gms | German Medical Science

The main function of the proprotein convertase subtilisin/kexin (PCSK) type 9 (PCSK9) is the proteolytic maturation of secreted proteins such as hormones, cytokines, growth factors, and cell surface receptors. PCSK9 is expressed mainly in the liver, the intestine, the kidney, and the central nervous system.

Apart from acting as a chaperone to transport the precursor form of the LDL receptor (LDLR) from the endoplasmic reticulum, intracellular PCSK9 plays a role in regulating the expression of the mature LDLR by inducing intracellular degradation of the LDLR prior to its transport to the cell surface membrane. If not degraded intracellularly, the mature LDLR is transported to the cell surface, where it resides in clathrin-coated pits because of its interaction with the autosomal recessive hypercholesterolemia (ARH) adapter protein. The LDLR undergoes endocytosis in the presence or absence of its ligand, entering the endocytic recycling compartment. The change in pH within this compartment allows dissociation of the LDLR from its ligand, which then becomes degraded in the lysosome while the LDLR recycles. The main role of secreted extracelluar PCSK9 is to post-translationally regulate the number of cell surface LDLR. Secreted PCSK9 binds to the epidermal growth factor repeat A (EGF-A) region of the LDLR and the formed PCSK9–LDLR complex is internalized again by clathrin-mediated endocytosis; the complex is then routed to the sorting endosome/lysosome. At the acidic pH of the endosome/lysosome, an additional interaction between the ligand-binding domain of the LDLR and the C-terminal domain of PCSK9 occurs; as a consequence PCSK9 remains bound to the LDLR and the LDLR fails to adopt a closed conformation which is required for LDLR recycling. Thus, by binding to the LDLR, PCSK9 disrupts the recycling of the LDLR leading to its degradation and subsequently a reduced number of available LDLRs.

Up to 30% of PCSK9 is bound to LDL cholesterol in mice and normolipidemic subjects. In mice, PCSK9 is also bound to high density lipoprotein (HDL). PCSK9 is cleaved by furin as well as protein convertases (PC) 5/6 and both forms of PCSK9 can be measured in human plasma. Furin-cleaved PCSK9 (55 kDa) is still active and binds to the LDLR, however, with a 2-fold reduced activity.

PCSK9 binds to a variety of other proteins, one of them being annexin A2 which is present in the nucleus, the cytosol and the cell membrane in a variety of cells. In annexin A2 knockout mice plasma PCSK9 levels are doubled resulting in reduced LDLR expression and an increase in LDL cholesterol, thus annexin A2 is viewed as endogenous inhibitor of PCSK9.

A number of transcription factors or cofactors regulate the PCSK9 gene expression, including sterol-response element binding proteins (SREBP-1/2). Since the PCSK9 gene is regulated by sterols through SREBP2, low dietary cholesterol concentrations potently suppresses its expression and PCSK9 protein levels decrease in the course of fasting and increase after feeding in animals and humans. SREBP2 also controls LDLR expression. Statins increase the transcription factor SREBP2 thereby increasing PCSK9 expression dose-dependently and to a greater extent than LDLR expression. Statins not only enhance the monomeric but also the heterodimeric form of PCSK9.

The gain of function (GOF) mutation of PCSK9 causes severe hypercholesterolemia and development of profound atherosclerotic lesions in mice and pigs. PCSK9 overexpression increases LDL-C concentration in mice and accelerates the development of atherosclerosis, the latter being absent in LDLR-knockout mice. On the contrary, development of atherosclerosis is slowed down by inactivation of the PCSK9 gene in mice. Treating mice with increased atherogenesis with different doses of a PCSK9 inhibitor alone and in combination with atorvastatin dose-dependently decreased serum cholesterol, reduced atherosclerotic lesion size and improved plaque morphology. Development of atherosclerosis involves endothelial cell apoptosis and accumulation of foam cells, both of which an be triggered by oxidized LDL-C (oxLDL-C). Indeed, oxLDL-C increases PCSK9 expression in macrophages and the oxLDL-C induced apoptosis is reduced in human umbilical vein endothelial cells by silencing PCSK9, an effect being related to less caspase 9 and 3 activation. Also cholesterol uptake of THP-1 macrophages and foam cell formation as well as oxLDL-C/NFkB- induced inflammation are attenuated by PCSK9 silencing.

In line with the above cell and animal experiments, the plasma PCSK9 concentration correlates with the intima-media thickness in patients, and GOF-mutations of PCSK9 increase not only the LDL-C concentration but also the intima-media thickness over time compared to normal subjects. Plasma PCSK9 concentrations are predictive for 4-5 year major cardiovascular event rate and PCSK9 serum concentrations correlate with cardiovascular risk.

Apart from its binding to LDLR, PCSK9 also interact with other receptors such as the very low density lipoprotein receptor (VLDLR), the LDLR related protein 1 (LRP1), the apoprotein E receptor (ApoER) as well as CD81 on hepatocytes (hepatitis C virus receptor) and CD36 on macrophages. VLDLR may contribute to Lipoprotein a (LP(a)) clearance. Clinical studies show that inhibition of PCSK9 potently lowers Lp(a), which is a marker of cardiovascular risk.