Role of ERK1/ERK2 and p70S6K pathway in insulin signalling of protein synthesis

Sequestosome 1/p62, a scaffolding protein, is a newly identified partner of IRS-1 protein

Defects in the insulin-signaling pathway may lead to the development of skeletal muscle insulin resistance, which is one of the earliest abnormalities detected in individuals with the metabolic syndrome and predisposes them to develop type 2 diabetes. Previous studies have shown that deletion of the mouse sequestosome 1/p62 gene results in mature-onset obesity that progresses to insulin and leptin resistance and, ultimately, type 2 diabetes. Sequestosome 1/p62 is involved in receptor-mediated signal transduction and functions as an intracellular signal modulator or adaptor protein. Insulin receptor substrate-1 (IRS-1) plays a central role in transducing the insulin signal via phosphorylation, protein-protein interactions, and protein modifications. Mapping studies demonstrated that the SH(2) domain at the amino terminus of sequestosome 1/p62 interacts with IRS-1 upon insulin stimulation. Further, IRS-1 interacts with p62 through its YMXM motifs at Tyr-608, Tyr-628, and/or Tyr-658 in a manner similar to its interaction with p85 of phosphoinositol 3-kinase. Overexpression of p62 increased phosphorylation of Akt, GLUT4 translocation, and glucose uptake, providing evidence that p62 participates in the insulin-signaling pathway through its interactions with IRS-1.

The role of myostatin in muscle wasting: an overview

Myostatin is an extracellular cytokine mostly expressed in skeletal muscles and known to play a crucial role in the negative regulation of muscle mass. Upon the binding to activin type IIB receptor, myostatin can initiate several different signalling cascades resulting in the upregulation of the atrogenes and downregulation of the important for myogenesis genes. Muscle size is regulated via a complex interplay of myostatin signalling with the insulin-like growth factor 1/phosphatidylinositol 3-kinase/Akt pathway responsible for increase in protein synthesis in muscle. Therefore, the regulation of muscle weight is a process in which myostatin plays a central role but the mechanism of its action and signalling cascades are not fully understood. Myostatin upregulation was observed in the pathogenesis of muscle wasting during cachexia associated with different diseases (i.e. cancer, heart failure, HIV). Characterisation of myostatin signalling is therefore a perspective direction in the treatment development for cachexia. The current review covers the present knowledge about myostatin signalling pathways leading to muscle wasting and the state of therapy approaches via the regulation of myostatin and/or its downstream targets in cachexia.

Mechanisms involved in the enhancement of mammalian target of rapamycin signalling and hypertrophy in skeletal muscle of myostatin-deficient mice

Myostatin deficiency leads to both an increased rate of protein synthesis and skeletal muscle hypertrophy. However, the mechanisms involved in mediating these effects are not yet fully understood. Here, we demonstrate that genetic loss of myostatin leads to enhanced muscle expression of both protein kinase B and mammalian target of rapamycin/S6K signalling components, consistent with their elevated activity. This is associated with a reduction in the expression of PGC1alpha and COX IV, proteins which play important roles in maintaining mitochondrial function. Furthermore, we show that these changes in signalling and protein expression are largely independent of alterations in intramuscular amino acid content. Our findings, therefore, reveal potential new mechanisms and further contribute to our understanding of myostatin-regulated skeletal muscle growth and function.

Mechano-transduction to muscle protein synthesis is modulated by FAK

We examined the involvement of focal adhesion kinase (FAK) in mechano-regulated signalling to protein synthesis by combining muscle-targeted transgenesis with a physiological model for un- and reloading of hindlimbs. Transfections of mouse tibialis anterior muscle with a FAK expression construct increased FAK protein 1.6-fold versus empty transfection in the contralateral leg and elevated FAK concentration at the sarcolemma. Altered activation status of phosphotransfer enzymes and downstream translation factors showed that FAK overexpression was functionally important. FAK auto-phosphorylation on Y397 was enhanced between 1 and 6 h of reloading and preceded the activation of p70S6K after 24 h of reloading. Akt and translation initiation factors 4E-BP1 and 2A, which reside up- or downstream of p70S6K, respectively, showed no FAK-modulated regulation. The findings identify FAK as an upstream element of the mechano-sensory pathway of p70S6K activation whose Akt-independent regulation intervenes in control of muscle mass by mechanical stimuli in humans.

Identification of 80K-H as a protein involved in GLUT4 vesicle trafficking

PKCzeta (protein kinase Czeta) is a serine/threonine protein kinase controlled by insulin, various growth factors and phosphoinositide 3-kinase. It has been implicated in controlling glucose transport in response to insulin by the translocation of GLUT4-(glucose transporter 4) containing vesicles to the plasma membrane in stimulated cells. How PKCzeta modulates GLUT4 vesicle trafficking remains unknown. A yeast two-hybrid screen using full-length human PKCzeta identified 80K-H protein as an interactor with PKCzeta. GST (glutathione S-transferase) pull-down assays with GST-tagged 80K-H constructs confirmed the interaction and showed that the N-terminal portion of 80K-H was not required for the interaction. Immunoprecipitates of endogenous PKCzeta from Cho cells, 3T3-L1 adipocytes or L6 myotubes contained endogenous 80K-H, demonstrating a physiological interaction. Insulin stimulation enhanced the association 3-5-fold. Immunoprecipitates of endogenous 80K-H contained endogenous munc18c and immunoprecipitates of endogenous munc18c contained endogenous PKCzeta, with insulin markedly increasing the amount of co-immunoprecipitated protein in each case. These results show that insulin triggers interactions in vivo between PKCzeta, 80K-H and munc18c. Overexpression of 80K-H constructs mimicked the action of insulin in stimulating both glucose uptake and translocation of Myc-tagged GLUT4 in Cho cells, with the level of effect proportional to the ability of the constructs to associate with munc18c. These results identify 80K-H as a new player involved in GLUT4 vesicle transport and identify a link between a kinase involved in the insulin signalling cascade, PKCzeta, and a known component of the GLUT4 vesicle trafficking pathway, munc18c. The results suggest a model whereby insulin triggers the formation of a PKCzeta-80K-H-munc18c complex that enhances GLUT4 translocation to the plasma membrane.

Protein kinase-zeta interacts with munc18c: role in GLUT4 trafficking

AIMS/HYPOTHESIS

Insulin-stimulated glucose transport requires a signalling cascade through kinases protein kinase (PK) Czeta/lambda and PKB that leads to movement of GLUT4 vesicles to the plasma membrane. The aim of this study was to identify missing links between the upstream insulin-regulated kinases and the GLUT4 vesicle trafficking system.

MATERIALS AND METHODS

A yeast two-hybrid screen was conducted, using as bait full-length mouse munc18c, a protein known to be part of the GLUT4 vesicle trafficking machinery.

RESULTS

The yeast two-hybrid screen identified PKCzeta as a novel interactor with munc18c. Glutathione S transferase (GST) pull-downs with GST-tagged munc18c constructs confirmed the interaction, mapped a key region of munc18c that binds PKCzeta to residues 295-338 and showed that the N-terminal region of PKCzeta was required for the interaction. Endogenous munc18c was shown to associate with endogenous PKCzeta in vivo in various cell types. Importantly, insulin stimulation increased the association by approximately three-fold. Moreover, disruption of PKCzeta binding to munc18c by deletion of residues 295-338 of munc18c or deletion of the N-terminal region of PKCzeta markedly inhibited the ability of insulin to stimulate glucose uptake or GLUT4 translocation.

CONCLUSIONS/INTERPRETATION

We have identified a physiological interaction between munc18c and PKCzeta that is insulin-regulated. This establishes a link between a kinase (PKCzeta) involved in the insulin signalling cascade and a known component of the GLUT4 vesicle trafficking pathway (munc18c). The results indicate that PKCzeta regulates munc18c and suggest a model whereby insulin triggers the docking of PKCzeta to munc18c, resulting in enhanced GLUT4 translocation to the plasma membrane.

Gene expression and tyrosine kinase activity of insulin receptor in uterine leiomyoma and matched myometrium

OBJECTIVE

Our objective was to determine the insulin receptor (IR) mRNA levels and IR tyrosine kinase activity in normal myometrium and leiomyoma.

DESIGN

Experimental study.

SETTING

Academic research center.

PATIENTS

Five women with leiomyoma submitted to hysterectomy.

INTERVENTION

Plasma membrane fraction of human myometrium and leiomyoma were prepared and samples were incubated with and without insulin. mRNA was isolated and RT-PCR with specific primers was performed.

MAIN OUTCOME MEASURE

Western blots of plasma membranes incubated with and without insulin were performed. Chemoluminescent methods followed by densitometry were used to assess IR autophosphorylation. RT-PCR with specific primers for IR gene sequences was used to determine IR mRNA levels.

RESULTS

IR mRNA levels in myometrium (0.634+/-0.038) and in leiomyoma (0.649+/-0.047; p=0.813) were not different. The degree of insulin-stimulated IR autophosphorylation (relative optical density of the 95 kDa band) was also not different between myometrium (1.496+/-0.310) and leiomyoma (1.593+/-0.129; p=0.650).

CONCLUSIONS

There was no difference in IR receptor expression and IR autophosphorylation between normal myometrium and leiomyoma. Other steps of insulin signaling chain may participate in the altered proliferation of leiomyomas.

Call volume and insulin signaling

Perturbations of cell hydration as provoked by changes in ambient osmolarity or under isoosmotic conditions by hormones, second messengers, intracellular substrate accumulation, or reactive oxygen intermediates critically contribute to the physiological regulation of cell function. In general an increase in cell hydration stimulates anabolic metabolism and proliferation and provides cytoprotection, whereas cellular dehydration leads to a catabolic situation and sensitizes cells to apoptotic stimuli. Insulin produces cell swelling by inducing a net K+ and Na+ accumulation inside the cell, which results from a concerted activation of Na+/H+ exchange, Na+/K+/2Cl- symport, and the Na+/K(+)-ATPase. In the liver, insulin-induced cell swelling is critical for stimulation of glycogen and protein synthesis as well as inhibition of autophagic proteolysis. These insulin effects can largely be mimicked by hypoosmotic cell swelling, pointing to a role of cell swelling as a trigger of signal transduction. This article discusses insulin-induced signal transduction upstream of swelling and introduces the hypothesis that cell swelling as a signal amplifyer represents an essential component in insulin signaling, which contributes to the full response to insulin at the level of signal transduction and function. Cellular dehydration impairs insulin signaling and may be a major cause of insulin resistance, which develops in systemic hyperosmolarity, nutrient deprivation, uremia, oxidative challenges, and unbalanced production of insulin-counteracting hormones. Hydration changes affect cell functions at multiple levels (such as transcriptom, proteom, phosphoproteom, and the metabolom) and a system biological approach may allow us to develop a more holistic view on the hydration dependence of insulin signaling in the future.

Activation of multiple signaling modules is critical in angiotensin IV-induced lung endothelial cell proliferation

Signaling events involving angiotensin IV (ANG IV)-mediated pulmonary artery endothelial cell (PAEC) proliferation were examined. ANG IV significantly increased upstream phosphatidylinositide (PI) 3-kinase (PI3K), PI-dependent kinase-1 (PDK-1), extracellular signal-related kinases (ERK1/2), and protein kinase B-alpha/Akt (PKB-alpha) activities, as well as downstream p70 ribosomal S6 kinase (p70S6K) activities and/or phosphorylation of these proteins. ANG IV also significantly increased 5-bromo-2'-deoxy-uridine incorporation into newly synthesized DNA in a concentration- and time-dependent manner. Pretreatment of cells with wortmannin and LY-294002, inhibitors of PI3K, or rapamycin, an inhibitor of the mammalian target of rapamycin kinase and p70S6K, diminished the ANG IV-mediated activation of PDK-1 and PKB-alpha as well as phosphorylation of p70S6K. Although an inhibitor of mitogen-activated protein kinase kinase, PD-98059, but not rapamycin, blocked ANG IV-induced phosphorylation of ERK1/2, both PD-98059 and rapamycin independently caused partial reduction in ANG IV-mediated cell proliferation. However, simultaneous treatment with PD-98059 and rapamycin resulted in total inhibition of ANG IV-induced cell proliferation. These results demonstrate that ANG IV-induced DNA synthesis is regulated in a coordinated fashion involving multiple signaling modules in PAEC.

Differential effects of des IGF-1 on Erks, AKT-1 and P70 S6K activation in mouse skeletal and cardiac muscle

Alterations in the degree of the phosphorylation of ERKI/2, Akt-1 and p70 S6K in mouse skeletal and cardiac muscle was examined in vivo following an intraperitoneal injection of des IGF-I. Plasma levels of insulin, IGF-I and glucose were measured. The administration of des IGF-I had no effect on plasma levels of insulin, or IGF-I, but plasma glucose levels were decreased about 50% (p < 0.01). In both skeletal and cardiac muscle, des IGF-I increased the phosphorylation of Akt-1 at Ser 473 (p < 0.01) with no change in the phosphorylation of p44 and p42 MAP kinases at Thr202/Tyr204. The phosphorylation of p70 S6K at Thr421/Ser424 was increased in skeletal muscle (p < 0.01), but not in cardiac muscle. The phosphorylation of the nuclear transcription factor CREB phosphorylation at Ser 133 was not significantly changed in either skeletal or cardiac muscle. Des IGF-I increased the phosphorylation of the transcription factor FKHR in cardiac muscle only (p < 0.05). These data demonstrate that the administration of des IGF-I had differential effects on the activation of the MAP kinase and PI 3-kinase pathways in mouse skeletal and cardiac muscle.

Adenovirus-mediated overexpression of dominant negative epidermal growth factor receptor-CD533 as a gene therapeutic approach radiosensitizes human carcinoma and malignant glioma cells

PURPOSE

Epidermal growth factor receptor (EGFR) and other members of the ErbB family of receptor tyrosine kinases (RTK) mediate autocrine growth regulation in a wide spectrum of human tumor cells. We have previously demonstrated that in stably transfected mammary carcinoma cells a dominant negative (DN) mutant of EGFR, EGFR-CD533 is a potent inhibitor of EGFR and its cytoprotective signaling after exposure to ionizing radiation. In the present study, we further investigate the capacity of a genetic approach, using replication-incompetent adenovirus (Ad)-mediated transfer of EGFR-CD533 (Ad-EGFR-CD533), to enhance the radiosensitivity in vitro of four cell lines representative of three major cancer phenotypes.

METHODS AND MATERIALS

The cell lines MDA-MB-231 and T-47D mammary carcinoma, A-431 squamous carcinoma, and U-373 MG malignant glioma cells were used. The ErbB expression profiles and the EGFR tyrosine phosphorylation (Tyr-P) levels following irradiation were quantified by Western blotting. The relative radiosensitivities of tumor cells were assessed by standard colony formation assays after infection with control vector (Ad-LacZ) or Ad-EGFR-CD533.

RESULTS

The expression profiles demonstrated varying levels of EGFR, ErbB2, ErbB3, and ErbB4 expression. The overexpression of EGFR-CD533 after infection with Ad-EGFR-CD533 completely inhibited the radiation-induced stimulation of EGFR Tyr-P relative to the immediate 2.4- to 3.1-fold increases in EGFR Tyr-P in control infected cells (Ad-LacZ). Ad-EGFR-CD533-infected cells demonstrated significant (p < 0.001) radiosensitization over a range of radiation doses (1-8 Gy), yielding dose-enhancement ratios (DER) between 1.4 and 1.7. This radiosensitization was maintained under conditions of repeated radiation exposures, using 3 x 2 Gy, yielding DERs of 1.6 and 1.7 for MDA-MB-231 and U-373 cells, respectively.

CONCLUSIONS

Overexpression of EGFR-CD533 significantly sensitizes human carcinoma and glioma cells to single and repeated radiation exposures irrespective of their ErbB expression levels. Therefore, transduction of human tumor cells with EGFR-CD533 holds promise as a gene therapeutic approach for the radiosensitization of neoplastic cells that are growth-regulated by EGFR or other ErbB receptors.

Retinoblastoma protein phosphorylation via PI 3-kinase and mTOR pathway regulates adipocyte differentiation

In the early phase of adipocyte differentiation, transient increase of DNA synthesis, called clonal expansion, and transient hyperphosphorylation of retinoblastoma protein (Rb) are observed. We investigated the role of these phenomena in insulin-induced adipocyte differentiation of 3T3-L1 cells. Insulin-induced clonal expansion, Rb phosphorylation and adipocyte differentiation were all inhibited by the PI 3-kinase inhibitors and rapamycin, but not the MEK inhibitor, whereas the MEK inhibitor, but not PI 3-kinase inhibitors or rapamycin, decreased c-fos induction. We conclude that insulin induces hyperphosphorylation of Rb via PI 3-kinase and mTOR dependent pathway, which promotes clonal expansion and adipocyte differentiation of 3T3-L1 cells.