Role of insulin receptor substrate-1 serine 307 phosphorylation and adiponectin in adipose tissue insulin resistance in late pregnancy

Insulin resistance is a hallmark of late pregnancy both in human and rat. Adipose tissue is one of the tissues that most actively contributes to this reduced insulin sensitivity. The aim of the present study was to characterize the molecular mechanisms of insulin resistance in adipose tissue at late pregnancy. To this end, we analyzed the insulin signaling cascade in lumbar adipose tissue of nonpregnant and pregnant (d 20) rats both under basal and insulin-stimulated conditions. We found that the levels of relevant signaling proteins, such as insulin receptor (IR), IR substrate-1 (IRS-1), phosphatidylinositol 3-kinase, 3-phosphoinositide-dependent kinase-1, ERK1/2, and phosphatase and tensin homolog (PTEN) did not change at late pregnancy. However, insulin-stimulated tyrosine phosphorylation of both IR and IRS-1 were significantly decreased, coincident with decreased IRS-1/p85 association and impaired phosphorylation of AKR mouse thymoma viral protooncogene (Akt) and ERK1/2. This impaired activation of IRS-1 occurred together with an increase of IRS-1 phosphorylation at serine 307 and a decrease in adiponectin levels. To corroborate the role of IRS-1 in adipose tissue insulin resistance during pregnancy, we treated pregnant rats with the antidiabetic drug englitazone. Englitazone improved glucose tolerance, and this pharmacological reversal of insulin resistance was paralleled by an increase of adiponectin levels in adipose tissue as well as by a reduction of IRS-1 serine phosphorylation. Furthermore, the impaired insulin-stimulated tyrosine phosphorylation of IRS-1 in adipose tissue of pregnant animals could be restored ex vivo by treating isolated adipocytes with adiponectin. Together, our findings support a role for adiponectin and serine phosphorylation of IRS-1 in the modulation of insulin resistance in adipose tissue at late pregnancy.

Characterisation of the phagocytic uptake of Aspergillus fumigatus conidia by macrophages

Aspergillus fumigatus is an opportunistic fungal pathogen responsible for severe, life-threatening infections in immunocompromised patients. Airborne conidia are the infectious agent and can reach the lower parts of the respiratory system. In the lung, phagocytes represent the first line of defence. Resident macrophages are able to track down, engulf and kill the invading spores. Phagocytosis of the conidia is therefore a prerequisite for their efficient elimination. Using human and murine macrophages we analysed the phagocytic uptake of A. fumigatus conidia. We found that conidial phagocytosis is an actin-depending process that additionally requires myosin motor, phosphoinositide-3-phosphate kinase and tyrosine kinase activity. Both broad range tyrosine kinase inhibitors and inhibitors that specifically block src kinases had a strong impact on the conidial uptake. Immunofluorescence data demonstrate the recruitment of tyrosine-phosphorylated proteins to the vicinity of engulfed conidia. Uptake of the conidia was accompanied by a strong and local reorganisation of the actin cytoskeleton, whereas no prominent reorganisation was apparent for the microtubules. Both confocal immunofluorescence and electron microscopic data revealed the presence of large ruffle-like structures engaged in the uptake of conidia. This suggests that the internalisation of A. fumigatus spores can be mediated by a process resembling macropinocytosis, which is furthermore supported by the detection of intracellular conidia within spacious vacuoles. Taken together, our data provide new insights into the internalisation of A. fumigatus spores by macrophages, a key process in the early immune defence against an Aspergillus infection.

A discrete period of FGF-induced Erk1/2 signalling is required for vertebrate neural specification

Neural tissue formation is induced by growth factors that activate networks of signal transduction cascades that ultimately lead to the expression of early neural genes, including transcription factors of the SoxB family. Here, we report that fibroblast growth factor (FGF)-induced Erk1/2 (Mapk3 and Mapk1, respectively) mitogen-activated protein kinase (MAPK), but not phosphatidylinositol 3'-OH kinase (PI3K, Pik3r1), signalling is required for neural specification in mouse embryonic stem (ES) cells and in the chick embryo. Further, blocking Erk1/2 inhibits the onset of key SoxB genes in both mouse ES cells (Sox1) and chick embryos (Sox2 and Sox3) and, in both contexts, Erk1/2 signalling is required during only a narrow time window, as neural specification takes place. In the absence of Erk1/2 signalling, differentiation of ES cells stalls following Fgf5 upregulation. Using differentiating ES cells as a model for neural specification, we demonstrate that sustained Erk1/2 activation controls the transition from an Fgf5-positive, primitive ectoderm-like cell state to a neural progenitor cell state without attenuating bone morphogenetic protein (BMP) signalling and we also define the minimum period of Erk1/2 activity required to mediate this key developmental step. Together, these findings identify a conserved, specific and stage-dependent requirement for Erk1/2 signalling downstream of FGF-induced neural specification in higher vertebrates and provide insight into the signalling dynamics governing this process.

MAPK-ERK activation in kidney of male rats chronically fed ochratoxin A at a dose causing a significant incidence of renal carcinoma

Kidney samples of male Fischer 344 (F-344) rats fed a carcinogenic dose of OTA over 7 days, 21 days and 12 months were analysed for various cell signalling proteins known to be potentially involved in chemical carcinogenicity. OTA was found to increase the phosphorylation of atypical-PKC. This was correlated with a selective downstream activation of the MAP-kinase extracellular regulated kinases isoforms 1 and 2 (ERK1/2) and of their substrates ELK1/2 and p90RSK. Moreover, analysis of effectors acting upstream of PKC indicated a possible mobilisation of the insulin-like growth factor-1 receptor (lGFr) and phosphoinositide-dependent kinase-1 (PDK1) system. An increased histone deacetylase (HDAC) enzymatic activity associated with enhanced HDAC3 protein expression was also observed. These findings are potentially relevant with respect to the understanding of OTA nephrocarcinogenicity. HDAC-induced gene silencing has previously been shown to play a role in tumour development. Furthermore, PKC and the MEK-ERK MAP-kinase pathways are known to play important roles in cell proliferation, cell survival, anti-apoptotic activity and renal cancer development.

Reduced Ca2+ entry and suicidal death of erythrocytes in PDK1 hypomorphic mice

The phosphoinositide-dependent kinase PDK1 is a key element in the phosphoinositol-3-kinase signalling pathway, which is involved in the regulation of ion channels, transporters, cell volume and cell survival. Eryptosis, the suicidal death of erythrocytes, is characterized by decrease in cell volume, cell membrane blebbing and phospholipids scrambling with phosphatidylserine exposure at the cell surface. Oxidative stress, osmotic shock or Cl- removal trigger eryptosis by activation of Ca2+-permeable cation channels and subsequent increase in cytosolic Ca2+ activity. To explore the impact of PDK1 for erythrocyte survival, eryptosis was analysed in hypomorphic mice (pdk1hm) expressing only some 25% of PDK1 and in their wild-type littermates (pdk1wt). Cell volume was estimated from forward scatter and phosphatidylserine exposure from annexin-V binding in fluorescence activated cell sorter analysis. Forward scatter was smaller in pdk1hm than in pdk1wt erythrocytes. Oxidative stress (100 microM tert-butylhydroperoxide), osmotic shock (+300 mM sucrose) and Cl- removal (replacement of Cl- with gluconate) all decreased forward scatter and increased the percentage of annexin-V-binding erythrocytes from both pdk1hm and pdk1wt mice. After treatment, the forward scatter was similar in both genotypes, but the percentage of annexin-V binding was significantly smaller in pdk1hm than in pdk1wt erythrocytes. According to Fluo-3 fluorescence, cytosolic Ca2+ activity was significantly smaller in pdk1hm than in pdk1wt erythrocytes. Treatment with Ca2+-ionophore ionomycin (1 microM) was followed by an increase in annexin-V binding to similar levels in pdk1hm and pdk1wt erythrocytes. The experiments reveal that PDK1 deficiency is associated with decreased Ca2+ entry into erythrocytes and thus with blunted eryptotic effects of oxidative stress, osmotic shock and Cl- removal.

Platelet-derived growth factor reorganizes the actin cytoskeleton through 3-phosphoinositide-dependent and 3-phosphoinositide-independent mechanisms in human mesangial cells

BACKGROUND

Platelet-derived growth factor (PDGF) is a potent activator of mesangial cell proliferation and migration. Although phosphoinositide 3-kinase (PI3K) enzymes are important downstream targets of the PDGF receptor, the contribution made by their 3-phosphoinositide products in the reorganization of actin cytoskeleton and focal adhesions has been questioned.

METHODS AND RESULTS

Pharmacological inhibition of the PI3K activity blocks PDGF-induced migration of human primary mesangial cells using an in vitro scrape wound healing assay. Acute (<10 min) inhibition of the PI3K activity did not alter the effect of PDGF on either stress fibre dissolution or reorganization of focal adhesions. However, at later times (>30 min), PDGF-stimulated responses were inhibited. In contrast, PDGF-stimulated membrane ruffling remained insensitive to PI3K inhibitors throughout. Inhibition of protein kinase C and Erk also attenuated PDGF-stimulated mesangial cell migration; however, neither signaling pathway was responsible for the initial effects on filamentous actin and focal adhesions.

CONCLUSIONS

We propose that following PDGF stimulation of mesangial cells, reorganization of the actin cytoskeleton occurs in a biphasic manner. The mechanism responsible for mesangial cell migration that occurs immediately following PDGF stimulation may serve to 'prime' for the subsequent 3-phosphoinositide-, protein-kinase-C-, and Erk-dependent migration.

Chemopreventive and bioenergetic signaling effects of PDK1/Akt pathway inhibition in a transgenic mouse model of prostate cancer

The phosphoinositide-dependent kinase 1 (PDK1)/Akt pathway is an important regulator of multiple biological processes including cell growth, survival, and glucose metabolism. In light of the mechanistic link between Akt signaling and prostate tumorigenesis, we evaluated the chemopreventive relevance of inhibiting this pathway in the transgenic adenocarcinoma of the model prostate (TRAMP) mouse with OSU03012, a celecoxib-derived, but COX-2-inactive, PDK1 inhibitor. Beginning at ten weeks of age when prostatic intraepithelial neoplasia (PIN) lesions are well developed, TRAMP mice received OSU03012 via daily oral gavage for 8 weeks. The drug treatment significantly decreased the weight of all 4 prostate lobes as well as the grade of epithelial proliferation in the dorsal and lateral lobes compared to vehicle-treated control mice. The incidences of carcinoma and metastasis were decreased, although not to statistically significant levels. Treated mice lost body fat and failed to gain weight independent of food intake. This change and periportal hepatocellular atrophy can be linked to sustained PDK1 inhibition through downstream inactivation of glycogen synthase. Centrilobular hepatocellular hypertrophy and necrosis of Type II skeletal myofibers were also compound-related effects. We conclude that targeting of the PDK1/Akt pathway has chemopreventive relevance in prostate cancer and causes other in vivo effects mediated in part by an alteration of bioenergetic signaling.

Rapamycin limits formation of active eukaryotic initiation factor 4F complex following meal feeding in rat hearts

Feeding promotes protein synthesis in cardiac muscle through a stimulation of the messenger RNA translation initiation phase of protein synthesis by enhancing assembly of active eukaryotic initiation factor (eIF)4F complex. The experiments reported herein examined the potential role for a rapamycin-sensitive signaling pathway in increasing formation of active eIF4G-eIF4E complex during meal feeding. Hearts from male Sprague-Dawley rats fed a meal consisting of rat nonpurified diet were sampled prior to and 3 h following the meal in the presence or absence of treatment with rapamycin, an inhibitor of the mammalian target of rapamycin (mTOR) complex 1. Rapamycin prevented the meal feeding-induced stimulation of myocardial protein synthesis. Inhibition of mTOR with rapamycin decreased the association of rapamycin-associated TOR protein with mTOR and prevented the feeding-induced assembly of eIF4G-eIF4E complex. In contrast, the abundance of eIF4E binding protein-1 (4E-BP1)-eIF4E complex was unaffected by either meal feeding or rapamycin. Pretreatment with rapamycin completely prevented the feeding-induced phosphorylation of eIF4G(Ser(1108)), whereas the inhibitor only partially attenuated meal feeding-induced 70-kDa ribosomal protein S6 kinase1(Thr(389)) phosphorylation and extent of 4E-BP1 in the gamma-form. Meal feeding-induced phosphorylation of protein kinase B on either Ser(473) or Thr(308) was unaffected by rapamycin. These findings suggest the extent of phosphorylation of eIF4G following meal feeding occurs by a rapamycin-sensitive mechanism in cardiac muscle. Furthermore, the rapamycin-sensitive reductions in phosphorylation of eIF4G may also lead to decreased formation of active eIF4G-eIF4E complex.

Insulin signaling in mouse oocytes

Continuous exposure of follicles/oocytes to elevated levels of insulin compromises embryonic developmental competence, although the underlying cellular mechanisms are unknown. The objectives of the present study were to determine whether mouse oocytes have insulin receptors and a functional insulin signaling cascade, and whether insulin exposure during oocyte growth or maturation influences meiotic progression and chromatin remodeling. Immunoblot and immunocytochemical analyses of germinal vesicle-intact (GVI) oocytes demonstrated the presence of insulin receptor-beta. Insulin receptor expression in oocytes was increased by gonadotropin stimulation, and remained elevated throughout meiotic maturation. Fully grown GVI oocytes contained 3-phosphoinositide-dependent protein kinase-1 (PDPK1), thymoma viral proto-oncogene 1 (AKT1), and glycogen synthase kinase 3 (GSK3). In vitro maturation of GVI oocytes in 5 microg/ml insulin had no influence on meiotic progression or the incidence of normal metaphase II (MII) chromosome condensation. Treatment of oocytes during maturation had no effect on GSK3A/B protein expression or phosphorylation of S21/9. However, the culturing of preantral follicles for 10 days with 5 microg/ml insulin increased the phosphorylation of oocyte GSK3B, indicating GSK3 inactivation. The rates of development to metaphase I (MI) were similar for oocytes obtained from insulin-treated follicles and controls, whereas the incidence of abnormal MI chromatin condensation was significantly higher in oocytes obtained from follicles cultured with insulin compared to those cultured without insulin. These results demonstrate that oocytes contain a functional insulin signaling pathway, and that insulin exposure during oocyte growth results in chromatin remodeling aberrations. These findings begin to elucidate the mechanisms by which chronic elevated insulin influences oocyte meiosis, chromatin remodeling, and embryonic developmental competence.

Activation loop phosphorylation-independent kinase activity of human protein kinase C zeta

Atypical protein kinase C zeta (PKCzeta) plays an important role in cell proliferation and survival. PKCzeta and its truncated form containing only the kinase domain, CATzeta, have been reported to be activated by the phosphorylation of threonine 410 in the activation loop. We expressed both the full length PKCzeta and CATzeta in a baculovirus/insect cell over-expression system and purified the proteins for biochemical characterization. Ion exchange chromatography of CATzeta revealed three species with different levels of phosphorylation at Thr-410 and allowed the isolation of the CATzeta protein devoid of phosphorylation at Thr-410. All three species of CATzeta were active and their activity was not correlated with phosphorylation at Thr-410, indicating that the kinase activity of CATzeta did not depend solely on activation loop phosphorylation. Tyrosine phosphorylation was detected in all three species of CATzeta and the full length PKCzeta. Homology structural modeling of PKCzeta revealed a conserved, predicted-to-be phosphorylated tyrosine residue, Tyr-428, in the close proximity of the RD motif of the catalytic loop and of Thr-410 in the activation loop. The structural analysis indicated that phospho-Tyr-428 would interact with two key, positively-charged residues to form a triad conformation similar to that formed by phospho-Thr-410. Based on these observations, it is possible that the Thr-410 phosphorylation-independent kinase activity of CATzeta is regulated by the phosphorylation of Tyr-428. This alternative mode of PKCzeta activation is supported by the observed stimulation of PKCzeta kinase activity upon phosphorylation at the equivalent site by Abl, and may be involved in resistance to drug-induced apoptosis.

Conventional protein kinase C isoenzymes undergo dephosphorylation in neutrophil-like HL-60 cells treated by chelerythrine or sanguinarine

The quaternary benzo[c]phenanthridine alkaloid chelerythrine is widely used as an inhibitor of protein kinase C (PKC). However, in biological systems chelerythrine interacts with an array of proteins. In this study, we examined the effects of chelerythrine and sanguinarine on conventional PKCs (cPKCs) and PKC upstream kinase, phosphoinositide-dependent protein kinase 1 (PDK1), under complete inhibition conditions of PKC-dependent oxidative burst. In neutrophil-like HL-60 cells, sanguinarine and chelerythrine inhibited N-formyl-Met-Leu-Phe, phorbol 12-myristate 13-acetate (PMA)-, and A23187-induced oxidative burst with IC(50) values not exceeding 4.6 micromol/L, but the inhibition of PMA-stimulated cPKC activity in intact cells required at least fivefold higher alkaloid concentrations. At concentrations below 10 micromol/L, sanguinarine and chelerythrine prevented phosphorylation of approximately 80 kDa protein and sequestered approximately 60 kDa phosphoprotein in cytosol. Moreover, neither sanguinarine nor chelerythrine impaired PMA-stimulated translocation of autophosphorylated PKCalpha/betaII isoenzymes, but both alkaloids induced dephosphorylation of the turn motif in PKCalpha/betaII. The dephosphorylation did not occur in unstimulated cells and it was not accompanied by PKC degradation. Furthermore, cell treatment with sanguinarine or chelerythrine resulted in phosphorylation of approximately 70 kDa protein by PDK1. We conclude that PKC-dependent cellular events are affected by chelerythrine primarily by multiple protein interactions rather than by inhibition of PKC activity.

Phosphoinositide-dependent protein kinase-1 (PDK1)-independent activation of the protein kinase C substrate, protein kinase D

Phosphoinoisitide dependent kinase l (PDK1) is proposed to phosphorylate a key threonine residue within the catalytic domain of the protein kinase C (PKC) superfamily that controls the stability and catalytic competence of these kinases. Hence, in PDK1-null embryonic stem cells intracellular levels of PKCα, PKCβ1, PKCγ, and PKCε are strikingly reduced. Although PDK1-null cells have reduced endogenous PKC levels they are not completely devoid of PKCs and the integrity of downstream PKC effector pathways in the absence of PDK1 has not been determined. In the present report, the PDK1 requirement for controlling the phosphorylation and activity of a well characterised substrate for PKCs, the serine kinase protein kinase D, has been examined. The data show that in embryonic stem cells and thymocytes loss of PDK1 does not prevent PKC-mediated phosphorylation and activation of protein kinase D. These results reveal that loss of PDK1 does not functionally inactivate all PKC-mediated signal transduction.

ASCT2 silencing regulates mammalian target-of-rapamycin growth and survival signaling in human hepatoma cells

System ASC amino acid transporter-2 (ASCT2) was previously demonstrated to be essential for human hepatoma cell growth and survival, as its silencing via inducible antisense RNA expression results in complete apoptosis within 48 h by a mechanism that transcends its role in amino acid delivery. To gain mechanistic insights into the reliance of cancerous liver cells on ASCT2, the aim of this study was to determine the early consequences of its silencing on the growth and survival signaling that presage apoptosis. Induced antisense ASCT2 RNA in SK-Hep1 cells led to >90% suppression of ASCT2 mRNA by 6 h and inhibition of mammalian target-of-rapamycin (mTOR)/raptor (mTOR complex-1; mTORC1) signaling by 8 h, as manifested by diminished p70 ribosomal protein S6 kinase-1 and eukaryotic initiation factor-4E (eIF4E) binding protein-1 phosphorylation, while protein synthesis rates declined by nearly 50% despite no measurable decreases in the cap binding protein eIF4G or cellular ribosomal protein content. Depressed mTORC1 signaling occurred before detectable reduction in ASCT2 activity but coincided with a 30% decline in total cellular ASCT2 protein. By 12 h after ASCT2 silencing, further decrements were observed in protein synthesis rates and ASCT2 protein and activity, each by ∼50%, while signaling from mTOR/rictor (mTOR complex-2; mTORC2) was stimulated as indexed by enhanced phosphorylation of the Akt/PKB kinase on serine-473 and of its proapoptotic substrate Bad on serine-136. These results suggest that ASCT2 silencing inhibits mTORC1 signaling to the translational machinery followed by an mTORC2-initiated survival response, establishing a link between amino acid transporter expression and mTOR function.

Indolinone based phosphoinositide-dependent kinase-1 (PDK1) inhibitors. Part 2: Optimization of BX-517

Based on the lead compound BX-517, a series of C-4' substituted indolinones have been synthesized and evaluated for PDK1 inhibition. Modification at C-4' of the pyrrole afforded potent compounds (7b and 7d) with improved solubility and ADME properties. In this letter, we describe the synthesis, selectivity profile, and pharmacokinetic data of selected compounds.

Indolinone based phosphoinositide-dependent kinase-1 (PDK1) inhibitors. Part 1: Design, synthesis and biological activity

HTS screening identified 1 with micromolar inhibitory activity against PDK1. Optimization of 1 afforded 4i (BX-517) which has single-digit nanomolar activity against PDK1 and excellent selectivity against PKA.

Notch-induced T cell development requires phosphoinositide-dependent kinase 1

Phosphoinositide-dependent kinase l (PDK1) phosphorylates and activates multiple AGC serine kinases, including protein kinase B (PKB), p70Ribosomal S6 kinase (S6K) and p90Ribosomal S6 kinase (RSK). PDK1 is required for thymocyte differentiation and proliferation, and herein, we explore the molecular basis for these essential functions of PDK1 in T lymphocyte development. A key finding is that PDK1 is required for the expression of key nutrient receptors in T cell progenitors: CD71 the transferrin receptor and CD98 a subunit of L-amino acid transporters. PDK1 is also essential for Notch-mediated trophic and proliferative responses in thymocytes. A PDK1 mutant PDK1 L155E, which supports activation of PKB but no other AGC kinases, can restore CD71 and CD98 expression in pre-T cells and restore thymocyte differentiation. However, PDK1 L155E is insufficient for thymocyte proliferation. The role of PDK1 in thymus development thus extends beyond its ability to regulate PKB. In addition, PDK1 phosphorylation of AGC kinases such as S6K and RSK is also necessary for thymocyte development.

Forkhead protein FKHR and its phosphorylated form p-FKHR in human prostate cancer

In vitro studies suggest that the proapoptotic function of forkhead protein FKHR is probably inactivated by means of phosphorylation through the protein kinase B pathway. However, the clinical significance of FKHR in prostate cancer remains unclear. Six hundred forty radical prostatectomies were used for building tissue microarrays. Slides were stained with antibodies against FKHR and phosphorylated FKHR (p-FKHR). Correlations with clinicopathologic parameters were analyzed by Spearman rank test. Cox regression test and Kaplan-Meier test were used to determine the probability of disease recurrence, which is defined as a serum prostate-specific antigen (PSA) level greater than 0.4 ng/mL after radical prostatectomy. Nuclear FKHR level was higher in normal prostate than in benign prostatic hyperplasia and prostate cancer (P = .0000). Nuclear expression of FKHR was correlated with preoperative PSA level (rho = 0.108, P = .029), extracapsular extension (rho = 0.137, P = .005), and seminal vesicle invasion (rho = 0.101, P = .039). FKHR expression was not a significant indicator of biochemical failure by either univariate or multivariate analysis. Nuclear p-FKHR expression correlated with patients' age (rho = 0.179, P = .0003), Gleason score (rho = 0.130, P = .0083), extracapsular extension (rho = 0.227, P = .0000), clinical stage (Union Internationale Contre le Cancer system) (rho = 0.166, P = .0007), and lymph node status (rho = 0.101, P = .0401). Cytoplasmic p-FKHR correlated with patients' age (rho = 0.146, P = .0030) and clinical stage (rho = 0.117, P = .0180). Cytoplasmic p-FKHR was a significant indicator of biochemical recurrence (P = .0164; hazard ratio, 1.114-2.929). Nuclear p-FKHR strongly correlated with phosphorylated protein kinase B (rho = 0.368, P = .0000), androgen receptor (rho = 0.385, P = .0000), and Skp-2 (rho = 0.170, P = .0036). Our data suggest that the proapoptotic role of FKHR is probably regulated by several signaling pathways in prostate cancer.

Metabolic targeting of hypoxia and HIF1 in solid tumors can enhance cytotoxic chemotherapy

Solid tumors frequently contain large regions with low oxygen concentrations (hypoxia). The hypoxic microenvironment induces adaptive changes to tumor cell metabolism, and this alteration can further distort the local microenvironment. The net result of these tumor-specific changes is a microenvironment that inhibits many standard cytotoxic anticancer therapies and predicts for a poor clinical outcome. Pharmacologic targeting of the unique metabolism of solid tumors could alter the tumor microenvironment to provide more favorable conditions for anti-tumor therapy. Here, we describe a strategy in which the mitochondrial metabolism of tumor cells is increased by pharmacologic inhibition of hypoxia-inducible factor 1 (HIF1) or its target gene pyruvate dehydrogenase kinase 1 (PDK1). This acute increase in oxygen consumption leads to a corresponding decrease in tumor oxygenation. Whereas decreased oxygenation could reduce the effectiveness of some traditional therapies, we show that it dramatically increases the effectiveness of a hypoxia-specific cytotoxin. This treatment strategy should provide a high degree of tumor specificity for increasing the effectiveness of hypoxic cytotoxins, as it depends on the activation of HIF1 and the presence of hypoxia, conditions that are present only in the tumor, and not the normal tissue.

Reconstitution of modular PDK1 functions on trans-splicing of the regulatory PH and catalytic kinase domains

The serine-threonine protein kinases PDK1 and PKB each contain a pleckstrin homology (PH) domain that binds the membrane-bound phosphatidylinositol 3,4,5-triphosphate [PI(3,4,5)P3] second messenger and is required for PDK1-catalyzed phosphorylation and activation of PKB. While X-ray structures have been reported for the individual regulatory PH and catalytic kinase domain constructs of both PDK1 and PKB, diffraction quality crystals of full length constructs have yet to be obtained, likely due to conformational heterogeneity. In developing alternative approaches to understanding the potential role of conformational dynamics in regulating PKB phosphorylation by PDK1, an efficient in vitro method for protein trans-splicing was developed, which utilizes the N- and C-terminal split inteins of the gene dnaE from Nostoc punctiforme [(N)NpuDnaE] and Synechocystis sp. strain PCC6803 [(C)SspDnaE], respectively. For conjugating the regulatory PH domain to the catalytic kinase domain of PDK1, the recombinant trans-splicing fusion constructs KINASE(AEY)-(N)NpuDnaE-His6 and GST-His6-(C)SspDnaE-(CMN)PH were designed, PCR assembled, overexpressed, and affinity purified. The cross-reacting (N)NpuDnaE and (C)SspDnaE inteins generated full length spliced-PDK1 with kobs = (2.8 +/- 0.3) x 10(-5) s(-1) and with < or =5% of any competing trans-cleavage reactions. Spliced-PDK1 was efficiently purified to > or =95% homogeneity from the reaction mixture by subsequent His6 affinity and ion exchange chromatography steps. In vitro kinase assays and phosphopeptide mapping studies confirmed that spliced-PDK1 retained the ability to colocalize and selectively phosphorylate Thr-309 of PKBbeta in a PI(3,4,5)P3-dependent manner. The high-level production and reconstitution of functional spliced-PDK1 establishes the feasibility of incorporating domain-specific biophysical probes for spectroscopic studies of regulatory PH domain mediated catalytic specificity.

Molecular mechanisms of the diabetogenic effects of arsenic: inhibition of insulin signaling by arsenite and methylarsonous acid

BACKGROUND

Increased prevalences of diabetes mellitus have been reported among individuals chronically exposed to inorganic arsenic (iAs). However, the mechanisms underlying the diabetogenic effects of iAs have not been characterized. We have previously shown that trivalent metabolites of iAs, arsenite (iAs(III)) and methylarsonous acid (MAs(III)) inhibit insulin-stimulated glucose uptake (ISGU) in 3T3-L1 adipocytes by suppressing the insulin-dependent phosphorylation of protein kinase B (PKB/Akt).

OBJECTIVES

Our goal was to identify the molecular mechanisms responsible for the suppression of PKB/Akt phosphorylation by iAs(III) and MAs(III).

METHODS

The effects of iAs(III) and MAs(III) on components of the insulin-activated signal transduction pathway that regulate PKB/Akt phosphorylation were examined in 3T3-L1 adipocytes.

RESULTS

Subtoxic concentrations of iAs(III) or MAs(III) had little or no effect on the activity of phosphatidylinositol 3-kinase (PI-3K), which synthesizes phosphatidylinositol-3,4,5-triphosphate (PIP(3)), or on phosphorylation of PTEN (phosphatase and tensin homolog deleted on chromosome ten), a PIP(3) phosphatase. Neither iAs(III) nor MAs(III) interfered with the phosphorylation of 3-phosphoinositide-dependent kinase-1 (PDK-1) located downstream from PI-3K. However, PDK-1 activity was inhibited by both iAs(III) and MAs(III). Consistent with these findings, PDK-1-catalyzed phosphorylation of PKB/Akt(Thr308) and PKB/Akt activity were suppressed in exposed cells. In addition, PKB/Akt(Ser473) phosphorylation, which is catalyzed by a putative PDK-2, was also suppressed. Notably, expression of constitutively active PKB/Akt restored the normal ISGU pattern in adipocytes treated with either iAs(III) or MAs(III).

CONCLUSIONS

These results suggest that inhibition of the PDK-1/PKB/Akt-mediated transduction step is the key mechanism for the inhibition of ISGU in adipocytes exposed to iAs(III) or MAs(III), and possibly for impaired glucose tolerance associated with human exposures to iAs.

Phosphoinositide 3-kinase signalling regulates early development and developmental haemopoiesis

Phosphoinositide 3-kinase (PI3K)-dependent signalling regulates a wide variety of cellular functions including proliferation and differentiation. Disruption of class I(A) PI3K isoforms has implicated PI3K-mediated signalling in development of the early embryo and lymphohaemopoietic system. We have used embryonic stem (ES) cells as an in vitro model to study the involvement of PI3K-dependent signalling during early development and haemopoiesis. Both pharmacological inhibition and genetic manipulation of PI3K-dependent signalling demonstrate that PI3K-mediated signals, most likely via 3-phosphoinositide-dependent protein kinase 1 (PDK1), are required for proliferation of cells within developing embryoid bodies (EBs). Surprisingly, the haemopoietic potential of EB-derived cells was not blocked upon PI3K inhibition but rather enhanced, correlating with modest increases in expression of haemopoietic marker genes. By contrast, PDK1-deficient EB-derived progeny failed to generate terminally differentiated haemopoietic lineages. This deficiency appeared to be due to a requirement for PI3K signalling during the proliferative phase of blast-colony-forming cell (BL-CFC) expansion, rather than as a result of effects on differentiation per se. We also demonstrate that PI3K-dependent signalling is required for optimal generation of erythroid and myeloid progenitors and their differentiation into mature haemopoietic colony types. These data demonstrate that PI3K-dependent signals play important roles at different stages of haemopoietic development.

CD4 ligation induces activation of protein kinase C zeta and phosphoinositide-dependent-protein kinase-1, two kinases required for down-regulation of LFA-1-mediated adhesion

We previously showed that CD4 binding induced a down-regulation of LFA-1-dependent-antigen-independent adhesion of T and B lymphocytes in a phosphatidylinositol-3-kinase (PI3K)-dependent manner. We now show in A201-CD4 (+) T cell lines, that anti-CD4 Ab increases activation of phosphoinositide-dependent-protein-kinase 1 (PDK1) or PKC zeta, two main effectors down-stream from PI3K. CD4 binding also increases interactions between PI3K and activated PKCzeta and PDK1. Both events are dependent on CD4/p56Lck association, since they are not detected when p56Lck is unable to bind a truncated form of CD4 in transfected T cell lines. We also show using antisense oligonucleotides that both kinases are necessary for down-regulating LFA-1-dependent adhesion induced by CD4 signalling. We also suggest a role of PDK1 in the recruitment of the phosphatase SHP-2 in a multiprotein complex induced by anti-CD4 Ab. This study thus provides further insights into the mechanism underlying the CD4 triggered regulation of LFA-1-mediated adhesion.

Restoration of glucokinase expression in the liver normalizes postprandial glucose disposal in mice with hepatic deficiency of PDK1

Phosphoinositide-dependent kinase-1 (PDK1) is implicated in the metabolic effects of insulin as a key mediator of phosphoinositide 3-kinase-dependent signaling. Here we show that mice with liver-specific PDK1 deficiency manifest various defects in the metabolic actions of insulin in the liver as well as a type 2 diabetes-like phenotype characterized by marked hyperinsulinemia and postprandial hyperglycemia. The hepatic abundance of glucokinase, an important determinant of glucose flux and glucose-evoked signaling in hepatocytes, was substantially reduced in these mice. Restoration of hepatic glucokinase expression, with the use of an adenoviral vector, induced insulin-like effects in the liver and almost completely normalized the fasting hyperinsulinemia and postprandial hyperglycemia in these animals. These results indicate that, if the hepatic abundance of glucokinase is maintained, ingested glucose is normally disposed of even in the absence of acute activation of proximal insulin signaling, such as the activation of Akt, in the liver.

Metabolic modulation induced by chronic hypoxia in rats using a comparative proteomic analysis of skeletal muscle tissue

Hypoxia-induced changes of rat skeletal muscle were investigated by two-dimensional difference in-gel electrophoresis (2D-DIGE) and mass spectrometry. The results indicated that proteins involved in the TCA cycle, ATP production, and electron transport are down-regulated, whereas glycolytic enzymes and deaminases involved in ATP and AMP production were up-regulated. Up-regulation of the hypoxia markers hypoxia inducible factor 1 (HIF-1alpha) and pyruvate dehydrogenase kinase 1 (PDK1) was also observed, suggesting that in vivo adaptation to hypoxia requires an active metabolic switch. The kinase protein, mammalian target of rapamycin (mTOR), which has been implicated in the regulation of protein synthesis in hypoxia, appears unchanged, suggesting that its activity, in this system, is not controlled by oxygen partial pressure.

Essential role of PDK1 in regulating endothelial cell migration

The serine/threonine protein kinase phosphoinositide-dependent kinase 1 (PDK1) plays a central role in cellular signaling by phosphorylating members of the AGC family of kinases, including PKB/Akt. We now present evidence showing that PDK1 is essential for the motility of vascular endothelial cells (ECs) and that it is involved in the regulation of their chemotaxis. ECs differentiated from mouse embryonic stem cells lacking PDK1 completely lost their ability to migrate in vitro in response to vascular endothelial growth factor-A (VEGF-A). In addition, PDK1^−/− embryoid bodies exhibit evident developmental and vascular defects that can be attributed to a reduced cell migration. Moreover, the overexpression of PDK1 increased the EC migration induced by VEGF-A. We propose a model of spatial distribution of PDK1 and Akt in which the synthesis of phosphatidylinositol 3,4,5 triphosphate at plasma membrane by activation of phosphoinositide 3-kinase recruits both proteins at the leading edge of the polarized ECs and promotes cell chemotaxis. These findings establish a mechanism for the spatial localization of PDK1 and its substrate Akt to regulate directional migration.