Proprotein convertase subtilisin/kexin type 9 (PCSK9) is a secreted protein that regulates circulating low-density lipoprotein cholesterol (LDL-C) through the hepatic LDL receptor degradation pathway. It is a known therapeutic target to further lower LDL-C in patients suffering from dyslipidemia/hypercholesterolemia and who are already on maximal statin therapy. Currently there are several anti-PCSK9 therapies either in development or on the market. However, potent LDL lowering effects of anti-PCSK9 therapies in patients are not always as expected.
Circulating PCSK9 is known to exist in a free and bound form in the circulation, and in particular has been shown to bind LDL and lipoprotein(a) (Lp(a)). The association of circulating PCSK9 with lipoproteins is expected to impact on the capacity of PCSK9 to degrade the LDL receptor.
Prof Mayr and his team’s recent findings have shown for the very first time that PCSK9 is predominantly associated with high-density lipoprotein (HDL) in human plasma. Their findings that PCSK9 can bind multiple lipoprotein classes, show that PCSK9-lipoprotein compartmentalisation could be an underappreciated regulator of its biological activity and susceptibility to therapy by anti- PCSK9 therapies.
The team has developed a method of assessing unbound PCSK9 in a subject based on their novel findings. Specifically, depleting lipoproteins from a sample allows assessment of the level of ‘free’ vs lipoprotein bound PCSK9 by ELISA. Patients that may have more “free” PCSK9 may be the ones that benefit the most from PCSK9 inhibition therapy than the ones whose PCSK9 is mostly bound. PCSK9-lipoprotein interactions are expected to alter activity and therapeutic targeting.
The Science
King’s team has investigated PCSK9-lipoprotein association using nuclear magnetic resonance (NMR)-based lipoprotein profiling, quantitative multiplexed proteomics and targeted lipidomics in the setting of cardiovascular disease (CVD) and postprandial lipaemia.
In a large collection of HDL samples (n=170, unpublished data) from patients with CVD, they assessed the impact of PCSK9 on the protein and lipid composition of HDL across multiple CVD phenotypes and gained novel insights into HDL remodelling during postprandial hyperlipaemia, with PCSK9 dynamics emerging as a central feature. The potential associations of PCSK9 with different human lipoproteins was first determined by immuno-capture in 20 healthy volunteers. Measurement of lipoprotein-associated PCSK9 suggested a predominant association of PCSK9 with HDL (Figure 1). King’s team has produced 5 lines of evidence to support its conclusion: immuno-capture of PCSK9-associated lipoproteins, plasma PCSK9 concentrations after ApoB and after HDL depletion, and enrichment of PCSK9 on HDL after immuno isolation from plasma and from an ApoB-depleted OptiPrepTM fraction.