3481043
Ligation of cell surface GRP78 with antibody directed against the COOH-terminal domain of GRP78 suppresses Ras/MAPK and PI 3-kinase/AKT signaling while promoting caspase activation in human prostate cancer cells
We have previously shown that treatment of prostate cancer and melanoma cells expressing GRP782 on their cell surface with antibody directed against the COOH-terminal domain of GRP78 up-regulates and activates p53 causing decreased cell proliferation and up-regulated apoptosis. In this report, we demonstrate that treatment of 1-LN prostate cancer cells with this antibody decreases cell surface expression of GRP78, AktThr308 and AktSer473 kinase activities and reduces phosphorylation of FOXO, and GSK3β. This treatment also suppresses activation of ERK1/2, p38 MAPK, and MKK3/6; however, it up-regulates MKK4 activity. JNK, as determined by its phosphorylation state, is subsequently activated, triggering apoptosis. Incubation of cells with antibody reduced levels of anti-apoptotic Bcl-2, while elevating pro-apoptotic BAD, BAX, and BAK expression as well as cleaved caspases-3, -7, -8, and -9. Silencing GRP78 or p53 gene expression by RNAi prior to antibody treatment abrogated these effects. We conclude that antibody directed against the COOH-terminal domain of GRP78 may prove useful as a pan suppressor of proliferative/survival signaling in cancer cells expressing GRP78 on their cell surface.
24665772
Androgen receptor in prostate cancer.
The normal development and maintenance of the prostate is dependent on androgen acting through the androgen receptor (AR). AR remains important in the development and progression of prostate cancer. AR expression is maintained throughout prostate cancer progression, and the majority of androgen-independent or hormone refractory prostate cancers express AR. Mutation of AR, especially mutations that result in a relaxation of AR ligand specificity, may contribute to the progression of prostate cancer and the failure of endocrine therapy by allowing AR transcriptional activation in response to antiandrogens or other endogenous hormones. Similarly, alterations in the relative expression of AR coregulators have been found to occur with prostate cancer progression and may contribute to differences in AR ligand specificity or transcriptional activity. Prostate cancer progression is also associated with increased growth factor production and an altered response to growth factors by prostate cancer cells. The kinase signal transduction cascades initiated by mitogenic growth factors modulate the transcriptional activity of AR and the interaction between AR and AR coactivators. The inhibition of AR activity through mechanisms in addition to androgen ablation, such as modulation of signal transduction pathways, may delay prostate cancer progression.
14631331
Prostate cancer cell proliferation in vitro is modulated by antibodies against glucose-regulated protein 78 isolated from patient serum.
Circulating autoantibodies against the glucose-regulated protein of 78 kDa (GRP78) are present at high levels in prostate cancer patients and are a biomarker of aggressive tumor behavior. We purified the anti-GRP78 IgGs and examined their effect on 1-LN, PC-3, DU145, and LnCap human prostate cancer cells. We also evaluated its effects on the breast cancer MDA-MB231 and melanoma DM413 cell lines. The anti-GRP78 antibody binds only to cells expressing GRP78 on the surface, to a site also recognized by its physiologic agonist, activated alpha(2)-macroglobulin (alpha(2)M*). This antibody is completely specific for a peptide, including the primary amino acid sequence CNVKSDKSC, which contains a tertiary structural motif mimicking an epitope in GRP78. Tertiary structural analysis suggested the linear GRP78 primary amino acid sequence LIGRTWNDPSVQQDIKFL (Leu(98)-Leu(115)) as the putative binding site, containing the tertiary structual arrangement described above, which was confirmed experimentally. The anti-GRP78 antibodies from prostate cancer patients recognize almost exclusively this epitope. We produced animal antibodies against both these peptides, and they are able to mimic the effects of the human antibody. Our experiments also suggest this epitope as highly immunogenic, thereby explaining the specificity of the immune response against this epitope in GRP78, observed in humans. Using 1-LN cells as a model, we show that anti-GRP78 IgG purified from the sera of these patients mimics the proproliferative effects induced by alpha(2)M* via the common receptor, GRP78. Furthermore, increasing concentrations of human anti-GRP78 IgG show a dose-dependent protective effect on apoptosis induced by tumor necrosis factor alpha.
343677
Signal Transduction by the JNK Group of MAP Kinases
MAP kinases are evolutionarily conserved proteins that are activated by a protein kinase cascade, including a MAP kinase kinase kinase, which phosphorylates a MAP kinase kinase, which in turn activates the MAP kinase by phosphorylation on Thr and Tyr residues. The primary sequence surrounding these phosphorylation sites serves to distinguish three major groups of mammalian MAP kinases. These include the Ras-activated ERK MAP kinases, which are characterized by the sequence TEY and the two stress-activated MAP kinases: p38 with the sequence TGY, and the c-Jun NH2-terminal kinases (JNK) with the sequence TPY. This review will focus on the JNK group of MAP kinases.
5725389
MEKK1/JNK signaling stabilizes and activates p53.
Activation of the tumor suppressor p53 by stress and damage stimuli often correlates with induction of stress kinases, Jun-NH2 kinase (JNK). As JNK association with p53 plays an important role in p53 stability, in the present study we have elucidated the relationship between the JNK-signaling pathway and p53 stability and activity. Expression of a constitutively active form of JNKK upstream kinase, mitogen-activated protein kinase kinase kinase (DeltaMEKK1), increased the level of the exogenously transfected form of p53 in p53 null (10.1) cells as well as of endogenous p53 in MCF7 breast cancer cells. Increased p53 level by forced expression of DeltaMEKK1 coincided with a decrease in p53 ubiquitination in vivo and with prolonged p53 half-life. Computerized modeling of the JNK-binding site (amino acids 97-116; p7 region) enabled us to design mutations of exposed residues within this region. Respective mutations (p53(101-5-8)) and deletion (p53(Deltap7)) forms of p53 did not exhibit the same increase in p53 levels upon DeltaMEKK1 expression. In vitro phosphorylation of p53 by JNK abolished Mdm2 binding and targeting of p53 ubiquitination. Similarly, DeltaMEKK1 expression increased p53 phosphorylation by immunopurified JNK and dissociated p53-Mdm2 complexes. Transcriptional activity of p53, as measured via mdm2 promoter-driven luciferase, exhibited a substantial increase in DeltaMEKK1-expressing cells. Cotransfection of p53 and DeltaMEKK1 into p53 null cells potentiated p53-dependent apoptosis, suggesting that MEKK1 effectors contribute to the ability of p53 to mediate programmed cell death. Our results point to the role of MEKK1-JNK signaling in p53 stability, transcriptional activities, and apoptotic capacity as part of the cellular response to stress.
10688571
Role of JNK activation in apoptosis: A double-edged sword
ABSTRACTJNK is a key regulator of many cellular events, including programmed cell death (apoptosis). In the absence of NF-κB activation, prolonged JNK activation contributes to TNF-α induced apoptosis. JNK is also essential for UV induced apoptosis. However, recent studies reveal that JNK can suppress apoptosis in IL-3-dependent hematopoietic cells via phosphorylation of the proapoptotic Bcl-2 family protein BAD. Thus, JNK has pro- or antiapoptotic functions, depending on cell type, nature of the death stimulus, duration of its activation and the activity of other signaling pathways.
81296
Cooperation between JNK1 and JNK2 in activation of p53 apoptotic pathway
FDH (10-formyltetrahydrofolate dehydrogenase) is strongly downregulated in tumors while its elevation suppresses proliferation of cancer cells and induces p53-dependent apoptosis. We have previously shown that FDH induces phosphorylation of p53 at Ser6, which is a required step in the activation of apoptosis. In the present study, we report that FDH-induced p53 phosphorylation is carried out by JNK1 and JNK2 (c-Jun N-terminal kinases) working in concert. We have demonstrated that FDH induces phosphorylation of JNK1 and JNK2, while treatment of FDH-expressing cells with JNK inhibitor SP600125, as well as knockdown of JNK1 or JNK2 by siRNA, prevents phosphorylation of p53 at Ser6 and protects cells from apoptosis. Interestingly, the knockdown of JNK1 abolished phosphorylation of JNK2 in response to FDH, while knockdown of JNK2 did not prevent JNK1 phosphorylation. Pull-down assay with the p53-specific antibody has shown that JNK2, but not JNK1, is physically associated with p53. Our studies revealed a novel mechanism in which phosphorylation of JNK2 is mediated by JNK1 before phosphorylation of p53, and then p53 is directly phosphorylated by JNK2 at Ser6.
17382391
BCL-2 Is Phosphorylated and Inactivated by an ASK1/Jun N-Terminal Protein Kinase Pathway Normally Activated at G2/M
ABSTRACT Multiple signal transduction pathways are capable of modifying BCL-2 family members to reset susceptibility to apoptosis. We used two-dimensional peptide mapping and sequencing to identify three residues (Ser70, Ser87, and Thr69) within the unstructured loop of BCL-2 that were phosphorylated in response to microtubule-damaging agents, which also arrest cells at G2/M. Changing these sites to alanine conferred more antiapoptotic activity on BCL-2 following physiologic death signals as well as paclitaxel, indicating that phosphorylation is inactivating. An examination of cycling cells enriched by elutriation for distinct phases of the cell cycle revealed that BCL-2 was phosphorylated at the G2/M phase of the cell cycle. G2/M-phase cells proved more susceptible to death signals, and phosphorylation of BCL-2 appeared to be responsible, as a Ser70Ala substitution restored resistance to apoptosis. We noted that ASK1 and JNK1 were normally activated at G2/M phase, and JNK was capable of phosphorylating BCL-2. Expression of a series of wild-type and dominant-negative kinases indicated an ASK1/Jun N-terminal protein kinase 1 (JNK1) pathway phosphorylated BCL-2 in vivo. Moreover, the combination of dominant negative ASK1, (dnASK1), dnMKK7, and dnJNK1 inhibited paclitaxel-induced BCL-2 phosphorylation. Thus, stress response kinases phosphorylate BCL-2 during cell cycle progression as a normal physiologic process to inactivate BCL-2 at G2/M.
22480427
JNK signaling in apoptosis
Jun N-terminal kinases or JNKs play a critical role in death receptor-initiated extrinsic as well as mitochondrial intrinsic apoptotic pathways. JNKs activate apoptotic signaling by the upregulation of pro-apoptotic genes through the transactivation of specific transcription factors or by directly modulating the activities of mitochondrial pro- and antiapoptotic proteins through distinct phosphorylation events. This review analyses our present understanding of the role of JNK in apoptotic signaling and the various mechanisms by which JNK promotes apoptosis.
38844586
Requirement of JNK for stress-induced activation of the cytochrome c-mediated death pathway.
The c-Jun NH2-terminal kinase (JNK) is activated when cells are exposed to ultraviolet (UV) radiation. However, the functional consequence of JNK activation in UV-irradiated cells has not been established. It is shown here that JNK is required for UV-induced apoptosis in primary murine embryonic fibroblasts. Fibroblasts with simultaneous targeted disruptions of all the functional Jnk genes were protected against UV-stimulated apoptosis. The absence of JNK caused a defect in the mitochondrial death signaling pathway, including the failure to release cytochrome c. These data indicate that mitochondria are influenced by proapoptotic signal transduction through the JNK pathway.
9872565
ERK and p38 MAPK-Activated Protein Kinases: a Family of Protein Kinases with Diverse Biological Functions
SUMMARY Conserved signaling pathways that activate the mitogen-activated protein kinases (MAPKs) are involved in relaying extracellular stimulations to intracellular responses. The MAPKs coordinately regulate cell proliferation, differentiation, motility, and survival, which are functions also known to be mediated by members of a growing family of MAPK-activated protein kinases (MKs; formerly known as MAPKAP kinases). The MKs are related serine/threonine kinases that respond to mitogenic and stress stimuli through proline-directed phosphorylation and activation of the kinase domain by extracellular signal-regulated kinases 1 and 2 and p38 MAPKs. There are currently 11 vertebrate MKs in five subfamilies based on primary sequence homology: the ribosomal S6 kinases, the mitogen- and stress-activated kinases, the MAPK-interacting kinases, MAPK-activated protein kinases 2 and 3, and MK5. In the last 5 years, several MK substrates have been identified, which has helped tremendously to identify the biological role of the members of this family. Together with data from the study of MK-knockout mice, the identities of the MK substrates indicate that they play important roles in diverse biological processes, including mRNA translation, cell proliferation and survival, and the nuclear genomic response to mitogens and cellular stresses. In this article, we review the existing data on the MKs and discuss their physiological functions based on recent discoveries.
30314467
Activation of AKT kinases in cancer: implications for therapeutic targeting.
The AKT1, AKT2, and AKT3 kinases have emerged as critical mediators of signal transduction pathways downstream of activated tyrosine kinases and phosphatidylinositol 3-kinase. An ever-increasing list of AKT substrates has precisely defined the multiple functions of this kinase family in normal physiology and disease states. Cellular processes regulated by AKT include cell proliferation and survival, cell size and response to nutrient availability, intermediary metabolism, angiogenesis, and tissue invasion. All these processes represent hallmarks of cancer, and a burgeoning literature has defined the importance of AKT alterations in human cancer and experimental models of tumorigenesis, continuing the legacy represented by the original identification of v-Akt as the transforming oncogene of a murine retrovirus. Many oncoproteins and tumor suppressors intersect in the AKT pathway, finely regulating cellular functions at the interface of signal transduction and classical metabolic regulation. This careful balance is altered in human cancer by a variety of activating and inactivating mechanisms that target both AKT and interrelated proteins. Reprogramming of this altered circuitry by pharmacologic modulation of the AKT pathway represents a powerful strategy for rational cancer therapy. In this review, we summarize a large body of data, from many types of cancer, indicating that AKT activation is one of the most common molecular alterations in human malignancy. We also review mechanisms of activation of AKT kinases, examples of therapeutic modulation of the AKT pathway in animal models, and the current status of efforts to target molecular components of the AKT pathway for cancer therapy and, possibly, cancer prevention.
33500799
Cellular survival: a play in three Akts.
The programmed cell death that occurs as part of normal mammalian development was first observed nearly a century ago (Collin 1906). It has since been established that approximately half of all neurons in the neuroaxis and >99.9% of the total number of cells generated during the course of a human lifetime go on to die through a process of apoptosis (for review, see Datta and Greenberg 1998; Vaux and Korsmeyer 1999). The induction of developmental cell death is a highly regulated process and can be suppressed by a variety of extracellular stimuli. The purification in the 1950s of the nerve growth factor (NGF), which promotes the survival of sympathetic neurons, set the stage for the discovery that peptide trophic factors promote the survival of a wide variety of cell types in vitro and in vivo (Levi-Montalcini 1987). The profound biological consequences of growth factor (GF) suppression of apoptosis are exemplified by the critical role of target-derived neurotrophins in the survival of neurons and the maintenance of functional neuronal circuits. (Pettmann and Henderson 1998). Recently, the ability of trophic factors to promote survival have been attributed, at least in part, to the phosphatidylinositide 38-OH kinase (PI3K)/c-Akt kinase cascade. Several targets of the PI3K/c-Akt signaling pathway have been recently identified that may underlie the ability of this regulatory cascade to promote survival. These substrates include two components of the intrinsic cell death machinery, BAD and caspase 9, transcription factors of the forkhead family, and a kinase, IKK, that regulates the NF-kB transcription factor. This article reviews the mechanisms by which survival factors regulate the PI3K/c-Akt cascade, the evidence that activation of the PI3K/c-Akt pathway promotes cell survival, and the current spectrum of c-Akt targets and their roles in mediating c-Akt-dependent cell survival.
45837814
Phosphorylation and Regulation of Akt/PKB by the Rictor-mTOR Complex
Deregulation of Akt/protein kinase B (PKB) is implicated in the pathogenesis of cancer and diabetes. Akt/PKB activation requires the phosphorylation of Thr308 in the activation loop by the phosphoinositide-dependent kinase 1 (PDK1) and Ser473 within the carboxyl-terminal hydrophobic motif by an unknown kinase. We show that in Drosophila and human cells the target of rapamycin (TOR) kinase and its associated protein rictor are necessary for Ser473 phosphorylation and that a reduction in rictor or mammalian TOR (mTOR) expression inhibited an Akt/PKB effector. The rictor-mTOR complex directly phosphorylated Akt/PKB on Ser473 in vitro and facilitated Thr308 phosphorylation by PDK1. Rictor-mTOR may serve as a drug target in tumors that have lost the expression of PTEN, a tumor suppressor that opposes Akt/PKB activation.
46166152
The two TORCs and Akt.
The regulatory circuits that control the activities of the two distinct target of rapamycin (TOR) complexes, TORC1 and TORC2, and of Akt have been a focus of intense research in recent years. It has become increasingly evident that these regulatory circuits control some of the most fundamental aspects of metabolism, cell growth, proliferation, survival, and differentiation at both the cellular and organismal levels. As such, they also play a pivotal role in the genesis of diseases including cancer, diabetes, aging, and degenerative diseases. This review highlights recent developments aimed at deciphering the interplay between Akt and mTORCs as well as their role in embryonic development and in cancer.
9466184
AKT/PKB Signaling: Navigating Downstream
The serine/threonine kinase Akt, also known as protein kinase B (PKB), is a central node in cell signaling downstream of growth factors, cytokines, and other cellular stimuli. Aberrant loss or gain of Akt activation underlies the pathophysiological properties of a variety of complex diseases, including type-2 diabetes and cancer. Here, we review the molecular properties of Akt and the approaches used to characterize its true cellular targets. In addition, we discuss those Akt substrates that are most likely to contribute to the diverse cellular roles of Akt, which include cell survival, growth, proliferation, angiogenesis, metabolism, and migration.
205530601
The PTEN–PI3K pathway: of feedbacks and cross-talks
The tumor suppressor PTEN was originally identified as a negative regulator of the phosphoinositide 3-kinase (PI3K) signaling, a main regulator of cell growth, metabolism and survival. Yet this function of PTEN is extremely relevant for its tumor-suppressive ability, albeit the recent characterization of many PI3K-independent tumor-suppressive activities. PI3K-mediated PIP3 production leads to the activation of the canonical AKT-mTORC1 pathway. The implications of this signaling cascade in health and disease have been underscored by the high number of regulators within the pathway whose alterations give rise to different malignancies, including familiar syndromes, metabolic dysfunctions and cancer. Moreover, PI3K is tightly buffered at multiple levels by downstream components, which have turned this signaling pathway literally upside down. PI3K and its downstream components in turn cross-talk with a number of other pathways, thereby leading to a complex network of signals that may have dramatic consequences when perturbed. Here, we review the current status of the PTEN–PI3K signaling pathway with special emphasis on the most recent data on targets and regulation of the PTEN–PI3K axis. This provides novel provocative therapeutic implications based on the targeted modulation of PI3K-cross-talking signals.
26667352
The deficiency of Akt1 is sufficient to suppress tumor development in Pten+/- mice.
The tumor suppressor PTEN is frequently inactivated in human cancers. A major downstream effector of PTEN is Akt, which is hyperactivated via PTEN inactivation. It is not known, however, whether diminished Akt activity is sufficient to inhibit tumorigenesis initiated by Pten deficiency. Here we showed that the deficiency of Akt1 is sufficient to dramatically inhibit tumor development in Pten+/- mice. Akt1 deficiency had a profound effect on endometrium and prostate neoplasia, two types of human cancer, in which PTEN is frequently mutated, and also affected thyroid and adrenal medulla tumors and intestinal polyps. Even haplodeficiency of Akt1 was sufficient to markedly attenuate the development of high-grade prostate intraepithelial neoplasia (PIN) and endometrial carcinoma. These results have significant implications for cancer therapy.
26552894
Levels of PTEN protein modulate Akt phosphorylation on serine 473, but not on threonine 308, in IGF-II-overexpressing rhabdomyosarcomas cells
Constitutive activation of Akt has been found in many types of human cancer, and is believed to promote proliferation and increased cell survival thereby contributing to cancer progression. In this study, we examined Akt phosphorylation on Ser473 and Thr308 in seven IGF-II-overexpressing rhabdomyosarcomas (RMS) cells. All the RMS cell lines tested had high levels of Akt phosphorylation on Thr308, whereas three cell lines (Rh5, Rh18, and CTR) had a much lower level of Akt phosphorylation on Ser473. To determine whether the difference in Akt phosphorylation on Ser473, but not on Thr308, observed among cell lines is a cell-specific phenomenon or due to other factors, which possibly downregulate Akt phosphorylation, we examined expression of PTEN protein, which acts as a negative regulator of the PI3K/Akt signaling pathway through its ability to dephosphorylate phosphatidylinositol 3,4,5-triphosphate (PIP3). The levels of PTEN expression inversely correlate with Akt phosphorylation on Ser473, but not on Thr308. Consistent with this finding, transfection of wild-type PTEN into RMS and mouse myoblast C2C12 cells resulted in reduced Akt phosphorylation on Ser473, but not on Thr308. Our data suggest that Ser473 may be a key target residue for PTEN to modulate the effects of IGF-II on activating the PI3K/Akt pathway in RMS cells. A better understanding of the pathway in RMS will likely contribute to insights into the biology of the RMS tumorigenesis and hopefully lead to novel therapeutic options.
4379637
Identification of a tumour suppressor network opposing nuclear Akt function
The proto-oncogene AKT (also known as PKB) is activated in many human cancers, mostly owing to loss of the PTEN tumour suppressor. In such tumours, AKT becomes enriched at cell membranes where it is activated by phosphorylation. Yet many targets inhibited by phosphorylated AKT (for example, the FOXO transcription factors) are nuclear; it has remained unclear how relevant nuclear phosphorylated AKT (pAKT) function is for tumorigenesis. Here we show that the PMLtumour suppressor prevents cancer by inactivating pAKT inside the nucleus. We find in a mouse model that Pml loss markedly accelerates tumour onset, incidence and progression in Pten-heterozygous mutants, and leads to female sterility with features that recapitulate the phenotype of Foxo3a knockout mice. We show that Pml deficiency on its own leads to tumorigenesis in the prostate, a tissue that is exquisitely sensitive to pAkt levels, and demonstrate that Pml specifically recruits the Akt phosphatase PP2a as well as pAkt into Pml nuclear bodies. Notably, we find that Pml-null cells are impaired in PP2a phosphatase activity towards Akt, and thus accumulate nuclear pAkt. As a consequence, the progressive reduction in Pml dose leads to inactivation of Foxo3a-mediated transcription of proapoptotic Bim and the cell cycle inhibitor p27kip1. Our results demonstrate that Pml orchestrates a nuclear tumour suppressor network for inactivation of nuclear pAkt, and thus highlight the importance of AKT compartmentalization in human cancer pathogenesis and treatment.
8137975
GRP78 induction in cancer: therapeutic and prognostic implications.
Cancer cells adapt to chronic stress in the tumor microenvironment by inducing the expression of GRP78/BiP, a major endoplasmic reticulum chaperone with Ca(2+)-binding and antiapoptotic properties. GRP78 promotes tumor proliferation, survival, metastasis, and resistance to a wide variety of therapies. Thus, GRP78 expression may serve as a biomarker for tumor behavior and treatment response. Combination therapy suppressing GRP78 expression may represent a novel approach toward eradication of residual tumors. Furthermore, the recent discovery of GRP78 on the cell surface of cancer cells but not in normal tissues suggests that targeted therapy against cancer via surface GRP78 may be feasible.
5790906
Pten null prostate tumorigenesis and AKT activation are blocked by targeted knockout of ER chaperone GRP78/BiP in prostate epithelium
GRP78/BiP has recently emerged as a novel biomarker for aggressive prostate cancer. Here, we report that homozygous deletion of Grp78 specifically in mouse prostate epithelium suppresses prostate tumorigenesis without affecting postnatal prostate development and growth. Mouse prostates with double conditional knockout of Grp78 and Pten exhibit normal histology and cytology, in contrast to the invasive adenocarcinoma in mouse prostates with Pten inactivation. AKT activation in Pten null prostate epithelium is inhibited by Grp78 homozygous deletion, corresponding with suppression of AKT phosphorylation by GRP78 knockdown in prostate cancer cell line. Thus, inactivation of GRP78 may represent a previously undescribed approach to stop prostate cancer and potentially other cancers resulting from the loss of PTEN tumor suppression and/or activation of the oncogenic AKT.
43669493
Activation and Cross-talk between Akt, NF-κB, and Unfolded Protein Response Signaling in 1-LN Prostate Cancer Cells Consequent to Ligation of Cell Surface-associated GRP78*
Binding of activated forms of the proteinase inhibitor α2-macroglobulin (α2M*) to cell surface-associated GRP78 on 1-LN human prostate cancer cells causes their proliferation. We have now examined the interplay between Akt activation, regulation of apoptosis, the unfolded protein response, and activation of NF-κBin α2M*-induced proliferation of 1-LN cells. Exposure of cells to α2M* (50 pm) induced phosphatidylinositol 3-kinase-dependent activation of Akt by phosphorylation at Thr-308 and Ser-473 with a concomitant 60-80% increase in Akt-associated kinase activity. ERK1/2 and p38 MAPK were also activated, but there was only a marginal effect on JNK activation. Treatment of 1-LN cells with α2M* down-regulated apoptosis and promoted NF-κB activation as shown by increases of Bcl-2, p-BadSer-136, p-FOXO1Ser-253, p-GSK3βSer-9, XIAP, NF-κB, cyclin D1, GADD45β, p-ASK1Ser-83, and TRAF2 in a time of incubation-dependent manner. α2M* treatment of 1-LN cells, however, showed no increase in the activation of caspase -3, -9, or -12. Under these conditions, we observed increased unfolded protein response signaling as evidenced by elevated levels of GRP78, IRE1α, XBP-1, ATF4, ATF6, p-PERK, p-eIF2α, and GADD34 and reduced levels of GADD153. Silencing of GRP78 gene expression by RNAi suppressed activation of AktThr-308, AktSer-473, and IκB kinase α kinase. The effects of α2M* on the NF-κB activation, antiapoptotic signaling, unfolded protein response signaling, and proapoptotic signaling were also reversed by this treatment. In conclusion, α2M* promotes cellular proliferation of 1-LN prostate cancer cells by activating MAPK and Akt-dependent signaling, down-regulating apoptotic signaling, and activating unfolded protein response signaling.
25784397
Binding of Activated α2-Macroglobulin to Its Cell Surface Receptor GRP78 in 1-LN Prostate Cancer Cells Regulates PAK-2-dependent Activation of LIMK*
Two characteristics of highly malignant cells are their increased motility and secretion of proteinases allowing these cells to penetrate surrounding basement membranes and metastasize. Activation of 21-kDa activated kinases (PAKs) is an important mechanism for increasing cell motility. Recently, we reported that binding of receptor-recognized forms of the proteinase inhibitor α2-macroglobulin (α2M*) to GRP78 on the cell surface of 1-LN human prostate cancer cells induces mitogenic signaling and cellular proliferation. In the current study, we have examined the ability of α2M* to activate PAK-1 and PAK-2. Exposure of 1-LN cells to α2M* caused a 2- to 3-fold increase in phosphorylated PAK-2 and a similar increase in its kinase activity toward myelin basic protein. By contrast, the phosphorylation of PAK-1 was only negligibly affected. Silencing the expression of the GRP78 gene, using either of two different mRNA sequences, greatly attenuated the appearance of phosphorylated PAK-2 in α2M*-stimulated cells. Treatment of 1-LN cells with α2M* caused translocation of PAK-2 in association with NCK to the cell surface as evidenced by the coimmunoprecipitation of PAK-2 and NCK in the GRP78 immunoprecipitate from plasma membranes. α2M*-induced activation of PAK-2 was inhibited by prior incubation of the cells with specific inhibitors of tyrosine kinases and phosphatidylinositol 3-kinase. PAK-2 activation was accompanied by significant increases in the levels of phosphorylated LIMK and phosphorylated cofilin. Silencing the expression of the PAK-2 gene greatly attenuated the phosphorylation of LIMK. In conclusion, we show for the first time the activation of PAK-2 in 1-LN prostate cancer cells by a proteinase inhibitor, α2-macroglobulin. These studies suggest a mechanism by which α2M* enhances the metastatic potential of these cells.
37306841
Potentiation of signal transduction mitogenesis and cellular proliferation upon binding of receptor-recognized forms of alpha2-macroglobulin to 1-LN prostate cancer cells.
The alpha2-macroglobulin signalling receptor is upregulated in highly metastatic 1-LN prostate cancer cells. Stimulation of 1-LN cells with activated alpha2-macroglobulin (alpha2M*) caused a two- to threefold increase in [3H]thymidine uptake and cell number. These events require the Ras-dependent MAPK and PI 3-kinase/Akt signalling cascades. Incubation of 1-LN cells with alpha2M* induced Grb2, shc, sos and Raf-1 expression, as well as phosphorylation of MEK 1/2, ERK 1/2, p38 MAPK and JNK. This treatment also increased PI 3-kinase activation, PDK1 expression, Akt phosphorylation and p70s6k phosphorylation. Levels of the early gene products c-fos protein and thymidylate synthase were comparably increased. Exposure of 1-LN cells to alpha2M* significantly raised the levels of phosphorylated CREB by about 15-20 min and phosphorylated p53 by about 60-90 min of incubation. We conclude that the growth regulatory effects of ligating the alpha2M* signalling receptor on 1-LN cells are exerted via the onset and crosstalk between the Ras-dependent MAPK and PI 3-kinase/Akt signalling cascades.
8342720
The role of Grp 78 in alpha 2-macroglobulin-induced signal transduction. Evidence from RNA interference that the low density lipoprotein receptor-related protein is associated with, but not necessary for, GRP 78-mediated signal transduction.
The low density lipoprotein receptor-related protein (LRP) is a scavenger receptor that binds to many proteins, some of which trigger signal transduction. Receptor-recognized forms of alpha(2)-Macroglobulin (alpha(2)M*) bind to LRP, but the pattern of signal transduction differs significantly from that observed with other LRP ligands. For example, neither Ni(2+) nor the receptor-associated protein, which blocks binding of all known ligands to LRP, block alpha(2)M*-induced signal transduction. In the current study, we employed alpha(2)-macroglobulin (alpha(2)M)-agarose column chromatography to purify cell surface membrane binding proteins from 1-LN human prostate cancer cells and murine macrophages. The predominant binding protein purified from 1-LN prostate cancer cells was Grp 78 with small amounts of LRP, a fact that is consistent with our previous observations that there is little LRP present on the surface of these cells. The ratio of LRP:Grp 78 is much higher in macrophages. Flow cytometry was employed to demonstrate the presence of Grp 78 on the cell surface of 1-LN cells. Purified Grp 78 binds to alpha(2)M* with high affinity (K(d) approximately 150 pm). A monoclonal antibody directed against Grp 78 both abolished alpha(2)M*-induced signal transduction and co-precipitated LRP. Ligand blotting with alpha(2)M* showed binding to both Grp 78 and LRP heavy chains in these preparations. Use of RNA interference to silence LRP expression had no effect on alpha(2)M*-mediated signaling. We conclude that Grp 78 is essential for alpha(2)M*-induced signal transduction and that a "co-receptor" relationship exists with LRP like that seen with several other ligands and receptors such as the uPA/uPAR (urinary type plasminogen activator or urokinase/uPA receptor) system.
206974236
p53 ubiquitination: Mdm2 and beyond.
Although early studies have suggested that the oncoprotein Mdm2 is the primary E3 ubiquitin ligase for the p53 tumor suppressor, an increasing amount of data suggests that p53 ubiquitination and degradation are more complex than once thought. The discoveries of MdmX, HAUSP, ARF, COP1, Pirh2, and ARF-BP1 continue to uncover the multiple facets of this pathway. There is no question that Mdm2 plays a pivotal role in downregulating p53 activities in numerous cellular settings. Nevertheless, growing evidence challenges the conventional view that Mdm2 is essential for p53 turnover.
12424151
p38 MAP-kinases pathway regulation, function and role in human diseases.
Mammalian p38 mitogen-activated protein kinases (MAPKs) are activated by a wide range of cellular stresses as well as in response to inflammatory cytokines. There are four members of the p38MAPK family (p38alpha, p38beta, p38gamma and p38delta) which are about 60% identical in their amino acid sequence but differ in their expression patterns, substrate specificities and sensitivities to chemical inhibitors such as SB203580. A large body of evidences indicates that p38MAPK activity is critical for normal immune and inflammatory response. The p38MAPK pathway is a key regulator of pro-inflammatory cytokines biosynthesis at the transcriptional and translational levels, which makes different components of this pathway potential targets for the treatment of autoimmune and inflammatory diseases. However, recent studies have shed light on the broad effect of p38MAPK activation in the control of many other aspects of the physiology of the cell, such as control of cell cycle or cytoskeleton remodelling. Here we focus on these emergent roles of p38MAPKs and their implication in different pathologies.
6533618
Role of mitogen-activated protein kinase kinase 4 in cancer
Mitogen-activated protein (MAP) kinase kinase 4 (MKK4) is a component of stress activated MAP kinase signaling modules. It directly phosphorylates and activates the c-Jun N-terminal kinase (JNK) and p38 families of MAP kinases in response to environmental stress, pro-inflammatory cytokines and developmental cues. MKK4 is ubiquitously expressed and the targeted deletion of the Mkk4 gene in mice results in early embryonic lethality. Further studies in mice have indicated a role for MKK4 in liver formation, the immune system and cardiac hypertrophy. In humans, it is reported that loss of function mutations in the MKK4 gene are found in approximately 5% of tumors from a variety of tissues, suggesting it may have a tumor suppression function. Furthermore, MKK4 has been identified as a suppressor of metastasis of prostate and ovarian cancers. However, the role of MKK4 in cancer development appears complex as other studies support a pro-oncogenic role for MKK4 and JNK. Here we review the biochemical and functional properties of MKK4 and discuss the likely mechanisms by which it may regulate the steps leading to the formation of cancers.
22395647
JNK- and p38 Kinase-mediated Phosphorylation of Bax Leads to Its Activation and Mitochondrial Translocation and to Apoptosis of Human Hepatoma HepG2 Cells*
Mitochondrial translocation of pro-apoptotic Bax prior to apoptosis is well established after treatment with many cell death stimulants or under apoptosis-inducing conditions. The mechanism of mitochondrial translocation of Bax is, however, still unknown. The aim of this work was to investigate the mechanism of Bax activation and mitochondrial translocation to initiate apoptosis of human hepatoma HepG2 and porcine kidney LLC-PK1 cells exposed to various cell death agonists. Phosphorylation of Bax by JNK and p38 kinase activated after treatment with staurosporine, H2O2, etoposide, and UV light was demonstrated by the shift in the pI value of Bax on two-dimensional gels and confirmed by metabolic labeling with inorganic [32P]phosphate in HepG2 cells. Specific inhibitors of JNK and p38 kinase significantly inhibited Bax phosphorylation and mitochondrial translocation and apoptosis of HepG2 cells. A specific small interfering RNA to MAPKK4 (the upstream protein kinase of JNK and p38 kinase) markedly decreased the levels of MAPKK4 and MAPKK3/6, blocked the activation of JNK or p38 kinase, and inhibited Bax phosphorylation. However, the negative control small interfering RNA did not cause these changes. Confocal microscopy of various Bax mutants showed differential rates of mitochondrial translocation of Bax before and after staurosporine treatment. Among the Bax mutants, T167D did not translocate to mitochondria after staurosporine exposure, suggesting that Thr167 is a potential phosphorylation site. In conclusion, our results demonstrate, for the first time, that Bax is phosphorylated by stress-activated JNK and/or p38 kinase and that phosphorylation of Bax leads to mitochondrial translocation prior to apoptosis.
34046920
The complexity of p53 stabilization and activation
A number of proteins are activated by stress stimuli but none so spectacularly or with the degree of complexity as the tumour suppressor p53 (human p53 gene or protein). Once stabilized, p53 is responsible for the transcriptional activation of a series of proteins involved in cell cycle control, apoptosis and senescence. This protein is present at low levels in resting cells but after exposure to DNA-damaging agents and other stress stimuli it is stabilized and activated by a series of post-translational modifications that free it from MDM2 (mouse double minute 2 but used interchangeably to denote human also), a ubiquination ligase that ubiquitinates it prior to proteasome degradation. The stability of p53 is also influenced by a series of other interacting proteins. In this review, we discuss the post-translational modifications to p53 in response to different stresses and the consequences of these changes.
17464318
MAP kinase signalling pathways in cancer
Cancer can be perceived as a disease of communication between and within cells. The aberrations are pleiotropic, but mitogen-activated protein kinase (MAPK) pathways feature prominently. Here, we discuss recent findings and hypotheses on the role of MAPK pathways in cancer. Cancerous mutations in MAPK pathways are frequently mostly affecting Ras and B-Raf in the extracellular signal-regulated kinase pathway. Stress-activated pathways, such as Jun N-terminal kinase and p38, largely seem to counteract malignant transformation. The balance and integration between these signals may widely vary in different tumours, but are important for the outcome and the sensitivity to drug therapy.
3345796
Targeting the Raf-MEK-ERK mitogen-activated protein kinase cascade for the treatment of cancer
Mitogen-activated protein kinase (MAPK) cascades are key signaling pathways involved in the regulation of normal cell proliferation, survival and differentiation. Aberrant regulation of MAPK cascades contribute to cancer and other human diseases. In particular, the extracellular signal-regulated kinase (ERK) MAPK pathway has been the subject of intense research scrutiny leading to the development of pharmacologic inhibitors for the treatment of cancer. ERK is a downstream component of an evolutionarily conserved signaling module that is activated by the Raf serine/threonine kinases. Raf activates the MAPK/ERK kinase (MEK)1/2 dual-specificity protein kinases, which then activate ERK1/2. The mutational activation of Raf in human cancers supports the important role of this pathway in human oncogenesis. Additionally, the Raf-MEK-ERK pathway is a key downstream effector of the Ras small GTPase, the most frequently mutated oncogene in human cancers. Finally, Ras is a key downstream effector of the epidermal growth factor receptor (EGFR), which is mutationally activated and/or overexpressed in a wide variety of human cancers. ERK activation also promotes upregulated expression of EGFR ligands, promoting an autocrine growth loop critical for tumor growth. Thus, the EGFR-Ras-Raf-MEK-ERK signaling network has been the subject of intense research and pharmaceutical scrutiny to identify novel target-based approaches for cancer treatment. In this review, we summarize the current status of the different approaches and targets that are under evaluation and development for the therapeutic intervention of this key signaling pathway in human disease.
12111219
Molecular interpretation of ERK signal duration by immediate early gene products
The duration of intracellular signalling is associated with distinct biological responses, but how cells interpret differences in signal duration are unknown. We show that the immediate early gene product c-Fos functions as a sensor for ERK1 (extracellular-signal-regulated kinase 1) and ERK2 signal duration. When ERK activation is transient, its activity declines before the c-Fos protein accumulates, and under these conditions c-Fos is unstable. However, when ERK signalling is sustained, c-Fos is phosphorylated by still-active ERK and RSK (90K-ribosomal S6 kinase). Carboxy-terminal phosphorylation stabilizes c-Fos and primes additional phosphorylation by exposing a docking site for ERK, termed the FXFP (DEF) domain. Mutating the DEF domain disrupts the c-Fos sensor and c-Fos-mediated signalling. Other immediate early gene products that control cell cycle progression, neuronal differentiation and circadium rhythms also contain putative DEF domains, indicating that multiple sensors exist for sustained ERK signalling. Together, our data identify a general mechanism by which cells can interpret differences in ERK activation kinetics.
1318544
Coordinating ERK/MAPK signalling through scaffolds and inhibitors
The pathway from Ras through Raf and MEK (MAPK and ERK kinase) to ERK/MAPK (extracellular signal-regulated kinase/mitogen-activated protein kinase) regulates many fundamental cellular processes. Recently, a number of scaffolding proteins and endogenous inhibitors have been identified, and their important roles in regulating signalling through this pathway are now emerging. Some scaffolds augment the signal flux, but also mediate crosstalk with other pathways; certain adaptors target MEK–ERK/MAPK complexes to subcellular localizations; others provide regulated inhibition. Computational modelling indicates that, together, these modulators can determine the dynamic biological behaviour of the pathway.
205494277
The RSK family of kinases: emerging roles in cellular signalling
The 90 kDa ribosomal S6 kinase (RSK) family of proteins is a group of highly conserved Ser/Thr kinases that regulate diverse cellular processes, such as cell growth, cell motility, cell survival and cell proliferation. RSKs are downstream effectors of the Ras–extracellular signal-regulated kinase (ERK)/mitogen-activated protein kinase (MAPK) signalling cascade. Significant advances in the field of RSK and ERK/MAPK signalling have occurred in the past few years, including biological insights and the discovery of novel substrates and new RSK regulatory mechanisms. Collectively, these data expand the current models of RSK signalling and highlight potential directions of research in RSK-mediated survival, growth, proliferation and migration.
52853715
A new tumor-specific variant of GRP78 as target for antibody-based therapy.
The chaperone GRP78 is a member of the heat-shock protein 70 (HSP70) family and is responsible for cellular homeostasis by preventing stress-induced apoptosis. GRP78 is expressed in all cells of the body. In malignant cells, which are permanently exposed to environmental stress, GRP78 is overexpressed and increased levels can be found in the cytoplasm and on the cell membrane. Thus, GRP78 promotes tumor proliferation, survival, metastases and resistance to a wide variety of therapies. Like other tumor-specific membrane molecules, GRP78 can also be present on cancer cells in a variant form. This modification qualifies it as a target for immune surveillance and antibody responses. The fully human monoclonal IgM antibody, SAM-6, was isolated from a gastric cancer patient and it binds to a new variant of GRP78 with a molecular weight of 82 kDa. The epitope is an O-linked carbohydrate moiety and is specific for malignant cells. These data show that cancer-specific modifications of cell-surface protection molecules are (a) subject of an immune response and (b) ideal targets for new therapeutical approaches.
22410084
Phage display derived human monoclonal antibodies isolated by binding to the surface of live primary breast cancer cells recognize GRP78.
Clinical trials using monoclonal antibodies (mAb) against cell-surface markers have yielded encouraging therapeutic results in several cancer types. Generally, however, anticancer antibodies are only efficient against a subpopulation of cancers, and there is a strong need for identification of novel targets and human antibodies against them. We have isolated single-chain human mAbs from a large naïve antibody phage display library by panning on a single-cell suspension of freshly isolated live cancer cells from a human breast cancer specimen, and these antibodies were shown to specifically recognize cancer-associated cell-surface proteins. One of the isolated human antibody fragments, Ab39, recognizes a cell-surface antigen expressed on a subpopulation of cancer cell lines of different origins. Immunohistochemical analysis of a large panel of cancerous and normal tissues showed that Ab39 bound strongly to several cancers, including 45% breast carcinomas, 35% lung cancers, and 86% melanomas, but showed no or weak binding to normal tissues. A yeast two-hybrid screen of a large human testis cDNA library identified the glucose-regulated protein of 78 kDa (GRP78) as the antigen recognized by Ab39. The interaction was confirmed by colocalization studies and antibody competition experiments that also mapped the epitope recognized by Ab39 to the COOH terminus of GRP78. The expression of GRP78 on the surface of cancer cells, but not normal cells, makes it an attractive target for cancer therapies including mAb-based immunotherapy. Our results suggest that the human antibody Ab39 may be a useful starting point for further genetic optimization that could render it a useful diagnostic and therapeutic reagent for a variety of cancers.
25680702
The glamour and gloom of glycogen synthase kinase-3.
Glycogen synthase kinase-3 (GSK3) is now recognized as a key component of a surprisingly large number of cellular processes and diseases. Several mechanisms play a part in controlling the actions of GSK3, including phosphorylation, protein complex formation, and subcellular distribution. These are used to control and direct the far-reaching influences of GSK3 on cellular structure, growth, motility and apoptosis. Dysregulation of GSK3 is linked to several prevalent pathological conditions, such as diabetes and/or insulin resistance, and Alzheimer's disease. Therefore, much effort is currently directed towards understanding the functions and control of GSK3, and identifying methods capable of diminishing the deleterious impact of GSK3 in pathological conditions.
206834443
The biology of cancer: metabolic reprogramming fuels cell growth and proliferation.
Cell proliferation requires nutrients, energy, and biosynthetic activity to duplicate all macromolecular components during each passage through the cell cycle. It is therefore not surprising that metabolic activities in proliferating cells are fundamentally different from those in nonproliferating cells. This review examines the idea that several core fluxes, including aerobic glycolysis, de novo lipid biosynthesis, and glutamine-dependent anaplerosis, form a stereotyped platform supporting proliferation of diverse cell types. We also consider regulation of these fluxes by cellular mediators of signal transduction and gene expression, including the phosphatidylinositol 3-kinase (PI3K)/Akt/mTOR system, hypoxia-inducible factor 1 (HIF-1), and Myc, during physiologic cell proliferation and tumorigenesis.
25230246
Mitochondrial hexokinases, novel mediators of the antiapoptotic effects of growth factors and Akt
Cell survival has been closely linked to both trophic growth factor signaling and cellular metabolism. Such couplings have obvious physiologic and pathophysiologic implications, but their underlying molecular bases remain incompletely defined. As a common mediator of both the metabolic and anti-apoptotic effects of growth factors, the serine/threonine kinase Akt – also known as protein kinase B or PKB – is capable of regulating and coordinating these inter-related processes. The glucose dependence of the antiapoptotic effects of growth factors and Akt plus a strong correlation between Akt-regulated mitochondrial hexokinase association and apoptotic susceptibility suggest a major role for hexokinases in these effects. Mitochondrial hexokinases catalyse the first obligatory step of glucose metabolism and directly couple extramitochondrial glycolysis to intramitochondrial oxidative phosphorylation, and are thus well suited to play this role. The ability of Akt to regulate energy metabolism appears to have evolutionarily preceded the capacity to control cell survival. This suggests that Akt-dependent metabolic regulatory functions may have given rise to glucose-dependent antiapoptotic effects that evolved as an adaptive sensing system involving hexokinases and serve to ensure mitochondrial homeostasis, thereby coupling metabolism to cell survival. We hypothesize that the enlistment of Akt and hexokinase in the control of mammalian cell apoptosis evolved as a response to the recruitment of mitochondria to the apoptotic cascade. The central importance of mitochondrial hexokinases in cell survival also suggests that they may represent viable therapeutic targets in cancer.
45173733
Is Akt the "Warburg kinase"?-Akt-energy metabolism interactions and oncogenesis.
The serine/threonine kinase Akt - also known as protein kinase B (PKB) - has emerged as one of the most frequently activated protein kinases in human cancer. In fact, most, if not all, tumors ultimately find a way to activate this important kinase. As such, Akt activation constitutes a hallmark of most cancer cells, and such ubiquity presumably connotes important roles in tumor genesis and/or progression. Likewise, the hypermetabolic nature of cancer cells and their increased reliance on "aerobic glycolysis", as originally described by Otto Warburg and colleagues, are considered metabolic hallmarks of cancer cells. In this review, we address the specific contributions of Akt activation to the signature metabolic features of cancer cells, including the so-called "Warburg effect".
21907100
Targeting AKT/mTOR and ERK MAPK signaling inhibits hormone-refractory prostate cancer in a preclinical mouse model.
The AKT/mammalian target of rapamycin (AKT/mTOR) and ERK MAPK signaling pathways have been shown to cooperate in prostate cancer progression and the transition to androgen-independent disease. We have now tested the effects of combinatorial inhibition of these pathways on prostate tumorigenicity by performing preclinical studies using a genetically engineered mouse model of prostate cancer. We report here that combination therapy using rapamycin, an inhibitor of mTOR, and PD0325901, an inhibitor of MAPK kinase 1 (MEK; the kinase directly upstream of ERK), inhibited cell growth in cultured prostate cancer cell lines and tumor growth particularly for androgen-independent prostate tumors in the mouse model. We further showed that such inhibition leads to inhibition of proliferation and upregulated expression of the apoptotic regulator Bcl-2-interacting mediator of cell death (Bim). Furthermore, analyses of human prostate cancer tissue microarrays demonstrated that AKT/mTOR and ERK MAPK signaling pathways are often coordinately deregulated during prostate cancer progression in humans. We therefore propose that combination therapy targeting AKT/mTOR and ERK MAPK signaling pathways may be an effective treatment for patients with advanced prostate cancer, in particular those with hormone-refractory disease.