Sensitization of Mesothelioma Cells to Tumor Necrosis Factor–Related Apoptosis–Inducing Ligand–Induced Apoptosis by Heat Stress via the Inhibition of the 3-Phosphoinositide-Dependent Kinase 1/Akt Pathway

Heat stress may enhance the effect of apoptosis-inducing agents in resistant tumor cells. One such agent is the tumor necrosis factor-related apoptosis-inducing ligand (TRAIL), which has attracted intense interest for its ability to induce apoptosis in tumors without affecting nonmalignant cells. We therefore tested whether heat stress potentiates TRAIL-induced apoptosis in mesothelioma cells, its cell type being resistant to TRAIL alone. We found that heat stress enhanced the apoptosis caused by TRAIL but not by chemotherapy. To explain this potentiation, we found that heat stress decreased Akt phosphorylation via the dissociation of heat shock protein 90 (Hsp90) from its client protein 3-phosphoinositide-dependent kinase 1 (PDK-1), a major Akt kinase. The role of Hsp90 and the Akt pathway was confirmed by showing that inhibitors of Hsp90 and the phosphatidyilinositol-3 kinase/Akt pathway reproduced the effect of heat stress on TRAIL-induced apoptosis and that the effect of inhibiting Hsp90 on TRAIL-induced apoptosis could be overcome by activating the Akt pathway with a constitutively active construct of the Akt kinase PDK-1. The effect of heat stress involved multiple steps of the apoptotic machinery. Heat stress potentiated the death receptor pathway, as shown by an increase in TRAIL-induced caspase 8 cleavage. Nonetheless, knockdown of Bid, the main intermediary molecule from the death receptor pathway to the mitochondria, inhibited the effect of heat stress, showing that mitochondrial amplification was required for potentiation by heat stress. In summary, these results support the novel concept that heat stress inhibits the Akt pathway by dissociating PDK-1 from its chaperone Hsp90, leading to potentiation of TRAIL-induced apoptosis in resistant malignant cells.

PTEN augments doxorubicin-induced apoptosis in PTEN-null Ishikawa cells

To investigate whether PTEN can augment doxorubicin-induced apoptosis in PTEN-null Ishikawa cells. We previously demonstrated that Ishikawa cells do not possess functional PTEN protein because of protein truncations. Clones expressing the steady-state level of the PTEN protein from PTEN-null Ishikawa cells have been established and were used in this study. Doxorubicin is a commonly used anticancer drug in endometrial carcinoma. The cytotoxic effect of doxorubicin was evaluated using the methyl thiazoleterazolium (MTT) assay. We used the Hoechst 33258 staining to confirm the induction of apoptosis. Immunoprecipitation and Western blot analysis were performed to evaluate the effects of doxorubicin on phosphorylation of Bcl-2 antagonist of cell death (Bad) and protein kinase B (Akt/PKB). Doxorubicin induced death of all cell lines in a dose-dependent manner, but the death was more significant in PTEN-expressing clones than in parent Ishikawa cells. A low concentration of doxorubicin (0.1 muM) did not affect apoptosis in PTEN-null Ishikawa cells, but it induced apoptosis in PTEN-expressing clones. A high concentration (1 microM) induced apoptosis in all cell lines, but the percentages of apoptotic cells were higher in PTEN-expressing clones than in parent Ishikawa cells. In the clones, phospho-Akt/PKB and phospho-Bad (Ser-136) were downregulated. Doxorubicin reduced the levels of phospho-Akt/PKB and phospho-Bad (Ser-136) in all the cell lines, but the reduction was most significant in the PTEN-expressing clones. Our present results indicate that PTEN transfection significantly enhances doxorubicin chemosensitivity through effective induction of apoptosis by downregulation of the PI3K/Akt/PKB signaling pathway in Ishikawa cells.

Dissociation between the translocation and the activation of Akt in fMLP-stimulated human neutrophils--effect of prostaglandin E2

PGE(2) and other cAMP-elevating agents are known to down-regulate most functions stimulated by fMLP in human polymorphonuclear neutrophils. We reported previously that the inhibitory potential of PGE(2) resides in its capacity to suppress fMLP-stimulated PI-3Kgamma activation via the PGE(2) receptor EP(2) and hence, to decrease phosphatidylinositol 3,4,5-triphosphate [PI(3,4,5)P(3)] formation. Akt activity is stimulated by fMLP through phosphorylation on threonine 308 (Thr308) and serine 473 (Ser473) by 3-phosphoinositide-dependent kinase 1 (PDK1) and MAPK-AP kinase (APK)-APK-2 (MAPKAPK-2), respectively, in a PI-3K-dependent manner. Despite the suppression of fMLP-induced PI-3Kgamma activation observed in the presence of PGE(2), we show that Akt is fully phosphorylated on Thr308 and Ser473. However, fMLP-induced Akt translocation is decreased markedly in this context. PGE(2) does not affect the phosphorylation of MAPKAPK-2 but decreases the translocation of PDK1 induced by fMLP. Other cAMP-elevating agents such as adenosine (Ado) similarly block the fMLP-induced PI-3Kgamma activation process but do not inhibit Akt phosphorylation. However, Akt activity stimulated by fMLP is down-regulated slightly by agonists that elevate cAMP levels. Whereas protein kinase A is not involved in the maintenance of Akt phosphorylation, it is required for the inhibition of Akt translocation by PGE(2). Moreover, inhibition of fMLP-stimulated PI-3Kdelta activity by the selective inhibitor IC87114 only partially affects the late phase of Akt phosphorylation in the presence of PGE(2). Taken together, these results suggest that cAMP-elevating agents, such as PGE(2) or Ado, are able to induce an alternative mechanism of Akt activation by fMLP in which the translocation of Akt to PI(3,4,5)P(3)-enriched membranes is not required prior to its phosphorylation.

Apoptosis and inactivation of the PI3-kinase pathway by tetrocarcin A in breast cancers

A survival kinase, Akt, is a downstream factor in the phosphatidylinositide-3'-kinase-dependent pathway, which mediates many biological responses including glucose uptake, protein synthesis and the regulation of proliferation and apoptosis, which is assumed to contribute to acquisition of malignant properties of human cancers. Here we find that an anti-tumor antibiotic, tetrocarcin A, directly induces apoptosis of human breast cancer cells. The apoptosis is accompanied by the activation of a proteolytic cascade of caspases including caspase-3 and -9, and concomitantly decreases phosphorylation of Akt, PDK1, and PTEN, a tumor suppressor that regulates the activity of Akt through the dephosphorylation of polyphosphoinositides. Tetrocarcin A affected neither expression of Akt, PDK1, or PTEN, nor did it affect the expression of Bcl family members including Bcl-2, Bcl-X(L), and Bax. These results suggest that tetrocarcin A could be a potent chemotherapeutic agent for human breast cancer targeting the phosphatidylinositide-3'-kinase/Akt signaling pathway.

3-Phosphoinositide-dependent PDK1 negatively regulates transforming growth factor-beta-induced signaling in a kinase-dependent manner through physical interaction with Smad proteins

We have reported previously that PDK1 physically interacts with STRAP, a transforming growth factor-beta (TGF-beta) receptor-interacting protein, and enhances STRAP-induced inhibition of TGF-beta signaling. In this study we show that PDK1 coimmunoprecipitates with Smad proteins, including Smad2, Smad3, Smad4, and Smad7, and that this association is mediated by the pleckstrin homology domain of PDK1. The association between PDK1 and Smad proteins is increased by insulin treatment but decreased by TGF-beta treatment. Analysis of the interacting proteins shows that Smad proteins enhance PDK1 kinase activity by removing 14-3-3, a negative regulator of PDK1, from the PDK1-14-3-3 complex. Knockdown of endogenous Smad proteins, including Smad3 and Smad7, by transfection with small interfering RNA produced the opposite trend and decreased PDK1 activity, protein kinase B/Akt phosphorylation, and Bad phosphorylation. Moreover, coexpression of Smad proteins and wild-type PDK1 inhibits TGF-beta-induced transcription, as well as TGF-beta-mediated biological functions, such as apoptosis and cell growth arrest. Inhibition was dose-dependent on PDK1, but no inhibition was observed in the presence of an inactive kinase-dead PDK1 mutant. In addition, confocal microscopy showed that wild-type PDK1 prevents translocation of Smad3 and Smad4 from the cytoplasm to the nucleus, as well as the redistribution of Smad7 from the nucleus to the cytoplasm in response to TGF-beta. Taken together, our results suggest that PDK1 negatively regulates TGF-beta-mediated signaling in a PDK1 kinase-dependent manner via a direct physical interaction with Smad proteins and that Smad proteins can act as potential positive regulators of PDK1.

A chemical approach to the identification of tensin-binding proteins

Many protein-protein interactions are mediated by small modular domains, which recognize short peptide motifs in their partner proteins. However, for the great majority of these domains, the identity of their partner proteins remains unknown. In this work, a chemical/bioinformatics approach has been developed to identify phosphotyrosyl (pY) proteins that bind to tensin, a protein involved in the formation of actin cytoskeleton and signal transduction. A pY peptide library was chemically synthesized and screened against the Src homology 2 (SH2) domain of tensin to identify the peptide motifs that bind to the SH2 domain. Next, protein databases were searched for proteins containing the SH2 domain-binding peptide motifs. Finally, the potential tensin-binding proteins were confirmed (or disproved) by in vitro pull-down and coimmunoprecipitation assays. This procedure identified phosphoinositide-dependent kinase-1 and downstream of tyrosine kinase 2 as novel tensin-binding proteins. In addition, a cell-permeable pY peptide was designed as tensin SH2 domain inhibitor, which caused the disruption of actin filaments in NIH 3T3 cells. This method should be generally applicable to other modular domains that recognize small peptide motifs.

TRAIL signalling: decisions between life and death

The TNF-related apoptosis-inducing ligand, TRAIL, has been shown to selectively kill tumour cells. This property has made TRAIL and agonistic antibodies against its death inducing receptors (TRAIL-R1 and TRAIL-R2) to some of the most promising novel biotherapeutic agents for cancer therapy. Here we review the signalling pathways initiated by the apoptosis- as well as the non-apoptosis-inducing receptors, TRAIL-R3 and TRAIL-R4. The TRAIL "death-inducing signalling complex" (DISC) transmits the apoptotic signal. DISC formation leads to activation of a protease cascade, finally resulting in cell death. The TRAIL death receptor-mediated "extrinsic" pathway and the "intrinsic" pathway, which is controlled by the interaction of members of the Bcl-2 family, interact with each other in the decision about life or death of a cell. Apoptotic and non-apoptotic signalling is influenced by the NF-kappaB, PKB/Akt and the MAPK signalling pathways. In this review we intend to summarise the most important findings on the TRAIL signalling network and the interplay in the decisions between life and death of a tumor cell.

Differential activation of the PI 3-kinase effectors AKT/PKB and p70 S6 kinase by compound 48/80 is mediated by PKCα

The secretagogue compound 48/80 (c48/80) is a well known activator of calcium mediated processes and PKCs, and is a potent inducer of mast cell degranulation. As the latter process is a phosphoinositide 3-kinase (PI 3-kinase) mediated event, we wished to address whether or not c48/80 was an activator of PI 3-kinases. The data presented here reveal that c48/80 is an effective activator of PI 3-kinases as judged by the increased phosphorylation of PKB and p70(S6K) in fibroblasts in a PI 3-kinase dependent fashion. Compound 48/80 effectively translocates PKB to the plasma membrane and induces phosphorylation at serine 473 (S473), detected by fluorescence imaging of fixed cells. At higher concentrations the secretagogue is inhibitory towards PKB phosphorylation on S473. Conversely, p70(S6K) phosphorylation on T389 is unaffected at high doses. We provide evidence that the differential effect on the two PI 3-kinase effectors is due to activation of PKCalpha by c48/80, itself a PI 3-kinase dependent process. We conclude that compound 48/80 is an effective activator of PI 3-kinase dependent pathways, leading to the activation of effectors including PKB/Akt, p70(S6K) and PKCalpha. The latter is only activated by higher doses of c48/80 resulting in an inhibition of the c48/80 induced PKB phosphorylation, thus explaining the observed biphasic activation profile for PKB in response to this secretagogue.

The role of PKC and PDK1 in monocyte lineage specification by Ras

Although hyperactivation of Ras is a common feature of myeloid malignancy, its role in subverting hematopoiesis is unclear. We have examined the influence of Ras on normal human uncommitted myeloid subsets and show that expression of this oncogene strongly favors monocyte lineage selection in bipotential granulocyte/macrophage progenitors while inhibiting colony formation in other uncommitted subsets. Ras also promoted monocytic differentiation but not the proliferation of these cells. The mechanism through which Ras drives monocyte lineage selection was dependent on PKC activity and Ras was found to promote the expression, membrane translocation, and phosphorylation of conventional and novel PKC isoforms. We further show that Ras promoted the expression of the AGC kinase master regulator, PDK1, which maintains the stability and activity of PKC isoforms. Consistent with this, overexpression of PDK1 itself promoted monocyte colony formation and translocation of PKC. Overexpression of PDK1 was found to be a common feature of acute myeloid leukemia (45% of patients) and was closely associated with hyperphosphorylation of PKC. These data demonstrate that Ras is able to promote monocyte lineage selection via PKC and show for the first time the involvement of the kinase master regulator, PDK1, in both lineage specification and in human leukemia.

Identification of ChChd3 as a novel substrate of the cAMP-dependent protein kinase (PKA) using an analog-sensitive catalytic subunit

Due to the numerous kinases in the cell, many with overlapping substrates, it is difficult to find novel substrates for a specific kinase. To identify novel substrates of cAMP-dependent protein kinase (PKA), the PKA catalytic subunit was engineered to accept bulky N(6)-substituted ATP analogs, using a chemical genetics approach initially pioneered with v-Src (1). Methionine 120 was mutated to glycine in the ATP-binding pocket of the catalytic subunit. To express the stable mutant C-subunit in Escherichia coli required co-expression with PDK1. This mutant protein was active and fully phosphorylated on Thr(197) and Ser(338). Based on its kinetic properties, the engineered C-subunit preferred N(6)(benzyl)-ATP and N(6)(phenethyl)-ATP over other ATP analogs, but still retained a 30 microm K(m) for ATP. This mutant recombinant C-subunit was used to identify three novel PKA substrates. One protein, a novel mitochondrial ChChd protein, ChChd3, was identified, suggesting that PKA may regulate mitochondria proteins.

Ligand activation of peroxisome proliferator-activated receptor-beta/delta(PPARbeta/delta) inhibits cell growth of human N/TERT-1 keratinocytes

The functional role of peroxisome proliferator-activated receptor-beta(PPARbeta; also referred to as PPARdelta) in epidermal cell growth remains controversial. Recent evidence suggests that ligand activation of PPARbeta/delta increases cell growth and inhibits apoptosis in epidermal cells. In contrast, other reports suggest that ligand activation of PPARbeta/delta leads to the induction of terminal differentiation and inhibition of cell growth. In the present study, the effect of the highly specific PPARbeta/delta ligand GW0742 on cell growth was examined using a human keratinocyte cell line (N/TERT-1) and mouse primary keratinocytes. Ligand activation of PPARbeta/delta with GW0742 prevented cell cycle progression from G1 to S phase and attenuated cell proliferation in N/TERT-1 cells. Despite specifically activating PPARbeta/delta as revealed by target gene induction, no changes in PTEN, PDK and ILK expression or downstream phosphorylation of Akt were found in either N/TERT-1 cells or primary keratinocytes. Further, altered cell growth resulting from serum withdrawal and the induction of caspase-3 activity by ultraviolet radiation were unchanged in the absence of PPARbeta/delta expression and/or the presence of GW0742. While no changes in the expression of mRNAs encoding cell cycle control proteins were found in response to GW0742, a significant decrease in the level of ERK phosphorylation was observed. Results from these studies demonstrate that ligand activation of PPARbeta/delta does not lead to an anti-apoptotic effect in either human or mouse keratinocytes, but rather, leads to inhibition of cell growth likely through the induction of terminal differentiation.

IL-4 induces a wide-spectrum intracellular signaling cascade in CD8+T cells

IL-4 has distinct effects on the differentiation and functional properties of CD8+ T cells. In vivo studies have shown that it is critical for the development of protective memory responses against tumors and infections by Leishmania and Plasmodium parasites. The intracellular signaling events mediated by IL-4/IL-4 receptor (IL-4R) interactions on CD4+ T cells have been studied extensively; however, the nature of IL-4-induced signaling on CD8+ T cells has not been characterized. Using naïve, activated, as well as differentiated CD8+ T cells, we show that IL-4 has a strong in vivo and in vitro antiapoptotic effect on activated and resting CD8+ T cells. We demonstrate that IL-4 induces the phosphorylation of the IL-4R, which is followed by the activation of at least two distinct intracellular signaling cascades: the Jak1/STAT6 and the insulin receptor substrate/PI-3K/protein kinase B pathways. We also found that IL-4 induces the Jak3-mediated phosphorylation and nuclear migration of STAT1, STAT3, and STAT5 in naïve, activated, as well as differentiated, IFN-gamma-producing CD8+ T cells. The induction of this broad signaling activity in CD8+ T cells coincides with a transcriptional activity of suppressors of cytokine signaling genes, which are decreased significantly in comparison with CD4+ T cells. To our knowledge, this report constitutes the first comprehensive analysis of the signaling events that shape CD8+ T cell responses to IL-4.

Akt-PDK1 complex mediates epidermal growth factor-induced membrane protrusion through Ral activation

We studied the spatiotemporal regulation of Akt (also called protein kinase B), phosphatidylinositol-3,4-bisphosphate [PtdIns(3,4)P2], and phosphatidylinositol-3,4,5-trisphosphate [PtdIns(3,4,5)P3] by using probes based on the principle of fluorescence resonance energy transfer. On epidermal growth factor (EGF) stimulation, the amount of PtdIns(3,4,5)P3 was increased diffusely in the plasma membrane, whereas that of PtdIns(3,4)P2 was increased more in the nascent lamellipodia than in the plasma membrane of the central region. The distribution and time course of Akt activation were similar to that of increased PtdIns(3,4)P2 levels, which were most prominent in the nascent lamellipodia. Moreover, we found that upon EGF stimulation 3-phosphoinositide-dependent protein kinase-1 (PDK1) was also recruited to nascent lamellipodia in an Akt-dependent manner. Because PDK1 is known to activate Ral GTPase and because Ral is required for EGF-induced lamellipodial protrusion, we speculated that the PDK1-Akt complex may be indispensable for the induction of lamellipodia. In agreement with this idea, EGF-induced lamellipodia formation was promoted by the overexpression of Akt and inhibited by an Akt inhibitor or a Ral-binding domain of Sec5. These results identified the Akt-PDK1 complex as an upstream positive regulator of Ral GTPase in the induction of lamellipodial protrusion.

Requirement for Akt-mediated survival in cell transformation by the dbl oncogene

The dbl oncogene product is the founding member of a large family of oncogenic proteins that function by activating the small GTP-binding proteins Cdc42, Rac and Rho. Through its substrate GTPases, Dbl transduces proliferative signals from cell-surface receptors to diverse cellular effectors and signaling pathways. The mechanisms by which these multiple signals are integrated, as well as their relative contribution to Dbl-induced cell transformation, are presently poorly understood. We investigated the role of the survival regulators PI3-kinase and Akt in Dbl-induced cell transformation. We found that Dbl induced the phosphorylation of Akt on threonine 308, through the GTPases Rac and Cdc42 and in a PI3-kinase dependent manner. Pharmacological or biochemical interference with this pathway lead to a marked, dose-dependent inhibition of the focus formation activity exhibited by Dbl-expressing cells. Dbl expression stimulated the phosphorylation of the anti-apoptotic Akt substrate Bad, and caused a marked decrease in basal levels of apoptosis. Finally, we found that activated Cdc42 existed in cells in complex with phosphoionositide-dependent kinase-1 (PDK1), the downstream mediator of PI3-kinase action. The data indicate that Dbl signaling stimulate the formation of a novel survival complex, through which anti-apoptotic signals are generated and propagated.

Estrogen-related receptors stimulate pyruvate dehydrogenase kinase isoform 4 gene expression

The pyruvate dehydrogenase complex (PDC) catalyzes the conversion of pyruvate to acetyl-CoA in mitochondria and is a key regulatory enzyme in the oxidation of glucose to acetyl-CoA. Phosphorylation of PDC by the pyruvate dehydrogenase kinases (PDK2 and PDK4) inhibits PDC activity. Expression of the PDK genes is elevated in diabetes, leading to the decreased oxidation of pyruvate to acetyl-CoA. In these studies we have investigated the transcriptional regulation of the PDK4 gene by the estrogen-related receptors (ERRalpha and ERRgamma). The ERRs are orphan nuclear receptors whose physiological roles include the induction of fatty acid oxidation in heart and muscle. Previously, we found that the peroxisome proliferator-activated receptor gamma coactivator (PGC-1alpha) stimulates the expression of PDK4. Here we report that ERRalpha and ERRgamma stimulate the PDK4 gene in hepatoma cells, suggesting a novel role for ERRs in controlling pyruvate metabolism. In addition, both ERR isoforms recruit PGC-1alpha to the PDK4 promoter. Insulin, which decreases the expression of the PDK4 gene, inhibits the induction of PDK4 by ERRalpha and ERRgamma. The forkhead transcription factor (FoxO1) binds the PDK4 gene and contributes to the induction of PDK4 by ERRs and PGC-1alpha. Insulin suppresses PDK4 expression in part through the dissociation of FoxO1 and PGC-1alpha from the PDK4 promoter. Our data demonstrate a key role for the ERRs in the induction of hepatic PDK4 gene expression.

Fused protein of deltaPKC activation loop and PDK1-interacting fragment (deltaAL-PIF) functions as a pseudosubstrate and an inhibitory molecule for PDK1 when expressed in cells

To elucidate the role of 3-phosphoinositide-dependent protein kinase-1 (PDK1) in cellular signaling, we constructed and expressed a pseudosubstrate of PDK1, designated as deltaAL-PIF, and characterized its properties in cultured cells. deltaAL-PIF consists of two fused proteins of the protein kinase Cdelta (deltaPKC) activation loop (deltaAL) and PDK1-interacting fragment (PIF). The phosphorylation of deltaAL-PIF was detected with anti-deltaPKC phospho-Thr505-specific antibody and was increased in proportion to the expression level of co-expressed GST-PDK1, indicating that it acts as a pseudosubstrate of PDK1. In cells expressing deltaAL-PIF, basal phosphorylation level at the activation loop of PKBalpha, deltaPKC and gammaPKC was reduced, compared with that in control cells, suggesting that deltaAL-PIF functions as an inhibitory molecule for PDK1. deltaAL-PIF affected the stability, translocation and endogenous activity of PKCs. These effects of deltaAL-PIF on gammaPKC properties were confirmed by investigation using conditioned PDK1 knockout cells. Furthermore, apoptosis frequently occurred in cells expressing deltaAL-PIF for 3 days. These findings revealed that deltaAL-PIF served as an effective pseudosubstrate and an inhibitory molecule for PDK1, suggesting that this molecule can be used as a tool for investigating PDK-mediated cellular functions as well as being applicable for anti-cancer therapy.

Microscopic Modes and Free Energies of 3-Phosphoinositide-Dependent Kinase-1 (PDK1) Binding with Celecoxib and Other Inhibitors

Celecoxib, also known as Celebrex (approved by FDA in 1998) and remembered as the fastest-selling drug in history, was used as a cyclooxygenase-2 (COX-2) selective inhibitor having both anti-inflammatory and anticancer activities. Most recent studies have revealed that the apoptotic activity of celecoxib (and its derivatives) is actually independent of the COX-2 inhibitory activity and that celecoxib also inhibits the kinase activity of 3-phosphoinositide-dependent protein kinase-1 (PDK1), suggesting that the well-known anticancer activity of celecoxib is not due to the inhibition of COX-2, but possibly is due to the inhibition of PDK1. It is highly desirable to develop new celecoxib derivatives as PDK1-specifc inhibitors to avoid the side effects of COX-2 inhibitors. To understand how PDK1 binds with celecoxib and its derivatives, we have performed extensive molecular docking and combined molecular dynamics (MD) simulations and molecular mechanics/Poisson-Boltzmann surface area (MM-PBSA) binding free energy calculations on eight representative PDK1 inhibitors, leading to the finding of a new, more favorable binding mode which is remarkably different from the previously proposed binding mode. Based on the determined most stable binding structures, the calculated binding free energies are all in good agreement with the corresponding experimental data, and the biological activity data available for celecoxib and its derivatives can be better interpreted. The obtained new insights, concerning both the binding mode and computational protocol, will be valuable not only for future rational design of novel, more potent PDK1-specific inhibitors as promising anticancer therapeutics, but also for rational design of drugs targeting other proteins.

The Arabidopsis Protein Kinase PTI1-2 Is Activated by Convergent Phosphatidic Acid and Oxidative Stress Signaling Pathways Downstream of PDK1 and OXI1

Arabidopsis PDK1 activity is regulated by binding to the lipid phosphatidic acid (PA) resulting in activation of the oxidative stress-response protein kinase OXI1/AGC2-1. Thus there is an inferred link between lipid signaling and oxidative stress signaling modules. Among a panel of hormones and stresses tested, we found that, in addition to PA, the fungal elicitor xylanase activated PDK1, suggesting that PDK1 has a role in plant pathogen defense mechanisms. The downstream OXI1 was activated by additional stress factors, including PA, H(2)O(2), and partially by xylanase. We have isolated an interacting partner of OXI1, a Ser/Thr kinase (PTI1-2), which is downstream of OXI1. Its sequence closely resembles the tomato Pti kinase, which has been implicated in the hypersensitive response, a localized programmed cell death that occurs at the site of pathogen infection. PTI1-2 is activated by the same stresses/elicitors as OXI1 and additionally flagellin. We have used RNA interference to knock out the expression of PDK1 and OXI1 and to study the effects on PTI1-2 activity. We show that specific lipid signaling pathways converge on PTI1-2 via the PDK1-OXI1 axis, whereas H(2)O(2) and flagellin signals to OXI1-PTI1-2 via a PDK1-independent pathway. PTI1-2 represents a new downstream component that integrates diverse lipid and reactive oxygen stress signals and functions closely with OXI1.

Investigation of interleukin 1beta-mediated regulation of NF-kappaB activation in colonic cells reveals divergence between PKB and PDK-transduced events

Recent work has highlighted a role for PDK1 in adaptive immunity, however its contribution to innate immunity has not been addressed. We have investigated the role of PKB and PDK1 in IL-1beta-induced NF-kappaB activation. Over-expression of either in HCT 116 and HEK 293T cells, effected a reproducible NF-kappaB activation. This was validated in a one-hybrid assay utilizing Gal4-RelA and Gal4-luciferase assay. N-tosyl phenylalanyl chloromethyl ketone (TPCK), wortmannin and Ly294002 inhibited IL-1beta-induced NF-kappaB activation in both systems indicating involvement of the PI3K axis in this response. p65 (Rel A) Ser536 phosphorylation was not affected by the PI3K inhibitors but was dose-dependently attenuated by TPCK. Evaluation of IKK-associated activity using GST-p65 substrate phosphorylation in immune complex assays, revealed that whilst TPCK attenuated this, neither of the PI3K inhibitors had any effect. Furthermore whilst TPCK inhibited IL-1beta-induced p65 DNA binding, this was not apparent with either of wortmannin or Ly294002. Similarly, over-expression of PDK1 but not PKB resulted in promotion of p65 DNA binding. Using a p65-S536A reporter construct, we found inhibition of only PDK1 over-expression-induced, but not PKB over-expression-induced NF-kappaB activation. This was supported using biochemical analysis in which immunoprecipitated IKKgamma from IL-1beta-activated cells was unable to phosphorylate a p65-S536A substrate, confirming this as the dominant IKK-dependent site. In further support of a dissociated response, we observed an attenuation of the Ser177/181 IKK phosphorylation by TPCK but not in response to PI3K inhibition. Our data reveals for the first time that PDK1 and PKB may differentially activate NF-kappaB, and that TPCK may subserve a useful anti-inflammatory function by inhibiting IKKbeta.

Reduced intestinal and renal amino acid transport in PDK1 hypomorphic mice

The phosphoinositide-dependent kinase PDK1 activates the serum- and glucocorticoid-inducible kinase isoforms SGK1, SGK2, and SGK3 and protein kinase B, which in turn are known to up-regulate a variety of sodium-coupled transporters. The present study was performed to explore the role of PDK1 in amino acid transport. As mice completely lacking functional PDK1 are not viable, mice expressing 10-25% of PDK1 (pdk1(hm)) were compared with their wild-type (WT) littermates (pdk1(wt)). Body weight was significantly less in pdk1(hm) than in pdk1(wt) mice. Despite lower body weight of pdk1(hm) mice, food and water intake were similar in pdk1(hm) and pdk1(wt) mice. According to Ussing chamber experiments, electrogenic transport of phenylalanine, cysteine, glutamine, proline, leucine, and tryptophan was significantly smaller in jejunum of pdk1(hm) mice than in pdk1(wt) mice. Similarly, electrogenic transport of phenylalanine, glutamine, and proline was significantly decreased in isolated perfused proximal tubules of pdk1(hm) mice. The urinary excretion of proline, valine, guanidinoacetate, methionine, phenylalanine, citrulline, glutamine/glutamate, and tryptophan was significantly larger in pdk1(hm) than in pdk1(wt) mice. According to immunoblotting of brush border membrane proteins prepared from kidney, expression of the Na+-dependent neutral amino acid transporter B(0)AT1 (SLC6A19), the glutamate transporter EAAC1/EAAT3 (SLC1A1), and the transporter for cationic amino acids and cystine b(0,+)AT (SLC7A9) was decreased but the Na+/proline cotransporter SIT (SLC6A20) was increased in pdk1(hm) mice. In conclusion, reduction of functional PDK1 leads to impairment of intestinal absorption and renal reabsorption of amino acids. The combined intestinal and renal loss of amino acids may contribute to the growth defect of PDK1-deficient mice.

Allosteric activation of the protein kinase PDK1 with low molecular weight compounds

Organisms rely heavily on protein phosphorylation to transduce intracellular signals. The phosphorylation of a protein often induces conformational changes, which are responsible for triggering downstream cellular events. Protein kinases are themselves frequently regulated by phosphorylation. Recently, we and others proposed the molecular mechanism by which phosphorylation at a hydrophobic motif (HM) regulates the conformation and activity of many members of the AGC group of protein kinases. Here we have developed specific, low molecular weight compounds, which target the HM/PIF-pocket and have the ability to allosterically activate phosphoinositide-dependent protein kinase 1 (PDK1) by modulating the phosphorylation-dependent conformational transition. The mechanism of action of these compounds was characterized by mutagenesis of PDK1, synthesis of compound analogs, interaction-displacement studies and isothermal titration calorimetry experiments. Our results raise the possibility of developing drugs that target the AGC kinases via a novel mode of action and may inspire future rational development of compounds with the ability to modulate phosphorylation-dependent conformational transitions in other proteins.

The yeast PH domain proteins Slm1 and Slm2 are targets of sphingolipid signaling during the response to heat stress

The PH domain-containing proteins Slm1 and Slm2 were previously identified as effectors of the phosphatidylinositol-4,5-bisphosphate (PI4,5P(2)) and TORC2 signaling pathways. Here, we demonstrate that Slm1 and Slm2 are also targets of sphingolipid signaling during the heat shock response. We show that upon depletion of cellular sphingolipid levels, Slm1 function becomes essential for survival under heat stress. We further demonstrate that Slm proteins are regulated by a phosphorylation/dephosphorylation cycle involving the sphingolipid-activated protein kinases Pkh1 and Pkh2 and the calcium/calmodulin-dependent protein phosphatase calcineurin. By using a combination of mass spectrometry and mutational analysis, we identified serine residue 659 in Slm1 as a site of phosphorylation. Characterization of Slm1 mutants that mimic dephosphorylated and phosphorylated states demonstrated that phosphorylation at serine 659 is vital for survival under heat stress and promotes the proper polarization of the actin cytoskeleton. Finally, we present evidence that Slm proteins are also required for the trafficking of the raft-associated arginine permease Can1 to the plasma membrane, a process that requires sphingolipid synthesis and actin polymerization. Together with previous work, our findings suggest that Slm proteins are subject to regulation by multiple signals, including PI4,5P(2), TORC2, and sphingolipids, and may thus integrate inputs from different signaling pathways to temporally and spatially control actin polarization.

Impaired intestinal NHE3 activity in the PDK1 hypomorphic mouse

In vitro experiments have demonstrated the stimulating effect of serum- and glucocorticoid-inducible kinase (SGK)1 on the activity of the Na+/H+ exchanger (NHE3). SGK1 requires activation by phosphoinositide-dependent kinase (PDK)1, which may thus similarly play a role in the regulation of NHE3-dependent epithelial electrolyte transport. The present study was performed to explore the role of PDK1 in the regulation of NHE3 activity. Because mice completely lacking functional PDK1 are not viable, hypomorphic mice expressing approximately 20% of PDK1 (pdk1(hm)) were compared with their wild-type littermates (pdk1(wt)). NHE3 activity in the intestine and PDK1-overexpressing HEK-293 cells was estimated by utilizing 2',7'-bis(2-carboxyethyl)-5(6)-carboxyfluorescein fluorescence for the determination of intracellular pH. NHE activity was reflected by the Na+-dependent pH recovery from an ammonium prepulse (DeltapH(NHE)). The pH changes after an ammonium pulse allowed the calculation of cellular buffer capacity, which was not significantly different between pdk1(hm) and pdk1(wt) mice. DeltapH(NHE) was in pdk1(hm) mice, only 30 +/- 6% of the value obtained in pdk1(wt) mice. Conversely, DeltapH(NHE) was 32 +/- 7% larger in PDK1-overexpressing HEK-293 cells than in HEK-293 cells expressing the empty vector. The difference between pdk1(hm) and pdk1(wt) mice and between PDK1-overexpressing and empty vector-transfected HEK cells, respectively, was completely abolished in the presence of the NHE3 inhibitor S3226 (10 microM). In conclusion, defective PDK1 expression leads to significant impairment of NHE3 activity in the intestine, pointing to a role of PDK1-dependent signaling in the regulation of NHE-mediated electrolyte transport.

Functions and metabolism of sphingolipids in Saccharomyces cerevisiae

We describe recent advances in understanding sphingolipid functions and metabolism in the baker's yeast Saccharomyces cerevisiae. One milestone has been reached in yeast sphingolipid research with the complete or nearly complete identification of genes involved in sphingolipid synthesis and breakdown. Other advances include roles for sphingolipid long-chain bases as signaling molecules that regulate growth, responses to heat stress, cell wall synthesis and repair, endocytosis and dynamics of the actin cytoskeleton. We touch briefly on other sphingolipid functions so that readers unfamiliar with the field will gain a broader view of sphingolipid research. These functions include roles in protein trafficking/exocytosis, lipid rafts or microdomains, calcium homeostasis, longevity and cellular aging, nutrient uptake, cross-talk with other lipids and the interaction of sphingolipids and antifungal drugs.