BackgroundProprotein convertase subtilisin kexin-like 9 (PCSK9) is a secreted glycoprotein that is transcriptionally regulated by cholesterol status. It modulates levels of circulating low density lipoprotein cholesterol (LDLC) by negatively regulating low density lipoprotein receptor (LDLR) levels. PCSK9 variants that result in 'gain of function' have been linked to autosomal dominant hypercholesterolemia, while significant protection from coronary artery disease has been documented in individuals who carry 'loss of function' PCSK9 variants. PCSK9 circulates in human plasma, and we previously reported that plasma PCSK9 is positively correlated with total cholesterol and LDLC in men.ResultsHerein, we report the effects of two lipid-modulating therapies, namely statins and fibrates, on PCSK9 plasma levels in human subjects. We also document their effects on endogenous PCSK9 and LDLR expression in a human hepatocyte cell line, HepG2, using immunoprecipitation and immunoblot analyses. Changes in plasma PCSK9 following fenofibrate or gemfibrozil treatments (fibric acid derivatives) were inversely correlated with changes in LDLC levels (r = -0.558, p = 0.013). Atorvastatin administration (HMGCoA reductase inhibitor) significantly increased plasma PCSK9 (7.40%, p = 0.033) and these changes were inversely correlated with changes in LDLC levels (r = -0.393, p = 0.012). Immunoblot analyses of endogenous PCSK9 and LDLR expression by HepG2 cells in response to statins and fibrates showed that LDLR is more upregulated than PCSK9 by simvastatin (2.6× vs 1.5×, respectively at 10 μM), while fenofibrate did not induce changes in either.ConclusionThese results suggest that in vivo (1) statins directly increase PCSK9 expression while (2) fibrates affect PCSK9 expression indirectly through its modulation of cholesterol levels and (3) that these therapies could be improved by combination with a PCSK9 inhibitor, constituting a novel hypercholesterolemic therapy, since PCSK9 was significantly upregulated by both treatments.

a b s t r a c tProprotein convertase subtilisin/kexin type 9 (PCSK9), a liver-secreted plasma enzyme, restricts hepatic uptake of low-density lipoprotein (LDL) cholesterol by promoting the degradation of LDL receptors (LDLR). PCSK9 and LDLR are also expressed in insulin-producing pancreatic islet b cells, possibly affecting the function of these cells. Here we show that, compared to control mice, PCSK9-null male mice over 4 months of age carried more LDLR and less insulin in their pancreas; they were hypoinsulinemic, hyperglycemic and glucose-intolerant; their islets exhibited signs of malformation, apoptosis and inflammation. Collectively, these observations suggest that PCSK9 may be necessary for the normal function of pancreatic islets.

BACKGROUND: PCSK9 (proprotein convertase subtilisin/kexin type 9) is a polymorphic gene whose protein product regulates plasma LDL cholesterol (LDLC) concentrations by shuttling liver LDL receptors (LDLRs) for degradation. PCSK9 variants that cause a gain or loss of PCSK9 function are associated with hyper-or hypocholesterolemia, which increases or reduces the risk of cardiovascular disease, respectively. We studied the clinical and molecular characteristics of a novel PCSK9 loss-of-function sequence variant in a white French-Canadian family.

Proprotein convertase subtilisin/kexin 9 (PCSK9) is a secreted glycoprotein that regulates the degradation of the low‐density lipoprotein receptor. Single nucleotide polymorphisms in its gene associate with both hypercholesterolemia and hypocholesterolemia, and studies have shown a significant reduction in the risk of coronary heart disease for ‘loss‐of‐function’ PCSK9 carriers. Previously, we reported that proPCSK9 undergoes autocatalytic processing of its prodomain in the endoplasmic reticulum and that its inhibitory prosegment remains associated with it following secretion. Herein, we used a combination of mass spectrometry and radiolabeling to report that PCSK9 is phosphorylated at two sites: Ser47 in its propeptide and Ser688 in its C‐terminal domain. Site‐directed mutagenesis suggested that a Golgi casein kinase‐like kinase is responsible for PCSK9 phosphorylation, based on the consensus site, SXE/S(p). PCSK9 phosphorylation was cell‐type specific and occurs physiologically because human plasma PCSK9 is phosphorylated. Interestingly, we show that the naturally occurring ‘loss‐of‐function’ variant PCSK9(R46L) exhibits significantly decreased propeptide phosphorylation in the Huh7 liver cell line by 34% (P < 0.0001). PCSK9(R46L) and the engineered, unphosphorylated variant PCSK9(E49A) are cleaved following Ser47, suggesting that phosphorylation protects the propeptide against proteolysis. Phosphorylation may therefore play an important regulatory role in PCSK9 function. These findings will be important for the future design of PCSK9 inhibitors.