Akt Substrate Research Articles

Metabolic and Transcriptional Changes in Cultured Muscle Stem Cells from Low Birth Weight Subjects.

Developmental programming of human muscle stem cells could in part explain why individuals born with low birth weight (LBW) have an increased risk of developing type 2 diabetes (T2D) later in life. We hypothesized that immature muscle stem cell functions including abnormal differentiation potential and metabolic function could link LBW with the risk of developing T2D. Design/Settings/Participants: We recruited 23 young men with LBW and 16 age-matched control subjects with normal birth weight. Biopsies were obtained from vastus lateralis, and muscle stem cells were isolated and cultured into fully differentiated myotubes. We studied glucose uptake, glucose transporters, insulin signaling, key transcriptional markers of myotube maturity, selected site-specific DNA methylation, and mitochondrial gene expression. We found reduced glucose uptake as well as decreased levels of glucose transporter-1 and -4 mRNA and of the Akt substrate of 160-kDa mRNA and protein in myotubes from LBW individuals compared with normal birth weight individuals. The myogenic differentiation markers, myogenin and myosin heavy chain 1 and 2, were decreased during late differentiation in LBW myotubes. Additionally, mRNA levels of the peroxisome proliferator-activated receptor-γ coactivator-1α and cytochrome c oxidase polypeptide 7A were reduced in LBW myotubes. Decreased gene expression was not explained by changes in DNA methylation levels. We demonstrate transcriptional and metabolic alterations in cultured primary satellite cells isolated from LBW individuals after several cell divisions, pointing toward a retained intrinsic defect conserved in these myotubes.

Molecular mechanism linking BDNF/TrkB signaling with the NMDA receptor in memory: the role of Girdin in the CNS.

It is well known that synaptic plasticity is the cellular mechanism underlying learning and memory. Activity-dependent synaptic changes in electrical properties and morphology, including synaptogenesis, lead to alterations of synaptic strength, which is associated with long-term potentiation (LTP). Brain-derived neurotrophic factor (BDNF)/tropomyosin-related kinase B (TrkB) signaling is involved in learning and memory formation by regulating synaptic plasticity. The phosphatidylinositol 3-kinase (PI3-K)/Akt pathway is one of the key signaling cascades downstream BDNF/TrkB and is believed to modulate N-methyl-d-aspartate (NMDA) receptor-mediated synaptic plasticity. However, the molecular mechanism underlying the connection between these two key players in synaptic plasticity remains largely unknown. Girders of actin filament (Girdin), an Akt substrate that directly binds to actin filaments, has been shown to play a role in neuronal migration and neuronal development. Recently, we identified Girdin as a key molecule involved in regulating long-term memory. It was demonstrated that phosphorylation of Girdin by Akt contributed to the maintenance of LTP by linking the BDNF/TrkB signaling pathway with NMDA receptor activity. These findings indicate that Girdin plays a pivotal role in a variety of processes in the CNS. Here, we review recent advances in our understanding about the roles of Girdin in the CNS and focus particularly on neuronal migration and memory.

Insulin signaling and skeletal muscle atrophy and autophagy in transition dairy cows either overfed energy or fed a controlled energy diet prepartum.

During periods of negative energy balance, mobilization of muscle is a physiologic process providing energy and amino acids. This is important in transition dairy cows experiencing negative energy and protein balance postpartum. Overconsumption of energy during late pregnancy affects resting glucose and insulin concentrations peripartum and increases the risk for hyperketonemia postpartum, but the effects on muscle tissue are not fully understood. Skeletal muscle accounts for the majority of insulin-dependent glucose utilization in ruminants. Our objective was to study peripartal skeletal muscle insulin signaling as well as muscle accretion and atrophy in cows with excess energy consumption prepartum. Skeletal muscle biopsies were obtained 28 and 10 days prepartum, as well as 4 and 21 days postpartum from 24 Holstein cows. Biopsies were taken immediately before and 60 min after intravenous glucose challenge causing endogenous release of insulin. Gene expression of IGF-1, myostatin, and atrogin-1, as well as immunoblot analysis of atrogin-1, muRF1, ubiquitinated proteins, LC3, and phosphorylation of AKT, ERK and mTORC1 substrate 4EBP1 was performed. Excess energy consumption in late pregnancy did not lead to changes in insulin-dependent molecular regulation of muscle accretion or atrophy compared with the controlled energy group. In both groups, phosphorylation of AKT and mTORC1 substrate was significantly decreased postpartum whereas proteasome activity and macroautopagy were upregulated. This study showed that in addition to the proteasome pathway of muscle atrophy, macroautophagy is upregulated in postpartum negative energy and protein balance regardless of dietary energy strategy prepartum and was higher in cows overfed energy throughout the study period.

Reduced DPP4 activity improves insulin signaling in primary human adipocytes

DPP4 is a ubiquitously expressed cell surface protease which is also released to the circulation as soluble DPP4 (sDPP4). Recently, we identified DPP4 as a novel adipokine oversecreted in obesity and thus potentially linking obesity to the metabolic syndrome. Furthermore, sDPP4 impairs insulin signaling in an autocrine and paracrine fashion in different cell types. However, it is still unknown which functional role DPP4 might play in adipocytes. Therefore, primary human adipocytes were treated with a specific DPP4 siRNA. Adipocyte differentiation was not affected by DPP4 silencing. Interestingly, DPP4 reduction improved insulin responsiveness of adipocytes at the level of insulin receptor, proteinkinase B (Akt) and Akt substrate of 160 kDa. To investigate whether the observed effects could be attributed to the enzymatic activity of DPP4, human adipocytes were treated with the DPP4 inhibitors sitagliptin and saxagliptin. Our data show that insulin-stimulated activation of Akt is augmented by DPP4 inhibitor treatment. Based on our previous observation that sDPP4 induces insulin resistance in adipocytes, and that adipose DPP4 levels are higher in obese insulin-resistant patients, we now suggest that the abundance of DPP4 might be a regulator of adipocyte insulin signaling.

Pyruvate Kinase M2 Activates mTORC1 by Phosphorylating AKT1S1.

In cancer cells, the mammalian target of rapamycin complex 1 (mTORC1) that requires hormonal and nutrient signals for its activation, is constitutively activated. We found that overexpression of pyruvate kinase M2 (PKM2) activates mTORC1 signaling through phosphorylating mTORC1 inhibitor AKT1 substrate 1 (AKT1S1). An unbiased quantitative phosphoproteomic survey identified 974 PKM2 substrates, including serine202 and serine203 (S202/203) of AKT1S1, in the proteome of renal cell carcinoma (RCC). Phosphorylation of S202/203 of AKT1S1 by PKM2 released AKT1S1 from raptor and facilitated its binding to 14-3-3, resulted in hormonal- and nutrient-signals independent activation of mTORC1 signaling and led accelerated oncogenic growth and autophagy inhibition in cancer cells. Decreasing S202/203 phosphorylation by TEPP-46 treatment reversed these effects. In RCCs and breast cancers, PKM2 overexpression was correlated with elevated S202/203 phosphorylation, activated mTORC1 and inhibited autophagy. Our results provided the first phosphorylome of PKM2 and revealed a constitutive mTORC1 activating mechanism in cancer cells.

Calorie restriction leads to greater Akt2 activity and glucose uptake by insulin-stimulated skeletal muscle from old rats.

Skeletal muscle insulin resistance is associated with many common age-related diseases, but moderate calorie restriction (CR) can substantially elevate glucose uptake by insulin-stimulated skeletal muscle from both young and old rats. The current study evaluated the isolated epitrochlearis muscle from ∼24.5-mo-old rats that were either fed ad libitum (AL) or subjected to CR (consuming ∼65% of ad libitum, AL, intake beginning at ∼22.5 mo old). Some muscles were also incubated with MK-2206, a potent and selective Akt inhibitor. The most important results were that in isolated muscles, CR vs. AL resulted in 1) greater insulin-stimulated glucose uptake 2) that was accompanied by significantly increased insulin-mediated activation of Akt2, as indicated by greater phosphorylation on both Thr(309) and Ser(474) along with greater Akt2 activity, 3) concomitant with enhanced phosphorylation of several Akt substrates, including an Akt substrate of 160 kDa on Thr(642) and Ser(588), filamin C on Ser(2213) and proline-rich Akt substrate of 40 kDa on Thr(246), but not TBC1D1 on Thr(596); and 4) each of the CR effects was eliminated by MK-2206. These data provide compelling new evidence linking greater Akt2 activation to the CR-induced elevation of insulin-stimulated glucose uptake by muscle from old animals.

Two new competing pathways establish the threshold for cyclin-B-Cdk1 activation at the meiotic G2/M transition.

ABSTRACTExtracellular ligands control biological phenomena. Cells distinguish physiological stimuli from weak noise stimuli by establishing a ligand-concentration threshold. Hormonal control of the meiotic G2/M transition in oocytes is essential for reproduction. However, the mechanism for threshold establishment is unclear. In starfish oocytes, maturation-inducing hormones activate the PI3K–Akt pathway through the Gβγ complex of heterotrimeric G-proteins. Akt directly phosphorylates both Cdc25 phosphatase and Myt1 kinase, resulting in activation of cyclin-B–Cdk1, which then induces meiotic G2/M transition. Here, we show that cyclin-B–Cdk1 is partially activated after subthreshold hormonal stimuli, but this triggers negative feedback, resulting in dephosphorylation of Akt sites on Cdc25 and Myt1, thereby canceling the signal. We also identified phosphatase activity towards Akt substrates that exists independent of stimuli. In contrast to these negative regulatory activities, an atypical Gβγ-dependent pathway enhances PI3K–Akt-dependent phosphorylation. Based on these findings, we propose a model for threshold establishment in which hormonal dose-dependent competition between these new pathways establishes a threshold; the atypical Gβγ-pathway becomes predominant over Cdk-dependent negative feedback when the stimulus exceeds this threshold. Our findings provide a regulatory connection between cell cycle and signal transduction machineries.

Quercetin oppositely regulates insulin‐mediated glucose disposal in skeletal muscle under normal and inflammatory conditions: The dual roles of AMPK activation

Quercetin is a dietary flavonoid whose role in the regulation of the activity of insulin remains controversial. Our study aimed to investigate how quercetin and its major metabolite quercetin-3-glucuronide (Q-3-G) regulate insulin-mediated glucose disposal in skeletal muscle under normal and inflammatory conditions. Under normal conditions, quercetin impaired glucose and insulin tolerance and attenuated insulin-mediated phosphorylation of Akt substrate of 160kDa (AS160) and TBC1D1 without affecting Akt activity in male Institute of Cancer Research (ICR) mice. However, under inflammatory conditions, quercetin exhibited an opposite effect in these animals. In C2C12 cells, quercetin also decreased insulin-stimulated AS160 and TBC1D1 phosphorylation and glucose uptake in the absence of an inflammatory insult, whereas it improved the action of insulin under inflammatory conditions. Knockdown of adenosine 5'-monophosphate-activated protein kinase α (AMPKα) blocked the differential effects of quercetin under both conditions. Unlike quercetin, Q-3-G had no influence on insulin-induced phosphorylation of AS160 and TBC1D1 and glucose uptake in C2C12 myotubes under normal conditions. Q-3-G displayed a similar regulation with quercetin in glucose disposal under inflammatory conditions. Quercetin suppressed insulin-mediated glucose disposal in skeletal muscle tissue/cells under normal conditions while it ameliorated impaired glucose uptake under inflammatory conditions with activation of AMPK. In contrast, Q-3-G ameliorated insulin resistance in skeletal cells under inflammatory conditions without affecting glucose disposal under normal conditions.

ATP-site binding inhibitor effectively targets mTORC1 and mTORC2 complexes in glioblastoma.

The PI3K-AKT-mTOR signaling axis is central to the transformed phenotype of glioblastoma (GBM) cells, due to frequent loss of tumor suppressor PTEN (phosphatase and tensin homolog deleted on chromosome 10). The mechanistic target of rapamycin (mTOR) kinase is present in two cellular multi-protein complexes, mTORC1 and mTORC2, which have distinct subunit composition, substrates and mechanisms of action. Targeting the mTOR protein is a promising strategy for GBM therapy. However, neither of these complexes is fully inhibited by the allosteric inhibitor of mTOR, rapamycin or its analogs. Herein, we provide evidence that the combined inhibition of mTORC1/2, using the ATP-competitive binding inhibitor PP242, would effectively suppress GBM growth and dissemination as compared to an allosteric binding inhibitor of mTOR. GBM cells treated with PP242 demonstrated significantly decreased activation of mTORC1 and mTORC2, as shown by reduced phosphorylation of their substrate levels, p70 S6K(Thr389) and AKT(Ser473), respectively, in a dose-dependent manner. Furthermore, insulin induced activation of these kinases was abrogated by pretreatment with PP242 as compared with rapamycin. Unlike rapamycin, PP242 modestly activates extracellular regulated kinase (ERK1/2), as shown by expression of pERK(Thr202/Tyr204). Cell proliferation and S-phase entry of GBM cells was significantly suppressed by PP242, which was more pronounced compared to rapamycin treatment. Lastly, PP242 significantly suppressed the migration of GBM cells, which was associated with a change in cellular behavior rather than cytoskeleton loss. In conclusion, these results underscore the potential therapeutic use of the PP242, a novel ATP-competitive binding inhibitor of mTORC1/2 kinase, in suppression of GBM growth and dissemination.

Aberrant Activation of the PI3K/mTOR Pathway Promotes Resistance to Sorafenib in AML

Therapy directed against oncogenic FLT3 has been shown to induce response in patients with AML, but these responses are almost always transient. To address the mechanism of FLT3 inhibitor resistance, we generated two resistant MV4-11 and MOLM-13 cell lines by sustained treatment with the FLT3 inhibitor sorafenib. MV4-11 cells express only FLT3-ITD, while MOLM-13 cells express wild-type FLT3 and FLT3-ITD. Both cell lines are dependent on FLT3 activation as sorafenib, PKC-412, and AC220, but not imatinib, dasatinib, nilotinib or bosutinib, inhibit cell survival in both cell lines. After treatment with sorafenib for 90 days, we observed that both cell lines displayed resistance to sorafenib as well as to AC220 suggesting that sustained treatment with an FLT3 inhibitor results in acquired resistance to multiple FLT3 inhibitors.To test whether sorafenib was still effective in FLT3 inhibition, we treated sensitive and resistant cells with sorafenib or DMSO and then stimulated with FLT3 ligand (FL). While DMSO-treated resistant cells responded to FL as expected, sorafenib-treated cells displayed poor FLT3 activation, suggesting that sorafenib was still capable of inhibiting FLT3 activation in the resistant cells. Surprisingly we observed that the resistant cells treated with DMSO had a much more robust response to ligand in terms of FLT3 activation. Similar results were observed with AKT and ERK activation, as sorafenib-treated cells poorly responded to FL-stimulation. Furthermore, resistant cells treated with sorafenib could still form colonies similar to DMSO-treated cells indicating that these cells were no longer dependent on FLT3 activation, although sorafenib could partially block FLT3 activation.To determine whether secondary mutations occurred in FLT3, we sequenced the whole coding region using Sanger sequencing. Except for a mutation in the extracellular domain in all four cell lines no mutations were detected in the inhibitor binding site. Similar to Sanger sequencing, mass spectroscopic analysis of affinity-enriched FLT3 indicated no differences in the intracellular part of FLT3 between sensitive and resistant cells. Because we observed an unexpected activation of FLT3 in resistant cells stimulated with FL, we hypothesized that certain FLT3 residues remain hyper-tyrosine phosphorylated. To test that we used phospho-specific antibodies against known FLT3 tyrosine phosphorylation sites. Although we observed an increase in total FLT3 tyrosine phosphorylation, we were unable to identify a single site that was selectively hyper-phosphorylated. Instead, all sites remained slightly more phosphorylated compared to control cells.Gene expression analysis of mRNA from sensitive and resistant cell lines using ANOVA and significance analysis of microarrays (SAM) suggested an enrichment of the PI3K/mTOR pathway in the resistant phenotype. In addition to pathway enrichment, using a phospho-protein antibody array, we found that phosphorylation of the mTOR substrates S6K and AKT were selectively increased in resistant cells. We also observed an increase in STAT3 phosphorylation. Elevated STAT3 phosphorylation was probably due to the previously described upregulation of JAK3 expression in sorafenib-resistant AML. We also observed enrichment of an mTOR signature in AML blasts from eight patients with sorafenib-resistant AML (GSE35907) and in AML blasts from patients expressing FLT3-ITD compared to those lacking FLT3-ITD (525 samples, GSE14468).Furthermore, a selective PI3K/mTOR inhibitor, gedatolisib, efficiently blocked proliferation, colony and tumor formation, and induced apoptosis in the resistant cell lines. Treatment of cells with a higher concentration of gedatolisib did not alter the phosphorylation of other signaling proteins except for AKT and S6K which was detected by a phospho-specific antibody array. The array data was further verified with western blotting using phospho-specific antibodies against AKT, ERK1/2, p38 and S6K. These results suggest that gedatolisib efficiently blocks the downstream effectors PI3K/mTOR without affecting other signaling pathways.Taken together, our data suggest that aberrant activation of the PI3K/mTOR pathway in FLT3-ITD dependent AML results in resistance to drugs targeting FLT3 and treatment with selective PI3K/mTOR inhibitors are likely to be effective in overcoming acquired resistance in AML patients. DisclosuresStegmaier:Novartis Pharmaceuticals: Consultancy.

Endothelial Cell Protein C Receptor Promotes Apoptosis in Malignant Pleural Mesothelioma Cells

Our recent studies showed that endothelial cell protein C receptor (EPCR) functions as a crucial negative regulator of cancer progression in malignant pleural mesothelioma (MPM) (Keshava et al., Cancer Research 73: 3963-73, 2013). In these studies, the introduction of EPCR expression to aggressive MPM cells lacking EPCR completely attenuated their tumorigenicity whereas the knock-down of EPCR expression in non-aggressive MPM cells that constitutively express EPCR increased their tumorigenicity. This study also revealed that EPCR in MPM cells promotes tumor cell apoptosis in vivo. These data are quite intriguing as EPCR-mediated cell signaling typically activates cell survival and anti-apoptotic pathways and is shown to promote tumor growth in breast and lung cancers. The aim of the present study is to understand the mechanism(s) by which EPCR promotes tumor cell apoptosis in MPM. Evaluation of thoracic fluids obtained from nude mice implanted with MPM cells orthotopically showed a significant increase in the levels of mTNFα and mIFNγ in mice implanted with REN (+EPCR) cells compared to mice implanted with parental REN MPM or REN cells transfected with the vector control [REN(z)]. Analysis of tumor cell apoptosis of cultured cells by TUNEL assay showed that REN (+EPCR) cells were highly susceptible to TNFα+IFNγ-induced cell death compared to parental REN or REN(z) cells (80% vs. 20% cell death at 72 h). In agreement with the concept that EPCR in MPM cells promotes the apoptosis, M9K MPM cells that constitutively express EPCR (non-aggressive MPM cell line) were highly susceptible to TNFα+IFNγ-induced cell death; the knock-down of EPCR in these cells rendered them resistant to cytokine-induced cell death. EPCR-induced tumor cell apoptosis in MPM cells was not dependent on activated protein C (APC). Deletion of the cytoplasmic tail of EPCR failed to diminish its cytotoxic effect. Analysis of REN and REN(+EPCR) tumor cell extracts using PathScan intracellular signaling antibody array kit (Cell Signaling Technology, MA) and immunoblot analysis with specific antibodies against proteins involved in apoptotic pathways revealed that EPCR expression in MPM cells, in comparison to MPM cells lacking EPCR, resulted in a sustained cleavage of poly ADP ribose polymerase (PARP) and an enhanced activation of caspase 3 in response to TNFα+ IFNγ treatment. In addition, REN(+EPCR) cells exhibited an increased activation of caspase 8 and a decreased phosphorylation of anti-apoptotic protein BAD. Further signaling studies showed a delayed activation of NF-kB, suppression of activation of STAT3 and AKT in REN(+EPCR) cells compared to REN cells in response to TNFα+ IFNγ treatment. In addition, EPCR expression suppressed the constitutive activation of the proline-rich AKT substrate, 40 kDa (PRAS40). Overall, our studies indicate that EPCR in MPM cells controls the progression of MPM by exerting its effect at multiple levels. First, EPCR in MPM cells elicits elaboration of cytokines TNFα and IFNγ in vivo and then renders the MPM cells susceptible to TNFα+ IFNγ-induced cell death. The EPCR-mediated cytotoxic effect in MPM cells is independent of APC and the cytoplasmic tail and involves the suppression of STAT3 and AKT activation. At present, it is unclear exactly how EPCR initiates the activation of the apoptotic pathway in MPM cells and whether it involves the interaction of EPCR with the endogenous ligand(s) specific to MPM cells. Studies are underway to identify EPCR interacting proteins in MPM cells. DisclosuresNo relevant conflicts of interest to declare.

Abstract A121: Constitutive K-Ras activity independent of K-RAS mutational status leads to resistance to anti-EGFR molecular targeting approaches and MEK-dependent reactivation of Akt following PI3K inhibition

Abstract Non-small cell lung cancer (NSCLC) cells expressing K-RAS mutation are resistant to anti-EGFR antibody cetuximab. Oncogenic Ras regulates basal effector PI3K/Akt and MAPK/ERK pathways. In this study, we investigated impact of K-RAS activity independent of its mutational status on targetibility of EGFR as well as the PI3K/Akt and MAPK/ERK pathways. A panel of five NSCLC cells (A549, H460, H661, SK-MES-1, HTB-182) and five head and neck squamous carcinoma (HNSCC) cells (SAS, FaDu, UT-SCC-5, UT-SCC-15, UT-SCC-5R (UT-SCC-5R are UT5 cells acquired resistance to cetuximab) were studied. K-Ras was constitutively activate not only in K-RAS mutated (K-RASmut) A549 and H460 cells, but also in K-RAS wild-type (K-RASwt) SAS and UT-SCC-5R cells. Enhanced K-Ras-GTP either endogenously or following overexpression of K-RAS(V12) was associated with significant shortening of population doubling time and increased clonogenic activity of tu-mor cells from both origin. Constitutive activation of K-Ras independent of K-RAS muta-tional status led to desensitization of both K-RASmut and K-RASwt cells to anti-EGFR an-tibody cetuximab and EGFR-tyrosine kinase inhibitors (TKI) erlotinib and AG1478, tested by standard clonogenic assay. Analyzing EGFR downstream pathways revealed that lack of response to the EGFR-TK inhibitors is through activation of the PI3K/Akt pathway. Thus, inhibition of PI3K was supposed to be an efficient approach to block clonogenic activity. However, PI3K inhibitor completely blocked clonogenic activity only in cells expressing low level of K-RASwt from both origins. In K-RASmut NSCLC cells or in HNSCC cells with overexpression of K-RASwt a partial response to PI-103 was observed. Analyzing pathway activity revealed that PI-103 does not inhibit Akt activity after 24 h pre-treatment while up to 4 h after post-treatment with PI-103, phosphorylation of Akt and Akt substrates (PRAS40 and GSK3αβ) are completely blocked. Lack of effect of PI103 at 24 h post-treatment was shown to be through K-RAS/MEK/ERK dependent reactivation of Akt. In line with this observation, targeting MEK by PD98059 significantly improved the anti-clonogenic activity of PI-103. Although, PD98059 blocked phosphorylation of ERK1/2, no marked effect was observed on clonogenic activity. Thus, EGFR or PI3K, but not MEK are appropriate target to inhibit clonogenic activity of tumor cells expressing low level of K-RASwt. In tumor cells with constitutive K-RAS activity, dual targeting of PI3K and MEK is an efficient approach to inhibit clonogenicity. Supported by grants from the Deutsche Forschungsgemeinschaft [Ro527/7-1 and SFB-773-TP B02] awarded to HPR, GRK 1302/2 (T11) awarded to MT/HPR. Citation Format: Mahmoud Toulany, Mohammad Saki, H. Peter Rodemann. Constitutive K-Ras activity independent of K-RAS mutational status leads to resistance to anti-EGFR molecular targeting approaches and MEK-dependent reactivation of Akt following PI3K inhibition. [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference: Molecular Targets and Cancer Therapeutics; 2015 Nov 5-9; Boston, MA. Philadelphia (PA): AACR; Mol Cancer Ther 2015;14(12 Suppl 2):Abstract nr A121.

Jicama (Pachyrhizus erosus) extract increases insulin sensitivity and regulates hepatic glucose in C57BL/Ksj-db/db mice.

This study investigated the effect of jicama extract on hyperglycemia and insulin sensitivity in an animal model of type 2 diabetes. Male C57BL/Ksj-db/db mice were divided into groups subsequently fed a regular diet (controls), or diet supplemented with jicama extract, and rosiglitazone. After 6 weeks, blood levels of glucose and glycosylated hemoglobin were significantly lower in animals administered the jicama extract than the control group. Additionally, glucose and insulin tolerance tests showed that jicama extract increased insulin sensitivity. The homeostatic index of insulin resistance was lower in the jicama extract-treated group than in the diabetic control group. Administration of jicama extract significantly enhanced the expressions of the phosphorylated AMP-activated protein kinase and Akt substrate of 160 kDa, and plasma membrane glucose transporter type 4 in skeletal muscle. Jicama extract administration also decreased the expressions of glucose 6-phosphatase and phosphoenol pyruvate carboxykinase in the liver. Jicama extract may increases insulin sensitivity and inhibites the gluconeogenesis in the liver.

AMPK, a metabolic sensor, is involved in isoeugenol-induced glucose uptake in muscle cells.

Isoeugenol exerts various beneficial effects on human health. However, the mechanisms underlying these effects are poorly understood. In this study, we observed that isoeugenol activated AMP-activated protein kinase (AMPK) and increased glucose uptake in rat L6 myotubes. Isoeugenol-induced increase in intracellular calcium concentration and glucose uptake was inhibited by STO-609, an inhibitor of calcium/calmodulin-dependent protein kinase kinase (CaMKK). Isoeugenol also increased the phosphorylation of protein kinase C-α (PKCα). Chelation of calcium with BAPTA-AM blocked isoeugenol-induced AMPK phosphorylation and glucose uptake. Isoeugenol stimulated p38MAPK phosphorylation that was inhibited after pretreatment with compound C, an AMPK inhibitor. Isoeugenol also increased glucose transporter type 4 (GLUT4) expression and its translocation to the plasma membrane. GLUT4 translocation was not observed after the inhibition of AMPK and CaMKK. In addition, isoeugenol activated the Akt substrate 160 (AS160) pathway, which is downstream of the p38MAPK pathway. Knockdown of the gene encoding AS160 inhibited isoeugenol-induced glucose uptake. Together, these results indicate that isoeugenol exerts beneficial health effects by activating the AMPK/p38MAPK/AS160 pathways in skeletal muscle.

Phosphorylation and activation of ubiquitin-specific protease-14 by Akt regulates the ubiquitin-proteasome system.

Regulation of ubiquitin-proteasome system (UPS), which controls the turnover of short-lived proteins in eukaryotic cells, is critical in maintaining cellular proteostasis. Here we show that USP14, a major deubiquitinating enzyme that regulates the UPS, is a substrate of Akt, a serine/threonine-specific protein kinase critical in mediating intracellular signaling transducer for growth factors. We report that Akt-mediated phosphorylation of USP14 at Ser432, which normally blocks its catalytic site in the inactive conformation, activates its deubiquitinating activity in vitro and in cells. We also demonstrate that phosphorylation of USP14 is critical for Akt to regulate proteasome activity and consequently global protein degradation. Since Akt can be activated by a wide range of growth factors and is under negative control by phosphoinosotide phosphatase PTEN, we suggest that regulation of UPS by Akt-mediated phosphorylation of USP14 may provide a common mechanism for growth factors to control global proteostasis and for promoting tumorigenesis in PTEN-negative cancer cells.

Ischemia-Induced Changes of PRAS40 and p-PRAS40 Immunoreactivities in the Gerbil Hippocampal CA1 Region After Transient Cerebral Ischemia.

Proline-rich Akt substrate of 40-kDa (PRAS40) is one of the important interactive linkers between Akt and mTOR signaling pathways. The increase of PRAS40 is related with the reduction of brain damage induced by cerebral ischemia. In the present study, we investigated time-dependent changes in PRAS40 and phospho-PRAS40 (p-PRAS40) immunoreactivities in the hippocampal CA1 region of the gerbil after 5min of transient cerebral ischemia. PRAS40 immunoreactivity in the CA1 region was decreased in pyramidal neurons from 12h after ischemic insult in a time-dependent manner, and, at 5days post-ischemia, PRAS40 immunoreactivity was newly expressed in astrocytes. p-PRAS40 immunoreactivity in the CA1 pyramidal neurons was hardly found 12h and apparently detected again 1 and 2days after ischemic insult. At 5days post-ischemia, p-PRAS40 immunoreactivity in the CA1 pyramidal neurons was not found. These results indicate that ischemia-induced changes in PRAS40 and p-PRAS40 immunoreactivities in CA1 pyramidal neurons and astrocytes may be closely associated with delayed neuronal death in the hippocampal CA1 region following transient cerebral ischemia.

Mechanisms for greater insulin-stimulated glucose uptake in normal and insulin-resistant skeletal muscle after acute exercise.

Enhanced skeletal muscle and whole body insulin sensitivity can persist for up to 24-48 h after one exercise session. This review focuses on potential mechanisms for greater postexercise and insulin-stimulated glucose uptake (ISGU) by muscle in individuals with normal or reduced insulin sensitivity. A model is proposed for the processes underlying this improvement; i.e., triggers initiate events that activate subsequent memory elements, which store information that is relayed to mediators, which translate memory into action by controlling an end effector that directly executes increased insulin-stimulated glucose transport. Several candidates are potential triggers or memory elements, but none have been conclusively verified. Regarding potential mediators in both normal and insulin-resistant individuals, elevated postexercise ISGU with a physiological insulin dose coincides with greater Akt substrate of 160 kDa (AS160) phosphorylation without improved proximal insulin signaling at steps from insulin receptor binding to Akt activity. Causality remains to be established between greater AS160 phosphorylation and improved ISGU. The end effector for normal individuals is increased GLUT4 translocation, but this remains untested for insulin-resistant individuals postexercise. Following exercise, insulin-resistant individuals can attain ISGU values similar to nonexercising healthy controls, but after a comparable exercise protocol performed by both groups, ISGU for the insulin-resistant group has been consistently reported to be below postexercise values for the healthy group. Further research is required to fully understand the mechanisms underlying the improved postexercise ISGU in individuals with normal or subnormal insulin sensitivity and to explain the disparity between these groups after similar exercise.

Akt Links Insulin Signaling to Albumin Endocytosis in Proximal Tubule Epithelial Cells.

Diabetes mellitus (DM) has become an epidemic, causing a significant decline in quality of life of individuals due to its multisystem involvement. Kidney is an important target organ in DM accounting for the majority of patients requiring renal replacement therapy at dialysis units. Microalbuminuria (MA) has been a valuable tool to predict end-organ damage in DM but its low sensitivity has driven research efforts to seek other alternatives. Albumin is taken up by albumin receptors, megalin and cubilin in the proximal tubule epithelial cells. We demonstrated that insulin at physiological concentrations induce albumin endocytosis through activation of protein kinase B (Akt) in proximal tubule epithelial cells. Inhibition of Akt by a phosphorylation deficient construct abrogated insulin induced albumin endocytosis suggesting a role for Akt in insulin-induced albumin endocytosis. Furthermore we demonstrated a novel interaction between Akt substrate 160kDa (AS160) and cytoplasmic tail of megalin. Mice with type 1 DM (T1D) displayed decreased Akt, megalin, cubilin and AS160 expression in their kidneys in association with urinary cubilin shedding preceding significant MA. Patients with T1D who have developed MA in the EDC (The Pittsburgh Epidemiology of Diabetes Complications) study demonstrated urinary cubilin shedding prior to development of MA. We hypothesize that perturbed insulin-Akt cascade in DM leads to alterations in trafficking of megalin and cubilin, which results in urinary cubilin shedding as a prelude to MA in early diabetic nephropathy. We propose that utilization of urinary cubilin shedding, as a urinary biomarker, will allow us to detect and intervene in diabetic nephropathy (DN) at an earlier stage.

Kinome Screen Identifies PFKFB3 and Glucose Metabolism as Important Regulators of the Insulin/Insulin-like Growth Factor (IGF)-1 Signaling Pathway

The insulin/insulin-like growth factor (IGF)-1 signaling pathway (ISP) plays a fundamental role in long term health in a range of organisms. Protein kinases including Akt and ERK are intimately involved in the ISP. To identify other kinases that may participate in this pathway or intersect with it in a regulatory manner, we performed a whole kinome (779 kinases) siRNA screen for positive or negative regulators of the ISP, using GLUT4 translocation to the cell surface as an output for pathway activity. We identified PFKFB3, a positive regulator of glycolysis that is highly expressed in cancer cells and adipocytes, as a positive ISP regulator. Pharmacological inhibition of PFKFB3 suppressed insulin-stimulated glucose uptake, GLUT4 translocation, and Akt signaling in 3T3-L1 adipocytes. In contrast, overexpression of PFKFB3 in HEK293 cells potentiated insulin-dependent phosphorylation of Akt and Akt substrates. Furthermore, pharmacological modulation of glycolysis in 3T3-L1 adipocytes affected Akt phosphorylation. These data add to an emerging body of evidence that metabolism plays a central role in regulating numerous biological processes including the ISP. Our findings have important implications for diseases such as type 2 diabetes and cancer that are characterized by marked disruption of both metabolism and growth factor signaling.

AIF inhibits tumor metastasis by protecting PTEN from oxidation.

Apoptosis-inducing factor (AIF) exerts dual roles on cell death and survival, but its substrates as a putative oxidoreductase and roles in tumorigenesis remain elusive. Here, we report that AIF physically interacts with and inhibits the oxidation of phosphatase and tensin homolog on chromosome ten (PTEN), a tumor suppressor susceptible for oxidation-mediated inactivation. More intriguingly, we also identify PTEN as a mitochondrial protein and the ectopic expression of mitochondrial targeting sequence-carrying PTEN almost completely inhibits Akt phosphorylation in PTEN-deficient cells. AIF knockdown causes oxidation-mediated inactivation of the lipid phosphatase activity of PTEN, with ensuing activation of Akt kinase, phosphorylation of the Akt substrate GSK-3β, and activation of β-catenin signaling in cancer cells. Through its effect on β-catenin signaling, AIF inhibits epithelial–mesenchymal transition (EMT) and metastasis of cancer cells in vitro and in orthotopically implanted xenografts. Accordingly, the expression of AIF is correlated with the survival of human patients with cancers of multiple origins. These results identify PTEN as the substrate of AIF oxidoreductase and reveal a novel function for AIF in controlling tumor metastasis.