Role of the PH domain in regulating in vitro autophosphorylation events required for reconstitution of PDK1 catalytic activity

In addition to its catalytic domain, phosphoinsositide-dependent protein kinase-1 (PDK1) contains a C-terminal pleckstrin homology (PH) domain, which binds the membrane-bound phosphatidylinositol (3,4,5)-triphosphate [PI(3,4,5)P3] second messenger. Here, we report in vitro kinetic, phosphopeptide mapping, and oligomerization studies that address the role of the PH domain in regulating specific autophosphorylation events, which are required for PDK1 catalytic activation. First, 'inactive' unphosphorylated forms of N-terminal His6 tagged full length (His6-PDK1) and catalytic domain constructs [His6-PDK1(Delta PH)] were generated by treatment with Lambda protein phosphatase (lambda PP). Reconstitution of lambda PP-treated His6-PDK1(Delta PH) catalytic activity required activation loop Ser-241 phosphorylation, which occurred only upon trans-addition of 'active' PDK1 with an apparent bimolecular rate constant of (app)k1(S241) = 374+/-29 M(-1) s(-1). In contrast, full length lambda PP-treated His6-PDK1 catalyzed Ser-241 cis-autophosphorylation with an apparent first-order rate constant of (app)k1(S241) = (5.0+/-1.5) x 10(-4) s(-1) but remained 'inactive'. Reconstitution of lambda PP-treated His(6)-PDK1 catalytic activity occurred only when autophosphorylated in the presence of PI(3,4,5)P3 containing vesicles. PI(3,4,5)P3 binding to the PH domain activated apparent first-order Ser-241 autophosphorylation by 20-fold [(app)k1(S241) = (1.1+/-0.1) x 10(-2) s(-1)] and also promoted biphasic Thr-513 trans-autophosphorylation [(app)k2(T513) = (4.9+/-1.1) x 10(2) M(-1) s(-1) and(app)k3(T513) = (1.5+/-0.2) x 10(3) M(-1) s(-1)]. The results of mutagenesis studies suggest that Thr-513 phosphorylation may cause dissociation of autoinhibitory contacts formed between the contiguous regulatory PH and catalytic kinase domains.

The mTOR/PI3K and MAPK pathways converge on eIF4B to control its phosphorylation and activity

The eukaryotic translation initiation factor 4B (eIF4B) plays a critical role in recruiting the 40S ribosomal subunit to the mRNA. In response to insulin, eIF4B is phosphorylated on Ser422 by S6K in a rapamycin-sensitive manner. Here we demonstrate that the p90 ribosomal protein S6 kinase (RSK) phosphorylates eIF4B on the same residue. The relative contribution of the RSK and S6K modules to the phosphorylation of eIF4B is growth factor-dependent, and the two phosphorylation events exhibit very different kinetics. The S6K and RSK proteins are members of the AGC protein kinase family, and require PDK1 phosphorylation for activation. Consistent with this requirement, phosphorylation of eIF4B Ser422 is abrogated in PDK1 null embryonic stem cells. Phosphorylation of eIF4B on Ser422 by RSK and S6K is physiologically significant, as it increases the interaction of eIF4B with the eukaryotic translation initiation factor 3.

Fine tuning PDK1 activity by phosphorylation at Ser163

3-Phosphoinositide-dependent protein kinase-1 (PDK1) mediates phosphorylation and activation of members of the AGC protein kinase family and plays an essential role in insulin signaling and action. However, whether and how PDK1 activity is regulated in cells remains largely uncharacterized. In the present study, we show that PDK1 undergoes insulin-stimulated and phosphatidylinositol 3-kinase-dependent phosphorylation at Ser244 in the activation loop and at a novel site: Ser163 in the hinge region between the two lobes of the kinase domain. Sequence alignment studies revealed that the residue corresponding to Ser163 of PDK1 in all other AGC kinases is glutamate, suggesting that a negative charge at this site may be important for PDK1 function. Replacing Ser163 with a negatively charged residue, glutamate, led to a 2-fold increase in PDK1 activity. Molecular modeling studies suggested that phosphorylated Ser163 may form additional hydrogen bonds with Tyr149 and Gln223. In support of this, mutation of Tyr149 to Ala is sufficient to reduce PDK1 activity. Taken together, our results suggest that PDK1 phosphorylation of Ser163 may provide a mechanism to fine-tune PDK1 activity and function in cells.

Interfering with cell-survival signalling as a treatment strategy for prostate cancer

Authors from Australia describe how interfering with cell survival is increasingly being chosen as a method of developing a treatment strategy for hormone-resistant prostate cancer. The authors show how several developmental drug candidates have preclinical and clinical activity against cell survival proteins, and that these might be worth considering as possible clinical entities in this condition. The commonly stated argument that laparoscopy has advanced the cause of donor nephrectomy in renal transplantation is examined by authors from the UK. They present a review of the published reports and show that the evidence base is poor for drawing a conclusion as to whether laparoscopic or open nephrectomy is best.

Impaired intestinal and renal glucose transport in PDK-1 hypomorphic mice

The phosphoinositide-dependent kinase-1 (PDK-1) activates the serum- and glucocorticoid-inducible kinase and protein kinase B isoforms, which, in turn, are known to stimulate the renal and intestinal Na+-dependent glucose transporter 1. The present study has been performed to explore the role of PDK-1 in electrogenic glucose transport in small intestine and proximal renal tubules. To this end, mice expressing approximately 20% of PDK-1 (pdk1hm) were compared with their wild-type littermates (pdk1wt). According to Ussing chamber experiments, electrogenic glucose transport was significantly smaller in the jejunum of pdk1hm than of pdk1wt mice. Similarly, proximal tubular electrogenic glucose transport in isolated, perfused renal tubule segments was decreased in pdk1hm compared with pdk1wt mice. Intraperitoneal injection of 3 g/kg body wt glucose resulted in a similar increase of plasma glucose concentration in pdk1hm and in pdk1wt mice but led to a higher increase of urinary glucose excretion in pdk1hm mice. In conclusion, reduction of functional PDK-1 leads to impairment of electrogenic intestinal glucose absorption and renal glucose reabsorption. The experiments disclose a novel element of glucose transport regulation in kidney and small intestine.

Steady-state kinetic mechanism of PDK1

PDK1 catalyzes phosphorylation of Thr in the conserved activation loop region of a number of its downstream AGC kinase family members. In addition to the consensus sequence at the site of phosphorylation, a number of PDK1 substrates contain a PIF sequence (PDK1-interacting fragment), which binds and activates the kinase domain of PDK1 (PDK1(deltaPH)). To gain further insight to PIF-dependent catalysis, steady-state kinetic and inhibition studies were performed for His6-PDK1(deltaPH)-catalyzed phosphorylation of PDK1-Tide (Tide), which contains an extended "PIF" sequence C-terminal to the consensus sequence for PDK1 phosphorylation. In two-substrate kinetics, a large degree of negative binding synergism was observed to occur on formation of the active ternary complex (alphaKd(ATP) = 40 microM and alphaKd(Tide) = 80 microM) from individual transitory binary complexes (Kd(ATP) = 0.6 microM and Kd(Tide) = 1 microM). On varying ATP concentrations, the ADP product and the (T/E)-PDK1-Tide product analog (p'Tide) behaved as competitive and noncompetitive inhibitors, respectively; on varying Tide concentrations, ADP and p'Tide behaved as noncompetitive and competitive inhibitors, respectively. Also, negative binding synergism was associated with formation of dead-end inhibited ternary complexes. Time progress curves in pre-steady-state studies under "saturating" or kcat conditions showed (i) no burst or lag phenomena, (ii) no change in reaction velocity when adenosine 5'-O-(thiotriphosphate) was used as a phosphate donor, and (iii) no change in reaction velocity on increasing relative microviscosity (0 < or = eta/eta0 < or = 3). Taken together, PDK1-catalyzed trans-phosphorylation of PDK1-Tide approximates a Rapid Equilibrium Random Bi Bi system, where motions in the central ternary complex are largely rate-determining.

Signal transduction mechanism involved in Clostridium perfringens alpha-toxin-induced superoxide anion generation in rabbit neutrophils

Clostridium perfringens alpha-toxin induces the generation of superoxide anion (O2(-)) via production of 1,2-diacylglycerol (DG) in rabbit neutrophils. The mechanism of the generation, however, remains poorly understood. Here we report a novel mechanism for the toxin-induced production of O2(-) in rabbit neutrophils. Treatment of the cells with the toxin resulted in tyrosine phosphorylation of a protein of about 140 kDa. The protein reacted with anti-TrkA (nerve growth factor high-affinity receptor) antibody and bound nerve growth factor. Anti-TrkA antibody inhibited the production of O2(-) and binding of the toxin to the protein. The toxin induced phosphorylation of 3-phosphoinositide-dependent protein kinase 1 (PDK1). K252a, an inhibitor of TrkA receptor, and LY294002, an inhibitor of phosphatidylinositol 3-kinase (PI3K), reduced the toxin-induced production of O2(-) and phosphorylation of PDK1, but not the formation of DG. These inhibitors inhibited the toxin-induced phosphorylation of protein kinase C theta (PKCtheta). U73122, a phospholipase C (PLC) inhibitor, and pertussis toxin inhibited the toxin-induced generation of O2(-) and formation of DG, but not the phosphorylation of PDK1. These observations show that the toxin independently induces production of DG through activation of endogenous PLC and phosphorylation of PDK1 via the TrkA receptor signaling pathway and that these events synergistically activate PKCtheta in stimulating an increase in O2(-). In addition, we show the participation of mitogen-activated protein kinase-associated signaling events via activation of PKCtheta in the toxin-induced generation of O2(-).

Upstream signaling of protein kinase C-epsilon in xenon-induced pharmacological preconditioning. Implication of mitochondrial adenosine triphosphate dependent potassium channels and phosphatidylinositol-dependent kinase-1

Xenon elicits preconditioning of the myocardium via protein kinase C-epsilon. We determined the implication of (1) the mitochondrial adenosinetriphosphate dependent potassium (K(ATP)) channels and (2) the 3'phosphatidylinositol-dependent kinase-1 (PDK-1) in activating protein kinase C-epsilon. For infarct size measurements, anaesthetized rats were subjected to 25 min of coronary artery occlusion followed by 120 min of reperfusion. Rats received xenon 70% during three 5-min periods before ischaemia with or without the K(ATP) channel blocker 5-hydroxydecanoate or Wortmannin as PI3K/PDK-1 inhibitor. For Western blot, hearts were excised at five time points after xenon preconditioning (Control, 15, 25, 35, 45 min). Infarct size was reduced from 42+/-6% (mean+/-S.D.) to 27+/-8% after xenon preconditioning (P<0.05). Western blot revealed an increased activation of PKC-epsilon after 45 min and of PDK-1 after 25 min during xenon preconditioning. 5-hydroxydecanoate and Wortmannin blocked both effects. PKC-epsilon is activated downstream of mitochondrial K(ATP) channels and PDK-1. Both pathways are functionally involved in xenon preconditioning.

Altered caveolin-3 expression disrupts PI(3) kinase signaling leading to death of cultured muscle cells

Caveolae and their coat proteins, caveolins, co-ordinate multiple signaling pathways. Caveolin-3 is a muscle-specific caveolin isoform that is deficient in limb girdle muscular dystrophy type 1 C (LGMD1C). Paradoxically, overexpression of this protein also causes muscle degeneration in vivo. We hypothesize that altered membrane expression of caveolin-3 in muscle cells causes a degenerative phenotype by disrupting the co-ordination of signaling pathways that are critical to the maintenance of cell survival. Here, we show for the first time that, in normal muscle cells subjected to oxidative stress, the phosphatidylinositol (3) kinase (PI(3) kinase)-associated proteins PDK1 and Akt associate with caveolae where they bind to caveolin-3, and that normal activation of this pathway promotes cell survival. Either increased or decreased expression of caveolin-3 at the membrane caused an increased susceptibility to oxidative stress, and myotube survival was markedly improved by PI(3) kinase inhibition. This occurred concomitantly with altered phosphorylation of the pro-apoptotic proteins GSK3beta and Bad, despite normal levels of Akt activation. Taken together, our results demonstrate that altered caveolin-3 expression can change the outcome of PI(3) kinase activation from cell survival to cell death. These findings indicate that normal expression and localization of caveolin-3 are required to appropriately co-ordinate PI(3) kinase/Akt-mediated cell survival signaling, and suggest that this pathway may be an effective therapeutic target for the treatment of muscular dystrophies associated with caveolin-3 mutations.

Simultaneous inhibition of PDK1/AKT and Fms-like tyrosine kinase 3 signaling by a small-molecule KP372-1 induces mitochondrial dysfunction and apoptosis in acute myelogenous leukemia

Phosphoinositol-3-kinase (PI3K)/protein kinase B (AKT) and Fms-like tyrosine kinase 3 (FLT3) signaling are aberrantly activated in acute myelogenous leukemia (AML) cells. Constitutively activated AKT and FLT3 regulate leukemia cell survival and resistance to chemotherapy. In this study, we investigated the effects of the novel multiple kinase inhibitor KP372-1 on the survival of AML cell lines and primary AML samples. KP372-1 directly inhibited the kinase activity of AKT, PDK1, and FLT3 in a concentration-dependent manner. Western blot analysis indicated that KP372-1 decreased the phosphorylation of AKT on both Ser(473) and Thr(308); abrogated the phosphorylation of p70S6 kinase, BAD, and Foxo3a via PI3K/AKT signaling; and down-regulated expression of PIM-1 through direct inhibition of FLT3. Treatment of AML cell lines with KP372-1 resulted in rapid generation of reactive oxygen species and stimulation of oxygen consumption, followed by mitochondrial depolarization, caspase activation, and phosphatidylserine externalization. KP372-1 induced pronounced apoptosis in AML cell lines and primary samples irrespective of their FLT3 status, but not in normal CD34(+) cells. Moreover, KP372-1 markedly decreased the colony-forming ability of primary AML samples (IC(50) < 200 nmol/L) with minimal cytotoxic effects on normal progenitor cells. Taken together, our results show that the simultaneous inhibition of critical prosurvival kinases by KP372-1 leads to mitochondrial dysfunction and apoptosis of AML but not normal hematopoietic progenitor cells.

Targeting survivin via PI3K but not c-akt/PKB by anticancer drugs in immature neutrophils

Myelosuppression is the most common unwanted side effect associated with the administration of anticancer drugs, and infections remain a common cause of death in chemotherapy-treated patients. Several mechanisms of the cytotoxicity of these drugs have been proposed and may synergistically operate in a given cell. Survivin expression has been associated with cancer, but recent reports suggest that this molecule is also expressed in several immature and mature hematopoietic cells. Here, we provide evidence that treatment of immature neutrophils with anticancer drugs reduced endogenous survivin levels causing apoptosis. The anticancer drugs did not directly target survivin, instead they blocked the activity of phosphatidylinositol-3-OH kinase, which regulated survivin expression and apoptosis in these cells. Strikingly, and in contrast to other cells, this pathway did not involve the serine/threonine kinase c-akt/PKB. Moreover, in combination with anticancer drug therapy, rapamycin did not induce increased myelosuppression in an experimental lymphoma mouse model. These data suggest that drugs that block either c-akt/PKB or signaling molecules located distal to c-akt/PKB may preferentially induce apoptosis of cancer cells as they exhibit no cytotoxicity for immature neutrophils.

Protein kinase Calpha regulates insulin receptor signaling in skeletal muscle

Certain PKC isoforms are stimulated by insulin and interact with IR as well as with IRS, but it is still not clear if specific PKC isoforms regulate IR signaling directly or through IRS-1. PKCalpha may regulate IRS activity in response to insulin. We investigated the possibility that PKCalpha may be important in insulin signaling. Studies were conducted on skeletal muscle in adult mice and on L6 skeletal cells. PKCalpha is constitutively associated with IRS-1, and insulin stimulation of PKCalpha causes disassociation of the two proteins within 5 min. Blockade of PKCalpha inhibited insulin-induced disassociation of PKCalpha from IRS1. Selective inhibition of PKCalpha increased the ability of insulin to reduce blood glucose levels. Insulin stimulation activates PKB and increases the association of PKCalpha with PKB. Blockade of PKCalpha increased threonine phosphorylation of PKB. We suggest that PKCalpha regulates insulin signaling in skeletal muscle through its disassociation from IRS-1 and association with PKB.

Delineating the mechanism by which selenium deactivates Akt in prostate cancer cells

The up-regulation of the phosphatidylinositol 3-kinase (PI3K)/Akt pathway is prevalent in many cancers. This phenomenon makes PI3K and Akt fruitful targets for cancer therapy and/or prevention because they are mediators of cell survival signaling. Although the suppression of phospho-Akt by selenium has been reported previously, little information is available on whether selenium modulates primarily the PI3K-phosphoinositide-dependent kinase 1 (PDK1) side of Akt phosphorylation or the phosphatase side of Akt dephosphorylation. The present study was aimed at addressing these questions in PC-3 prostate cancer cells which are phosphatase and tensin homologue-null. Our results showed that selenium decreased Akt phosphorylation at Thr308 (by PDK1) and Ser473 (by an unidentified kinase); the Thr308 site was more sensitive to selenium inhibition than the Ser473 site. The protein levels of PI3K and phospho-PDK1 were not affected by selenium. However, the activity of PI3K was reduced by 30% in selenium-treated cells, thus discouraging the recruitment of PDK1 and Akt to the membrane due to low phosphatidylinositol-3,4,5-trisphosphate formation by PI3K. Consistent with the above interpretation, the membrane localization of PDK1 and Akt was significantly diminished as shown by Western blotting. In the presence of a calcium chelator or a specific inhibitor of calcineurin (a calcium-dependent phosphatase), the suppressive effect of selenium on phospho-Akt(Ser473) was greatly reduced. The finding suggests that selenium-mediated dephosphorylation of Akt via calcineurin is likely to be an additional mechanism in regulating the status of phospho-Akt.

OSU-03012 in the treatment of glioblastoma

In an article presented in this issue of Molecular Pharmacology, Yacoub et al. (p. 589) examine the actions of 2-amino-N{4-5-(2-phenanthrenyl)-3-(trifluoromethyl)-1H-pyrazol-1-yl]phenyl}-acetamide (OSU-03012) on both primary and glioblastoma cell lines. The authors found that OSU-03012 could induce tumor cell death by itself but also acted as a strong sensitizing agent to radiotherapy-induced cell death. Glioblastoma cells were also more sensitive to this compound than nontransformed astrocytes. Radiation-induced cell death was refractory to small interfering RNA-directed inhibition of PDK1 but not OSU-03012. These results indicate that OSU-03012, which has been thought to primarily mediate antitumor effects via the inhibition of PDK1, has actions independent of PDK1. Furthermore, the authors demonstrated that the effects of OSU-03012 were independent of ERB-B1-vIII and PTEN expression. These are important findings because they start to identify a new mechanism to sensitize glioblastoma cells and also suggest that OSU-03012 could be combined with existing inhibitors to further sensitize tumor cells. In glioblastoma cells, OSU-03012 seemed to induce apoptosis via endoplasmic reticulum stress-induced PERK-dependent signaling. OSU-03012-induced death of the glioblastoma was only weakly suppressed by the pan-caspase inhibitor, N-benzyloxycarbonyl-Val-Ala-Asp, suggesting that OSU-03012-induced cell death was largely caspase-independent. Overall, these are exciting results and suggest that new more effective treatment options may be obtainable for people suffering from these deadly tumors.

Intravenous administration of amino acids during anesthesia stimulates muscle protein synthesis and heat accumulation in the body

The present study was conducted to determine the contribution of muscle protein synthesis to the prevention of anesthesia-induced hypothermia by intravenous administration of an amino acid (AA) mixture. We examined the changes of intraperitoneal temperature (Tcore) and the rates of protein synthesis (K(s)) and the phosphorylation states of translation initiation regulators and their upstream signaling components in skeletal muscle in conscious (Nor) or propofol-anesthetized (Ane) rats after a 3-h intravenous administration of a balanced AA mixture or saline (Sal). Compared with Sal administration, the AA mixture administration markedly attenuated the decrease in Tcore in rats during anesthesia, whereas Tcore in the Nor-AA group became slightly elevated during treatment. Stimulation of muscle protein synthesis resulting from AA administration was observed in each case, although K(s) remained lower in the Ane-AA group than in the Nor-Sal group. AA administration during anesthesia significantly increased insulin concentrations to levels approximately 6-fold greater than in the Nor-AA group and enhanced phosphorylation of eukaryotic initiation factor 4E-binding protein-1 (4E-BP1) and ribosomal protein S6 protein kinase relative to all other groups and treatments. The alterations in the Ane-AA group were accompanied by hyperphosphorylation of protein kinase B and the mammalian target of rapamycin (mTOR). These results suggest that administration of an AA mixture during anesthesia stimulates muscle protein synthesis via insulin-mTOR-dependent activation of translation initiation regulators caused by markedly elevated insulin and, thereby, facilitates thermal accumulation in the body.

Epidermal growth factor receptor expression and activation in neuroendocrine tumours

Epidermal growth factor receptor (EGFR) is expressed in many cancers and is associated with poor prognosis. EGFR activation pathways have been well characterised using tumour cell lines and are known to involve EGFR activation through autophosphorylation. Phosphorylation of downstream signalling molecules, such as ERK1/2 (extra-cellular regulated kinase 1 and 2) and PKB/Akt (protein kinase B), leads to enhanced tumour cell survival and proliferation. Although EGFR expression has been determined in neuroendocrine tumour tissue, its activation and subsequent effects on the downstream signalling molecules, ERK1/2 and Akt, have not been studied. We therefore planned to determine the role of EGFR in neuroendocrine tumours (NETs) by determining its pattern of expression and activation, and the subsequent activation of downstream signalling molecules ERK1/2 and Akt. Paraffin-embedded tumour tissue was available from 98 patients with NETs (39 foregut, 42 midgut, four hindgut, five paragangliomas, and four of unknown origin). Immunohistochemical evaluation was performed for the expression of EGFR, p-EGFR, p-Akt, and p-ERK1/2. Ninety-six percent of tumour samples were positive for EGFR expression; 63% were positive for activated EGFR; 76% were positive for activated Akt; and 96% were positive for activated ERK1/2. Importantly, the histological score for the activation of Akt and ERK1/2 correlated with the histological score for activated EGFR. These data provide a rationale for considering EGFR inhibitors in the treatment of NETs. Additionally, direct inhibition of Akt and ERK1/2 may provide further therapeutic options in the treatment of NETs in the future.

Exposure to music in the perinatal period enhances learning performance and alters BDNF/TrkB signaling in mice as adults

Music has been suggested to have a beneficial effect on various types of performance in humans. However, the physiological and molecular mechanism of this effect remains unclear. We examined the effect of music exposure during the perinatal period on learning behavior in adult mice, and measured the levels of brain-derived neurotrophic factor (BDNF) and its receptor, tyrosine kinase receptor B (TrkB), which play critical roles in synaptic plasticity. In addition, we measured the levels of 3-phosphoinositide-dependent protein kinase-1 (PDK1) and mitogen-activated protein kinase (MAPK), downstream targets of two main pathways in BDNF/TrkB signaling. Music-exposed mice completed a maze learning task with fewer errors than the white noise-exposed mice and had lower levels of BDNF and higher levels of TrkB and PDK1 in the cortex. MAPK levels were unchanged. Furthermore, TrkB and PDK1 protein levels in the cortex showed a significant negative correlation with the number of errors on the maze. These results suggest that perinatal exposure of mice to music has an influence on BDNF/TrkB signaling and its intracellular signaling pathway targets, including PDK1, and thus may induce improved learning and memory functions.

Vascular endothelial growth factor-induced signaling pathways in endothelial cells that mediate overexpression of the chemokine IFN-gamma-inducible protein of 10 kDa in vitro and in vivo

Vascular endothelial growth factor (VEGF), an angiogenesis factor, has recently been found to have potent proinflammatory properties in vivo. However, the mechanism by which it mediates inflammation is poorly understood. In this study, we have evaluated the function of VEGF on the induced expression and function of the T cell chemoattractant chemokine IFN-gamma-inducible protein of 10 kDa (IP-10). In vitro, we find that VEGF augments the effect of IFN-gamma on the induction of IP-10 mRNA and protein expression in endothelial cells. Moreover, we show that VEGF and IFN-gamma regulate the activation of the IP-10 promoter, and that the kinases PI3K, phosphoinositide-dependent kinase 1, and Akt act as intermediary signaling molecules for cytokine-inducible IP-10 transcriptional activation in endothelial cells. To examine whether VEGF is functional for IP-10 expression in vivo, Chinese hamster ovary cells that were designed to secrete VEGF were injected s.c. into the skin of nude mice and were found to mediate a time-dependent increase in IP-10 mRNA. This response was reduced in animals treated systemically with the PI3K inhibitor wortmannin. When the Chinese hamster ovary cells expressing VEGF plasmid were injected s.c. into C57BL/6 wild-type or CXCR3-/- mice, they elicited an inflammatory reaction in wild-type but not in CXCR3-/- mice. Collectively, these findings indicate that VEGF-induced augmentation of IP-10 expression is a major mechanism underlying its proinflammatory function.

Phosphorylation of TNF-alpha converting enzyme by gastrin-releasing peptide induces amphiregulin release and EGF receptor activation

G protein-coupled receptors induce EGF receptor (EGFR) signaling, leading to the proliferation and invasion of cancer cells. Elucidation of the mechanism of EGFR activation by G protein-coupled receptors may identify new signaling paradigms. A gastrin-releasing peptide (GRP)/GRP receptor-mediated autocrine pathway was previously described in squamous cell carcinoma of head and neck. In the present study, we demonstrate that TNF-alpha converting enzyme (TACE), a disintegrin and metalloproteinse-17, undergoes a Src-dependent phosphorylation that regulates release of the EGFR ligand amphiregulin upon GRP treatment. Further investigation reveals the phosphatidylinositol 3-kinase (PI3-K) as the intermediate of c-Src and TACE, contributing to their association and TACE phosphorylation. Phosphoinositide-dependent kinase 1 (PDK1), a downstream target of PI3-K, has been identified as the previously undescribed kinase to directly phosphorylate TACE upon GRP treatment. These findings suggest a signaling cascade of GRP-Src-PI3-K-PDK1-TACE-amphiregulin-EGFR with multiple points of interaction, translocation, and phosphorylation. Furthermore, knockdown of PDK1 augmented the antitumor effects of the EGFR inhibitor erlotinib, indicating PDK1 as a therapeutic target to improve the clinical response to EGFR inhibitors.

Ablation of PDK1 in pancreatic beta cells induces diabetes as a result of loss of beta cell mass

The total mass of islets of Langerhans is reduced in individuals with type 2 diabetes, possibly contributing to the pathogenesis of this condition. Although the regulation of islet mass is complex, recent studies have suggested the importance of a signaling pathway that includes the insulin or insulin-like growth factor-1 receptors, insulin receptor substrate and phosphatidylinositol (PI) 3-kinase. 3-Phosphoinositide-dependent protein kinase 1 (PDK1) is a serine-threonine kinase that mediates signaling downstream of PI 3-kinase. Here we show that mice that lack PDK1 specifically in pancreatic beta cells (betaPdk1-/- mice) develop progressive hyperglycemia as a result of a loss of islet mass. The mice show reductions in islet density as well as in the number and size of cells. Haploinsufficiency of the gene for the transcription factor Foxo1 resulted in a marked increase in the number, but not the size, of cells and resulted in the restoration of glucose homeostasis in betaPdk1-/- mice. These results suggest that PDK1 is important in maintenance of pancreatic cell mass and glucose homeostasis.

OSU-03012 promotes caspase-independent but PERK-, cathepsin B-, BID-, and AIF-dependent killing of transformed cells

We determined one mechanism by which the putative phosphoinositide-dependent kinase (PDK)-1 inhibitor 2-amino-N-{4-[5-(2-phenanthrenyl)-3-(trifluoromethyl)-1H-pyrazol-1-yl]-phenyl}acetamide (OSU-03012) killed primary human glioma and other transformed cells. OSU-03012 caused a dose-dependent induction of cell death that was not altered by p53 mutation, expression of ERBB1 vIII, or loss of phosphatase and tensin homolog deleted on chromosome 10 function. OSU-03012 promoted cell killing to a greater extent in glioma cells than in nontransformed astrocytes. OSU-03012 and ionizing radiation caused an additive, caspase-independent elevation in cell killing in 96-h viability assays and true radiosensitization in colony formation assays. In a cell type-specific manner, combined exposure to OSU-03012 with a mitogen-activated protein kinase kinase 1/2 inhibitor, phosphoinositide 3-kinase/AKT inhibitors, or parallel molecular interventions resulted in a greater than additive induction of cell killing that was independent of AKT activity and caspase function. OSU-03012 lethality as a single agent or when combined with signaling modulators was not modified in cells lacking expression of BIM or of BAX/BAK. OSU-03012 promoted the release of cathepsin B from the lysosomal compartment and release of AIF from mitochondria. Loss of BH3-interacting domain (BID) function, overexpression of BCL(XL), and inhibition of cathepsin B function suppressed cell killing and apoptosis-inducing factor (AIF) release from mitochondria. In protein kinase R-like endoplasmic reticulum kinase-/- cells, the lethality of OSU-03012 was attenuated which correlated with reduced cleavage of BID and with suppression of cathepsin B and AIF release into the cytosol. Our data demonstrate that OSU-03012 promotes glioma cell killing that is dependent on endoplasmic reticulum stress, lysosomal dysfunction, and BID-dependent release of AIF from mitochondria, and whose lethality is enhanced by irradiation or by inhibition of protective signaling pathways.

Comment on "PDK1 nucleates T cell receptor-induced signaling complex for NF-kappaB activation"

We observe that protein kinase C (PKC) is phosphorylated on the activation loop at threonine 538 (Thr-538) before T cell activation. Our results are inconsistent with the conclusions of Lee et al. (Reports, 1 April 2005, p. 114) that the Thr-538 phosphorylation of PKC is regulated by T cell receptor activation. Other mechanisms, such as autophosphorylation of Thr-219, might orchestrate the cellular function of PKC in T cells.

Phosphorylation and activation of PINOID by the phospholipid signaling kinase 3-phosphoinositide-dependent protein kinase 1 (PDK1) in Arabidopsis

Activity of the serine-threonine protein kinase PINOID (PID) has been implicated in the asymmetrical localization of the membrane-associated PINFORMED (PIN) family of auxin transport facilitators. However, the means by which PID regulates PIN protein distribution is unknown. We have used recombinant PID protein to dissect the regulation of PID activity in vitro. We demonstrate that intramolecular PID autophosphorylation is required for the ability of PID to phosphorylate an exogenous substrate. PID-like mammalian AGC kinases act in a phosphorylation cascade initiated by the phospholipid-associated kinase, 3-phosphoinositide-dependent protein kinase 1 (PDK1), which binds to the C-terminal hydrophobic PDK1-interacting fragment (PIF) domain found in PDK1 substrates. We find that Arabidopsis PDK1 interacts with PID, and that transphosphorylation by PDK1 increases PID autophosphorylation. We show that a PID activation loop serine is required for PDK1-dependent PID phosphorylation. This activation is rapid and requires the PIF domain. Cell extracts from flowers and seedling shoots dramatically increase PID phosphorylation in a tissue-specific manner. A PID protein variant in which the PIF domain was mutated failed to be activated by the seedling shoot extracts. PID immunoprecipitated from Arabidopsis cells in which PDK1 expression was inhibited by RNAi showed a dramatic reduction in transphosphorylation of myelin basic protein substrate. These results indicate that AtPDK1 is a potent enhancer of PID activity and provide evidence that phospholipid signaling may play a role in the signaling processes controlling polar auxin transport.

3-phosphoinositide-dependent protein kinase-1 (PDK1) promotes invasion and activation of matrix metalloproteinases

Background

Metastasis is a major cause of morbidity and mortality in breast cancer with tumor cell invasion playing a crucial role in the metastatic process. PDK1 is a key molecule that couples PI3K to cell proliferation and survival signals in response to growth factor receptor activation, and is oncogenic when expressed in mouse mammary epithelial cells. We now present evidence showing that PDK1-expressing cells exhibit enhanced anchorage-dependent and -independent cell growth and are highly invasive when grown on Matrigel. These properties correlate with induction of MMP-2 activity, increased MT1-MMP expression and a unique gene expression profile.

Methods

Invasion assays in Matrigel, MMP-2 zymogram analysis, gene microarray analysis and mammary isografts were used to characterize the invasive and proliferative function of cells expressing PDK1. Tissue microarray analysis of human breast cancers was used to measure PDK1 expression in invasive tumors by IHC.

Results

Enhanced invasion on Matrigel in PDK1-expressing cells was accompanied by increased MMP-2 activity resulting from stabilization against proteasomal degradation. Increased MMP-2 activity was accompanied by elevated levels of MT1-MMP, which is involved in generating active MMP-2. Gene microarray analysis identified increased expression of the ECM-associated genes decorin and type I procollagen, whose gene products are substrates of MT1-MMP. Mammary fat pad isografts of PDK1-expressing cells produced invasive adenocarcinomas. Tissue microarray analysis of human invasive breast cancer indicated that PDK1pSer241 was strongly expressed in 90% of samples.

Conclusion

These results indicate that PDK1 serves as an important effector of mammary epithelial cell growth and invasion in the transformed phenotype. PDK1 mediates its effect in part by MT1-MMP induction, which in turn activates MMP-2 and modulates the ECM proteins decorin and collagen. The presence of increased PDK1 expression in the majority of invasive breast cancers suggests its importance in the metastatic process.

Phosphoinositide-dependent kinase l (PDK1) haplo-insufficiency inhibits production of alpha/beta (α/β) but not gamma delta (γ/δ) T lymphocytes

In the present study, we have explored the impact of deleting a single allele of PDK1 in T cell progenitors on alpha/beta and gamma/delta T cell development. The data show that deleting a single allele of PDK1 allows differentiation of alpha/beta T cells but prevents their proliferative expansion in the thymus. Accordingly, mice with T cells that are haplo-insufficient for PDK1 have reduced numbers of thymocytes and alpha/beta peripheral T cells. T cell progenitors also give rise to gamma/delta T cells but in contrast to the loss of alpha/beta T cells in T-PDK1 null and haplo-insufficient mice, there were increased numbers of gamma/delta T cells. The production of alpha/beta T cells is dependent on the proliferative expansion of thymocytes and is determined by a balance between the frequency with which cells enter the proliferative phase of the cell cycle and rates of cell death. Herein, we show that PDK1 haplo-insufficient thymocytes have no defects in their ability to enter the cell cycle but show increased apoptosis. PDK1 thus plays a determining role in the development of alpha/beta T lymphocytes but does not limit gamma/delta T cell development.