disease. Lp(a) resembles low-density lipoprotein (LDL), but is distinguished by the covalent addition of apolipoprotein(a) (apo(a)) to the apolipoproteinB-100 (apoB-100) moiety of LDL. Apo(a) shares homology with the zymogen plasminogen and Lp(a)/apo(a) can inhibit plasminogen activation to the serine protease plasmin on the fibrin surface. In addition, apo(a)/Lp(a) can inhibit fibrinolysis through attenuation of Glu1-plasminogen to Lys78-plasminogen conversion: a key positive feedback mechanism in accelerating plasmin formation. Cellular receptors for Lp(a)/apo(a) are suggested to contribute to the pathophysiological mechanisms of Lp(a) such as inhibition of pericellular plasminogen activation as well as Lp(a) clearance from plasma. The roles and identities of these receptors, however, remain elusive. The work in this dissertation evaluates the roles of receptors implicated in the ability of apo(a)/Lp(a) to inhibit pericellular plasminogen activation as well as in Lp(a) clearance by hepatocytes. Apo(a)/Lp(a) was found to potently inhibit pericellular plasminogen activation on vascular and blood cells through attenuating Glu1- to Lys78-plasminogen conversion. For both these effects, critical roles for the strong lysine binding site in kringle IV type 10 as well as the kringle V domain within apo(a) were identified; there was also no dependency on apo(a) isoform size. We found that the urokinase receptor and integrins αMβ2 and αVβ3 all contribute to plasminogen activation and apo(a) mediated inhibition of plasminogen activation on the cell surfaces of vascular and blood cells, with only a minor role for receptors containing carboxyl-terminal lysines. In vivo evidence suggests a potential role for proprotein convertase subtilisin/kexin type 9 (PCSK9) in Lp(a) clearance through an unidentified receptor. Effects on the number of LDL-receptor (LDLR) molecules were found to underlie the ability of PCSK9 to modulate Lp(a) catabolism in hepatic cells and fibroblasts. This process was dependent on the apoB-100 component of Lp(a) and on clathrin-mediated endocytosis. Taken together, our data contribute importantly to our understanding of the mechanisms of Lp(a) pathogenicity with regard to inhibition of pericellular plasminogen activation and highlight a novel role for PCSK9 activity and the LDLR in modulating Lp(a) catabolism.

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