PGC-1α, un co-activateur transcriptionnel impliqué dans le métabolisme

Transcriptional coactivators can be important targets for physiologic regulation. PPARgamma coactivator-1alpha (PGC-1alpha), in cooperation with several transcription factors, has emerged as a key regulator of several aspects of mammalian energy metabolism including mitochondrial biogenesis, adaptive thermogenesis in brown adipose tissue, glucose uptake, fiber type-switching in skeletal muscle, gluconeogenesis in liver and insulin secretion from pancreas. Recent studies have shown a reduced expression of PGC-1alpha in skeletal muscle of diabetic and prediabetic humans. Moreover, expression of PGC-1alpha in white fat cells activates a broad program of adaptive thermogenesis characteristic of brown fat cells. PGC-1alpha could be a target for antiobesity or diabetes drugs. The aim of this article was to summarize the molecular mechanisms and biological programs controlled by the transcriptional coactivator PGC-1alpha.

The sweet thing about Type 1 diabetes: A cryoprotective evolutionary adaptation

The reasons for the uneven worldwide distribution of Type 1 diabetes mellitus have yet to be fully explained. Epidemiological studies have shown a higher prevalence of Type 1 diabetes in northern Europe, particularly in Scandinavian countries, and Sardinia. Recent animal research has uncovered the importance of the generation of elevated levels of glucose, glycerol and other sugar derivatives as a physiological means for cold adaptation. High concentrations of these substances depress the freezing point of body fluids and prevent the formation of ice crystals in cells through supercooling, thus acting as a cryoprotectant or antifreeze for vital organs as well as in their muscle tissue. In this paper, we hypothesize that factors predisposing to elevated levels of glucose, glycerol and other sugar derivatives may have been selected for, in part, as adaptive measures in exceedingly cold climates. This cryoprotective adaptation would have protected ancestral northern Europeans from the effects of suddenly increasingly colder climates, such as those believed to have arisen around 14,000 years ago and culminating in the Younger Dryas. When life expectancy was short, factors predisposing to Type 1 diabetes provided a survival advantage. However, deleterious consequences of this condition have become significant only in more modern times, as life expectancy has increased, thus outweighing their protective value. Examples of evolutionary adaptations conferring selection advantages against human pathogens that result in deleterious effects have been previously reported as epidemic pathogenic selection (EPS). Such proposed examples include the cystic fibrosis mutations in the CFTR gene bestowing resistance to Salmonella typhi and hemochromatosis mutations conferring protection against iron-seeking intracellular pathogens. This paper is one of the first accounts of a metabolic disorder providing a selection advantage not against a pathogenic stressor alone, but rather against a climatic change. We thus believe that the concept of EPS should now include environmental factors that may be nonorganismal in nature. In so doing we propose that factors resulting in Type 1 diabetes be considered a result of environmental pathogenic selection (EnPS).

Fast apoptosis and erythroid differentiation induced by imatinib mesylate in JURL-MK1 cells

We compare the effects of Imatinib mesylate (Glivec) on chronic myeloid leukemia derived cell lines K562 and JURL-MK1. In both cell lines, the cell cycle arrests in G(1)/G(0) phase within 24 h after the addition of 1 microM Imatinib. This is followed by a decrease of Ki-67 expression and the induction of apoptosis. In JURL-MK1 cells, the apoptosis is faster in comparison with K562 cells: the caspase-3 activity reaches the peak value (20 to 30 fold of the control) after about 40 h and the apoptosis proceeds to its culmination point, the DNA fragmentation, within 48 h following 1 microM Imatinib addition. Unlike K562 cells, JURL-MK1 cells possess a probably functional p53 protein inducible by TPA (tetradecanoyl phorbol acetate) or UV-B irradiation. However, no increase in p53 expression was observed in Imatinib-treated JURL-MK1 cells indicating that the difference in the apoptosis rate between the two cell lines is not due to the lack of p53 in K562 cells. Imatinib also triggers erythroid differentiation both in JURL-MK1 and K562 cells. Glycophorin A expression occurred simultaneously with the apoptosis, even at the single cell level. In K562 cells, but not in JURL-MK1 cells, the differentiation process involved increased hemoglobin synthesis. However, during spontaneous evolution of JURL-MK1 cells in culture, the effects produced by Imatinib progressively changed from the fast apoptosis to the more complete erythroid differentiation. We suggest that the apoptosis and the erythroid differentiation are parallel effects of Imatinib and their relative contributions, kinetics and completeness are related to the differentiation stage of the treated cells.

Cross-talk between the insulin and leptin signaling systems in rat hypothalamus

OBJECTIVE

To investigate whether insulin and leptin share common intracellular signal transduction pathways and to determine whether these hormonal signaling systems modulate each other's action in rat hypothalamus.

RESEARCH METHODS AND PROCEDURES

Male Wistar rats were studied after chronic implantation of an intracerebroventricular catheter into the third ventricle. Immunoprecipitation and immunoblotting were used to examine the activation of insulin and leptin signaling molecules in the rat hypothalamus.

RESULTS

Insulin alone is able to produce molecular activation of insulin receptor substrates (IRSs)/phosphatidylinositol 3-kinase (PI 3-kinase)/Akt and mitogen-activated protein (MAP) kinase signaling pathways in hypothalamus, whereas leptin alone activates MAP kinase and IRSs/PI 3-kinase signaling with no effect on Akt. Combined infusion of leptin and insulin provokes a dual action. There was no quantitative potentialization of any single hormone's action on the elements of the insulin signaling pathway, IRSs/PI 3-kinase/Akt, and MAP kinase. Conversely, leptin plus insulin leads to quantitative potentialization of molecular signaling through the Janus kinase/signal transducer and activator of transcription pathway.

DISCUSSION

We provide evidence for a convergence of leptin and insulin signaling at the level of IRSs-PI 3-kinase and a divergence at the level of Akt. Moreover, our results indicate a direct and positive cross-talk between insulin and leptin at the level of Janus kinase 2 and signal transducer and activator of transcription 3 tyrosine phosphorylation. This mechanism may serve to potentiate the activity of both insulin and leptin pathways and to increase stimulation in physiological processes such as the control of food intake and body weight, which are under the combined control of insulin and leptin.

Identification of the cytoplasmic domains of CXCR4 involved in Jak2 and STAT3 phosphorylation

The chemokine SDF-1alpha transduces G(i)-dependent and -independent signals through CXCR4. Activation of Jak2/STAT3, a G(i)-independent signaling pathway, which plays a major role in survival signals, is known to be activated after SDF-1alpha binding to CXCR4 but the domains of CXCR4 involved in this signaling remain unexplored. Using human embryonic kidney HEK-293 cells stably expressing wild-type or mutated forms of CXCR4, we demonstrated that STAT3 phosphorylation requires the N-terminal part of the third intracellular loop (ICL3) and the tyrosine 157 present at the end of the second intracellular loop (ICL2) of CXCR4. In contrast, neither the conserved Tyr(135) in the DRY motif at the N terminus of ICL2 nor the Tyr(65) and Tyr(76) in the first intracellular loop (ICL1) are involved in this activation. ICL3, which does not contain any tyrosine residues, is needed to activate Jak2. These results demonstrate that two separate domains of CXCR4 are involved in Jak2/STAT3 signaling. The N-terminal part of ICL3 is needed to activate Jak2 after SDF-1alpha binding to CXCR4, leading to phosphorylation of only one cytoplasmic Tyr, present at the C terminus of ICL2, which triggers STAT3 activation. This work has profound implications for the understanding of CXCR4-transduced signaling.

Leptin and insulin stimulation of signalling pathways in arcuate nucleus neurones: PI3K dependent actin reorganization and KATP channel activation

Background

Leptin and insulin are long-term regulators of body weight. They act in hypothalamic centres to modulate the function of specific neuronal subtypes, by altering transcriptional control of releasable peptides and by modifying neuronal electrical activity. A key cellular signalling intermediate, implicated in control of food intake by these hormones, is the enzyme phosphoinositide 3-kinase. In this study we have explored further the linkage between this enzyme and other cellular mediators of leptin and insulin action on rat arcuate nucleus neurones and the mouse hypothalamic cell line, GT1-7.

Results

Leptin and insulin increased the levels of various phosphorylated signalling intermediates, associated with the JAK2-STAT3, MAPK and PI3K cascades in the arcuate nucleus. Inhibitors of PI3K were shown to reduce the hormone driven phosphorylation through the PI3K and MAPK pathways. Using isolated arcuate neurones, leptin and insulin were demonstrated to increase the activity of K_ATP channels in a PI3K dependent manner, and to increase levels of PtdIns(3,4,5)P₃. K_ATP activation by these hormones in arcuate neurones was also sensitive to the presence of the actin filament stabilising toxin, jasplakinolide. Using confocal imaging of fluorescently labelled actin and direct analysis of G- and F-actin concentration in GT1-7 cells, leptin was demonstrated directly to induce a re-organization of cellular actin, by increasing levels of globular actin at the expense of filamentous actin in a PI3-kinase dependent manner. Leptin stimulated PI3-kinase activity in GT1-7 cells and an increase in PtdIns(3,4,5)P₃ could be detected, which was prevented by PI3K inhibitors.

Conclusions

Leptin and insulin mediated phosphorylation of cellular signalling intermediates and of K_ATP channel activation in arcuate neurones is sensitive to PI3K inhibition, thus strengthening further the likely importance of this enzyme in leptin and insulin mediated energy homeostasis control. The sensitivity of leptin and insulin stimulation of K_ATP channel opening in arcuate neurones to jasplakinolide indicates that cytoskeletal remodelling may be an important contributor to the cellular signalling mechanisms of these hormones in hypothalamic neurones. This hypothesis is reinforced by the finding that leptin induces actin filament depolymerization, in a PI3K dependent manner in a mouse hypothalamic cell line.

Characterization of mice doubly transgenic for parathyroid hormone-related protein and murine placental lactogen: a novel role for placental lactogen in pancreatic beta-cell survival

Transgenic overexpression of either parathyroid hormone-related peptide (PTHrP) or mouse placental lactogen type 1 (mPL1) in pancreatic beta-cells, using the rat insulin II promoter (RIP), results in islet hyperplasia either through prolonged beta-cell survival or through increased beta-cell proliferation and hypertrophy, respectively. For determining whether the two proteins might exert complementary, additive, or synergistic effects on islet mass and function when simultaneously overexpressed in beta-cells in vivo, RIP-PTHrP and RIP-mPL1 mice were crossed to generate mice doubly transgenic for PTHrP and mPL1. These double-transgenic mice displayed marked islet hyperplasia (threefold), hypoglycemia, increased beta-cell proliferation (threefold), and resistance to the diabetogenic and cytotoxic effects of streptozotocin compared with their normal siblings. Although the phenotype of the double-transgenic mice was neither additive nor synergistic relative to their single-transgenic counterparts, it was indeed complementary, yielding the maximal salutary phenotypic features of both individual transgenes. Finally, mPL1, for the first time, was shown to exert a protective effect on the survival of beta-cells, placing it among the few proteins that can improve function and proliferation and prolong the survival of beta-cells. Placental lactogen 1 is an attractive target for future therapeutic strategies in diabetes.

beta3-Adrenergic-dependent and -independent mechanisms participate in cold-induced modulation of insulin signal transduction in brown adipose tissue of rats

During cold exposure, homeothermic animals mobilize glucose with higher efficiency than at thermoneutrality. An interaction between the insulin signal transduction machinery and high sympathetic tonus is thought to play an important role in this phenomenon. In the present study, rats were exposed to cold during 8 days and treated, or not, with a beta3-adrenergic agonist, BRL37344 sodium 4-2-2-(3-chlorophenyl)-2-hydroxyethyl amino propyl phenoxy-acetic acid sodium (BRL37344), or antagonist, SR59230A 3-(2-ethylphenoxy)-[(1S)-1,2,3,4-tetrahydronaphth-1-ylamino]-(2S)-2-propanol oxalate (SR59230A), to evaluate the cross-talk between insulin and beta3-adrenergic intracellular signaling in brown adipose tissue. The drugs did not modify food ingestion, body temperature, and body weight in control and cold-exposed rats. Treatment of control rats with BRL37344 led to higher insulin-induced tyrosine phosphorylation of the insulin receptors, insulin receptor substrate (IRS)-1 and ERK, higher insulin-induced IRS-1/PI3-kinase association, and higher [Ser(473)] phosphorylation of Akt. Cold exposure alone promoted higher insulin-induced tyrosine phosphorylation of the insulin receptors, IRS-1, IRS-2, and ERK, and higher insulin-induced IRS-1 and IRS-2/PI3-kinase association. Except for the regulation of ERK, SR59230A abolished all the cold-induced effects upon the insulin signal transduction pathway. However, this antagonist only partially inhibited the cold-induced increase of glucose uptake. Thus, the sympathetic tonus generated during cold-exposure acts, in brown adipose tissue, through the beta3-adrenergic receptor and modulates insulin signal transduction, with the exception of ERK. However, insulin-independent mechanisms other than beta3-adrenergic activation participate in cold-induced glucose uptake in brown adipose tissue of rats.

Molecular mechanisms of insulin resistance that impact cardiovascular biology

Insulin resistance is concomitant with type 2 diabetes, obesity, hypertension, and other features of the metabolic syndrome. Because insulin resistance is associated with cardiovascular disease, both scientists and physicians have taken great interest in this disorder. Insulin resistance is associated with compensatory hyperinsulinemia, but individual contributions of either of these two conditions remain incompletely understood and a subject of intense investigation. One possibility is that in an attempt to overcome the inhibition within the metabolic insulin-signaling pathway, hyperinsulinemia may continue to stimulate the mitogenic insulin-signaling pathway, thus exerting its detrimental influence. Here we discuss some of the effects of insulin resistance and mechanisms of potentially detrimental influence of hyperinsulinemia in the presence of metabolic insulin resistance.

Leptinomimetic effects of the AMP kinase activator AICAR in leptin-resistant rats: prevention of diabetes and ectopic lipid deposition

AIMS/HYPOTHESIS

Leptin has been shown to activate AMP-activated protein kinase (AMPK), an enzyme that regulates the activities of key enzymes of lipid synthesis and metabolism. We assess here (i) whether AMPK activity is diminished in rodents deficient in leptin or the leptin receptor, and (ii) the effects of treating the diabetes-prone, leptin-receptor-deficient Zucker Diabetic Fatty (ZDF) rat with an AMPK activator.

METHODS

AMPK activity and related parameters were measured in muscle and or liver of fa/fa and ZDF rats and ob/ob mice. We also explored the effect of treatment with the AMPK activator 5-aminoimidazole 4-carboxamide 1-beta-D ribofuranoside (AICAR) (7.4 mmol/l, on Monday, Wednesday and Friday for 15 weeks, beginning at 7 weeks of age) on the phenotype of the ZDF rat.

RESULTS

AMPK activity was diminished in muscle and/or liver of fa/fa (leptin-receptor-deficient, non-diabetic) and ZDF (leptin-receptor-deficient, diabetes-prone) rats and ob/ob mice (leptin-deficient). ZDF rats that had free access to food became hyperglycaemic (22.2 mmol/l) and hyperphagic after 2 to 5 weeks and remained so during the remainder of the study. Treatment of ZDF rats with AICAR prevented the development of diabetes, as well as increases of triglyceride content in liver, muscle and the pancreatic islets. It also attenuated the morphological abnormalities observed in the islets of untreated rats. Rats diet-matched with the AICAR-treated animals developed diabetes of intermediate severity and showed decreases in triglyceride content in the islets, but not in liver or muscle.

CONCLUSIONS/INTERPRETATION

The results indicate that a deficiency of leptin or the leptin receptor is associated with a decrease in AMPK activity in muscle and/or liver. They also suggest that treatment with an AMPK activator prevents the development of diabetes and ectopic lipid accumulation in the ZDF rat.

Alterations in mitogen-activated protein kinase kinase and extracellular regulated kinase signaling in theca cells contribute to excessive androgen production in polycystic ovary syndrome

We have investigated the involvement of the MAPK signaling pathway in increased androgen biosynthesis and CYP17 gene expression in women with polycystic ovary syndrome (PCOS). A comparison of MAPK kinase (MEK1/2) and ERK1/2 phosphorylation in propagated normal and PCOS theca cells, revealed that MEK1/2 phosphorylation was decreased more than 70%, and ERK1/2 phosphorylation was reduced 50% in PCOS cells as compared with normal cells. Infection with dominant-negative MEK1 increased CYP17 mRNA and dehydroepiandrosterone (DHEA) abundance, whereas constitutively active MEK1 reduced DHEA production and CYP17 mRNA abundance. Similarly, the MEK inhibitor, PD98059, increased CYP17 mRNA accumulation and CYP17 promoter activity to levels observed in PCOS cells. Remarkably, in theca cells maintained in the complete absence of insulin, ERK1/2 phosphorylation was decreased in PCOS theca cells as compared with normal theca cells, and CYP17 mRNA and DHEA synthesis were increased in PCOS theca cells. These studies demonstrate that in PCOS cells reduced levels of activated MEK1/2 and ERK1/2 are correlated with increased androgen production, irrespective of the insulin concentration. These findings implicate alterations in the MAPK pathway in the pathogenesis of excessive ovarian androgen production in PCOS.

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Hypothalamic PI3K and MAPK differentially mediate regional sympathetic activation to insulin

The action of insulin in the central nervous system produces sympathetic nervous system activation (also called sympathoactivation), although the neuronal intracellular mechanisms that mediate this are unclear. We hypothesized that PI3K and MAPK, the major pathways involved in insulin receptor signaling, mediate sympathetic nerve responses to insulin. Intracerebroventricular administration of insulin in rat increased multifiber sympathetic nerve activity to the hindlimb, brown adipose tissue (BAT), adrenal gland, and kidney. Ex vivo biochemical studies of mediobasal hypothalamic tissue revealed that insulin stimulated the association of insulin receptor substrate-1 with the p85alpha subunit of PI3K and also tyrosine phosphorylation of p42 and p44 subunits of MAPK in the hypothalamus. In order to determine whether PI3K and/or MAPK were involved in insulin-mediated sympathoactivation, we tested the effect of specific inhibitors of PI3K (LY294002 and wortmannin) and MAPK (PD98059 and U0126) on regional sympathetic responses to insulin. Interestingly, regional sympathoactivation to insulin was differentially affected by blockade of PI3K and MAPK. Inhibition of PI3K specifically blocked insulin-induced sympathoactivation to the hindlimb, while inhibition of MAPK specifically blocked insulin-induced sympathoactivation to BAT. Sympathoactivation to corticotrophin-releasing factor, however, was not affected by inhibition of PI3K and MAPK. These data demonstrate that PI3K and MAPK are specific and regionally selective mediators of the action of insulin on the sympathetic nervous system.

Cold-induced PGC-1alpha expression modulates muscle glucose uptake through an insulin receptor/Akt-independent, AMPK-dependent pathway

Peroxisome proliferator-activated receptor-gamma coactivator-1alpha (PGC-1alpha) participates in control of expression of genes involved in adaptive thermogenesis, muscle fiber type differentiation, and fuel homeostasis. The objective of the present study was to evaluate the participation of cold-induced PGC-1alpha expression in muscle fiber type-specific activity of proteins that belong to the insulin-signaling pathway. Rats were exposed to 4 degrees C for 4 days and acutely treated with insulin in the presence or absence of an antisense oligonucleotide to PGC-1alpha. Cold exposure promoted a significant increase of PGC-1alpha and uncoupling protein-3 protein expression in type I and type II fibers of gastrocnemius muscle. In addition, cold exposure led to higher glucose uptake during a hyperinsulinemic clamp, which was accompanied by higher expression and membrane localization of GLUT4 in both muscle fiber types. Cold exposure promoted significantly lower insulin-induced tyrosine phosphorylation of the insulin receptor (IR) and Ser473 phosphorylation of acute transforming retrovirus thymoma (Akt) and an insulin-independent increase of Thr172 phosphorylation of adenosine 5'-monophosphate-activated protein kinase (AMPK). Inhibition of PGC-1alpha expression in cold-exposed rats by antisense oligonucleotide treatment diminished glucose clearance rates during a hyperinsulinemic clamp and reduced expression and membrane localization of GLUT4. Reduction of PGC-1alpha expression resulted in no modification of insulin-induced tyrosine phosphorylation of the IR and Ser473 phosphorylation of Akt. Finally, reduction of PGC-1alpha resulted in lower Thr172 phosphorylation of AMPK. Thus cold-induced hyperexpression of PGC-1alpha participates in control of skeletal muscle glucose uptake through a mechanism that controls GLUT4 expression and subcellular localization independent of the IR and Akt activities but dependent on AMPK.

Effects of vanadate on the activities of mice glucokinase and hexokinase

This study aimed at acquiring knowledge on the hypoglycemic mechanisms of sodium metavanadate (SMV) showed that the liver glucokinase and muscle hexokinase activities increased rapidly after oral SMV was given, and that the blood glucose level was correlated closely with the activities of the two enzymes but not with the insulin level; which indicated that SMV could improve the altered glucose phosphorylation in diabetic mice independently of stimulating insulin secretion. This was probably one of the mechanisms of hypoglycemic effects of SMV.

Insulin and its evolving partnership with leptin in the hypothalamic control of energy homeostasis

Despite an alarming increase in the burden of obesity worldwide, body adiposity seems to be a regulated physiological variable. Regulation of adiposity occurs through a classical endocrine feedback loop, in which the pancreatic beta-cell-derived hormone insulin and the adipocyte-derived hormone leptin signal the status of body energy stores to the hypothalamus. Recent advances in our understanding of the signal transduction mechanisms used by insulin and leptin in the hypothalamus to modulate neuronal firing suggest that intracellular cross-talk occurs at several levels and is a potentially important determinant of regulated body weight. These pathways are thus an attractive target for pharmacological intervention in the treatment of obesity.

A prototype antibody microarray platform to monitor changes in protein tyrosine phosphorylation

Reversible protein phosphorylation is a key regulatory process in all living cells. Deregulation of modification control mechanisms, especially in the case of tyrosine, may lead to malignant transformation and disease. Phosphotyrosine (p-Tyr) accounts for only 0.05% of the total cellular phospho-amino acid content, yet plays an unusually prominent role in eukaryotic signaling, development, and growth. Tracking temporal and positional p-Tyr changes across the cellular proteome, i.e. tyrosine phosphoproteomics, is therefore tremendously valuable. Here, we describe and evaluate a prototype antibody (Ab) microarray platform to monitor changes in protein Tyr phosphorylation. Availability permitting, a virtually unlimited number of Abs, each recognizing a specific cellular protein, may be arrayed on a chip, incubated with total cell or tissue extracts or with biological fluids, and then probed with a fluorescently labeled p-Tyr-specific monoclonal Ab, PY-KD1, specifically generated for this assay as part of the current study. The optimized protocol allowed detection of changes in the Tyr phosphorylation state of selected proteins using submicrogram to low nanogram of total protein extract, amounts that may conceivably be obtained from a thousand to a hundred thousand cells, or less, depending on the cell or tissue type. The assay platform was evaluated by assessing changes in a rationally selected subset of the Tyr phosphoproteome of Bcr-Abl-expressing cells treated with a specific inhibitor, Gleevec, and of epidermal growth factor (EGF)-treated HeLa cells. The results, ratiometric rather than strictly quantitative in nature, conformed with previous identifications of several Bcr-Abl and EGF receptor targets, and associated proteins, as detected by exhaustive mass spectrometric analyses. The Ab microarray method described here offers advantages of low sample and reagent consumption, scalability, detection multiplexing, and potential compatibility with microfluidic devices and automation. The system may hold particular promise for dissecting signaling pathways, molecular classification of tumors, and profiling of novel target-cancer drugs.

RNAi-based Analysis of CAP, Cbl, and CrkII Function in the Regulation of GLUT4 by Insulin

Stimulation of glucose transport by insulin in cultured adipocytes through translocation of intracellular GLUT4 glucose transporters to the plasma membrane has been suggested to require phosphatidylinositol (PI) 3-kinase-dependent and independent mechanisms. To test the involvement of a PI 3-kinase-independent pathway leading to activation of the TC10 GTPase, the putative intermediates CAP, c-Cbl, Cbl-b, and CrkII were selectively depleted in 3T3-L1 adipocytes using highly efficient small interfering (si) RNAs. Simultaneous depletion of the ubiquitination factors c-Cbl plus Cbl-b in cultured adipocytes had the expected effect of delaying dephosphorylation of EGF receptors upon removal of EGF. However, siRNA-mediated gene silencing of both Cbl isoforms or CAP or CrkII in these cells failed to attenuate insulin-stimulated deoxyglucose transport or Myc-tagged GLUT4-GFP translocation at either sub-maximal or maximal concentrations of insulin. The dose-response relationship for insulin stimulation of deoxyglucose transport in primary adipocytes derived from c-Cbl knock-out mice was also identical to insulin action on adipocytes from wild type mice. These data are consistent with the hypothesis that CAP, Cbl iso-forms, and CrkII are not required components of insulin signaling to GLUT4 transporters.

Exercise training increases glycogen synthase activity and GLUT4 expression but not insulin signaling in overweight nondiabetic and type 2 diabetic subjects

Exercise training improves insulin sensitivity in subjects with and without type 2 diabetes. However, the mechanism by which this occurs is unclear. The present study was undertaken to determine how improved insulin signaling, GLUT4 expression, and glycogen synthase activity contribute to this improvement. Euglycemic clamps with indirect calorimetry and muscle biopsies were performed before and after 8 weeks of exercise training in 16 insulin-resistant nondiabetic subjects and 6 type 2 diabetic patients. Training increased peak aerobic capacity (Vo(2peak)) in both nondiabetic (from 34 +/- 2 to 39 +/- 2 mL O(2)/kg fat-free mass [FFM]/min, 14% +/- 2%, P <.001) and diabetic (from 26 +/- 3 to 34 +/- 3 mL O(2)/kg FFM/min, 32% +/- 4%) subjects. Training also increased insulin-stimulated glucose disposal in nondiabetic (from 6.2 +/- 0.5 to 7.1 +/- 0.7 mg/kg FFM/min) and diabetic subjects (from 4.3 +/- 0.6 to 5.5 +/- 0.6 mg/kg FFM/min). Total glycogen synthase activity was increased by 46% +/- 17% and 45% +/- 12% in nondiabetic and diabetic subjects, respectively, in response to training (P <.01 v before training). Moreover, after training, glycogen synthase fractional velocity was correlated with insulin-stimulated glucose storage (r = 0.53, P <.05) and the training-induced improvement in glucose disposal was accounted for primarily by increased insulin-stimulated glucose storage. Training also increased GLUT4 protein by 38% +/- 8% and 22% +/- 10% in nondiabetic and diabetic subjects, respectively (P <.05 v. before training). Akt protein expression, which was decreased by 29% +/- 3% (P <.05) in the diabetic subjects before training (compared to the nondiabetics), increased significantly in both groups (P <.001). In contrast, exercise training did not enhance the ability of insulin to stimulate insulin receptor substrate-1 (IRS-1)-associated phosphatidylinositol 3 (PI 3)-kinase activity. The present data are consistent with a working model whereby 8 weeks of exercise training increases insulin-stimulated glucose disposal primarily by increasing GLUT4 protein expression without enhancing insulin-stimulated PI 3-kinase signaling, and that once the glucose enters the myocyte, increased glycogen synthase activity preferentially shunts it into glycogen synthesis.

SH2-B promotes insulin receptor substrate 1 (IRS1)- and IRS2-mediated activation of the phosphatidylinositol 3-kinase pathway in response to leptin

Leptin regulates energy homeostasis primarily by binding and activating its long form receptor (LRb). Deficiency of either leptin or LRb causes morbid obesity. Leptin stimulates LRb-associated JAK2, thus initiating multiple pathways including the Stat3 and phosphatidylinositol (PI) 3-kinase pathways that mediate leptin biological actions. Here we report that SH2-B, a JAK2-interacting protein, promotes activation of the PI 3-kinase pathway by recruiting insulin receptor substrate 1 (IRS1) and IRS2 in response to leptin. SH2-B directly bound, via its PH and SH2 domain, to both IRS1 and IRS2 both in vitro and in intact cells and mediated formation of a JAK2/SH2-B/IRS1 or IRS2 tertiary complex. Consequently, SH2-B dramatically enhanced leptin-stimulated tyrosine phosphorylation of IRS1 and IRS2 in HEK293 cells stably expressing LRb, thus promoting association of IRS1 and IRS2 with the p85 regulatory subunit of PI 3-kinase and phosphorylation and activation of Akt. SH2-B mutants with lower affinity for IRS1 and IRS2 exhibited reduced ability to promote association of JAK2 with IRS1, tyrosine phosphorylation of IRS1, and association of IRS1 with p85 in response to leptin. Moreover, deletion of the SH2-B gene impaired leptin-stimulated tyrosine phosphorylation of endogenous IRS1 in mouse embryonic fibroblasts (MEF), which was reversed by reintroduction of SH2-B. Similarly, SH2-B promoted growth hormone-stimulated tyrosine phosphorylation of IRS1 in both HEK293 and MEF cells. Our data suggest that SH2-B is a novel mediator of the PI 3-kinase pathway in response to leptin or other hormones and cytokines that activate JAK2.

Increased insulin receptor substrate 1 serine phosphorylation and stress-activated protein kinase/c-Jun N-terminal kinase activation associated with vascular insulin resistance in spontaneously hypertensive rats

Insulin resistance is associated with cardiovascular disease. Impaired insulin receptor substrate (IRS)-mediated signal transduction is a major contributor to insulin resistance. Recently, IRS-1 phosphorylation at serine 307 by stress-activated protein kinase/c-Jun N-terminal kinase (SAPK/JNK) has been highlighted as a molecular event that causes insulin resistance. We investigated IRS-1-mediated insulin signaling, IRS-1 phosphorylation at serine 307, and SAPK/JNK activation status in the aorta of spontaneously hypertensive rats (SHR) by immunoprecipitation and immunoblotting. Insulin-stimulated tyrosine phosphorylation of insulin receptor and IRS-1 in SHR was decreased to 55% (P<0.01) and 40% (P<0.01) of the levels in Wistar-Kyoto rats (WKY), respectively. Insulin-stimulated IRS-1-associated phosphatidylinositol 3-kinase activation in SHR was reduced to 28% of the level in WKY (P<0.0001). Immunoblot analysis revealed that phosphorylated IRS-1 at serine 307 in SHR was increased to 261% (P<0.001) of the level in WKY. Phosphorylated (activated) SAPK/JNK in SHR was increased to 223% of the level in WKY (P<0.01). Serine-phosphorylated IRS-1 that was immunoprecipitated from the aorta of SHR was capable of inhibiting in vitro tyrosine phosphorylation by recombinant insulin receptor compared with WKY-derived IRS-1. These findings demonstrate that insulin resistance in the aorta of SHR was associated with elevated IRS-1 phosphorylation at serine 307 and increased SAPK/JNK activation. The present study suggests that increased SAPK/JNK activation may play an important role in the pathogenesis of vascular insulin resistance via inhibitory serine phosphorylation of IRS-1.

Ovarian insufficiency and early pregnancy loss induced by activation of the innate immune system

We describe a murine model of early pregnancy failure induced by systemic activation of the CD40 immune costimulatory pathway. Although fetal loss involved an NK cell intermediate, it was not due to lymphocyte-mediated destruction of the fetus and placenta. Rather, pregnancy failure resulted from impaired progesterone synthesis by the corpus luteum of the ovary, an endocrine defect in turn associated with ovarian resistance to the gonadotropic effects of prolactin. Pregnancy failure also required the proinflammatory cytokine TNF-alpha and correlated with the luteal induction of the prolactin receptor signaling inhibitors suppressor of cytokine signaling 1 (Socs1) and Socs3. Such links between immune activation and reproductive endocrine dysfunction may be relevant to pregnancy loss and other clinical disorders of reproduction.

Aspirin inhibits serine phosphorylation of IRS-1 in muscle and adipose tissue of septic rats

Whole body insulin resistance has been demonstrated in septic patients and in infected animals. In this study, we demonstrate that sepsis induces insulin resistance and that pretreatment with aspirin inhibits sepsis-induced insulin resistance. Sepsis was observed to lead to serine phosphorylation of IRS-1, a phenomenon which was reversed by aspirin in muscle and WAT, in parallel with a reduction in JNK activity. In addition, our data show an impairment of insulin activation of IR and IRS-1 tyrosine phosphorylation in septic rats and, consistent with the reduction of IRS-1 serine phosphorylation observed in septic animals pretreated with aspirin, there was an increase in IRS-1 protein levels and tyrosine phosphorylation in muscle and WAT. Overall, these results provide important new insights into the mechanism of sepsis-induced insulin resistance.

Effects of aerobic exercise training on the protein kinase B (PKB)/mammalian target of rapamycin (mTOR) signaling pathway in aged skeletal muscle

The protein kinase B (PKB)/mammalian target of rapamycin (mTOR) signaling pathway is thought to play a critical role in the regulation of protein synthesis and skeletal muscle mass. The purpose of the present study was to determine the effects of voluntary wheel running on the PKB/mTOR signaling pathway in muscles from aged mice (20-22 months). The total levels of mTOR were 65% higher in gastrocnemius muscles from aged mice subjected to wheel running compared to aged sedentary mice (p-0.002) PKB phosphorlation on Ser473 was 45% higher in gastrocnemius muscles from aged mice subjected to wheel running compared to aged sedentary mice (p=0.01) The total abundance of PKB was 50% higher in gastrocnemius muscles from wheel running mice compared to aged controls (p=0.005). Three months of wheel running did not alter the total amount of p70 S6K in gastrocnemius muscle. Protein synthesis, as assessed by [(14)C]phenylalanine incorporation into protein was significantly higher in soleus muscles from aged mice subjected to wheel running compared to aged sedentary mice (p-0.001) These findings indicate the aerobic exercise training may attenuate the age-related decline in protein synthesis, a process that appears to be due, in part, to increases in mTOR and PKB.

Disruption of cortical actin in skeletal muscle demonstrates an essential role of the cytoskeleton in glucose transporter 4 translocation in insulin-sensitive tissues

Cell culture work suggests that signaling to polymerize cortical filamentous actin (F-actin) represents a required pathway for the optimal redistribution of the insulin-responsive glucose transporter, GLUT4, to the plasma membrane. Recent in vitro study further suggests that the actin-regulatory neural Wiskott-Aldrich syndrome protein (N-WASP) mediates the effect of insulin on the actin filament network. Here we tested whether similar cytoskeletal mechanics are essential for insulin-regulated glucose transport in isolated rat epitrochlearis skeletal muscle. Microscopic analysis revealed that cortical F-actin is markedly diminished in muscle exposed to latrunculin B. Depolymerization of cortical F-actin with latrunculin B caused a time- and concentration-dependent decline in 2-deoxyglucose transport. The loss of cortical F-actin and glucose transport was paralleled by a decline in insulin-stimulated GLUT4 translocation, as assessed by photolabeling of cell surface GLUT4 with Bio-LC-ATB-BMPA. Although latrunculin B impaired insulin-stimulated GLUT4 translocation and glucose transport, activation of phosphatidylinositol 3-kinase and Akt by insulin was not rendered ineffective. In contrast, the ability of insulin to elicit the cortical F-actin localization of N-WASP was abrogated. These data provide the first evidence that actin cytoskeletal mechanics are an essential feature of the glucose transport process in intact skeletal muscle. Furthermore, these findings support a distal actin-based role for N-WASP in insulin action in vivo.

Expression and functional activity of PPARgamma in pancreatic beta cells

Rosiglitazone is an agonist of peroxisome proliferator activated receptor-gamma (PPARgamma) and ameliorates insulin resistance in type II diabetes. In addition, it may also promote increased pancreatic beta-cell viability, although it is not known whether this effect is mediated by a direct action on the beta cell. We have investigated this possibility. Semiquantitative real-time reverse transcription-polymerase chain reaction analysis (Taqman) revealed that freshly isolated rat islets and the clonal beta-cell line, BRIN-BD11, express PPARgamma, as well as PPARalpha and PPARdelta. The levels of expression of PPARgamma were estimated by reference to adipose tissue and were found to represent approximately 60% (islets) and 30% (BRIN-BD11) of that found in freshly isolated visceral adipose tissue. Western blotting confirmed the presence of immunoreactive PPARgamma in rat (and human) islets and in BRIN-BD11 cells. Transfection of BRIN-BD11 cells with a PPARgamma-sensitive luciferase reporter construct was used to evaluate the functional competence of the endogenous PPARgamma. Luciferase activity was modestly increased by the putative endogenous ligand, 15-deoxy-Delta12,14 prostaglandin J2 (15dPGJ2). Rosiglitazone also caused activation of the luciferase reporter construct but this effect required concentrations of the drug (50-100 microm) that are beyond the expected therapeutic range. This suggests that PPARgamma is relatively insensitive to activation by rosiglitazone in BRIN-BD11 cells. Exposure of BRIN-BD11 cells to the lipotoxic effector, palmitate, caused a marked loss of viability. This was attenuated by treatment of the cells with either actinomycin D or cycloheximide suggesting that a pathway of programmed cell death was involved. Rosiglitazone failed to protect BRIN-BD11 cells from the toxic actions of palmitate at concentrations up to 50 microm. Similar results were obtained with a range of other PPARgamma agonists. Taken together, the present data suggest that, at least under in vitro conditions, thiazolidinediones do not exert direct protective effects against fatty acid-mediated cytotoxicity in pancreatic beta cells.

cDNA cloning and functional characterization of a novel splice variant of c-Cbl-associated protein from mouse skeletal muscle

c-Cbl-associated protein (CAP) is an SH3-containing adapter protein that binds to the proto-oncogene c-Cbl. Recent work suggests that signaling through these molecules is involved in the regulation of insulin-stimulated glucose uptake in 3T3-L1 adipocytes. Skeletal muscle is the major site of insulin-stimulated glucose disposal but there have been no reports of CAP function in this tissue. Using RT-PCR of mouse skeletal muscle RNA, we discovered a novel splice variant of CAP (CAPSM; GenBank Accession No. AF521593) that is different from the adipocyte form by inclusion of a novel 168 bp fragment. This fragment encodes a peptide sequence that shows very high similarity with exon 25 of the human homologue of CAP (SORBS1). To understand the function of CAPSM in glucose uptake regulation, L6 myotubes were transfected with either CAPSM or a truncated CAPSM devoid of all three SH3-binding domains (CAPDeltaSH3), which prevents CAP association with c-Cbl. Transfection with CAPDeltaSH3 decreased insulin-stimulated 2-deoxyglucose (2-DG) uptake and reduced c-Cbl phosphorylation. In contrast, transfection of L6 myotubes with CAPDeltaSH3 had no effect on dinitrophenol (DNP)- or hypoxia-stimulated glucose uptake, stimuli that work through insulin-independent mechanisms for the regulation of glucose uptake. These data demonstrate the existence of a novel CAP isoform expressed in skeletal muscle, and suggest the involvement of the CAP/Cbl pathway in the regulation of insulin-stimulated glucose uptake in L6 myotubes.

Regulation of luteinizing hormone/human chorionic gonadotropin receptor expression: a perspective

The LH/hCG receptor, a member of the G protein coupled receptor family mediates the cellular actions of LH in the ovary. A considerable amount of information regarding its structure, mechanism of activation, and regulation of expression has emerged in recent years. Here we provide a brief overview of the current information on the structural organization of the receptor and the mechanism of receptor mediated signaling as well as an in-depth discussion on recent developments pertaining to the regulation of receptor expression. Specifically, we describe studies from our laboratory showing that the posttranscriptional regulation of the receptor involves an LH/hCG receptor mRNA-binding protein. We also propose a model to explain the loss of steady-state LH/hCG receptor mRNA levels during receptor down-regulation.

Brain insulin and feeding: a bi-directional communication

Insulin and specific insulin receptors are found widely distributed in the central nervous system (CNS) networks related in particular to energy homeostasis. This review highlights the complex regulatory loop between dietary nutrients, brain insulin and feeding. It is well documented that brain insulin has a negative, anorexigenic effect on food intake. At present, a specific role for brain insulin on cognitive functions related to feeding is emerging. The balance between orexigenic and anorexigenic pathways in the hypothalamus is crucial for the maintenance of energy homeostasis in animals and humans. The ingestion of nutrients triggers neurochemical events that signal nutrient and energy availability in the CNS, down regulate stimulators, activate anorexigenic factors, including brain insulin, and result in reduced eating. The effects of insulin in the CNS are under a multilevel control of food-intake peripherally and in the CNS, via the metabolic, endocrine and neural modifications induced by nutrients. Single meals as well as glucose and serotonin are able to regulate insulin release directly in the hypothalamus and may be of importance for its biological effects. Central mechanisms operating in glucose-induced insulin release show some analogy with the mechanisms operating in the pancreas. Leptin and melanocortins, peptides that down regulate food intake and are largely affected by nutrients, are highly interactive with insulin in the CNS probably via the neurotransmitter serotonin. In the hypothalamus, insulin and leptin share a common signaling pathway involved in food intake, namely the insulin receptor substrate, phosphatidylinositol 3-kinase pathway. Over or under-feeding, unbalanced single meals or diets, in particular diets enriched in fat, modify the amount of insulin actively transported into the brain, the release of brain insulin, the expression of insulin messenger RNA and potentially disrupt insulin signaling in the CNS. This impairment may result in disorders related to feeding behavior and energy homeostasis leading to profound dysregulations, obesity or diabetes.

Role of the p38 mitogen-activated protein kinase pathway in the generation of the effects of imatinib mesylate (STI571) in BCR-ABL-expressing cells

Imatinib mesylate (STI571), a specific inhibitor of the BCR-ABL tyrosine kinase, exhibits potent antileukemic effects in vitro and in vivo. Despite the well established role of STI571 in the treatment of chronic myelogenous leukemia, the precise mechanisms by which inhibition of BCR-ABL tyrosine kinase activity results in generation of antileukemic responses remain unknown. In the present study we provide evidence that treatment of CML-derived BCR-ABL-expressing leukemia cells with STI571 results in activation of the p38 mitogen-activated protein (MAP) kinase signaling pathway. Our data indicate that STI571 induces phosphorylation of the p38 and activation of its kinase domain, in KT-1 cells and other BCR-ABL-expressing cell lines. We also identify the kinases MAP kinase-activated protein kinase-2 and Msk1 as two downstream effectors of p38, activated during inhibition of BCR-ABL activity by STI571. Importantly, pharmacological inhibition of p38 reverses the growth inhibitory effects of STI571 on primary leukemic colony-forming unit granulocyte/macrophage progenitors from patients with CML. Altogether, our data establish that activation of the p38 MAP kinase signaling cascade plays an important role in the generation of the effects of STI571 on BCR-ABL-expressing cells. They also suggest that, in addition to activation of mitogenic pathways, BCR-ABL promotes leukemogenesis by suppressing the function of growth inhibitory signaling cascades.

Angiotensin II impairs the insulin signaling pathway promoting production of nitric oxide by inducing phosphorylation of insulin receptor substrate-1 on Ser312 and Ser616 in human umbilical vein endothelial cells

It has been suggested that serine (Ser) phosphorylation of insulin receptor substrate-1 (IRS-1) decreases the ability of IRS-1 to be phosphorylated on tyrosine, thereby attenuating insulin signaling. There is evidence that angiotensin II (AII) may impair insulin signaling to the IRS-1/phosphatydilinositol 3-kinase (PI 3-kinase) pathway by enhancing Ser phosphorylation. Insulin stimulates NO production by a pathway involving IRS-1/PI3-kinase/Akt/endothelial NO synthase (eNOS). We addressed the question of whether AII affects insulin signaling involved in NO production in human umbilical vein endothelial cells and tested the hypothesis that the inhibitory effect of AII on insulin signaling was caused by increased site-specific Ser phosphorylation in IRS-1. Exposure of human umbilical vein endothelial cells to AII resulted in inhibition of insulin-stimulated production of NO. This event was associated with impaired IRS-1 phosphorylation at Tyr612 and Tyr632, two sites essential for engaging the p85 subunit of PI3-kinase, resulting in defective activation of PI 3-kinase, Akt, and eNOS. This inhibitory effect of AII was reversed by the type 1 receptor antagonist losartan. AII increased c-Jun N-terminal kinase (JNK) and extracellular signal-regulated kinase (ERK) 1/2 activity, which was associated with a concomitant increase in IRS-1 phosphorylation at Ser312 and Ser616, respectively. Inhibition of JNK and ERK1/2 activity reversed the negative effects of AII on insulin-stimulated NO production. Our data suggest that AII, acting via the type 1 receptor, increases IRS-1 phosphorylation at Ser312 and Ser616 via JNK and ERK1/2, respectively, thus impairing the vasodilator effects of insulin mediated by the IRS-1/PI 3-kinase/Akt/eNOS pathway.

Leptin: a multifaceted hormone in the central nervous system

It is well established that the adipocyte-derived hormone leptin is an important circulating satiety factor that regulates body weight and food intake via its actions on specific hypothalamic nuclei. However, there is growing evidence that leptin and its receptors are widely expressed throughout the brain, in regions not generally associated with energy homeostasis, such as cortex, cerebellum, brainstem, basal ganglia, and hippocampus. In this review the author discusses recent advances made in leptin neurobiology, with particular emphasis on the role of this endocrine peptide in normal and pathophysiological hippocampal function.

Hypothalamic control of energy balance: different peptides, different functions

Energy balance is maintained via a homeostatic system involving both the brain and the periphery. A key component of this system is the hypothalamus. Over the past two decades, major advances have been made in identifying an increasing number of peptides within the hypothalamus that contribute to the process of energy homeostasis. Under stable conditions, equilibrium exists between anabolic peptides that stimulate feeding behavior, as well as decrease energy expenditure and lipid utilization in favor of fat storage, and catabolic peptides that attenuate food intake, while stimulating sympathetic nervous system (SNS) activity and restricting fat deposition by increasing lipid metabolism. The equilibrium between these neuropeptides is dynamic in nature. It shifts across the day-night cycle and from day to day and also in response to dietary challenges as well as peripheral energy stores. These shifts occur in close relation to circulating levels of the hormones, leptin, insulin, ghrelin and corticosterone, and also the nutrients, glucose and lipids. These circulating factors together with neural processes are primary signals relaying information regarding the availability of fuels needed for current cellular demand, in addition to the level of stored fuels needed for long-term use. Together, these signals have profound impact on the expression and production of neuropeptides that, in turn, initiate the appropriate anabolic or catabolic responses for restoring equilibrium. In this review, we summarize the evidence obtained on nine peptides in the hypothalamus that have emerged as key players in this process. Data from behavioral, physiological, pharmacological and genetic studies are described and consolidated in an attempt to formulate a clear statement on the underlying function of each of these peptides and also on how they work together to create and maintain energy homeostasis.

Stress-hyperglycemia, insulin and immunomodulation in sepsis

Stress-hyperglycemia and insulin resistance are exceedingly common in critically ill patients, particularly those with sepsis. Multiple pathogenetic mechanisms are responsible for this metabolic syndrome; however, increased release of pro-inflammatory mediators and counter-regulatory hormones may play a pivotal role. Recent data suggests that hyperglycemia may potentiate the pro-inflammatory response while insulin has the opposite effect. Furthermore, emerging evidence suggests that tight glycemic control will improve the outcome of critically ill patients. This paper reviews the pathophysiology of stress hyperglycemia in the critically ill septic patient and outlines a treatment strategy for the management of this disorder.

A highly conserved NTRK3 C-terminal sequence in the ETV6-NTRK3 oncoprotein binds the phosphotyrosine binding domain of insulin receptor substrate-1: an essential interaction for transformation

Receptor tyrosine kinases are integral components of cellular signaling pathways and are frequently deregulated in malignancies. The NTRK family of neurotrophin receptors mediate neuronal cell survival and differentiation, but altered NTRK signaling has also been implicated in oncogenesis. The ETV6-NTRK3 (EN) gene fusion occurs in human pediatric spindle cell sarcomas and secretory breast carcinoma, and encodes the oligomerization domain of the ETV6 transcription factor fused to the protein-tyrosine kinase domain of NTRK3. The EN protein functions as a constitutively active protein-tyrosine kinase with potent transforming activity in multiple cell lineages, and EN constitutively activates both the Ras-MAPK and phosphatidylinositol 3-kinase-Akt pathways. EN transformation is associated with constitutive tyrosine phosphorylation of insulin receptor substrate-1 (IRS-1). Further, IRS-1 functions as the adaptor protein linking EN to downstream signaling pathways. However, the exact nature of the EN-IRS-1 interaction remains unknown. We now demonstrate that EN specifically binds the phosphotyrosine binding domain of IRS-1 via an interaction at the C terminus of EN. An EN mutant lacking the C-terminal 19 amino acids does not bind IRS-1 and lacks transforming ability. Moreover, expression of an IRS-1 polypeptide containing the phosphotyrosine binding domain acts in a dominant negative manner to inhibit EN transformation, and overexpression of IRS-1 potentiates EN transforming activity. These findings indicate that EN.IRS-1 complex formation through the NTRK3 C terminus is essential for EN transformation.

Liver-specific deletion of negative regulator Pten results in fatty liver and insulin hypersensitivity [corrected]

In the liver, insulin controls both lipid and glucose metabolism through its cell surface receptor and intracellular mediators such as phosphatidylinositol 3-kinase and serine-threonine kinase AKT. The insulin signaling pathway is further modulated by protein tyrosine phosphatase or lipid phosphatase. Here, we investigated the function of phosphatase and tension homologue deleted on chromosome 10 (PTEN), a negative regulator of the phosphatidylinositol 3-kinase/AKT pathway, by targeted deletion of Pten in murine liver. Deletion of Pten in the liver resulted in increased fatty acid synthesis, accompanied by hepatomegaly and fatty liver phenotype. Interestingly, Pten liver-specific deletion causes enhanced liver insulin action with improved systemic glucose tolerance. Thus, deletion of Pten in the liver may provide a valuable model that permits the study of the metabolic actions of insulin signaling in the liver, and PTEN may be a promising target for therapeutic intervention for type 2 diabetes.

Liver-specific deletion of negative regulator Pten results in fatty liver and insulin hypersensitivity [corrected]

In the liver, insulin controls both lipid and glucose metabolism through its cell surface receptor and intracellular mediators such as phosphatidylinositol 3-kinase and serine-threonine kinase AKT. The insulin signaling pathway is further modulated by protein tyrosine phosphatase or lipid phosphatase. Here, we investigated the function of phosphatase and tension homologue deleted on chromosome 10 (PTEN), a negative regulator of the phosphatidylinositol 3-kinase/AKT pathway, by targeted deletion of Pten in murine liver. Deletion of Pten in the liver resulted in increased fatty acid synthesis, accompanied by hepatomegaly and fatty liver phenotype. Interestingly, Pten liver-specific deletion causes enhanced liver insulin action with improved systemic glucose tolerance. Thus, deletion of Pten in the liver may provide a valuable model that permits the study of the metabolic actions of insulin signaling in the liver, and PTEN may be a promising target for therapeutic intervention for type 2 diabetes.

Regulation of Cbl-associated protein/Cbl pathway in muscle and adipose tissues of two animal models of insulin resistance

The phosphatidylinositol 3-kinase-independent pathway to induce glucose transport may involve the tyrosine phosphorylation of the protooncogene c-Cbl. In the present study, we examined whether acute exposure to insulin stimulates the tyrosine phosphorylation of Cbl and its association with Cbl-associated protein (CAP) in muscle and adipose tissue of rats in vivo. We report herein that insulin induces Cbl tyrosine phosphorylation and association with CAP in adipose tissue but not in muscle. We also examined the expression and tyrosyl-phosphorylation state of Cbl and CAP/Cbl association in adipose tissue of rats submitted to prolonged fasting and in monosodium glutamate (MSG)-insulin-resistant rats. An increase in Cbl phosphorylation is observed in the fat of MSG rats, parallel with an increase in association of CAP-Cbl as well as an augment in CAP and Cbl protein expression in the adipose tissue of these animals. These events are accompanied by a decrease in insulin-stimulated insulin receptor/ insulin receptor substrate (IRS)-1 tyrosine phosphorylation and an increase in the IRS-2/phosphatidylinositol 3-kinase/Akt/Foxo1 pathway. In adipocytes of fasted rats, there is a decrease in CAP and Cbl protein expression, insulin-induced Cbl phosphorylation, and the association with CAP. In parallel, there is also a decrease in the insulin receptor/IRSs/Akt/Foxo1 pathway. Thus, insulin is able to induce Cbl tyrosine phosphorylation and its association with CAP in the adipose tissue of normal rats. In addition, our data provide evidence that the CAP-Cbl pathway may have a role in the modulation of adiposity in fasting and in MSG-treated rats.

Decreased insulin secretion in islets from rats fed a low protein diet is associated with a reduced PKAalpha expression

A low protein diet has been shown to affect the amount and activity of several enzymes and to decrease insulin secretion by islets isolated from rats fed such a diet. To understand the mechanisms involved in this phenomenon, we investigated the effects of forskolin, a stimulator of adenylyl cyclase, on insulin secretion by pancreatic islets from rats fed a normal (17%; NP) or low (6%; LP) protein diet for 8 wk. Isolated islets were incubated for 1 h in Krebs-bicarbonate solution containing 8.3 mmol glucose/L, with or without 10 micromol forskolin/L. The forskolin-induced insulin secretion was higher in islets from NP rats than in those from LP rats (P<0.05). Western blotting revealed that the amount of the alpha catalytic subunit of protein kinase A (PKAalpha) was 35% lower in islets from LP rats than in islets from NP rats (P<0.05). Moreover, PKAalpha mRNA expression was reduced by 30% in islets from LP rats (P<0.05). Our results indicated a possible relationship between a low protein diet and a reduction in PKAalpha expression. These alterations in PKAalpha may be responsible in part for the decreased insulin secretion by islets from rats fed a low protein diet.

Exercise training attenuated the PKB and GSK-3 dephosphorylation in the myocardium of ZDF rats

Cardiac dysfunction is a severe secondary effect of Type 2 diabetes. Recruitment of the protein kinase B/glycogen synthase kinase-3 pathway represents an integral event in glucose homeostasis, albeit its regulation in the diabetic heart remains undefined. Thus the following study tested the hypothesis that the regulation of protein kinase B/glycogen synthase kinase-3 was altered in the myocardium of the Zucker diabetic fatty rat. Second, exercise has been shown to improve glucose homeostasis, and, in this regard, the effect of swimming training on the regulation of protein kinase B/glycogen synthase kinase-3 in the diabetic rat heart was examined. In the sedentary Zucker diabetic fatty rats, glucose levels were elevated, and cardiac glycogen content increased, compared with wild type. A 13-wk swimming regimen significantly reduced plasma glucose levels and cardiac glycogen content and partially normalized protein kinase B-serine473, protein kinase B-threonine308, and glycogen synthase kinase-3alpha phosphorylation in Zucker diabetic fatty rats. In conclusion, hyperglycemia and increased cardiac glycogen content in the Zucker diabetic fatty rats were associated with dysregulation of protein kinase B/glycogen synthase kinase-3 phosphorylation. These anomalies in the Zucker diabetic fatty rat were partially normalized with swimming. These data support the premise that exercise training may protect the heart against the deleterious consequences of diabetes.

Intracerebroventricular administration of insulin and glucose inhibits the anorectic action of leptin in rats

Obese individuals with glucose intolerance present with high serum levels of glucose, insulin, and leptin. These substances are potent inhibitors of feeding in the brain. Obese subjects still present with over-feeding despite elevation of the above factors. To elucidate the mechanism of this paradox, the effects of insulin and glucose on the anorectic action of leptin in the hypothalamus were examined. Adult male Sprague-Dawley rats (weighing 285-320 g) were pretreated with intracerebroventricular injection of insulin, glucose, or saline, followed by leptin (7.5 microg) or phosphate-buffered saline (PBS) injection into the third cerebral ventricle (icv). The cumulative food intakes were measured 24 hr after leptin icv. The tyrosine phosphorylation of signal transducer and activator transcription factor 3 (STAT3) in the hypothalamus was determined by Western blotting. In rats pretreated with saline and stimulated with leptin (saline/LEPTIN group), food intake diminished to about 50% of that of the saline/PBS group (P < 0.005). Food intake in the insulin/LEPTIN group was significantly higher compared with the saline/LEPTIN group (P < 0.005) and reached the level seen in the saline/PBS group. Similar data were obtained in glucose pretreatment experiments. Insulin and glucose icv resulted in reduction of leptin-induced STAT3 tyrosine phosphorylation compared with saline. Infusion of insulin and glucose icv did not alter peripheral blood glucose levels in all groups. High insulin or glucose levels in the brain could result in leptin resistance as manifested by food intake, which is probably due to the attenuation of STAT3 phosphorylation downstream the leptin receptor.

Leptin signaling in the hypothalamus: emphasis on energy homeostasis and leptin resistance

Leptin, the long-sought satiety factor of adipocytes origin, has emerged as one of the major signals that relay the status of fat stores to the hypothalamus and plays a significant role in energy homeostasis. Understanding the mechanisms of leptin signaling in the hypothalamus during normal and pathological conditions, such as obesity, has been the subject of intensive research during the last decade. It is now established that leptin action in the hypothalamus in regulation of food intake and body weight is mediated by a neural circuitry comprising of orexigenic and anorectic signals, including NPY, MCH, galanin, orexin, GALP, alpha-MSH, NT, and CRH. In addition to the conventional JAK2-STAT3 pathway, it has become evident that PI3K-PDE3B-cAMP pathway plays a critical role in leptin signaling in the hypothalamus. It is now established that central leptin resistance contributes to the development of diet-induced obesity and ageing associated obesity. Central leptin resistance also occurs due to hyperleptinimia produced by exogenous leptin infusion. A defective nutritional regulation of leptin receptor gene expression and reduced STAT3 signaling may be involved in the development of leptin resistance in DIO. However, leptin resistance in the hypothalamic neurons may occur despite an intact JAK2-STAT3 pathway of leptin signaling. Thus, in addition to defective JAK2-STAT3 pathway, defects in other leptin signaling pathways may be involved in leptin resistance. We hypothesize that defective regulation of PI3K-PDE3B-cAMP pathway may be one of the mechanisms behind the development of central leptin resistance seen in obesity.

Selective impairment of insulin signalling in the hypothalamus of obese Zucker rats

AIM/HYPOTHESIS

By acting in the brain, insulin suppresses food intake. However, little is known with regard to insulin signalling in the hypothalamus in insulin-resistant states.

METHODS

Western blotting, immunohistochemistry and polymerase chain reaction assays were combined to compare in vivo hypothalamic insulin signalling through the PI3-kinase and MAP kinase pathways between lean and obese Zucker rats.

RESULTS

Intracerebroventricular insulin infusion reduced food intake in lean rats to a greater extent than that observed in obese rats, and pre-treatment with PI3-kinase inhibitors prevented insulin-induced anorexia. The relative abundance of IRS-2 was considerably higher than that of IRS-1 in hypothalamus of both lean and obese rats. Insulin-stimulated phosphorylation of IR, IRS-1/2, the associations of PI 3-kinase to IRS-1/2 and phosphorylation of Akt in hypothalamus were decreased in obese rats compared to lean rats. These effects seem to be mediated by increased phosphoserine content of IR, IRS-1/2 and decreased protein levels of IRS-1/2 in obese rats. In contrast, insulin stimulated the phosphorylation of MAP kinase equally in lean and obese rats.

CONCLUSION/INTERPRETATION

This study provides direct measurements of insulin signalling in hypothalamus, and documents selective resistance to insulin signalling in hypothalamus of Zucker rats. These findings provide support for the hypothesis that insulin could have anti-obesity actions mediated by the PI3-kinase pathway, and that impaired insulin signalling in hypothalamus could play a role in the development of obesity in this animal model of insulin-resistance.

Skeletal muscle cells and adipocytes differ in their reliance on TC10 and Rac for insulin-induced actin remodeling

Insulin causes distinct cortical actin remodeling in muscle and fat cells, and interfering with actin dynamics halts glucose transporter 4 (GLUT4) translocation to the membrane. Phosphatidylinositol 3-kinase (PI3-K) and the small G protein Rac govern myocyte actin remodeling, whereas TC10 alpha contributes to adipocyte actin dynamics downstream of Cbl-associated protein (CAP) and Cbl, independently of PI3-K. Given the importance of insulin action in both cell types, it is paramount to determine whether signaling pathways and actin manifestations are cell type specific. We found CAP expression and insulin-mediated Cbl phosphorylation in differentiated myotubes but not in myoblasts. Unlike adipocytes, Cbl is phosphorylated on Y774 and Y731 in myotubes. TC10 alpha and beta-transcripts are amplified by RT-PCR in muscle cells, but the endogenous proteins are barely detectable using two unrelated antibodies. TC10 alpha transfected into myoblasts is activated by insulin despite the lack of CAP expression and Cbl phosphorylation. Moreover, dominant-negative TC10 alpha mutants do not prevent insulin-induced actin remodeling in either myoblasts or myotubes and do not interfere with insulin-mediated recruitment of c-myc epitope-tagged GLUT4 to the cell surface. In contrast to TC10 alpha, endogenous Rac is readily detectable in both muscle cells and adipocytes and binds GTP after insulin in a PI3-K-dependent manner. These data suggest that whereas individual components of the CAP to TC10 pathway are regulated by insulin, a functional TC10-dependent signaling pathway leading to actin remodeling and GLUT4 translocation may not operate in myocytes, as it does in adipocytes.

Hypothalamic melanin-concentrating hormone is induced by cold exposure and participates in the control of energy expenditure in rats

Short-term cold exposure of homeothermic animals leads to higher thermogenesis and food consumption accompanied by weight loss. An analysis of cDNA-macroarray was employed to identify candidate mRNA species that encode proteins involved in thermogenic adaptation to cold. A cDNA-macroarray analysis, confirmed by RT-PCR, immunoblot, and RIA, revealed that the hypothalamic expression of melanin-concentrating hormone (MCH) is enhanced by exposure of rats to cold environment. The blockade of hypothalamic MCH expression by antisense MCH oligonucleotide in cold-exposed rats promoted no changes in feeding behavior and body temperature. However, MCH blockade led to a significant drop in body weight, which was accompanied by decreased liver glycogen, increased relative body fat, increased absolute and relative interscapular brown adipose tissue mass, increased uncoupling protein 1 expression in brown adipose tissue, and increased consumption of lean body mass. Thus, increased hypothalamic MCH expression in rats exposed to cold may participate in the process that allows for efficient use of energy for heat production during thermogenic adaptation to cold.

Cold exposure induces tissue-specific modulation of the insulin-signalling pathway in Rattus norvegicus

Cold exposure provides a reproducible model of improved glucose turnover accompanied by reduced steady state and glucose-induced insulin levels. In the present report we performed immunoprecipitation and immunoblot studies to evaluate the initial and intermediate steps of the insulin-signalling pathway in white and brown adipose tissues, liver and skeletal muscle of rats exposed to cold. Basal and glucose-induced insulin secretion were significantly impaired, while glucose clearance rates during a glucose tolerance test and the constant for glucose decay during a 15 min insulin tolerance test were increased, indicating a significantly improved glucose turnover and insulin sensitivity in rats exposed to cold. Evaluation of protein levels and insulin-induced tyrosine (insulin receptor, insulin receptor substrates (IRS)-1 and -2, ERK (extracellular signal-related kinase)) or serine (Akt; protein kinase B) phosphorylation of proteins of the insulin signalling cascade revealed a tissue-specific pattern of regulation of the molecular events triggered by insulin such that in white adipose tissue and skeletal muscle an impaired molecular response to insulin was detected, while in brown adipose tissue an enhanced response to insulin was evident. In muscle and white and brown adipose tissues, increased 2-deoxy-D-glucose (2-DG) uptake was detected. Thus, during cold exposure there is a tissue-specific regulation of the insulin-signalling pathway, which seems to favour heat-producing brown adipose tissue. Nevertheless, muscle and white adipose tissue are able to take up large amounts of glucose, even in the face of an apparent molecular resistance to insulin.

Effect of intracerebroventricularly injected insulin on urinary sodium excretion by cerebroventricular streptozotocin-treated rats

Recent evidence suggests that insulin may influence many brain functions. It is known that intracerebroventricular (icv) injection of nondiabetogenic doses of streptozotocin (STZ) can damage insulin receptor signal transduction. In the present study, we examined the functional damage to the brain insulin receptors on central mechanisms regulating glomerular filtration rate and urinary sodium excretion, over four periods of 30 min, in response to 3 microl insulin or 0.15 NaCl (vehicle) injected icv in STZ-treated freely moving Wistar-Hannover rats (250-300 g). The icv cannula site was visually confirmed by 2% Evans blue infusion. Centrally administered insulin (42.0 ng/ micro l) increased the urinary output of sodium (from 855.6 85.1 to 2055 310.6 delta%/min; N = 11) and potassium (from 460.4 100 to 669 60.8 delta%/min; N = 11). The urinary sodium excretion response to icv insulin microinjection was markedly attenuated by previous central STZ (100 micro g/3 micro l) administration (from 628 45.8 to 617 87.6 delta%/min; N = 5) or by icv injection of a dopamine antagonist, haloperidol (4 micro g/3 micro l) (from 498 +/- 39.4 to 517 +/- 73.2 delta%/min; N = 5). Additionally, insulin-induced natriuresis occurred by increased post-proximal tubule sodium rejection, despite an unchanged glomerular filtration rate. Excluding the possibility of a direct action of STZ on central insulin receptor-carrying neurons, the current data suggest that the insulin-sensitive response may be processed through dopaminergic D1 receptors containing neuronal pathways.

Leptin signaling in the hypothalamus during chronic central leptin infusion

Using a rat model of chronic central leptin infusion in which neuropeptide Y neurons develop leptin resistance, we examined whether leptin signal transduction mechanism in the hypothalamus is altered during central leptin infusion. Adult male rats were infused chronically into the lateral cerebroventricle with leptin (160 ng/h) or vehicle via Alzet pumps for 16 d. In the leptin-infused group, the initial decrease in food intake was followed by a recovery to their preleptin levels by d 16, although food intake remained significantly lower than in artificial cerebrospinal fluid controls; and body weight gradually decreased reaching a nadir at d 11 and remained stabilized at lower level thereafter. Phosphorylated leptin receptor and phosphorylated signal transducer and activator of transcription-3 (p-STAT3) remained elevated in association with a sustained elevation in DNA-binding activity of STAT3 in the hypothalamus throughout 16-d period of leptin infusion. However, phosphorylated Janus kinase-2 was increased during the early part of leptin infusion but remained unaltered on d 16. Although hypothalamic suppressors of cytokine signaling-3 (SOCS3) mRNA levels were increased throughout leptin infusion, SOCS3 protein levels were increased only on d 16. This study demonstrates a sustained elevation in hypothalamic leptin receptor signaling through Janus kinase-STAT pathway despite an increased expression of SOCS3 during chronic central leptin infusion. We propose that an alteration in leptin signaling in the hypothalamus through pathways other than STAT3 and/or a defect in downstream of STAT3 signaling may be involved in food intake recovery seen after an initial decrease during chronic central leptin infusion.