Background Proprotein convertase subtilisin/kexin type 9 (PCSK9) modulates low-density lipoprotein (LDL) receptor (LDLR) degradation, thus influencing serum cholesterol levels. However, dysfunctional LDLR causes hypercholesterolemia without affecting PCSK9 clearance from the circulation. Methods and Results To study the reciprocal effects of PCSK9 and LDLR and the resultant effects on serum cholesterol, we produced transgenic mice expressing human (h) PCSK9. Although hPCSK9 was mainly expressed in the kidney, LDLR degradation was more evident in the liver. Adrenal LDLR levels were not affected, likely due to impaired PCSK9 retention in this tissue. In addition, hPCSK9 expression increased hepatic secretion of apoB-containing lipoproteins in an LDLR-independent fashion. Expression of hPCSK9 raised serum murine (m) PCSK9 levels by 4.3-fold in wild-type (WT) mice and not at all in LDLR−/− ice, where mPCSK9 levels were already 10-fold higher than in WT mice. In addition, LDLR+/− mice had 2.7-fold elevation in mPCSK9 levels and no elevation in cholesterol levels. Conversely, acute expression of hLDLR in transgenic mice caused a 70% decrease in serum mPCSK9 levels. Turnover studies using physiological levels of hPCSK9 showed rapid clearance in WT (half-life 5.2 min), faster in hLDLR transgenics (2.9 min), and much slower in LDLR−/− recipients (50.5 min). Supportive results were obtained using an in vitro system. Finally, up to 30% of serum hPCSK9 was associated with LDL regardless of LDLR expression. Conclusions Our results support a scenario where LDLR represents the main route of elimination of PCSK9, and a reciprocal regulation between these two proteins controls serum PCSK9 levels, hepatic LDLR expression, and serum LDL levels.

Proprotein Convertase Subtilisin/Kexin type 9 (PCSK9) promotes atherosclerosis by increasing low-density lipoprotein (LDL) cholesterol levels through degradation of hepatic LDL receptors (LDLR). Studies have described the systemic effects of PCSK9 on atherosclerosis, but whether PCSK9 has local and direct effects on the plaque in unknown. To study the local effect of human PCSK9 (hPCSK9) on atherosclerotic lesion composition independently of changes in serum cholesterol levels we generated chimeric mice expressing hPCSK9 exclusively from macrophages using marrow from hPCSK9 transgenic (hPCSK9tg) mice transplanted into apoE−/− and LDLR−/− mice, which were then placed on a high fat diet for 8 wk. We further characterized the effect of hPCSK9 expression on the inflammatory responses in the spleen and by mouse peritoneal macrophages (MPM) in vitro. We found that MPM from transgenic mice express both murine (m) Pcsk9 and hPCSK9 and that the latter reduces macrophage LDLR and LRP1 surface levels. hPCSK9 was detected in serum of mice transplanted with hPCSK9tg marrow, but did not influence lipid levels or atherosclerotic lesion size. However, marrow-derived PCSK9 progressively accumulated in lesions of apoE−/− recipient mice while increasing the infiltration of Ly6Chi inflammatory monocytes by 32% compared with controls. Expression of hPCSK9 also increased CD11b and Ly6Chi positive cell numbers in spleens of apoE−/− mice. In vitro, expression of hPCSK9 in LPS-stimulated macrophages increased mRNA levels of the pro-inflammatory markers Tnf and Il1b (40% and 45%, respectively) and suppressed those of the anti-inflammatory markers Il10 and Arg1 (30% and 44%, respectively). All PCSK9 effects were LDLR-dependent as PCSK9 protein was not detected in lesions of LDLR−/− recipient mice and did not affect macrophage or splenocyte inflammation. In conclusion, PCSK9 directly increases atherosclerotic lesion inflammation in an LDLR-dependent but cholesterol-independent mechanism, suggesting that therapeutic PCSK9 inhibition may have vascular benefits secondary to LDL reduction.

Background Proprotein convertase subtilisin kexin type 9 (PCSK9) promotes the degradation of the low-density lipoprotein receptor (LDLR), and its deficiency in humans results in low plasma LDL-cholesterol and protection against coronary heart disease (CHD). Recent evidence indicates that PCSK9 also modulates the metabolism of triglyceride-rich apolipoprotein B (apoB) lipoproteins (TRL), another important CHD risk factor. Here we studied effects of physiological levels of PCSK9 on intestinal TRL production and elucidated for the first time the cellular and molecular mechanisms involved. Methods and Results Treatment of human enterocytes (CaCo-2 cells) with recombinant human PCSK9 (10 μg/mL, 24 hours) increased cellular and secreted apoB48 and apoB100 by 40–55% each (p<0.01 vs. untreated cells), whereas acute deletion of PCSK9 expression reversed this effect. PCSK9 stimulation of apoB was due to: (1) a 1.5-fold increase in apoB mRNA (p<0.01); and (2) enhanced apoB protein stability through both LDLR-dependent and LDLR-independent mechanisms. PCSK9 decreased LDLR protein (p<0.01) and increased cellular apoB stability via activation of microsomal triglyceride transfer protein (MTP). PCSK9 also increased levels of the lipid-generating enzymes FAS, SCD and DGAT2 (p<0.05). In mice, human PCSK9 at physiologic levels increased intestinal MTP levels and activity regardless of LDLR expression. Conclusions PCSK9 markedly increases intestinal TRL apoB production through mechanisms mediated in part by transcriptional effects on apoB, MTP and lipogenic genes, and in part by post-transcriptional effects on the LDLR and MTP. These findings indicate that targeted PCSK9-based therapies may also be effective in the management of postprandial hypertriglyceridemia.