“New”-clear functions of PDK1: Beyond a master kinase?

Activation of cytosolic phosphoinositide-3 kinase (PI-3K) signaling pathway has been well established to regulate gene expression, cell cycle, and survival by feeding signals to the nucleus. In addition, strong evidences accumulated over the past few years indicate the presence of an autonomous inositol lipid metabolism and PI-3K signaling within the nucleus. Much less, however, is known about the role and regulation of this nuclear PI-3K pathway. Components of the PI-3K signaling pathway, including PI 3-kinase and its downstream kinase Akt, have been identified at the nuclear level. Consistent with the presence of a complete PI-3K signaling pathway in the nucleus, we have recently found that phosphoinositide-dependent kinase 1 (PDK1), a kinase functioning downstream of PI-3K and upstream of Akt, is a nucleo-cytoplasmic shuttling protein. In the present review, we update our current knowledge on the regulatory mechanisms and the functional roles of PDK1 nuclear translocation. We also summarize some of the kinase-independent activities of PDK1 in cell signaling.

The major target of the endogenously generated reactive oxygen species in response to insulin stimulation is phosphatase and tensin homolog and not phosphoinositide-3 kinase (PI-3 kinase) in the PI-3 kinase/Akt pathway

Phosphoinositide-3 kinase (PI-3 kinase) and its downstream signaling molecules PDK-1 and Akt were analyzed in SK-N-SH and SK-N-BE(2) human neuroblastoma cell lines. When cells were stimulated with insulin, PI-3 kinase was activated in both cell lines, whereas the translocation of PDK-1 to the membrane fraction and phosphorylated Akt were observed only in SK-N-SH cells. Analyses of the insulin-mediated reactive oxygen species (ROS) generation and Phosphatase and Tensin homolog (PTEN) oxidation indicate that PTEN oxidation occurred in SK-N-SH cells, which can produce ROS, but not in SK-N-BE(2) cells, which cannot increase ROS in response to insulin stimulation. When SK-N-SH cells were pretreated with the NADPH oxidase inhibitor diphenyleneiodonium chloride before insulin stimulation, insulin-mediated translocation of PDK-1 to the membrane fraction and phosphorylation of Akt were remarkably reduced, whereas PI-3 kinase activity was not changed significantly. These results indicate that not only PI-3 kinase activation but also inhibition of PTEN by ROS is needed to increase cellular level of phosphatidylinositol 3,4,5-trisphosphate for recruiting downstream signaling molecules such as PDK-1 and Akt in insulin-mediated signaling. Moreover, the ROS generated by insulin stimulation mainly contributes to the inactivation of PTEN and not to the activation of PI-3 kinase in the PI-3 kinase/Akt pathway.

P21-activated kinase-1 is necessary for depolarization-mediated neuronal survival

Cerebellar granule neurons undergo apoptosis when switched from culture medium containing high potassium (HK) to medium that contains low potassium (LK). HK treatment leads to an activation of p21-activated kinase-1 (PAK-1). Overexpression of a constitutively active form of PAK-1 protects against apoptosis in LK medium. Overexpression of a dominant-negative form of PAK-1 blocks survival in HK. Although PAK-1 is usually considered to be a downstream effector of Rac and Cdc42, we were unable to detect association between PAK-1 and either Rac1 or Cdc42 in cerebellar granule neurons. Interaction between PAK-1 and PDK1 is detected in granule neurons, although there is no change in the extent of interaction in neurons primed to die. Neuronal survival by PAK-1 overexpression is not inhibited by PD98059 or LY294002, which inhibit the activity of MEK and PI-3 kinase, respectively. The ability of PAK-1 to maintain neuronal survival is, however, blocked by ML-9, a compound known to inhibit Akt. Our results show that that PAK-1 is necessary for neuronal survival in HK and suggest that its neuroprotective action may be mediated by a GTPase-independent, but Akt-dependent, mechanism.

Development of a fluorescence polarization bead-based coupled assay to target different activity/conformation states of a protein kinase

Most of the protein kinase inhibitors being developed are directed toward the adenosine triphosphate (ATP) binding site that is highly conserved in many kinases. A major issue with these inhibitors is the specificity for a given kinase. Structure determination of several kinases has shown that protein kinases adopt distinct conformations in their inactive state, in contrast to their strikingly similar conformations in their active states. Hence, alternative assay formats that can identify compounds targeting the inactive form of a protein kinase are desirable. The authors describe the development and optimization of an Immobilized Metal Assay for Phosphochemicals (IMAP)-based couple d assay using PDK1 and inactive Akt-2 enzymes. PDK1 phosphorylates Akt-2 at Thr 309 in the catalytic domain, leading to enzymatic activation. Activation of Akt by PDK1 is measured by quantitating the phosphorylation of Akt-specific substrate peptide using the IMAP assay format. This IMAP-coupled assay has been formatted in a 384-well microplate format with a Z' of 0.73 suitable for high-throughput screening. This assay was evaluated by screening the biologically active sample set LOPAC trade mark and validated with the protein kinase C inhibitor staurosporine. The IC(50) value generated was comparable to the value obtained by the radioactive (33)P-gamma-ATP flashplate transfer assay. This coupled assay has the potential to identify compounds that target the inactive form of Akt and prevent its activation by PDK1, in addition to finding inhibitors of PDK1 and activated Akt enzymes.

RETRACTED: Contribution of the PI3K/Akt/PKB signal pathway to maintenance of self-renewal in human embryonic stem cells

Although basic fibroblast growth factor (FGF2) is generally included in the media for maintenance of human embryonic stem cells (hESCs), the action of FGF2 in these cells has not been well defined. Here, we determined the roles of FGF2 in maintaining hESC self-renewal. Withdrawal of FGF2 from the media led to acquisition of typical differentiated characteristics in hESCs. In the presence of FGF2, which is normally required for proliferation in an undifferentiated state, inhibition of phosphatidylinositol 3-kinase (PI3K)/Akt/PKB signal stimulated differentiation and attenuated the expression of extracellular matrix (ECM) molecules. We suggest that FGF2 maintains hESC self-renewal by supporting stable expression of ECM molecules through activation of the PI3K/Akt/PKB pathway.

Activation of the phosphoinositide 3-kinase-Akt-mammalian target of rapamycin signaling pathway is required for metabotropic glutamate receptor-dependent long-term depression

Hippocampal long-term depression (LTD) is a long-lasting decrease in synaptic strength that is most commonly studied at glutamatergic inputs to pyramidal cells in hippocampal area CA1. Activation of G-protein-coupled group I (including types 1 and 5) metabotropic glutamate receptors (mGluRs) by the pharmacological agonist (RS)-3,5-dihydroxyphenylglycine (DHPG) elicits LTD in area CA1 of the hippocampus. Recent reports have shown that de novo protein synthesis is necessary for DHPG-induced LTD. However, relatively little is known about the signaling pathways that couple mGluRs to translation initiation. In this study, we investigated whether the activation of the phosphoinositide 3-kinase (PI3K)-Akt-mammalian target of rapamycin (mTOR) pathway, which has been shown to regulate translation initiation, is necessary for mGluR-LTD induced by DHPG. We found that brief incubations of mouse hippocampal slices with DHPG resulted in increased phosphorylation of Akt and mTOR in hippocampal area CA1. Two structurally unrelated PI3K inhibitors, LY294002 and wortmannin, blocked the DHPG-induced increases in phosphorylation of Akt and mTOR. Biochemical fractionation studies showed that the DHPG-induced increase in the phosphorylation of Akt and mTOR could be detected in synaptoneurosome preparations, and immunohistochemical analysis revealed that similar increases could be detected in both stratum pyramidale and stratum radiatum in area CA1. Finally, we observed that both PI3K inhibitors and rapamycin, an mTOR inhibitor, prevented mGluR-LTD induced by DHPG. Together, our findings indicate that activation of the PI3K-Akt-mTOR signaling cascade is required for mGluR-LTD and suggest that this pathway may couple group I mGluRs to translation initiation in hippocampal area CA1.

PDK1 is required for the hormonal signaling pathway leading to meiotic resumption in starfish oocytes

Meiotic resumption is generally under the control of an extracellular maturation-inducing hormone. It is equivalent to the G2-M phase transition in somatic cell mitosis and is regulated by cyclin B-Cdc2 kinase. However, the complete signaling pathway from the hormone to cyclin B-Cdc2 is yet unclear in any organism. A model system to analyze meiotic resumption is the starfish oocyte, in which Akt/protein kinase B (PKB) plays a key mediator in hormonal signaling that leads to cyclin B-Cdc2 activation. Here we show in starfish oocytes that when PDK1 activity is inhibited by a neutralizing antibody, maturation-inducing hormone fails to induce cyclin B-Cdc2 activation at the meiotic G2-M phase transition, even though PDK2 activity becomes detectable. These observations assign a novel role to PDK1 for a hormonal signaling intermediate toward meiotic resumption. They further support that PDK2 is a molecule distinct from PDK1 and Akt, and that PDK2 activity is not sufficient for the full activation of Akt in the absence of PDK1 activity.

Insulin Substrates 1 and 2 Are Corequired for Activation of Atypical Protein Kinase C and Cbl-Dependent Phosphatidylinositol 3-Kinase during Insulin Action in Immortalized Brown Adipocytes†

Phosphatidylinositol 3-kinase (PI3K)-dependent activation of atypical protein kinase C (aPKC) is required for insulin-stimulated glucose transport. Although insulin receptor substrate-1 (IRS-1) and IRS-2, among other factors, activate PI3K, there is little information on the relative roles of IRS-1and IRS-2 during aPKC activation by insulin action in specific cell types. Presently, we have used immortalized brown adipocytes in which either IRS-1 or IRS-2 has been knocked out by recombinant methods to examine IRS-1 and IRS-2 requirements for activation of aPKC. We have also used these adipocytes to see if IRS-1 and IRS-2 are required for activation of Cbl, which is required for insulin-stimulated glucose transport and has been found to function upstream of both PI3K/aPKC and Crk during thiazolidinedione action in 3T3/L1 adipocytes [Miura et al. (2003) Biochemistry 42, 14335]. In brown adipocytes in which either IRS-1 or IRS-2 was knocked out, insulin-induced increases in aPKC activity and glucose transport were markedly diminished. These effects of insulin on aPKC and glucose transport were fully restored by retroviral-mediated expression of IRS-1 or IRS-2 in their respective knockout cells. Knockout of IRS-1 or IRS-2 also inhibited insulin-induced increases in Cbl binding to the p85 subunit of PI3K, which, along with IRS-1/2, may be required for activation of PI3K, aPKC, and glucose transport during insulin action in 3T3/L1 adipocytes. These findings provide evidence that directly links both IRS-1 and IRS-2 to aPKC activation in immortalized brown adipocytes, and further suggest that IRS-1 and IRS-2 are required for the activation of Cbl/PI3K during insulin action in these cells.

Protein phosphatase 2A negatively regulates insulin's metabolic signaling pathway by inhibiting Akt (protein kinase B) activity in 3T3-L1 adipocytes

Protein phosphatase 2A (PP2A) is a multimeric serine/threonine phosphatase which has multiple functions, including inhibition of the mitogen-activated protein (MAP) kinase pathway. Simian virus 40 small t antigen specifically inhibits PP2A function by binding to the PP2A regulatory subunit, interfering with the ability of PP2A to associate with its cellular substrates. We have reported that the expression of small t antigen inhibits PP2A association with Shc, leading to augmentation of insulin and epidermal growth factor-induced Shc phosphorylation with enhanced activation of the Ras/MAP kinase pathway. However, the potential involvement of PP2A in insulin's metabolic signaling pathway is presently unknown. To assess this, we overexpressed small t antigen in 3T3-L1 adipocytes by adenovirus-mediated gene transfer and found that the phosphorylation of Akt and its downstream target, glycogen synthase kinase 3beta, were enhanced both in the absence and in the presence of insulin. Furthermore, protein kinase C lambda (PKC lambda) activity was also augmented in small-t-antigen-expressing 3T3-L1 adipocytes. Consistent with this result, both basal and insulin-stimulated glucose uptake were enhanced in these cells. In support of this result, when inhibitory anti-PP2A antibody was microinjected into 3T3-L1 adipocytes, we found a twofold increase in GLUT4 translocation in the absence of insulin. The small-t-antigen-induced increase in Akt and PKC lambda activities was not inhibited by wortmannin, while the ability of small t antigen to enhance glucose transport was inhibited by dominant negative Akt (DN-Akt) expression and Akt small interfering RNA (siRNA) but not by DN-PKC lambda expression or PKC lambda siRNA. We conclude that PP2A is a negative regulator of insulin's metabolic signaling pathway by promoting dephosphorylation and inactivation of Akt and PKC lambda and that most of the effects of PP2A to inhibit glucose transport are mediated through Akt.

Puromycin-insensitive leucyl-specific aminopeptidase (PILSAP) binds and catalyzes PDK1, allowing VEGF-stimulated activation of S6K for endothelial cell proliferation and angiogenesis

Puromycin-insensitive leucyl-specific aminopeptidase (PILSAP) plays an important role in angiogenesis by regulating the proliferation and migration of endothelial cells (ECs). Here we characterize the mechanism by which PILSAP regulates the vascular endothelial growth factor (VEGF)–stimulated proliferation of ECs. The specific elimination of PILSAP expression or its enzymatic activity inhibited VEGF-stimulated G1/S transition in ECs. This G1 arrest correlated with reduced cyclin dependent kinase 4/6 (CDK4/6) activity and retinoblastoma (Rb) protein phosphorylation. Analyses of signaling molecules upstream of CDK4/6 revealed that S6 kinase (S6K) activation was affected by PILSAP, whereas that of phosphatidylinositol-3 kinase (PI3K), Akt, and extracellular signal-related kinase 1/2 (ERK1/2) was not. We further demonstrated that PILSAP bound phosphatidylinositol-dependent kinase 1 (PDK1) and removed 9 amino acids from its N-terminus, which allowed S6K to associate with PDK1 and PILSAP upon VEGF stimulation. We constructed mutant PILSAP, which lacked the aminopeptidase activity but bound PDK1. Mutant PILSAP abrogated S6K activation upon VEGF stimulation in a dominant-negative manner. An N-terminal truncated form of PDK1 abolished the dominant-negative effect of mutant PILSAP. Finally, the introduction of a mutated PILSAP gene in ECs inhibited angiogenesis and retarded tumor growth in vivo. These results indicate that PILSAP plays a crucial role in the cell cycle progression of ECs and angiogenesis via the binding and modification of PDK1.

Structural insights into the regulation of PDK1 by phosphoinositides and inositol phosphates

3-phosphoinositide-dependent protein kinase-1 (PDK1) phosphorylates and activates many kinases belonging to the AGC subfamily. PDK1 possesses a C-terminal pleckstrin homology (PH) domain that interacts with PtdIns(3,4,5)P3/PtdIns(3,4)P2 and with lower affinity to PtdIns(4,5)P2. We describe the crystal structure of the PDK1 PH domain, in the absence and presence of PtdIns(3,4,5)P3 and Ins(1,3,4,5)P4. The structures reveal a 'budded' PH domain fold, possessing an N-terminal extension forming an integral part of the overall fold, and display an unusually spacious ligand-binding site. Mutagenesis and lipid-binding studies were used to define the contribution of residues involved in phosphoinositide binding. Using a novel quantitative binding assay, we found that Ins(1,3,4,5,6)P5 and InsP6, which are present at micromolar levels in the cytosol, interact with full-length PDK1 with nanomolar affinities. Utilising the isolated PDK1 PH domain, which has reduced affinity for Ins(1,3,4,5,6)P5/InsP6, we perform localisation studies that suggest that these inositol phosphates serve to anchor a portion of cellular PDK1 in the cytosol, where it could activate its substrates such as p70 S6-kinase and p90 ribosomal S6 kinase that do not interact with phosphoinositides.

Interactions of LY333531 and other bisindolyl maleimide inhibitors with PDK1

LY333531, BIM-1, BIM-2, BIM-3, and BIM-8 are bisindolyl maleimide-based, nanomolar protein kinase C inhibitors. LY333531, a PKCbeta-specific inhibitor, is in clinical trials against diabetes and cardiac ventricular hypertrophy complications. Specificity analysis with a panel of 29 protein kinases reveals that these bisindolyl maleimide inhibitors also inhibit PDK1, a key kinase from the insulin signaling pathway, albeit in the lower microM range. To understand the molecular basis of inhibition, the PDK1 kinase domain was cocrystallized with these bisindolyl maleimide inhibitors. The inhibitor complexes represent the first structural description of this class of compounds, revealing their unusual nonplanar conformation within the ATP binding site and also explaining the higher inhibitory potential of LY33331 compared to the BIM compounds toward PDK1. A combination of site-directed mutagenesis and essential dynamics analysis gives further insight into PDK1 and also PKC inhibition by these compounds, and may aid inhibitor design.

Potentiation of signal transduction mitogenesis and cellular proliferation upon binding of receptor-recognized forms of alpha2-macroglobulin to 1-LN prostate cancer cells

The alpha2-macroglobulin signalling receptor is upregulated in highly metastatic 1-LN prostate cancer cells. Stimulation of 1-LN cells with activated alpha2-macroglobulin (alpha2M*) caused a two- to threefold increase in [3H]thymidine uptake and cell number. These events require the Ras-dependent MAPK and PI 3-kinase/Akt signalling cascades. Incubation of 1-LN cells with alpha2M* induced Grb2, shc, sos and Raf-1 expression, as well as phosphorylation of MEK 1/2, ERK 1/2, p38 MAPK and JNK. This treatment also increased PI 3-kinase activation, PDK1 expression, Akt phosphorylation and p70s6k phosphorylation. Levels of the early gene products c-fos protein and thymidylate synthase were comparably increased. Exposure of 1-LN cells to alpha2M* significantly raised the levels of phosphorylated CREB by about 15-20 min and phosphorylated p53 by about 60-90 min of incubation. We conclude that the growth regulatory effects of ligating the alpha2M* signalling receptor on 1-LN cells are exerted via the onset and crosstalk between the Ras-dependent MAPK and PI 3-kinase/Akt signalling cascades.

Phosphoinositide-dependent kinase-1 orthologues from five eukaryotes are activated by the hydrophobic motif in AGC kinases

Phosphoinositide-dependent kinase-1 (PDK1) mediates activation of many AGC kinases by docking onto a phosphorylated hydrophobic motif located C-terminal of the catalytic domain in the AGC kinase. The interaction shifts PDK1 into a conformation with increased catalytic activity and leads to autophosphorylation of PDK1. We demonstrate here that addition of a hydrophobic motif peptide increases the catalytic activity of PDK1 orthologues from Homo sapiens, Aplysia californica, Arabidopsis thaliana, Schizosaccharomyces pombe (ksg1), and Saccharomyces cerevisiae (Pkh1 and Pkh2) 2- to 12-fold. Furthermore, the hydrophobic motif peptide increases autophosphorylation of PDK1 from Homo sapiens, S. pombe, and S. cerevisiae (Phk2). Our results suggest that PDK1 interaction and activation by the hydrophobic motif of AGC kinases is a central mechanism in PDK1 function, which is conserved during eukaryotic evolution.

Involvement of 3-Phosphoinositide-dependent Protein Kinase-1 in the MEK/MAPK Signal Transduction Pathway

The phosphatidylinositide-3-OH kinase/3-phospho-inositide-dependent protein kinase-1 (PDK1)/Akt and the Raf/mitogen-activated protein kinase (MAPK/ERK) kinase (MEK)/mitogen-activated protein kinase (MAPK) pathways have central roles in the regulation of cell survival and proliferation. Despite their importance, however, the cross-talk between these two pathways has not been fully understood. Here we report that PDK1 promotes MAPK activation in a MEK-dependent manner. In vitro kinase assay revealed that the direct targets of PDK1 in the MAPK pathway were the upstream MAPK kinases MEK1 and MEK2. The identified PDK1 phosphorylation sites in MEK1 and MEK2 are Ser222 and Ser226, respectively, and are known to be essential for full activation. To date, these sites are thought to be phosphorylated by Raf kinases. However, PDK1 gene silencing using small interference RNA demonstrates that PDK1 is associated with maintaining the steady-state phosphorylated MEK level and cell growth. The small interference RNA-mediated down-regulation of PDK1 attenuated maximum MEK and MAPK activities but could not prolong MAPK signaling duration. Stable and transient expression of constitutively active MEK1 overcame these effects. Our results suggest a novel cross-talk between the phosphatidylinositide-3-OH kinase/PDK1/Akt pathway and the Raf/MEK/MAPK pathway.

Expression of a mutant IRS inhibits metabolic and mitogenic signalling of insulin in human adipocytes

Adipose tissue is a primary target of insulin, but knowledge about insulin signalling in human adipocytes is limited. We developed an electroporation technique for transfection of primary human adipocytes with a transfection efficiency of 15% +/- 5 (mean +/- S.D.). Human adipocytes were co-transfected with a mutant of IRS-3 (all four potential PI3-kinase binding motifs mutated: IRS-3F4) and HA-tagged protein kinase B (HA-PKB/Akt). HA-PKB/Akt was immunoprecipitated from cell lysates with anti-HA antibodies, resolved with SDS-PAGE, and immunoblotted with phospho-specific antibodies. We found that IRS-3F4 blocked insulin stimulation of HA-PKB/Akt phosphorylation and in further analyses also translocation of recombinant HA-tagged glucose transporter to the plasma membrane. IRS-3F4 also blocked insulin-induced activation of the transcription factor Elk-1. Our results demonstrate the critical importance of IRS for metabolic as well as mitogenic signalling by insulin. This method for transfection of primary human adipocytes will be useful for studying insulin signalling in human adipocytes with molecular biological techniques.

Regulation of insulin action by ceramide: dual mechanisms linking ceramide accumulation to the inhibition of Akt/protein kinase B

The sphingolipid ceramide negatively regulates insulin action by inhibiting Akt/protein kinase B (PKB), a serine/threonine kinase that is a central regulator of glucose uptake and anabolic metabolism. Despite considerable attention, the molecular mechanism accounting for this action of ceramide has remained both elusive and controversial. Herein we utilized deletion constructs encoding two different functional domains of Akt/PKB to identify which region of the enzyme conferred responsiveness to ceramide. Surprisingly the findings obtained with these separate domains reveal that ceramide blocks insulin stimulation of Akt/PKB by two independent mechanisms. First, using the isolated pleckstrin homology domain, we found that ceramide specifically blocks the translocation of Akt/PKB, but not its upstream activator phosphoinositide-dependent kinase-1, to the plasma membrane. Second, using a construct lacking this pleckstrin homology domain, which does not require translocation for activation, we found that ceramide stimulates the dephosphorylation of Akt/PKB by protein phosphatase 2A. Collectively these findings identify at least two independent mechanisms by which excessive ceramide accumulation in peripheral tissues could contribute to the development of insulin resistance. Moreover the results obtained provide a unifying theory to account for the numerous dissenting reports investigating the actions of ceramide toward Akt/PKB.

The adaptor protein Grb14 regulates the localization of 3-phosphoinositide-dependent kinase-1

The metabolic actions of insulin are transduced through the phosphatidylinositol 3-kinase pathway. A critical component of this pathway is 3-phosphoinositide-dependent kinase-1 (PDK-1), a PH domain-containing enzyme that catalyzes the activating phosphorylation for many AGC kinases, including Akt and protein kinase C isozymes. We used a directed proteomics-based approach to identify the adaptor protein Grb14, which binds the insulin receptor through an SH2 domain, as a novel PDK-1 binding partner. Interaction of these two proteins is constitutive and mediated by a PDK-1 binding motif on Grb14. Disruption of this motif by point mutation or deletion of the Grb14 SH2 domain prevents the insulin-triggered membrane translocation of PDK-1. The interaction of PDK-1 with Grb14 facilitates Akt function: disruption of the interaction by overexpression of a construct of Grb14 mutated in the PDK-1 binding motif significantly decreases insulin-dependent activation of Akt. Thus, Grb14 serves as an adaptor protein to recruit PDK-1 to activated insulin receptor, thus promoting Akt phosphorylation and transduction of the insulin signal.

Rescue of TNFalpha-inhibited neuronal cells by IGF-1 involves Akt and c-Jun N-terminal kinases

Proinflammatory cytokines, especially tumor necrosis factor alpha (TNFalpha), is a pleiotropic mediator of a diverse array of physiologic and neurologic functions and is upregulated during various inflammatory and neurodegenerative diseases. A common survival response during such situations is the increased expression of the hormone insulin-like growth factor 1 (IGF-1). Although it was thought previously that the mechanisms of TNFalpha and IGF-1 action were unrelated, it has been shown that low doses of TNFalpha can inhibit the survival effects of IGF-1 in mouse cerebellar granule neurons. We used a neuronal cell line SH-SY5Y, which underwent apoptosis in response to TNFalpha and this process could be reversed substantially by IGF-1. Crosstalk between signaling pathways of these two factors was found at various points downstream of their signal transduction. To determine the mechanisms of IGF-1-mediated rescue, we looked at the MAP kinases, which are known to be involved in IGF-1 as well as TNFalpha signaling. The c-Jun N-terminal kinase pathway, which is known normally to promote cell death, was found to actually promote survival of TNFalpha-mediated cell death. Inhibiting the c-Jun survival pathway completely reversed the rescue mediated by IGF-1. In addition, the Akt pathway played an equally important role in this rescue.

From the cyclooxygenase-2 inhibitor celecoxib to a novel class of 3-phosphoinositide-dependent protein kinase-1 inhibitors

The blockade of Akt activation through the inhibition of 3-phosphoinositide-dependent kinase-1 (PDK-1) represents a major signaling mechanism whereby celecoxib mediates apoptosis. Celecoxib, however, is a weak PDK-1 inhibitor (IC(50), 48 microM), requiring at least 30 microM to exhibit discernable effects on the growth of tumor cells in vitro. Here, we report the structure-based optimization of celecoxib to develop PDK-1 inhibitors with greater potency in enzyme inhibition and growth inhibition. Kinetics of PDK-1 inhibition by celecoxib with respect to ATP suggest that celecoxib derivatives inhibit PDK-1 by competing with ATP for binding, a mechanism reminiscent to that of many kinase inhibitors. Structure-activity analysis together with molecular modeling was used to generate compounds that were tested for their potency in inhibiting PDK-1 kinase activity and in inducing apoptosis in PC-3 prostate cancer cells. Docking of potent compounds into the ATP-binding site of PDK-1 was performed for lead optimization, leading to two compounds, OSU-03012 and OSU-03013, with IC(50) values in PDK-1 inhibition and apoptosis induction in the low microM range. Exposure of PC-3 cells to these agents led to Akt dephosphorylation and inhibition of p70 S6 kinase activity. Moreover, overexpression of constitutively active forms of PDK-1 and Akt partially protected OSU-03012-induced apoptosis. Screening in a panel of 60 cell lines and more extensive testing in PC-3 cells indicated that the mean concentration for total growth inhibition was approximately 3 microM for both agents. Considering the conserved role of PDK-1/Akt signaling in promoting tumorigenesis, these celecoxib analogs are of translational relevance for cancer prevention and therapy.

Induction of mitogenic signalling in the 1LN prostate cell line on exposure to submicromolar concentrations of cadmium+

Cadmium exposure increases the risk of prostate cancer. We now describe the effects of Cd2+ on signalling and proliferation in 1LN prostate cells. Cd2+ increased [3H]thymidine uptake and cell number twofold. Cd2+ elevated intracellular IP3, cytosolic-free Ca2+, phosphorylated MEK1/2, ERK1/2, p38 MAPK and JNK two- to threefold. Increased PDK1 and phosphorylation of the 85-kDa regulatory subunit of PI 3-kinase, Akt and p70s6k were also observed. Cd2+ treatment increased transcription factors NFkappaB and CREB, and the expression of c-fos and c-myc. Cd2+-induced increased uptake of [3H]thymidine was abolished by translational and transcriptional inhibitors, and Ca2+ channel blockers. Inhibition of phospholipase C and of Ca2+ binding to IP3 receptors inhibited Cd2+-induced DNA synthesis as did inhibition of tyrosine kinases, protein kinase C, PI 3-kinase, farnesyl transferase, MEK1/2, ERK1/2 and p38MAPK. Thus signalling events, which are triggered on exposure of 1LN cells to submicromolar concentrations of Cd2+, induce increased proliferation of these cells.

Olanzapine produces trophic effects in vitro and stimulates phosphorylation of Akt/PKB, ERK1/2, and the mitogen-activated protein kinase p38

Olanzapine has previously been shown to stimulate the growth of neuronal cells in culture. A major goal of the present studies was to determine if olanzapine also provided neuroprotection to pheochromocytoma (PC12) cells, SH-SY5Y neuroblastoma cells, and primary cultures of rat cortical neurons. Olanzapine was mitogenic and enhanced the survival of PC12 cells, SH-SY5Y cells and 3T3 preadipocytes, but not L6 myoblasts or myeloma cells. It protected neuronal cells from death induced by serum and glutamine deprivation, amyloid beta peptide (25-35), and fluphenazine. Molecular mechanisms of the neuroprotection by olanzapine were explored, specifically the activation of various protein kinase signaling pathways including Akt/protein kinase B (PKB), extracellular-regulated kinase (ERK), ERK1/2, and mitogen-activated protein kinase (MAPK), p38. Olanzapine treatment led to rapid phosphorylation of kinases from all three pathways in PC12 cells. Phosphorylation of Akt was blocked with selective inhibitors (wortmannin and LY294002), which implicates phosphoinositide 3-kinase (PI3K) in the signaling cascade. Short-term mitogenic effects of olanzapine were abolished with a selective inhibitor of Akt, but not by inhibition of the ERK pathway. Other antipsychotic drugs stimulated phosphorylation of a subset of the kinase panel, but not all three kinases. The present findings demonstrate that olanzapine has both mitogenic and neuroprotective effects in neuronal cells.

Nelfinavir-induced insulin resistance is associated with impaired plasma membrane recruitment of the PI 3-kinase effectors Akt/PKB and PKC-zeta

AIMS/HYPOTHESIS

Chronic exposure of 3T3-L1 adipocytes to the HIV protease inhibitor nelfinavir induces insulin resistance, recapitulating key metabolic alterations of adipose tissue in the lipodystrophy syndrome induced by these agents. Our goal was to identify the defect in the insulin signal transduction cascade leading to nelfinavir-induced insulin resistance.

METHODS

Fully differentiated 3T3-L1 adipocytes were exposed to 30 micro mol/l nelfinavir for 18 h, after which the amount, the phosphorylation and the localisation of key proteins in the insulin signalling cascade were evaluated.

RESULTS

Insulin-induced interaction of phosphatidylinositol 3'-kinase (PI 3-kinase) with IRS proteins was normal in cells treated with nelfinavir, as was IRS-1-associated PI 3-kinase activity. Yet insulin-induced phosphorylation of Akt/protein kinase B (PKB), p70S6 kinase and extracellular signal-regulated kinase 1/2 was significantly impaired. This could not be attributed to increased protein phosphatase 2A activity or to increased expression of phosphoinositide phosphatases (SHIP2 or PTEN). However, insulin failed to induce translocation of the PI 3-kinase effectors Akt/PKB and protein kinase C-zeta (PKC-zeta) to plasma membrane fractions of nelfinavir-treated adipocytes.

CONCLUSIONS/INTERPRETATION

We therefore conclude that nelfinavir induces a defect in the insulin signalling cascade downstream of the activation of PI 3-kinase. This defect manifests itself by impaired insulin-mediated recruitment of Akt/PKB and PKC-zeta to the plasma membrane.

The in vitro effects of H-89, a specific inhibitor of protein kinase A, in the human colonic carcinoma cell line Caco-2

SUMMARY

H-89 is a compound characterized in vitro as a potent and selective inhibitor of protein kinase A. In the present study, we observed that H-89 induced morphological transformation and caused growth inhibition of the human colon cancer cell line Caco-2 in a dose-dependent manner. However, another protein kinase A inhibitor, H-8, had no effect on Caco-2 cells. To evaluate the possible molecular mechanism of H-89-evoked effects in Caco-2 cells, we analysed the capacity of H-89 to regulate the protein kinase B (Akt/PKB) signalling pathway. H-89 treatment led to an activation of Akt/PKB in Caco-2 cells. This activation was phosphatidylinositol 3 (PI3)-kinase-dependent and promoted survival of Caco-2 cells because the PI3 kinase inhibitor LY294002 inhibited the Akt/PKB activation and induced apoptosis of Caco-2 cells. To test whether Akt/PKB activity promoted resistance to H-89-induced effects, LY294002 was added in combination with H-89. LY294002 greatly potentiated the H-89-induced growth inhibition and apoptosis of Caco-2 cells. These results suggest that the H-89-induced growth inhibition of Caco-2 cells is associated with phosphorylation of Akt/PKB protein and that the cells become more sensitive to H-89 and die by apoptosis upon inhibition of the PI3K/Akt pathway.