PDK1, the master regulator of AGC kinase signal transduction

The antiapoptotic effect of fibroblast growth factor-2 is mediated through nuclear factor-kappaB activation induced via interaction between Akt and IkappaB kinase-beta in breast cancer cells

Fibroblast growth factor-2 (FGF-2) is known for its mitogenic and motogenic effects on breast cancer cells. Here, we demonstrate that FGF-2 is also a potent stimulator of breast cancer cell survival, as it counteracts the apoptotic activity of the C2 ceramide analogue and various chemotherapeutic agents (5-fluorouracil, camptothecin, etoposide) in MCF-7, T47-D and BT-20 cells. The use of pharmacological inhibitors (PD98059, wortmannin, LY294002, SN50) and transfection with negative dominants (IkappaBm, p110(PI3K (phosphoinositide 3-kinase))*DeltaK, AktND) or small interfering RNA targeted against Akt indicated that PI3K/Akt and nuclear factor-kappaB (NF-kappaB), but not p42/p44 MAP-kinases, were required to stimulate FGF-2 antiapoptotic activity. The activation of NF-kappaB was dependent on PI3K/Akt, and using a combination of approaches based on immunoprecipitation, Western blotting and proteomics (two-dimensional electrophoresis and mass spectrometry), we identified the beta form of IkappaB kinase (IKKbeta) as a target of Akt signaling. The selective disruption of IKKbeta using small interfering RNA induced a potent inhibition of Akt-mediated activation of NF-kappaB and cell survival, indicating the functional involvement of IKKbeta in FGF-2 antiapoptotic signaling. Together, these results demonstrate Akt/IKKbeta interaction in NF-kappaB pathways, thereby emphasizing the potential of these proteins as therapeutic targets in breast cancer.

Arabidopsis PDK1: identification of sites important for activity and downstream phosphorylation of S6 kinase

The Arabidopsis thaliana protein kinase AtPDK1 was identified as a homologue of the mammalian 3-phosphoinositide-dependent protein kinase-1 (PDK1), which is involved in a number of physiological processes including cell growth and proliferation. We now show that AtPDK1, expressed in E. coli as a recombinant protein, undergoes autophosphorylation at several sites. Using mass spectrometry, three phosphorylated amino acid residues, Ser-177, Ser-276 and Ser-382, were identified, followed by mutational analyses to reveal their roles. These residues are not conserved in mammalian PDK1s. Mutation of Ser-276 in AtPDK1 to alanine resulted in an enzyme with no detectable autophosphorylation. Autophosphorylation was significantly reduced in the Ser177Ala mutant but was only slightly reduced in the Ser382Ala mutant. Other identified sites of importance for autophosphorylation and/or activity of AtPDK1 were Asp-167, Thr-176, and Thr-211. Sites in the mammalian PDK1 corresponding to Asp-167 and Thr-211 are essential for PDK1 autophosphorylation and activity. Autophosphorylation was absent in the Asp167Ala mutant while the Thr176Ala and The211Ala mutants exhibited very low but detectable autophosphorylation, pointing to both similarity and difference between mammalian and plant enzymes. We also demonstrate that AtS6k2, an A. thaliana homologue to the mammalian S6 kinases, is an in vitro target of AtPDK1. Our data clearly show that Asp-167, Thr-176, Ser-177, Thr-211, and Ser-276 in AtPDK1 are important for the downstream phosphorylation of AtS6k2. The results confirm that AtPDK1, like mammalian PDK1, needs phosphorylation at several sites for full downstream phosphorylation activity. Finally, we investigated A. thaliana 14-3-3 proteins as potential AtPDK1 regulatory proteins and the effect of phospholipids on the AtPDK1 activity. Nine of the 12 14-3-3 isoforms tested enhanced AtPDK1 activity whereas one isoform suppressed the activity. No significant effects on AtPDK1 activity by the various phospholipids (including phosphoinositides) were evident.

Recent advances in the protein kinase B signaling pathway

The phosphoinositide 3' kinase signaling pathway is activated in response to a plethora of growth factors and cytokines, and initiates a cascade of signaling events primarily via the induction of specific protein-serine/threonine kinases. Interest in the pathway has been driven by its frequent aberrant activation in disease and its impact on cell fate decisions owing to roles in survival signaling and metabolic control. There have been recent advances in our understanding of the primary components of this pathway, namely phosphoinositide-dependent kinase-1, protein kinase B and glycogen synthase kinase-3, including insights into their mechanisms of regulation, substrate proteins and cellular functions.

Serum- and glucocorticoid-regulated kinase 1 regulates ubiquitin ligase neural precursor cell-expressed, developmentally down-regulated protein 4-2 by inducing interaction with 14-3-3

Serum- and glucocorticoid-regulated kinase 1 (SGK1) is an aldosterone-regulated early response gene product that regulates the activity of several ion transport proteins, most notably that of the epithelial sodium channel (ENaC). Recent evidence has established that SGK1 phosphorylates and inhibits Nedd4-2 (neural precursor cell-expressed, developmentally down-regulated protein 4-2), a ubiquitin ligase that decreases cell surface expression of the channel and possibly stimulates its degradation. The mechanistic basis for this SGK1-induced Nedd4-2 inhibition is currently unknown. In this study we show that SGK1-mediated phosphorylation of Nedd4-2 induces its interaction with members of the 14-3-3 family of regulatory proteins. Through functional characterization of Nedd4-2-mutant proteins, we demonstrate that this interaction is required for SGK1-mediated inhibition of Nedd4-2. The concerted action of SGK1 and 14-3-3 appears to disrupt Nedd4-2-mediated ubiquitination of ENaC, thus providing a mechanism by which SGK1 modulates the ENaC-mediated Na(+) current. Finally, the expression pattern of 14-3-3 is also consistent with a functional role in distal nephron Na(+) transport. These results demonstrate a novel, physiologically significant role for 14-3-3 proteins in modulating ubiquitin ligase-dependent pathways in the control of epithelial ion transport.

PDK2: the missing piece in the receptor tyrosine kinase signaling pathway puzzle

Activation of members of the protein kinase AGC (cAMP dependent, cGMP dependent, and protein kinase C) family is regulated primarily by phosphorylation at two sites: a conserved threonine residue in the activation loop and a serine/threonine residue in a hydrophobic motif (HM) near the COOH terminus. Although phosphorylation of these kinases in the activation loop has been found to be mediated by phosphoinositide-dependent protein kinase-1 (PDK1), the kinase(s) that catalyzes AGC kinase phosphorylation in the HM remains uncharacterized. So far, at least 10 kinases have been suggested to function as an HM kinase or the so-called "PDK2," including mitogen-activated protein (MAP) kinase-activated protein kinase-2 (MK2), integrin-linked kinase (ILK), p38 MAP kinase, protein kinase Calpha (PKCalpha), PKCbeta, the NIMA-related kinase-6 (NEK6), the mammalian target of rapamycin (mTOR), the double-stranded DNA-dependent protein kinase (DNK-PK), and the ataxia telangiectasia mutated (ATM) gene product. However, whether any or all of these kinases act as a physiological HM kinase remains to be established. Nonetheless, available data suggest that multiple systems may be used in cells to regulate the activation of the AGC family kinases. It is possible that, unlike activation loop phosphorylation, phosphorylation of the HM site in the different AGC family kinases is mediated by distinct kinases. In addition, phosphorylation of the AGC family kinase at the HM site could be cell type, signaling pathway, and substrate specific. Identification and characterization of the bonafide HM kinase(s) will be essential to verify these hypotheses.

Role of the PDK1-PKB-GSK3 pathway in regulating glycogen synthase and glucose uptake in the heart

In order to investigate the importance of the PDK1-PKB-GSK3 signalling network in regulating glycogen synthase (GS) in the heart, we have employed tissue specific conditional knockout mice lacking PDK1 in muscle (mPDK1-/-), as well as knockin mice in which the protein kinase B (PKB) phosphorylation site on glycogen synthase kinase-3alpha (GSK3alpha) (Ser21) and GSK3beta (Ser9) is changed to Ala. We demonstrate that in hearts from mPDK1-/- or double GSK3alpha/GSK3beta knockin mice, insulin failed to stimulate the activity of GS or induce its dephosphorylation at residues that are phosphorylated by GSK3. We also establish that in the heart, both GSK3 isoforms participate in the regulation of GS, with GSK3beta playing a more prominent role. This contrasts with skeletal muscle where GSK3beta is the major regulator of insulin-induced GS activity. Despite the inability of insulin to stimulate glycogen synthesis in hearts from the mPDK1-/- or double GSK3alpha/GSK3beta knockin mice, these animals possessed normal levels of cardiac glycogen, demonstrating that total glycogen levels are regulated independently of insulin's ability to stimulate GS in the heart and that mechanisms such as allosteric activation of GS by glucose-6-phosphate and/or activation of GS by muscle contraction, could operate to maintain normal glycogen levels in these mice. We also demonstrate that in cardiomyocytes derived from the mPDK1-/- hearts, although the levels of glucose transporter type 4 (GLUT4) are increased 2-fold, insulin failed to stimulate glucose uptake, providing genetic evidence that PDK1 plays a crucial role in enabling insulin to promote glucose uptake in cardiac muscle.

Mobility of proteins associated with the plasma membrane by interaction with inositol lipids

Translocation of a protein to the plasma membrane in response to the generation of polyphosphoinositol lipids is believed to be an important component of cellular regulation, in part because it increases the effective concentration of that protein relative to other proteins in the same membrane by restricting it to a two-dimensional space. However, such a concept assumes that, once translocated, a protein retains the free mobility it had in the cytoplasm, and also that the possible existence of partitioned pools of inositol lipids does not restrict its sphere of influence. We have explored by fluorescence recovery after photobleaching (FRAP) the mobility of four green-fluorescent-protein-tagged proteins, GAP1(IP4BP) and GAP1(m), when they are either cytoplasmic or attached to the plasma membrane, and the PH domain of PI-PLCdelta(1) and ICAM as representative of, respectively, another inositol-lipid-anchored protein and a single-transmembrane-span-domain protein. The data from GAP1(m) and the PI-PLCdelta(1) PH domain show that, when proteins associate with inositol lipids in the plasma membrane, they retain a mobility similar to that in the cytoplasm, and probably also similar to the inositol lipid to which they are attached, suggesting a free diffusion within the plane of the membrane. Moreover, this free diffusion is similar whether they are bound to PtdIns(3,4,5)P(3) or to PtdIns(4,5)P(2), and no evidence was found by these criteria for restricted pools of PtdIns(4,5)P(2). The mobility of GAP1(IP4BP), which has been reported to associate with PtdIns(4,5)P(2) in the plasma membrane, is much lower, suggesting that it might interact with other cellular components. Moreover, the mobility of GAP1(IP4BP) is not detectably altered by the generation of either of its two potential regulators, Ins(1,3,4,5)P(4) or PtdIns(3,4,5)P(3).

Chronic activation of protein kinase Bβ/Akt2 leads to multinucleation and cell fusion in human epithelial kidney cells: events associated with tumorigenesis

Most cancers arise from the stepwise accumulation of genetic changes. There is also evidence for defects in the machinery and checkpoints for maintenance of normal diploid chromosome complements, resulting in genetic instability that helps fuel the accumulation of mutations that contribute to the development of cancer. The proto-oncogene protein kinase B (PKB/Akt), and its regulators including phosphatidylinositol 3' kinase and PTEN, has been shown to play critical roles in the regulation of multiple cellular functions such as transcription, cell survival, cell cycle progression, angiogenesis and cell motility--all of which are important to the malignant process. Here, we report the use of a membrane targeted PKBbeta, the activation of which is under the control of a 4-hydroxy-Tamoxifen-responsive estrogen-receptor (ER) ligand binding domain. Induction of PKBbeta-ER activity in human kidney epithelial cells (HEK293) resulted in changes in cellular growth, size, and in the appearance of aneuploid cells. Over time, in a PKBbeta-dependent manner, cells also underwent extensive multinucleation caused due to a combination of both endomitosis and cell fusion. These findings suggest that chronic activation of PKBbeta may contribute to genetic instability and autophagy, properties commonly found in tumor cells.

Role of EGF receptor transactivation in phosphoinositide 3-kinase-dependent activation of MAP kinase by GPCRs

Many G protein coupled receptors (GPCRs) cause phosphorylation of MAP kinases through transactivation of the epidermal growth factor receptor (EGF-R), leading to increased cell survival and growth, motility, and migration. Phosphoinositide 3-kinase (PI3K) is one of the important cell survival signaling molecules activated by EGF-R stimulation. However, the extent to which EGF-R transactivation is essential for GPCR agonist-stimulated PI3K activation is not known. Here we examined the mechanism of PI3K activation that elicits GPCR-mediated ERK1/2 activation by pathways dependent and/or independent of EGF-R transactivation in specific cell types. Immortalized hypothalamic neurons (GT1-7 cells) express endogenous gonadotropin-releasing hormone receptors (GnRH-R) and their stimulation causes marked phosphorylation of ERK1/2 and Akt (Ser 473) through transactivation of the EGF-R and recruitment of PI3K. In C9 hepatocytes, agonist activation of AT1 angiotensin II (AT1-R), lysophosphatidic acid (LPA), and EGF receptors caused phosphorylation of Akt through activation of the EGF-R in a PI3K-dependent manner. However, ERK1/2 activation by these agonists in these cells was independent of PI3K activation. In contrast, agonist stimulation of HEK 293 cells stably expressing AT1-R caused ERK1/2 phosphorylation that was independent of EGF-R transactivation but required PI3K activation. LPA signaling in these cells showed partial and complete dependence on EGF-R and PI3K, respectively. These data indicate that GPCR-induced ERK1/2 phosphorylation is dependent or independent of PI3K in specific cell types, and that the involvement of PI3K during ERK1/2 activation is not dependent solely on agonist-induced transactivation of the EGF-R.

IL-6 activates serum and glucocorticoid kinase via p38alpha mitogen-activated protein kinase pathway

Interleukin-6 (IL-6) has been implicated as an autocrine factor involved in growth of several human cancers, such as tumors arising from the biliary tract or cholangiocarcinoma. In malignant biliary tract epithelia, IL-6 activates the p38 MAPK pathway, which mediates a dominant survival signaling pathway. Serum and glucocorticoid-stimulated kinase (SGK) has been implicated as a survival kinase, but its role in survival signaling by IL-6 is unknown. After IL-6 stimulation, p38 MAPK activation preceded phosphorylation of SGK at Ser78. Pretreatment with the pharmacological inhibitors of p38 MAPK SB-203580 or SB-202190 blocked IL-6-induced SGK phosphorylation at Ser78 and SGK activation. Overexpression of p38alpha increased constitutive SGK phosphorylation at Ser78, whereas dominant negative p38alpha MAPK blocked IL-6-induced SGK phosphorylation and nuclear translocation. Interestingly, in addition to stimulating SGK phosphorylation, both IL-6 stimulation and p38alpha MAPK overexpression increased SGK mRNA and protein expression. An increase in p38 MAPK and SGK occurred following enforced expression of IL-6 in vivo. Furthermore, inhibition of SGK expression by siRNA increased toxicity due to chemotherapeutic drugs. Taken together, these data identify SGK as both a downstream kinase substrate as well as a transcriptionally regulated gene target of p38 MAPK in response to IL-6 and support a role of SGK during survival signaling by IL-6 in human cancers, such as cholangiocarcinoma.

Identification of filamin C as a new physiological substrate of PKBalpha using KESTREL

We detected a protein in rabbit skeletal muscle extracts that was phosphorylated rapidly by PKBa (protein kinase Ba), but not by SGK1 (serum- and glucocorticoid-induced kinase 1), and identified it as the cytoskeletal protein FLNc (filamin C). PKBa phosphorylated FLNc at Ser2213 in vitro, which lies in an insert not present in the FLNa and FLNb isoforms. Ser2213 became phosphorylated when C2C12 myoblasts were stimulated with insulin or epidermal growth factor, and phosphorylation was prevented by low concentrations of wortmannin, at which it is a relatively specific inhibitor of phosphoinositide 3-kinase. PD 184352 [an inhibitor of the classical MAPK (mitogen-activated protein kinase) cascade] and/or rapamycin [an inhibitor of mTOR (mammalian target of rapamycin)] had no effect. Insulin also induced the phosphorylation of FLNc at Ser2213 in cardiac muscle in vivo, but not in cardiac muscle that does not express PDK1 (3-phosphoinositide-dependent kinase 1), the upstream activator of PKB. These results identify the muscle-specific isoform FLNc as a new physiological substrate for PKB.

Roles of Pdk1p, a fission yeast protein related to phosphoinositide-dependent protein kinase, in the regulation of mitosis and cytokinesis

Proteins related to the phosphoinositide-dependent protein kinase family have been identified in the majority of eukaryotes. Although much is known about upstream mechanisms that regulate the PDK1-family of kinases in metazoans, how these kinases regulate cell growth and division remains unclear. Here, we characterize a fission yeast protein related to members of this family, which we have termed Pdk1p. Pdk1p localizes to the spindle pole body and the actomyosin ring in early mitotic cells. Cells deleted for pdk1 display multiple defects in mitosis and cytokinesis, all of which are exacerbated when the function of fission yeast polo kinase, Plo1p, is partially compromised. We conclude that Pdk1p functions in concert with Plo1p to regulate multiple processes such as the establishment of a bipolar mitotic spindle, transition to anaphase, placement of the actomyosin ring and proper execution of cytokinesis. We also present evidence that the effects of Pdk1p on cytokinesis are likely mediated via the fission yeast anillin-related protein, Mid1p, and the septation initiation network.

The activation of Akt/PKB signaling pathway and cell survival

Akt/PKB is a serine/threonine protein kinase that functions as a critical regulator of cell survival and proliferation. Akt/PKB family comprises three highly homologous members known as PKBalpha/Akt1, PKBbeta/Akt2 and PKBgamma/Akt3 in mammalian cells. Similar to many other protein kinases, Akt/PKB contains a conserved domain structure including a specific PH domain, a central kinase domain and a carboxyl-terminal regulatory domain that mediates the interaction between signaling molecules. Akt/PKB plays important roles in the signaling pathways in response to growth factors and other extracellular stimuli to regulate several cellular functions including nutrient metabolism, cell growth, apoptosis and survival. This review surveys recent developments in understanding the molecular mechanisms of Akt/PKB activation and its roles in cell survival in normal and cancer cells.

Phosphoinositide-dependent phosphorylation of PDK1 regulates nuclear translocation

3-phosphoinositide-dependent kinase 1 (PDK1) phosphorylates the activation loop of a number of protein serine/threonine kinases of the AGC kinase superfamily, including protein kinase B (PKB; also called Akt), serum and glucocorticoid-induced kinase, protein kinase C isoforms, and the p70 ribosomal S6 kinase. PDK1 contains a carboxyl-terminal pleckstrin homology domain, which targets phosphoinositide lipids at the plasma membrane and is central to the activation of PKB. However, PDK1 subcellular trafficking to other compartments is not well understood. We monitored the posttranslational modifications of PDK1 following insulin-like growth factor 1 stimulation. PDK1 underwent rapid and transient phosphorylation on S396, which was dependent upon plasma membrane localization. Phosphorylation of S396 was necessary for nuclear shuttling of PDK1, possibly through its influence on an adjacent nuclear export sequence. Thus, mitogen-stimulated phosphorylation of PDK1 provides a means for directed PDK1 subcellular trafficking, with potential implications for PDK1 signaling.

The sphingoid long chain base phytosphingosine activates AGC-type protein kinases in Saccharomyces cerevisiae including Ypk1, Ypk2, and Sch9

The Pkh1 protein kinase of Saccharomyces cerevisiae, a homolog of the mammalian 3-phosphoinositide-dependent kinase (PDK1), regulates downstream AGC-type protein kinases including Ypk1/2 and Pkc1, which control cell wall integrity, growth, and other processes. Phytosphingosine (PHS), a sphingoid long chain base, is hypothesized to be a lipid activator of Pkh1 and thereby controls the activity of Ypk1/2. Here we present biochemical evidence supporting this hypothesis, and in addition we demonstrate that PHS also stimulates autophosphorylation and activation of Ypk1/2. Greatest stimulation of Ypk1/2 phosphorylation and activity are achieved by inclusion of both PHS and Pkh1 in an in vitro kinase reaction. We also demonstrate for the first time that Pkh1 phosphorylates the Sch9 protein kinase in vitro and that such phosphorylation is stimulated by PHS. This is the first biochemical demonstration of Sch9 activators, and the results further support roles for long chain bases in heat stress resistance in addition to implying roles in chronological aging and cell size determination, since Sch9 functions in these processes. Thus, our data support a model in which PHS, rather than simply being an upstream activator of Pkh1, also activates kinases that are downstream targets of Pkh1 including Ypk1/2 and Sch9.

The tRNA methylase METTL1 is phosphorylated and inactivated by PKB and RSK in vitro and in cells

A substrate for protein kinase B (PKB)alpha in HeLa cell extracts was identified as methyltransferase-like protein-1 (METTL1), the orthologue of trm8, which catalyses the 7-methylguanosine modification of tRNA in Saccharomyces cerevisiae. PKB and ribosomal S6 kinase (RSK) both phosphorylated METTL1 at Ser27 in vitro. Ser27 became phosphorylated when HEK293 cells were stimulated with insulin-like growth factor-1 (IGF-1) and this was prevented by inhibition of phosphatidyinositol 3-kinase. The IGF-1-induced Ser27 phosphorylation did not occur in 3-phosphoinositide-dependent protein kinase-1 (PDK1)-deficient embryonic stem cells, but occurred normally in PDK1[L155E] cells, indicating that the effect of IGF-1 is mediated by PKB. METTL1 also became phosphorylated at Ser27 in response to phorbol-12-myristate 13-acetate and this was prevented by PD 184352 or pharmacological inhibition of RSK. Phosphorylation of METTL1 by PKB or RSK inactivated METTL1 in vitro, as did mutation of Ser27 to Asp or Glu. Expression of METTL1[S27D] or METTL1[S27E] did not rescue the growth phenotype of yeast lacking trm8. In contrast, expression of METTL1 or METTL1[S27A] partially rescued growth. These results demonstrate that METTL1 is inactivated by PKB and RSK in cells, and the potential implications of this finding are discussed.

Improved yields for baculovirus-mediated expression of human His(6)-PDK1 and His(6)-PKBbeta/Akt2 and characterization of phospho-specific isoforms for design of inhibitors that stabilize inactive conformations

PDK1 and PKB/Akt have a pleckstrin homology (PH) domain at the C-terminus and N-terminus, respectively, which stabilizes an unphosphorylated, autoinhibited conformation. Binding of the PH domain to a phospholipid second messenger causes relief of autoinhibition, which results in kinase phosphorylation and activation. Baculovirus-mediated expression in Sf9 insect cells of both His(6)-PDK1 and His(6)-PKBbeta/Akt2 were optimized, which significantly improved the yields (5-fold) of the affinity purified enzymes over previously reported values. Isoelectric focusing (IEF) and Western analyses indicated that the apparent V(max)=192+/-13 U/mg and K(m) (PDK-Tide)=55+/-10 microM of purified His(6)-PDK1 results from a mixture of at least three different phospho-specific isoforms (pI values of 6.8, 6.5, and 6.4). A purely unphosphorylated isoform of His(6)-PDK1 (pI=6.8) was generated by treatment with lambda protein phosphatase (lambdaPP), which decreased V(max) to 2.4+/-0.4 U/mg and increased K(m) (PDK-Tide) to 217+/-61 microM. Isoelectric focusing and Western analyses indicated that the apparent V(max)=0.21+/-0.03 U/mg and K(m) (Crosstide)=87+/-30 microM of purified His(6)-PKBbeta/Akt2 results from a mixture of the enzyme monophosphorylated either at Ser-474 ( approximately 90%) or at Thr-309 ( approximately 10%). A purely unphosphorylated isoform of His(6)-PKBbeta/Akt2 (pI=6.4) was generated by treatment with lambdaPP, which decreased V(max) approximately 2-fold. The optimization of high-level production and detailed characterization of purified and lambdaPP-treated His(6)-PDK1 and His(6)-PKBbeta/Akt2 will facilitate detailed structural and kinetic studies aimed at understanding the mechanism of second messenger-induced activation.

Novel small molecule inhibitors of 3-phosphoinositide-dependent kinase-1

The phosphoinositide 3-kinase/3-phosphoinositide-dependent kinase 1 (PDK1)/Akt signaling pathway plays a key role in cancer cell growth, survival, and tumor angiogenesis and represents a promising target for anticancer drugs. Here, we describe three potent PDK1 inhibitors, BX-795, BX-912, and BX-320 (IC(50) = 11-30 nm) and their initial biological characterization. The inhibitors blocked PDK1/Akt signaling in tumor cells and inhibited the anchorage-dependent growth of a variety of tumor cell lines in culture or induced apoptosis. A number of cancer cell lines with elevated Akt activity were >30-fold more sensitive to growth inhibition by PDK1 inhibitors in soft agar than on tissue culture plastic, consistent with the cell survival function of the PDK1/Akt signaling pathway, which is particularly important for unattached cells. BX-320 inhibited the growth of LOX melanoma tumors in the lungs of nude mice after injection of tumor cells into the tail vein. The effect of BX-320 on cancer cell growth in vitro and in vivo indicates that PDK1 inhibitors may have clinical utility as anticancer agents.

Role of T-loop phosphorylation in PDK1 activation, stability, and substrate binding

3-Phosphoinositide-dependent protein kinase-1 (PDK1) phosphorylates the T-loop of several AGC (cAMP-dependent, cGMP-dependent, protein kinase C) family protein kinases, resulting in their activation. Previous structural studies have revealed that the alpha C-helix, located in the small lobe of the kinase domain of PDK1, is a key regulatory element, as it links a substrate interacting site termed the hydrophobic motif (HM) pocket with the phosphorylated Ser-241 in the T-loop. In this study we have demonstrated by mutational analysis that interactions between the phosphorylated Ser-241 and the alpha C-helix are not required for PDK1 activity or substrate binding through the HM-pocket but are necessary for PDK1 to be activated or stabilized by a peptide that binds to this site. The structure of an inactive T-loop mutant of PDK1, in which Ser-241 is changed to Ala, was also determined. This structure, together with surface plasmon resonance binding studies, demonstrates that the PDK1(S241A)-inactive mutant possesses an intact HM-pocket as well as an ordered alpha C-helix. These findings reveal that the integrity of the alpha C-helix and HM-pocket in PDK1 is not regulated by T-loop phosphorylation.

Deficiency of PDK1 in liver results in glucose intolerance, impairment of insulin-regulated gene expression and liver failure

The liver plays an important role in insulin-regulated glucose homoeostasis. To study the function of the PDK1 (3-phosphoinositide-dependent protein kinase-1) signalling pathway in mediating insulin's actions in the liver, we employed CRE recombinase/loxP technology to generate L(liver)-PDK1-/- mice, which lack expression of PDK1 in hepatocytes and in which insulin failed to induce activation of PKB in liver. The L-PDK1-/- mice were not insulin-intolerant, possessed normal levels of blood glucose and insulin under normal feeding conditions, but were markedly glucose-intolerant when injected with glucose. The L-PDK1-/- mice also possessed 10-fold lower levels of hepatic glycogen compared with control littermates, and were unable to normalize their blood glucose levels within 2 h after injection of insulin. The glucose intolerance of the L-PDK1-/- mice may be due to an inability of glucose to suppress hepatic glucose output through the gluconeogenic pathway, since the mRNA encoding hepatic PEPCK (phosphoenolpyruvate carboxykinase), G6Pase (glucose-6-phosphatase) and SREBP1 (sterol-regulatory-element-binding protein 1), which regulate gluconeogenesis, are no longer controlled by feeding. Furthermore, three other insulin-controlled genes, namely IGFBP1 (insulin-like-growth-factor-binding protein-1), IRS2 (insulin receptor substrate 2) and glucokinase, were regulated abnormally by feeding in the liver of PDK1-deficient mice. Finally, the L-PDK1-/- mice died between 4-16 weeks of age due to liver failure. These results establish that the PDK1 signalling pathway plays an important role in regulating glucose homoeostasis and controlling expression of insulin-regulated genes. They suggest that a deficiency of the PDK1 pathway in the liver could contribute to development of diabetes, as well as to liver failure.

“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.

Lithium and bipolar mood disorder: the inositol-depletion hypothesis revisited

Inositol, a simple six-carbon sugar, forms the basis of a number of important intracellular signaling molecules. Over the last 35 years, a series of biochemical and cell biological experiments have shown that lithium (Li(+)) reduces the cellular concentration of myo-inositol and as a consequence attenuates signaling within the cell. Based on these observations, inositol-depletion was proposed as a therapeutic mechanism in the treatment of bipolar mood disorder. Recent results have added significant new dimensions to the original hypothesis. However, despite a number of clinical studies, this hypothesis still remains to be either proven or refuted. In this review of our current knowledge, I will consider where the inositol-depletion hypothesis stands today and how it may be further investigated in the future.

Frontline: The p85alpha isoform of phosphoinositide 3-kinase is essential for a subset of B cell receptor-initiated signaling responses

Phosphoinositide 3-kinase (PI3K) is a ubiquitously expressed signaling enzyme that plays an integral role in development and activation of B cells. B cell receptor (BCR)-driven proliferation is completely blocked either in cells lacking the p85alpha regulatory isoform of PI3K or in wild-type cells treated with pharmacological PI3K inhibitors. However, the contribution of p85alpha to early signaling events has not been fully investigated. Here we show that B cells lacking p85alpha have signaling impairments that are both quantitatively and qualitatively different from those in cells treated with PI3K inhibitors. Loss of p85alpha results in partial reductions in Ca2+ mobilization and IkappaB phosphorylation, whereas ERK phosphorylation is not diminished. Moreover, although Akt phosphorylation is partially reduced, phosphorylation of several proteins downstream of Akt is preserved. These partial impairments suggest that there are other routes to PI3K activation in B cells apart from p85alpha-associated catalytic subunits. Notably, addition of phorbol ester restores BCR-mediated proliferation in p85alpha-deficient cells but not wild-type cells treated with PI3K inhibitors. These findings suggest that the primary BCR signaling defect in B cells lacking p85alpha is a failure to activate diacylglycerol-regulated signaling enzymes, most likely protein kinase C.

Analysis of the LKB1-STRAD-MO25 complex

Mutations in the LKB1 tumour suppressor threonine kinase cause the inherited Peutz-Jeghers cancer syndrome and are also observed in some sporadic cancers. Recent work indicates that LKB1 exerts effects on metabolism, polarity and proliferation by phosphorylating and activating protein kinases belonging to the AMPK subfamily. In vivo, LKB1 forms a complex with STRAD, an inactive pseudokinase, and MO25, an armadillo repeat scaffolding-like protein. Binding of LKB1 to STRAD-MO25 activates LKB1 and re-localises it from the nucleus to the cytoplasm. To learn more about the inherent properties of the LKB1-STRAD-MO25 complex, we first investigated the activity of 34 point mutants of LKB1 found in human cancers and their ability to interact with STRAD and MO25. Interestingly, 12 of these mutants failed to interact with STRAD-MO25. Performing mutagenesis analysis, we defined two binding sites located on opposite surfaces of MO25alpha, which are required for the assembly of MO25alpha into a complex with STRADalpha and LKB1. In addition, we demonstrate that LKB1 does not require phosphorylation of its own T-loop to be activated by STRADalpha-MO25alpha, and discuss the possibility that this unusual mechanism of regulation arises from LKB1 functioning as an upstream kinase. Finally, we establish that STRADalpha, despite being catalytically inactive, is still capable of binding ATP with high affinity, but that this is not required for activation of LKB1. Taken together, our findings reinforce the functional importance of the binding of LKB1 to STRAD, and provide a greater understanding of the mechanism by which LKB1 is regulated and activated through its interaction with STRAD and MO25.

Analysis of insulin signalling by RNAi-based gene silencing

Using siRNA-mediated gene silencing in cultured adipocytes, we have dissected the insulin-signalling pathway leading to translocation of GLUT4 glucose transporters to the plasma membrane. RNAi (RNA interference)-based depletion of components in the putative TC10 pathway (CAP, CrkII and c-Cbl plus Cbl-b) or the phospholipase Cγ pathway failed to diminish insulin signalling to GLUT4. Within the phosphoinositide 3-kinase pathway, loss of the 5′-phosphatidylinositol 3,4,5-trisphosphate phosphatase SHIP2 was also without effect, whereas depletion of the 3′-phosphatase PTEN significantly enhanced insulin action. Downstream of phosphatidylinositol 3,4,5-trisphosphate and PDK1, silencing the genes encoding the protein kinases Akt1/PKBα, or CISK(SGK3) or protein kinases Cλ/ζ had little or no effect, but loss of Akt2/PKBβ significantly attenuated GLUT4 regulation by insulin. These results show that Akt2/PKBβ is the key downstream intermediate within the phosphoinositide 3-kinase pathway linked to insulin action on GLUT4 in cultured adipocytes, whereas PTEN is a potent negative regulator of this pathway.

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.

Insulin signaling and the regulation of glucose transport

Gaps remain in our understanding of the precise molecular mechanisms by which insulin regulates glucose uptake in fat and muscle cells. Recent evidence suggests that insulin action involves multiple pathways, each compartmentalized in discrete domains. Upon activation, the receptor catalyzes the tyrosine phosphorylation of a number of substrates. One family of these, the insulin receptor substrate (IRS) proteins, initiates activation of the phosphatidylinositol 3-kinase pathway, resulting in stimulation of protein kinases such as Akt and atypical protein kinase C. The receptor also phosphorylates the adapter protein APS, resulting in the activation of the G protein TC10, which resides in lipid rafts. TC10 can influence a number of cellular processes, including changes in the actin cytoskeleton, recruitment of effectors such as the adapter protein CIP4, and assembly of the exocyst complex. These pathways converge to control the recycling of the facilitative glucose transporter Glut4.

The in vivo role of PtdIns(3,4,5)P3 binding to PDK1 PH domain defined by knockin mutation

We generated homozygous knockin ES cells expressing a form of 3-phosphoinositide-dependent protein kinase-1 (PDK1) with a mutation in its pleckstrin homology (PH) domain that abolishes phosphatidylinositol 3,4,5-tris-phosphate (PtdIns(3,4,5)P3) binding, without affecting catalytic activity. In the knockin cells, protein kinase B (PKB) was not activated by IGF1, whereas ribosomal S6 kinase (RSK) was activated normally, indicating that PtdIns(3,4,5)P3 binding to PDK1 is required for PKB but not RSK activation. Interestingly, amino acids and Rheb, but not IGF1, activated S6K in the knockin cells, supporting the idea that PtdIns(3,4,5)P3 stimulates S6K through PKB-mediated activation of Rheb. Employing PDK1 knockin cells in which either the PtdIns(3,4,5)P3 binding or substrate-docking 'PIF pocket' was disrupted, we established the roles that these domains play in regulating phosphorylation and stabilisation of protein kinase C isoforms. Moreover, mouse PDK1 knockin embryos in which either the PH domain or PIF pocket was disrupted died displaying differing phenotypes between E10.5 and E11.5. Although PDK1 plays roles in regulating cell size, cells derived from PH domain or PIF pocket knockin embryos were of normal size. These experiments establish the roles of the PDK1 regulatory domains and illustrate the power of knockin technology to probe the physiological function of protein-lipid and protein-protein interactions.