Phosphorylation and activation of cAMP-dependent protein kinase by phosphoinositide-dependent protein kinase

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.

In vivo activation of protein kinase A in Schizosaccharomyces pombe requires threonine phosphorylation at its activation loop and is dependent on PDK1

Phosphoinositide-dependent protein kinase 1 (PDK1) plays a central role in cellular signaling by phosphorylating members of the AGC family of kinases. This family includes protein kinase C (PKC), protein kinase B (PKB), p70/p90 ribosomal S6 kinases (RSK and S6K), and the catalytic subunit of cAMP-dependent protein kinase (PKA). Although PDK1 phosphorylates and activates PKC, PKB, and RSK in vivo, PDK1 regulation of PKA remains controversial. We isolated ksg1, the fission yeast ortholog of mammalian PDK1, as a suppressor of growth defects caused by loss of the stress-activated MAP kinase, Spc1. Here, we demonstrate that Ksg1 is required for activation of PKA. Cells containing the ksg1.12 thermolabile allele exhibit pleiotropic phenotypes, including the failure to arrest in G(1) and an inability to conjugate. The ksg1.12 allele strongly suppresses defects associated with unregulated PKA. Pka1, the catalytic subunit of cAMP-dependent protein kinase, is phosphorylated in vivo at Thr-356, which is located in the activation loop of the kinase and corresponds to Thr-197 in mammalian PKA. Phosphorylation of Thr-356 is required for in vivo activation of Pka1 and is dependent upon Ksg1. These data provide experimental evidence that PKA is a physiological substrate for PDK1.

Activation-loop autophosphorylation is mediated by a novel transitional intermediate form of DYRKs

Autophosphorylation of a critical residue in the activation loop of several protein kinases is an essential maturation event required for full enzyme activity. However, the molecular mechanism by which this happens is unknown. We addressed this question for two dual-specificity tyrosine-phosphorylation-regulated protein kinases (DYRKs), as they autophosphorylate their activation loop on an essential tyrosine but phosphorylate their substrates on serine and threonine. Here we demonstrate that autophosphorylation of the critical activation-loop tyrosine is intramolecular and mediated by the nascent kinase passing through a transitory intermediate form. This DYRK intermediate differs in residue and substrate specificity, as well as sensitivity to small-molecule inhibitors, compared with its mature counterpart. The intermediate's characteristics are lost upon completion of translation, making the critical tyrosine autophosphorylation a "one-off" inceptive event. This mechanism is likely to be shared with other kinases.

Activation of the PI3K/Akt signal transduction pathway and increased levels of insulin receptor in protein repair-deficient mice

Protein L-isoaspartate (D-aspartate) O-methyltransferase is an enzyme that catalyses the repair of isoaspartyl damage in proteins. Mice lacking this enzyme (Pcmt1-/- mice) have a progressive increase in brain size compared with wild-type mice (Pcmt1+/+ mice), a phenotype that can be associated with alterations in the PI3K/Akt signal transduction pathway. Here we show that components of this pathway, including Akt, GSK3beta and PDK-1, are more highly phosphorylated in the brains of Pcmt1-/- mice, particularly in cells of the hippocampus, in comparison with Pcmt1+/+ mice. Examination of upstream elements of this pathway in the hippocampus revealed that Pcmt1-/- mice have increased activation of insulin-like growth factor-I (IGF-I) receptor and/or insulin receptor. Western blot analysis revealed an approximate 200% increase in insulin receptor protein levels and an approximate 50% increase in IGF-I receptor protein levels in the hippocampus of Pcmt1-/- mice. Higher levels of the insulin receptor protein were also found in other regions of the adult brain and in whole tissue extracts of brain, liver, heart and testes of both juvenile and adult Pcmt1-/- mice. There were no significant differences in plasma insulin levels for adult Pcmt1-/- mice during glucose tolerance tests. However, they did show higher peak levels of blood glucose, suggesting a mild impairment in glucose tolerance. We propose that Pcmt1-/- mice have altered regulation of the insulin pathway, possibly as a compensatory response to altered glucose uptake or metabolism or as an adaptive response to a general accumulation of isoaspartyl protein damage in the brain and other tissues.

Consequences of lysine 72 mutation on the phosphorylation and activation state of cAMP-dependent kinase

General strategies to obtain inactive kinases have utilized mutation of key conserved residues in the kinase core, and the equivalent Lys72 in cAMP-dependent kinase has often been used to generate a "dead" kinase. Here, we have analyzed the consequences of this mutation on kinase structure and function. Mutation of Lys72 to histidine (K72H) generated an inactive enzyme, which was unphosphorylated. Treatment with an exogenous kinase (PDK-1) resulted in a mutant that was phosphorylated only at Thr197 and remained inactive but nevertheless capable of binding ATP. Ser338 in K72H cannot be autophosphorylated, nor can it be phosphorylated in an intermolecular process by active wild type C-subunit. The Lys72 mutant, once phosphorylated on Thr197, can bind with high affinity to the RIalpha subunits. Thus a dead kinase can still act as a scaffold for binding substrates and inhibitors; it is only phosphoryl transfer that is defective. Using a potent inhibitor of C-subunit activity, H-89, Escherichia coli-expressed C-subunit was also obtained in its unphosphorylated state. This protein is able to mature into its active form in the presence of PDK-1 and is able to undergo secondary autophosphorylation on Ser338. Unlike the H-89-treated wild type protein, the mutant protein (K72H) cannot undergo the subsequent cis autophosphorylation following phosphorylation at Thr197. Using these two substrates and mammalian-expressed PDK-1, we can elucidate a possible two-step process for the activation of the C-subunit: initial phosphorylation on the activation loop at Thr197 by PDK-1, or a PDK-1-like enzyme, followed by second cis autophosphorylation step at Ser338.

Enhanced dephosphorylation of cAMP-dependent protein kinase by oxidation and thiol modification

The catalytic subunit of cAMP-dependent protein kinase (PKA) is phosphorylated at threonine 197 and serine 338. Phosphorylation of threonine 197, located in the activation loop, is required for coordinating the active site conformation and optimal enzymatic activity. However, this phosphorylation has not been widely appreciated as a regulatory site because of the apparent constitutive nature of the phosphorylation and the general resistance of the kinase to phosphatase treatment. We demonstrate here that the observed resistance of the catalytic subunit to dephosphorylation is due, in part, to the presence of the highly nucleophilic cysteine 199 located proximal to the phosphate on threonine 197. Experiments performed in vitro demonstrated that mutation (cysteine 199 to alanine), oxidation, such as by glutathionylation or internal disulfide bond formation, or alkylation of the C-subunit enhanced its ability to be dephosphorylated. Furthermore, rephosphorylation of reduced C-subunit by PDK1 created a cycle whereby the inactive kinase could be reactivated. To demonstrate that thiol modification of PKA can lead to enhanced dephosphorylation in vivo, PC12 cells were treated with N-ethylmaleimide (NEM). Such treatment resulted in complete PKA inactivation and dephosphorylation of threonine 197. This effect of NEM was contingent upon prior treatment of the cells with PKA activators, demonstrating the resistance of the holoenzyme to thiol alkylation-mediated dephosphorylation. Our results also demonstrated that NEM treatment of PC12 cells enhanced the dephosphorylation of the protein kinase Calpha activation loop, suggesting a common mechanism of regulation among members of the AGC family of kinases.

PDK1, the master regulator of AGC kinase signal transduction

The interaction of insulin and growth factors with their receptors on the outside surface of a cell, leads to the activation of phosphatidylinositol 3-kinase (PI 3-kinase) and generation of the phosphatidylinositol 3,4,5-trisphosphate (PtdIns(3,4,5)P3) second messenger at the inner surface of the cell membrane. One of the most studied signalling events controlled by PtdIns(3,4,5)P3, comprises the activation of a group of AGC family protein kinases, including isoforms of protein kinase B (PKB)/Akt, p70 ribosomal S6 kinase (S6K), serum- and glucocorticoid-induced protein kinase (SGK) and protein kinase C (PKC), which play crucial roles in regulating physiological processes relevant to metabolism, growth, proliferation and survival. Here, we review recent biochemical, genetic and structural studies on the 3-phosphoinositide-dependent protein kinase-1 (PDK1), which phosphorylates and activates the AGC kinase members regulated by PI 3-kinase. We also discuss whether inhibitors of PDK1 might have chemotherapeutic potential in the treatment of cancers in which the PDK1-regulated AGC kinases are constitutively activated.

Flow-dependent regulation of endothelial nitric oxide synthase: role of protein kinases

Vascular endothelial cells are directly and continuously exposed to fluid shear stress generated by blood flow. Shear stress regulates endothelial structure and function by controlling expression of mechanosensitive genes and production of vasoactive factors such as nitric oxide (NO). Though it is well known that shear stress stimulates NO production from endothelial nitric oxide synthase (eNOS), the underlying molecular mechanisms remain unclear and controversial. Shear-induced production of NO involves Ca2+/calmodulin-independent mechanisms, including phosphorylation of eNOS at several sites and its interaction with other proteins, including caveolin and heat shock protein-90. There have been conflicting results as to which protein kinases-protein kinase A, protein kinase B (Akt), other Ser/Thr protein kinases, or tyrosine kinases-are responsible for shear-dependent eNOS regulation. The functional significance of each phosphorylation site is still unclear. We have attempted to summarize the current status of understanding in shear-dependent eNOS regulation.

Scoring and identifying organism-specific functional patterns and putative phosphorylation sites in protein sequences using mutual information

The co-variance of amino acid positions within a multiple alignment of 294 protein kinases from mammals, plants, and bacteria was studied. Applying mutual information (MI), characteristic amino acid sites have been identified markedly discriminating the different organisms. The relation of surface accessibility of these sites in the 3D structure of a kinase and their MI content is studied. We extended the method to score a predicted phosphorylation site of this highly conserved catalytic protein kinase region. Based on this score mammalian and plant protein kinases were grouped together apart from the bacterial kinases. Thus, the presented method allows us to analyse putative phosphorylation sites in the context of their organism-specific origin.

RET/PTC (rearranged in transformation/papillary thyroid carcinomas) tyrosine kinase phosphorylates and activates phosphoinositide-dependent kinase 1 (PDK1): an alternative phosphatidylinositol 3-kinase-independent pathway to activate PDK1

Thyroid cancers are a leading cause of death due to endocrine malignancies. RET/PTC (rearranged in transformation/papillary thyroid carcinomas) gene rearrangements are the most frequent genetic alterations identified in papillary thyroid carcinoma. Although the oncogenic potential of RET/PTC is related to intrinsic tyrosine kinase activity, the substrates for this enzyme are yet to be identified. In this report, we show that phosphoinositide-dependent kinase 1 (PDK1), a pivotal serine/threonine kinase in growth factor-signaling pathways, is a target of RET/PTC. RET/PTC and PDK1 colocalize in the cytoplasm. RET/PTC phosphorylates a specific tyrosine (Y9) residue located in the N-terminal region of PDK1. Y9 phosphorylation of PDK1 by RET/PTC requires an intact catalytic kinase domain. The short (iso 9) and long forms (iso 51) of the RET/PTC kinases (RET/PTC1 and RET/PTC3) induce Y9 phosphorylation of PDK1. Moreover, Y9 phosphorylation of PDK1 by RET/PTC does not require phosphatidylinositol 3-kinase or Src activity. RET/PTC-induced phosphorylation of the Y9 residue results in increased PDK1 activity, decrease of cellular p53 levels, and repression of p53-dependent transactivation. In conclusion, RET/PTC-induced tyrosine phosphorylation of PDK1 may be one of the mechanisms by which it acts as an oncogenic tyrosine kinase in thyroid carcinogenesis.

Amino acids determining enzyme-substrate specificity in prokaryotic and eukaryotic protein kinases

The binding between a PK and its target is highly specific, despite the fact that many different PKs exhibit significant sequence and structure homology. There must be, then, specificity-determining residues (SDRs) that enable different PKs to recognize their unique substrate. Here we use and further develop a computational procedure to discover putative SDRs (PSDRs) in protein families, whereby a family of homologous proteins is split into orthologous proteins, which are assumed to have the same specificity, and paralogous proteins, which have different specificities. We reason that PSDRs must be similar among orthologs, whereas they must necessarily be different among paralogs. Our statistical procedure and evolutionary model identifies such residues by discriminating a functional signal from a phylogenetic one. As case studies we investigate the prokaryotic two-component system and the eukaryotic AGC (i.e., cAMP-dependent PK, cGMP-dependent PK, and PKC) PKs. Without using experimental data, we predict PSDRs in prokaryotic and eukaryotic PKs, and suggest precise mutations that may convert the specificity of one PK to another. We compare our predictions with current experimental results and obtain considerable agreement with them. Our analysis unifies much of existing data on PK specificity. Finally, we find PSDRs that are outside the active site. Based on our results, as well as structural and biochemical characterizations of eukaryotic PKs, we propose the testable hypothesis of "specificity via differential activation" as a way for the cell to control kinase specificity.

Genetic redox preconditioning differentially modulates AP-1 and NF kappa B responses following cardiac ischemia/reperfusion injury and protects against necrosis and apoptosis

Reactive oxygen species have been established as key mediators of cardiac injury following ischemia/reperfusion (I/R). We hypothesized that superoxide formation at different subcellular locations following cardiac I/R injury may differentially regulate cellular responses that determine pathophysiologic outcomes. Recombinant adenoviruses expressing Cu/ZnSOD or MnSOD were utilized to modulate superoxide levels in the cytoplasmic or mitochondrial compartments, respectively, prior to coronary artery I/R injury in the rat heart. Ectopic expression of both MnSOD and Cu/ZnSOD afforded protection from I/R injury, as evidenced by a significant reduction in serum creatine kinase levels, infarct size, malondialdehyde levels, and apoptotic cell death in comparison to controls. MnSOD and Cu/ZnSOD expression also significantly altered the kinetics of NF kappa B and AP-1 activation following I/R injury, characterized by a delayed induction of NF kappa B and abrogated AP-1 response. Western blot analysis of Bcl-2, Bcl-xL, Bad, Caspase 3, PDK1, and phospho-Akt also revealed SOD-mediated changes in gene expression consistent with protection and decreased apoptosis. These findings support the notion that both mitochondrial and cytoplasmic-derived SOD induce changes in AP-1 and NF kappa B activity, creating an antiapoptotic microenvironment within cardiomyocytes that affords protection following I/R injury.

Regulation of the ABC kinases by phosphorylation: protein kinase C as a paradigm

Phosphorylation plays a central role in regulating the activation and signalling lifetime of protein kinases A, B (also known as Akt) and C. These kinases share three conserved phosphorylation motifs: the activation loop segment, the turn motif and the hydrophobic motif. This review focuses on how phosphorylation at each of these sites regulates the maturation, signalling and down-regulation of PKC as a paradigm for how these sites control the function of the ABC kinases.

Expression of the serum- and glucocorticoid-inducible protein kinase, Sgk, is a cell survival response to multiple types of environmental stress stimuli in mammary epithelial cells

The effects of multiple stress stimuli on the cellular utilization of the serum- and glucocorticoid-inducible protein kinase (Sgk) were examined in NMuMg mammary epithelial cells exposed to hyperosmotic stress induced by the organic osmolyte sorbitol, heat shock, ultraviolet irradiation, oxidative stress induced by hydrogen peroxide, or to dexamethasone, a synthetic glucocorticoid that represents a general class of physiological stress hormones. Each of the stress stimuli induced Sgk protein expression with differences in the kinetics and duration of induction and in subcellular localization. The environmental stresses, but not dexamethasone, stimulated Sgk expression through a p38/MAPK-dependent pathway. In each case, a hyperphosphorylated active Sgk protein was produced under conditions in which Akt, the close homolog of Sgk, remained in its non-phosphorylated state. Ectopic expression of wild type Sgk or of the T256D/S422D mutant Sgk that mimics phosphorylation conferred protection against stress-induced cell death in NMuMg cells. In contrast, expression of the T256A/S422A Sgk phosphorylation site mutant has no effect on cell survival. Sgk is known to phosphorylate and negatively regulate pro-apoptotic forkhead transcription factor FKHRL1. The environmental stress stimuli that induce Sgk, but not dexamethasone, strongly inhibited the nuclear transcriptional activity and increased the cytoplasmic retention of FKHRL1. Also, the conditional IPTG inducible expression of wild type Sgk, but not of the kinase dead T256A mutant Sgk, protected Con8 mammary epithelial tumor cells from serum starvation-induced apoptosis. Taken together, our study establishes that induction of enzymatically active Sgk functions as a key cell survival component in response to different environmental stress stimuli.

Autophosphorylation kinetics of protein kinases

Protein kinases play a central role in cellular signal transduction, by transmitting biochemical information between activated membrane-bound receptors and physiological target proteins. In addition to phosphorylating other proteins, almost all protein kinases catalyse autophosphorylation reactions (i.e. reactions in which the kinase serves as its own substrate). The autophosphorylation reactions can be intramolecular or intermolecular. In the present study, a detailed kinetic analysis of the intermolecular autophosphorylation reaction is presented. On the basis of the kinetic equations, a new procedure is developed to evaluate the kinetic parameters of the autophosphorylation reaction. This method was used to analyse the intermolecular autophosphorylation of an S6/H4 kinase from human placenta. At a fixed ATP concentration of 0.125 mM, the apparent catalytic-centre activity (turnover number; k (cat)) and apparent Michaelis-Menten constant ( K (m)) for the autophosphorylation reaction were determined to be 0.91 min(-1) and 0.86 microM respectively.

Phosphorylation of the catalytic subunit of protein kinase A. Autophosphorylation versus phosphorylation by phosphoinositide-dependent kinase-1

The identification of phosphoinositide-dependent kinase-1 (PDK-1) as an activating kinase for members of the AGC family of kinases has led to its implication as the activating kinase for cAMP-dependent protein kinase. It has been established in vitro that PDK-1 can phosphorylate the catalytic (C) subunit (), but the Escherichia coli-expressed C-subunit undergoes autophosphorylation. To assess which of these mechanisms occurs in mammalian cells, a set of mutations was engineered flanking the site of PDK-1 phosphorylation, Thr-197, on the activation segment of the C-subunit. Two distinct requirements appeared for autophosphorylation and phosphorylation by PDK-1. Autophosphorylation was disrupted by mutations that compromised activity (Thr-201 and Gly-200) or altered substrate recognition (Arg-194). Conversely, only residues peripheral to Thr-197 altered PDK-1 phosphorylation, including a potential hydrophobic PDK-1 binding site at the C terminus. To address the in vivo requirements for phosphorylation, select mutant proteins were transfected into COS-7 cells, and their phosphorylation state was assessed with phospho-specific antibodies. The phosphorylation pattern of these mutant proteins indicates that autophosphorylation is not the maturation mechanism in the eukaryotic cell; instead, a heterologous kinase with properties resembling the in vitro characteristics of PDK-1 is responsible for in vivo phosphorylation of PKA.

The phospholipids sphingosine-1-phosphate and lysophosphatidic acid prevent apoptosis in osteoblastic cells via a signaling pathway involving G(i) proteins and phosphatidylinositol-3 kinase

The naturally occurring phospholipids lysophosphatidic acid (LPA) and sphingosine-1-phosphate (S1P) have recently emerged as bioactive compounds that exert mitogenic effects in many cell types, including osteoblasts. In the current study, we examined the ability of each of these compounds to influence osteoblast survival. Using terminal deoxynucleotidyl transferase-mediated deoxyuridine 5'-triphosphate nick-end labeling and DNA fragmentation assays, we found that both LPA and S1P dose-dependently inhibited (by at least 50% and 40%, respectively) the apoptosis induced by serum withdrawal in cultures of primary calvarial rat osteoblasts and SaOS-2 cells. The antiapoptotic effects were inhibited by pertussis toxin, wortmannin, and LY294002, implicating G(i) proteins and phosphatidylinositol-3 kinase (PI-3 kinase) in the signaling pathway that mediates phospholipid-induced osteoblast survival. Specific inhibitors of p42/44 MAPK signaling did not block LPA- or S1P-induced osteoblast survival. LPA and S1P induced PI-3 kinase-dependent activation of p70 S6 kinase, but rapamycin, a specific inhibitor of p70 S6 kinase activation, did not prevent phospholipid-induced osteoblast survival. LPA and S1P also inhibited apoptosis in Swiss 3T3 fibroblastic cells in a G(i) protein-dependent fashion. In fibroblastic cells, however, the antiapoptotic effects of S1P were sensitive to inhibition of both PI-3 kinase and p42/44 MAPK signaling, whereas those of LPA were partially abrogated by inhibitors of p42/44 MAPK signaling but not by PI-3 kinase inhibitors. These data demonstrate that LPA and S1P potently promote osteoblast survival in vitro, and that cell-type specificity exists in the antiapoptotic signaling pathways activated by phospholipids.

Shear stress stimulates phosphorylation of eNOS at Ser(635) by a protein kinase A-dependent mechanism

Shear stress stimulates nitric oxide (NO) production by phosphorylating endothelial NO synthase (eNOS) at Ser(1179) in a phosphoinositide-3-kinase (PI3K)- and protein kinase A (PKA)-dependent manner. The eNOS has additional potential phosphorylation sites, including Ser(116), Thr(497), and Ser(635). Here, we studied these potential phosphorylation sites in response to shear, vascular endothelial growth factor (VEGF), and 8-bromocAMP (8-BRcAMP) in bovine aortic endothelial cells (BAEC). All three stimuli induced phosphorylation of eNOS at Ser(635), which was consistently slower than that at Ser(1179). Thr(497) was rapidly dephosphorylated by 8-BRcAMP but not by shear and VEGF. None of the stimuli phosphorylated Ser(116). Whereas shear-stimulated Ser(635) phosphorylation was not affected by phosphoinositide-3-kinase inhibitors wortmannin and LY-294002, it was blocked by either treating the cells with a PKA inhibitor H89 or infecting them with a recombinant adenovirus-expressing PKA inhibitor. These results suggest that shear stress stimulates eNOS by two different mechanisms: 1) PKA- and PI3K-dependent and 2) PKA-dependent but PI3K-independent pathways. Phosphorylation of Ser(635) may play an important role in chronic regulation of eNOS in response to mechanical and humoral stimuli.

Duality of G protein-coupled mechanisms for beta-adrenergic activation of NKCC activity in skeletal muscle

Skeletal muscle Na(+)-K(+)-2Cl(-) cotransporter (NKCC) activity provides a potential mechanism for regulated K(+) uptake. beta-Adrenergic receptor (beta-AR) activation stimulates skeletal muscle NKCC activity in a MAPK pathway-dependent manner. We examined potential G protein-coupled pathways for beta-AR-stimulated NKCC activity. Inhibition of G(s)-coupled PKA blocked isoproterenol-stimulated NKCC activity in both the slow-twitch soleus muscle and the fast-twitch plantaris muscle. However, the PKA-activating agents cholera toxin, forskolin, and 8-bromo-cAMP (8-BrcAMP) were not sufficient to activate NKCC in the plantaris and partially stimulated NKCC activity in the soleus. Isoproterenol-stimulated NKCC activity in the soleus was abolished by pretreatment with pertussis toxin (PTX), indicating a G(i)-coupled mechanism. PTX did not affect the 8-BrcAMP-stimulated NKCC activity. PTX treatment also precluded the isoproterenol-mediated ERK1/2 MAPK phosphorylation in the soleus, consistent with NKCC's MAPK dependency. Inhibition of isoproterenol-stimulated ERK activity by PTX treatment was associated with an increase in Akt activation and phosphorylation of Raf-1 on the inhibitory residue Ser(259). These results demonstrate a novel, muscle phenotype-dependent mechanism for beta-AR-mediated NKCC activation that involves both G(s) and G(i) protein-coupled mechanisms.

Role of PKC isoforms in glucose transport in 3T3-L1 adipocytes: insignificance of atypical PKC

To elucidate the involvement of protein kinase C (PKC) isoforms in insulin-induced and phorbol ester-induced glucose transport, we expressed several PKC isoforms, conventional PKC-alpha, novel PKC-delta, and atypical PKC isoforms of PKC-lambda and PKC-zeta, and their mutants in 3T3-L1 adipocytes using an adenovirus-mediated gene transduction system. Endogenous expression and the activities of PKC-alpha and PKC-lambda/zeta, but not of PKC-delta, were detected in 3T3-L1 adipocytes. Overexpression of each wild-type PKC isoform induced a large amount of PKC activity in 3T3-L1 adipocytes. Phorbol 12-myristrate 13-acetate (PMA) activated PKC-alpha and exogenous PKC-delta but not atypical PKC-lambda/zeta. Insulin also activated the overexpressed PKC-delta but not PKC-alpha. Expression of the wild-type PKC-alpha or PKC-delta resulted in significant increases in glucose transport activity in the basal and PMA-stimulated states. Dominant-negative PKC-alpha expression, which inhibited the PMA activation of PKC-alpha, decreased in PMA-stimulated glucose transport. Glucose transport activity in the insulin-stimulated state was increased by the expression of PKC-delta but not of PKC-alpha. These findings demonstrate that both conventional and novel PKC isoforms are involved in PMA-stimulated glucose transport and that other novel PKC isoforms could participate in PMA-stimulated and insulin-stimulated glucose transport. Atypical PKC-lambda/zeta was not significantly activated by insulin, and expression of the wild-type, constitutively active, and dominant-negative mutants of atypical PKC did not affect either basal or insulin-stimulated glucose transport. Thus atypical PKC enzymes do not play a major role in insulin-stimulated glucose transport in 3T3-L1 adipocytes.

Differential transcriptional regulation by the alpha- and gamma-catalytic subunit isoforms of cAMP-dependent protein kinase

The C gamma and C alpha isoforms of the cAMP-dependent protein kinase (PKA) share 83% identity including all critical catalytic and substrate-binding residues defined to date. Compared to C alpha, C gamma has a different substrate specificity and a selective pseudosubstrate specificity, exhibiting inhibition by regulatory subunits, but not by the protein kinase inhibitor. In these studies, C gamma-mediated gene transcription regulation was compared with that of C alpha in four cell lines using transient transfection/dual luciferase assays. As compared to C gamma, C alpha more efficiently activated a cAMP-response element (CRE)-regulated fragment of the human alpha-glycoprotein hormone promoter which was coupled to a firefly luciferase reporter gene (pGH alpha-fluc). This occurred in Cos7, Y1, and Kin8 adrenal cells by 23-, 6.5-, and 1.4-fold, respectively. In contrast, C gamma, but not C alpha, activated the Sp1RE-regulated herpes simplex virus thymidine kinase promoter which was coupled to a Renilla luciferase reporter (pTK-rluc). In Sp1-deficient Sf9 cells, pGH alpha-fluc expression was maintained for both isoforms, but cotransfection with an Sp1 expression plasmid was necessary and sufficient for activation of pTK-rluc expression by C gamma. In all cell lines, cotransfection with a PDK1 expression plasmid enhanced the transcriptional activation of both C alpha and C gamma (1.5- to 3-fold), while a catalytically inactive PDK1 mutant (PDK.KD) did not. These results suggest that both C alpha and C gamma can activate CRE-responsive genes; however, C alpha does so with better efficiency than C gamma. In contrast to C alpha, C gamma activates transcription of genes containing pTK-like Sp1RE sites. Activation of different C subunit isoforms can provide a means to diversify cAMP-mediated transcription, possibly affecting cell phenotype.

Cell cycle-regulated phosphorylation of the Xenopus polo-like kinase Plx1

Polo-like kinases (Plks) control multiple important events during M phase progression, but little is known about their activation during the cell cycle. The activities of both mammalian Plk1 and Xenopus Plx1 peak during M phase, and this activation has been attributed to phosphorylation. However, no phosphorylation sites have previously been identified in any member of the Plk family. Here we have combined tryptic phosphopeptide mapping with mass spectrometry to identify four major phosphorylation sites in Xenopus Plx1. All four sites appear to be phosphorylated in a cell cycle-dependent manner. Phosphorylations at two sites (Ser-260 and Ser-326) most likely represent autophosphorylation events, whereas two other sites (Thr-201 and Ser-340) are targeted by upstream kinases. Several recombinant kinases were tested for their ability to phosphorylate Plx1 in vitro. Whereas xPlkk1 phosphorylated primarily Thr-10, Thr-201 was readily phosphorylated by protein kinase A, and Cdk1/cyclin B was identified as a likely kinase acting on Ser-340. Phosphorylation of Ser-340 was shown to be responsible for the retarded electrophoretic mobility of Plx1 during M phase, and phosphorylation of Thr-201 was identified as a major activating event.

Activation of SGK1 by HGF, Rac1 and integrin-mediated cell adhesion in MDCK cells: PI-3K-dependent and -independent pathways

The SGK1 protein belongs to the AGC gene family of kinases that are regulated by phosphorylation mediated by PDK1. SGK1 regulation is accomplished by several pathways including growth-factor and stress-mediated signaling. We have expanded the analysis of SGK1 regulation in epithelial cells. We used HA-tagged SGK1 to transiently transfect MDCK cells and study the regulation of SGK1 upon stimulation with HGF, cAMP or upon adhesion of the cells to immobilized fibronectin. In addition, we studied the regulation of SGK1 activity by small GTP-binding proteins of the Rho family.

Treatment of MDCK cells with HGF leads to a time-dependent activation of SGK1 that is blocked by wortmanin. This activation requires the conserved phosphorylation site present in the activation loop of the kinase (T256 in SGK1) and the phosphorylation site present in a hydrophobic domain at its C-terminus (S422 in SGK1), which are targets for PDK1/PDK2-mediated regulation of SGK1. We tested whether SGK1 could be activated by cAMP as it contains a putative PKA site. We were unable to demonstrate a significant activation of HA-SGK1 by cAMP stimulation under conditions where we detect cAMP-mediated phosphorylation of the transcription factor CREB.

Cotransfection of SGK1 with activated small GTP-binding proteins revealed that Rac1, but not Rho or Rap1, induces activation of SGK1. However, this activation was wortmanin insensitive and dominant-negative Rac1 did not inhibit the HGF-mediated activation of SGK1. Adhesion of MDCK cells to immobilized fibronectin also leads to activation of SGK1. However, it appears that the integrin-mediated activation of HA-SGK1 differs from AKT activation in the fact that AKT phosphorylation was blocked by wortmanin (or LY294002)whereas HA-SGK1 was not. The adhesion-dependent activation, however, requires the intact phosphorylation sites of SGK1. Co-transfection of HA-SGK1 with RacV12 results in increased activity in adherent cells compared with HA-SGK1 alone. Since RacN17 failed to inhibit adhesion dependent-activation of SGK1,it suggests that integrin activation is achieved by a parallel Rac-independent pathway.

The activation of SGK1 by HGF and integrin provides a link between HGF-mediated protection of MDCK from de-attachment induced apoptosis(anoikis). We demonstrate that dephosphorylation of the transcription factor FKRHL1 induced by cell de-attachment is prevented by activated SGK1,suggesting that SGK1 regulates cell survival pathways.

In summary, we demonstrate that SGK1 activation could be achieved through signaling pathways involved in the regulation of cell survival, cell-cell and cell-matrix interactions. SGK1 activation can be accomplished via HGF,PI-3K-dependent pathways and by integrin-mediated, PI-3K independent pathways. In addition, activation of SGK1 by the small GTP-binding protein Rac1 has been observed.

Inhibition of Xenopus oocyte meiotic maturation by catalytically inactive protein kinase A

Progesterone induces G2-arrested Xenopus oocytes to develop into fertilizable eggs in a process called meiotic maturation. Protein kinase A (PKA), the cAMP-dependent protein kinase, has long been known to be a potent inhibitor of meiotic maturation, but little information is available on how PKA functions. We have cloned two Xenopus PKA catalytic subunit isoforms, XPKAalpha and XPKAbeta. These proteins are 89% identical and both inhibit progesterone-induced meiotic maturation when overexpressed at low levels, suggesting that PKA activity is tightly regulated in the oocyte. Unexpectedly, catalytically inactive XPKA mutants are able to block progesterone-induced maturation as efficiently as the wild-type active XPKA. These mutants also block meiotic maturation induced by Mos, but are less efficient at inhibiting Cdc25C-induced maturation. Our results indicate that PKA can inhibit meiotic maturation by a novel mechanism, which does not require its kinase activity and is also independent of binding to the PKA regulatory subunits.

Differential signaling of cyclic AMP: opposing effects of exchange protein directly activated by cyclic AMP and cAMP-dependent protein kinase on protein kinase B activation

The recent discovery of Epac, a novel cAMP receptor protein, opens up a new dimension in studying cAMP-mediated cell signaling. It is conceivable that many of the cAMP functions previously attributed to cAMP-dependent protein kinase (PKA) are in fact also Epac-dependent. The finding of an additional intracellular cAMP receptor provides an opportunity to further dissect the divergent roles that cAMP exerts in different cell types. In this study, we probed cross-talk between cAMP signaling and the phosphatidylinositol 3-kinase/PKB pathways. Specifically, we examined the modulatory effects of cAMP on PKB activity by monitoring the specific roles that Epac and PKA play individually in regulating PKB activity. Our study suggests a complex regulatory scheme in which Epac and PKA mediate the opposing effects of cAMP on PKB regulation. Activation of Epac leads to a phosphatidylinositol 3-kinase-dependent PKB activation, while stimulation of PKA inhibits PKB activity. Furthermore, activation of PKB by Epac requires the proper subcellular targeting of Epac. The opposing effects of Epac and PKA on PKB activation provide a potential mechanism for the cell type-specific differential effects of cAMP. It is proposed that the net outcome of cAMP signaling is dependent upon the dynamic abundance and distribution of intracellular Epac and PKA.

Shear stress stimulates phosphorylation of endothelial nitric-oxide synthase at Ser1179 by Akt-independent mechanisms: role of protein kinase A

Recently, we have shown that shear stress stimulates NO(*) production by the protein kinase B/Akt (Akt)-dependent mechanisms in bovine aortic endothelial cells (BAEC) (Go, Y. M., Boo, Y. C., Park, H., Maland, M. C., Patel, R., Pritchard, K. A., Jr., Fujio, Y., Walsh, K., Darley-Usmar, V., and Jo, H. (2001) J. Appl. Physiol. 91, 1574-1581). Akt has been believed to regulate shear-dependent production of NO(*) by directly phosphorylating endothelial nitric-oxide synthase (eNOS) at the Ser(1179) residue (eNOS-S(1179)), but a critical evaluation using specific inhibitors or dominant negative mutants (Akt(AA) or Akt(AAA)) has not been reported. In addition, other kinases, including protein kinase A (PKA) and AMP kinase have also shown to phosphorylate eNOS-S(1179). Here, we show that shear-dependent phosphorylation of eNOS-S(1179) is mediated by an Akt-independent, but a PKA-dependent, mechanism. Expression of Akt(AA) or Akt(AAA) in BAEC by using recombinant adenoviral constructs inhibited phosphorylation of eNOS-S(1179) if cells were stimulated by vascular endothelial growth factor (VEGF), but not by shear stress. As shown before, expression of Akt(AA) inhibited shear-dependent NO(*) production, suggesting that Akt is still an important regulator in NO production. Further studies showed that a selective inhibitor of PKA, H89, inhibited shear-dependent phosphorylation of eNOS-S(1179) and NO(*) production. In contrast, H89 did not inhibit phosphorylation of eNOS-S(1179) induced by expressing a constitutively active Akt mutant (Akt(Myr)) in BAEC, showing that the inhibitor did not affect the Akt pathway. 8-Bromo-cAMP alone phosphorylated eNOS-S(1179) within 5 min without activating Akt, in an H89-sensitive manner. Collectively, these results demonstrate that shear stimulates phosphorylation of eNOS-S(1179) in a PKA-dependent, but Aktindependent manner, whereas the NO(*) production is regulated by the mechanisms dependent on both PKA and Akt. A coordinated interaction between Akt and PKA may be an important mechanism by which eNOS activity is regulated in response to physiological stimuli such as shear stress.

Vascular endothelial cells express isoforms of protein kinase A inhibitor

The expression and function of the endogenous inhibitor of cAMP-dependent protein kinase (PKI) in endothelial cells are unknown. In this study, overexpression of rabbit muscle PKI gene into endothelial cells inhibited the cAMP-mediated increase and exacerbated thrombin-induced decrease in endothelial barrier function. We investigated PKI expression in human pulmonary artery (HPAECs), foreskin microvessel (HMECs), and brain microvessel endothelial cells (HBMECs). RT-PCR using specific primers for human PKI alpha, human PKI gamma, and mouse PKI beta sequences detected PKI alpha and PKI gamma mRNA in all three cell types. Sequencing and BLAST analysis indicated that forward and reverse DNA strands for PKI alpha and PKI gamma were of >96% identity with database sequences. RNase protection assays showed protection of the 542 nucleotides in HBMEC and HPAEC PKI alpha mRNA and 240 nucleotides in HBMEC, HPAEC, and HMEC PKI gamma mRNA. Western blot analysis indicated that PKI gamma protein was detected in all three cell types, whereas PKI alpha was found in HBMECs. In summary, endothelial cells from three different vascular beds express PKI alpha and PKI gamma, which may be physiologically important in endothelial barrier function.

Identification of tyrosine phosphorylation sites on 3-phosphoinositide-dependent protein kinase-1 and their role in regulating kinase activity

3-Phosphoinositide-dependent protein kinase-1 (PDK1) plays a central role in signal transduction pathways that activate phosphoinositide 3-kinase. Despite its key role as an upstream activator of enzymes such as protein kinase B and p70 ribosomal protein S6 kinase, the regulatory mechanisms controlling PDK1 activity are poorly understood. PDK1 has been reported to be constitutively active in resting cells and not further activated by growth factor stimulation (Casamayor, A., Morrice, N. A., and Alessi, D. R. (1999) Biochem. J. 342, 287-292). Here, we report that PDK1 becomes tyrosine-phosphorylated and translocates to the plasma membrane in response to pervanadate and insulin. Following pervanadate treatment, PDK1 kinase activity increased 1.5- to 3-fold whereas the activity of PDK1 associated with the plasma membrane increased approximately 6-fold. The activity of PDK1 localized to the plasma membrane was also increased by insulin treatment. Three tyrosine phosphorylation sites of PDK1 (Tyr-9 and Tyr-373/376) were identified using in vivo labeling and mass spectrometry. Using site-directed mutants, we show that, although phosphorylation on Tyr-373/376 is important for PDK1 activity, phosphorylation on Tyr-9 has no effect on the activity of the kinase. Both of these residues can be phosphorylated by v-Src tyrosine kinase in vitro, and co-expression of v-Src leads to tyrosine phosphorylation and activation of PDK1. Thus, these data suggest that PDK1 activity is regulated by reversible phosphorylation, possibly by a member of the Src kinase family.

Two splice variants of protein kinase B gamma have different regulatory capacity depending on the presence or absence of the regulatory phosphorylation site serine 472 in the carboxyl-terminal hydrophobic domain

We have reported previously the cloning and characterization of human and mouse protein kinase B gamma (PKB gamma), the third member of the PKB family of second messenger-regulated serine/threonine kinases (Brodbeck, D., Cron, P., and Hemmings, B. A. (1999) J. Biol. Chem. 274, 9133--9136). Here we report the isolation of human and mouse PKB gamma 1, a splice variant lacking the second regulatory phosphorylation site Ser-472 in the hydrophobic C-terminal domain. Expression of PKB gamma 1 is low compared with PKB gamma, and it is regulated in different human tissues. We show that PKB gamma and PKB gamma 1 differ in their response to stimulation by insulin, pervanadate, peroxide, or okadaic acid. Activation of PKB gamma 1 requires phosphorylation at a single regulatory site Thr-305. Interestingly, this site is phosphorylated to a higher extent in PKB gamma compared with PKB gamma 1 upon maximal stimulation by pervanadate, and this is reflected in the respective specific kinase activities. Furthermore, upon insulin stimulation of transfected cells, PKB gamma 1 translocates to the plasma membrane to a lesser extent than PKB gamma. Taken together, these results suggest that phosphorylation of the hydrophobic motif at the extreme C terminus of PKB gamma may facilitate translocation of the kinase to the membrane and/or its phosphorylation on the activation loop site by phosphoinositide-dependent protein kinase-1.

Drosophila phosphoinositide-dependent kinase-1 regulates apoptosis and growth via the phosphoinositide 3-kinase-dependent signaling pathway

Phosphoinositide-dependent kinase-1 (PDK-1) is a central mediator of the cell signaling between phosphoinositide 3-kinase (PI3K) and various intracellular serine/threonine kinases including Akt/protein kinase B (PKB), p70 S6 kinases, and protein kinase C. Recent studies with cell transfection experiments have implied that PDK-1 may be involved in various cell functions including cell growth and apoptosis. However, despite its pivotal role in cellular signalings, the in vivo functions of PDK-1 in a multicellular system have rarely been investigated. Here, we have isolated Drosophila PDK-1 (dPDK-1) mutants and characterized the in vivo roles of the kinase. Drosophila deficient in the dPDK-1 gene exhibited lethality and an apoptotic phenotype in the embryonic stage. Conversely, overexpression of dPDK-1 increased cell and organ size in a Drosophila PI3K-dependent manner. dPDK-1 not only could activate Drosophila Akt/PKB (Dakt1), but also substitute the in vivo functions of its mammalian ortholog to activate Akt/PKB. This functional interaction between dPDK-1 and Dakt1 was further confirmed through genetic analyses in Drosophila. On the other hand, cAMP-dependent protein kinase, which has been proposed as a possible target of dPDK-1, did not interact with dPDK-1. In conclusion, our findings provide direct evidence that dPDK-1 regulates cell growth and apoptosis during Drosophila development via the PI3K-dependent signaling pathway and demonstrate our Drosophila system to be a powerful tool for elucidating the in vivo functions and targets of PDK-1.

Disruption of 3-phosphoinositide-dependent kinase 1 (PDK1) signaling by the anti-tumorigenic and anti-proliferative agent n-alpha-tosyl-l-phenylalanyl chloromethyl ketone

The anti-tumorigenic and anti-proliferative effects of N-alpha-tosyl-l-phenylalanyl chloromethyl ketone (TPCK) have been known for more than three decades. Yet little is known about the discrete cellular targets of TPCK controlling these effects. Previous work from our laboratory showed TPCK, like the immunosuppressant rapamycin, to be a potent inhibitor of the 70-kilodalton ribosomal S6 kinase 1 (S6K1), which mediates events involved in cell growth and proliferation. We show here that rapamycin and TPCK display distinct inhibitory mechanisms on S6K1 as a rapamycin-resistant form of S6K1 was TPCK-sensitive. Additionally, we show that TPCK inhibited the activation of the related kinase and proto-oncogene Akt. Upstream regulators of S6K1 and Akt include phosphoinositide 3-kinase (PI 3-K) and 3-phosphoinositide-dependent kinase 1 (PDK1). Whereas TPCK had no effect on either mitogen-regulated PI 3-K activity or total cellular PDK1 activity, TPCK prevented phosphorylation of the PDK1 regulatory sites in S6K1 and Akt. Furthermore, whereas both PDK1 and the mitogen-activated protein kinase (MAPK) are required for full activation of the 90-kilodalton ribosomal S6 kinase (RSK), TPCK inhibited RSK activation without inhibiting MAPK activation. Consistent with the capacity of RSK and Akt to mediate a cell survival signal, in part through phosphorylation of the pro-apoptotic protein BAD, TPCK reduced BAD phosphorylation and led to cell death in interleukin-3-dependent 32D cells. Finally, in agreement with results seen in embryonic stem cells lacking PDK1, protein kinase A activation was not inhibited by TPCK showing TPCK specificity for mitogen-regulated PDK1 signaling. TPCK inhibition of PDK1 signaling thus disables central kinase cascades governing diverse cellular processes including proliferation and survival and provides an explanation for its striking biological effects.

Protein kinase SGK mediates survival signals by phosphorylating the forkhead transcription factor FKHRL1 (FOXO3a)

Serum- and glucocorticoid-inducible kinases (SGKs) form a novel family of serine/threonine kinases that are activated in response to a variety of extracellular stimuli. SGKs are related to Akt (also called PKB), a serine/threonine kinase that plays a crucial role in promoting cell survival. Like Akt, SGKs are activated by the phosphoinositide-3 kinase (PI3K) and translocate to the nucleus upon growth factor stimulation. However the physiological substrates and cellular functions of SGKs remained to be identified. We hypothesized that SGKs regulate cellular functions in concert with Akt by phosphorylating common targets within the nucleus. The best-characterized nuclear substrates of Akt are transcription factors of the Forkhead family. Akt phosphorylates Forkhead transcription factors such as FKHRL1, leading to FKHRL1's exit from the nucleus and the consequent shutoff of FKHRL1 target genes. We show here that SGK1, like Akt, promotes cell survival and that it does so in part by phosphorylating and inactivating FKHRL1. However, SGK and Akt display differences with respect to the efficacy with which they phosphorylate the three regulatory sites on FKHRL1. While both kinases can phosphorylate Thr-32, SGK displays a marked preference for Ser-315 whereas Akt favors Ser-253. These findings suggest that SGK and Akt may coordinately regulate the function of FKHRL1 by phosphorylating this transcription factor at distinct sites. The efficient phosphorylation of these three sites on FKHRL1 by SGK and Akt appears to be critical to the ability of growth factors to suppress FKHRL1-dependent transcription, thereby preventing FKHRL1 from inducing cell cycle arrest and apoptosis. These findings indicate that SGK acts in concert with Akt to propagate the effects of PI3K activation within the nucleus and to mediate the biological outputs of PI3K signaling, including cell survival and cell cycle progression.

New signaling pathways for hormones and cyclic adenosine 3',5'-monophosphate action in endocrine cells

The glycoprotein hormones, ACTH, TSH, FSH, and LH regulate diverse functions in endocrine cells. Although cAMP and PKA have long been shown to mediate specific intracellular signaling events including the transcription of specific genes via the CREB-CBP complex, recent observations have indicated that PKA does not account for all of the intracellular targets of cAMP. For example, TSH stimulation of thyroid cell proliferation is not completely blocked by PKA inhibitors. TSH and FSH can stimulate PKB phosphorylation by a PKAindependent but PI3-K/PDK1-dependent pathway. An FSH inducible kinase, Sgk, has recently been shown to be a close relative of PKB. Sgk is also a target of PI3-K-PDK1 pathway, indicating that some effects previously ascribed to PKB may be mediated by this inducible kinase. The identification of novel cAMP-binding proteins that exhibit guanine nucleotide exchange (GEF) activity (cAMP-GEFS; Epacs) has open new doors for cAMP action that include activation of small GTPases such as Rap1a, Rap2, and possibly Ras. These GTPases are known activators of downstream kinase cascades, including p38MAPK and Erk1/2 as well as PI3-K. Thus, FSH and TSH activation of PKB and Sgk may occur via this alternative cAMP pathway that involves cAMP-GEFs and the activation of the PI3-K/PDK1 pathway.

Kinetic basis for activation of CDK2/cyclin A by phosphorylation

The activation of most protein kinases requires phosphorylation at a conserved site within a structurally defined segment termed the activation loop. A classic example is the regulation of the cell cycle control enzyme, CDK2/cyclin A, in which catalytic activation depends on phosphorylation at Thr(160) in CDK2. The structural consequences of phosphorylation have been revealed by x-ray crystallographic studies on CDK2/cyclin A and include changes in conformation, mainly of the activation loop. Here, we describe the kinetic basis for activation by phosphorylation in CDK2/cyclin A. Phosphorylation results in a 100,000-fold increase in catalytic efficiency and an approximate 1,000-fold increase in the overall turnover rate. The effects of phosphorylation on the individual steps in the catalytic reaction pathway were determined using solvent viscosometric techniques. It was found that the increase in catalytic power arises mainly from a 3,000-fold increase in the rate of the phosphoryl group transfer step with a more moderate increase in substrate binding affinity. In contrast, the rate of phosphoryl group transfer in the ATPase pathway was unaffected by phosphorylation, demonstrating that phosphorylation at Thr(160) does not serve to stabilize ATP in the ATPase reaction. Thus, we hypothesize that the role of phosphorylation in the kinase reaction may be to specifically stabilize the peptide phosphoacceptor group.

p21-activated kinase (PAK1) is phosphorylated and activated by 3-phosphoinositide-dependent kinase-1 (PDK1)

In this study, we show that phosphorylated 3-phosphoinositide-dependent kinase 1 (PDK1) phosphorylates p21-activated kinase 1 (PAK1) in the presence of sphingosine. We identify threonine 423, a conserved threonine in the activation loop of kinase subdomain VIII, as the PDK1 phosphorylation site on PAK1. Threonine 423 is a previously identified PAK1 autophosphorylation site that lies within a PAK consensus phosphorylation sequence. After pretreatment with phosphatases, autophosphorylation of PAK1 occurred at all major sites except threonine 423. A phosphothreonine 423-specific antibody detected phosphorylation of recombinant, catalytically inactive PAK1 after incubation with wild-type PAK1, indicating phosphorylation of threonine 423 occurs by an intermolecular mechanism. The biological significance of PDK1 phosphorylation of PAK1 at threonine 423 in vitro is supported by the observation that these two proteins interact in vivo and that PDK1-phosphorylated PAK1 has an increased activity toward substrate. An increase of phosphorylation of catalytically inactive PAK1 was observed in COS-7 cells expressing wild-type, but not catalytically inactive, PDK1 upon elevation of intracellular sphingosine levels. PDK1 phosphorylation of PAK1 was not blocked by pretreatment with wortmannin or when PDK1 was mutated to prevent phosphatidylinositol binding, indicating this process is independent of phosphatidylinositol 3-kinase activity. The data presented here provide evidence for a novel mechanism for PAK1 regulation and activation.

MEKK1 binds raf-1 and the ERK2 cascade components

Mitogen-activated protein (MAP) kinase cascades are involved in transmitting signals that are generated at the cell surface into the cytosol and nucleus and consist of three sequentially acting enzymes: a MAP kinase, an upstream MAP/extracellular signal-regulated protein kinase (ERK) kinase (MEK), and a MEK kinase (MEKK). Protein-protein interactions within these cascades provide a mechanism to control the localization and function of the proteins. MEKK1 is implicated in activation of the c-Jun N-terminal kinase/stress-activated protein kinase (JNK/SAPK) and ERK1/2 MAP kinase pathways. We showed previously that MEKK1 binds directly to JNK/SAPK. In this study we demonstrate that endogenous MEKK1 binds to endogenous ERK2, MEK1, and another MEKK level kinase, Raf-1, suggesting that it can assemble all three proteins of the ERK2 MAP kinase module.

The protein kinase A catalytic subunit Cbeta2: molecular characterization and distribution of the splice variant

Cbeta2, a 46 kDa splice variant of the Cbeta isoform, is the largest isoform so far described for catalytic subunits from cAMP-dependent protein kinase in mammals. It differs from Cbeta in the first 15 N-terminal residues which are replaced with a 62-residue domain with no similarity to other known proteins. The Cbeta2 protein was identified in cardiac tissue by MS, microsequencing and C-subunit-isoform-selective antibodies. The Cbeta2 protein has a very low abundance of about 2% of total affinity-purified C subunits from bovine cardiac tissue. This, and the similarity of its biochemical properties to Calpha and Cbeta, are probably some of the reasons why the Cbeta2 protein has escaped detection so far. The abundance of the Cbeta2 protein differs dramatically between tissues, with most protein detected in heart, liver and spleen, and the lowest level in testis. Cbeta2 protein shows kinase activity against synthetic substrates, and is inhibited by the protein kinase inhibitor peptide PKI(5-24). The degree of Cbeta2 removal from tissue extracts by binding to PKI(5-24) depends on the cAMP level, i.e. on the dissociation state of the holoenzyme. Two sites in the protein are phosphorylated: Thr-244 in the activation segment and Ser-385 close to the C-terminus. By affinity purification and immunodetection Cbeta2 was found in cattle, pig, rat, mouse and turkey tissue and in HeLa cells. In the cAMP-insensitive CHO 10260 cell line, which has normal Cbeta but is depleted of Calpha, stable transfection with Cbeta2 restored most of the cAMP-induced morphological changes. Cbeta2 is a ubiquitously expressed protein with characteristic properties of a cAMP-dependent protein kinase catalytic subunit.

AKT/PKB and other D3 phosphoinositide-regulated kinases: kinase activation by phosphoinositide-dependent phosphorylation

The protein kinase Akt/PKB is activated via a multistep process by a variety of signals. In the early steps of this process, PI-3 kinase-generated D3-phosphorylated phosphoinositides bind the Akt PH domain and induce the translocation of the kinase to the plasma membrane where it co-localizes with phosphoinositide-dependent kinase-1. By binding to the PH domains of both Akt and phosphoinositide-dependent kinase-1, D3-phosphorylated phosphoinositides appear to also induce conformational changes that permit phosphoinositide-dependent kinase-1 to phosphorylate the activation loop of Akt. The paradigm of Akt activation via phosphoinositide-dependent phosphorylation provided a framework for research into the mechanism of activation of other members of the AGC kinase group (p70S6K, PKC, and PKA) and members of the Tec tyrosine kinase family (TecI, TecII, Btk/Atk, Itk/Tsk/Emt, Txk/Rlk, and Bm/Etk). The result was the discovery that these kinases and Akt are activated by overlapping pathways. In this review, we present our current understanding of the regulation and function of the Akt kinase and we discuss the common and unique features of the activation processes of Akt and the AGC and Tec kinase families. In addition, we present an overview of the biosynthesis of phosphoinositides that contribute to the regulation of these kinases.

Sphingosine is a novel activator of 3-phosphoinositide-dependent kinase 1

3-Phosphoinositide-dependent kinase 1 (PDK1) has previously been shown to phosphorylate the activation loop of several AGC kinase family members. In this study, we show that p21-activated kinase 1, the activity of which is regulated by the GTP-bound form of Cdc42 and Rac and by sphingosine, is phosphorylated by PDK1. Phosphorylation of p21-activated kinase 1 by PDK1 occurred only in the presence of sphingosine, which increased PDK1 autophosphorylation 25-fold. Sphingosine increased PDK1 autophosphorylation in a concentration-dependent manner and significantly increased phosphate incorporation into known PDK1 substrates. Studies on the lipid requirement for PDK1 activation found that both sphingosine isoforms and stearylamine also increased PDK1 autophosphorylation. However, C(10)-sphingosine, octylamine, and stearic acid were unable to increase PDK1 autophosphorylation, indicating that both a positive charge and a lipid tail containing at least a C(10)-carbon backbone were required for PDK1 activation. Three PDK1 autophosphorylation sites were identified after stimulation with sphingosine in a serine-rich region located between the kinase domain and the pleckstrin homology domain using two-dimensional phosphopeptide maps and matrix assisted laser desorption/ionization mass spectroscopy. Increased phosphorylation of endogenous Akt at threonine 308 was observed in COS-7 cells expressing wild type PDK1, but not catalytically inactive PDK1, when cellular sphingosine levels were elevated by treatment with sphingomyelinase. Sphingosine thus appears to be a true PDK1 activator.

Sli2 (Ypk1), a homologue of mammalian protein kinase SGK, is a downstream kinase in the sphingolipid-mediated signaling pathway of yeast

ISP-1 is a new type of immunosuppressant, the structure of which is homologous to that of sphingosine. In a previous study, ISP-1 was found to inhibit mammalian serine palmitoyltransferase, the primary enzyme involved in sphingolipid biosynthesis, and to reduce the intracellular pool of sphingolipids. ISP-1 induces the apoptosis of cytotoxic T cells, which is triggered by decreases in the intracellular levels of sphingolipids. In this study, the inhibition of yeast (Saccharomyces cerevisiae) proliferation by ISP-1 was observed. This ISP-1-induced growth inhibition was also triggered by decreases in the intracellular levels of sphingolipids. In addition, DNA duplication without cytokinesis was detected in ISP-1-treated yeast cells on flow cytometry analysis. We have cloned multicopy suppressor genes of yeast which overcome the lethal sphingolipid depletion induced by ISP-1. One of these genes, SLI2, is synonymous with YPK1, which encodes a serine/threonine kinase. Kinase-dead mutants of YPK1 did not show any resistance to ISP-1, leading us to predict that the kinase activity of the Ypk1 protein should be essential for this resistance to ISP-1. Ypk1 protein overexpression had no effect on sphingolipid biosynthesis by the yeast. Furthermore, both the phosphorylation and intracellular localization of the Ypk1 protein were regulated by the intracellular sphingolipid levels. These data suggest that the Ypk1 protein is a downstream kinase in the sphingolipid-mediated signaling pathway of yeast. The Ypk1 protein was reported to be a functional homologue of the mammalian protein kinase SGK, which is a downstream kinase of 3-phosphoinositide-dependent kinase 1 (PDK1). PDK1 phosphotidylinositol (PI) is regulated by PI-3,4,5-triphosphate and PI-3,4-bisphosphate through the pleckstrin homology (PH) domain. Overexpression of mammalian SGK also overcomes the sphingolipid depletion in yeast. Taking both the inability to produce PI-3,4, 5-triphosphate and PI-3,4-bisphosphate and the lack of a PH domain in the yeast homologue of PDK1, the Pkh1 protein, into account, these findings further suggest that yeast may use sphingolipids instead of inositol phospholipids as lipid mediators.

Phosphorylation of protein kinase N by phosphoinositide-dependent protein kinase-1 mediates insulin signals to the actin cytoskeleton

Growth factors such as insulin regulate phosphatidylinositol 3-kinase-dependent actin cytoskeleton rearrangement in many types of cells. However, the mechanism by which the insulin signal is transmitted to the actin cytoskeleton remains largely unknown. Yeast two-hybrid screening revealed that the phosphatidylinositol 3-kinase downstream effector phosphoinositide-dependent protein kinase-1 (PDK1) interacted with protein kinase N (PKN), a Rho-binding Ser/Thr protein kinase potentially implicated in a variety of cellular events, including phosphorylation of cytoskeletal components. PDK1 and PKN interacted in vitro and in intact cells, and this interaction was mediated by the kinase domain of PDK1 and the carboxyl terminus of PKN. In addition to a direct interaction, PDK1 also phosphorylated Thr(774) in the activation loop and activated PKN. Insulin treatment or ectopic expression of the wild-type PDK1 or PKN, but not protein kinase Czeta, induced actin cytoskeleton reorganization and membrane ruffling in 3T3-L1 fibroblasts and Rat1 cells that stably express the insulin receptor (Rat1-IR). However, the insulin-stimulated actin cytoskeleton reorganization in Rat1-IR cells was prevented by expression of kinase-defective PDK1 or PDK1-phosphorylation site-mutated PKN. Thus, phosphorylation by PDK1 appears to be necessary for PKN to transduce signals from the insulin receptor to the actin cytoskeleton.

The PI3K-PDK1 connection: more than just a road to PKB

Phosphoinositide 3-kinases (PI3Ks) generate specific inositol lipids that have been implicated in the regulation of cell growth, proliferation, survival, differentiation and cytoskeletal changes. One of the best characterized targets of PI3K lipid products is the protein kinase Akt or protein kinase B (PKB). In quiescent cells, PKB resides in the cytosol in a low-activity conformation. Upon cellular stimulation, PKB is activated through recruitment to cellular membranes by PI3K lipid products and phosphorylation by 3'-phosphoinositide-dependent kinase-1 (PDK1). Here we review the mechanism by which PKB is activated and the downstream actions of this multifunctional kinase. We also discuss the evidence that PDK1 may be involved in the activation of protein kinases other than PKB, the mechanisms by which this activity of PDK1 could be regulated and the possibility that some of the currently postulated PKB substrates targets might in fact be phosphorylated by PDK1-regulated kinases other than PKB.

The role of 3-phosphoinositide-dependent protein kinase 1 in activating AGC kinases defined in embryonic stem cells

BACKGROUND

Protein kinase B (PKB), and the p70 and p90 ribosomal S6 kinases (p70 S6 kinase and p90 Rsk, respectively), are activated by phosphorylation of two residues, one in the 'T-loop' of the kinase domain and, the other, in the hydrophobic motif carboxy terminal to the kinase domain. The 3-phosphoinositide-dependent protein kinase 1 (PDK1) activates many AGC kinases in vitro by phosphorylating the T-loop residue, but whether PDK1 also phosphorylates the hydrophobic motif and whether all other AGC kinases are substrates for PDK1 is unknown.

RESULTS

Mouse embryonic stem (ES) cells in which both copies of the PDK1 gene were disrupted were viable. In PDK1(-/-) ES cells, PKB, p70 S6 kinase and p90 Rsk were not activated by stimuli that induced strong activation in PDK1(+/+) cells. Other AGC kinases - namely, protein kinase A (PKA), the mitogen- and stress-activated protein kinase 1 (MSK1) and the AMP-activated protein kinase (AMPK) - had normal activity or were activated normally in PDK1(-/-) cells. The insulin-like growth factor 1 (IGF1) induced PKB phosphorylation at its hydrophobic motif, but not at its T-loop residue, in PDK1(-/-) cells. IGF1 did not induce phosphorylation of p70 S6 kinase at its hydrophobic motif in PDK1(-/-) cells.

CONCLUSIONS

PDK1 mediates activation of PKB, p70 S6 kinase and p90 Rsk in vivo, but is not rate-limiting for activation of PKA, MSK1 and AMPK. Another kinase phosphorylates PKB at its hydrophobic motif in PDK1(-/-) cells. PDK1 phosphorylates the hydrophobic motif of p70 S6 kinase either directly or by activation of another kinase.

Rho GTPase control of protein kinase C-related protein kinase activation by 3-phosphoinositide-dependent protein kinase

The protein kinase C-related protein kinases (PRKs) have been shown to be under the control of the Rho GTPases and influenced by autophosphorylation. In analyzing the relationship between these inputs, it is shown that activation in vitro and in vivo involves the activation loop phosphorylation of PRK1/2 by 3-phosphoinositide-dependent protein kinase-1 (PDK1). Rho overexpression in cultured cells is shown to increase the activation loop phosphorylation of endogenous PRKs and is demonstrated to influence this process by controlling the ability of PRKs to bind to PDK1. The interaction of PRK1/2 with PDK1 is shown to be dependent upon Rho. Direct demonstration of ternary (Rho.PRK.PDK1) complex formation in situ is provided by the observation that PDK1 is recruited to RhoB-containing endosomes only if PRK is coexpressed. Furthermore, this in vivo complex is maintained after phosphoinositide 3-kinase inhibition. The control of PRKs by PDK1 thus evidences a novel strategy of substrate-directed control involving GTPases.

The PI3K-PDK1 connection: more than just a road to PKB

Phosphoinositide 3-kinases (PI3Ks) generate specific inositol lipids that have been implicated in the regulation of cell growth, proliferation, survival, differentiation and cytoskeletal changes. One of the best characterized targets of PI3K lipid products is the protein kinase Akt or protein kinase B (PKB). In quiescent cells, PKB resides in the cytosol in a low-activity conformation. Upon cellular stimulation, PKB is activated through recruitment to cellular membranes by PI3K lipid products and phosphorylation by 3'-phosphoinositide-dependent kinase-1 (PDK1). Here we review the mechanism by which PKB is activated and the downstream actions of this multifunctional kinase. We also discuss the evidence that PDK1 may be involved in the activation of protein kinases other than PKB, the mechanisms by which this activity of PDK1 could be regulated and the possibility that some of the currently postulated PKB substrates targets might in fact be phosphorylated by PDK1-regulated kinases other than PKB.

Phosphorylation of a Src kinase at the autophosphorylation site in the absence of Src kinase activity

Exposure of cells to oxidants increases the phosphorylation of the Src family tyrosine protein kinase Lck at Tyr-394, a conserved residue in the activation loop of the catalytic domain. Kinase-deficient Lck expressed in fibroblasts that do not express any endogenous Lck has been shown to be phosphorylated at Tyr-394 following H(2)O(2) treatment to an extent indistinguishable from that seen with wild type Lck. This finding indicates that a kinase other than Lck itself is capable of phosphorylating Tyr-394. Because fibroblasts express other Src family members, it remained to be determined whether the phosphorylation of Tyr-394 was carried out by another Src family kinase or by an unrelated tyrosine protein kinase. We examined here whether Tyr-394 in kinase-deficient Lck was phosphorylated following exposure of cells devoid of endogenous Src family kinase activity to H(2)O(2). Strikingly, treatment of such cells with H(2)O(2) led to the phosphorylation of Tyr-394 to an extent identical to that seen with wild type Lck, demonstrating that Src family kinases are not required for H(2)O(2)-induced phosphorylation of Lck. Furthermore, this efficient phosphorylation of Lck at Tyr-394 in non-lymphoid cells suggests the existence of an ubiquitous activator of Src family kinases.

Identification of a pocket in the PDK1 kinase domain that interacts with PIF and the C-terminal residues of PKA

The 3-phosphoinositide-dependent protein kinase-1 (PDK1) phosphorylates and activates a number of protein kinases of the AGC subfamily. The kinase domain of PDK1 interacts with a region of protein kinase C-related kinase-2 (PRK2), termed the PDK1-interacting fragment (PIF), through a hydrophobic motif. Here we identify a hydrophobic pocket in the small lobe of the PDK1 kinase domain, separate from the ATP- and substrate-binding sites, that interacts with PIF. Mutation of residues predicted to form part of this hydrophobic pocket either abolished or significantly diminished the affinity of PDK1 for PIF. PIF increased the rate at which PDK1 phosphorylated a synthetic dodecapeptide (T308tide), corresponding to the sequences surrounding the PDK1 phosphorylation site of PKB. This peptide is a poor substrate for PDK1, but a peptide comprising T308tide fused to the PDK1-binding motif of PIF was a vastly superior substrate for PDK1. Our results suggest that the PIF-binding pocket on the kinase domain of PDK1 acts as a 'docking site', enabling it to interact with and enhance the phosphorylation of its substrates.