PDK1 Activity Research Articles

Targeting 3-Phosphoinoside-Dependent Kinase-1 to Inhibit Insulin-Like Growth Factor-I Induced AKT and p70 S6 Kinase Activation in Breast Cancer Cells

Binding of IGF to IGF-IR activates PI3K to generate PIP3 which in turn recruits and activates proteins that contain a pleckstrin homology (PH) domain, including AKT and PDK1. PDK1 is highly expressed in breast tumor samples and breast cancer cell lines. Here we demonstrate that targeting PDK1 with the potent and selective PDK1 inhibitor PF-5177624 in the IGF-PI3K pathway blocks breast cancer cell proliferation and transformation. Breast cancer cell lines MCF7 and T47D, representing the luminal ER positive subtype and harboring PIK3CA mutations, were most responsive to IGF-I induction resulting in upregulated AKT and p70S6K phosphorylation via PDK1 activation. PF-5177624 downregulated AKT and p70S6K phosphorylation, blocked cell cycle progression, and decreased cell proliferation and transformation to block IGFR-I induced activation in breast cancer cells. These results may provide insight into clinical strategies for developing an IGFR-I inhibitor and/or a PDK1 inhibitor in luminal breast cancer patients.

Substrate-Selective Inhibition of Protein Kinase PDK1 by Small Compounds that Bind to the PIF-Pocket Allosteric Docking Site

The PIF-pocket of AGC protein kinases participates in the physiologic mechanism of regulation by acting as a docking site for substrates and as a switch for the transduction of the conformational changes needed for activation or inhibition. We describe the effects of compounds that bind to the PIF-pocket of PDK1. In vitro, PS210 is a potent activator of PDK1, and the crystal structure of the PDK1-ATP-PS210 complex shows that PS210 stimulates the closure of the kinase domain. However, in cells, the prodrug of PS210 (PS423) acts as a substrate-selective inhibitor of PDK1, inhibiting the phosphorylation and activation of S6K, which requires docking to the PIF-pocket, but not affecting PKB/Akt. This work describes a tool to study the dynamics of PDK1 activity and a potential approach for drug discovery.

PDK1 Protein Phosphorylation at Thr354 by Murine Protein Serine-Threonine Kinase 38 Contributes to Negative Regulation of PDK1 Protein Activity

Murine protein serine-threonine kinase 38 (MPK38) is a member of the AMP-activated protein kinase-related serine/threonine kinase family, which acts as cellular energy sensors. In this study, MPK38-induced PDK1 phosphorylation was examined to elucidate the biochemical mechanisms underlying phosphorylation-dependent regulation of 3-phosphoinositide-dependent protein kinase-1 (PDK1) activity. The results showed that MPK38 interacted with and inhibited PDK1 activity via Thr(354) phosphorylation. MPK38-PDK1 complex formation was mediated by the amino-terminal catalytic kinase domain of MPK38 and the pleckstrin homology domain of PDK1. This activity was dependent on insulin, a PI3K/PDK1 stimulator, as well as various apoptotic stimuli, including TNF-α, H(2)O(2), thapsigargin, and ionomycin. MPK38 inhibited PDK1 activity in a kinase-dependent manner and alleviated PDK1-mediated suppression of TGF-β (or ASK1) signaling, probably via the phosphorylation of PDK1 at Thr(354). In addition, MPK38-mediated inhibition of PDK1 activity was accompanied by the modulation of PDK1 binding to its positive and negative regulators, serine/threonine kinase receptor-associated protein and 14-3-3, respectively. Together, these findings suggest an important role for MPK38-mediated phosphorylation of PDK1 in the negative regulation of PDK1 activity.

Abstract 5151: Transcriptional regulation of the pyruvate dehydrogenase kinase Pdk2 by tumor suppressors

Abstract Conversion of pyruvate into lactate despite the presence of oxygen is a hallmark of the Warburg effect, and distinguishes the metabolism of many tumors from the metabolism of many normal cell types. Pdk2, a pyruvate dehydrogenase kinase, facilitates conversion of pyruvate into lactate by inhibiting the pyruvate dehydrogenase complex, which would otherwise convert pyruvate into acetyl-CoA. Previously, Pdk2 activity was thought to be controlled exclusively in a post-translational fashion, through inhibition or activation by metabolites. But in this study we show that two important tumor suppressor pathways, p53 and retinoblastoma, prevent the Warburg effect, and do so through repression of pdk2 transcription. Importantly, a heterologous promoter that drives physiological expression of Pdk2, but disallows repression by p53 or Rb, prevents these tumor suppressors from blocking the Warburg effect. Another consequence of pdk2 repression is increased production of reactive oxygen species, which stimulates p53-dependent apoptosis. We further show that when unable to downregulate Pdk2 expression, p53 can no longer cause apoptosis. Thus two critical features of tumor suppression, apoptosis and pyruvate metabolism, are mediated by repression of pdk2 transcription. This study proves that proper transcriptional regulation of the pdk2 gene is critical for tumor suppression. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 5151. doi:1538-7445.AM2012-5151

357 Comparative Effects of the Drugs Used for Pulmonary Hypertension Reversibility Testing: A Meta-Analysis

pyruvate dehydrogenase kinase (PDK) 2, catalyzing the phosphorylation of pyruvate dehydrogenase (PDH) and thus regulating the flux of pyruvate into the mitochondria, were investigated during SMC proliferation and the development of human IH. Methods and Materials: A novel human intimal hyperplasia (IH) model was established in which balloon-injured human internal mammary arteries (hMA) were transplanted into the abdominal aortic position of nude rats. Results: IH rapidly developed over 21 days and was morphologically and in immunohistochemistry identical to the neointima of human coronary arteries. Over 21 days, we found increased proliferative and decreased apoptotic activity within the IH lesion and luminal obliteration progressed to approx. 60%. Tissue PDK2 expression was significantly elevated and PDH was inhibited.PDGF was markedly elevated in the animal serum during neointima formation. Activation of SMC in vitro with PDGF caused increased PDK2 activity via both PI3K/Akt and Ras/Raf/Erkpathways. We observed an increase of the mitochondrial membrane potential ( m), a decreased release of mitochondrial reactive oxygen species (mROS), as well as enhanced cell proliferation. PDK2 inhibition using dichloroacetate(DCA) or PDK2 shRNA prevented PDGF-induced mitochondrial hyperpolarization and SMC proliferation.In vivo, oral DCAtreatment of rats or pre-implant PDK2 shRNA-transduction of the hMAs prevented the injuryinduced increase of SMC proliferation and significantly decreased IH development. Conclusions: PDK2 is the link between PDGF downstream pathways and mitochondrial metabolic modulation and its activation facilitates SMC proliferation. Pharmacological or genetic inhibition of PDK2 in SMC attenuates PDGF-induced increase in m, cell proliferation and the development of human IH. PDK2 may be a novel target for the prevention of restenosis.

P3-17-05: Beyond HER2 and Hormonal Agents: The Heat Shock Protein 90 Inhibitor Ganetespib as a Potential New Breast Cancer Therapy.

Abstract Background: Ganetespib is a fully synthetic and selective inhibitor of heat shock protein 90 (HSP90), a molecular chaperone recognized as a key facilitator of breast cancer initiation, progression and metastasis. Methods: Preclinical activity of ganetespib across the four major breast cancer subtypes and inflammatory breast cancer was assessed in vitro and in vivo. Modulation of cell proliferation and viability was determined both in monolayer and three-dimensional cultures. HSP90 client protein expression and activity was monitored by Western blot and protein array. To recapitulate clinical dosing, kinetics of client protein destabilization were measured following short exposures to drug in vitro. Anticancer activity of ganetespib was further investigated in vivo using breast cancer xenografts. Results: Ganetespib displayed potent, low nanomolar activity in luminal (A and B), basal (A and B) and inflammatory breast cancer cell lines grown as monolayers in vitro. BT-474 (HER2 amplified) luminal cells grown as mammospheres in 3D were equally as sensitive to ganetespib as those grown in monolayer. In luminal cells, ganetespib simultaneously disrupted multiple signaling components including the estrogen and progesterone receptor, several receptor and non-receptor tyrosine kinases, as well as the MAPK pathway. Further, ganetespib effectively inhibited AKT, PDK1 and SGK3 activity in PIK3CA mutant cells suggesting that HSP90 is essential for both AKT-dependent and AKT-independent signaling. Clinically relevant exposure times to ganetespib in vitro resulted in potent, long term destabilization of HER2. In the basal-like breast cancer cell line MDA-MB-231, enriched in CD44+CD24- stem like cells that commonly display chemotherapeutic resistance and activated JAK2/STAT3 signaling, ganetespib (50 nM) induced significant degradation of JAK2 concordant with loss of both tyrosine and serine phosphorylation of STAT3, followed by cell death. The potent anticancer activity in vitro translated in vivo, where ganetespib was effective in modulating breast cancer xenograft growth as a single agent in both luminal and basal-like breast cancer models. Finally, ganetespib has demonstrated encouraging signs of clinical activity in breast cancer patients, including confirmed partial responses in both a triple negative breast cancer patient and a HER2 positive breast cancer patient. Conclusions: Ganetespib is a highly potent HSP90 inhibitor that displays preclinical activity in breast cancer due to its ability to simultaneously perturb multiple oncogenic signaling pathways. Citation Information: Cancer Res 2011;71(24 Suppl):Abstract nr P3-17-05.

Oleanane triterpenoid CDDO-Me inhibits Akt activity without affecting PDK1 kinase or PP2A phosphatase activity in cancer cells

Our previous studies have shown that methyl-2-cyano-3,12-dioxooleana-1,9(11)-dien-28-oate (CDDO-Me), a oleanane synthetic triterpenoid induces apoptosis in prostate cancer cells by inhibiting the Akt/NF-κB/mTOR signaling cascade; however, the mechanism by which CDDO-Me inhibits Akt/NF-κB/mTOR signaling has remained undetermined. Present studies show that Akt plays a critical role in the response of prostate cancer cells to CDDO-Me. Silencing of Akt sensitized PC-3 cells to CDDO-Me, whereas its overexpression rendered them resistant to CDDO-Me. Evaluation of the effect of CDDO-Me on Akt which lies upstream of NF-κB and mTOR showed that CDDO-Me directly inhibits the Akt kinase activity in cell-free kinase activity assay and in vivo without modulating the activity of PDK1, the upstream kinase that phosphorylates and activates Akt. The inhibition of Akt activity resulted in inhibition of phosphorylation/inactivation of proapoptotic procaspase-9, Bad and Foxo3a. Further, inhibition of p-Akt by CDDO-Me was not attributable to an increase in the activity of protein phosphatase 2A (PP2A) or PH domain/leucine-rich repeat protein phosphatase1 (PHLPP1) both of which dephosphorylate p-Akt. These findings show that Akt is a direct target of CDDO-Me in the Akt/NF-κB/mTOR prosurvival signaling axis.

Characterization of a PDK1 Homologue from the Moss Physcomitrella patens

The serine/threonine protein kinase 3-phosphoinositide-dependent protein kinase 1 (PDK1) is a highly conserved eukaryotic kinase that is a central regulator of many AGC kinase subfamily members. Through its regulation of AGC kinases, PDK1 controls many basic cellular processes, from translation to cell survival. While many of these PDK1-regulated processes are conserved across kingdoms, it is not well understood how PDK1 may have evolved within kingdoms. In order to better understand PDK1 evolution within plants, we have isolated and characterized the PDK1 gene from the moss Physcomitrella patens (PpPDK1), a nonvascular representative of early land plants. PpPDK1 is similar to other plant PDK1s in that it can functionally complement a yeast PDK1 knockout line. However, unlike PDK1 from other plants, the P. patens PDK1 protein does not bind phospholipids due to a lack of the lipid-binding pleckstrin homology domain, which is used for lipid-mediated regulation of PDK1 activity. Sequence analysis of several PDK1 proteins suggests that lipid regulation of PDK1 may not commonly occur in algae and nonvascular land plants. PpPDK1 can phosphorylate AGC kinase substrates from tomato (Solanum lycopersicum) and P. patens at the predicted PDK1 phosphorylation site, indicating that the PpPDK1 substrate phosphorylation site is conserved with higher plants. We have also identified residues within the PpPDK1 kinase domain that affect kinase activity and show that a mutant with highly reduced kinase activity can still confer cell viability in both yeast and P. patens. These studies lay the foundation for further analysis of the evolution of PDK1 within plants.

Structure-based design of PDK1 inhibitors

A macrocyclic 2-anilino-4-phenyl-pyrimidine CDK/Flt3/JAK2 inhibitor was found to have moderate PDK1 activity. After docking into a PDK1 X-ray structure it was suggested that the pyrimidine ring could be substituted for a purine thereby increasing the number of hydrophobic contacts with the protein and forming an additional hydrogen bond to the kinase hinge. Deletion of the macrocyclic linker allowed a more rapid optimisation of the aromatic substituents as well as the introduction of an amino-amide solubility tag. This improved both binding to the enzyme and physiochemical properties without compromising ligand efficiency.

Increased 90-kDa ribosomal S6 kinase (Rsk) activity is protective against mutant huntingtin toxicity

BackgroundThe 90-kDa ribosomal S6 kinase (Rsk) family is involved in cell survival. Rsk activation is regulated by sequential phosphorylations controlled by extracellular signal-regulated kinase (ERK) 1/2 and 3-phosphoinositide-dependent protein kinase 1 (PDK1). Altered ERK1/2 and PDK1 phosphorylation have been described in Huntington's disease (HD), characterized by the expression of mutant huntingtin (mhtt) and striatal degeneration. However, the role of Rsk in this neurodegenerative disease remains unknown. Here, we analyzed the protein levels, activity and role of Rsk in in vivo and in vitro HD models.ResultsWe observed increased protein levels of Rsk1 and Rsk2 in the striatum of HdhQ111/Q111 and R6/1 mice, STHdhQ111/Q111 cells and striatal cells transfected with full-length mhtt. Analysis of the phosphorylation of Rsk in Hdh mice and STHdh cells showed reduced levels of phospho Ser-380 (dependent on ERK1/2), whereas phosphorylation at Ser-221 (dependent on PDK1) was increased. Moreover, we found that elevated Rsk activity in STHdhQ111/Q111 cells was mainly due to PDK1 activity, as assessed by transfection with Rsk mutant constructs. The increase of Rsk in STHdhQ111/Q111 cells occurred in the cytosol and in the nucleus, which results in enhanced phosphorylation of both cytosolic and nuclear Rsk targets. Finally, pharmacological inhibition of Rsk, knock-down and overexpression experiments indicated that Rsk activity exerts a protective effect against mhtt-induced cell death in STHdhQ7/Q7 cells transfected with mhtt.ConclusionThe increase of Rsk levels and activity would act as a compensatory mechanism with capacity to prevent mhtt-mediated cell death. We propose Rsk as a good target for neuroprotective therapies in HD.

Growth hormone modulation of EGF-induced PI3K-Akt pathway in mice liver

The epidermal growth factor (EGF) activates the phosphatidylinositol 3-kinase (PI3K)-Akt cascade among other signaling pathways. This route is involved in cell proliferation and survival, therefore, its dysregulation can promote cancer. Considering the relevance of the PI3K-Akt signaling in cell survival and in the pathogenesis of cancer, and that GH was reported to modulate EGFR expression and signaling, the objective of this study was to analyze the effects of increased GH levels on EGF-induced PI3K-Akt signaling.EGF-induced signaling was evaluated in the liver of GH-overexpressing transgenic mice and in their normal siblings. While Akt expression was increased in GH-overexpressing mice, EGF-induced phosphorylation of Akt, relative to its protein content, was diminished at Ser473 and inhibited at Thr308; consequently, mTOR, which is a substrate of Akt, was not activated by EGF. However, the activation of PDK1, a kinase involved in Akt phosphorylation at Thr308, was not reduced in transgenic mice. Kinetics studies of EGF-induced Akt phosphorylation showed that it is rapidly and transiently induced in GH-overexpressing mice compared with normal siblings. Thus, the expression and activity of phosphatases involved in the termination of the PI3K-Akt signaling were studied. In transgenic mice, neither PTEN nor PP2A were hyperactivated; however, EGF induced the rapid and transient association of SHP-2 to Gab1, which mediates association to EGFR and activation of PI3K. Rapid recruitment of SHP2, which would accelerate the termination of the proliferative signal induced, could be therefore contributing to the diminished EGF-induced activity of Akt in GH-overexpressing mice.

Phosphatidylserine Binding Is Essential for Plasma Membrane Recruitment and Signaling Function of 3-Phosphoinositide-dependent Kinase-1

3-Phosphoinositide-dependent kinase-1 (PDK1) is a ubiquitously expressed serine/threonine kinase that functions downstream of phosphoinositide 3-kinase. Although binding of 3'-phosphoinositides, phosphatidylinositol 3,4,5-trisphosphate and phosphatidylinositol 3,4-bisphosphate, to the pleckstrin homology (PH) domain of PDK1 is known to be essential for its interaction with and activation of downstream kinases, the mechanism by which PDK1 is recruited to the plasma membrane remains controversial. Our surface plasmon resonance analysis of the PDK1 PH domain and selected mutants shows that the PH domain specifically binds phosphatidylserine using a site that is separate from the canonical phosphoinositide-binding site. Further cell studies show that this specific phosphatidylserine binding is important for the plasma membrane localization and signaling function of PDK1.

The Phosphoinositide-3-Kinase–Akt Signaling Pathway Is Important for Staphylococcus aureus Internalization by Endothelial Cells

Internalization of Staphylococcus aureus in bovine endothelial cells (BEC) is increased by tumor necrosis factor alpha stimulation and NF-κB activation. Because the phosphoinositide-3-kinase (PI3K)-Akt signaling pathway also modulates NF-κB activity, we considered whether the internalization of S. aureus by BEC is associated with the activity of PI3K and Akt. We found a time- and multiplicity of infection-dependent phosphorylation of Akt on Ser473 in BEC infected with S. aureus. This phosphorylation was inhibited by LY294002 (LY), indicating the participation of PI3K. Inhibition of either PI3K with LY or wortmannin, or Akt with SH-5, strongly reduced the internalization of S. aureus. Transfection of BEC with a dominant-negative form of the Akt gene significantly decreased S. aureus internalization, whereas transfection with the constitutively active mutant increased the number of internalized bacterium. Inhibition of PDK1 activity with OSU-03012 did not affect the level of S. aureus internalization, demonstrating that phosphorylation of Akt on Thr308 is not important for this process. Compared to the untreated control, the adherence of S. aureus to the surface of BEC was unaltered when cells were transfected or incubated with the pharmacological inhibitors. Furthermore, Akt activation by internalized S. aureus triggered a time-dependent phosphorylation of glycogen synthase kinase-3α (GSK-3α) on Ser21 and GSK-3β on Ser9 that was partially inhibited with SH-5. Finally, treatment of BEC with LY prior to S. aureus infection inhibited the NF-κB p65 subunit phosphorylation on Ser536, indicating the involvement of PI3K. These results suggest that PI3K-Akt activity is important for the internalization of S. aureus and phosphorylation of GSK-3α, GSK-3β, and NF-κB.

The Deacetylase SIRT1 Promotes Membrane Localization and Activation of Akt and PDK1 During Tumorigenesis and Cardiac Hypertrophy

Signaling through the kinase Akt regulates many biological functions. Akt is activated during growth factor stimulation through a process that requires binding of Akt to phosphatidylinositol 3,4,5-trisphosphate (PIP(3)), which promotes membrane localization and phosphorylation of Akt by the upstream kinase PDK1 (phosphoinositide-dependent protein kinase 1). We show that Akt and PDK1 are acetylated at lysine residues in their pleckstrin homology domains, which mediate PIP(3) binding. Acetylation blocked binding of Akt and PDK1 to PIP(3), thereby preventing membrane localization and phosphorylation of Akt. Deacetylation by SIRT1 enhanced binding of Akt and PDK1 to PIP(3) and promoted their activation. Mice injected with cells expressing a mutant that mimicked a constitutively acetylated form of Akt developed smaller tumors than those injected with cells expressing wild-type Akt. Furthermore, impaired Akt activation in the hearts of SIRT1-deficient mice was associated with reduced cardiac hypertrophy in response to physical exercise and angiotensin II. These findings uncover a key posttranslational modification of Akt that is important for its oncogenic and hypertrophic activities.

Abstract 2514: Radiation-induced HIF-1α prevents apoptosis through reduction of ROS productions after irradiation in glioblastoma

Abstract Purpose/Objective(s): Glioblastoma is one of the most radioresistant tumors, and hypoxic regions are commonly observed. Molecular markers of hypoxia, Hypoxia inducible factor-1α (HIF-1α), a transcriptional factor, is strongly expressed in glioblastoma and associated with poor survival. Recent studies have shown that HIF-1α is induced by irradiation even if under normoxic conditions, although the mechanism still remains unclear. In this study, we firstly investigated the association radiation-induced HIF-1α with radioresistance in glioblastoma cells. Next, to investigate the function of HIF-1α, we focused Pyruvate dehydrogenase kinase (PDK1), one of the target genes of HIF-1α, which can decrease reactive oxygen species (ROS) productions in hypoxic conditions. We analyzed whether radiation-induced HIF-1α decreases ROS productions through activation of PDK1 after irradiation under normoxic conditions. Materials/Methods: Clonogenic survival assay was performed to investigate the effect of radiation on colony-forming ability in established cell lines (U87, LN18, and LN229) and primary cell lines (VC3 and MGH8). In these cell lines, HIF-1α and PDK1 expressions were analyzed by using Western blotting after irradiation. To investigate whether HIF-1α is associated with radiation resistance, we used two HIF-1α blockade strategies: stable knockdown cell lines of HIF-1α using lentivirus-based sh-RNA and YC-1 (A.G. Scientific, Inc.), a novel HIF-1α inhibitor. These blockade treatments were evaluated by clonogenic survival assay, MTS assay (Promega), and apoptotic proteins, such as cleaved caspase-3 and cleaved PARP. To measure ROS levels, CM-H2DCFDA (Invitrogen) was used at a dose of 10 µM. Results: Clonogenic survival assay showed that U87 and LN18 were more radioresistant than VC3, MGH8, and LN229. In this study, U87 and LN18 were defined as radioresistant cells and the others were radiosensitive cells. Under normoxic conditions radiation-induced HIF-1α was detected in radioresistant cells, but not in radiosensitive cells. Knockdown of HIF-1α decreased cell proliferation in clonogenic survival assay. YC-1 decreased the radiation-induced HIF-1α and decreased cell viability in MTS assay. These HIF-1α blockades treatments increased cleaved caspase-3 and cleaved PARP expression after irradiation. To evaluate the function of HIF-1α, PDK1 expression and ROS levels were analyzed. PDK1 was increased only in radioresistant cells that have radiation-induced HIF-1α. Knockdown of HIF-1α decreased PDK1 expressions and increased ROS levels after irradiation. Conclusions: We demonstrated that radiation-induced HIF-1α induces PDK1 expression and decreases ROS productions, resulting in prevention of apoptosis after irradiation. Additional studies are ongoing to determine the in vivo efficacy. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr 2514. doi:10.1158/1538-7445.AM2011-2514

Abstract 1005: Plk3 functions as an essential component of the survival and hypoxia regulatory pathway by direct phosphorylation and regulation of PTEN and HIF-1α

Abstract Plk3, a member of the Polo-like kinase family, plays an important role in regulating cell cycle progression and cell cycle checkpoints in response to genotoxic stresses. Plk3 is also strongly implicated in tumorigenesis because PLK3-/- mice develop tumors in multiple organs at an enhanced rate. We have previously shown that many of the tumors developed in the PLK3 null mice are large in size and are highly vascularized, suggesting that this kinase may be involved in regulating the angiogenesis pathway. By studying PLK3 null primary isogenic murine embryonic fibroblasts (MEFs), we tested the hypothesis that Plk3 functions as a component in the hypoxia signaling pathway. PLK3-/- MEFs contained a reduced level and activity of PTEN that closely correlated with increased PDK1 and Akt1 activation as well as decreased GSK3β activity. The activated Akt1 in PLK3 null MEFs was closely associated with enhanced stability of HIF-1α under hypoxic conditions. Purified recombinant Plk3 phosphorylated PTEN and HIF-1α in vitro and phospho-mutant analysis indicated that Plk3 directly regulates these proteins in vivo. Combined, our study identifies Plk3 as a new and essential player in the regulation of the hypoxia and angiogenesis signaling pathway. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr 1005. doi:10.1158/1538-7445.AM2011-1005

Regulation of AKT Phosphorylation at Ser473 and Thr308 by Endoplasmic Reticulum Stress Modulates Substrate Specificity in a Severity Dependent Manner

Endoplasmic reticulum (ER) stress is a common factor in the pathophysiology of diverse human diseases that are characterised by contrasting cellular behaviours, from proliferation in cancer to apoptosis in neurodegenerative disorders. Coincidently, dysregulation of AKT/PKB activity, which is the central regulator of cell growth, proliferation and survival, is often associated with the same diseases. Here, we demonstrate that ER stress modulates AKT substrate specificity in a severity-dependent manner, as shown by phospho-specific antibodies against known AKT targets. ER stress also reduces both total and phosphorylated AKT in a severity-dependent manner, without affecting activity of the upstream kinase PDK1. Normalisation to total AKT revealed that under ER stress phosphorylation of Thr308 is suppressed while that of Ser473 is increased. ER stress induces GRP78, and siRNA-mediated knock-down of GRP78 enhances phosphorylation at Ser473 by 3.6 fold, but not at Thr308. Substrate specificity is again altered. An in-situ proximity ligation assay revealed a physical interaction between GRP78 and AKT at the plasma membrane of cells following induction of ER stress. Staining was weak in cells with normal nuclear morphology but stronger in those displaying rounded, condensed nuclei. Co-immunoprecipitation of GRP78 and P-AKT(Ser473) confirmed the immuno-complex consists of non-phosphorylated AKT (Ser473 and Thr308). The interaction is likely specific as AKT did not bind to all molecular chaperones, and GRP78 did not bind to p70 S6 kinase. These findings provide one mechanistic explanation for how ER stress contributes to human pathologies demonstrating contrasting cell fates via modulation of AKT signalling.

5‐hydroxy‐2‐(4‐hydroxy‐3‐methoxyphenyl)‐3,7‐dimethoxy‐4H‐chromen‐4‐one (MSF‐2) suppresses fMLP‐mediated respiratory burst in human neutrophils by inhibiting phosphatidylinositol 3‐kinase activity

Respiratory burst mediates crucial bactericidal mechanism in neutrophils. However, undesirable respiratory burst leads to pathological inflammation and tissue damage. This study investigates the effect and the underlying mechanism of 5-hydroxy-2-(4-hydroxy-3-methoxyphenyl)-3,7-dimethoxy-4H-chromen-4-one (MSF-2), a lignan extracted from the fruit of Melicope Semecarprifolia, on fMLP-induced respiratory burst in human neutrophils and suggests a possible therapeutic approach to ameliorate disease associated with neutrophil hyperactivation. MSF-2 inhibited fMLP-induced neutrophil superoxide anion production, cathepsin G release and migration in human neutrophils isolated from healthy volunteers, reflecting inhibition of phosphatidylinositol 3-kinase (PI3K) activation. Specifically, PI3K/AKT activation results in migration, degranulation and superoxide anion production in neutrophils. MSF-2 suppresses PI3K activation and phosphatidylinositol (3,4,5)-trisphosphate (PIP3) production, and consequently inhibits downstream activation of PDK1 and AKT. Further, PI3K also stimulates respiratory burst via PLC-dependent elevation of intracellular calcium. MSF-2 reduces fMLP-mediated PLCγ2 activation and intracellular calcium accumulation notably through extracellular calcium influx in a PI3K and PLC-dependent manner. However, MSF-2 is not a competitive or allosteric antagonist of fMLP. Additionally, in an in vivo study, MSF-2 prevents fMLP-induced neutrophil infiltration and inflammation in mice. In conclusion, MSF-2 opposes fMLP-mediated neutrophil activation and inflammation by inhibiting PI3K activation and subsequent activation of AKT and PLCγ2.

PDH activation during in vitro muscle contractions in PDH kinase 2 knockout mice: effect of PDH kinase 1 compensation

Pyruvate dehydrogenase (PDH) plays an important role in regulating carbohydrate oxidation in skeletal muscle. PDH is deactivated by a set of PDH kinases (PDK1, PDK2, PDK3, PDK4), with PDK2 and PDK4 being the most predominant isoforms in skeletal muscle. Although PDK2 is the most abundant isoform, few studies have examined its physiological role. The role of PDK2 on PDH activation (PDHa) at rest and during muscle stimulation at 10 and 40 Hz (eliciting low- and moderate-intensity muscle contractions, respectively) in isolated extensor digitorum longus muscles was studied in PDK2 knockout (PDK2KO) and wild-type (WT) mice (n = 5 per group). PDHa activity was unexpectedly 35 and 77% lower in PDK2KO than WT muscle (P = 0.043), while total PDK activity was nearly fourfold lower in PDK2KO muscle (P = 0.006). During 40-Hz contractions, initial force was lower in PDK2KO than WT muscle (P < 0.001) but fatigued similarly to ∼75% of initial force by 3 min. There were no differences in initial force or rate of fatigue during 10-Hz contractions. PDK1 compensated for the lack of PDK2 and was 1.8-fold higher in PDK2KO than WT muscle (P = 0.019). This likely contributed to ensuring that resting PDHa activity was similar between the groups and accounts for the lower PDH activation during muscle contraction, as PDK1 is a very potent inhibitor of the PDH complex. Increased PDK1 expression appears to be regulated by hypoxia inducible factor-1α, which was 3.5-fold higher in PDK2KO muscle. It is clear that PDK2 activity is essential, even at rest, in regulation of carbohydrate oxidation and production of reducing equivalents for the electron transport chain. In addition, these results underscore the importance of the overall kinetics of the PDK isoform population, rather than total PDK activity, in determining transformation of the PDH complex and PDHa activity during muscle contraction.

Study of the PDK1/AKT signaling pathway using selective PDK1 inhibitors, HCS, and enhanced biochemical assays

The PI3K/AKT signaling pathway has an important regulatory role in cancer cell growth and tumorigenesis. Signal transduction through this pathway requires the assembly and activation of PDK1 and AKT at the plasma membrane. On activation of the pathway, PDK1 and AKT1/2 translocate to the membrane and bind to phosphatidylinositol-(3,4,5)-trisphosphate (PIP 3) through interaction with their pleckstrin-homology domains. A biochemical method was developed to measure the kinase activity of PDK1 and AKT1/2, utilizing nickel-chelating coated lipid vesicles as a way to mimic the membrane environment. The presence of these vesicles in the reaction buffer enhanced the specific activity of the His-tagged PDK1 (full-length, and the truncated kinase domain) and the activity of the full-length His-tagged AKT1 and AKT2 when assayed in a cascade-type reaction. This enhanced biochemical assay is also suitable for measuring the inhibition of PDK1 by several selective compounds from the carbonyl-4-amino-pyrrolopyrimidine (CAP) series. One of these inhibitors, PF-5168899, was further evaluated using a high content cell-based assay in the presence of CHO cells engineered with GFP-PDK1.