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PURPOSE OF REVIEW: The purpose of this review is to highlight the recent findings of one of the most promising therapeutic targets in low-density lipoprotein (LDL) cholesterol management, proprotein convertase subtilisin/kexin type 9 (PCSK9). RECENT FINDINGS: Endoplasmic reticulum (ER) cargo receptor, surfeit locus protein 4 (SURF4) interacts with PCSK9 and regulates its exit from ER and its secretion. Once secreted, PCSK9 binds to heparin sulfate proteoglycans on the hepatocyte surface and this binding is required for PCSK9–LDL receptor (LDLR) complex formation and LDLR degradation. Post-transcriptionally, recent work has shown that PCSK9 gets degraded in the lysosomes by activation of the glucagon receptor signaling, providing more data on the hormonal regulation of PCSK9. Finally, human studies with PCSK9 inhibitors offered more evidence on their benefits and safe use. SUMMARY: Recent work on the regulation of PCSK9 has enhanced our understanding of its biology, which may provide important information for future PCSK9-based therapies.

RATIONALE: Glucagon is a key hormone that regulates the adaptive metabolic responses to fasting. In addition to maintaining glucose homeostasis, glucagon participates in the regulation of cholesterol metabolism, however, the molecular pathways underlying this effect are incompletely understood. OBJECTIVE: We sought to determine the role of hepatic glucagon receptor (Gcgr) signaling in plasma cholesterol regulation and identify its underlying molecular mechanisms. METHODS AND RESULTS: We show that Gcgr signaling plays an essential role in low-density lipoprotein (LDL) cholesterol homeostasis through regulating the proprotein convertase subtilisin/kexin type 9 (PCSK9) levels. Silencing of hepatic Gcgr or inhibition of glucagon action increased hepatic and plasma PCSK9 and resulted in lower LDL receptor protein and increased plasma LDL-cholesterol. Conversely, treatment of WT mice with glucagon lowered LDL-cholesterol levels, whereas this response was abrogated in Pcsk9(−/−) and Ldlr(−/−) mice. Our gain- and loss-of-function studies identified exchange protein activated by cAMP-2 (Epac2) and Ras-related protein-1 (Rap1) as the downstream mediators of glucagon’s action on PCSK9 homeostasis. Moreover, mechanistic studies revealed that glucagon affected the half-life of PCSK9 protein without changing the level of its mRNA, indicating that Gcgr signaling regulates PCSK9 degradation. CONCLUSIONS: These findings provide novel insights into the molecular interplay between hepatic glucagon signaling and lipid metabolism and describe a new post-transcriptional mechanism of PCSK9 regulation.

We previously demonstrated that hepatic activation of a small G protein of the Ras family, Rap1a, is suppressed in obesity, which results in increased hepatic glucose production and glucose intolerance in obese mice. Here, we show that Rap1a inhibition in obese mice liver also results in fatty liver formation, which is characteristic of the diabetic liver. Specifically, we report that Rap1a activity is decreased in the livers of patients with non-alcoholic steatohepatitis (NASH) and mouse models of non-alcoholic fatty liver disease (NAFLD) and NASH. Restoring hepatic Rap1a activity by overexpressing a constitutively active mutant form of Rap1a lowered the mature, processed form of lipogenic transcription factor, Srebp1, without an effect on the unprocessed Srebp1 and suppressed hepatic TG accumulation, whereas liver Rap1a deficiency increased Srebp1 processing and exacerbated steatosis. Mechanistically, we show that mTORC1, which promotes Srebp1 cleavage, is hyperactivated upon Rap1a deficiency despite disturbed insulin signaling. In proof-of-principle studies, we found that treatment of obese mice with a small molecule activator of Rap1a (8-pCPT) or inhibiting Rap1a's endogenous inhibitor, Rap1Gap, recapitulated our hepatic gain-of-function model and resulted in improved hepatic steatosis and lowered lipogenic genes. Thus, hepatic Rap1a serves as a signaling molecule that suppresses both hepatic gluconeogenesis and steatosis, and inhibition of its activity in the liver contributes to the pathogenesis of glucose intolerance and NAFLD/NASH development.

Low-density lipoprotein cholesterol (LDL-C) lowering is the main goal of atherosclerotic cardiovascular disease prevention, and proprotein convertase subtilisin/kexin type 9 (PCSK9) inhibition is now a validated therapeutic strategy that lowers serum LDL-C and reduces coronary events. Ironically, the most widely used medicine to lower cholesterol, statins, has been shown to increase circulating PCSK9 levels, which limits their efficacy. Here, we show that geranylgeranyl isoprenoids and hepatic Rap1a regulate both basal and statin induced expression of PCSK9 and contribute to LDL-C homeostasis. Rap1a prenylation and activity is inhibited upon statin treatment, and statin mediated PCSK9 induction is dependent on geranylgeranyl synthesis and hepatic Rap1a. Accordingly, treatment of mice with a small molecule activator of Rap1a lowered PCSK9 protein and plasma cholesterol and inhibited statin mediated PCSK9 induction in hepatocytes. The mechanism involves inhibition of the downstream RhoA-ROCK pathway and regulation of PCSK9 at the post transcriptional level. These data further identify Rap1a as a novel regulator of PCSK9 protein and show that blocking Rap1a prenylation through lowering geranylgeranyl levels contributes to statin-mediated induction of PCSK9.

Background–: Type 2 diabetes is associated with an increased risk of atherosclerotic cardiovascular disease. It has been suggested that insulin resistance underlies this link, possibly by altering the functions of cells in the artery wall. We aimed to test whether improving systemic insulin sensitivity reduces atherosclerosis., Methods–: We used mice that are established to have improved systemic insulin sensitivity: those lacking FoxO transcription factors in hepatocytes. Three hepatic FoxO isoforms (FoxO1, FoxO3, and FoxO4) function together to promote hepatic glucose output, and ablating them lowers glucose production, lowers circulating glucose and insulin, and improves systemic insulin sensitivity. We made these mice susceptible to atherosclerosis in two different ways, by injecting them with gain-of-function AAV8.mPcsk9 D377Y and by crossing with Ldlr -/- mice., Results–: We verified that hepatic FoxO ablation improves systemic insulin sensitivity in these atherosclerotic settings. We observed that FoxO deficiency caused no reductions in atherosclerosis, and in some cases increased atherosclerosis. These phenotypes coincided with large increases in circulating triglycerides in FoxO-ablated mice., Conclusions–: These findings suggest that systemic insulin sensitization is insufficient to reduce atherosclerosis., Competing Interests: Conflict of interest statement. The authors have declared that no conflict of interest exists.