Yasushi Ishigaki

Neuronal Pathway from the Liver Modulates Energy Expenditure and Systemic Insulin Sensitivity

Coordinated control of energy metabolism and glucose homeostasis requires communication between organs and tissues. We identified a neuronal pathway that participates in the cross talk between the liver and adipose tissue. By studying a mouse model, we showed that adenovirus-mediated expression of peroxisome proliferator–activated receptor (PPAR)–g2 in the liver induces acute hepatic steatosis while markedly decreasing peripheral adiposity. These changes were accompanied by increased energy expenditure and improved systemic insulin sensitivity. Hepatic vagotomy and selective afferent blockage of the hepatic vagus revealed that the effects on peripheral tissues involve the afferent vagal nerve. Furthermore, an antidiabetic thiazolidinedione, a PPARg agonist, enhanced this pathway. This neuronal pathway from the liver may function to protect against metabolic perturbation induced by excessive energy storage.

Regulation of Pancreatic β Cell Mass by Neuronal Signals from the Liver

Metabolic regulation in mammals requires communication between multiple organs and tissues. The rise in the incidence of obesity and associated metabolic disorders, including type 2 diabetes, has renewed interest in interorgan communication. We used mouse models to explore the mechanism whereby obesity enhances pancreatic β cell mass, pathophysiological compensation for insulin resistance. We found that hepatic activation of extracellular regulated kinase (ERK) signaling induced pancreatic β cell proliferation through a neuronal-mediated relay of metabolic signals. This metabolic relay from the liver to the pancreas is involved in obesity-induced islet expansion. In mouse models of insulin-deficient diabetes, liver-selective activation of ERK signaling increased β cell mass and normalized serum glucose levels. Thus, interorgan metabolic relay systems may serve as valuable targets in regenerative treatments for diabetes.

Impact of Plasma Oxidized Low-Density Lipoprotein Removal on Atherosclerosis

Background— Several clinical studies of statin therapy have demonstrated that lowering low-density lipoprotein (LDL) cholesterol prevents atherosclerotic progression and decreases cardiovascular mortality. In addition, oxidized LDL (oxLDL) is suggested to play roles in the formation and progression of atherosclerosis. However, whether lowering oxLDL alone, rather than total LDL, affects atherogenesis remains unclear. Methods and Results— To clarify the atherogenic impact of oxLDL, lectin-like oxLDL receptor 1 (LOX-1), an oxLDL receptor, was expressed ectopically in the liver with adenovirus administration in apolipoprotein E–deficient mice at 46 weeks of age. Hepatic LOX-1 expression enhanced hepatic oxLDL uptake, indicating functional expression of LOX-1 in the liver. Although plasma total cholesterol, triglyceride, and LDL cholesterol levels were unaffected, plasma oxLDL was markedly and transiently decreased in LOX-1 mice. In controls, atherosclerotic lesions, detected by Oil Red O staining, were markedly increased (by 38%) during the 4-week period after adenoviral administration. In contrast, atherosclerotic progression was almost completely inhibited by hepatic LOX-1 expression. In addition, plasma monocyte chemotactic protein-1 and lipid peroxide levels were decreased, whereas adiponectin was increased, suggesting decreased systemic oxidative stress. Thus, LOX1 expressed in the livers of apolipoprotein E–deficient mice transiently removes oxLDL from circulating blood and possibly decreases systemic oxidative stress, resulting in complete prevention of atherosclerotic progression despite the persistence of severe LDL hypercholesterolemia and hypertriglyceridemia. Conclusions— OxLDL has a major atherogenic impact, and oxLDL removal is a promising therapeutic strategy against atherosclerosis.

Circulating oxidized LDL: a biomarker and a pathogenic factor.

Purpose of review Oxidized LDL (oxLDL) contributes to many atherogenic steps in the vascular wall, but the significance of oxLDL in circulating blood remains unclear. Recent progress in procedures for measuring both human and murine oxLDL has provided growing evidence of the importance of circulating oxLDL. Recent findings Circulating oxLDL is elevated in patients with advanced atherosclerosis, such as coronary heart disease and ischemic stroke, and also reflects early atherosclerotic changes and metabolic disorders including diabetes and obesity. In-vitro exposure to oxLDL increased mononuclear cell nuclear factor-κB activity, suggesting a pathogenic role of circulating oxLDL in exacerbation of oxidative stress. In addition, adenoviral administration of secreted scavenger receptor-A1, which functions as a decoy, suppresses foam cell formation in LDL receptor-deficient mice via a blockade of modified LDL incorporation into macrophages. Furthermore, when lectin-like oxLDL receptor-1 was ectopically expressed in the liver, circulating oxLDL was reduced, resulting in complete prevention of atherosclerotic progression in apolipoprotein E-deficient mice. Thus, circulating oxLDL impacts atherogenic formation. Summary The roles of circulating oxLDL in atherosclerotic pathogenesis are now attracting considerable attention. OxLDL removal from circulating blood is a promising therapeutic strategy against atherosclerosis.

Involvement of Apolipoprotein E in Excess Fat Accumulation and Insulin Resistance

Although apolipoprotein E (apoE) is well known to play a major role in lipid metabolism, its role in glucose and energy homeostasis remains unclear. Herein, we established apoE-deficient genetically obese Ay (apoE−/−;Ay/+) mice. ApoE deficiency in Ay mice prevented the development of obesity, with decreased fat accumulation in the liver and adipose tissues. ApoE−/−;Ay/+ mice exhibited better glucose tolerance than apoE+/+;Ay/+ mice. Insulin tolerance testing and hyperinsulinemic-euglycemic clamp study revealed marked improvement of insulin sensitivity, despite increased plasma free fatty acid levels. These metabolic phenotypes were reversed by adenoviral replenishment of apoE protein, indicating circulating apoE to be involved in increased adiposity and obesity-related metabolic disorders. Uptake of apoE-lacking VLDL into the liver and adipocytes was markedly inhibited, but adipocytes in apoE−/−;Ay/+ mice exhibited normal differentiation, suggesting that apoE-dependent VLDL transport is involved in the development of obesity, i.e., surplus fat accumulation. Interestingly, apoE−/−;Ay/+ mice exhibited decreased food intake and increased energy expenditure. Pair-feeding experiments indicate these phenomena to both contribute to the obesity-resistant phenotypes associated with apoE deficiency. Thus, apoE is involved in maintaining energy homeostasis. ApoE-dependent excess fat accumulation is a promising therapeutic target for the metabolic syndrome.

Blockade of the Nuclear Factor-κB Pathway in the Endothelium Prevents Insulin Resistance and Prolongs Life Spans

Background— Nuclear factor-&kgr;B (NF-&kgr;B) signaling plays critical roles in physiological and pathological processes such as responses to inflammation and oxidative stress. Methods and Results— To examine the role of endothelial NF-&kgr;B signaling in vivo, we generated transgenic mice expressing dominant-negative I&kgr;B under the Tie2 promoter/enhancer (E-DNI&kgr;B mice). These mice exhibited functional inhibition of NF-&kgr;B signaling specifically in endothelial cells. Although E-DNI&kgr;B mice displayed no overt phenotypic changes when young and lean, they were protected from the development of insulin resistance associated with obesity, whether diet- or genetics-induced. Obesity-induced macrophage infiltration into adipose tissue and plasma oxidative stress markers were decreased and blood flow and mitochondrial content in muscle and active-phase locomotor activity were increased in E-DNI&kgr;B mice. In addition to inhibition of obesity-related metabolic deteriorations, blockade of endothelial NF-&kgr;B signaling prevented age-related insulin resistance and vascular senescence and, notably, prolonged life span. These antiaging phenotypes were also associated with decreased oxidative stress markers, increased muscle blood flow, enhanced active-phase locomotor activity, and aortic upregulation of mitochondrial sirtuin-related proteins. Conclusions— The endothelium plays important roles in obesity- and age-related disorders through intracellular NF-&kgr;B signaling, thereby ultimately affecting life span. Endothelial NF-&kgr;B signaling is a potential target for treating the metabolic syndrome and for antiaging strategies.

Cold exposure suppresses serum adiponectin levels through sympathetic nerve activation in mice.

Objective: Several lines of evidence suggest important roles for adiponectin in glucose and lipid metabolism and atherosclerosis. However, the mechanisms regulating serum adiponectin levels and adiponectin production are still not completely understood. Our aim was to determine whether adiponectin synthesis is physiologically regulated by the sympathetic nervous system (SNS).

Blockade of the NF-κB Pathway in the Endothelium Prevents Insulin Resistance and Prolongs Lifespans

Background— Nuclear factor-&kgr;B (NF-&kgr;B) signaling plays critical roles in physiological and pathological processes such as responses to inflammation and oxidative stress. Methods and Results— To examine the role of endothelial NF-&kgr;B signaling in vivo, we generated transgenic mice expressing dominant-negative I&kgr;B under the Tie2 promoter/enhancer (E-DNI&kgr;B mice). These mice exhibited functional inhibition of NF-&kgr;B signaling specifically in endothelial cells. Although E-DNI&kgr;B mice displayed no overt phenotypic changes when young and lean, they were protected from the development of insulin resistance associated with obesity, whether diet- or genetics-induced. Obesity-induced macrophage infiltration into adipose tissue and plasma oxidative stress markers were decreased and blood flow and mitochondrial content in muscle and active-phase locomotor activity were increased in E-DNI&kgr;B mice. In addition to inhibition of obesity-related metabolic deteriorations, blockade of endothelial NF-&kgr;B signaling prevented age-related insulin resistance and vascular senescence and, notably, prolonged life span. These antiaging phenotypes were also associated with decreased oxidative stress markers, increased muscle blood flow, enhanced active-phase locomotor activity, and aortic upregulation of mitochondrial sirtuin-related proteins. Conclusions— The endothelium plays important roles in obesity- and age-related disorders through intracellular NF-&kgr;B signaling, thereby ultimately affecting life span. Endothelial NF-&kgr;B signaling is a potential target for treating the metabolic syndrome and for antiaging strategies.

Obesity alters circadian expressions of molecular clock genes in the brainstem.

Major components of energy homeostasis, including feeding behavior and glucose and lipid metabolism, are subject to circadian rhythms. Recent studies have suggested that dysfunctions of molecular clock genes are involved in the development of obesity and diabetes. To examine whether metabolic states per se alter the circadian clock in the central nervous system (CNS), we analyzed the daily mRNA expression profiles of core clock genes in the caudal brainstem nucleus of the solitary tract (NTS). In lean C57BL/6 mice, transcript levels of the core clock genes (Npas2, Bmal1, Per1, Per2 and Rev-erbalpha) clearly showed 24-h rhythmicity. On the other hand, the expression profiles of Bmal1 and Rev-erbalpha were attenuated in mice with high fat diet-induced obesity as well as genetically obese KK-A(y) and ob/ob mice. Clock expression levels were increased in mice with high fat diet-induced obesity and Cry1 expression levels were decreased in KK-A(y) and ob/ob mice. In addition, peroxisome proliferator-activated receptor alpha (PPARalpha), which reportedly increases the BMAL1 transcriptional level, was up-regulated in the NTS of these murine models of obesity and insulin resistance, suggesting involvement of PPARalpha in the attenuation of circadian rhythms in the NTS in obese states. Furthermore, a circadian expression profile of a downstream target of clock genes, the large conductance Ca(2+)-activated K(+)channel, was disturbed in the NTS of these murine obesity models. These perturbations might contribute to neuronal dysfunction in obese states. This is the first report showing that obesity perturbs the circadian expressions of core clock genes in the CNS.

Atf6α-null mice are glucose intolerant due to pancreatic β-cell failure on a high-fat diet but partially resistant to diet-induced insulin resistance

Activating transcription factor 6α (ATF6α) is essential for the endoplasmic reticulum (ER) stress response. Since recent studies suggested that ER stress is involved in the pathogenesis of type 2 diabetes mellitus, we have analyzed Atf6α-null (Atf6α(-/-)) mice challenged with metabolic overload or genetic manipulations. Atf6α(-/-) mice were fed a high-fat diet to create diet-induced obese (DO) mice, and were subjected to examination of glucose homeostasis with biochemical and morphological analysis of the pancreatic β-cell and liver tissues. Atf6α-null mice were also crossed with genetic models of diabetes caused either by insulin resistance (Agouti obese mice) or by impaired insulin secretion (Ins2(WT/C96Y) mice). Atf6α(-/-) DO mice were less glucose tolerant with blunted insulin secretion compared to littermates on a high-fat diet. Pancreatic insulin content was lower in Atf6α(-/-) DO mice with the swollen β-cell ER, a typical feature of cells with ER stress. In the liver of Atf6α(-/-) DO mice, XBP-1 splicing was increased, suggesting that higher ER stress was present. ATF6-deficient mice showed increased mRNA expressions of glucose-6-phosphatase and SREBP1c associated with a tendency for a higher degree of steatosis in the liver. However, Atf6α(-/-) DO mice exhibited higher insulin sensitivity with lower serum triglyceride levels. Similar phenotypes were observed in ATF6α-deficient Agouti mice. In addition, ATF6α-deficiency accelerated reduction in pancreatic insulin content in Ins2(WT/C96Y) mice. These data suggested that ATF6α contributes to both prevention and promotion of diabetes; it protects β-cells from ER stress and suppresses hepatosteatosis, but plays a role in the development of hyperlipidemia and insulin resistance.