Bezafibrate (BEZ), a pan activator of peroxisome proliferator–activated receptors (PPARs), has been generally used to treat hyperlipidemia for decades. Clinical trials with type 2 diabetes patients indicated that BEZ also has beneficial effects on glucose metabolism, although the underlying mechanisms of these effects remain elusive. Even less is known about a potential role for BEZ in treating type 1 diabetes. Here we show that BEZ markedly improves hyperglycemia and glucose and insulin tolerance in mice with streptozotocin (STZ)-induced diabetes, an insulin-deficient mouse model of type 1 diabetes. BEZ treatment of STZ mice significantly suppressed the hepatic expression of genes that are annotated in inflammatory processes, whereas the expression of PPAR and insulin target gene transcripts was increased. Furthermore, BEZ-treated mice also exhibited improved metabolic flexibility as well as an enhanced mitochondrial mass and function in the liver. Finally, we show that the number of pancreatic islets and the area of insulin-positive cells tended to be higher in BEZ-treated mice. Our data suggest that BEZ may improve impaired glucose metabolism by augmenting hepatic mitochondrial performance, suppressing hepatic inflammatory pathways, and improving insulin sensitivity and metabolic flexibility. Thus, BEZ treatment might also be useful for patients with impaired glucose tolerance or diabetes.

Recent data suggest that brains of patients with Alzheimer's disease (AD) are insulin and insulin-like growth factor-1 (IGF-1) resistant. So far, there have been two different approaches to investigate possible therapeutic implications of modulating cerebral insulin/IGF-1 signaling (IIS) in AD. One approach is peripheral or intranasal administration of insulin or IGF-1. Intranasal and peripheral insulin administration has been shown to improve memory in patients with AD. Additionally, peripheral IGF-1 administration resulted in decreased amyloid-beta (Aβ) levels in brains of AD mouse models accompanied by elevated Aβ levels in the cerebrospinal fluid (CSF). Insulin and IGF-1 regulate multicargotransporters influencing trafficking of several molecules including Aβ from the brain to the blood as well as to the CSF and possibly vice versa. Furthermore, insulin and related peptides regulate neurovascular coupling changing regional blood flow. Thus, positive effects of peripheral insulin/IGF-1 administration on AD pathology might be due to changes in the blood-brain-barrier (BBB) and/or in the transport between the CSF/blood and the brain. Clinical and experimental data suggest that increased serum insulin and IGF-1 levels do not necessarily correlate with an upregulation of neuronal insulin/IGF-1 receptor signaling. Therefore, the second approach in investigating the role of neuronal IIS for the pathogenesis of AD analyzes knockout mice lacking components of the IIS in AD models. Haploinsufficiency of the IGF- 1 receptor (IGF-1R) (IGF-1R(+/-) mice) as well as neuronal deficiency of the insulin receptor (IR) (nIR(-/-) mice) or IGF-1R (nIGF-1R(-/-) mice) leads to delayed Aβ accumulation when crossed with mouse models for AD. Furthermore, insulin receptor substrate (IRS)-2 knockout mice (IRS-2(-/-) mice) show reduced Aβ levels in an Alzheimer background. These data suggest beneficial effects of decreased neuronal IIS on Alzheimer-pathology and question the therapeutic outcome of long-term administration of insulin or IGF-1 in patients with AD. Whether the observed phenomenon of cerebral insulin and IGF-1 resistance even at an early stage of Alzheimer's disease is cause, consequence or possibly counter-regulation to AD-pathology needs further investigation and should lead to critical discussions. The current review discusses the pros and cons of targeting insulin/IGF-1 signaling as therapeutic approach for AD.

The insulin receptor substrates (IRS) are adapter proteins mediating insulin's and IGF1's intracellular effects. Recent data suggest that IRS2 in the central nervous system (CNS) is involved in regulating fuel metabolism as well as memory formation. The present study aims to specifically define the role of chronically increased IRS2-mediated signal transduction in the CNS. We generated transgenic mice overexpressing IRS2 specifically in neurons (nIRS2 (tg)) and analyzed these in respect to energy metabolism, learning, and memory. Western blot (WB) analysis of nIRS2 (tg) brain lysates revealed increased IRS2 downstream signaling. Histopathological investigation of nIRS2 (tg) mice proved unaltered brain development and structure. Interestingly, nIRS2 (tg) mice showed decreased voluntary locomotoric activity during dark phase accompanied with decreased energy expenditure (EE) leading to increased fat mass. Accordingly, nIRS2 (tg) mice develop insulin resistance and glucose intolerance during aging. Exploratory behavior, motor function as well as food and water intake were unchanged in nIRS2 (tg) mice. Surprisingly, increased IRS2-mediated signals did not change spatial working memory in the T-maze task. Since FoxO1 is a key mediator of IRS2-transmitted signals, we additionally generated mice expressing a dominant negative mutant of FoxO1 (FoxO1DN) specifically in neurons. This mutant mimics the effect of increased IRS2 signaling on FoxO-mediated transcription. Interestingly, the phenotype observed in nIRS2 (tg) mice was not present in FoxO1DN mice. Therefore, increased neuronal IRS2 signaling causes decreased locomotoric activity in the presence of unaltered exploratory behavior and motor coordination that might lead to increased fat mass, insulin resistance, and glucose intolerance during aging independent of FoxO1-mediated transcription.

Fragestellung: Patienten mit Diabetes mellitus Typ 2 (T2DM) leiden häufig unter kognitiven Einschränkungen und haben ein erhöhtes Risiko für die Entwicklung einer Demenz. Die zugrunde liegenden molekularen Mechanismen sind zurzeit jedoch noch unklar. Sowohl bei Morbus Alzheimer als auch T2DM ist die Insulinrezeptor(IR)- und Insulin-like-growth factor-1- Rezeptor(IGF-1R)- Signaltransduktion im Gehirn der Patienten gestört und möglicherweise bei der Entstehung T2DM-assoziierter neurodegenerativer Erkrankungen beteiligt. Daher haben wir die Expression des IR, des IGF-1R, der Insulinrezeptor- Substrate (IRS)-1 und -2 sowie die Aktivität des phosphatidylinositol 3 (PI3)- und mitogen-activated protein (MAP) Kinase- Signalwegs in 6 verschiedenen Regionen des murinen Gehirns im Alter von 6 – 100 Wochen untersucht.

The concentration of serum testosterone is mainly regulated by the testicular function, which is under control of the central hypothalamic-pituitary-gonadal axis. A certain amount of testosterone is converted into beta-estradiol by adipose tissue. Obesity in men is often associated with decreased androgen levels. The aim of the present study was to examine the effect of caloric restriction on serum testosterone levels in obese men. Dietary intervention study was performed with a very low calorie diet (800 kcal/d) for 12 weeks. Thirteen obese human male subjects (median body mass index: 42.7 kg/m(2)) were included. Body composition was assessed by impedance analysis. Insulin sensitivity was estimated by leptin-to-adiponectin ratio (LAR). Testosterone (T), beta-estradiol, albumin, sex hormone-binding globulin (SHBG), LH, and FSH serum concentrations were measured by enzyme immunoassays. Statistical analysis was performed on baseline and values after 3 months. Caloric restriction significantly increased total testosterone (6.97 nmol/l to 13.21 nmol/l; p = 0.001) and SHBG (22.11 nmol/l to 42.12 nmol/l; p = 0.001) concentrations in serum. This is caused by a significant improvement of the testicular function (LH/T: 0.36-0.20; p = 0.005) and a significant reduction of the T/beta-estradiol conversion rate (73.59-104.29; p = 0.003). There was a signifi cant negative correlation of improvement of testicular function and LAR (r(s) = -0.683 (p = 0.042)). In obese men caloric restriction significantly increases the serum testosterone concentration. This is achieved by 2 distinct mechanisms, that is, improvement of testicular function and reduced conversion of testosterone to beta-estradiol by aromatase activity of the adipose tissue.

In brains from patients with Alzheimer’s disease (AD), expression of insulin receptor (IR), insulin-like growth factor-1 receptor (IGF-1R), and insulin receptor substrate proteins is downregulated. A key step in the pathogenesis of AD is the accumulation of amyloid precursor protein (APP) cleavage products, β-amyloid (Aβ)1-42 and Aβ1–40. Recently, we and others have shown that central IGF-1 resistance reduces Aβ accumulation as well as Aβ toxicity and promotes survival. To define the role of IR in this context, we crossed neuron-specific IR knockout mice (nIR−/−) with Tg2576 mice, a well-established mouse model of an AD-like pathology. Here, we show that neuronal IR deficiency in Tg2576 (nIR−/−Tg2576) mice leads to markedly decreased Aβ burden but does not rescue premature mortality of Tg2576 mice. Analyzing APP C-terminal fragments (CTF) revealed decreased α-/β-CTFs in the brains of nIR−/−Tg2576 mice suggesting decreased APP processing. Cell based experiments showed that inhibition of the PI3-kinase pathway suppresses endosomal APP cleavage and decreases α- as well as β-secretase activity. Deletion of only one copy of the neuronal IGF-1R partially rescues the premature mortality of Tg2576 mice without altering total amyloid load. Analysis of Tg2576 mice expressing either a dominant negative or constitutively active form of forkhead box-O (FoxO)1 did not reveal any alteration of amyloid burden, APP processing and did not rescue premature mortality in these mice. Thus, our findings identified IR signaling as a potent regulator of Aβ accumulation in vivo. But exclusively decreased IGF-1R expression reduces AD-associated mortality independent of β-amyloid accumulation and FoxO1-mediated transcription.

Background & Aims: To determine if diabetic and insulin-resistant states cause mitochondrial dysfunction in liver or if there is long term adaptation of mitochondrial function to these states, mice were (i) fed with a high-fat diet to induce obesity and T2D (HFD), (ii) had a genetic defect in insulin signaling causing whole body insulin resistance, but not full blown T2D (IR/IRS-1(+/-) mice), or (iii) were analyzed after treatment with streptozocin (STZ) to induce a T1D-like state.Methods: Hepatic lipid levels were measured by thin layer chromatography. Mitochondrial respiratory chain (RC) levels and function were determined by Western blot, spectrophotometric, oxygen consumption and proton motive force analysis. Gene expression was analyzed by real-time PCR and microarray.Results: HFD caused insulin resistance and hepatic lipid accumulation, but RC was largely unchanged. Livers from insulin resistant IR/IRS-1(+/-) mice had normal lipid contents and a normal RC, but mitochondria were less well coupled. Livers from severely hyperglycemic and hypoinsulinemic STZ mice had massively depleted lipid levels, but RC abundance was unchanged. However, liver mitochondria isolated from these animals showed increased abundance and activity of the RC, which was better coupled.Conclusions: Insulin resistance, induced either by obesity or genetic manipulation and steatosis do not cause mitochondrial dysfunction in mouse liver. Also, mitochondrial dysfunction is not a prerequisite for liver steatosis. However, severe insulin deficiency and high blood glucose levels lead to an enhanced performance and better coupling of the RC. This may represent an adaptation to fuel overload and the high energy-requirement of an unsuppressed gluconeogenesis. (C) 2013 European Association for the Study of the Liver. Published by Elsevier B. V. All rights reserved.

Diabetic nephropathy ( DN ) is a progressive fibrotic condition that may lead to end‐stage renal disease and kidney failure. Transforming growth factor‐β1 and bone morphogenetic protein‐7 ( BMP 7) have been shown to induce DN ‐like changes in the kidney and protect the kidney from such changes, respectively. Recent data identified insulin action at the level of the nephron as a crucial factor in the development and progression of DN . Insulin requires a family of insulin receptor substrate ( IRS ) proteins for its physiological effects, and many reports have highlighted the role of insulin and IRS proteins in kidney physiology and disease. Here, we observed IRS 2 expression predominantly in the developing and adult kidney epithelium in mouse and human. BMP 7 treatment of human kidney proximal tubule epithelial cells ( HK ‐2 cells) increases IRS 2 transcription. In addition, BMP 7 treatment of HK ‐2 cells induces an electrophoretic shift in IRS 2 migration on SDS / PAGE , and increased association with phosphatidylinositol‐3‐kinase, probably due to increased tyrosine/serine phosphorylation. In a cohort of DN patients with a range of chronic kidney disease severity, IRS 2 m RNA levels were elevated approximately ninefold, with the majority of IRS 2 staining evident in the kidney tubules in DN patients. These data show that IRS 2 is expressed in the kidney epithelium and may play a role in the downstream protective events triggered by BMP 7 in the kidney. The specific up‐regulation of IRS 2 in the kidney tubules of DN patients also indicates a novel role for IRS 2 as a marker and/or mediator of human DN progression. Structured digital abstract IRS2 physically interacts with p85α by anti bait coimmunoprecipitation ( View interaction )

Fragestellung: Restenosen nach Ballondilatation und Stentimplantation sind besonders bei Diabetikern ein klinisches Problem. Dabei proliferieren glatte Gefäßmuskelzellen und wandern in die Intima ein (Neointimabildung). Insulin und IGF-1, deren intrazelluläre Signale über die Insulinrezeptor-Substrate (IRS)-1 und IRS-2 vermittelt werden, wirken in verschiedenen Zelltypen proliferationsfördernd und antiapoptotisch. Die Bedeutung der einzelnen IRS-Proteine in diesem Kontext ist jedoch unklar. Daher wurde die Bedeutung einer Überexpression von IRS-2 und damit einer vermehrten Insulin/IGF-1 Signaltransduktion in glatten Gefäßmuskelzellen auf die Bildung einer Restenose nach Ballondilatation untersucht.

Epidemiological studies suggest that being obese in midlife is a risk factor for cognitive decline and dementia in later life. Hyperinsulinemia is one of the most frequent endocrine features in overweight people which results in insulin desensitization. Thus, chronically high insulin levels have been identified as risk factor for dementia. Accordingly, chronically high insulin levels might be harmful for brain function. Furthermore, insulin and IGF-1-induced signaling is reduced in the brains of patients suffering from Alzheimer’s disease (AD). Interestingly, studies in rodents suggest that reduced insulin receptor (IR) and insulin-like growth factor-1 receptor (IGF-1R) signaling decrease AD pathology, that is, β-amyloid toxicity. Data obtained in C. elegans indicate that the beneficial effect mediated via reduced IR/IGF-1R signaling might partially be induced via the forkhead-box O transcription factors (FoxO). In the mammalian brain, there are FoxO1, FoxO3a, and FoxO6 expressed. Surprisingly, high-fat diet specifically reduces the expression of FoxO3a and FoxO6 suggesting that IR/IGF-1 → FoxO-mediated transcription is involved in the pathogenesis of obesity-associated cognitive impairment. Therefore, the function of FoxO1 and FoxO3a has been investigated in animal models of Alzheimer’s disease in detail. The current paper focuses on the role of IR/IGF-1 signaling and IR/IGF-1 → FoxO-mediated transcription for the pathogenesis of obesity-associated dementia.

Fragestellung: Klinische Studien zeigen einen Zusammenhang zwischen Typ 2 Diabetes (T2D) und einer verminderten kognitiven Leistung. Zudem konnte post mortem bei Patienten mit T2D eine niedrigere Expression der Proteine des Insulinsignalwegs (IS) nachgewiesen werden. Eine kurzfristige Insulinapplikation führt zu einer kognitiven Leistungsverbesserung in Tiermodellen sowie in gesunden und kognitiv alterierten Patienten. Welche Bedeutung eine permanente Steigerung des zentralen IS auf die kognitive Leistung hat, ist bisher unklar. Daher wurden transgene Mäuse generiert, die neuronen-spezifisch das Insulinrezeptor-Substrat (IRS)-2 überexprimieren (nIRS2tg).

Chronic peripheral hyperinsulinemia is one of the main characteristics of type 2 diabetes accompanied by impaired glucose homeostasis and obesity resulting from increased food intake and decreased physical activity. Patients with type 2 diabetes have a higher risk of cognitive decline and neurodegenerative diseases e.g. Alzheimer's disease (AD). Furthermore, obesity or hyperinsulinemia alone already increase the probability of cognitive decline possibly progressing to AD. Tau hyperphosphorylation is one of the pathological hallmarks of AD and so called tauopathies. Aim of the present study was to analyze the influence of obesity-associated hyperinsulinemia on tau phosphorylation without changes in glucose homeostasis. 15% high fat diet fed over 12-16 weeks induced 2.4-fold increased plasma insulin levels without changing glucose tolerance. However, this diet did not lead to substantial differences in tau phosphorylation in the brain of C57Bl/6 mice. Additionally, chronic hyperinsulinemia did not influence downstream insulin receptor signaling and the expression of the tau kinases (e.g. ERK-1/-2, Akt, GSK-3 beta, CDK5 or jNK) and tau phosphatases (e.g. PP2A) in the murine central nervous system. Thus, we successfully induced hyperinsulinemia without causing glucose intolerance in our experimental animals but this did not influence central insulin receptor signaling or tau phosphorylation. (C) 2012 Elsevier Ireland Ltd. All rights reserved.

Fragestellung: Patienten mit Diabetes mellitus Typ 2 (T2D) haben ein erhöhtes Risiko für die Entwicklung einer Demenz. Weitestgehend unklar ist bislang welche neuronalen Prozesse und molekularen Mechanismen eine Rolle spielen. Die Insulinrezeptor(IR)- und Insulin-like-growth factor-1- Rezeptor(IGF1R)-Signaltransduktion ist in Gehirnen von Patienten mit Morbus Alzheimer sowie T2D gestört und scheint daher bei der Entstehung Diabetes-assoziierter neurodegenerativer Erkrankungen wesentlich beteiligt zu sein. Daher haben wir die Expression des IR, des IGF1R sowie der Insulinrezeptor-Substrate (IRS)-1 und -2 im frontalen, parietalen und occipitalen Kortex sowie im Hippocampus, Thalamus und Cerebellum des murinen Gehirns während des Alterns untersucht. Methodik: Die Gehirne von jeweils 3männlichen und 3 weiblichen C57BL/6- Mäusen wurden im Alter von 6, 16, 60 und 100 Wochen mittels Realtime-PCR Analysen auf die mRNA-Expression des IR, IGF1R, IRS-1 und IRS-2 untersucht sowie mittels Western Blots und Immunpräzipitation auf die Proteinebene. Die Analysen erfolgten für frontalen, parietalen und occipitalen Kortex sowie Hippocampus, Thalamus und Cerebellum. Ergebnisse: Auf mRNA-Ebene zeigte der IR einen Expressionsgipfel im Alter von 16 Wochen, der weitestgehend auf die weiblichen Tiere zurückzuführen war. Dieser Gipfel konnte auf Proteinebene im Hippocampus nachvollzogen werden nicht jedoch in den anderen Hirnregionen. Die mRNA-Expression des IGF1R stieg in beiden Geschlechtern während des Alterns signifikant an. Auf Proteinebene zeigte sich in den kortikalen Regionen sowie dem Hippocampus eine maximale IGF1R-Expression im Alter von 100 Wochen. IRS-1 mRNA-Konzentrationen zeigten einen signifikanten Abfall während des Alterns. Dieser ließ sich auch auf Proteinebene in allen untersuchten Hirnregionen nachweisen. Ähnlich wie bereits für den IR beschrieben, stellte sich bei der mRNA-Expression des IRS-2 ein Gipfel bei 16 Wochen dar, welcher sowohl bei den männlichen als auch weiblichen Tieren nachweisbar war. Schlussfolgerungen: Der IR und IGF1R sowie IRS-1 und -2 zeigen im murinen Gehirn ein altersabhängiges und teilweise geschlechtsspezifisches Expressionsmuster. Der IR zeigt in den Weibchen und IRS-2 in Weibchen und Männchen ein Expressionsmaximum bei 16 Wochen. IRS-1 ist im Alter signifikant weniger und der IGF1R signifikant stärker exprimiert.

Fragestellung: Post mortem konnte in Gehirnen von Patienten mit Diabetes mellitus Typ 2 (T2D) eine niedrigere Expression der Proteine des Insulinsignalwegs (IS) nachgewiesen werden. Eine kurzfristige Insulinapplikation führt zu einer kognitiven Leistungsverbesserung in Tiermodellen sowie in gesunden Menschen und kognitiv eingeschränkten Patienten. Welche Bedeutung eine chronische Steigerung des zentralen IS auf das Verhalten, die kognitive Leistung sowie den peripheren Glukosemetabolismus hat, ist bisher unklar. Daher wurden transgene Mäuse generiert, die neuronen-spezifisch das Insulinrezeptor-Substrat (IRS)-2 überexprimieren (nIRS2tg). Methodik: Es wurden transgene Mäuse etabliert, die mithilfe des LoxP-Cre-Systems in postmitotischen Nervenzellen des ZNS IRS-2 überexprimieren. nIRS-2tg und Wildtyp (WT) Mäuse wurden morphologisch, metabolisch (Glukose-/Insulintoleranztest; indirekte Kalorimetrie) und biochemisch charakterisiert. Des Weiteren wurden Verhaltenstests bezüglich des Explorations- (Elevated O-Maze, Open field) und Lernverhaltens (Morris Water-Maze) sowie der motorischen Koordination (RotaRod) durchgeführt. Ergebnisse: Die neuronale Überexpression von IRS-2 ließ sich mittels Immunhistochemie und Western Blots in nIRS2tg Mäusen nachweisen. Heterozygote nIRS2tg Mäuse zeigten einen erhöhten Körperfettanteil sowie eine vermehrte epigonadale Fettansammlung gegenüber WT Mäusen im Alter von 60 Wochen. Zudem zeigte sich im Alter von 100 Wochen eine gestörte Glukosetoleranz der transgenen Tiere. Der gesteigerte Körperfettanteil ließ sich in homozygoten nIRS2tg Mäusen noch deutlicher beobachten. Auffällig war ebenfalls eine verminderte Lokomotion während der Dunkel-Phase der nIRS-2tg Mäuse; die zirkadiane Rhythmik unterschied sich jedoch nicht vom WT. Damit übereinstimmend hatten die nIRS-2tg Mäuse einen niedrigeren Energieverbrauch während der Dunkel-Phase. Im Explorationsverhalten und der motorischen Koordination zeigten die nIRS2tg Mäuse keine Unterschiede. Schlussfolgerungen: Durch die neuronale Überexpression von IRS-2 kommt es zu einer verminderten Lokomotion sowie zu einem verminderten Energieverbrauch in der Dunkel-Phase. Wahrscheinlich ist die verminderte körperliche Aktivität die Ursache der beobachteten Zunahme des viszeralen Köperfettanteils sowie der veränderten Glukosetoleranz.

The role of mitochondrial dysfunction in the development of insulin resistance and type 2 diabetes remains controversial. In order to specifically define the relationship between insulin receptor (InsR) signaling, insulin resistance, hyperglycemia, hyperlipidemia and mitochondrial function, we analyzed mitochondrial performance of insulin-sensitive, slow-oxidative muscle in four different mouse models. In obese but normoglycemic ob/ob mice as well as in obese but diabetic mice under high-fat diet, mitochondrial performance remained unchanged even though intramyocellular diacylglycerols (DAGs), triacylglycerols (TAGs), and ceramides accumulated. In contrast, in muscle-specific InsR knockout (MIRKO) and streptozotocin (STZ)-treated hypoinsulinemic, hyperglycemic mice, levels of mitochondrial respiratory chain complexes and mitochondrial function were markedly reduced. In STZ, but not in MIRKO mice, this was caused by reduced transcription of mitochondrial genes mediated via decreased PGC-1α expression. We conclude that mitochondrial dysfunction is not causally involved in the pathogenesis of obesity-associated insulin resistance under normoglycemic conditions. However, obesity-associated type 2 diabetes and accumulation of DAGs or TAGs is not associated with impaired mitochondrial function. In contrast, chronic hypoinsulinemia and hyperglycemia as seen in STZ-treated mice as well as InsR deficiency in muscle of MIRKO mice lead to mitochondrial dysfunction. We postulate that decreased mitochondrial mass and/or performance in skeletal muscle of non-diabetic, obese or type 2 diabetic, obese patients observed in clinical studies must be explained by genetic predisposition, physical inactivity, or other still unknown factors.

BACKGROUND:Recent data suggest that insulin-like growth factor (IGF)-1 resistance in neurons prolongs longevity. In C. elegans this effect is mediated via DAF-16 the ortholog of the mammalian FoxO transcription factors. 3 different FoxO transcription factors (FoxOs) are expressed in rodent CNS: FoxO1, FoxO3a and FoxO6.METHODS:To define whether the different FoxOs are region-, sex- and age-specifically expressed, we analyzed FoxO mRNA levels in different brain regions from 6, 16, 60 and 100 weeks old mice using realtime-PCR. In addition, we fed mice a high fat diet (HFD) to experimentally induce obesity and diabetes and analyzed FoxO mRNA in the different brain regions.RESULTS:Interestingly, FoxO1 was predominantly expressed in the hippocampus whereas FoxO3a was quantitatively the most abundant FoxO in the neocortex. During aging, FoxO1 expression peaked in all brain regions at 16 weeks and FoxO6 showed its highest expression at 60 weeks in the parietal and occipital cortex. In 6 weeks old mice FoxO6 expression was higher in male compared to female mice in the hippocampus and all cortical regions. Surprisingly, in HFD animals FoxO3a was significantly less expressed in the cerebellum and all cortical regions compared to control animals. Even more dramatic, FoxO6 expression dropped about 80% in all brain regions in response to HFD.CONCLUSION:Thus, FoxOs in the CNS showed a highly distinct expression, which in addition was age- and sex-dependent. In contrast to FoxO1, FoxO3a and FoxO6 were specifically diminished in the CNS of HFD animals possibly contributing to the reduced lifespan observed in these animals.

Patients with type 2 diabetes (T2DM) have a two- to three-fold increased risk for Alzheimer's disease (AD), the most common form of dementia. Vascular complications might explain partially the increased incidence of neurodegeneration in patients with T2DM. Alternatively, neuronal resistance for insulin/insulin-like growth factor-1 (IGF- 1) might represent a molecular link between T2DM and AD, characterizing AD as "brain-type diabetes". According to this hypothesis, brains from AD patients showed substantially downregulated expression of the Insulin receptor (IR), the IGF-1 receptor (IGF-1R), and the insulin receptor substrate (IRS) proteins. Similar changes in insulin/IGF-1 signaling (IIS) have been described in animals fed a high fat diet and human T2DM, suggesting that decreased IIS might be involved in the pathogenesis of both T2DM and AD. In contrast, type 2 diabetic patients suffering from AD accumulate less β-amyloid (Aβ) compared to non-diabetic AD patients raising the question, whether the changes in IIS are cause, consequence, or compensatory counterregulation to neurodegeneration. Recent data in C. elegans showed that reducing IIS decreases Aβ toxicity. This effect is accomplished via two transcription factors downstream of IIS, DAF-16 and HSF- 1: The first detoxification path leads to degradation of the toxic misassemblies and is mediated via HSF-1. The second mechanism mediates the formation of low toxic, high molecular weight aggregates from highly toxic small molecular weight aggregates regulated by DAF-16 suggesting that Insulin/IGF-1 transmitted signals influence Aβ proteotoxicity. The current review discusses possible implications of recent findings in humans and model organisms for the understanding and possible therapeutic approaches of diabetes associated dementia.

Hyperglycemia in patients with type 2 diabetes causes multiple neuronal complications, e.g., diabetic polyneuropathy, cognitive decline, and embryonic neural crest defects due to increased apoptosis. Possible mechanisms of neuronal response to increased glucose burden are still a matter of debate. Insulin and insulin-like growth factor-1 (IGF-1) receptor signaling inhibits glucose-induced caspase-3 activation and apoptotic cell death. The insulin receptor substrates (IRS) are intracellular adapter proteins mediating insulin's and IGF-1's intracellular effects. Even though all IRS proteins have similar function and structure, recent data suggest different actions of IRS-1 and IRS-2 in mediating their anti-apoptotic effects in glucose neurotoxicity. We therefore investigated the role of IRS-1/-2 in glucose-induced caspase-3 activation using human neuroblastoma cells. Overexpression of IRS-1 or IRS-2 caused complete resistance to glucose-induced caspase-3 cleavage. Inhibition of PI3-kinase reversed this protective effect of IRS-1 or IRS-2. However, MAP-kinases inhibition had only minor impact. IRS overexpression increased MnSOD abundance as well as BAD phosphorylation while Bim and BAX levels remained unchanged. Since Akt promotes cell survival at least partially via phosphorylation and inhibition of downstream forkhead box-O (FoxO) transcription factors, we generated neuroblastoma cells stably overexpressing a dominant negative mutant of FoxO1 mimicking activation of the insulin/IGF-1 pathway on FoxO-mediated transcription. Using these cells we showed that FoxO1 is not involved in neuronal protection mediated by increased IRS-1/-2 expression. Thus, overexpression of both IRS-1 and IRS-2 induces complete resistance to glucose-induced caspase-3 activation via PI3-kinase mediated BAD phosphorylation and MnSOD expression independent of FoxO1.