Domain swapping used to investigate the mechanism of protein kinase B regulation by 3-phosphoinositide-dependent protein kinase 1 and Ser473 kinase

Hyperoside Protects Trastuzumab-Induced Cardiotoxicity via Activating the PI3K/Akt Signaling Pathway

PURPOSE

Trastuzumab is a landmark agent in the treatment of human epidermal growth factor receptor-2(HER2)-positive breast cancer. Nevertheless, trastuzumab also comes with unexpected cardiac side effects. Hyperoside is a natural product that serves beneficial roles in cardiovascular disease. This study aimed to explore the effect and mechanism of hyperoside in trastuzumab-induced cardiotoxicity.

METHODS

A female C57BL/6 mice cardiotoxicity model was established via intraperitoneally injecting with trastuzumab (10 mg/kg/day, once every other day, cumulative dosage to 40 mg/kg) with or without hyperoside (15 or 30 mg/kg/day) administration. In vitro, the H9c2 cells were exposed to 1 μM trastuzumab with or without hyperoside (100 or 200 μM) administration. Cardiac function was evaluated by echocardiographic, myocardial enzymes levels, and pathological section examinations. TUNEL staining and Annexin V-FITC/ propidium iodide flow cytometry were used to analyze the cardiomyocyte apoptosis.

RESULTS

Compared to the control group, the LVEF, LVFS was decreased and the concentrations of cTnT, CK, CK-MB and LDH in mice were significantly increased after treatment with trastuzumab. Collagen deposition and cardiomyocyte hypertrophy were observed in the myocardium of the trastuzumab group. However, these changes were all reversed by different doses of hyperoside. In addition, hyperoside attenuated trastuzumab-induced myocardium apoptosis and H9c2 cells apoptosis through inhibiting the expressions of cleaved caspase-3 and Bax. Trastuzumab abolished the PI3K/Akt signaling pathway in mice and H9c2 cells, while co-treatment of hyperoside effectively increased the ratio of p-Akt/Akt.

CONCLUSION

Hyperoside inhibited trastuzumab-induced cardiotoxicity through activating the PI3K/Akt signaling pathway. Hyperoside may be a promising therapeutic approach to trastuzumab-induced cardiotoxicity.

A RAC-alpha serine/threonine-protein kinase (CgAKT1) involved in the synthesis of CgIFNLP in oyster Crassostrea gigas

The RAC-alpha serine/threonine-protein kinase (AKT) is one of the most important protein kinases involved in many biological processes in eukaryotes. In the present study, a novel AKT homologue named CgAKT1 was identified from the Pacific oyster Crassostrea gigas. The open reading frame (ORF) of CgAKT1 cDNA was of 1482 bp encoding a peptide with 493 amino acid residues. There were classical domains in the predicted CgAKT1 protein, including an N-terminal pleckstrin homology domain, a central catalytic domain and a C-terminal hydrophobic domain. The mRNA transcripts of CgAKT1 were detected in all the examined tissues of C. gigas with higher level in gills (8.24-fold of that in mantle, p < 0.05) and haemocytes (3.62-fold of that in mantle, p < 0.05). After poly (I:C) stimulation, the mRNA expression of CgAKT1 decreased significantly in haemocytes from 3 h (0.44-fold of that in the control group, p < 0.001) to 24 h (0.20-fold of that in the control group, p < 0.001), and then increased significantly at 48 h (3.65-fold of that in the control group, p < 0.05). The expression level of CgAKT1 mRNA increased significantly at 6 h after rCgIFNLP stimulation, which was 3.60-fold of that in the control group (p < 0.001). The Alexa Fluor 488 positive signals of CgAKT1 protein were found to be distributed in the cytoplasm and cell membrane of haemocytes, while those in the cytoplasm became weaker after poly (I:C) stimulation. In CgAKT1-RNAi oysters, the mRNA expression of cyclic GMP-AMP synthase (CgcGAS) and TANK-binding kinase 1 (CgTBK1) did not change significantly, but the mRNA expression level of stimulator of interferon gene (CgSTING), interferon regulatory factor-1 (CgIRF-1), interferon regulatory factor-8 (CgIRF-8) and IFN-like protein (CgIFNLP) increased significantly, which was 1.40-fold, 1.53-fold, 1.72-fold and 1.99-fold of that in EGFP-RNAi oysters (p < 0.05), respectively. In CgIFNLP-RNAi oysters, the transcripts of CgAKT1 decreased significantly compared to those in EGFP-RNAi oysters (0.16-fold, p < 0.01). Moreover, the expression of p-CgTBK1, CgSTING and CgIFNLP at the protein level in the oysters treated with p-AKT1 activator (SC-79) was significantly suppressed after poly (I:C) stimulation. After the transfection of CgAKT1, the expression of p-cGAS protein in HEK293T cells increased significantly, while the cyclic GMP-AMP in the cells and the interferon (IFN-β) in the cell culture fluid decreased significantly compared with that in the control group. These results indicated that CgAKT1 might play a negative role in antiviral immunity of oyster by regulating the synthesis of CgIFNLP.

The Inositol Phosphate System-A Coordinator of Metabolic Adaptability

All cells rely on nutrients to supply energy and carbon building blocks to support cellular processes. Over time, eukaryotes have developed increasingly complex systems to integrate information about available nutrients with the internal state of energy stores to activate the necessary processes to meet the immediate and ongoing needs of the cell. One such system is the network of soluble and membrane-associated inositol phosphates that coordinate the cellular responses to nutrient uptake and utilization from growth factor signaling to energy homeostasis. In this review, we discuss the coordinated interactions of the inositol polyphosphates, inositol pyrophosphates, and phosphoinositides in major metabolic signaling pathways to illustrate the central importance of the inositol phosphate signaling network in nutrient responses.

Interplay between PI3K/AKT pathway and heart disorders

The PI3K/AKT signaling has crucial role in the regulation of numerous physiological functions through activation of downstream effectors and modulation of cell cycle transition, growth and proliferation. This pathway participates in the pathogenesis of several human disorders such as heart diseases through regulation of size and survival of cardiomyocytes, angiogenic processes as well as inflammatory responses. Moreover, PI3K/AKT pathway participates in the process of myocardial injury induced by a number of substances such as H₂O₂, Mercury, lipopolysaccharides, adriamycin, doxorubicin and epirubicin. In this review, we describe the contribution of this pathway in the pathoetiology of myocardial ischemia/reperfusion injury and myocardial infarction, heart failure, cardiac hypertrophy, cardiomyopathy and toxins-induced cardiac injury.

Seasonal expressions of ERα, ERβ, EGF, EGFR, PI3K and Akt in the scent glands of the muskrats (Ondatra zibethicus)

Epidermal growth factor (EGF) is an important autocrine and/or paracrine mediator of steroid hormones to stimulate growth and differentiation in mammals. The aim of this study is to investigate seasonal expressions of estrogen receptor α (ERα), estrogen receptor β (ERβ), EGF, epidermal growth factor receptor (EGFR), phosphatidylinositol 3-kinase (PI3K) and protein kinase B (Akt) in the scent glands of the muskrats during the breeding and non-breeding seasons. Histologically, three types of cells including the glandular cells, interstitial cells and epithelial cells were identified in the scent glands in both seasons. Immunohistochemical results showed that ERα, ERβ, EGF, EGFR, PI3K and Akt were presented in the different types of cells of the scent glands during the breeding and non-breeding seasons. Transcriptome data of the scent glandular tissues from muskrats in the breeding and non-breeding seasons showed that differential seasonal changes might be related to the estrogen-EGFR signaling pathway. The gene expression levels of ERα, ERβ, EGF, EGFR, PI3K were increased, while the gene expression level of Akt were decreased in the breeding season than those in the non-breeding season. Besides, the concentrations of 17β-estradiol (E₂) in the serum and the scent glandular tissues were remarkably higher in the breeding season than those of the non-breeding season. Taken together, our results suggested that EGFR signaling pathway may coordinate with ERs signaling to regulate the seasonal changes of the scent glandular functions.

Insulin signaling in health and disease

The molecular mechanisms of cellular insulin action have been the focus of much investigation since the discovery of the hormone 100 years ago. Insulin action is impaired in metabolic syndrome, a condition known as insulin resistance. The actions of the hormone are initiated by binding to its receptor on the surface of target cells. The receptor is an α2β2 heterodimer that binds to insulin with high affinity, resulting in the activation of its tyrosine kinase activity. Once activated, the receptor can phosphorylate a number of intracellular substrates that initiate discrete signaling pathways. The tyrosine phosphorylation of some substrates activates phosphatidylinositol-3-kinase (PI3K), which produces polyphosphoinositides that interact with protein kinases, leading to activation of the kinase Akt. Phosphorylation of Shc leads to activation of the Ras/MAP kinase pathway. Phosphorylation of SH2B2 and of Cbl initiates activation of G proteins such as TC10. Activation of Akt and other protein kinases produces phosphorylation of a variety of substrates, including transcription factors, GTPase-activating proteins, and other kinases that control key metabolic events. Among the cellular processes controlled by insulin are vesicle trafficking, activities of metabolic enzymes, transcriptional factors, and degradation of insulin itself. Together these complex processes are coordinated to ensure glucose homeostasis.

Genetic code expansion and live cell imaging reveal that Thr-308 phosphorylation is irreplaceable and sufficient for Akt1 activity

The proto-oncogene Akt/protein kinase B (PKB) is a pivotal signal transducer for growth and survival. Growth factor stimulation leads to Akt phosphorylation at two regulatory sites (Thr-308 and Ser-473), acutely activating Akt signaling. Delineating the exact role of each regulatory site is, however, technically challenging and has remained elusive. Here, we used genetic code expansion to produce site-specifically phosphorylated Akt1 to dissect the contribution of each regulatory site to Akt1 activity. We achieved recombinant production of full-length Akt1 containing site-specific pThr and pSer residues for the first time. Our analysis of Akt1 site-specifically phosphorylated at either or both sites revealed that phosphorylation at both sites increases the apparent catalytic rate 1500-fold relative to unphosphorylated Akt1, an increase attributable primarily to phosphorylation at Thr-308. Live imaging of COS-7 cells confirmed that phosphorylation of Thr-308, but not Ser-473, is required for cellular activation of Akt. We found in vitro and in the cell that pThr-308 function cannot be mimicked with acidic residues, nor could unphosphorylated Thr-308 be mimicked by an Ala mutation. An Akt1 variant with pSer-308 achieved only partial enzymatic and cellular signaling activity, revealing a critical interaction between the γ-methyl group of pThr-308 and Cys-310 in the Akt1 active site. Thus, pThr-308 is necessary and sufficient to stimulate Akt signaling in cells, and the common use of phosphomimetics is not appropriate for studying the biology of Akt signaling. Our data also indicate that pThr-308 should be regarded as the primary diagnostic marker of Akt activity.

Differential regulation of the pro-inflammatory biomarker, YKL-40/CHI3L1, by PTEN/Phosphoinositide 3-kinase and JAK2/STAT3 pathways in glioblastoma

Constitutive activation of the phosphoinositide 3-kinase/AKT signaling pathway is frequently observed in high-grade gliomas with high frequency of losing PTEN tumor suppressor. To identify transcriptomic profiles associated with a hyperactivated PI3K pathway, RNA-sequencing analysis was performed in a glioblastoma cell line stably expressing PTEN. RNA-sequencing revealed enriched transcripts of pro-inflammatory mediators, and among the genes that displayed high differential expression was the secreted glycoprotein YKL-40. Treatment with chemical inhibitors that target the PI3K/AKT pathway elicited differential effects on YKL-40 expression in selected GBM cell lines, indicating that its expression displayed tumor cell-specific variations. This variability appeared to be correlated with the ability to transactivate the immune signaling molecules JAK2 and STAT3. In summary, the differential expression of the immunomodulatory molecule YKL-40 may affect the treatment efficacy of PI3K/AKT-based pathway inhibitors in glioblastoma.

Involvement of PI3K, Akt and RhoA in Oestradiol Regulation of Cardiac iNOS Expression

BACKGROUND

Oestradiol is an important regulatory factor with several positive effects on the cardiovascular (CV) system. We evaluated the molecular mechanism of the in vivo effects of oestradiol on the regulation of cardiac inducible nitric oxide (NO) synthase (iNOS) expression and activity.

METHODS

Male Wistar rats were treated with oestradiol (40 mg/kg, intraperitoneally) and after 24 h the animals were sacrificed. The concentrations of NO and L-Arginine (L-Arg) were determined spectrophotometrically. For protein expressions of iNOS, p65 subunit of nuclear factor-κB (NFκB-p65), Ras homolog gene family-member A (RhoA), angiotensin II receptor type 1 (AT1R), insulin receptor substrate 1 (IRS-1), p85, p110 and protein kinase B (Akt), Western blot method was used. Coimmunoprecipitation was used for measuring the association of IRS-1 with the p85 subunit of phosphatidylinositol- 3-kinase (PI3K). The expression of iNOS messenger ribonucleic acid (mRNA) was measured with the quantitative real-time polymerase chain reaction (qRT-PCR). Immunohistochemical analysis of the tissue was used to detect localization and expression of iNOS in heart tissue.

RESULTS

Oestradiol treatment reduced L-Arg concentration (p<0.01), iNOS mRNA (p<0.01) and protein (p<0.001) expression, level of RhoA (p<0.05) and AT1R (p<0.001) protein. In contrast, plasma NO (p<0.05), Akt phosphorylation at Thr308 (p<0.05) and protein level of p85 (p<0.001) increased after oestradiol treatment.

CONCLUSION

Our results suggest that oestradiol in vivo regulates cardiac iNOS expression via the PI3K/Akt signaling pathway, through attenuation of RhoA and AT1R.

The multifaceted role of PIP2 in leukocyte biology

Phosphatidylinositol 4,5-bisphosphate (PIP2) represents about 1 % of plasma membrane phospholipids and behaves as a pleiotropic regulator of a striking number of fundamental cellular processes. In recent years, an increasing body of literature has highlighted an essential role of PIP2 in multiple aspects of leukocyte biology. In this emerging picture, PIP2 is envisaged as a signalling intermediate itself and as a membrane-bound regulator and a scaffold of proteins with specific PIP2 binding domains. Indeed PIP2 plays a key role in several functions. These include directional migration in neutrophils, integrin-dependent adhesion in T lymphocytes, phagocytosis in macrophages, lysosomes secretion and trafficking at immune synapse in cytolytic effectors and secretory cells, calcium signals and gene transcription in B lymphocytes, natural killer cells and mast cells. The coordination of these different aspects relies on the spatio-temporal organisation of distinct PIP2 pools, generated by the main PIP2 generating enzyme, phosphatidylinositol 4-phosphate 5-kinase (PIP5K). Three different isoforms of PIP5K, named α, β and γ, and different splice variants have been described in leukocyte populations. The isoform-specific coupling of specific isoforms of PIP5K to different families of activating receptors, including integrins, Fc receptors, toll-like receptors and chemokine receptors, is starting to be reported. Furthermore, PIP2 is turned over by multiple metabolising enzymes including phospholipase C (PLC) γ and phosphatidylinositol 3-kinase (PI3K) which, along with Rho family small G proteins, is widely involved in strategic functions within the immune system. The interplay between PIP2, lipid-modifying enzymes and small G protein-regulated signals is also discussed.

ChAkt1 involvement in orchestrating the immune and heat shock responses in Crassostrea hongkongensis: Molecular cloning and functional characterization

G-protein-coupled receptors (GPCRs) are the largest class of cell-surface receptors and play crucial roles in virtually every organ system. As one of the major downstream effectors of GPCRs, Akt can acquire information from the receptors and coordinate intracellular responses for many signaling pathways, through which the serine/threonine kinase masters numerous aspects of biological processes, such as cell survival, growth, proliferation, migration, angiogenesis, and metabolism. In the present study, we have characterized the first Akt1 ortholog in mollusks using the Hong Kong oyster, Crassostrea hongkongensis (designed ChAkt1). The full-length cDNA is 2223 bp and encodes a putative protein of 493 amino acids that contains an amino-terminal pleckstin homology (PH) domain, a central catalytic domain, and a carboxy-terminal regulatory domain. Quantitative real-time PCR analysis showed that ChAkt1 mRNA is broadly expressed in various tissues and during different stages of the oyster's embryonic and larval development. Upon exposure to two stressors (microbial infection and heat shock), the expression level of ChAkt1 mRNA increases significantly. Furthermore, ChAkt1 is located in the cytoplasm in HEK293T cells, where the over-expression of ChAkt1 regulates the transcriptional activities of NF-κB and p53 reporter genes. Taken together, our results indicate that ChAkt1 most likely plays a central role in response to various stimuli in oysters and has a particular response to microbial pathogens and high temperature.

Dual inhibition of CDK4/Rb and PI3K/AKT/mTOR pathways by ON123300 induces synthetic lethality in mantle cell lymphomas

This study describes the characterization of a novel kinase inhibitor, ON123300, which inhibits CDK4/6 and PI3K-δ and exhibits potent activity against mantle cell lymphomas (MCLs) both in vitro and in vivo. We examined the effects of PD0332991 and ON 123300 on cell cycle progression, modulation of the Rb and PI3K/AKT pathways, and the induction of apoptosis in MCL cell lines and patient-derived samples. When Granta 519 and Z138C cells were incubated with PD0332991 and ON123300, both compounds were equally efficient in their ability to inhibit the phosphorylation of Rb family proteins. However, only ON123300 inhibited the phosphorylation of proteins associated with the PI3K/AKT pathway. Cells treated with PD0332991 rapidly accumulated in the G₀/G₁ phase of cell cycle as a function of increasing concentration. Although ON123300-treated cells arrested similarly at lower concentrations, higher concentrations resulted in the induction of apoptosis, which was not observed in PD 0332991-treated samples. Mouse xenograft assays also showed a strong inhibition of MCL tumor growth in ON123300-treated animals. Finally, treatment of ibrutinib-sensitive and resistant patient-derived MCLs with ON123300 also triggered apoptosis and inhibition of the Rb and PI3K/AKT pathways, suggesting that this compound might be an effective agent in MCL, including ibrutinib-resistant forms of the disease.

NOX4 pathway as a source of selective insulin resistance and responsiveness

OBJECTIVE

Type 2 diabetes mellitus and related syndromes exhibit a deadly triad of dyslipoproteinemia, which leads to atherosclerosis; hyperglycemia, which causes microvascular disease; and hypertension. These features share a common, but unexplained, origin-namely, pathway-selective insulin resistance and responsiveness. Here, we undertook a comprehensive characterization of pathway-selective insulin resistance and responsiveness in liver and hepatocytes by examining 18 downstream targets of the insulin receptor, surveying the AKT, ERK, and NAD(P)H oxidase 4 pathways.

METHODS AND RESULTS

Injection of insulin into hyperphagic, obese type 2 diabetic db/db mice failed to inactivate hepatic protein tyrosine phosphatase gene family members, a crucial action of NAD(P)H oxidase 4 previously thought to be required for all signaling through AKT and ERK. Insulin-stimulated type 2 diabetic livers unexpectedly produced an unusual form of AKT that was phosphorylated at Thr308 (pT308), with only weak insulin-stimulated phosphorylation at Ser473. Remarkably, knockdown or inhibition of NAD(P)H oxidase 4 in cultured hepatocytes recapitulated the entire complicated pattern of pathway-selective insulin resistance and responsiveness seen in vivo-namely, monophosphorylated pT308-AKT, impaired insulin-stimulated pathways for lowering plasma lipids and glucose, but continued lipogenic pathways and robust ERK activation.

CONCLUSIONS

Functional disturbance of a single molecule, NAD(P)H oxidase 4, is sufficient to induce the key harmful features of deranged insulin signaling in type 2 diabetes mellitus, obesity, and other conditions associated with hyperinsulinemia and pathway-selective insulin resistance and responsiveness.

Insulin stimulation of Akt/PKB phosphorylation in the placenta of preeclampsia patients

CONTEXT AND OBJECTIVE

Preeclampsia is a multi-systemic disease and one of the most frequent severe health problems during pregnancy. Binding of insulin triggers phosphorylation and activates cytoplasmic substrates such as phosphatidylinositol 3 kinase (PI3K). Phosphorylation of membrane phosphoinositide 2 (PIP2) to phosphoinositide 3 (PIP3) by PI3K starts Akt/PKB activation. Defects in phosphorylation of the insulin receptor and its substrates have an important role in insulin resistance. Studies have shown that insulin resistance is associated with preeclampsia and its pathophysiology. The aim here was to investigate insulin stimulation of the Akt/PKB pathway in the placenta, in normal and preeclampsia parturients.

DESIGN AND SETTING

Cross-sectional study in a tertiary public university hospital.

METHODS

Placentas were collected from 12 normal and 12 preeclampsia patients. These were stimulated and analyzed using Western blot to quantify the Akt/PKB phosphorylation.

RESULTS

The insulin stimulation was confirmed through comparing the stimulated group (1.14 ± 0.10) with the non-stimulated group (0.91 ± 0.08; P < 0.001). The phosphorylation of Akt/PKB did not differ between the placenta of the normal patients (1.26 ± 0.16) and those of the preeclampsia patients (1.01 ± 0.11; P = 0.237).

CONCLUSIONS

In vitro insulin stimulation of the human placenta has been well established. There was no difference in Akt/PKB phosphorylation, after stimulation with insulin, between placentas of normal and preeclampsia patients. Nevertheless, it cannot be ruled out that the Akt/PKB signaling pathway may have a role in the pathophysiology of preeclampsia, since the substrates of Akt/PKB still need to be investigated.

3DSwap: curated knowledgebase of proteins involved in 3D domain swapping

Three-dimensional domain swapping is a unique protein structural phenomenon where two or more protein chains in a protein oligomer share a common structural segment between individual chains. This phenomenon is observed in an array of protein structures in oligomeric conformation. Protein structures in swapped conformations perform diverse functional roles and are also associated with deposition diseases in humans. We have performed in-depth literature curation and structural bioinformatics analyses to develop an integrated knowledgebase of proteins involved in 3D domain swapping. The hallmark of 3D domain swapping is the presence of distinct structural segments such as the hinge and swapped regions. We have curated the literature to delineate the boundaries of these regions. In addition, we have defined several new concepts like ‘secondary major interface’ to represent the interface properties arising as a result of 3D domain swapping, and a new quantitative measure for the ‘extent of swapping’ in structures. The catalog of proteins reported in 3DSwap knowledgebase has been generated using an integrated structural bioinformatics workflow of database searches, literature curation, by structure visualization and sequence–structure–function analyses. The current version of the 3DSwap knowledgebase reports 293 protein structures, the analysis of such a compendium of protein structures will further the understanding molecular factors driving 3D domain swapping.

Database URL:http://caps.ncbs.res.in/3dswap

Does metformin influence the insulin-, IGF I- and IGF II-receptor gene expression and Akt phosphorylation in human decidualized endometrial stromal cells?

OBJECTIVE

To assess the effects of metformin on insulin-, IGF I-, and IGF II-receptor gene expression and Akt phosphorylation in decidualized human endometrial stromal cells (ESC) after stimulation with insulin, IGF I and II.

STUDY DESIGN

ESC were isolated from healthy, regularly cycling women and after two passages decidualized with estrogen/progesterone±metformin. Cells were incubated with insulin, IGF I or IGF II for 1, 5, and 10 min to assess Akt phosphorylation by Western blot. To investigate the insulin-, IGF I- and IGF II-receptor gene expression ESC were incubated with insulin, IGF I or IGF II for 6 and 24h.

RESULTS

Insulin- and IGF I-receptor gene expression in ESC changed significantly after incubation with insulin, IGF I or IGF II. This was further augmented in metformin pretreated cells, while IGF II-receptor gene expression changed particularly after pretreatment with metformin. Akt phosphorylation peaked after 5 min insulin, IGF I and IGF II stimulation in ESC in both control (control 0.08 ± 0.03 vs. insulin 0.74 ± 0.19, IGF I 0.68 ± 0.22, IGF II 0.53 ± 0.13, p<0.05) and metformin pretreated cells (control 0.03 ± 0.01 vs. insulin 0.75 ± 0.11, IGF I 0.74 ± 0.15, IGF II 0.67 ± 0.09, p<0.005). However, there was no significant difference between the control and metformin pretreated group.

CONCLUSION

Insulin, IGF I and IGF II lead to changes in their receptor gene expression and induced Akt phosphorylation in ESC. These effects were further highlighted in the presence of metformin.

Multiple implications of 3-phosphoinositide-dependent protein kinase 1 in human cancer

3-phosphoinositide-dependent protein kinase-1 (PDK1) is a central mediator of cellular signaling between phosphoinositide-3 kinase and various intracellular serine/threonine kinases, including protein kinase B, p70 ribosomal S6 kinase, serum and glucocorticoid-inducible kinase, and protein kinase C. PDK1 activates members of the AGC family of protein kinases by phosphorylating serine/threonine residues in the activation loop. Here, we review the regulatory mechanisms of PDK1 and its roles in cancer. PDK1 is activated by autophosphorylation in the activation loop and other serine residues, as well as by phosphorylation of Tyr-9 and Tyr-373/376. Src appears to recognize PDK1 following tyrosine phosphorylation. The role of heat shock protein 90 in regulating PDK1 stability and PDK1-Src complex formation are also discussed. Furthermore, we summarize the subcellular distribution of PDK1. Finally, an important role for PDK1 in cancer chemotherapy is proposed. In conclusion, a better understanding of its molecular regulatory mechanisms in various signaling pathways will help to explain how PDK1 acts as an oncogenic kinase in various cancers, and will contribute to the development of novel cancer chemotherapies.

Insights into Protein Sequence and Structure-Derived Features Mediating 3D Domain Swapping Mechanism using Support Vector Machine Based Approach

3-dimensional domain swapping is a mechanism where two or more protein molecules form higher order oligomers by exchanging identical or similar subunits. Recently, this phenomenon has received much attention in the context of prions and neurodegenerative diseases, due to its role in the functional regulation, formation of higher oligomers, protein misfolding, aggregation etc. While 3-dimensional domain swap mechanism can be detected from three-dimensional structures, it remains a formidable challenge to derive common sequence or structural patterns from proteins involved in swapping. We have developed a SVM-based classifier to predict domain swapping events using a set of features derived from sequence and structural data. The SVM classifier was trained on features derived from 150 proteins reported to be involved in 3D domain swapping and 150 proteins not known to be involved in swapped conformation or related to proteins involved in swapping phenomenon. The testing was performed using 63 proteins from the positive dataset and 63 proteins from the negative dataset. We obtained 76.33% accuracy from training and 73.81% accuracy from testing. Due to high diversity in the sequence, structure and functions of proteins involved in domain swapping, availability of such an algorithm to predict swapping events from sequence and structure-derived features will be an initial step towards identification of more putative proteins that may be involved in swapping or proteins involved in deposition disease. Further, the top features emerging in our feature selection method may be analysed further to understand their roles in the mechanism of domain swapping.

Modulation in activation and expression of phosphatase and tensin homolog on chromosome ten, Akt1, and 3-phosphoinositide-dependent kinase 1: further evidence demonstrating altered phosphoinositide 3-kinase signaling in postmortem brain of suicide subjects

BACKGROUND

Phosphoinositide 3-kinase (PI3-K) signaling plays a crucial role in neuronal growth and plasticity. Recently, we demonstrated that suicide brain is associated with decreased activation and expression of selective catalytic and regulatory subunits of PI3-K. The present investigation examined the regulation and functional significance of compromised PI3-K in suicide brain at the level of upstream phosphatase and tensin homologue on chromosome ten (PTEN) and downstream substrates 3-phosphoinositide-dependent kinase 1 (PDK1) and Akt.

METHODS

Messenger RNA expression of Akt1, Akt3, PTEN, and PDK1 by competitive reverse transcription polymerase polymerase chain reaction; protein expression of Akt1, Akt3, PTEN, PDK1, phosphorylated Akt1 (Ser473 and Thr308), phosphorylated PDK1, and phosphorylated PTEN by Western blot; and catalytic activities of Akt1, Akt3, and PDK1 by enzymatic assays were determined in prefrontal cortex and hippocampus obtained from suicide subjects and nonpsychiatric control subjects.

RESULTS

No significant changes in the expression of Akt1 or Akt3 were observed; however, catalytic activity of Akt1, but not of Akt3, was decreased in prefrontal cortex and hippocampus of suicide subjects, which was associated with decreased phosphorylation of Akt1 at Ser473 and Thr308. The catalytic activity of PDK1 and the level of phosphorylated PDK1 were also decreased in both brain areas without any change in expression levels of PDK1. On the other hand, messenger RNA and protein expression of PTEN was increased, whereas the level of phosphorylated PTEN was decreased.

CONCLUSIONS

Our study demonstrates abnormalities in PI3-K signaling at several levels in brain of suicide subjects and suggests the possible involvement of aberrant PI3-K/Akt signaling in the pathogenic mechanisms of suicide.

12-O-tetradecanoylphorbol-1, 3-acetate induces the negative regulation of protein kinase B by protein kinase Calpha during gastric cancer cell apoptosis

The PKB signaling pathway is essential for cell survival and the inhibition of apoptosis, but its functional mechanisms have not been fully explored. Previously, we reported that TPA effectively inhibited PKB activity and caused PKB degradation, which was correlated with the repression of PKB phosphorylation at Ser473. In this study, we focus on how PKB is regulated by TPA in gastric cancer cells. One of the TPA targets, PKCα, was found to mediate the inhibition of PKB phosphorylation and degredation caused by TPA. Furthermore, TPA induced the import of PKCα into the nucleus, where PKCα exerted an inhibitory effect on PKB expression and phosphorylation. As a result, cancer cell proliferation was arrested. Our study characterizes a novel function of PKCα in mediating the negative regulation of PKB by TPA, and suggests a potential application in the clinical treatment of gastric cancer.

Early activation of the beta-catenin pathway in osteocytes is mediated by nitric oxide, phosphatidyl inositol-3 kinase/Akt, and focal adhesion kinase

Bone mechanotransduction is vital for skeletal integrity. Osteocytes are thought to be the cellular structures that sense physical forces and transform these signals into a biological response. The Wnt/beta-catenin signaling pathway has been identified as one of the signaling pathways that is activated in response to mechanical loading, but the molecular events that lead to an activation of this pathway in osteocytes are not well understood. We assessed whether nitric oxide, focal adhesion kinase, and/or the phosphatidyl inositol-3 kinase/Akt signaling pathway mediate loading-induced beta-catenin pathway activation in MLO-Y4 osteocytes. We found that mechanical stimulation by pulsating fluid flow (PFF, 0.7+/-0.3 Pa, 5 Hz) for 30 min induced beta-catenin stabilization and activation of the Wnt/beta-catenin signaling pathway. The PFF-induced stabilization of beta-catenin and activation of the beta-catenin signaling pathway was abolished by adding focal kinase inhibitor FAK inhibitor-14 (50 microM), or phosphatidyl inositol-3 kinase inhibitor LY-294002 (50 microM). Addition of nitric oxide synthase inhibitor L-NAME (1.0mM) also abolished PFF-induced stabilization of beta-catenin. This suggests that mechanical loading activates the beta-catenin signaling pathway by a mechanism involving nitric oxide, focal adhesion kinase, and the Akt signaling pathway. These data provide a framework for understanding the role of beta-catenin in mechanical adaptation of bone.

mTORC2 is the hydrophobic motif kinase for SGK1

The activation of the AGC (protein kinase A/protein kinase G/protein kinase C)-family kinase SGK1 (serum- and glucocorticoid-induced kinase 1) by insulin via PI3K (phosphoinositide 3-kinase) signalling has been appreciated for almost 10 years. PDK1 (phosphoinositide-dependent protein kinase 1), a kinase that phosphorylates the SGK1 catalytic domain at Thr(256), is known to play a critical role in SGK1 activation. However, the identity of the protein kinase(s) responsible for phosphorylation of Ser(422), a site outside the catalytic domain (the so-called hydrophobic motif, or HM) that promotes activation of the kinase by PDK1, was unclear. In work reported in this issue of the Biochemical Journal, García-Martínez and Alessi have revealed the identity of a 'PDK2' kinase that catalyses Ser(422) phosphorylation as mTORC2 (mammalian target of rapamycin complex 2), a multiprotein kinase that phosphorylates a similar site in PKB (protein kinase B).

Epigallocatechin-3-gallate diminishes CCL2 expression in human osteoblastic cells via up-regulation of phosphatidylinositol 3-Kinase/Akt/Raf-1 interaction: a potential therapeutic benefit for arthritis

OBJECTIVE

To assess the effects of epigallocatechin-3-gallate (EGCG) on oncostatin M (OSM)-induced CCL2 synthesis and the associated signaling pathways in human osteoblastic cells. The therapeutic effect of EGCG on collagen-induced arthritis (CIA) in rats was also studied.

METHODS

CCL2 and c-Fos messenger RNA expression was analyzed by Northern blotting. The modulating effects of EGCG on the activation of Raf-1, Akt, and phosphatidylinositol 3-kinase (PI 3-kinase) were examined by coimmunoprecipitation, Western blotting, and PI 3-kinase activity assay. Interactions between c-Fos and CCL2 promoter were evaluated by electrophoretic mobility shift assay (EMSA) and chromatin immunoprecipitation (ChIP) assay. The effect of EGCG on CIA in rats was examined clinically and immunohistochemically.

RESULTS

EGCG inhibited OSM-stimulated CCL2 expression in primary human osteoblasts and MG-63 cells. In MG-63 cells, EGCG alleviated the OSM-induced phosphorylation of Raf-1 at Ser338 but restored the dephosphorylation of Raf-1 at Ser259. EGCG increased the activity of PI 3-kinase, the level of phosphorylated Akt (Ser473), and binding between Raf-1 and active Akt. EMSA and ChIP assay revealed that EGCG attenuated activator protein 1 (AP-1)-CCL2 promoter interaction, possibly by reducing c-Fos synthesis. Codistribution of CD68+ macrophages and CCL2+ osteoblasts in osteolytic areas was obvious in the CIA model. Administration of EGCG markedly diminished the severity of CIA, macrophage infiltration, and the amount of CCL2-synthesizing osteoblasts.

CONCLUSION

By stimulating PI 3-kinase activity, EGCG promoted Akt/Raf-1 crosstalk, resulting in decreased AP-1 binding to CCL2 promoter, and finally reduced CCL2 production in osteoblasts. EGCG alleviated the severity of CIA, probably by suppressing CCL2 synthesis in osteoblasts to diminish macrophage infiltration. Our data support the therapeutic potential of EGCG on arthritis.

Molecular imaging of phosphorylation events for drug development

PURPOSE

Protein phosphorylation mediated by protein kinases controls numerous cellular processes. A genetically encoded, generalizable split firefly luciferase (FL)-assisted complementation system was developed for noninvasive monitoring phosphorylation events and efficacies of kinase inhibitors in cell culture and in small living subjects by optical bioluminescence imaging.

PROCEDURES

An Akt sensor (AST) was constructed to monitor Akt phosphorylation and the effect of different PI-3K and Akt inhibitors. Specificity of AST was determined using a non-phosphorylable mutant sensor containing an alanine substitution (ASA).

RESULTS

The PI-3K inhibitor LY294002 and Akt kinase inhibitor perifosine led to temporal- and dose-dependent increases in complemented FL activities in 293T human kidney cancer cells stably expressing AST (293T/AST) but not in 293T/ASA cells. Inhibition of endogenous Akt phosphorylation and kinase activities by perifosine also correlated with increase in complemented FL activities in 293T/AST cells but not in 293T/ASA cells. Treatment of nude mice bearing 293T/AST xenografts with perifosine led to a 2-fold increase in complemented FL activities compared to that of 293T/ASA xenografts. Our system was used to screen a small chemical library for novel modulators of Akt kinase activity.

CONCLUSION

This generalizable approach for noninvasive monitoring of phosphorylation events will accelerate the discovery and validation of novel kinase inhibitors and modulators of phosphorylation events.

Activation Mechanism of Protein Kinase B by DNA-dependent Protein Kinase Involved in the DNA Repair System

DNA-dependent protein kinase (DNA-PK) is involved in joining DNA double-strand breaks induced by ionizing radiation or V(D)J recombination and is activated by DNA ends and composed of a DNA binding subunit, Ku, and a catalytic subunit, DNA-PKcs. It has been suggested that DNA-PK might be 2^nd upstream kinase for protein kinase B (PKB). In this report, we showed that Ser473 phosphorylation in the hydrophobic-motif of PKB is blocked in DNA-PK knockout mouse embryonic fibroblast cells (MEFs) following insulin stimulation, while there is no effect on Ser473 phosphorylation in DNA-PK wild type MEF cells. The observation is further confirmed in human glioblastoma cells expressing a mutant form of DNA-PK (M059J) and a wild-type of DNA-PK (M059K), indicating that DNA-PK is indeed important for PKB activation. Furthermore, the treatment of cells with doxorubicin, DNA-damage inducing agent, leads to PKB phosphorylation on Ser473 in control MEF cells while there is no response in DNA-PK knockout MEF cells. Together, these results proposed that DNA-PK has a potential role in insulin signaling as well as DNA-repair signaling pathway.

Positive receptor feedback during lineage commitment can generate ultrasensitivity to ligand and confer robustness to a bistable switch

Cytokines and lineage-specific transcription factors are critical molecular effectors for terminal differentiation during hematopoiesis. Intrinsic transcription factor activity is often believed to drive commitment and differentiation, whereas cytokine receptor signals have been implicated in the regulation of cell proliferation, survival, and differentiation. In erythropoiesis, recent experimental findings provide direct evidence that erythropoietin (Epo) can generate commitment cues via the erythropoietin receptor (EpoR); specifically, EpoR signaling leads to activation of the transcription factor GATA-1, which then triggers transcription of erythrocyte-specific genes. In particular, activated GATA-1 induces two positive feedback loops in the system through the enhanced expression of both inactive GATA-1 and EpoR, the latter of which is externally regulatable by Epo. Based upon this network architecture, we present a mathematical model of GATA-1 activation by EpoR, which bidirectionally links a lineage-specific receptor and transcription factor. Our deterministic model offers insight into stimulus-response relationships between Epo and several downstream effectors. In addition to the survival signals that EpoR provides, steady-state analysis of our model suggests that receptor upregulation during lineage commitment can also generate ultrasensitivity to Epo and bistability in GATA-1 activity. These system-level properties can induce a switch-like characteristic during differentiation and provide robustness to the mature state. The topology also suggests a novel mechanism for achieving robust bistability in a purely deterministic manner without molecular cooperativity. The analytical solution of a generalized, minimal model is provided and the significance of each of the two positive feedback loops is elucidated through bifurcation analysis. This network topology, or variations thereof, may link other receptor-transcription factor pairs and may therefore be of general relevance in cellular decision-making.

TOR regulation of AGC kinases in yeast and mammals

The TOR (target of rapamycin), an atypical protein kinase, is evolutionarily conserved from yeast to man. Pharmacological studies using rapamycin to inhibit TOR and yeast genetic studies have provided key insights on the function of TOR in growth regulation. One of the first bona fide cellular targets of TOR was the mammalian protein kinase p70 S6K (p70 S6 kinase), a member of a family of kinases called AGC (protein kinase A/protein kinase G/protein kinase C-family) kinases, which include PKA (cAMP-dependent protein kinase A), PKG (cGMP-dependent kinase) and PKC (protein kinase C). AGC kinases are also highly conserved and play a myriad of roles in cellular growth, proliferation and survival. The AGC kinases are regulated by a common scheme that involves phosphorylation of the kinase activation loop by PDK1 (phosphoinositide-dependent kinase 1), and phosphorylation at one or more sites at the C-terminal tail. The identification of two distinct TOR protein complexes, TORC1 (TOR complex 1) and TORC2, with different sensitivities to rapamycin, revealed that TOR, as part of either complex, can mediate phosphorylation at the C-terminal tail for optimal activation of a number of AGC kinases. Together, these studies elucidated that a fundamental function of TOR conserved throughout evolution may be to balance growth versus survival signals by regulating AGC kinases in response to nutrients and environmental conditions. This present review highlights this emerging function of TOR that is conserved from budding and fission yeast to mammals.

Regulation of 3-phosphoinositide-dependent protein kinase-1 (PDK1) by Src involves tyrosine phosphorylation of PDK1 and Src homology 2 domain binding

3-Phosphoinositide-dependent protein kinase-1 (PDK1) appears to play a central regulatory role in many cell signalings between phosphoinositide-3 kinase and various intracellular serine/threonine kinases. In resting cells, PDK1 is known to be constitutively active and is further activated by tyrosine phosphorylation (Tyr(9) and Tyr(373/376)) following the treatment of the cell with insulin or pervanadate. However, little is known about the mechanisms for this additional activation of PDK1. Here, we report that the SH2 domain of Src, Crk, and GAP recognized tyrosine-phosphorylated PDK1 in vitro. Destabilization of PDK1 induced by geldanamycin (a Hsp90 inhibitor) was partially blocked in HEK 293 cells expressing PDK1-Y9F. Co-expression of Hsp90 enhanced PDK1-Src complex formation and led to further increased PDK1 activity toward PKB and SGK. Immunohistochemical analysis with anti-phospho-Tyr(9) antibodies showed that the level of Tyr(9) phosphorylation was markedly increased in tumor samples compared with normal. Taken together, these data suggest that phosphorylation of PDK1 on Tyr(9), distinct from Tyr(373/376), is important for PDK1/Src complex formation, leading to PDK1 activation. Furthermore, Tyr(9) phosphorylation is critical for the stabilization of both PDK1 and the PDK1/Src complex via Hsp90-mediated protection of PDK1 degradation.

EGFR and beta1 integrins utilize different signaling pathways to activate Akt

Akt, also called PKB, is a serine/threonine kinase that plays a major role in cell survival. It can be activated by several cellular receptors, including integrins and growth factor receptors, in PI3K-dependent manners. In this study, we analyzed the two current models for Akt activation upon beta1 integrin-mediated adhesion: via focal adhesion kinase and via transactivation of the EGF receptor. Distinct differences in the pathways leading to phosphorylation and activation of Akt from stimulated beta1 integrins and EGF receptor were observed, including opposing sensitivity to the tyrosine kinase inhibitors PP2 and Gefitinib. Using knockout cells and integrin mutant cells, we show that beta1 integrins can induce phosphorylation of Akt at Ser473 and Thr308 and Akt kinase activity independently of the EGF receptor activity, focal adhesion kinase, and the Src family members. In contrast to stimulation with EGF, beta1 integrin-mediated adhesion did not induce Akt tyrosine phosphorylation. Moreover, tyrosine phosphorylation of Akt was found not to be required for its catalytic activity. The results identify a previously unrecognized mechanism by which beta1 integrins activate the PI3K/Akt pathway.

Enhancement of ajoene-induced apoptosis by conjugated linoleic acid in 3T3-L1 adipocytes

Ajoene has been shown to induce apoptosis in 3T3-L1 adipocytes. In this report the effects on apoptosis of combinations of ajoene and trans-10, cis-12 conjugated linoleic acid (t10,c12CLA) in 3T3-L1 adipocytes were investigated. Although t10,c12CLA alone had no effect, ajoene plus t10,c12CLA reduced cell viability more than ajoene alone at 24 h (59.1 vs. 85.9% of control, respectively; p<0.05). Compared to treatment with t10,c12CLA, ajoene increased apoptosis 218% after 24 h (p<0.01), whereas ajoene plus t10,c12CLA increased apoptosis 122% over that caused by ajoene alone (p<0.01). Immunoblotting analysis also indicated that ajoene plus t10,c12CLA caused a greater increase in phosphorylation of c-Jun N-terminal kinase (JNK) and Bax expression and a greater release of mitochondrial proteins (cytochrome c, AIF) than additive responses to each compound alone. Ajoene plus t10,c12CLA also increased ROS production more than that resulting from ajoene treatment alone (264 vs 204% after 40 min, respectively; p<0.01). Furthermore, the antioxidant NAC prevented ROS generation and apoptosis by ajoene plus t10,c12CLA. Interestingly, the combination of ajoene and t10,c12CLA increased NF-kappaB activation and decreased the level of phosphorylated Akt more than each compound alone. Altogether, our observations indicate that t10,c12CLA potentiates the effect of ajoene on apoptosis in 3T3-L1 adipocytes.

Intramolecular and intermolecular interactions of protein kinase B define its activation in vivo

Protein kinase B (PKB/Akt) is a pivotal regulator of diverse metabolic, phenotypic, and antiapoptotic cellular controls and has been shown to be a key player in cancer progression. Here, using fluorescent reporters, we shown in cells that, contrary to in vitro analyses, 3-phosphoinositide-dependent protein kinase 1 (PDK1) is complexed to its substrate, PKB. The use of Förster resonance energy transfer detected by both frequency domain and two-photon time domain fluorescence lifetime imaging microscopy has lead to novel in vivo findings. The preactivation complex of PKB and PDK1 is maintained in an inactive state through a PKB intramolecular interaction between its pleckstrin homology (PH) and kinase domains, in a "PH-in" conformer. This domain-domain interaction prevents the PKB activation loop from being phosphorylated by PDK1. The interactive regions for this intramolecular PKB interaction were predicted through molecular modeling and tested through mutagenesis, supporting the derived model. Physiologically, agonist-induced phosphorylation of PKB by PDK1 occurs coincident to plasma membrane recruitment, and we further shown here that this process is associated with a conformational change in PKB at the membrane, producing a "PH-out" conformer and enabling PDK1 access the activation loop. The active, phosphorylated, "PH-out" conformer can dissociate from the membrane and retain this conformation to phosphorylate substrates distal to the membrane. These in vivo studies provide a new model for the mechanism of activation of PKB. This study takes a crucial widely studied regulator (physiology and pathology) and addresses the fundamental question of the dynamic in vivo behaviour of PKB with a detailed molecular mechanism. This has important implications not only in extending our understanding of this oncogenic protein kinase but also in opening up distinct opportunities for therapeutic intervention.

Radiation-induced cell signaling: inside-out and outside-in

Exposure of tumor cells to clinically relevant doses of ionizing radiation causes DNA damage as well as mitochondria-dependent generation of reactive oxygen species. DNA damage causes activation of ataxia telangiectasia mutated and ataxia telangiectasia mutated and Rad3-related protein, which induce cell cycle checkpoints and also modulate the activation of prosurvival and proapoptotic signaling pathways, such as extracellular signal-regulated kinase 1/2 (ERK1/2) and c-Jun NH(2)-terminal kinase 1/2, respectively. Radiation causes a rapid reactive oxygen species-dependent activation of ERBB family and other tyrosine kinases, leading to activation of RAS proteins and multiple protective downstream signaling pathways (e.g., AKT and ERK1/2), which alter transcription factor function and the apoptotic threshold of cells. The initial radiation-induced activation of ERK1/2 can promote the cleavage and release of paracrine ligands, which cause a temporally delayed reactivation of receptors and intracellular signaling pathways in irradiated and unirradiated bystander cells. Hence, signals from within the cell can promote activation of membrane-associated receptors, which signal back into the cytosol: signaling from inside the cell outward to receptors and then inward again via kinase pathways. However, cytosolic signaling can also cause release of membrane-associated paracrine factors, and thus, paracrine signals from outside of the cell can promote activation of growth factor receptors: signaling from the outside inward. The ultimate consequence of these signaling events after multiple exposures may be to reprogram the irradiated and affected bystander cells in terms of their expression levels of growth-regulatory and cell survival proteins, resulting in altered mitogenic rates and thresholds at which genotoxic stresses cause cell death. Inhibition of signaling in one and/or multiple survival pathways enhances radiosensitivity. Prolonged inhibition of any one of these pathways, however, gives rise to lineages of cells, which have become resistant to the inhibitor drug, by evolutionary selection for the clonal outgrowth of cells with point mutations in the specific targeted protein that make the target protein drug resistant or by the reprogramming of multiple signaling processes within all cells, to maintain viability. Thus, tumor cells are dynamic with respect to their reliance on specific cell signaling pathways to exist and rapidly adapt to repeated toxic challenges in an attempt to maintain tumor cell survival.

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.

Differential roles of phosphoinositide-dependent protein kinase-1 and akt1 expression and phosphorylation in breast cancer cell resistance to Paclitaxel, Doxorubicin, and gemcitabine

3-Phosphoinositide-dependent protein kinase-1 (PDK1) and Akt1 are two closely related components of the phosphatidylinositol-3 kinase (PI3K) pathway, which is aberrantly regulated in breast cancer. Despite the importance of PDK1, few studies have evaluated it as a potential target for cancer therapy compared with studies of Akt1. We hypothesized that PDK1 is a superior target in the PI3K pathway. To test this, we first used a mouse mammary cell line retrovirally infected to express human PDK1 or Akt1 for comparative studies of treatment with paclitaxel, doxorubicin, and gemcitabine. Overexpression of PDK1 or Akt1 conferred similar resistance to treatment with paclitaxel or doxorubicin compared with control cells. However, the PDK1-overexpressing cells were more resistant to gemcitabine than were the Akt1-overexpressing cells. We next correlated the expression and activation-specific phosphorylation of PDK1 and Akt1 with the cytotoxic effects of the same agents in several human breast cancer cell lines. Cells with high levels of phosphorylated PDK1 were more resistant to gemcitabine-induced apoptosis than cells expressing high levels of phosphorylated Akt1. To further validate this observation, we used small interfering RNA oligonucleotides to selectively knock down PDK1 or Akt1 expression in MCF7 human breast cancer cells. We found that knockdown of PDK1 expression sensitized MCF7 cells to gemcitabine-induced apoptosis more effectively than did knockdown of Akt1 expression in the same cells. Our findings show that PDK1 may be a superior alternative to Akt1 as a target for sensitizing breast cancer cells to chemotherapeutic agents, particularly gemcitabine.

Targeting survivin via PI3K but not c-akt/PKB by anticancer drugs in immature neutrophils

Myelosuppression is the most common unwanted side effect associated with the administration of anticancer drugs, and infections remain a common cause of death in chemotherapy-treated patients. Several mechanisms of the cytotoxicity of these drugs have been proposed and may synergistically operate in a given cell. Survivin expression has been associated with cancer, but recent reports suggest that this molecule is also expressed in several immature and mature hematopoietic cells. Here, we provide evidence that treatment of immature neutrophils with anticancer drugs reduced endogenous survivin levels causing apoptosis. The anticancer drugs did not directly target survivin, instead they blocked the activity of phosphatidylinositol-3-OH kinase, which regulated survivin expression and apoptosis in these cells. Strikingly, and in contrast to other cells, this pathway did not involve the serine/threonine kinase c-akt/PKB. Moreover, in combination with anticancer drug therapy, rapamycin did not induce increased myelosuppression in an experimental lymphoma mouse model. These data suggest that drugs that block either c-akt/PKB or signaling molecules located distal to c-akt/PKB may preferentially induce apoptosis of cancer cells as they exhibit no cytotoxicity for immature neutrophils.

Autophosphorylation of Akt at threonine 72 and serine 246. A potential mechanism of regulation of Akt kinase activity

Activation of the serine/threonine protein kinase Akt is a multistep process. We here propose that the kinase activity of Akt is regulated via autophosphorylation in trans at two putative sites (threonine 72 and serine 246) that lie in the characteristic Akt substrate motif (RXRXX(S/T)). Incubation of Akt immunoprecipitated from transfected cells with a pre-activated Akt recombinant protein and gamma-32P-labeled ATP led to marked incorporation of radioactivity in wild-type Akt but not Akt/T72A/S246A mutant. Western blot analysis using a phosphorylated Akt substrate-specific antibody of Akt immunoprecipitated from transfected cells confirmed the autophosphorylation of wild-type Akt but not Akt/T72A/S246A mutant in insulin-like growth factor-1 (IGF-1)-stimulated cells. Autophosphorylation of Akt on Thr-72 and Ser-246 appeared to require prior phosphorylation of Akt on Thr-308 and Ser-473. Compared with wild-type Akt, Akt/T72A/S246A mutant exhibited markedly reduced basal Akt kinase activity and response to cellular stimulation by insulin-like growth factor-1, and also conferred less cellular resistance to doxorubicin-induced apoptosis. The findings from these pilot studies suggest that Akt regulates its kinase activity through autophosphorylation. Further investigation of this potential novel regulatory mechanism by which Akt performs its cellular functions is warranted.

Structure-based design of isoquinoline-5-sulfonamide inhibitors of protein kinase B

Structure-based drug design of novel isoquinoline-5-sulfonamide inhibitors of PKB as potential antitumour agents was investigated. Constrained pyrrolidine analogues that mimicked the bound conformation of linear prototypes were identified and investigated by co-crystal structure determinations with the related protein PKA. Detailed variation in the binding modes between inhibitors with similar overall conformations was observed. Potent PKB inhibitors from this series inhibited GSK3beta phosphorylation in cellular assays, consistent with inhibition of PKB kinase activity in cells.

Human NDR kinases are rapidly activated by MOB proteins through recruitment to the plasma membrane and phosphorylation

Human nuclear Dbf2-related kinases (NDRs) are up-regulated in certain cancer types, yet their precise function(s) and regulatory mechanism(s) still remain to be defined. Here, we show that active (phosphorylated on Thr444) and inactive human NDRs are both mainly cytoplasmic. Moreover, NDR kinases colocalize at the plasma membrane with human MOBs (hMOBs), which are recently described coactivators of human NDR in vitro. Strikingly, membrane targeting of NDR results in a constitutively active kinase due to phosphorylation on Ser281 and Thr444 that is further activated upon coexpression of hMOBs. Membrane-targeted hMOBs also robustly promoted activation of NDR. We further demonstrate that the in vivo activation of human NDR by membrane-bound hMOBs is dependent on their interaction and occurs solely at the membrane. By using a chimeric molecule of hMOB, which allows inducible membrane translocation, we found that NDR phosphorylation and activation at the membrane occur a few minutes after association of hMOB with membranous structures. We provide insight into a potential in vivo mechanism of NDR activation through rapid recruitment to the plasma membrane mediated by hMOBs.

mTOR.RICTOR is the Ser473 kinase for Akt/protein kinase B in 3T3-L1 adipocytes

The insulin-signaling pathway leading to the activation of Akt/protein kinase B has been well characterized except for a single step, the phosphorylation of Akt at Ser-473. Double-stranded DNA-dependent protein kinase (DNA-PK), ataxia telangiectasia mutated (ATM) gene product, integrin-linked kinase (ILK), protein kinase Calpha (PKCalpha), and mammalian target of rapamycin (mTOR), when complexed to rapamycin-insensitive companion of mTOR (RICTOR), have all been identified as playing a critical role in Akt Ser-473 phosphorylation. However, the apparently disparate results reported in these studies are difficult to evaluate, given that different stimuli and cell types were examined and that all of the candidate proteins have never been systematically studied in a single system. Additionally, none of these studies were performed in a classical insulin-responsive cell type or tissue such as muscle or fat. We therefore examined each of these candidates in 3T3-L1 adipocytes. In vitro kinase assays, using different subcellular fractions of 3T3-L1 adipocytes, revealed that phosphatidylinositol 3,4,5-trisphosphate-stimulated Ser-473 phosphorylation correlated well with the amount of DNA-PK, mTOR, and RICTOR but did not correlate with levels of ATM, ILK, and PKCalpha. PKCalpha was completely absent from compartments with Ser-473 phosphorylation activity. Although purified DNA-PK could phosphorylate a peptide derived from Akt that contains amino acid Ser-473, it could not phosphorylate full-length Akt2. Vesicles immunoprecipitated from low density microsomes using antibodies directed against mTOR or RICTOR had phosphatidylinositol 3,4,5-trisphosphate-stimulated Ser-473 activity that was sensitive to wortmannin but not staurosporine. In contrast, immunopurified low density microsome vesicles containing ILK could not phosphorylate Akt on Ser-473 in vitro. Small interference RNA knockdown of RICTOR, but not DNA-PK, ATM, or ILK, suppressed insulin-activated Ser-473 phosphorylation and, to a lesser extent, Thr-308 phosphorylation in 3T3-L1 adipocytes. Based on our cell-free kinase and small interference RNA results, we conclude that mTOR complexed to RICTOR is the Ser-473 kinase in 3T3-L1 adipocytes.

Functional and therapeutic significance of Akt deregulation in malignant melanoma

Identification of specific genes or signaling pathways involved in development of melanoma could lead to new therapies that target and correct these defects. Recent studies have revealed deregulation of the Akt signaling pathway occurring in 43-67% of melanomas. Akt kinase family members, Akt1/PKBalpha, Akt2/PKBbeta and Akt3/PKBgamma, share extensive structural similarity and perform common as well as unique functions within cells. The Akt signaling cascade initiates at the cell surface when growth factors or other extracellular stimuli activate phosphoinositide 3-kinase (PI3K). Activated PI3K generates a lipid second messenger, phosphatidylinositol-3,4,5-trisphosphate (PIP3), causing translocation of Akt to the plasma membrane where it becomes phosphorylated and activated. The balance of cellular PIP3 is regulated primarily by a phosphatase called PTEN that reduces PIP3 levels thereby lowering Akt activity. In melanomas, decreased PTEN activity elevates PIP3 levels resulting in Akt activation. Active Akt then phosphorylates downstream cellular proteins that promote melanoma cell proliferation and survival. Recently, Akt3 was discovered to be the predominant isoform activated in sporadic melanomas. Levels of activity increased during melanoma progression with metastatic melanomas having the highest activity. Although mechanisms of Akt3 activation remain to be fully characterized, overexpression of Akt3 and decreased PTEN activity play important roles in this process. Targeted reduction of Akt3 activity decreased survival of melanoma tumor cells leading to inhibition of tumor development, which may be therapeutically effective for shrinking tumors in melanoma patients. This review surveys recent developments in Akt deregulation in melanoma and its potential as a selective therapeutic target in patients in the advanced stages of this disease.

Phosphoinositide-dependent phosphorylation of PDK1 regulates nuclear translocation

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

Full Activation of PKB/Akt in Response to Insulin or Ionizing Radiation Is Mediated through ATM

The gene mutated in ataxia telangiectasia, ATM, has been implicated in several cell functions such as cell cycle control and response to DNA damage and insulin. PKB/Akt has also been implicated in the cellular response to insulin, gamma-radiation, and cell cycle control. Interestingly, lack of PKB/Akt function in vivo is able to mimic some phenotypic abnormalities associated with ataxia telangiectasia (AT). Here we show that ATM is a major determinant of full PKB/Akt activation in response to insulin or gamma-radiation. This effect is mediated through the phosphatidylinositol 3-kinase domain of ATM that specifically affects Akt serine 473 phosphorylation. This conclusion was inferred from the results obtained in transient transfection assays using exogenous PKB/Akt and ATM in Cos cells. Moreover, the use of ATM inhibitors or small interfering RNA confirmed our observation. Further supporting these results, we also observed that biological responses tightly regulated by Akt, such as transcription factor of the forkhead family activity after insulin treatment or gamma-radiation response, were altered in cell lines derived from AT patients and knockout mice for ATM in which phosphorylation in serine 473 was almost abolished. This study proposes new clues in the search of the unknown PDK2 and new explanations for the radiosensitivity or insulin intolerance described more than 30 years ago in AT patients.

Hepatocyte growth factor enhances protein phosphatase Cdc25A inhibitor compound 5-induced hepatoma cell growth inhibition via Akt-mediated MAPK pathway

We have previously shown that Compound 5 (Cpd 5), an inhibitor of protein phosphatase Cdc25A, inhibits Hep3B human hepatoma cell growth. We now show that hepatocyte growth factor (HGF), a hepatocyte growth stimulant, can strongly enhance Cpd 5-induced growth inhibition in Hep3B cells, and this enhancement in cell growth inhibition is correlated with a much stronger ERK phosphorylation when compared to cells treated with Cpd 5 or HGF separately. We found that HGF/Cpd 5-induced ERK phosphorylation and cell growth inhibition were mediated by Akt (protein kinase B) pathway, since combination HGF/Cpd 5 treatment of Hep3B cells inhibited Akt phosphorylation at Ser-473 and its kinase activity, which led to the suppression of Raf-1 phosphorylation at Ser-259. The suppression of Raf-1 Ser-259 phosphorylation caused the induction of Raf-1 kinase activity, as well as hyper-ERK phosphorylation. Transient transfection of Hep3B cells with dominant negative Akt c-DNA further enhanced both Cpd 5- and HGF/Cpd 5-induced ERK phosphorylation, while over-expression of wild-type Akt c-DNA diminished their effects. In contrast, HGF antagonized the growth inhibitory actions of Cpd 5 on normal rat hepatocytes, thus showing a selective effect on tumor cells compared to normal cells. Our data suggest that Akt kinase negatively regulates MAPK activity at the Akt-Raf level. Suppression of Akt activity by either combination HGF/Cpd 5 treatment or by dominant negative Akt c-DNA transfection antagonizes the Akt inhibitory effect on Raf-1, resulting in an enhancement of Cpd 5-induced MAPK activation and cell growth inhibition.

Putting the rap on Akt

The protein kinase Akt is activated in a wide variety of cancers, and this activation results in enhanced resistance to apoptosis through multiple mechanisms. This article reviews the control of Akt activation by the opposing actions of the oncogene phosphoinositide 3-kinase (PI3-K) and the tumor suppressor phosphatase and tensin homolog deleted on chromosome 10. The activation of Akt by transforming mutations, such as the amplification of HER-2/neu in breast cancer and the formation of the BCR/ABL fusion gene in chronic myelogenous leukemia, seems to be essential for the transforming activity of these oncogenes. We discuss several of the proposed mechanisms for the antiapoptotic effect of activated Akt, including the inhibition of the proapoptotic protein Bad, downregulation of death receptors, and enhancement of the glycolytic rate. Increased glycolysis is seen in many malignancies and forms the basis for the increasing use of positron emission tomography imaging for diagnosis and staging. Finally, we discuss rapamycin and its analogs, which are now in trials as antineoplastic therapy; these agents show particular promise in tumors in which Akt has been activated.

Protein kinase C betaII regulates Akt phosphorylation on Ser-473 in a cell type- and stimulus-specific fashion

Akt (= protein kinase B), a subfamily of the AGC serine/threonine kinases, plays critical roles in survival, proliferation, glucose metabolism, and other cellular functions. Akt activation requires the recruitment of the enzyme to the plasma membrane by interacting with membrane-bound lipid products of phosphatidylinositol 3-kinase. Membrane-bound Akt is then phosphorylated at two sites for its full activation; Thr-308 in the activation loop of the kinase domain is phosphorylated by 3-phosphoinositide-dependent kinase-1 (PDK1) and Ser-473 in the C-terminal hydrophobic motif by a putative kinase PDK2. The identity of PDK2 has been elusive. Here we present evidence that conventional isoforms of protein kinase C (PKC), particularly PKCbetaII, can regulate Akt activity by directly phosphorylating Ser-473 in vitro and in IgE/antigen-stimulated mast cells. By contrast, PKCbeta is not required for Ser-473 phosphorylation in mast cells stimulated with stem cell factor or interleukin-3, in serum-stimulated fibroblasts, or in antigen receptor-stimulated T or B lymphocytes. Therefore, PKCbetaII appears to work as a cell type- and stimulus-specific PDK2.

Transformation potency of ErbB heterodimer signaling is determined by B-Raf kinase

Cellular transformation occurs only in cells that express both ErbB1 and ErbB4 receptors, but not in cells expressing only one or the other of these receptors. However, when both receptors are coexpressed and ligand-stimulated, they interact with virtually the same adaptor/effector proteins as when expressed singly. To reveal the underlying regulatory mechanism of the kinase/phosphatase network in ErbB homo- and heterodimer receptor signaling, extracellular signal-regulated kinase (ERK) and Akt activities were evaluated in the presence of several enzyme inhibitors in ligand-induced cells expressing ErbB1 (E1), ErbB4 (E4), and ErbB1/ErbB4 (E1/4) receptor. The PP2A inhibitor okadaic acid showed receptor-specific inhibitory profiles for ERK and Akt activities. Moreover, B-Raf isolated only from E1/4 cells could induce in vitro phosphorylation for MEK; this B-Raf kinase activity was abolished by pretreatment of the cells with okadaic acid. Our study further showed that the E1/4 cell-specific B-Raf activity was stimulated by PLC gamma and subsequent Rap1 activation. The present study suggests that B-Raf kinase, which was specifically activated in the cells coexpressing ErbB1 and ErbB4 receptors, elevates total ERK activity within the cell and, therefore, can induce cellular transformation.