Insulin-stimulated glucose uptake does not require p38 mitogen-activated protein kinase in adipose tissue or skeletal muscle

CaMKK2 is not involved in contraction-stimulated AMPK activation and glucose uptake in skeletal muscle

OBJECTIVE

The AMP-activated protein kinase (AMPK) gets activated in response to energetic stress such as contractions and plays a vital role in regulating various metabolic processes such as insulin-independent glucose uptake in skeletal muscle. The main upstream kinase that activates AMPK through phosphorylation of α-AMPK Thr172 in skeletal muscle is LKB1, however some studies have suggested that Ca2+/calmodulin-dependent protein kinase kinase 2 (CaMKK2) acts as an alternative kinase to activate AMPK. We aimed to establish whether CaMKK2 is involved in activation of AMPK and promotion of glucose uptake following contractions in skeletal muscle.

METHODS

A recently developed CaMKK2 inhibitor (SGC-CAMKK2-1) alongside a structurally related but inactive compound (SGC-CAMKK2-1N), as well as CaMKK2 knock-out (KO) mice were used. In vitro kinase inhibition selectivity and efficacy assays, as well as cellular inhibition efficacy analyses of CaMKK inhibitors (STO-609 and SGC-CAMKK2-1) were performed. Phosphorylation and activity of AMPK following contractions (ex vivo) in mouse skeletal muscles treated with/without CaMKK inhibitors or isolated from wild-type (WT)/CaMKK2 KO mice were assessed. Camkk2 mRNA in mouse tissues was measured by qPCR. CaMKK2 protein expression was assessed by immunoblotting with or without prior enrichment of calmodulin-binding proteins from skeletal muscle extracts, as well as by mass spectrometry-based proteomics of mouse skeletal muscle and C2C12 myotubes.

RESULTS

STO-609 and SGC-CAMKK2-1 were equally potent and effective in inhibiting CaMKK2 in cell-free and cell-based assays, but SGC-CAMKK2-1 was much more selective. Contraction-stimulated phosphorylation and activation of AMPK were not affected with CaMKK inhibitors or in CaMKK2 null muscles. Contraction-stimulated glucose uptake was comparable between WT and CaMKK2 KO muscle. Both CaMKK inhibitors (STO-609 and SGC-CAMKK2-1) and the inactive compound (SGC-CAMKK2-1N) significantly inhibited contraction-stimulated glucose uptake. SGC-CAMKK2-1 also inhibited glucose uptake induced by a pharmacological AMPK activator or insulin. Relatively low levels of Camkk2 mRNA were detected in mouse skeletal muscle, but neither CaMKK2 protein nor its derived peptides were detectable in mouse skeletal muscle tissue.

CONCLUSIONS

We demonstrate that pharmacological inhibition or genetic loss of CaMKK2 does not affect contraction-stimulated AMPK phosphorylation and activation, as well as glucose uptake in skeletal muscle. Previously observed inhibitory effect of STO-609 on AMPK activity and glucose uptake is likely due to off-target effects. CaMKK2 protein is either absent from adult murine skeletal muscle or below the detection limit of currently available methods.

AMP activated kinase negatively regulates hepatic Fetuin-A via p38 MAPK-C/EBPβ/E3 Ubiquitin Ligase Signaling pathway

Fetuin-A (Fet-A) is a liver-secreted phosphorylated protein, known to impair insulin signaling, which has been shown to be associated with obesity, insulin resistance, and incident diabetes. Fet-A interacts with the insulin-stimulated insulin receptor (IR) and inhibits IR tyrosine kinase activity and glucose uptake. It has been shown that high glucose increases Fet-A expression through the ERK1/2 signaling pathway. However, factors that downregulate Fet-A expression and their potential mechanisms are unclear. We examined the effect of AMP-activated protein kinase (AMPK) on high-glucose induced Fet-A expression in HepG2 cells, Hep3B cells and primary rat hepatocytes. High glucose increased Fet-A and phosphorylated (Ser312) fetuin-A (pFet-A) expression, which are known to impair insulin signaling. AICAR-induced AMPK activation significantly down-regulated high glucose-induced Fet-A expression and secretion of pFet-A while treatment with Compound C (AMPK inhibitor), SB202190 (p38 MAPK inhibitor) or p38 MAPK siRNA transfection prevented AICAR-induced downregulation of Fet-A expression. In addition, activation of p38 MAPK, by anisomycin, decreased the hepatic expression of Fet-A. Further, we our studies have shown that short-term effect of AICAR-treatment on Fet-A expression was mediated by proteosomal degradation, and long-term treatment of AICAR was associated with decrease in hepatic expression of C/EBP beta, an important transcription factor involved in the regulation of Fet-A. Taken together, our studies implicate a critical role for AMPK-p38 MAPK-C/EBPb-ubiquitin-proteosomal axis in the regulation of the expression of hepatic Fet-A.

Investigation of the specificity and mechanism of action of the ULK1/AMPK inhibitor SBI-0206965

SBI-0206965, originally identified as an inhibitor of the autophagy initiator kinase ULK1, has recently been reported as a more potent and selective AMP-activated protein kinase (AMPK) inhibitor relative to the widely used, but promiscuous inhibitor Compound C/Dorsomorphin. Here, we studied the effects of SBI-0206965 on AMPK signalling and metabolic readouts in multiple cell types, including hepatocytes, skeletal muscle cells and adipocytes. We observed SBI-0206965 dose dependently attenuated AMPK activator (991)-stimulated ACC phosphorylation and inhibition of lipogenesis in hepatocytes. SBI-0206965 (≥25 μM) modestly inhibited AMPK signalling in C2C12 myotubes, but also inhibited insulin signalling, insulin-mediated/AMPK-independent glucose uptake, and AICA-riboside uptake. We performed an extended screen of SBI-0206965 against a panel of 140 human protein kinases in vitro, which showed SBI-0206965 inhibits several kinases, including members of AMPK-related kinases (NUAK1, MARK3/4), equally or more potently than AMPK or ULK1. This screen, together with molecular modelling, revealed that most SBI-0206965-sensitive kinases contain a large gatekeeper residue with a preference for methionine at this position. We observed that mutation of the gatekeeper methionine to a smaller side chain amino acid (threonine) rendered AMPK and ULK1 resistant to SBI-0206965 inhibition. These results demonstrate that although SBI-0206965 has utility for delineating AMPK or ULK1 signalling and cellular functions, the compound potently inhibits several other kinases and critical cellular functions such as glucose and nucleoside uptake. Our study demonstrates a role for the gatekeeper residue as a determinant of the inhibitor sensitivity and inhibitor-resistant mutant forms could be exploited as potential controls to probe specific cellular effects of SBI-0206965.

Adipocyte integrin-linked kinase plays a key role in the development of diet-induced adipose insulin resistance in male mice

Objective

Increased deposition of the extracellular matrix (ECM) in adipose tissue (AT) during obesity contributes to insulin resistance. The integrin receptors transmit changes in the extracellular environment causing corresponding intracellular adaptations. Integrin-linked kinase (ILK), an adaptor protein, is a central hub for intracellular signaling of integrins. This study determined the role of ILK in adipose function and insulin resistance.

Methods

The pathogenic role of ILK in obesity and insulin resistance was studied in human adipose tissue and adipocyte-specific ILK-deficient mice (ILK^lox/loxAdCre). ILK^lox/loxAdCre mice together with wild-type littermates (ILK^lox/lox) were fed a chow diet or 60% high-fat (HF) diet for 16 weeks. In vivo insulin sensitivity was determined by hyperinsulinemic-euglycemic clamps.

Results

AT ILK expression was increased by HF diet feeding in mice and increased in visceral fat of morbidly obese humans. The HF-fed ILK^lox/loxAdCre mice displayed reduced fat mass and improved glucose tolerance relative to the HF-fed ILK^lox/lox mice. During a hyperinsulinemic-euglycemic clamp, the HF-fed ILK^lox/loxAdCre mice exhibited partially improved insulin resistance in AT. Lipolysis was suppressed to a greater extent by insulin and glucose uptake in brown AT (BAT) increased. Increased inhibition of lipolysis may have been attributed to increased vascularization in white AT, while increased glucose uptake in BAT was associated with increased Akt phosphorylation and P38/JNK dephosphorylation. Notably, AT insulin sensitivity in lean mice was not affected by ILK deletion. Moreover, reduced fat mass in the HF-fed ILK^lox/loxAdCre mice may have been attributed to decreased free fatty acid uptake into adipocytes via the downregulation of CD36 gene expression. Consistent with the results in the mice, knockdown and knockout of ILK in 3T3-L1 cells decreased lipid accumulation and CD36 gene expression during adipogenesis.

Conclusions

These data show that adipocyte ILK is important for regulating HF diet-mediated insulin resistance in AT in a manner consistent with AT function.

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ILK protein increased in visceral adipose tissue of obese humans and mice.

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Mice lacking adipocyte ILK had less fat and improved glucose tolerance in obesity.

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Adipocyte ILK deletion improved anti-lipolytic action of insulin in obese mice.

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Adipocyte ILK deletion stimulated brown adipose tissue glucose uptake in obese mice.

Glucocorticoid Receptor and Adipocyte Biology

Glucocorticoids are steroid hormones that play a key role in metabolic adaptations during stress, such as fasting and starvation, in order to maintain plasma glucose levels. Excess and chronic glucocorticoid exposure, however, causes metabolic syndrome including insulin resistance, dyslipidemia, and hyperglycemia. Studies in animal models of metabolic disorders frequently demonstrate that suppressing glucocorticoid signaling improves insulin sensitivity and metabolic profiles. Glucocorticoids convey their signals through an intracellular glucocorticoid receptor (GR), which is a transcriptional regulator. The adipocyte is one cell type that contributes to whole body metabolic homeostasis under the influence of GR. Glucocorticoids' functions on adipose tissues are complex. Depending on various physiological or pathophysiological states as well as distinct fat depots, glucocorticoids can either increase or decrease lipid storage in adipose tissues. In rodents, glucocorticoids have been shown to reduce the thermogenic activity of brown adipocytes. However, in human acute glucocorticoid exposure, glucocorticoids act to promote thermogenesis. In this article, we will review the recent studies on the mechanisms underlying the complex metabolic functions of GR in adipocytes. These include studies of the metabolic outcomes of adipocyte specific GR knockout mice and identification of novel GR primary target genes that mediate glucocorticoid action in adipocytes.

Effect of a p38 MAPK inhibitor on FFA-induced hepatic insulin resistance in vivo

The mechanisms whereby prolonged plasma free fatty acids elevation, as found in obesity, causes hepatic insulin resistance are not fully clarified. We herein investigated whether inhibition of p38 mitogen-activated protein kinase (MAPK) prevented hepatic insulin resistance following prolonged lipid infusion. Chronically cannulated rats were subdivided into one of four intravenous (i.v.) treatments that lasted 48 h: Saline (5.5 μl min⁻¹), Intralipid plus heparin (IH, 20% Intralipid+20 U ml⁻¹ heparin; 5.5 μl min⁻¹), IH+p38 MAPK inhibitor (SB239063) and SB239063 alone. During the last 2 h of treatment, a hyperinsulinemic (5 mU kg⁻¹ min⁻¹) euglycemic clamp together with [3-³H] glucose methodology was carried out to distinguish hepatic from peripheral insulin sensitivity. We found that SB239063 prevented IH-induced hepatic insulin resistance, but not peripheral insulin resistance. SB239063 also prevented IH-induced phosphorylation of activating transcription factor 2 (ATF2), a marker of p38 MAPK activity, in the liver. Moreover, in another lipid infusion model in mice, SB239063 prevented hepatic but not peripheral insulin resistance caused by 48 h combined ethyloleate plus ethylpalmitate infusion. Our results suggest that inhibition of p38 MAPK may be a useful strategy in alleviating hepatic insulin resistance in obesity-associated disorders.

Pentacyclic Triterpenoids from Astilbe rivularis that Enhance Glucose Uptake via the Activation of Akt and Erk1/2 in C2C12 Myotubes

Glucose uptake into insulin-sensitive tissues is important for the regulation of blood glucose. This study has investigated whether the pentacyclic triterpenoids substituted with a carboxylic acid at the C-27 position isolated from Astilbe rivularis can enhance glucose uptake and subsequently to also examine their underlying molecular mechanisms. The structure of the new pentacyclic triterpenoid 1 was assigned by spectroscopic data interpretation. To evaluate the activity of compounds 1 and 2, glucose uptake and glucose transporter 4 (GLUT4) translocation were measured in C2C12 myotubes. The C-27-carboxylated triterpenoids 1 and 2 significantly increased basal and insulin-stimulated glucose uptake and GLUT4 translocation to plasma membrane. Both compounds stimulated the phosphorylation of insulin receptor substrate-1 (IRS-1), protein kinase B (Akt), and extracellular signal-regulated kinase 1/2 (Erk1/2). Pretreatment with the Akt inhibitor triciribine or the Erk1/2 inhibitor U0126 decreased the ability of both compounds to enhance basal- and insulin-stimulated glucose uptake and stimulate GLUT4 translocation. These results indicate that compounds 1 and 2 activated both the IRS-1/Akt and Erk1/2 pathways and subsequently stimulated GLUT4 translocation, leading to enhanced glucose uptake. Thus, these observations suggest that C-27-carboxylated-pentacyclic triterpenoids may serve as scaffolds for development as agents for the management of blood glucose levels in disease states such as diabetes.

Activation of p38 in C2C12 myotubes following ATP depletion depends on extracellular glucose

Muscle cells adjust their glucose metabolism in response to myriad stimuli, and particular attention has been paid to glucose metabolism after contraction, ATP depletion, and insulin stimulation. Each of these requires translocation of GLUT4 to the cell membrane, and may require activation of glucose transporters by p38. In contrast, AICAR stimulates glucose transport without activation of p38, suggesting that p38 activation may be an indirect consequence of accelerated glucose transport or metabolism. This study was designed to investigate the contribution of AMPK and p38 to ATP homeostasis and glucose metabolism to test the hypothesis that p38 reflects glycolytic activity rather than controls glucose uptake. Treating mature myotubes with rotenone caused transient ATP depletion in 15 min with recovery by 120 min, associated with increased lactate production. Both ACC and p38 were rapidly phosphorylated, but ACC remained phosphorylated while p38 phosphorylation declined as ATP recovered. AMPK inhibition blocked ATP recovery, lactate production, and phosphorylation of p38 and ACC. Inhibition of p38 had little effect. AICAR induced ACC phosphorylation, but not lactate production or p38 phosphorylation. Finally, removing extracellular glucose potentiated rotenone-induced AMPK activation, but reduced lactate generation, ATP recovery and p38 activation. Thus, glucose metabolism is highly sensitive to ATP homeostasis via AMPK activity, but p38 activity is dispensable. Although p38 is strongly phosphorylated during ATP depletion, this appears to be an indirect consequence of accelerated glycolysis.

Insulin sensitizing effect of 3 Indian medicinal plants: an in vitro study

OBJECTIVE

Measurement of glucose uptake into peripheral tissue is an important mechanism to assess Insulin sensitivity. The present in vitro study was conducted to evaluate the Insulin sensitizing activity of Phyllanthus emblica (Pe), Tinospora cordifolia (Tc) and Curcuma longa (Cl) by assessing glucose uptake activity in a 3T3L1 adipocyte model.

MATERIALS AND METHODS

The 3T3 L1 fibroblast cells were differentiated to adipocytes, using a cocktail of insulin, isobutyl-1-methylxanthine and dexamethazone. These adipocytes were initially treated with different concentrations of the selected plants following which 2-deoxy glucose uptake was estimated using a radioactive assay. The effects of plants on glucose uptake both in the presence and absence of insulin was evaluated and compared with pioglitazone, a known insulin sensitizer.

RESULTS

Pe and Tc per se significantly stimulated glucose uptake in 3T3-L1 adipocytes in a dose dependent manner with maximal effect at higher concentrations (200 μg/ml). The effect of both Pe and Tc at 200 μg/ml was comparable to insulin and greater than pioglitazone. Cl per se stimulated glucose uptake with maximal effect at 50 μg/ml. However, this effect was lesser as compared to insulin with higher concentrations inhibiting glucose uptake. When combined with insulin, an antagonist effect was observed between Pe, Tc and insulin indicating a possible plant-drug interaction while Cl in combination with insulin showed an increase in the glucose uptake as compared to Cl alone.

CONCLUSION

The results suggest that one of the mechanisms for the anti-diabetic effect of Pe, Cl and Tc may be through an insulin sensitizing effect (stimulation of glucose uptake into adipocytes). Further studies using other target sites viz. skeletal muscle and hepatocytes models and in an insulin resistant state would help substantiate this conclusion.

Insulin-stimulated glucose uptake occurs in specialized cells within the cumulus oocyte complex

The oocyte exists within the mammalian follicle surrounded by somatic cumulus cells. These cumulus cells metabolize the majority of the glucose within the cumulus oocyte complex and provide energy substrates and intermediates such as pyruvate to the oocyte. The insulin receptor is present in cumulus cells and oocytes; however, it is unknown whether insulin-stimulated glucose uptake occurs in either cell type. Insulin-stimulated glucose uptake is thought to be unique to adipocytes, skeletal and cardiac muscle, and the blastocyst. Here, we show for the first time that many of the components required for insulin signaling are present in both cumulus cells and oocytes. We performed a set of experiments on mouse cumulus cells and oocytes and human cumulus cells using the nonmetabolizable glucose analog 2-deoxy-d-glucose to measure basal and insulin-stimulated glucose uptake. We show that insulin-stimulated glucose uptake occurs in both compact and expanded cumulus cells of mice, as well as in human cumulus cells. Oocytes, however, do not display insulin-stimulated glucose uptake. Insulin-stimulated glucose uptake in cumulus cells is mediated through phosphatidylinositol 3-kinase signaling as shown by inhibition of insulin-stimulated glucose uptake and Akt phosphorylation with the specific phosphatidylinositol 3-kinase inhibitor, LY294002. To test the effect of systemic in vivo insulin resistance on insulin sensitivity in the cumulus cell, cumulus cells from high fat-fed, insulin-resistant mice and women with polycystic ovary syndrome were examined. Both sets of cells displayed blunted insulin-stimulated glucose uptake. Our studies identify another tissue that, through a classical insulin-signaling pathway, demonstrates insulin-stimulated glucose uptake. Moreover, these findings suggest insulin resistance occurs in these cells under conditions of systemic insulin resistance.

Mouse resistin modulates adipogenesis and glucose uptake in 3T3-L1 preadipocytes through the ROR1 receptor

Mouse resistin, a cysteine-rich protein primarily secreted from mature adipocytes, is involved in insulin resistance and type 2 diabetes. Human resistin, however, is mainly secreted by immune mononuclear cells, and it competes with lipopolysaccharide for the binding to Toll-like receptor 4, which could mediate some of the well-known proinflammatory effects of resistin in humans. In addition, resistin has been involved in the regulation of many cell differentiation and proliferation processes, suggesting that different receptors could be involved in mediating its numerous effects. Thus, a recent work identifies an isoform of Decorin (Δ Decorin) as a functional resistin receptor in adipocyte progenitors that may regulate white adipose tissue expansion. Our work shows that the mouse receptor tyrosine kinase-like orphan receptor (ROR)1 could mediate some of the described functions of resistin in 3T3-L1 adipogenesis and glucose uptake. We have demonstrated an interaction of mouse resistin with specific domains of the extracellular region of the ROR1 receptor. This interaction results in the inhibition of ROR1 phosphorylation, modulates ERK1/2 phosphorylation, and regulates suppressor of cytokine signaling 3, glucose transporter 4, and glucose transporter 1 expression. Moreover, mouse resistin modulates glucose uptake and promotes adipogenesis of 3T3-L1 cells through ROR1. In summary, our results identify mouse resistin as a potential inhibitory ligand for the receptor ROR1 and demonstrate, for the first time, that ROR1 plays an important role in adipogenesis and glucose homeostasis in 3T3-L1 cells. These data open a new line of research that could explain important questions about the resistin mechanism of action in adipogenesis and in the development of insulin resistance.

The p38 MAPK pathway is essential for skeletogenesis and bone homeostasis in mice

Nearly every extracellular ligand that has been found to play a role in regulating bone biology acts, at least in part, through MAPK pathways. Nevertheless, much remains to be learned about the contribution of MAPKs to osteoblast biology in vivo. Here we report that the p38 MAPK pathway is required for normal skeletogenesis in mice, as mice with deletion of any of the MAPK pathway member-encoding genes MAPK kinase 3 (Mkk3), Mkk6, p38a, or p38b displayed profoundly reduced bone mass secondary to defective osteoblast differentiation. Among the MAPK kinase kinase (MAP3K) family, we identified TGF-beta-activated kinase 1 (TAK1; also known as MAP3K7) as the critical activator upstream of p38 in osteoblasts. Osteoblast-specific deletion of Tak1 resulted in clavicular hypoplasia and delayed fontanelle fusion, a phenotype similar to the cleidocranial dysplasia observed in humans haploinsufficient for the transcription factor runt-related transcription factor 2 (Runx2). Mechanistic analysis revealed that the TAK1-MKK3/6-p38 MAPK axis phosphorylated Runx2, promoting its association with the coactivator CREB-binding protein (CBP), which was required to regulate osteoblast genetic programs. These findings reveal an in vivo function for p38beta and establish that MAPK signaling is essential for bone formation in vivo. These results also suggest that selective p38beta agonists may represent attractive therapeutic agents to prevent bone loss associated with osteoporosis and aging.

Attenuation of oxidant-induced muscle insulin resistance and p38 MAPK by exercise training

We have recently shown that direct exposure to an oxidant stress induces resistance to insulin in glucose transport activity in intact rat skeletal muscle. In this study, we evaluated the effectiveness of prior exercise training in attenuating oxidative stress-induced insulin resistance. Male Sprague-Dawley rats either remained sedentary or underwent a treadmill-running regimen for 6 weeks. Isolated soleus muscles were incubated in the absence or presence of hydrogen peroxide (H(2)O(2)) (50-70 microM) with or without insulin for 2 h. In the sedentary animals, H(2)O(2) significantly inhibited insulin action on glucose transport activity and phosphorylation of Akt (Ser(473)), by 28 and 24%, respectively, and substantially activated the phosphorylation levels of p38 MAPK (Thr(180)/Tyr(182)) by 43% and SAPK/JNK (Thr(183)/Tyr(185)) by 111%. Interestingly, the inhibitory effects of H(2)O(2) on insulin-stimulated glucose transport and Akt (Ser(473)) phosphorylation were attenuated by 43 and 75% in exercise-trained muscles. Additionally, the phosphorylation level of p38 MAPK (Thr(180)/Tyr(182)) triggered by oxidative stress was reduced by 59% in the exercise-trained muscle. We have demonstrated for the first time in mammalian skeletal muscle that endurance exercise training can partially protect against glucose transport resistance to insulin induced by oxidative stress, and this benefit of exercise training is at least in part mediated through the insulin signaling pathway and stress-activated signaling elements.

Amino acid regulation of autophagosome formation

Amino acids are not only substrates for various metabolic pathways, but can also serve as signaling molecules controlling signal transduction pathways. One of these signaling pathways is mTOR-dependent and is activated by amino acids (leucine in particular) in synergy with insulin. Activation of this pathway inhibits autophagy. Because activation of mTOR-mediated signaling also stimulates protein synthesis, it appears that protein synthesis and autophagic protein degradation are reciprocally controlled by the same signaling pathway. Recent developments indicate that amino acid-stimulated mTOR-dependent signaling is subject to complex regulation. The mechanism by which amino acids stimulate mTORdependent signaling (and other signaling pathways), and its molecular connection with the autophagic machinery, is still unknown.

Exercise training increases components of the c-Cbl-associated protein/c-Cbl signaling cascade in muscle of obese Zucker rats

The purpose of this investigation was to determine whether alterations in the c-Cbl-associated protein/c-Cbl pathway and/or p38-mitogen-activated protein kinase (p38 MAP kinase) were associated with improved skeletal muscle insulin responsiveness in exercise-trained obese Zucker rats. Obese Zucker rats ran 5 d/wk on a motorized treadmill for 90 minutes over a 7-week period. Age-matched obese Zucker rats (OB-SED) and their lean littermates (LN-SED) were obtained to serve as nontrained controls. Twenty-four (OB-EX-24 h) or 48 hours (OB-EX-48 h) after the last exercise bout, the trained rats were studied via the hind limb perfusion technique in the presence of insulin. Insulin-stimulated glucose uptake was significantly decreased across the skeletal muscle of OB-SED rats compared with LN-SED, but was normalized in the obese rats by 7 weeks of training. The insulin-stimulated plasma membrane protein concentrations of TC10 and glucose transporter 4 were reduced in the sedentary Zuckers, but both proteins were increased by the training protocol. Training did not increase insulin-stimulated p38 MAP kinase protein concentration, nor did it have an effect on insulin-stimulated p38 MAP kinase phosphorylation at the plasma membrane. These results suggest that skeletal muscle insulin resistance is associated with reduced expression of TC10 and that this deficiency can be corrected with exercise training.

Two phases of palmitate-induced insulin resistance in skeletal muscle: impaired GLUT4 translocation is followed by a reduced GLUT4 intrinsic activity

We examined, in soleus muscle, the effects of prolonged palmitate exposure (0, 6, 12, 18 h) on insulin-stimulated glucose transport, intramuscular lipid accumulation and oxidation, activation of selected insulin-signaling proteins, and the insulin-stimulated translocation of GLUT4. Insulin-stimulated glucose transport was progressively reduced after 6 h (-33%), 12 h (-66%), and 18 h (-89%) of palmitate exposure. These decrements were closely associated with concurrent reductions in palmitate oxidation at 6 h (-40%), 12 h (-60%), and 18 h (-67%). In contrast, intramuscular ceramide (+24%) and diacylglycerol (+32%) concentrations, insulin-stimulated AS160 (-36%) and PRAS40 (-33%) phosphorylations, and Akt (-40%), PKCtheta (-50%), and GLUT4 translocation (-40%) to the plasma membrane were all maximally altered within the first 6 h of palmitate treatment. No further changes were observed in any of these parameters after 12 and 18 h of palmitate exposure. Thus, the intrinsic activity of GLUT4 was markedly reduced after 12 and 18 h of palmitate treatment. During this reduced GLUT4 intrinsic activity phase at 12 and 18 h, the reduction in glucose transport was twofold greater compared with the early phase (< or =6 h), when only GLUT4 translocation was impaired. Our study indicates that palmitate-induced insulin resistance is provoked by two distinct mechanisms: 1) an early phase (< or =6 h), during which lipid-mediated impairments in insulin signaling and GLUT4 translocation reduce insulin-stimulated glucose transport, followed by 2) a later phase (12 and 18 h), during which the intrinsic activity of GLUT4 is markedly reduced independently of any further alterations in intramuscular lipid accumulation, insulin signaling and GLUT4 translocation.

In vivo functions of mitogen-activated protein kinases: conclusions from knock-in and knock-out mice

Multicellular organisms achieve intercellular communication by means of signalling molecules whose effect on the target cell is mediated by signal transduction pathways. Such pathways relay, amplify and integrate signals to elicit appropriate biological responses. Protein kinases form crucial intermediate components of numerous signalling pathways. One group of protein kinases, the mitogen-activated protein kinases (MAP kinases) are kinases involved in signalling pathways that respond primarily to mitogens and stress stimuli. In vitro studies revealed that the MAP kinases are implicated in several cellular processes, including cell division, differentiation, cell survival/apoptosis, gene expression, motility and metabolism. As such, dysfunction of specific MAP kinases is associated with diseases such as cancer and immunological disorders. However, the genuine in vivo functions of many MAP kinases remain elusive. Genetically modified mouse models deficient in a specific MAP kinase or expressing a constitutive active or a dominant negative variant of a particular MAP kinase offer valuable tools for elucidating the biological role of these protein kinases. In this review, we focus on the current status of MAP kinase knock-in and knock-out mouse models and their phenotypes. Moreover, examples of the application of MAP kinase transgenic mice for validating therapeutic properties of specific MAP kinase inhibitors, and for investigating the role of MAP kinase in pathogen-host interactions will be discussed.

Arsenite modulates cardiac substrate preference by translocation of GLUT4, but not CD36, independent of mitogen-activated protein kinase signaling

The protein thiol-modifying agent arsenite, a potent activator of stress signaling, was used to examine the involvement of MAPKs in the regulation of cardiac substrate uptake. Arsenite strongly induced p38 MAPK phosphorylation in isolated rat cardiac myocytes but also moderately enhanced phosphorylation of p42/44 ERK and p70 S6K. At the level of cardiomyocytic substrate use, arsenite enhanced glucose uptake dose dependently up to 5.1-fold but failed to stimulate long-chain fatty acid uptake. At the substrate transporter level, arsenite stimulated the translocation of GLUT4 to the sarcolemma but failed to recruit CD36 or FABPpm. Because arsenite did not influence the intrinsic activity of glucose transporters, GLUT4 translocation is entirely responsible for the selective increase in glucose uptake by arsenite. Moreover, the nonadditivity of arsenite-induced glucose uptake and insulin-induced glucose uptake indicates that arsenite recruits GLUT4 from insulin-responsive intracellular stores. Inhibitor studies with SB203580/SB202190, PD98059, and rapamycin indicate that activation of p38 MAPK, p42/44 ERK, and p70 S6K, respectively, are not involved in arsenite-induced glucose uptake. In addition, all these kinases do not play a role in regulation of cardiac glucose and long-chain fatty acid uptake by insulin. Hence, arsenite's selective stimulation of glucose uptake appears unrelated to its signaling actions, suggesting that arsenite acts via thiol modification of a putative intracellular protein target of arsenite within insulin-responsive GLUT4-containing stores. Because of arsenite's selective stimulation of cardiac glucose uptake, identification of this putative target of arsenite within the GLUT4-storage compartment may indicate whether it is a target for future strategies in prevention of diabetic cardiomyopathy.