Hepatic FoxOs regulate lipid metabolism via modulation of expression of the nicotinamide phosphoribosyltransferase gene

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.

Oleate ameliorates palmitate-induced reduction of NAMPT activity and NAD levels in primary human hepatocytes and hepatocarcinoma cells

BACKGROUND

Nicotinamide phosphoribosyltransferase (NAMPT) and nicotinamide adenine dinucleotide (NAD) levels are crucial for liver function. The saturated fatty acid palmitate and the unsaturated fatty acid oleate are the main free fatty acids in adipose tissue and human diet. We asked how these fatty acids affect cell survival, NAMPT and NAD levels in HepG2 cells and primary human hepatocytes.

METHODS

HepG2 cells were stimulated with palmitate (0.5mM), oleate (1mM) or a combination of both (0.5mM/1mM) as well as nicotinamide mononucleotide (NMN) (0.5 mM) or the specific NAMPT inhibitor FK866 (10nM). Cell survival was measured by WST-1 assay and Annexin V/propidium iodide staining. NAD levels were determined by NAD/NADH Assay or HPLC. Protein and mRNA levels were analysed by Western blot analyses and qPCR, respectively. NAMPT enzyme activity was measured using radiolabelled 14C-nicotinamide. Lipids were stained by Oil red O staining.

RESULTS

Palmitate significantly reduced cell survival and induced apoptosis at physiological doses. NAMPT activity and NAD levels significantly declined after 48h of palmitate. In addition, NAMPT mRNA expression was enhanced which was associated with increased NAMPT release into the supernatant, while intracellular NAMPT protein levels remained stable. Oleate alone did not influence cell viability and NAMPT activity but ameliorated the negative impact of palmitate on cell survival, NAMPT activity and NAD levels, as well as the increased NAMPT mRNA expression and secretion. NMN was able to normalize intracellular NAD levels but did not ameliorate cell viability after co-stimulation with palmitate. FK866, a specific NAMPT inhibitor did not influence lipid accumulation after oleate-treatment.

CONCLUSIONS

Palmitate targets NAMPT activity with a consequent cellular depletion of NAD. Oleate protects from palmitate-induced apoptosis and variation of NAMPT and NAD levels. Palmitate-induced cell stress leads to an increase of NAMPT mRNA and accumulation in the supernatant. However, the proapoptotic action of palmitate seems not to be mediated by decreased NAD levels.

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.

Inhibition of nicotinamide phosphoribosyltransferase and depletion of nicotinamide adenine dinucleotide contribute to arsenic trioxide suppression of oral squamous cell carcinoma

Emerging evidence suggests that increased nicotinamide phosphoribosyltransferase (NAMPT) expression is associated with the development and prognosis of many cancers, but it remains unknown regarding its role in oral squamous cell carcinoma (OSCC). In the present study, the results from tissue microarray showed that NAMPT was overexpressed in OSCC patients and its expression level was directly correlated with differential grades of cancer. Interestingly, treatment of OSCC cells with chemotherapy agent arsenic trioxide (ATO) decreased the levels of NAMPT protein and increased cellular death in an ATO dose- and time-dependent manner. Most importantly, combination of low concentration ATO with FK866 (a NAMPT inhibitor) exerted enhanced inhibitive effect on NAMPT protein and mRNA expressions, leading to synergistic cytotoxicity on cancer cells through increasing cell apoptosis and depleting intracellular nicotinamide adenine dinucleotide levels. These findings demonstrate the crucial role of NAMPT in the prognosis of OSCC and reveal inhibition of NAMPT as a novel mechanism of ATO in suppressing cancer cell growth. Our results suggest that ATO can significantly enhance therapeutic efficacy of NAMPT inhibitor, and combined treatment may be a novel and effective therapeutic strategy for OSCC patients.

Regulation of Nampt expression by transcriptional coactivator NCOA6 in pancreatic β-cells

Nuclear receptor coactivator 6 (NCOA6) is a transcriptional coactivator and crucial for insulin secretion and glucose metabolism in pancreatic β-cells. However, the regulatory mechanism of β-cell function by NCOA6 is largely unknown. In this study, we found that the transcript levels of nicotinamide phosphoribosyltransferase (Nampt) were decreased in islets of NCOA6^+/- mice compared with NCOA6^+/+ mice. Moreover, NCOA6 overexpression increased the levels of Nampt transcripts in the mouse pancreatic β-cell line NIT-1. Promoter analyses showed that transcriptional activity of the Nampt promoter was stimulated by cooperation of sterol regulatory element binding protein-1c (SREBP-1c) and NCOA6. Additional studies using mutant promoters demonstrated that SREBP-1c activates Nampt promoter through the sterol regulatory element (SRE), but not through the E-box. Using chromatin immunoprecipitation assay, NCOA6 was also shown to be directly recruited to the SRE region of the Nampt promoter. Furthermore, treatment with nicotinamide mononucleotide (NMN), a product of the Nampt reaction and a key NAD⁺ intermediate, ameliorates glucose-stimulated insulin secretion from NCOA6^+/- islets. These results suggest that NCOA6 stimulates insulin secretion, at least partially, by modulating Nampt expression in pancreatic β-cells.

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.

Adipose tissue NAD+ biology in obesity and insulin resistance: From mechanism to therapy

Nicotinamide adenine dinucleotide (NAD⁺ ) biosynthetic pathway, mediated by nicotinamide phosphoribosyltransferase (NAMPT), a key NAD⁺ biosynthetic enzyme, plays a pivotal role in controlling many biological processes, such as metabolism, circadian rhythm, inflammation, and aging. Over the past decade, NAMPT-mediated NAD⁺ biosynthesis, together with its key downstream mediator, namely the NAD⁺ -dependent protein deacetylase SIRT1, has been demonstrated to regulate glucose and lipid metabolism in a tissue-dependent manner. These discoveries have provided novel mechanistic and therapeutic insights into obesity and its metabolic complications, such as insulin resistance, an important risk factor for developing type 2 diabetes and cardiovascular disease. This review will focus on the importance of adipose tissue NAMPT-mediated NAD⁺ biosynthesis and SIRT1 in the pathophysiology of obesity and insulin resistance. We will also critically explore translational and clinical aspects of adipose tissue NAD⁺ biology.

Nicotinamide protects hepatocytes against palmitate-induced lipotoxicity via SIRT1-dependent autophagy induction

Lipotoxicity induced by saturated fatty acids (SFAs) plays a pathological role in the development of non-alcoholic fatty liver disease (NAFLD); however, the exact mechanism remains to be clearly elucidated. Palmitate is the most abundant SFA in the circulation and major lipotoxic inducer. Accumulating evidence supports that autophagy induction is protective against palmitate-induced cell death in a variety of cell types, including hepatocytes. Nicotinamide is the amide form of nicotinic acid (vitamin B3, Niacin) and a dietary supplementation as a source of vitamin B3. We previously reported that nicotinamide endowed hepatocytes resistance to palmitate-induced ER stress via up-regulating SIRT1, with cAMP/PKA/CREB pathway activation being a fundamental mechanism. This study was undertaken to investigate the potential anti-lipotoxic effect of nicotinamide and to elucidate underlying mechanism(s). Our data demonstrated that nicotinamide supplementation protected hepatocytes against palmitate-induced cell death. Mechanistic investigations revealed that nicotinamide supplementation activated autophagy in hepatocytes. Importantly, we showed that the anti-lipotoxic property of nicotinamide was abolished by autophagy inhibitors, suggesting that autophagy induction plays a mechanistic role in nicotinamide's anti-lipotoxic effect. Furthermore, we showed that SIRT1 inhibition blunted autophagy induction in response to nicotinamide supplementation and similarly abrogated the anti-lipotoxic effect conferred by nicotinamide supplementation. In conclusion, our data suggest that nicotinamide protects against palmitate-induced hepatotoxicity via SIRT1-dependent autophagy induction and that nicotinamide supplementation may represent a therapeutic choice for NAFLD.

Nicotinamide adenine dinucleotide biosynthesis promotes liver regeneration

The regenerative capacity of the liver is essential for recovery from surgical resection or injuries induced by trauma or toxins. During liver regeneration, the concentration of nicotinamide adenine dinucleotide (NAD) falls, at least in part due to metabolic competition for precursors. To test whether NAD availability restricts the rate of liver regeneration, we supplied nicotinamide riboside (NR), an NAD precursor, in the drinking water of mice subjected to partial hepatectomy. NR increased DNA synthesis, mitotic index, and mass restoration in the regenerating livers. Intriguingly, NR also ameliorated the steatosis that normally accompanies liver regeneration. To distinguish the role of hepatocyte NAD levels from any systemic effects of NR, we generated mice overexpressing nicotinamide phosphoribosyltransferase, a rate-limiting enzyme for NAD synthesis, specifically in the liver. Nicotinamide phosphoribosyltransferase overexpressing mice were mildly hyperglycemic at baseline and, similar to mice treated with NR, exhibited enhanced liver regeneration and reduced steatosis following partial hepatectomy. Conversely, mice lacking nicotinamide phosphoribosyltransferase in hepatocytes exhibited impaired regenerative capacity that was completely rescued by administering NR.

CONCLUSION

NAD availability is limiting during liver regeneration, and supplementation with precursors such as NR may be therapeutic in settings of acute liver injury. (Hepatology 2017;65:616-630).

NAD+ salvage pathway in cancer metabolism and therapy

Nicotinamide adenine dinucleotide (NAD⁺) is an essential coenzyme for various physiological processes including energy metabolism, DNA repair, cell growth, and cell death. Many of these pathways are typically dysregulated in cancer cells, making NAD⁺ an intriguing target for cancer therapeutics. NAD⁺ is mainly synthesized by the NAD⁺ salvage pathway in cancer cells, and not surprisingly, the pharmacological targeting of the NAD⁺ salvage pathway causes cancer cell cytotoxicity in vitro and in vivo. Several studies have described the precise consequences of NAD⁺ depletion on cancer biology, and have demonstrated that NAD+ depletion results in depletion of energy levels through lowered rates of glycolysis, reduced citric acid cycle activity, and decreased oxidative phosphorylation. Additionally, depletion of NAD⁺ causes sensitization of cancer cells to oxidative damage by disruption of the anti-oxidant defense system, decreased cell proliferation, and initiation of cell death through manipulation of cell signaling pathways (e.g., SIRT1 and p53). Recently, studies have explored the effect of well-known cancer therapeutics in combination with pharmacological depletion of NAD⁺ levels, and found in many cases a synergistic effect on cancer cell cytotoxicity. In this context, we will discuss the effects of NAD⁺ salvage pathway inhibition on cancer cell biology and provide insight on this pathway as a novel anti-cancer therapeutic target.

Nonalcoholic fatty liver disease: cause or consequence of type 2 diabetes?

Growing epidemiological evidence suggests that nonalcoholic fatty liver disease (NAFLD) is an early predictor of and determinant for the development of type 2 diabetes and other features of the metabolic syndrome. This finding may have important clinical implications for the diagnosis, prevention and treatment of type 2 diabetes and its chronic complications. However, given the complex and bi-directional relationships between NAFLD, insulin resistance and chronic hyperglycaemia, it is extremely difficult to distinguish whether NAFLD is a cause or a consequence of insulin resistance and type 2 diabetes. Indeed, at the molecular level, hepatic lipogenesis and hepatic glucose production depend on differentially regulated branches of the insulin signalling pathway. Furthermore, genetic studies suggest that excess hepatic fat is associated with progressive liver disease, but does not always increase the risk of incident type 2 diabetes. Here, we will briefly review the epidemiological, pathophysiological and molecular evidence linking NAFLD to the development of type 2 diabetes. We will also discuss some recent genetic and therapeutic advances that seem to challenge a causal role of NAFLD in the pathogenesis type 2 diabetes, and propose a working hypothesis to explain this apparent conundrum. In conclusion, progressive liver disease and type 2 diabetes are divergent though inter-related consequences of insulin resistance and the metabolic syndrome.

Targeting hepatic glucose metabolism in the treatment of type 2 diabetes

Type 2 diabetes mellitus is characterized by the dysregulation of glucose homeostasis, resulting in hyperglycaemia. Although current diabetes treatments have exhibited some success in lowering blood glucose levels, their effect is not always sustained and their use may be associated with undesirable side effects, such as hypoglycaemia. Novel antidiabetic drugs, which may be used in combination with existing therapies, are therefore needed. The potential of specifically targeting the liver to normalize blood glucose levels has not been fully exploited. Here, we review the molecular mechanisms controlling hepatic gluconeogenesis and glycogen storage, and assess the prospect of therapeutically targeting associated pathways to treat type 2 diabetes.

Hepatic NAD(+) deficiency as a therapeutic target for non-alcoholic fatty liver disease in ageing

BACKGROUND AND PURPOSE

Ageing is an important risk factor of non-alcoholic fatty liver disease (NAFLD). Here, we investigated whether the deficiency of nicotinamide adenine dinucleotide (NAD(+) ), a ubiquitous coenzyme, links ageing with NAFLD.

EXPERIMENTAL APPROACH

Hepatic concentrations of NAD(+) , protein levels of nicotinamide phosphoribosyltransferase (NAMPT) and several other critical enzymes regulating NAD(+) biosynthesis, were compared in middle-aged and aged mice or patients. The influences of NAD(+) decline on the steatosis and steatohepatitis were evaluated in wild-type and H247A dominant-negative, enzymically-inactive NAMPT transgenic mice (DN-NAMPT) given normal or high-fat diet (HFD).

KEY RESULTS

Hepatic NAD(+) level decreased in aged mice and humans. NAMPT-controlled NAD(+) salvage, but not de novo biosynthesis pathway, was compromised in liver of elderly mice and humans. Given normal chow, middle-age DN-NAMPT mice displayed systemic NAD(+) reduction and had moderate NAFLD phenotypes, including lipid accumulation, enhanced oxidative stress, triggered inflammation and impaired insulin sensitivity in liver. All these NAFLD phenotypes, especially release of pro-inflammatory factors, Kupffer cell accumulation, monocytes infiltration, NLRP3 inflammasome pathway and hepatic fibrosis (Masson's staining and α-SMA staining), deteriorated further under HFD challenge. Oral administration of nicotinamide riboside, a natural NAD(+) precursor, completely corrected these NAFLD phenotypes induced by NAD(+) deficiency alone or HFD, whereas adenovirus-mediated SIRT1 overexpression only partially rescued these phenotypes.

CONCLUSIONS AND IMPLICATIONS

These results provide the first evidence that ageing-associated NAD(+) deficiency is a critical risk factor for NAFLD, and suggest that supplementation with NAD(+) substrates may be a promising therapeutic strategy to prevent and treat NAFLD.

Nicotinamide phosphoribosyltransferase (Nampt) in carcinogenesis: new clinical opportunities

INTRODUCTION

Nicotinamide phosphoribosyltransferase (Nampt) is the rate-limiting enzyme that catalyzes the first step in the mammalian nicotinamide adenine dinucleotide (NAD) salvage pathway. Aberrant NAD metabolism was associated with oncogenic signal transduction, suggesting the critical roles of Nampt in tumorigenesis and metastasis. Additionally, Nampt can be secreted out of the cell, and this extracellular form of Nampt (eNampt) was shown to induce inflammation and angiogenesis due to its cytokine activity, which may also be involved in carcinogenesis.

AREAS COVERED

This article reviews recent advances in the studies of Nampt in carcinogenesis, with a special highlight on Nampt inhibitors and future clinical application, including cancer diagnosis, prognosis and therapy. Expert commentary: Nampt not only maintains the balance of cellular metabolism, but also has a profound influence on multiple aspects of carcinogenesis. Therefore, elucidation of these mechanisms opens the door for future clinical applications targeting this protein. Additional studies are needed to address important questions including the relationship between extracellular Nampt and carcinogenesis.

Intestinal inflammation requires FOXO3 and prostaglandin E2-dependent lipogenesis and elevated lipid droplets

Intestinal inflammation has been recently characterized by the dysregulation of lipids as metabolic and energy sources, revealing a novel feature of its pathophysiology. Because intracellular lipids, stored in dynamic lipid droplets (LDs), provide energy for cellular needs, we investigated whether they play a role in intestinal inflammation. In the inflamed intestine of mice, elevated LDs were found in colonic and infiltrating immune cells as shown by staining for the LD coat protein PLIN2 and for lipids with BODIPY. In colonic cells, TNF stimulated LD increases by receptor signaling rely on phosphatidylinositol 3-kinase activation. Downstream, TNF triggered a negative regulatory loop between LDs and the transcription factor FOXO3. This was shown in the colon of Foxo3-deficient mice, where elevation in PLIN2 and lipids were further facilitated by inflammation and were more prominent relative to wild-type, whereas, in colonic cells, inhibition of lipogenesis blocked the TNF-mediated loss of FOXO3. Furthermore, blockade of PGE2 synthesis abrogated TNF-stimulated increases in LDs and FOXO3 inactivation. We found in colonic tissue of Foxo3-deficient mice higher levels of cyclooxygenase-2, a mediator of prostaglandin E2 (PGE2) synthesis, supporting involvement of PGE2 in the LD-FOXO3 regulatory loop. Ultimately, TNF-stimulated lipogenesis leading to elevated LDs facilitated NF-κB-mediated increases in IL-8 protein, which is associated with the surface of LDs found in the lumina of the endoplasmic reticulum and Golgi apparatus. This novel immunometabolic mechanism of colonic inflammation involving elevated LDs could provide opportunities for new treatment options.

A conjugated fatty acid present at high levels in bitter melon seed favorably affects lipid metabolism in hepatocytes by increasing NAD(+)/NADH ratio and activating PPARα, AMPK and SIRT1 signaling pathway

α-Eleostearic acid (α-ESA), or the cis-9, trans-11, trans-13 isomer of conjugated linolenic acid, is a special fatty acid present at high levels in bitter melon seed oil. The aim of this study was to examine the effect of α-ESA on hepatic lipid metabolism. Using H4IIEC3 hepatoma cell line, we showed that α-ESA significantly lowered intracellular triglyceride accumulation compared to α-linolenic acid (LN), used as a fatty acid control, in a dose- and time-dependent manner. The effects of α-ESA on enzyme activities and mRNA profiles in H4IIEC3 cells suggested that enhanced fatty acid oxidation and lowered lipogenesis were involved in α-ESA-mediated triglyceride lowering effects. In addition, α-ESA triggered AMP-activated protein kinase (AMPK) activation without altering sirtuin 1 (SIRT1) protein levels. When cells were treated with vehicle control (VC), LN alone (LN; 100μmol/L) or in combination with α-ESA (LN+α-ESA; 75+25μmol/L) for 24h, acetylation of forkhead box protein O1 was decreased, while the NAD(+)/NADH ratio, mRNA levels of NAMPT and PTGR1 and enzyme activity of nicotinamide phosphoribosyltransferase were increased by LN+α-ESA treatment compared to treatment with LN alone, suggesting that α-ESA activates SIRT1 by increasing NAD(+) synthesis and NAD(P)H consumption. The antisteatosis effect of α-ESA was confirmed in mice treated with a high-sucrose diet supplemented with 1% α-ESA for 5weeks. We conclude that α-ESA favorably affects hepatic lipid metabolism by increasing cellular NAD(+)/NADH ratio and activating PPARα, AMPK and SIRT1 signaling pathways.

Nicotinamide ameliorates palmitate-induced ER stress in hepatocytes via cAMP/PKA/CREB pathway-dependent Sirt1 upregulation

Nicotinamide (NAM) is the amide of nicotinic acid and a predominant precursor for NAD(+) biosynthesis via the salvage pathway. Sirt1 is a NAD(+)-dependent deacetylase, playing an important role in regulating cellular functions. Although hepatoprotective effect of NAM has been reported, the underlying mechanism remains elusive. ER stress, induced by saturated fatty acids, in specific palmitate, plays a pathological role in the development of nonalcoholic fatty liver disease. This study aims to determine the effect of NAM on palmitate-induced ER stress in hepatocytes and to elucidate molecular mechanisms behind. Both HepG2 cells and primary mouse hepatocytes were exposed to palmitate (conjugated to BSA at a 2:1 M ratio), NAM, or their combination for different durations. Cellular NAD(+) level, Sirt1 expression/activity, ER stress, as well as cAMP/PKA/CREB pathway activation were determined. NAM increased Sirt1 expression and enzymatic activity, which contributes to the ameliorative effect of NAM on palmitate-triggered ER stress. NAM increased intracellular NAD(+) level in hepatocytes, however, blocking the salvage pathway, a pathway for NAD(+) synthesis from NAM, only partially prevented NAM-induced Sirt1 upregulation while completely prevented NAD+ increase in response to NAM. Further mechanistic investigations revealed that NAM elevated intracellular cAMP level via suppressing PDE activity, leading to downstream PKA and CREB activation. Importantly, cAMP/PKA/CREB pathway blockade abolished not only NAM-induced Sirt1 upregulation, but also its protective effect against ER stress. Our results demonstrate that NAM protects hepatocytes against palmitate-induced ER stress in hepatocytes via upregulating Sirt1. Activation of the cAMP/PKA/CREB pathway plays a key role in NAM-induced Sirt1 upregulation.

The new biology of diabetes

Until recently, type 2 diabetes was seen as a disease caused by an impaired ability of insulin to promote the uptake and utilisation of glucose. Work on forkhead box protein O (FOXO) transcription factors revealed new aspects of insulin action that have led us to articulate a liver- and beta cell-centric narrative of diabetes pathophysiology and treatment. FOXO integrate a surprisingly diverse subset of biological functions to promote metabolic flexibility. In the liver, they controls the glucokinase/glucose-6-phosphatase switch and bile acid pool composition, directing carbons to glucose or lipid utilisation, thus providing a unifying mechanism for the two abnormalities of the diabetic liver: excessive glucose production and increased lipid synthesis and secretion. Moreover, FOXO are necessary to maintain beta cell differentiation, and diabetes development is associated with a gradual loss of FOXO function that brings about beta cell dedifferentiation. We proposed that dedifferentiation is the main cause of beta cell failure and conversion into non-beta endocrine cells, and that treatment should restore beta cell differentiation. Our studies investigating these proposals have revealed new dimensions to the pathophysiology of diabetes that can be leveraged to design new therapies.

The diabetes gene Zfp69 modulates hepatic insulin sensitivity in mice

AIMS/HYPOTHESIS

Zfp69 was previously identified by positional cloning as a candidate gene for obesity-associated diabetes. C57BL/6J and New Zealand obese (NZO) mice carry a loss-of-function mutation due to the integration of a retrotransposon. On the NZO background, the Zfp69 locus caused severe hyperglycaemia and loss of beta cells. To provide direct evidence for a causal role of Zfp69, we investigated the effects of its overexpression on both a lean [B6-Tg(Zfp69)] and an obese [NZO/B6-Tg(Zfp69)] background.

METHODS

Zfp69 transgenic mice were generated by integrating the cDNA into the ROSA locus of the C57BL/6 genome and characterised.

RESULTS

B6-Tg(Zfp69) mice were normoglycaemic, developed hyperinsulinaemia, and exhibited increased expression of G6pc and Pck1 and slightly reduced phospho-Akt levels in the liver. During OGTTs, glucose clearance was normal but insulin levels were significantly higher in the B6-Tg(Zfp69) than in control mice. The liver fat content and plasma triacylglycerol levels were significantly increased in B6-Tg(Zfp69) and NZO/B6-Tg(Zfp69) mice on a high-fat diet compared with controls. Liver transcriptome analysis of B6-Tg(Zfp69) mice revealed a downregulation of genes involved in glucose and lipid metabolism. Specifically, expression of Nampt, Lpin2, Map2k6, Gys2, Bnip3, Fitm2, Slc2a2, Ppargc1α and Insr was significantly decreased in the liver of B6-Tg(Zfp69) mice compared with wild-type animals. However, overexpression of Zfp69 did not induce overt diabetes with hyperglycaemia and beta cell loss.

CONCLUSIONS/INTERPRETATION

Zfp69 mediates hyperlipidaemia, liver fat accumulation and mild insulin resistance. However, it does not induce type 2 diabetes, suggesting that the diabetogenic effect of the Zfp69 locus requires synergy with other as yet unidentified genes.

Hepatic NAD salvage pathway is enhanced in mice on a high-fat diet

Nicotinamide phosphoribosyltransferase (Nampt) is the rate-limiting enzyme for NAD salvage and the abundance of Nampt has been shown to be altered in non-alcoholic fatty liver disease. It is, however, unknown how hepatic Nampt is regulated in response to accumulation of lipids in the liver of mice fed a high-fat diet (HFD). HFD mice gained more weight, stored more hepatic lipids and had an impaired glucose tolerance compared with control mice. NAD levels as well as Nampt mRNA expression, protein abundance and activity were significantly increased in HFD mice. Enhanced NAD levels were associated with deacetylation of p53 and Nfκb indicating increased activation of Sirt1. Despite impaired glucose tolerance and increased hepatic lipid levels in HFD mice, NAD metabolism was significantly enhanced. Thus, improved NAD metabolism may be a compensatory mechanism to protect against negative impact of hepatic lipid accumulation.

Physiological and pathophysiological roles of NAMPT and NAD metabolism

Nicotinamide phosphoribosyltransferase (NAMPT) is a regulator of the intracellular nicotinamide adenine dinucleotide (NAD) pool. NAD is an essential coenzyme involved in cellular redox reactions and is a substrate for NAD-dependent enzymes. In various metabolic disorders and during ageing, levels of NAD are decreased. Through its NAD-biosynthetic activity, NAMPT influences the activity of NAD-dependent enzymes, thereby regulating cellular metabolism. In addition to its enzymatic function, extracellular NAMPT (eNAMPT) has cytokine-like activity. Abnormal levels of eNAMPT are associated with various metabolic disorders. NAMPT is able to modulate processes involved in the pathogenesis of obesity and related disorders such as nonalcoholic fatty liver disease (NAFLD) and type 2 diabetes mellitus (T2DM) by influencing the oxidative stress response, apoptosis, lipid and glucose metabolism, inflammation and insulin resistance. NAMPT also has a crucial role in cancer cell metabolism, is often overexpressed in tumour tissues and is an experimental target for antitumour therapies. In this Review, we discuss current understanding of the functions of NAMPT and highlight progress made in identifying the physiological role of NAMPT and its relevance in various human diseases and conditions, such as obesity, NAFLD, T2DM, cancer and ageing.

Sirtuin and metabolic kidney disease

Sirtuin is a nicotinamide adenine dinucleotide–dependent deacetylase. One of its isoforms, Sirt1, is a key molecule in glucose, lipid, and energy metabolism. The renal protective effects of Sirt1 are found in various models of renal disorders with metabolic impairment, such as diabetic nephropathy. Protective effects include the maintenance of glomerular barrier function, anti–fibrosis effects, anti–oxidative stress effects, and regulation of mitochondria function and energy metabolism. Various target molecules subject to direct deacetylation or epigenetic gene regulation have been identified as effectors of the renal protective function of sirtuin. Recently, it was demonstrated that Sirt1 expression decreases in proximal tubules before albuminuria in a mouse model of diabetic nephropathy, and that albuminuria is suppressed in proximal tubule–specific mice overexpressing Sirt1. These findings suggest that decreased Sirt1 expression in proximal tubular cells causes abnormal nicotine metabolism and reduces the supply of nicotinamide mononucleotide from renal tubules to glomeruli. This further decreases expression of Sirt1 in glomerular podocytes and increases expression of a tight junction protein, claudin-1, which results in albuminuria. Activators of the sirtuin family of proteins, including resveratrol, may be important in the development of new therapeutic strategies for treating metabolic kidney diseases, including diabetic nephropathy.

A mutant allele encoding DNA binding-deficient FoxO1 differentially regulates hepatic glucose and lipid metabolism

Insulin signaling in the liver blunts glucose production and stimulates triglyceride biosynthesis. FoxO1 is required for cAMP induction of hepatic glucose production and is permissive for the effect of insulin to suppress this process. Moreover, FoxO1 ablation increases lipogenesis. In this study, we investigated the pleiotropic actions of FoxO1 on glucose and lipid metabolism. To this end, we reconstituted FoxO1 function in mice with a liver-specific deletion of Foxo1 using targeted knock-in of an allele encoding a DNA binding-deficient FoxO1 mutant (L-DBD). Chow-reared L-DBD mice showed defects in hepatic glucose production but normal liver triglyceride content despite increased rates of de novo lipogenesis and impaired fatty acid oxidation in isolated hepatocytes. Gene expression studies indicated that FoxO1 regulates the expression of glucokinase via a cell-nonautonomous coregulatory mechanism, while its regulation of glucose-6-phosphatase proceeds via a cell-autonomous action as a direct transcriptional activator. These conclusions support a differential regulation of hepatic glucose and lipid metabolism by FoxO1 based on the mechanism by which it alters the expression of key target genes involved in each process.

Sirtuin 1 signaling and alcoholic fatty liver disease

Alcoholic fatty liver disease (AFLD) is one of the most prevalent forms of liver disease worldwide and can progress to inflammation (hepatitis), fibrosis/cirrhosis, and ultimately lead to end stage liver injury. The mechanisms, by which ethanol consumption leads to AFLD, are complicated and multiple, and remain incompletely understood. Nevertheless, understanding its pathogenesis will facilitate the development of effective pharmacological or nutritional therapies for treating human AFLD. Chronic ethanol consumption causes steatosis and inflammation in rodents or humans by disturbing several important hepatic transcriptional regulators, including AMP-activated kinase (AMPK), lipin-1, sterol regulatory element binding protein 1 (SREBP-1), PPARγ co-activator-1α (PGC-1α), and nuclear transcription factor-κB (NF-κB). Remarkably, the effects of ethanol on these regulators are mediated in whole or in part by inhibition of a central signaling molecule, sirtuin 1 (SIRT1), which is a nicotinamide adenine dinucleotide (NAD(+), NADH)-dependent class III protein deacetylase. In recent years, SIRT1 has emerged as a pivotal molecule controlling the pathways of hepatic lipid metabolism, inflammatory responses and in the development of AFLD in rodents and in humans. Ethanol-mediated SIRT1 inhibition suppresses or stimulates the activities of above described transcriptional regulators and co-regulators, thereby deregulating diverse lipid metabolism and inflammatory response pathways including lipogenesis, fatty acid β-oxidation, lipoprotein uptake and secretion and expression of pro-inflammatory cytokines in the liver. This review aims to highlight our current understanding of SIRT1 regulatory mechanisms and its response to ethanol-induced toxicity, thus, affirming significant role of SIRT1 signaling in the development of AFLD.

Regulation of NAD biosynthetic enzymes modulates NAD-sensing processes to shape mammalian cell physiology under varying biological cues

In addition to its role as a redox coenzyme, NAD is a substrate of various enzymes that split the molecule to either catalyze covalent modifications of target proteins or convert NAD into biologically active metabolites. The coenzyme bioavailability may be significantly affected by these reactions, with ensuing major impact on energy metabolism, cell survival, and aging. Moreover, through the activity of the NAD-dependent deacetylating sirtuins, NAD behaves as a beacon molecule that reports the cell metabolic state, and accordingly modulates transcriptional responses and metabolic adaptations. In this view, NAD biosynthesis emerges as a highly regulated process: it enables cells to preserve NAD homeostasis in response to significant NAD-consuming events and it can be modulated by various stimuli to induce, via NAD level changes, suitable NAD-mediated metabolic responses. Here we review the current knowledge on the regulation of mammalian NAD biosynthesis, with focus on the relevant rate-limiting enzymes. This article is part of a Special Issue entitled: Cofactor-dependent proteins: evolution, chemical diversity and bio-applications.

Integrated control of hepatic lipogenesis versus glucose production requires FoxO transcription factors

Insulin integrates hepatic glucose and lipid metabolism, directing nutrients to storage as glycogen and triglyceride. In type 2 diabetes, levels of the former are low and the latter are exaggerated, posing a pathophysiologic and therapeutic conundrum. A branching model of insulin signaling, with FoxO1 presiding over glucose production and Srebp–1c regulating lipogenesis, provides a potential explanation. Here we illustrate an alternative mechanism that integrates glucose production and lipogenesis under the unifying control of FoxO. Liver–specific ablation of three FoxOs (L–FoxO1,3,4) prevents the induction of glucose–6–phosphatase and the repression of glucokinase during fasting, thus increasing lipogenesis at the expense of glucose production. We document a similar pattern in the early phases of diet-induced insulin resistance, and propose that FoxOs are required to enable the liver to direct nutritionally derived carbons to glucose vs. lipid metabolism. Our data underscore the heterogeneity of hepatic insulin resistance during progression from the metabolic syndrome to overt diabetes, and the conceptual challenge of designing therapies that curtail glucose production without promoting hepatic lipid accumulation.

Nicotinamide mononucleotide, an intermediate of NAD+ synthesis, protects the heart from ischemia and reperfusion

Nicotinamide phosphoribosyltransferase (Nampt), the rate-limiting enzyme for nicotinamide adenine dinucleotide (NAD⁺) synthesis, and Sirt1, an NAD⁺-dependent histone deacetylase, protect the heart against ischemia/reperfusion (I/R). It remains unknown whether Nampt mediates the protective effect of ischemic preconditioning (IPC), whether nicotinamide mononucleotide (NMN, 500 mg/kg), a product of Nampt in the NAD⁺ salvage pathway, mimics the effect of IPC, or whether caloric restriction (CR) upregulates Nampt and protects the heart through a Sirt1-dependent mechanism. IPC upregulated Nampt protein, and the protective effect of IPC against ischemia (30 minutes) and reperfusion (24 hours) was attenuated at both early and late phases in Nampt +/− mice, suggesting that Nampt plays an essential role in mediating the protective effect of IPC. In order to mimic the effect of Nampt, NMN was administered by intraperitoneal injection. NMN significantly increased the level of NAD⁺ in the heart at baseline and prevented a decrease in NAD⁺ during ischemia. NMN protected the heart from I/R injury when it was applied once 30 minutes before ischemia or 4 times just before and during reperfusion, suggesting that exogenous NMN protects the heart from I/R injury in both ischemic and reperfusion phases. The protective effect of NMN was accompanied by decreases in acetylation of FoxO1, but it was not obvious in Sirt1 KO mice, suggesting that the effect of NMN is mediated through activation of Sirt1. Compared to control diet (90% calories), CR (60% calories for 6 weeks) in mice led to a significant reduction in I/R injury, accompanied by upregulation of Nampt. The protective effect of CR against I/R injury was not significant in cardiac-specific Sirt1 KO mice, suggesting that the protective effect of CR is in part mediated through the Nampt-Sirt1 pathway. In conclusion, exogenous application of NMN and CR protects the heart by both mimicking IPC and activating Sirt1.

Insulin-FOXO3 signaling modulates circadian rhythms via regulation of clock transcription

Circadian rhythms are responsive to external and internal cues, light and metabolism being among the most important. In mammals, the light signal is sensed by the retina and transmitted to the suprachiasmatic nucleus (SCN) master clock [1], where it is integrated into the molecular oscillator via regulation of clock gene transcription. The SCN synchronizes peripheral oscillators, an effect that can be overruled by incoming metabolic signals [2]. As a consequence, peripheral oscillators can be uncoupled from the master clock when light and metabolic signals are not in phase. The signaling pathways responsible for coupling metabolic cues to the molecular clock are being rapidly uncovered [3-5]. Here we show that insulin-phosphatidylinositol 3-kinase (PI3K)-Forkhead box class O3 (FOXO3) signaling is required for circadian rhythmicity in the liver via regulation of Clock. Knockdown of FoxO3 dampens circadian amplitude, an effect that is rescued by overexpression of Clock. Subsequently, we show binding of FOXO3 to two Daf-binding elements (DBEs) located in the Clock promoter area, implicating Clock as a transcriptional target of FOXO3. Transcriptional oscillation of both core clock and output genes in the liver of FOXO3-deficient mice is affected, indicating a disrupted hepatic circadian rhythmicity. Finally, we show that insulin, a major regulator of FOXO activity [6-9], regulates Clock levels in a PI3K- and FOXO3-dependent manner. Our data point to a key role of the insulin-FOXO3-Clock signaling pathway in the modulation of circadian rhythms.

High altitude exposure alters gene expression levels of DNA repair enzymes, and modulates fatty acid metabolism by SIRT4 induction in human skeletal muscle

We hypothesized that high altitude exposure and physical activity associated with the attack to Mt Everest could alter mRNA levels of DNA repair and metabolic enzymes and cause oxidative stress-related challenges in human skeletal muscle. Therefore, we have tested eight male mountaineers (25-40 years old) before and after five weeks of exposure to high altitude, which included attacks to peaks above 8000m. Data gained from biopsy samples from vastus lateralis revealed increased mRNA levels of both cytosolic and mitochondrial superoxide dismutase. On the other hand 8-oxoguanine DNA glycosylase (OGG1) mRNA levels tended to decrease while Ku70 mRNA levels and SIRT6 decreased with altitude exposure. The levels of SIRT1 and SIRT3 mRNA did not change significantly. However, SIRT4 mRNA level increased significantly, which could indicate decreases in fatty acid metabolism, since SIRT4 is one of the important regulators of this process. Within the limitations of this human study, data suggest that combined effects of high altitude exposure and physical activity climbing to Mt. Everest, could jeopardize the integrity of the particular chromosome.

Rosiglitazone and fenofibrate exacerbate liver steatosis in a mouse model of obesity and hyperlipidemia. A transcriptomic and metabolomic study

Peroxisome proliferator-activated receptors (PPAR) play an important role in the regulation of lipid and glucose metabolism, inflammatory, and vascular responses. We show the effect of treatment with two PPAR agonists, fenofibrate (FF) and rosiglitazone (RSG), on ob/ob and LDLR-double deficient mice, by combined gene-expression and metabolomic analyses. Male mice were daily treated for 12 weeks with RSG (10 mg·kg(1-)·day(-1) per os (p.o.), n = 8) and FF (50 mg·kg(1-)·day(-1) p.o., n = 8). Twelve untreated ob/ob and LDLR-double deficient mice were used as controls. To integrate the transcriptomic and metabolomic results, we designed a hierarchical algorithm, based on the average linkage method in clustering. Data were also interpreted with the Ingenuity Pathway Analysis program. FF and RSG treatments significantly increased the hepatic triglyceride content in the liver when compared with the control group, and the treatments induced an increase in the number and size of hepatic lipid droplets. Both drugs simultaneously activate pro-steatotic and antisteatotic metabolic pathways with a well-ordered result of aggravation of the hepatic lipid accumulation. The present study is a cautionary note not only to researchers on the basic mechanism of the action of PPAR activators but also to the use of these compounds in clinical practice.

Regulation of FOXO3 by phosphorylation and methylation in hepatitis C virus infection and alcohol exposure

Hepatitis C virus (HCV) infection produces chronic liver injury that is significantly exacerbated by alcohol consumption. While multiple mechanisms contribute to this synergy, a viral-induced loss of antioxidant responses has been shown to play an important role. This study examined the effects of HCV infection and alcohol on the regulation of the transcription factor FOXO3, an important regulator of Mn-superoxide dismutase (SOD2) expression, a tumor suppressor, and a component of the hepatic antioxidant response system. FOXO3 was activated by either HCV or alcohol alone but suppressed by the combination. To understand this paradoxical result, we applied a capillary isoelectric focusing (IEF) method to determine the pattern of FOXO3 posttranslational modifications (PTMs) induced by HCV and alcohol. We observed the presence of multiple different nuclear and cytosolic species of FOXO3 and used antiphosphoserine, acetyl-lysine, methylarginine, and ubiquitin antibodies to identify the PTM patterns present in each species. HCV caused multiple changes including phosphorylation of FOXO3 at S-574, a novel c-Jun N-terminal kinase (JNK) site, which promoted nuclear translocation and transcription. Ethanol suppressed arginine-methylation of FOXO3 promoting nuclear export and degradation of the JNK phosphorylated form. Human liver biopsy samples showed the presence of the HCV-specific form of FOXO3 in HCV-infected livers but not in normal liver or nonalcoholic steatohepatitis.

CONCLUSION

The development of this novel IEF method for the simultaneous quantification of differently modified FOXO3 species allowed us to demonstrate how HCV and alcohol combine to modify a complex pattern of FOXO3 PTMs that contribute to pathogenesis. This approach will allow further dissection of the role of protein PTMs in viral liver disease.

Critical physiological and pathological functions of Forkhead Box O tumor suppressors

The Forkhead box, subclass O (FOXO) proteins are critical transcription factors, ubiquitously expressed in the human body. These proteins are characterized by a remarkable functional diversity, being involved in cell cycle arrest, apoptosis, oxidative detoxification, DNA damage repair, stem cell maintenance, cell differentiation, cell metabolism, angiogenesis, cardiac development, aging and others. In addition, FOXO have critical implications in both normal and cancer stem cell biology. New strategies to modulate FOXO expression and activity may now be developed since the discovery of novel FOXO regulators and non-coding RNAs (such as microRNAs) targeting FOXO transcription factors. This review focuses on physiological and pathological functions of FOXO proteins and on their action as fine regulators of cell fate and context-dependent cell decisions. A better understanding of the structure and critical functions of FOXO transcription factors and tumor suppressors may contribute to the development of novel therapies for cancer and other diseases.

Hepatic notch signaling correlates with insulin resistance and nonalcoholic fatty liver disease

Hepatic Notch signaling is inappropriately activated in obese/insulin-resistant mouse models. Genetic or pharmacologic inhibition of hepatic Notch signaling in obese mice simultaneously improves glucose tolerance and reduces hepatic triglyceride content. As such, we predicted that Notch signaling in human liver would be positively associated with insulin resistance and hepatic steatosis. Here, we systematically survey Notch signaling in liver biopsy specimens, and show active Notch signaling in lean and obese adults, with expression of multiple Notch receptors and ligands. In morbidly obese patients undergoing bariatric surgery, we show that Notch activation positively correlates with glucose-6-phosphatase (G6PC) and phosphoenolpyruvate carboxykinase (PCK1) expression, key regulators of hepatic glucose output. We used immunofluorescence to identify active Notch signaling in hepatocytes and show highest activity in hyperglycemia, which we confirmed is a direct effect of hyperglycemia and insulin resistance. In a validation cohort of leaner individuals undergoing percutaneous liver biopsy for suspected nonalcoholic fatty liver disease (NAFLD), Notch activity showed independent positive association with insulin resistance and hepatic steatosis. Notably, Notch activity showed stronger correlation with the NAFLD activity score and alanine aminotransferase levels than with steatosis alone, suggesting that Notch activity is associated with nonalcoholic steatohepatitis. In summary, this study establishes that Notch signaling is activated in and may represent a therapeutic target for patients with obesity-related liver disease.

Hepatitis C and alcohol exacerbate liver injury by suppression of FOXO3

Hepatitis C virus (HCV) infection exacerbates alcoholic liver injury by mechanisms that include enhanced oxidative stress. The forkhead box transcription factor FOXO3 is an important component of the antioxidant stress response that can be altered by HCV. To test whether FOXO3 is protective for alcoholic liver injury, we fed alcohol to FOXO3(-/-) mice. After 3 weeks, one third of these mice developed severe hepatic steatosis, neutrophilic infiltration, and >10-fold alanine aminotransferase (ALT) elevations. In cell culture, either alcohol or HCV infection alone increased FOXO3 transcriptional activity and expression of target genes, but the combination of HCV and alcohol together caused loss of nuclear FOXO3 and decreased its transcriptional activity. This was accompanied by increased phosphorylation of FOXO3. Mice expressing HCV structural proteins on a background of reduced expression of superoxide dismutase 2 (SOD2; Sod2(+/-)) also had increased liver sensitivity to alcohol, with elevated ALT, steatosis, and lobular inflammation. Elevated ALT was associated with an alcohol-induced decrease in SOD2 and redistribution of FOXO3 to the cytosol. These results demonstrate that FOXO3 functions as a protective factor preventing alcoholic liver injury. The combination of HCV and alcohol, but not either condition alone, inactivates FOXO3, causing a decrease in expression of its target genes and an increase in liver injury. Modulation of the FOXO3 pathway is a potential therapeutic approach for HCV-alcohol-induced liver injury.

Advanced glycation endproducts induce fibrogenic activity in nonalcoholic steatohepatitis by modulating TNF-α-converting enzyme activity in mice

Advanced glycation endproducts (AGEs) accumulate in patients with diabetes, yet the link between AGEs and inflammatory and fibrogenic activity in nonalcoholic steatohepatitis (NASH) has not been explored. Tumor necrosis factor alpha (TNF-α)-converting enzyme (TACE) is at the center of inflammatory processes. Because the main natural regulator of TACE activity is the tissue inhibitor of metalloproteinase 3 (Timp3), we hypothesized that AGEs induce TACE through nicotinamide adenine dinucleotide phosphate reduced oxidase 2 (NOX2); and the down-regulation of Sirtuin 1 (Sirt1)/Timp3 pathways mediate fibrogenic activity in NASH. The role of NOX2, Sirt1, Timp3, and TACE was evaluated in choline-deficient L-amino acid defined (CDAA) or Western diet (WD)-fed wild-type (WT) and NOX2(-/-) mice. To restore Timp3, mice were injected with adenovirus (Ad)-Timp3. Sirt1 and Timp3 expressions were studied in livers from NASH patients, and we found that their levels were significantly lower than in healthy controls. In WT mice on the CDAA or WD, Sirt1 and Timp3 expressions were lower, whereas production of reactive oxidative species and TACE activity significantly increased with an increase in active TNF-α production as well as induction of fibrogenic transcripts. Ad-Timp3 injection resulted in a significant decline in TACE activity, procollagen α1 (I), alpha smooth muscle actin (α-SMA) and transforming growth factor beta (TGF-β) expression. NOX2(-/-) mice on the CDAA or WD had no significant change in Sirt1, Timp3, and TACE activity or the fibrosis markers assessed. In vitro, AGE exposure decreased Sirt1 and Timp3 in hepatic stellate cells by a NOX2-dependent pathway, and TACE was induced after exposure to AGEs.

CONCLUSION

TACE activation is central to the pathogenesis of NASH and is mediated by AGEs through NOX2 induction and down-regulation of Sirt1/Timp3 pathways.

20-Hydroxyecdysone-induced transcriptional activity of FoxO upregulates brummer and acid lipase-1 and promotes lipolysis in Bombyx fat body

In a previous study, we have shown that the molting hormone, 20-hydroxyecdysone (20E), reduces insect food consumption resulting in fat body lipolysis during the non-feeding molting and pupation stages, and assumed that the transcription factor FoxO is involved in this process. To verify this hypothesis, we cloned foxO from the silkworm, Bombyx mori. During molting and pupation, FoxO is highly expressed and predominantly localizes in the nuclei of fat body cells. 20E induced foxO mRNA expression and FoxO nuclear localization resulting in an increase of FoxO transcriptional activity. RNAi of foxO prior to the 4th larval molting downregulated two lipase genes--the insect adipose triacylglycerol lipase homologue, brummer, and an acid lipase, acid lipase-1, in the fat body. Overexpression of the constitutively-active form of foxO (foxO(CA)) upregulated brummer and acid lipase-1 in both the fat body and Bombyx Bm-12 cells. Putative FoxO-response elements (FREs) are present in the promoter regions of brummer and acid lipase-1, and mutation of the FREs attenuated their FoxO-induced luciferase activities. ChIP assay revealed that FoxO binds directly to those FREs. Moreover, foxO(CA) overexpression in vivo doubled lipid concentration in the hemolymph, increased total lipase activity, and slightly but significantly reduced lipid content in the fat body. Taken together, we conclude that 20E increases the transcriptional activity of FoxO which, in turn, upregulates brummer and acid lipase-1 and induces lipolysis in the Bombyx fat body during molting and pupation.

The importance of NAMPT/NAD/SIRT1 in the systemic regulation of metabolism and ageing

Ageing is associated with a variety of pathophysiological changes, including development of insulin resistance, progressive decline in β-cell function and chronic inflammation, all of which affect metabolic homeostasis in response to nutritional and environmental stimuli. SIRT1, the mammalian nicotinamide adenine dinucleotide (NAD)-dependent protein deacetylase, and nicotinamide phosphoribosyltransferase (NAMPT), the rate-limiting NAD biosynthetic enzyme, together comprise a novel systemic regulatory network, named the 'NAD World', that orchestrates physiological responses to internal and external perturbations and maintains the robustness of the physiological system in mammals. In the past decade, an accumulating body of evidence has demonstrated that SIRT1 and NAMPT, two essential components in the NAD World, play a critical role in regulating insulin sensitivity and insulin secretion throughout the body. In this article, we will summarize the physiological significance of SIRT1 and NAMPT-mediated NAD biosynthesis in metabolic regulation and discuss the ideas of functional hierarchy and frailty in determining the robustness of the system. We will also discuss the potential of key NAD intermediates as effective nutriceuticals for the prevention and the treatment of age-associated metabolic complications, such as type 2 diabetes.

Deletion of hepatic FoxO1/3/4 genes in mice significantly impacts on glucose metabolism through downregulation of gluconeogenesis and upregulation of glycolysis

Forkhead transcription factors FoxO1/3/4 have pleiotrophic functions including anti-oxidative stress and metabolism. With regard to glucose metabolism, most studies have been focused on FoxO1. To further investigate their hepatic functions, we generated liver-specific FoxO1/3/4 knockout mice (LTKO) and examined their collective impacts on glucose homeostasis under physiological and pathological conditions. As compared to wild-type mice, LTKO mice had lower blood glucose levels under both fasting and non-fasting conditions and they manifested better glucose and pyruvate tolerance on regular chow diet. After challenged by a high-fat diet, wild-type mice developed type 2 diabetes, but LTKO mice remained euglycemic and insulin-sensitive. To understand the underlying mechanisms, we examined the roles of SIRT6 (Sirtuin 6) and Gck (glucokinase) in the FoxO-mediated glucose metabolism. Interestingly, ectopic expression of SIRT6 in the liver only reduced gluconeogenesis in wild-type but not LTKO mice whereas knockdown of Gck caused glucose intolerance in both wild-type and LTKO mice. The data suggest that both decreased gluconeogenesis and increased glycolysis may contribute to the overall glucose phenotype in the LTKO mice. Collectively, FoxO1/3/4 transcription factors play important roles in hepatic glucose homeostasis.

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.

Forkhead box class O transcription factors in liver function and disease

The forkhead box transcription factor class O (FOXO) family represents a group of transcription factors that is required for a number of stress-related transcriptional programs including antioxidant response, gluconeogenesis, cell cycle control, apoptosis, and autophagy. The liver utilizes several FOXO-dependent pathways to adapt to its routine cycles of feeding and fasting and to respond to the stresses induced by disease. FOXO1 is a direct transcriptional regulator of gluconeogenesis, reciprocally regulated by insulin, and has profound effects on hepatic lipid metabolism. FOXO3 is required for antioxidant responses and autophagy and is altered in hepatitis C infection and fatty liver. Emerging evidence suggests dysregulation of FOXO3 in some hepatocellular carcinomas. FOXOs are notable for the extensive number of functionally significant posttranslational modifications that they undergo. Recent advances in our understanding how FOXOs are regulated are providing a more detailed picture of how specific combinations of posttranslational modifications alter both nuclear translocation as well as transcriptional specificity under different conditions. This review summarizes emerging knowledge of FOXO function in the liver, FOXO changes in liver disease, and the posttranslational modifications responsible for these effects.

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

Cholesterol homeostasis is crucial for cellular function and organismal health. The key regulator for the cholesterol biosynthesis is sterol-regulatory element binding protein (SREBP)-2. The biochemical process and physiological function of SREBP-2 have been well characterized; however, it is not clear how this gene is epigenetically regulated. Here we have identified sirtuin (Sirt)6 as a critical factor for Srebp2 gene regulation. Hepatic deficiency of Sirt6 in mice leads to elevated cholesterol levels. On the mechanistic level, Sirt6 is recruited by forkhead box O (FoxO)3 to the Srebp2 gene promoter where Sirt6 deacetylates histone H3 at lysines 9 and 56, thereby promoting a repressive chromatin state. Remarkably, Sirt6 or FoxO3 overexpression improves hypercholesterolemia in diet-induced or genetically obese mice. In summary, our data suggest an important role of hepatic Sirt6 and FoxO3 in the regulation of cholesterol homeostasis.

Inhibition of Notch uncouples Akt activation from hepatic lipid accumulation by decreasing mTorc1 stability

Increased hepatic lipid content is an early correlate of insulin resistance, and can be caused by nutrient-induced mTor activation. The latter increases basal Akt activity, leading to a self-perpetuating lipogenic cycle. We have previously shown that the developmental Notch pathway has metabolic functions in adult liver. Acute or chronic inhibition of Notch dampens hepatic glucose production and increases Akt tone, and might therefore be predicted to increase hepatic lipid content. Surprisingly, we show that constitutive liver-specific ablation of Notch signaling, or its acute inhibition with a decoy Notch1 receptor, prevents hepatosteatosis by blocking mTorc1. Conversely, Notch gain-of-function causes fatty liver through constitutive activation of mTorc1, an effect reversible by rapamycin treatment. We demonstrate that Notch signaling increases mTorc1 complex stability, augmenting mTorc1 function and Srebp1c-mediated lipogenesis. The data identify Notch as a therapeutically actionable branch point of metabolic signaling, where hepatic Akt activation can be uncoupled from steatosis.

Pharmacological promotion of autophagy alleviates steatosis and injury in alcoholic and non-alcoholic fatty liver conditions in mice

BACKGROUND & AIMS

Pharmacological approaches can potentially improve fatty liver condition in alcoholic and non-alcoholic fatty liver diseases. The salutary effects of reducing lipid synthesis or promoting lipid oxidation have been well reported, but the benefits of increasing lipid degradation have yet to be well explored. Macroautophagy is a cellular degradation process that can remove subcellular organelles including lipid droplets. We thus investigated whether pharmacological modulation of macroautophagy could be an effective approach to alleviate fatty liver condition and liver injury.

METHODS

C57BL/6 mice were given ethanol via intraperitoneal injection (acute) or by a 4-week oral feeding regime (chronic), or high fat diet for 12 weeks. An autophagy enhancer, carbamazepine or rapamycin, or an autophagy inhibitor, chloroquine, was given before sacrifice. Activation of autophagy, level of hepatic steatosis, and blood levels of triglycerides, liver enzyme, glucose and insulin were measured.

RESULTS

In both acute and chronic ethanol condition, macroautophagy was activated. Carbamazepine, as well as rapamycin, enhanced ethanol-induced macroautophagy in hepatocytes in vitro and in vivo. Hepatic steatosis and liver injury were exacerbated by chloroquine, but alleviated by carbamazepine. The protective effects of carbamazepine and rapamycin in reducing steatosis and in improving insulin sensitivity were also demonstrated in high fat diet-induced non-alcoholic fatty liver condition.

CONCLUSIONS

These findings indicate that pharmacological modulation of macroautophagy in the liver can be an effective strategy for reducing fatty liver condition and liver injury.

Pharmacological promotion of autophagy alleviates steatosis and injury in alcoholic and non-alcoholic fatty liver conditions in mice

BACKGROUND & AIMS

Pharmacological approaches can potentially improve fatty liver condition in alcoholic and non-alcoholic fatty liver diseases. The salutary effects of reducing lipid synthesis or promoting lipid oxidation have been well reported, but the benefits of increasing lipid degradation have yet to be well explored. Macroautophagy is a cellular degradation process that can remove subcellular organelles including lipid droplets. We thus investigated whether pharmacological modulation of macroautophagy could be an effective approach to alleviate fatty liver condition and liver injury.

METHODS

C57BL/6 mice were given ethanol via intraperitoneal injection (acute) or by a 4-week oral feeding regime (chronic), or high fat diet for 12 weeks. An autophagy enhancer, carbamazepine or rapamycin, or an autophagy inhibitor, chloroquine, was given before sacrifice. Activation of autophagy, level of hepatic steatosis, and blood levels of triglycerides, liver enzyme, glucose and insulin were measured.

RESULTS

In both acute and chronic ethanol condition, macroautophagy was activated. Carbamazepine, as well as rapamycin, enhanced ethanol-induced macroautophagy in hepatocytes in vitro and in vivo. Hepatic steatosis and liver injury were exacerbated by chloroquine, but alleviated by carbamazepine. The protective effects of carbamazepine and rapamycin in reducing steatosis and in improving insulin sensitivity were also demonstrated in high fat diet-induced non-alcoholic fatty liver condition.

CONCLUSIONS

These findings indicate that pharmacological modulation of macroautophagy in the liver can be an effective strategy for reducing fatty liver condition and liver injury.