Hepatic SREBP-2 and cholesterol biosynthesis are regulated by FoxO3 and Sirt6

Proteasome inhibition protects against diet-induced gallstone formation through modulation of cholesterol and bile acid homeostasis

Gallstone disease is one of the most prevalent and costly gastrointestinal disorders worldwide. Gallstones are formed in the biliary system by cholesterol secretions in bile, which result from excess cholesterol, a deficiency in bile salts or a combination of the two. The present study examined the effects of proteasome inhibition on gallstone formation using the proteasome inhibitors bortezomib (BT) and carfilzomib (CF). C57BL/6J mice were fed a lithogenic diet to generate gallstones and injected with BT or CF for 12 weeks. After 12 weeks of the lithogenic diet, 8 out of the 10 mice in the control group had developed gallstones, whereas none of the mice who received proteasome inhibitors had developed gallstones. Notably, the expression of genes associated with cholesterol synthesis (sterol regulatory element‑binding protein‑2 and 3‑hydroxy‑3‑methylglutaryl‑CoA reductase), cholesterol secretion [ATP‑binding cassette subfamily G member 5 (ABCG5) and ABCG8] and bile acid synthesis [cytochrome P450 family 7 subfamily A member 1 (Cyp7a1), Cyp7b1, Cyp27a1 and Cyp8b1] was reduced in the livers of mice injected with BT or CF. Cyp7a1 encodes cholesterol 7α‑hydroxylase, the rate‑limiting enzyme in the synthesis of bile acid from cholesterol. The present study therefore measured the expression levels of transcription factors that are known to inhibit Cyp7a1 expression, namely farnesoid X receptor (FXR), pregnane X receptor (PXR) and small heterodimer partner (SHP). Although FXR, PXR and SHP expression was predicted to increase in the presence of proteasome inhibitors, the expression levels were actually reduced; thus, it was concluded that they were not involved in the proteasome inhibition‑induced regulation of Cyp7a1. Further investigation of the mitogen‑activated protein kinase and protein kinase A (PKA) signaling pathways in human hepatoma cells revealed that proteasome inhibition‑induced c‑Jun N‑terminal kinase (JNK) phosphorylation reduced CYP7A1 and CYP27A1 expression. In addition, reduced PKA phosphorylation as a result of proteasome inhibition regulated ABCG5 and ABCG8 expression. In conclusion, these findings suggest that proteasome inhibition regulates cholesterol and biliary metabolism via the JNK and PKA pathways, and is a promising therapeutic strategy to prevent gallstone disease.

SREBP-regulated lipid metabolism: convergent physiology - divergent pathophysiology

Cellular lipid metabolism and homeostasis are controlled by sterol regulatory-element binding proteins (SREBPs). In addition to performing canonical functions in the transcriptional regulation of genes involved in the biosynthesis and uptake of lipids, genome-wide system analyses have revealed that these versatile transcription factors act as important nodes of convergence and divergence within biological signalling networks. Thus, they are involved in myriad physiological and pathophysiological processes, highlighting the importance of lipid metabolism in biology. Changes in cell metabolism and growth are reciprocally linked through SREBPs. Anabolic and growth signalling pathways branch off and connect to multiple steps of SREBP activation and form complex regulatory networks. In addition, SREBPs are implicated in numerous pathogenic processes such as endoplasmic reticulum stress, inflammation, autophagy and apoptosis, and in this way, they contribute to obesity, dyslipidaemia, diabetes mellitus, nonalcoholic fatty liver disease, nonalcoholic steatohepatitis, chronic kidney disease, neurodegenerative diseases and cancers. This Review aims to provide a comprehensive understanding of the role of SREBPs in physiology and pathophysiology at the cell, organ and organism levels.

SIRT6 Is Essential for Adipocyte Differentiation by Regulating Mitotic Clonal Expansion

Preadipocytes initiate differentiation into adipocytes through a cascade of events. Mitotic clonal expansion, as one of the earliest events, is essential for adipogenesis. However, the underlying mechanisms that regulate mitotic clonal expansion remain elusive. SIRT6 is a member of the evolutionarily conserved sirtuin family of nicotinamide adenine dinucleotide (NAD)+-dependent protein deacetylases. Here, we show that SIRT6 deficiency in preadipocytes blocks their adipogenesis. Analysis of gene expression during adipogenesis reveals that KIF5C, which belongs to the kinesin family, is negatively regulated by SIRT6. Furthermore, we show that KIF5C is a negative factor for adipogenesis through interacting with CK2α', a catalytic subunit of CK2. This interaction blocks CK2α' nuclear translocation and CK2 kinase activity and inhibits mitotic clonal expansion during adipogenesis. These findings reveal a crucial role of SIRT6 in adipogenesis and provide potential therapeutic targets for obesity.

Mechanisms controlling the anti-neoplastic functions of FoxO proteins

The Forkhead box O (FoxO) proteins comprise a family of evolutionarily conserved transcription factors that predominantly function as tumor suppressors. These proteins assume diverse roles in the cellular anti-neoplastic response, including regulation of apoptosis and autophagy, cancer metabolism, cell-cycle arrest, oxidative stress and the DNA damage response. More recently, FoxO proteins have been implicated in cancer immunity and cancer stem-cell (CSC) homeostasis. Interestingly, in some sporadic sub-populations, FoxO protein function may also be manipulated by factors such as β-catenin whereby they instead can facilitate cancer progression via maintenance of CSC properties or promoting drug resistance or metastasis and invasion. This review highlights the essential biological functions of FoxOs and explores the areas that may be exploited in FoxO protein signaling pathways in the development of novel cancer therapeutic agents.

FOXO transcription factors in non-alcoholic fatty liver disease

Non-alcoholic fatty liver disease (NAFLD) is a chronic progressive liver disorder that begins with simple hepatic steatosis and progresses to non-alcoholic steatohepatitis, fibrosis, cirrhosis, and even liver cancer. As the global prevalence of NAFLD rises, it is increasingly important that we understand its pathogenesis and develop effective therapies for this chronic disease. Forkhead box O (FOXO) transcription factors are key downstream regulators in the insulin/insulin-like growth factor 1 (IGF1) signaling pathway, and have been implicated in a range of cellular functions including the regulation of glucose, triglyceride, and cholesterol homeostasis. The role of FOXOs in the modulation of immune response and inflammation is complex, with reports of both pro- and anti-inflammatory effects. FOXOs are reported to protect against hepatic fibrosis by inhibiting proliferation and transdifferentiation of hepatic stellate cells. Mice that are deficient in hepatic FOXOs are more susceptible to non-alcoholic steatohepatitis than wild-type controls. In summary, FOXOs play a critical role in maintaining metabolic and cellular homeostasis in the liver, and dysregulation of FOXOs may be involved in NAFLD development.

SIRT6 promotes osteogenic differentiation of mesenchymal stem cells through BMP signaling

SIRT6 has been identified as an H3K9 deacetylase and a critical regulator of genome stability, telomere integrity, and metabolic homeostasis. Sirt6-deficient mice displayed dramatic phenotypes including profound lymphopenia, loss of subcutaneous fat, lordokyphosis and low bone marrow density. Here, we report that SIRT6 regulates osteogenic differentiation independent of its deacetylase activity in vitro. Further mechanistic studies showed that SIRT6 involves the cell fate determination by modulating bone morphogenetic protein (BMP) signaling. Unexpectedly, this modulation depends upon P300/CBP-associated factor (PCAF). In addition, we observed impaired SIRT6 expression in bone marrow mesenchymal stem cells and in bone sections of ovariectomized mice. Taken together, our present study provide new insights into mechanisms of SIRT6-regulated MSC function beyond its H3K9 deacetylase activity.

A novel role for CRTC2 in hepatic cholesterol synthesis through SREBP-2

Cholesterol synthesis is regulated by the transcription factor sterol regulatory element binding protein 2 (SREBP-2) and its target gene 3-hydroxy-3-methylglutaryl-coenzyme A reductase (HMGCR), which is the rate-limiting enzyme in cholesterol synthesis. Cyclic adenosine monophosphate-responsive element (CRE) binding protein-regulated transcription coactivator (CRTC) 2 is the master regulator of glucose metabolism. However, the effect of CRTC2 on cholesterol and its potential molecular mechanism remain unclear. Here, we demonstrated that CRTC2 expression and liver cholesterol content were increased in patients with high serum cholesterol levels who underwent resection of liver hemangiomas, as well as in mice fed a 4% cholesterol diet. Mice with adenovirus-mediated CRTC2 overexpression also showed elevated lipid levels in both serum and liver tissues. Intriguingly, hepatic de novo cholesterol synthesis was markedly increased under these conditions. In contrast, CRTC2 ablation in mice fed a 4% cholesterol diet (18 weeks) showed decreased lipid levels in serum and liver tissues compared with those in littermate wild-type mice. The expression of lipogenic genes (SREBP-2 and HMGCR) was consistent with hepatic CRTC2 levels. In vivo imaging showed enhanced adenovirus-mediated HMGCR-luciferase activity in adenovirus-mediated CRTC2 mouse livers; however, the activity was attenuated after mutation of CRE or sterol regulatory element sequences in the HMGCR reporter construct. The effect of CRTC2 on HMGCR in mouse livers was alleviated upon SREBP-2 knockdown. CRTC2 modulated SREBP-2 transcription by CRE binding protein, which recognizes the half-site CRE sequence in the SREBP-2 promoter. CRTC2 reduced the nuclear protein expression of forkhead box O1 and subsequently increased SREBP-2 transcription by binding insulin response element 1, rather than insulin response element 2, in the SREBP-2 promoter.

CONCLUSION

CRTC2 regulates the transcription of SREBP-2 by interfering with the recognition of insulin response element 1 in the SREBP-2 promoter by forkhead box O1, thus inducing SREBP-2/HMGCR signaling and subsequently facilitating hepatic cholesterol synthesis. (Hepatology 2017;66:481-497).

Lipogenic transcription factor ChREBP mediates fructose-induced metabolic adaptations to prevent hepatotoxicity

Epidemiologic and animal studies implicate overconsumption of fructose in the development of nonalcoholic fatty liver disease, but the molecular mechanisms underlying fructose-induced chronic liver diseases remain largely unknown. Here, we have presented evidence supporting the essential function of the lipogenic transcription factor carbohydrate response element-binding protein (ChREBP) in mediating adaptive responses to fructose and protecting against fructose-induced hepatotoxicity. In WT mice, a high-fructose diet (HFrD) activated hepatic lipogenesis in a ChREBP-dependent manner; however, in Chrebp-KO mice, a HFrD induced steatohepatitis. In Chrebp-KO mouse livers, a HFrD reduced levels of molecular chaperones and activated the C/EBP homologous protein-dependent (CHOP-dependent) unfolded protein response, whereas administration of a chemical chaperone or Chop shRNA rescued liver injury. Elevated expression levels of cholesterol biosynthesis genes in HFrD-fed Chrebp-KO livers were paralleled by an increased nuclear abundance of sterol regulatory element-binding protein 2 (SREBP2). Atorvastatin-mediated inhibition of hepatic cholesterol biosynthesis or depletion of hepatic Srebp2 reversed fructose-induced liver injury in Chrebp-KO mice. Mechanistically, we determined that ChREBP binds to nuclear SREBP2 to promote its ubiquitination and destabilization in cultured cells. Therefore, our findings demonstrate that ChREBP provides hepatoprotection against a HFrD by preventing overactivation of cholesterol biosynthesis and the subsequent CHOP-mediated, proapoptotic unfolded protein response. Our findings also identified a role for ChREBP in regulating SREBP2-dependent cholesterol metabolism.

Fabp4-Cre-mediated Sirt6 deletion impairs adipose tissue function and metabolic homeostasis in mice

SIRT6 is a member of sirtuin family of deacetylases involved in diverse processes including genome stability, metabolic homeostasis and anti-inflammation. However, its function in the adipose tissue is not well understood. To examine the metabolic function of SIRT6 in the adipose tissue, we generated two mouse models that are deficient in Sirt6 using the Cre-lox approach. Two commonly used Cre lines that are driven by either the mouse Fabp4 or Adipoq gene promoter were chosen for this study. The Sirt6-knockout mice generated by the Fabp4-Cre line (Sirt6^f/f:Fabp4-Cre) had a significant increase in both body weight and fat mass and exhibited glucose intolerance and insulin resistance as compared with the control wild-type mice. At the molecular levels, the Sirt6^f/f :Fabp4-Cre-knockout mice had increased expression of inflammatory genes including F4/80, TNFα, IL-6 and MCP-1 in both white and brown adipose tissues. Moreover, the knockout mice showed decreased expression of the adiponectin gene in the white adipose tissue and UCP1 in the brown adipose tissue, respectively. In contrast, the Sirt6 knockout mice generated by the Adipoq-Cre line (Sirt6^f/f :Adipoq-Cre) only had modest insulin resistance. In conclusion, our data suggest that the function of SIRT6 in the Fabp4-Cre-expressing cells in addition to mature adipocytes plays a critical role in body weight maintenance and metabolic homeostasis.

Multiple pathways of SIRT6 at the crossroads in the control of longevity, cancer, and cardiovascular diseases

Sirtuin 6 (SIRT6) is a member of the sirtuin family NAD⁺-dependent deacetylases with multiple roles in controlling organism homeostasis, lifespan, and diseases. Due to its complex and opposite functional roles, this sirtuin is considered a two-edged sword in health and disease. Indeed, SIRT6 improves longevity, similarly to the founding yeast member, silent information regulator-2 (Sir2), and modulates genome stability, telomere integrity, transcription, and DNA repair. Its deficiency is associated with chronic inflammation, diabetes, cardiac hypertrophy, obesity, liver dysfunction, muscle/adipocyte disorders, and cancer. Besides, pieces of evidence showed that SIRT6 is a promoter of specific oncogenic pathways, thus disclosing its dual role regarding cancer development. Collectively, these findings suggest that multiple mechanisms, to date not entirely known, underlie the intriguing roles of SIRT6. Here we provide an overview of the current molecular mechanisms through which SIRT6 controls cancer and heart diseases, and describe its recent implications in the atherosclerotic plaque development.

Peptostreptococcus anaerobius Induces Intracellular Cholesterol Biosynthesis in Colon Cells to Induce Proliferation and Causes Dysplasia in Mice

BACKGROUND & AIMS

Stool samples from patients with colorectal cancer (CRC) have a higher abundance of Peptostreptococcus anaerobius than stool from individuals without CRC, based on metagenome sequencing. We investigated whether P anaerobius contributes to colon tumor formation in mice and its possible mechanisms of carcinogenesis.

METHODS

We performed quantitative polymerase chain reaction analyses to measure P anaerobius in 112 stool samples and 255 colon biopsies from patients with CRC or advanced adenoma and from healthy individuals (controls) undergoing colonoscopy examination at hospitals in Hong Kong and Beijing. C57BL/6 mice were given broad-spectrum antibiotics, followed by a single dose of azoxymethane, to induce colon tumor formation. Three days later, mice were given P anaerobius or Esherichia coli MG1655 (control bacteria), via gavage, for 6 weeks. Some mice were also given the nicotinamide adenine dinucleotide phosphate oxidase inhibitor apocynin. Intestine tissues were collected and analyzed histologically. The colon epithelial cell line NCM460 and colon cancer cell lines HT-29 and Caco-2 were exposed to P anaerobius or control bacteria; cells were analyzed by immunoblot, proliferation, and bacterial attachment analyses and compared in gene expression profiling studies. Gene expression was knocked down in these cell lines with small interfering RNAs.

RESULTS

P anaerobius was significantly enriched in stool samples from patients with CRC and in biopsies from patients with colorectal adenoma or CRC compared with controls. Mice depleted of bacteria and exposed to azoxymethane and P anaerobius had a higher incidence of intestinal dysplasia (63%) compared with mice not given the bacteria (8.3%; P < .01). P anaerobius mainly colonized the colon compared with the rest of the intestine. Colon cells exposed to P anaerobius had significantly higher levels of proliferation than control cells. We found genes that regulate cholesterol biosynthesis, Toll-like receptor (TLR) signaling, and AMP-activated protein kinase signaling to be significantly up-regulated in cells exposed to P anaerobius. Total cholesterol levels were significantly increased in colon cell lines exposed to P anaerobius via activation of sterol regulatory element-binding protein 2. P anaerobius interacted with TLR2 and TLR4 to increase intracellular levels of reactive oxidative species, which promoted cholesterol synthesis and cell proliferation. Depletion of reactive oxidative species by knockdown of TLR2 or TLR4, or incubation of cells with an antioxidant, prevented P anaerobius from inducing cholesterol biosynthesis and proliferation.

CONCLUSIONS

Levels of P anaerobius are increased in human colon tumor tissues and adenomas compared with non-tumor tissues; this bacteria increases colon dysplasia in a mouse model of CRC. P anaerobius interacts with TLR2 and TLR4 on colon cells to increase levels of reactive oxidative species, which promotes cholesterol synthesis and cell proliferation.

Physiological and therapeutic regulation of PCSK9 activity in cardiovascular disease

Ischemic heart disease is the main cause of death worldwide and is accelerated by increased levels of low-density lipoprotein cholesterol (LDL-C). Proprotein convertase subtilisin/kexin type 9 (PCSK9) is a potent circulating regulator of LDL-C through its ability to induce degradation of the LDL receptor (LDLR) in the lysosome of hepatocytes. Only in the last few years, a number of breakthroughs in the understanding of PCSK9 biology have been reported illustrating how PCSK9 activity is tightly regulated at several levels by factors influencing its transcription, secretion, or by extracellular inactivation and clearance. Two humanized antibodies directed against the LDLR-binding site in PCSK9 received approval by the European and US authorities and additional PCSK9 directed therapeutics are climbing up the phases of clinical trials. The first outcome data of the PCSK9 inhibitor evolocumab reported a significant reduction in the composite endpoint (cardiovascular death, myocardial infarction, or stroke) and further outcome data are awaited. Meanwhile, it became evident that PCSK9 has (patho)physiological roles in several cardiovascular cells. In this review, we summarize and discuss the recent biological and clinical data on PCSK9, the regulation of PCSK9, its extra-hepatic activities focusing on cardiovascular cells, molecular concepts to target PCSK9, and finally briefly summarize the data of recent clinical studies.

Pharmacological Sirt6 inhibition improves glucose tolerance in a type 2 diabetes mouse model

Sirtuin 6 (SIRT6) is a sirtuin family member involved in a wide range of physiologic and disease processes, including cancer and glucose homeostasis. Based on the roles played by SIRT6 in different organs, including its ability to repress the expression of glucose transporters and glycolytic enzymes, inhibiting SIRT6 has been proposed as an approach for treating type 2 diabetes mellitus (T2DM). However, so far, the lack of small-molecule Sirt6 inhibitors has hampered the conduct of in vivo studies to assess the viability of this strategy. We took advantage of a recently identified SIRT6 inhibitor, compound 1, to study the effect of pharmacological Sirt6 inhibition in a mouse model of T2DM (i.e., in high-fat-diet-fed animals). The administration of the Sirt6 inhibitor for 10 d was well tolerated and improved oral glucose tolerance, it increased the expression of the glucose transporters GLUT1 and -4 in the muscle and enhanced the activity of the glycolytic pathway. Sirt6 inhibition also resulted in reduced insulin, triglycerides, and cholesterol levels in plasma. This study represents the first in vivo study of a SIRT6 inhibitor and provides the proof-of-concept that targeting SIRT6 may be a viable strategy for improving glycemic control in T2DM.-Sociali, G., Magnone, M., Ravera, S., Damonte, P., Vigliarolo, T., Von Holtey, M., Vellone, V. G., Millo, E., Caffa, I., Cea, M., Parenti, M. D., Del Rio, A., Murone, M., Mostoslavsky, R., Grozio, A., Nencioni, A., Bruzzone S. Pharmacological Sirt6 inhibition improves glucose tolerance in a type 2 diabetes mouse model.

FOXO transcription factors protect against the diet-induced fatty liver disease

Forkhead O transcription factors (FOXOs) have been implicated in glucose and lipid homeostasis; however, the role of FOXOs in the development of nonalcoholic fatty liver disease (NAFLD) is not well understood. In this study, we designed experiments to examine the effects of two different diets-very high fat diet (HFD) and moderately high fat plus cholesterol diet (HFC)-on wildtype (WT) and liver-specific Foxo1/3/4 triple knockout mice (LTKO). Both diets induced severe hepatic steatosis in the LTKO mice as compared to WT controls. However, the HFC diet led to more severe liver injury and fibrosis compared to the HFD diet. At the molecular levels, hepatic Foxo1/3/4 deficiency triggered a significant increase in the expression of inflammatory and fibrotic genes including Emr1, Ccl2, Col1a1, Tgfb, Pdgfrb, and Timp1. Thus, our data suggest that FOXO transcription factors play a salutary role in the protection against the diet-induced fatty liver disease.

SIRT6: Novel Mechanisms and Links to Aging and Disease

SIRT6, a member of the Sirtuin family of NAD⁺-dependent enzymes, has established roles in chromatin signaling and genome maintenance. Through these functions, SIRT6 protects against aging-associated pathologies including metabolic disease and cancer, and can promote longevity in mice. Research from the past few years revealed that SIRT6 is a complex enzyme with multiple substrates and catalytic activities, and uncovered novel SIRT6 functions in the maintenance of organismal health span. Here, we review these new discoveries and models of SIRT6 biology in four areas: heterochromatin stabilization and silencing; stem cell biology; cancer initiation and progression; and regulation of metabolic homeostasis. We discuss the possible implications of these findings for therapeutic interventions in aging and aging-related disease processes.

Perilipin-2 Deletion Impairs Hepatic Lipid Accumulation by Interfering with Sterol Regulatory Element-binding Protein (SREBP) Activation and Altering the Hepatic Lipidome

Perilipin-2 (PLIN2) is a constitutively associated cytoplasmic lipid droplet coat protein that has been implicated in fatty liver formation in non-alcoholic fatty liver disease. Mice with or without whole-body deletion of perilipin-2 (Plin2-null) were fed either Western or control diets for 30 weeks. Perilipin-2 deletion prevents obesity and insulin resistance in Western diet-fed mice and dramatically reduces hepatic triglyceride and cholesterol levels in mice fed Western or control diets. Gene and protein expression studies reveal that PLIN2 deletion suppressed SREBP-1 and SREBP-2 target genes involved in de novo lipogenesis and cholesterol biosynthetic pathways in livers of mice on either diet. GC-MS lipidomics demonstrate that this reduction correlated with profound alterations in the hepatic lipidome with significant reductions in both desaturation and elongation of hepatic neutral lipid species. To examine the possibility that lipidomic actions of PLIN2 deletion contribute to suppression of SREBP activation, we isolated endoplasmic reticulum membrane fractions from long-term Western diet-fed wild type (WT) and Plin2-null mice. Lipidomic analyses reveal that endoplasmic reticulum membranes from Plin2-null mice are markedly enriched in ω-3 and ω-6 long-chain polyunsaturated fatty acids, which others have shown inhibit SREBP activation and de novo lipogenesis. Our results identify PLIN2 as a determinant of global changes in the hepatic lipidome and suggest the hypothesis that these actions contribute to SREBP-regulated de novo lipogenesis involved in non-alcoholic fatty liver disease.

Tanshinone IIA Modulates Low Density Lipoprotein Uptake via Down-Regulation of PCSK9 Gene Expression in HepG2 Cells

Tanshinone IIA, one of the most pharmacologically bioactive phytochemicals isolated from Salvia miltiorrhiza Bunge, possesses several biological activities such as anti-inflammation, anti-cancer, neuroprotection and hypolipidemic activities. In this study, we aim to investigate the hypocholesterolemic effect of tanshinone IIA in hepatic cells. We demonstrated that tanshinone IIA significantly increased the amount of low-density lipoprotein receptor (LDLR) and LDL uptake activity in HepG2 cells at the post-transcriptional regulation. We further demonstrated that tanshinone IIA inhibited the expression of proprotein convertase subtilisin/kexin type 9 (PCSK9) mRNA and mature protein, which may lead to an increase the cell-surface LDLR in hepatic cells. We further identified a regulatory DNA element involved in the tanshinone IIA-mediated PCSK9 down-regulation, which is located between the -411 and -336 positions of the PCSK9 promoter. Moreover, we found that tanshinone IIA markedly increased the nuclear forkhead box O3a (FoxO3a) level, enhanced FoxO3a/PCSK9 promoter complexes formation and decreased the PCSK9 promoter binding capacity of hepatocyte nuclear factor 1α (HNF-1α), resulting in suppression of PCSK9 gene expression. Finally, we found that the statin-induced PCSK9 overexpression was attenuated and the LDLR activity was elevated in a synergic manner by combination of tanshinone IIA treatment in HepG2 cells. Overall, our results reveal that the tanshinone IIA modulates LDLR level and activity via down-regulation of PCSK9 expression in hepatic cells. Our current findings provide a molecular basis of tanshinone IIA to develop PCSK9 inhibitors for cholesterol management.

MicroRNAs in heart failure: Non-coding regulators of metabolic function

Heart failure (HF) is the inability of the heart to provide sufficient cardiac output for the energy demands of the body. Over the last decades, our understanding of the role of microRNAs (miRNAs), a class of small non-coding RNA regulators of gene expression at the post-transcriptional level, in cardiovascular diseases has expanded at a rapid rate. Importantly, multiple miRNAs have been specifically implicated in the progression of HF. Growing evidence suggests that miRNAs regulate central metabolic pathways and thus are highly implicated in the maintenance of energy homeostasis. In this review, we highlight recent discoveries of the mechanistic role of miRNAs in regulating metabolic functions in HF, with specific focus on the implication of miRNAs in metabolic rearrangements, discuss the potential value of miRNA profiles as novel HF biomarkers, and summarize the recent investigations on therapeutic approaches using miRNAs in heart disease. This article is part of a Special Issue entitled: The role of post-translational protein modifications on heart and vascular metabolism edited by Jason R.B. Dyck & Jan F.C. Glatz.

Deletion of sirtuin 6 accelerates endothelial dysfunction and atherosclerosis in apolipoprotein E-deficient mice

Sirtuin 6 (SIRT6) is a chromatin-associated deacetylase that plays a leading role in genomic stability and aging. However, the precise role of SIRT6 in atherosclerosis, an aging-associated cardiovascular disease, remains elusive. This study aims at defining the role of SIRT6 in atherosclerotic lesion development. SIRT6 messenger RNA and protein expression are markedly decreased in atherosclerotic aortas of apolipoprotein E-deficient (ApoE(-/-)) mice fed a high-cholesterol diet. SIRT6 was knocked down in ApoE(-/-) mice using small hairpin RNAs (shRNAs) lentivirus injection. SIRT6-shRNA-treated ApoE(-/-) mice showed impaired endothelium-dependent vasodilation, increased plaque size (in aortic sinus, aortic root and en face aorta), and augmented plaque vulnerability (evidenced by increased necrotic core areas and macrophage accumulation and reduced collagen content). At the cellular level, SIRT6 depletion by RNA interference in human umbilical vein endothelial cells significantly increased monocyte adhesion to endothelial cells by inducing the expression of intracellular adhesion molecule-1. Consistently, intracellular adhesion molecule-1 expression was significantly upregulated in aortic endothelium of SIRT6-shRNA-treated ApoE(-/-) mice compared with controls. In sum, the aforementioned findings suggest that SIRT6 is a primary negative regulation factor in endothelial dysfunction and atherosclerosis development. As a result, SIRT6 is a promising therapeutic target for treating atherosclerosis and its cardiovascular complications.

Cholesterol metabolism: A review of how ageing disrupts the biological mechanisms responsible for its regulation

Cholesterol plays a vital role in the human body as a precursor of steroid hormones and bile acids, in addition to providing structure to cell membranes. Whole body cholesterol metabolism is maintained by a highly coordinated balancing act between cholesterol ingestion, synthesis, absorption, and excretion. The aim of this review is to discuss how ageing interacts with these processes. Firstly, we will present an overview of cholesterol metabolism. Following this, we discuss how the biological mechanisms which underpin cholesterol metabolism are effected by ageing. Included in this discussion are lipoprotein dynamics, cholesterol absorption/synthesis and the enterohepatic circulation/synthesis of bile acids. Moreover, we discuss the role of oxidative stress in the pathological progression of atherosclerosis and also discuss how cholesterol biosynthesis is effected by both the mammalian target of rapamycin and sirtuin pathways. Next, we examine how diet and alterations to the gut microbiome can be used to mitigate the impact ageing has on cholesterol metabolism. We conclude by discussing how mathematical models of cholesterol metabolism can be used to identify therapeutic interventions.

Two tagSNPs rs352493 and rs3760908 within SIRT6 Gene Are Associated with the Severity of Coronary Artery Disease in a Chinese Han Population

SIRT6 has been demonstrated to exert protective effects on endothelial cells and is closely associated with lipid metabolism, glucose metabolism, and obesity, indicating an important role in the pathogenesis and progression of coronary artery disease (CAD). Nonetheless, the biological significance of SIRT6 variants on CAD is far to be elucidated. Here we aimed to investigate the influence of SIRT6 polymorphisms on individual susceptibility and severity of CAD. Multivariate logistic regression analysis exhibited no significant association between these five polymorphisms and CAD risk in the genotype and allele frequencies. However, we found that the rs352493 polymorphism in SIRT6 exhibited a significant effect on the severity of CAD; C allele (χ² = 7.793, adjusted P = 0.013) and the combined CC/CT genotypes (χ² = 5.609, adjusted P = 0.031) presented the greater CAD severity. In addition, A allele (χ² = 5.208, adjusted P = 0.046) and AA (χ² = 4.842, adjusted P = 0.054) of rs3760908 were also associated with greater CAD severity in Chinese subjects. Our data provided the first evidence that SIRT6 tagSNPs rs352493 and rs3760908 play significant roles in the severity of CAD in Chinese Han subjects, which might be useful predictors of the severity of CAD.

Sequence Variants of SIRT6 Gene Promoter in Myocardial Infarction

AIMS

Coronary artery disease (CAD), including myocardial infarction (MI), is a common complex disease caused by atherosclerosis. Although more than 50 genetic variants have been associated with CAD, these loci collectively account for only 10% of CAD cases. Genetic variants of low and rare frequencies have been proposed as the main causes of CAD. SIRT6, one of the highly conserved NAD-dependent class III deacetylases, has been implicated in cardiovascular diseases. Considering the important roles that SIRT6 plays in the cardiovascular system, inflammation, and lipid and cholesterol metabolism, genetic variants were hypothesized to contribute to MI development.

METHODS

The promoter regions of the SIRT6 gene were genetically analyzed in large cohorts of MI patients (n = 371) and ethnically-matched controls (n = 383).

RESULTS

A total of 15 DNA sequence variants (DSVs) were identified, including seven single-nucleotide polymorphisms (SNPs). Two novel heterozygous DSVs, g.4183823G>C and g.4183742G>A, were identified in two MI patients but in none of the controls. Two SNPs, g.4183685T>C (rs4359565) and g.4182942C>A (rs3760905), were found in MI patients with significantly higher frequencies compared with controls.

CONCLUSIONS

These DSVs identified in MI patients may alter the transcriptional activity of the SIRT6 gene promoter and alter SIRT6 levels which might contribute to the risk of MI.

Noncoding RNAs in Regulation of Cancer Metabolic Reprogramming

Since the description of the Warburg effect 90 years ago, metabolic reprogramming has been gradually recognized as a major hallmark of cancer cells. Mounting evidence now indicates that cancer is a kind of metabolic disease, quite distinct from conventional perception. While metabolic alterations in cancer cells have been extensively observed in glucose, lipid, and amino acid metabolisms, its underlying regulatory mechanisms are still poorly understood. Noncoding RNA, also known as the "dark matter in life," functions through various mechanisms at RNA level regulating different biological pathways. The last two decades have witnessed the booming of noncoding RNA study on microRNA (miRNA), long noncoding RNA (lncRNA), circular RNA (circRNA), PIWI-interacting RNA (piRNA), etc. In this chapter, we will discuss the regulatory roles of noncoding RNAs on cancer metabolism.

Sirtuin 6 regulates glucose-stimulated insulin secretion in mouse pancreatic beta cells

AIMS/HYPOTHESIS

Sirtuin 6 (SIRT6) has been implicated in ageing, DNA repair and metabolism; however, its function in pancreatic beta cells is unclear. The aim of this study is to elucidate the role of SIRT6 in pancreatic beta cells.

METHODS

To investigate the function of SIRT6 in pancreatic beta cells, we performed Sirt6 gene knockdown in MIN6 cells and generated pancreatic- and beta cell-specific Sirt6 knockout mice. Islet morphology and glucose-stimulated insulin secretion (GSIS) were analysed. Glycolysis and oxygen consumption rates in SIRT6-deficient beta cells were measured. Cytosolic calcium was monitored using the Fura-2-AM fluorescent probe (Invitrogen, Grand Island, NY, USA). Mitochondria were analysed by immunoblots and electron microscopy.

RESULTS

Sirt6 knockdown in MIN6 beta cells led to a significant decrease in GSIS. Pancreatic beta cell Sirt6 knockout mice showed a ~50% decrease in GSIS. The knockout mouse islets had lower ATP levels compared with the wild-type controls. Mitochondrial oxygen consumption rates were significantly decreased in the SIRT6-deficient beta cells. Cytosolic calcium dynamics in response to glucose or potassium chloride were attenuated in the Sirt6 knockout islets. Numbers of damaged mitochondria were increased and mitochondrial complex levels were decreased in the SIRT6-deficient islets.

CONCLUSIONS/INTERPRETATION

These data suggest that SIRT6 is important for GSIS from pancreatic beta cells and activation of SIRT6 may be useful to improve insulin secretion in diabetes.

Clinically Relevant Mechanisms of Lipid Synthesis, Transport, and Storage

Lipids not only are fundamental nutrients but also serve as basic structural components of cells and as multifunctional signaling molecules. Lipid metabolism pathways underlie basic processes in health and disease and are the targets of novel therapeutics. In this review, we explore the molecular control of lipid synthesis, trafficking, and storage, with a focus on clinically relevant pathways. To illustrate the clinical relevance of molecular lipid regulation, we highlight how these biochemical processes contribute to the pathogenesis of nonalcoholic fatty liver disease, a component of the metabolic syndrome and a paradigmatic example of lipid dysregulation.

Redox regulation of FoxO transcription factors

Transcription factors of the forkhead box, class O (FoxO) family are important regulators of the cellular stress response and promote the cellular antioxidant defense. On one hand, FoxOs stimulate the transcription of genes coding for antioxidant proteins located in different subcellular compartments, such as in mitochondria (i.e. superoxide dismutase-2, peroxiredoxins 3 and 5) and peroxisomes (catalase), as well as for antioxidant proteins found extracellularly in plasma (e.g., selenoprotein P and ceruloplasmin). On the other hand, reactive oxygen species (ROS) as well as other stressful stimuli that elicit the formation of ROS, may modulate FoxO activity at multiple levels, including posttranslational modifications of FoxOs (such as phosphorylation and acetylation), interaction with coregulators, alterations in FoxO subcellular localization, protein synthesis and stability. Moreover, transcriptional and posttranscriptional control of the expression of genes coding for FoxOs is sensitive to ROS. Here, we review these aspects of FoxO biology focusing on redox regulation of FoxO signaling, and with emphasis on the interplay between ROS and FoxOs under various physiological and pathophysiological conditions. Of particular interest are the dual role played by FoxOs in cancer development and their key role in whole body nutrient homeostasis, modulating metabolic adaptations and/or disturbances in response to low vs. high nutrient intake. Examples discussed here include calorie restriction and starvation as well as adipogenesis, obesity and type 2 diabetes.

Sestrin 3 protein enhances hepatic insulin sensitivity by direct activation of the mTORC2-Akt signaling

Sestrin proteins have been implicated in multiple biological processes including resistance to oxidative and genotoxic stresses, protection against aging-related pathologies, and promotion of metabolic homeostasis; however, the underlying mechanisms are incompletely understood. Some evidence suggests that sestrins may inhibit mTORC1 (mechanistic target of rapamycin complex 1) through inhibition of RagA/B GTPases or activation of AMPK; however, whether sestrins are also involved in mTORC2 regulation and function is unclear. To investigate the functions and mechanisms of Sestrin 3 (Sesn3), we generated Sesn3 liver-specific transgenic and knockout mice. Our data show that Sesn3 liver-specific knockout mice exhibit insulin resistance and glucose intolerance, and Sesn3 transgenic mice were protected against insulin resistance induced by a high-fat diet. Using AMPK liver-specific knockout mice, we demonstrate that the Sesn3 insulin-sensitizing effect is largely independent of AMPK. Biochemical analysis reveals that Sesn3 interacts with and activates mTORC2 and subsequently stimulates Akt phosphorylation at Ser473. These findings suggest that Sesn3 can activate Akt via mTORC2 to regulate hepatic insulin sensitivity and glucose metabolism.

Fibroblast growth factor 21 prevents atherosclerosis by suppression of hepatic sterol regulatory element-binding protein-2 and induction of adiponectin in mice

Supplemental Digital Content is available in the text.

Background—

Fibroblast growth factor 21 (FGF21) is a metabolic hormone with pleiotropic effects on glucose and lipid metabolism and insulin sensitivity. It acts as a key downstream target of both peroxisome proliferator-activated receptor α and γ, the agonists of which have been used for lipid lowering and insulin sensitization, respectively. However, the role of FGF21 in the cardiovascular system remains elusive.

Methods and Results—

The roles of FGF21 in atherosclerosis were investigated by evaluating the impact of FGF21 deficiency and replenishment with recombinant FGF21 in apolipoprotein E^−/− mice. FGF21 deficiency causes a marked exacerbation of atherosclerotic plaque formation and premature death in apolipoprotein E^−/− mice, which is accompanied by hypoadiponectinemia and severe hypercholesterolemia. Replenishment of FGF21 protects against atherosclerosis in apolipoprotein E^−/−mice via 2 independent mechanisms, inducing the adipocyte production of adiponectin, which in turn acts on the blood vessels to inhibit neointima formation and macrophage inflammation, and suppressing the hepatic expression of the transcription factor sterol regulatory element-binding protein-2, thereby leading to reduced cholesterol synthesis and attenuation of hypercholesterolemia. Chronic treatment with adiponectin partially reverses atherosclerosis without obvious effects on hypercholesterolemia in FGF21-deficient apolipoprotein E^−/− mice. By contrast, the cholesterol-lowering effects of FGF21 are abrogated by hepatic expression of sterol regulatory element-binding protein-2.

Conclusions—

FGF21 protects against atherosclerosis via fine tuning the multiorgan crosstalk among liver, adipose tissue, and blood vessels.

Interplay between sirtuins, MYC and hypoxia-inducible factor in cancer-associated metabolic reprogramming

In the early twentieth century, Otto Heinrich Warburg described an elevated rate of glycolysis occurring in cancer cells, even in the presence of atmospheric oxygen (the Warburg effect). Despite the inefficiency of ATP generation through glycolysis, the breakdown of glucose into lactate provides cancer cells with a number of advantages, including the ability to withstand fluctuations in oxygen levels, and the production of intermediates that serve as building blocks to support rapid proliferation. Recent evidence from many cancer types supports the notion that pervasive metabolic reprogramming in cancer and stromal cells is a crucial feature of neoplastic transformation. Two key transcription factors that play major roles in this metabolic reprogramming are hypoxia inducible factor-1 (HIF1) and MYC. Sirtuin-family deacetylases regulate diverse biological processes, including many aspects of tumor biology. Recently, the sirtuin SIRT6 has been shown to inhibit the transcriptional output of both HIF1 and MYC, and to function as a tumor suppressor. In this Review, we highlight the importance of HIF1 and MYC in regulating tumor metabolism and their regulation by sirtuins, with a main focus on SIRT6.

Sirtuins, metabolism, and DNA repair

Cells evolve to actively coordinate nutrient availability with cellular activity in order to maintain metabolic homeostasis. In addition, active pathways to repair DNA damage are crucial to avoid deleterious genomic instability. In recent years, it has become increasingly clear that availability of intermediate metabolites may play an important role in DNA repair, suggesting that these two seemingly distant cellular activities may be highly coordinated. The sirtuin family of proteins now described as deacylases (they can also remove acyl groups other than acetyl moieties), it appears to have evolved to control both metabolism and DNA repair. In this review, we discuss recent advances that lay the foundation to understanding the role of sirtuins in these two biological processes, and the potential crosstalk to coordinate them.

Sirtuins: guardians of mammalian healthspan

The first link between sirtuins and longevity was made 15 years ago in yeast. These initial studies sparked efforts by many laboratories working in diverse model organisms to elucidate the relations between sirtuins, lifespan, and age-associated dysfunction. Here, we discuss the current understanding of how sirtuins relate to aging. We focus primarily on mammalian sirtuins SIRT1, SIRT3, and SIRT6, the three sirtuins for which the most relevant data are available. Strikingly, a large body of evidence now indicates that these and other mammalian sirtuins suppress a variety of age-related pathologies and promote healthspan. Moreover, increased expression of SIRT1 or SIRT6 extends mouse lifespan. Overall, these data point to important roles for sirtuins in promoting mammalian health, and perhaps in modulating the aging process.

PCSK9: a key modulator of cardiovascular health

Since the discovery of proprotein convertase subtilisin kexin 9 (PCSK9) in 2003, this PC has attracted a lot of attention from the scientific community and pharmaceutical companies. Secreted into the plasma by the liver, the proteinase K-like serine protease PCSK9 binds the low-density lipoprotein (LDL) receptor at the surface of hepatocytes, thereby preventing its recycling and enhancing its degradation in endosomes/lysosomes, resulting in reduced LDL-cholesterol clearance. Surprisingly, in a nonenzymatic fashion, PCSK9 enhances the intracellular degradation of all its target proteins. Rare gain-of-function PCSK9 variants lead to higher levels of LDL-cholesterol and increased risk of cardiovascular disease; more common loss-of-function PCSK9 variants are associated with reductions in both LDL-cholesterol and risk of cardiovascular disease. It took 9 years to elaborate powerful new PCSK9-based therapeutic approaches to reduce circulating levels of LDL-cholesterol. Presently, PCSK9 monoclonal antibodies that inhibit its function on the LDL receptor are evaluated in phase III clinical trials. This review will address the biochemical, genetic, and clinical aspects associated with PCSK9's biology and pathophysiology in cells, rodent and human, with emphasis on the clinical benefits of silencing the expression/activity of PCSK9 as a new modality in the treatment of hypercholesterolemia and associated pathologies.

SIRT1 and other sirtuins in metabolism

Sirtuins such as SIRT1 are conserved protein NAD(+)-dependent deacylases and thus their function is intrinsically linked to cellular metabolism. Over the past two decades, accumulating evidence has indicated that sirtuins are not only important energy status sensors but also protect cells against metabolic stresses. Sirtuins regulate the aging process and are themselves regulated by diet and environmental stress. The versatile functions of sirtuins including, more specifically, SIRT1 are supported by their diverse cellular location allowing cells to sense changes in energy levels in the nucleus, cytoplasm, and mitochondrion. SIRT1 plays a critical role in metabolic health by deacetylating many target proteins in numerous tissues, including liver, muscle, adipose tissue, heart, and endothelium. This sirtuin also exerts important systemic effects via the hypothalamus. This review will cover these topics and suggest that strategies to maintain sirtuin activity may be on the horizon to forestall diseases of aging.

Chromatin and beyond: the multitasking roles for SIRT6

In recent years there has been a large expansion in our understanding of SIRT6 biology including its structure, regulation, biochemical activity, and biological roles. SIRT6 functions as an ADP-ribosylase and NAD(+)-dependent deacylase of both acetyl groups and long-chain fatty-acyl groups. Through these functions SIRT6 impacts upon cellular homeostasis by regulating DNA repair, telomere maintenance, and glucose and lipid metabolism, thus affecting diseases such diabetes, obesity, heart disease, and cancer. Such roles may contribute to the overall longevity and health of the organism. Until recently, the known functions of SIRT6 were largely restricted to the chromatin. In this article we seek to describe and expand this knowledge with recent advances in understanding the mechanisms of SIRT6 action and their implications for human biology and disease.

FoxO4 interacts with the sterol regulatory factor SREBP2 and the hypoxia inducible factor HIF2α at the CYP51 promoter

The late steps of cholesterol biosynthesis are oxygen demanding, requiring eleven oxygen molecules per synthesized cholesterol molecule. A key enzymatic reaction, which occurs at the top of the Bloch and Kandutsch-Russell pathways, is the demethylation of lanosterol and dihydrolanosterol (DHL). This reaction is catalyzed by lanosterol 14α demethylase (CYP51) and requires three oxygen molecules. Thus, it is the first step in the distal pathway to be susceptible to oxygen deprivation. Having previously identified that the forkhead transcription factor 4 (FoxO4) represses CYP51 expression, we aimed to characterize its role at the CYP51 promoter. Hypoxia-treated 3T3L1 cells showed decreased cholesterol biosynthesis, accumulation of lanosterol/DHL, and stimulation of FoxO4 expression and its cytoplasmic translocation to the nucleus. Transfection assays with a CYP51 promoter reporter gene revealed that FoxO4 and sterol regulatory element binding protein (SREBP)2 exert a stimulatory effect, whereas FoxO4 and the hypoxia inducible factor (HIF)2α repress CYP51 promoter activity. Electromobility shift, chromatin immunoprecipitation, pull-down, and coimmunoprecipitation assays show that FoxO4 interacts with SREBP2 and HIF2α to modulate CYP51 promoter activity. We also show an inverse correlation between FoxO4 and CYP51 in adipose tissue of ob/ob mice and mouse fetal cortical neurons exposed to hypoxia. Overall, these studies demonstrate a role for FoxO4 in the regulation of CYP51 expression.

FoxO3 transcription factor and Sirt6 deacetylase regulate low density lipoprotein (LDL)-cholesterol homeostasis via control of the proprotein convertase subtilisin/kexin type 9 (Pcsk9) gene expression

Elevated LDL-cholesterol is a risk factor for the development of cardiovascular disease. Thus, proper control of LDL-cholesterol homeostasis is critical for organismal health. Genetic analysis has identified PCSK9 (proprotein convertase subtilisin/kexin type 9) as a crucial gene in the regulation of LDL-cholesterol via control of LDL receptor degradation. Although biochemical characteristics and clinical implications of PCSK9 have been extensively investigated, epigenetic regulation of this gene is largely unknown. In this work we have discovered that Sirt6, an NAD(+)-dependent histone deacetylase, plays a critical role in the regulation of the Pcsk9 gene expression in mice. Hepatic Sirt6 deficiency leads to elevated Pcsk9 gene expression and LDL-cholesterol as well. Mechanistically, we have demonstrated that Sirt6 can be recruited by forkhead transcription factor FoxO3 to the proximal promoter region of the Pcsk9 gene and deacetylates histone H3 at lysines 9 and 56, thereby suppressing the gene expression. Also remarkably, overexpression of Sirt6 in high fat diet-fed mice lowers LDL-cholesterol. Overall, our data suggest that FoxO3 and Sirt6, two longevity genes, can reduce LDL-cholesterol levels through regulation of the Pcsk9 gene.