Abstract 3071: Ras-related Protein 1a-mediated Regulation Of Proprotein Convertase Subtilisin/kexin Type 9

Abstract

Background & Aims: Proprotein convertase subtilisin/kexin type 9 (PCSK9) has recently been identified as a therapeutic target to regulate plasma low-density lipoprotein cholesterol (LDL-C) levels. PCSK9 binds to the LDL receptor (LDLR) and targets it for lysosomal degradation, hindering LDL-C uptake from the circulation. Despite the use of anti-PCSK9 therapies, the molecular mechanisms by which PCSK9 gets regulated are not fully understood. In this regard, we recently discovered that activation of hepatic small GTPase, Ras-related protein 1a (Rap1a), decreases PCSK9 levels and subsequently lowers plasma LDL-C. In addition, the most widely used medicine to lower cholesterol, statins, have been shown to increase circulating PCSK9 levels, which limits their efficacy; however, our understanding of the link between statins and plasma PCSK9 is limited. Hypothesis & Aims: As statins block the synthesis of geranylgeranyl isoprenoids (GGPP), which are required for Rap1a activation, we explored the hypothesis that Rap1a is involved in statin-induced expression of PCSK9. We further explored the mechanisms of PCSK9 regulation by Rap1a. Methods: Mice were treated orally with statins and geranylgeraniol, the precursor of GGPP by daily oral gavage. Rap1a activator, 8-pCPT-2’-OMe-cAMP, was administered via i.p. injection. Hepatic PCSK9, plasma PCSK9, and LDL-C levels were assayed. For in vitro experiments, primary hepatocytes were isolated from wild-type mice and treated with vehicle, simvastatin, or simvastatin in combination with GGPP or 8-pCPT-2’-OMe-cAMP. PCSK9 levels were measured. Results: Statin treatment combined with GGOH lowered plasma PCSK9 and LDL-C in mice (174 ± 25 vs 109 ± 16 ng/mL and 24 ± 1.8 vs 19 ± 1.4 mg/dL, respectively, n=4-5, mean ± SEM, p < 0.05). Rap1a activator, 8-pCPT-2’-OMe-cAMP, also lowered statin-induced PCSK9 in vitro. Mechanistically, we found that Rap1a inhibits the downstream RhoA-ROCK pathway and regulates PCSK9 at the post-transcriptional level. Conclusions: In conclusion, our data reveal that Rap1a is a novel regulator of PCSK9 protein and blocking Rap1a prenylation through lowering geranylgeranyl levels contributes to statin-mediated induction of PCSK9.