Zuhier Awan

Since the discovery of proprotein convertase subtilisin kexin 9 (PCSK9) in 2003, this PC has attracted a lot of attention from the scientific community and pharmaceutical companies. Secreted into the plasma by the liver, the proteinase K-like serine protease PCSK9 binds the low-density lipoprotein (LDL) receptor at the surface of hepatocytes, thereby preventing its recycling and enhancing its degradation in endosomes/lysosomes, resulting in reduced LDL-cholesterol clearance. Surprisingly, in a nonenzymatic fashion, PCSK9 enhances the intracellular degradation of all its target proteins. Rare gain-of-function PCSK9 variants lead to higher levels of LDL-cholesterol and increased risk of cardiovascular disease; more common loss-of-function PCSK9 variants are associated with reductions in both LDL-cholesterol and risk of cardiovascular disease. It took 9 years to elaborate powerful new PCSK9-based therapeutic approaches to reduce circulating levels of LDL-cholesterol. Presently, PCSK9 monoclonal antibodies that inhibit its function on the LDL receptor are evaluated in phase III clinical trials. This review will address the biochemical, genetic, and clinical aspects associated with PCSK9's biology and pathophysiology in cells, rodent and human, with emphasis on the clinical benefits of silencing the expression/activity of PCSK9 as a new modality in the treatment of hypercholesterolemia and associated pathologies.

OBJECTIVE: Proprotein convertase subtilisin/kexin 9 (PCSK9) promotes the degradation of the low-density lipoprotein receptor (LDLR), and its gene is the third locus implicated in familial hypercholesterolemia. Herein, we investigated the role of PCSK9 in adipose tissue metabolism. METHODS AND RESULTS: At 6 months of age, Pcsk9(-/-) mice accumulated ≈80% more visceral adipose tissue than wild-type mice. This was associated with adipocyte hypertrophy and increased in vivo fatty acid uptake and ex vivo triglyceride synthesis. Moreover, adipocyte hypertrophy was also observed in Pcsk9(-/-) Ldlr(-/-) mice, indicating that the LDLR is not implicated. Rather, we show here by immunohistochemistry that Pcsk9(-/-) males and females exhibit 4- and ≈ 40-fold higher cell surface levels of very-low-density lipoprotein receptor (VLDLR) in perigonadal depots, respectively. Expression of PCSK9 in the liver of Pcsk9(-/-) females reestablished both circulating PCSK9 and normal VLDLR levels. In contrast, specific inactivation of PCSK9 in the liver of wild-type females led to ≈ 50-fold higher levels of perigonadal VLDLR. CONCLUSIONS: In vivo, endogenous PCSK9 regulates VLDLR protein levels in adipose tissue. This regulation is achieved by circulating PCSK9 that originates entirely in the liver. PCSK9 is thus pivotal in fat metabolism: it maintains high circulating cholesterol levels via hepatic LDLR degradation, but it also limits visceral adipogenesis likely via adipose VLDLR regulation.

Proprotein convertase subtilisin/kexin Type 9 (PCSK9) is now identified as an important and major player in hypercholesterolaemia and atherosclerosis pathophysiology. PCSK9, through promoting lysosomal degradation of hepatic low-density lipoprotein (LDL) receptor, can decrease the clearance of plasma LDLs, leading to hypercholesterolaemia and consequent atherosclerotic plaque formation. Hypercholesterolaemia has been found to promote systemic and vascular inflammation, which can cause atherosclerotic lesion formation and progression and subsequent incidence of cardiovascular disease. Recent studies have shown the involvement of PCSK9 in the inflammatory pathway of atherosclerosis. Although trials with PCSK9 inhibitors have not shown any alteration in plasma C-reactive protein levels, there is accumulating evidence showing lessened inflammatory response in the arterial wall that could attenuate atherosclerotic plaque development beyond the established LDL-lowering effect of PCSK9 inhibition. In this review, we represent mounting evidence indicating that PCSK9 can locally increase vascular inflammation and contribute to atherosclerotic plaque progression in patients with hypercholesterolaemia.

BACKGROUND: Although statin therapy is known to increase concentrations of PCSK9, whether this effect is related to the magnitude of LDL reduction is uncertain. This study was undertaken to understand the extent of this effect and examine the relationship between PCSK9 and LDL cholesterol (LDL-C) reduction. METHODS: We measured plasma PCSK9 concentrations by ELISA at baseline and at 1 year in 500 men and 500 women participating in the Justification for Use of Statins in Prevention: an Intervention Trial Evaluating Rosuvastatin (JUPITER) trial that randomly allocated participants to rosuvastatin 20 mg daily or placebo. We also evaluated rs11591147, a single nucleotide polymorphism known to have an impact on plasma PCSK9 concentrations. RESULTS: At baseline, median (interquartile range) PCSK9 concentrations were higher in women [73 (62-90)] ng/mL than in men [69 (57-81) ng/mL] (P<0.005). During 1 year, there was no change in PCSK9 concentrations in the placebo arm, suggesting stability in time. In contrast, the rosuvastatin increased PCSK9 by 35% in women [101 (82-117) ng/mL] and 28% in men [89 (71-109) ng/mL] (P<0.0001). Among those allocated to rosuvastatin, greater reductions in LDL-C were associated with greater increases in PCSK9 on both absolute and relative scales (r=-0.15, P<0.0005). Furthermore PCSK9 (rs11591147) did not alter the magnitude of LDL-C reduction associated with rosuvastatin use. CONCLUSIONS: In this randomized trial, rosuvastatin increased plasma concentration of PCSK9 in proportion to the magnitude of LDL-C reduction; the LDL-C response to statin could not be inferred by PCSK9 concentrations.