Geranylgeranyl isoprenoids and hepatic Rap1a regulate basal and statin-induced expression of PCSK9.

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.

Hepatic Glucagon Signaling Regulates PCSK9 and Low-Density Lipoprotein Cholesterol.

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 Gcgr (glucagon receptor) signaling in plasma cholesterol regulation and identify its underlying molecular mechanisms. METHODS AND RESULTS: We show that Gcgr signaling plays an essential role in LDL-C (low-density lipoprotein cholesterol) homeostasis through regulating the PCSK9 (proprotein convertase subtilisin/kexin type 9) levels. Silencing of hepatic Gcgr or inhibition of glucagon action increased hepatic and plasma PCSK9 and resulted in lower LDLR (LDL receptor) protein and increased plasma LDL-C. Conversely, treatment of wild-type (WT) mice with glucagon lowered LDL-C levels, whereas this response was abrogated in Pcsk9-/- and Ldlr-/- mice. Our gain- and loss-of-function studies identified Epac2 (exchange protein activated by cAMP-2) and Rap1 (Ras-related protein-1) 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 posttranscriptional mechanism of PCSK9 regulation.

The association of serum total bile acid with non-alcoholic fatty liver disease in Chinese adults: a cross sectional study.

BACKGROUND: Non-alcoholic fatty liver disease (NAFLD) is currently the major cause of chronic liver disease globally. Bile acids (BAs) have emerged as relevant signaling molecules that are associated with NAFLD development. This study was aimed to examine the association of serum total bile acids (TBAs) with NAFLD in a large population of Chinese subjects. METHODS: This cross sectional study recruited 152,336 participants from the Second Xiangya Hospital, China. NAFLD was diagnosed based on the presence of hepatic steatosis on ultrasonography, without significant alcohol consumption and other known causes for chronic liver disease. A multivariate logistic regression model was used to test for the association of serum TBAs with NAFLD, adjusting for conventional risk factors of NAFLD. RESULTS: A total of 27.4% of the participants had NAFLD. Patients with NAFLD had slightly higher TBA levels than those without, 3.4 vs. 3.0 µmol/L (p < 0.001). However, TBA levels were not associated with NAFLD in the multivariate logistic regression model, which adjusted for age, gender and other acknowledged risk factors for NAFLD (OR = 1.00. 95% CI: 1.00-1.00, p = 0.797). CONCLUSIONS: We found that the serum TBA levels were not associated with NAFLD. Future studies in a large population, focusing on serum BA composition may improve the understating of the role of BAs in NAFLD.

Proprotein convertase subtilisin/kexin type 9 and lipid metabolism.

PURPOSE OF REVIEW: The purpose of this review is to highlight the recent findings of one of the most promising therapeutic targets in LDL cholesterol (LDL-C) management, proprotein convertase subtilisin/kexin type 9 (PCSK9). RECENT FINDINGS: Endoplasmic reticulum cargo receptor, surfeit locus protein 4 interacts with PCSK9 and regulates its exit from endoplasmic reticulum 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. Posttranscriptionally, recent work has shown that PCSK9 gets degraded in 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.

Global deletion of MGL in mice delays lipid absorption and alters energy homeostasis and diet-induced obesity.

Monoacylglycerol lipase (MGL) is a ubiquitously expressed enzyme that catalyzes the hydrolysis of monoacylglycerols (MGs) to yield FFAs and glycerol. MGL contributes to energy homeostasis through the mobilization of fat stores and also via the degradation of the endocannabinoid 2-arachidonoyl glycerol. To further examine the role of MG metabolism in energy homeostasis, MGL(-/-) mice were fed either a 10% (kilocalories) low-fat diet (LFD) or a 45% (kilocalories) high-fat diet (HFD) for 12 weeks. Profound increases of MG species in the MGL(-/-) mice compared with WT control mice were found. Weight gain over the 12 weeks was blunted in both diet groups. MGL(-/-) mice were leaner than WT mice at both baseline and after 12 weeks of LFD feeding. Circulating lipids were decreased in HFD-fed MGL(-/-) mice, as were the levels of several plasma peptides involved in glucose homeostasis and energy balance. Interestingly, MGL(-/-) mice had markedly reduced intestinal TG secretion following an oral fat challenge, suggesting delayed lipid absorption. Overall, the results indicate that global MGL deletion leads to systemic changes that produce a leaner phenotype and an improved serum metabolic profile.

Geranylgeranyl Isoprenoids and Hepatic Rap1a Regulate Basal and Statin-Induced Expression of PCSK9.

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.

Hepatocyte Rap1a contributes to obesity- and statin-associated hyperglycemia.

Excessive hepatic glucose production contributes to the development of hyperglycemia and is a key feature of type 2 diabetes. Here, we report that activation of hepatocyte Rap1a suppresses gluconeogenic gene expression and glucose production, whereas Rap1a silencing stimulates them. Rap1a activation is suppressed in obese mouse liver, and restoring its activity improves glucose intolerance. As Rap1a's membrane localization and activation depends on its geranylgeranylation, which is inhibited by statins, we show that statin-treated hepatocytes and the human liver have lower active-Rap1a levels. Similar to Rap1a inhibition, statins stimulate hepatic gluconeogenesis and increase fasting blood glucose in obese mice. Geranylgeraniol treatment, which acts as the precursor for geranylgeranyl isoprenoids, restores Rap1a activity and improves statin-mediated glucose intolerance. Mechanistically, Rap1a activation induces actin polymerization, which suppresses gluconeogenesis by Akt-mediated FoxO1 inhibition. Thus, Rap1a regulates hepatic glucose homeostasis, and blocking its activity, via lowering geranylgeranyl isoprenoids, contributes to statin-induced glucose intolerance.

The Many Faces of Purpura: Vancomycin-Induced Leukocytoclastic Vasculitis.

Leukocytoclastic vasculitis is a rare form of immune-mediated vasculitis that might be caused by infections or autoimmune diseases or might be precipitated by specific medications. We describe a 65-year-old patient, who was receiving vancomycin for a methicillin-sensitive Staphylococcus aureus permacath infection. Vancomycin was chosen due to medication non-adherence and the patient's desire to receive antimicrobial therapy in conjunction with his scheduled dialysis sessions. The patient's medical history was notable for untreated hepatitis C infection and end-stage renal disease, requiring hemodialysis three times a week. Vancomycin was administered during dialysis sessions. After one week of therapy, the patient developed bilateral lower extremity purpura. Skin biopsy was suggestive of leukocytoclastic vasculitis with an absence of intravascular thrombi. Serum cryoglobulins were negative, making cryoglobulinemia due to HCV infection unlikely. Following cessation of vancomycin therapy, the rash gradually disappeared with scarring in the form of post-purpuric hyperpigmentation. Despite its widespread use, vancomycin is a rare cause of leukocytoclastic vasculitis. Clinicians should keep in mind a wide range of differential diagnosis of bilateral lower extremity purpura as treatment differs depending on its underlying etiology.