p120 GAP modulates Ras activation of Jun kinases and transformation

Prognostic Gene Signature for Squamous Cell Carcinoma with a Higher Risk for Treatment Failure and Accelerated MEK-ERK Pathway Activity

Simple Summary

Squamous cell carcinoma (SCC) is the most prevalent type of human cancer worldwide and represents the majority of head and neck tumors. As SCC from aerodigestive or genitourinary tracts share not only common etiology and histological features but also molecular patterns, the major objectives of this study were the establishment of a pan-SCC-related prognostic gene signature by an integrative analysis of multi-omics data and the elucidation of underlying oncogenic pathway activities as potential vulnerabilities for a more efficient and less toxic therapy. Our approach delivers a reliable molecular classifier to identify HNSCC and other SCC patients at higher risk for treatment failure with tumors characterized by a more prominent MAPK activity, who might benefit from a targeted treatment with MEK inhibitors.

Abstract

Squamous cell carcinoma (SCC) is the most prevalent histological type of human cancer, including head and neck squamous cell carcinoma (HNSCC). However, reliable prognostic gene signatures for SCC and underlying genetic and/or epigenetic principles are still unclear. We identified 37 prognostic candidate genes by best cutoff computation based on survival in a pan-SCC cohort (n = 1334) of The Cancer Genome Atlas (TCGA), whose expression stratified not only the pan-SCC cohort but also independent HNSCC validation cohorts into three distinct prognostic subgroups. The most relevant prognostic genes were prioritized by a Least Absolute Shrinkage and Selection Operator Cox regression model and were used to identify subgroups with high or low risks for unfavorable survival. An integrative analysis of multi-omics data identified FN1, SEMA3A, CDH2, FBN1, COL5A1, and ADAM12 as key nodes in a regulatory network related to the prognostic phenotype. An in-silico drug screen predicted two MEK inhibitors (Trametinib and Selumetinib) as effective compounds for high-risk SCC based on the Cancer Cell Line Encyclopedia, which is supported by a higher p-MEK1/2 immunohistochemical staining of high-risk HNSCC. In conclusion, our data identified a molecular classifier for high-risk HNSCC as well as other SCC patients, who might benefit from treatment with MEK inhibitors.

The Crossroads between RAS and RHO Signaling Pathways in Cellular Transformation, Motility and Contraction

Ras and Rho proteins are GTP-regulated molecular switches that control multiple signaling pathways in eukaryotic cells. Ras was among the first identified oncogenes, and it appears mutated in many forms of human cancer. It mainly promotes proliferation and survival through the MAPK pathway and the PI3K/AKT pathways, respectively. However, the myriad proteins close to the plasma membrane that activate or inhibit Ras make it a major regulator of many apparently unrelated pathways. On the other hand, Rho is weakly oncogenic by itself, but it critically regulates microfilament dynamics; that is, actin polymerization, disassembly and contraction. Polymerization is driven mainly by the Arp2/3 complex and formins, whereas contraction depends on myosin mini-filament assembly and activity. These two pathways intersect at numerous points: from Ras-dependent triggering of Rho activators, some of which act through PI3K, to mechanical feedback driven by actomyosin action. Here, we describe the main points of connection between the Ras and Rho pathways as they coordinately drive oncogenic transformation. We emphasize the biochemical crosstalk that drives actomyosin contraction driven by Ras in a Rho-dependent manner. We also describe possible routes of mechanical feedback through which myosin II activation may control Ras/Rho activation.

40 Years of RAS-A Historic Overview

It has been over forty years since the isolation of the first human oncogene (HRAS), a crucial milestone in cancer research made possible through the combined efforts of a few selected research groups at the beginning of the 1980s. Those initial discoveries led to a quantitative leap in our understanding of cancer biology and set up the onset of the field of molecular oncology. The following four decades of RAS research have produced a huge pool of new knowledge about the RAS family of small GTPases, including how they regulate signaling pathways controlling many cellular physiological processes, or how oncogenic mutations trigger pathological conditions, including developmental syndromes or many cancer types. However, despite the extensive body of available basic knowledge, specific effective treatments for RAS-driven cancers are still lacking. Hopefully, recent advances involving the discovery of novel pockets on the RAS surface as well as highly specific small-molecule inhibitors able to block its interaction with effectors and/or activators may lead to the development of new, effective treatments for cancer. This review intends to provide a quick, summarized historical overview of the main milestones in RAS research spanning from the initial discovery of the viral RAS oncogenes in rodent tumors to the latest attempts at targeting RAS oncogenes in various human cancers.

ERK1 Directly Interacts With JNK1 Leading to Regulation of JNK1/c-Jun Activity and Cell Transformation

ERK1 and ERK2 share a great deal of homology and have been presumed to have similar functions. Available antibodies recognize both isoforms making the elucidation of functional differences challenging. Mitogen-activated protein (MAP) kinase networks are commonly depicted in the literature as linear and sequential phosphorylation cascades; however, the activation of these pathways is not mutually exclusive. Little doubt exists that MAP kinases engage in crosstalk, but the extent or the direct effect of these "conversations" is unclear. Here, we report the possible points of direct interaction as "crosstalk" points between ERK1 and JNK1 and a potential mechanism for ERK1 function in repressing Ras/JNK-mediated cell transformation. ERK1, but not ERK2, directly interacts with and antagonizes JNK1 phosphorylation and activity, resulting in suppression of neoplastic cell transformation mediated by the Ras/JNK/c-Jun signaling pathway. Interestingly, ERK1 phosphorylation was increased in normal tissues compared to liver cancer tissues. Furthermore, predominant JNK/c-Jun activation was observed in liver cancer tissues. These phenomena can provide evidence for the existence of a functional association between ERK and JNK signaling pathways during in vivo tumorigenesis. Overall, our findings provide new evidence supporting the paradigm of an ERK1/JNK1 antagonistic interaction as a novel mechanism of trans-regulation between different MAP kinase signaling modules. J. Cell. Biochem. 118: 2357-2370, 2017. © 2017 Wiley Periodicals, Inc.

Rac1b enhances cell survival through activation of the JNK2/c-JUN/Cyclin-D1 and AKT2/MCL1 pathways

Rac1b is a constitutively activated, alternatively spliced form of the small GTPase Rac1. Previous studies showed that Rac1b promotes cell proliferation and inhibits apoptosis. In the present study, we used microarray analysis to detect genes differentially expressed in HEK293T cells and SW480 human colon cancer cells stably overexpressing Rac1b. We found that the pro-proliferation genes JNK2, c-JUN and cyclin-D1 as well as anti-apoptotic AKT2 and MCL1 were all upregulated in both lines. Rac1b promoted cell proliferation and inhibited apoptosis by activating the JNK2/c-JUN/cyclin-D1 and AKT2/MCL1 pathways, respectively. Very low Rac1b levels were detected in the colonic epithelium of wild-type Sprague-Dawley rats. Knockout of the rat Rac1 gene exon-3b or knockdown of endogenous Rac1b in HT29 human colon cancer cells downregulated only the AKT2/MCL1 pathway. Our study revealed that very low levels of endogenous Rac1b inhibit apoptosis, while Rac1b upregulation both promotes cell proliferation and inhibits apoptosis. It is likely the AKT2/MCL1 pathway is more sensitive to Rac1b regulation.

miR182 activates the Ras-MEK-ERK pathway in human oral cavity squamous cell carcinoma by suppressing RASA1 and SPRED1

Purpose

The constitutive activation of the Ras–MEK–ERK signaling pathway in oral cavity squamous cell carcinoma (OSCC) has been found to be tightly controlled at multiple levels under physiological conditions. RASA1 and SPRED1 are two important negative regulators of this pathway, but the exact regulating mechanism remains unclear. In this study, we aimed to explore the potential regulating mechanisms involved in the Ras–MEK–ERK signaling pathway in OSCC.

Materials and methods

MicroRNA (miRNA) expression was detected by quantitative reverse-transcription polymerase chain reaction. The protein levels of RASA1, SPRED1, and signaling proteins were detected by Western blot. Cell growth was determined using CCK-8 reagent, colony formation was stained by crystal violet, and cell invasion was tested using transwell chambers. Cell apoptosis and the cell cycle were then analyzed by flow cytometry. The binding of miR182 with RASA1 or SPRED1 was evaluated by luciferase reporter assays on a dual-luciferase reporter system.

Results

The expression of miR182 was found to be upregulated significantly in malignant oral carcinoma tissues compared with the adjacent nonmalignant tissues, and was inversely correlated with protein levels of RASA1 and SPRED1. Overexpression of miR182 in OSCC cell lines sustained Ras–MEK–ERK signaling-pathway activation, and promoted cell proliferation, cell-cycle progression, colony formation, and invasion capacity, whereas miR182 downregulation alleviated these properties significantly in vitro. Furthermore, we demonstrated that miR182 exerted its oncogenic role in OSCC by directly targeting and suppressing RASA1 and SPRED1.

Conclusion

Our results bring new insights into the important role of miR182 in the activation of the Ras–MEK–ERK signaling pathway, and suggest that miR182 may be used as a potential target for treatment of OSCC, prompting further investigation into miRNA antisense oligonucleotides for cancer therapy.

MicroRNA-31 initiates lung tumorigenesis and promotes mutant KRAS-driven lung cancer

MicroRNA (miR) are important regulators of gene expression, and aberrant miR expression has been linked to oncogenesis; however, little is understood about their contribution to lung tumorigenesis. Here, we determined that miR-31 is overexpressed in human lung adenocarcinoma and this overexpression independently correlates with decreased patient survival. We developed a transgenic mouse model that allows for lung-specific expression of miR-31 to test the oncogenic potential of miR-31 in the lung. Using this model, we observed that miR-31 induction results in lung hyperplasia, followed by adenoma formation and later adenocarcinoma development. Moreover, induced expression of miR-31 in mice cooperated with mutant KRAS to accelerate lung tumorigenesis. We determined that miR-31 regulates lung epithelial cell growth and identified 6 negative regulators of RAS/MAPK signaling as direct targets of miR-31. Our study distinguishes miR-31 as a driver of lung tumorigenesis that promotes mutant KRAS-mediated oncogenesis and reveals that miR-31 directly targets and reduces expression of negative regulators of RAS/MAPK signaling.

Functional cross-talk between ras and rho pathways: a Ras-specific GTPase-activating protein (p120RasGAP) competitively inhibits the RhoGAP activity of deleted in liver cancer (DLC) tumor suppressor by masking the catalytic arginine finger

The three deleted in liver cancer genes (DLC1-3) encode Rho-specific GTPase-activating proteins (RhoGAPs). Their expression is frequently silenced in a variety of cancers. The RhoGAP activity, which is required for full DLC-dependent tumor suppressor activity, can be inhibited by the Src homology 3 (SH3) domain of a Ras-specific GAP (p120RasGAP). Here, we comprehensively investigated the molecular mechanism underlying cross-talk between two distinct regulators of small GTP-binding proteins using structural and biochemical methods. We demonstrate that only the SH3 domain of p120 selectively inhibits the RhoGAP activity of all three DLC isoforms as compared with a large set of other representative SH3 or RhoGAP proteins. Structural and mutational analyses provide new insights into a putative interaction mode of the p120 SH3 domain with the DLC1 RhoGAP domain that is atypical and does not follow the classical PXXP-directed interaction. Hence, p120 associates with the DLC1 RhoGAP domain by targeting the catalytic arginine finger and thus by competitively and very potently inhibiting RhoGAP activity. The novel findings of this study shed light on the molecular mechanisms underlying the DLC inhibitory effects of p120 and suggest a functional cross-talk between Ras and Rho proteins at the level of regulatory proteins.

Thymosin β 4 mediates oligodendrocyte differentiation by upregulating p38 MAPK

Thymosin beta 4 (Tβ4), a G-actin sequestering peptide, increases oligodendrogenesis and improves functional outcome in models of neurological injury. The molecular mechanisms of Tβ4 mediated oligodendrogenesis are unclear. The p38 mitogen-activated protein kinase (p38MAPK) regulates oligodendrocyte (OL) differentiation and myelin gene expression in other models. Therefore, we investigated p38MAPK signaling pathways. We used primary rat neural progenitor cells (NPCs) and a mouse oligodendrocyte progenitor cell (OPC) line (N20.1 cells) to investigate the molecular mechanisms of Tβ4-enhanced oligodendrogenesis. NPCs were isolated from rat subventricular zone (SVZ) of the lateral ventricles (n = 12). Primary NPCs and N20.1 cells were grown in the presence of 0, 25, and 50 ng/mL of Tβ4 (RegeneRx Biopharmaceuticals Inc, Rockville, MD) for 14 days. Quantitative real-time PCR and Western blot data showed significant induction of both expression and phosphorylation of p38MAPK with simultaneous inhibition of phosphorylation of extracellular signal regulated kinase (ERK1), c-Jun N-terminal kinase 1 (JNK1), leading to reduction of phosphorylation of c-Jun, a potent negative regulator of transcription of myelin genes. These effects were reversed with transfection of Tβ4siRNA. Our data indicate that Tβ4 treatment induces OL differentiation by inducing p38MAPK with parallel inactivation of ERK1 and JNK1, thus preventing the accumulation of phosphorylated c-Jun.

p120RasGAP-mediated activation of c-Src is critical for oncogenic Ras to induce tumor invasion

Ras genes are the most common targets for somatic gain-of-function mutations in human cancers. In this study, we found a high incidence of correlation between Ras oncogenic mutations and c-Src activation in human cancer cells. We showed that oncogenic Ras induces c-Src activation mainly on the Golgi complex and endoplasmic reticulum. Moreover, we identified p120RasGAP as an effector for oncogenic Ras to activate c-Src. The recruitment of p120RasGAP to the Golgi complex by oncogenic Ras facilitated its interaction with c-Src, thereby leading to c-Src activation, and this p120RasGAP-mediated activation of c-Src was important for tumor invasion induced by oncogenic Ras. Collectively, our findings unveil a relationship between oncogenic Ras, p120RasGAP, and c-Src, suggesting a critical role for c-Src in cancers evoked by oncogenic mutations in Ras genes.

NF-κB, JNK, and TLR Signaling Pathways in Hepatocarcinogenesis

Hepatocellular carcinoma (HCC) is the third largest cause of cancer deaths worldwide. The role of molecular changes in HCC have been used to identify prognostic markers and chemopreventive or therapeutic targets. It seems that toll-like receptors (TLRs) as well as the nuclear factor (NF)-κB, and JNK pathways are critical regulators for the production of the cytokines associated with tumor promotion. The cross-talk between an inflammatory cell and a neoplastic cell, which is instigated by the activation of NF-κB and JNKs, is critical for tumor organization. JNKs also regulate cell proliferation and act as oncogenes, making them the main tumor-promoting protein kinases. TLRs play roles in cytokine and hepatomitogen expression mainly in myeloid cells and may promote liver tumorigenesis. A better understanding of these signaling pathways in the liver will help us understand the mechanism of hepatocarcinogenesis and provide a new therapeutic target for HCC.

Mechanisms of regulation of oligodendrocyte development by p38 mitogen-activated protein kinase

Many extracellular and intrinsic factors regulate oligodendrocyte development, but their signaling pathways remain poorly understood. Although the p38 mitogen-activated protein kinase (MAPK)-dependent pathway is implicated in oligodendrocyte progenitor cell (OPC) lineage progression, its molecular targets involved in myelinogenesis are mostly unidentified. We have analyzed mechanisms by which p38MAPK regulates oligodendrocyte development and demonstrate that p38MAPK inhibition prevents OPC lineage progression and inhibits MBP (myelin basic protein) promoter activity and Sox10 function. In white-matter tissue, differential levels of MAPK phosphorylation are observed in oligodendrocyte lineage cells. Phosphorylated p38MAPK was found in CC1- and CNP-expressing differentiated oligodendrocytes of the adult brain and was temporally associated with a decline in the levels of phosphorylated extracellular signal-regulated kinase (ERK) in cells of this lineage. PDGF stimulates the phosphorylation of ERK, p38MAPK, and c-Jun N-terminal kinase (JNK), and p38MAPK inhibition was associated with increased ERK, JNK, and c-Jun phosphorylation. In the presence of PDGF, simultaneous inhibition of p38MAPK and either MAPK kinase (MEK) or JNK significantly alleviates the repression of myelin gene expression and lineage progression induced by p38MAPK inhibition alone. Dominant-negative c-Jun reverses the inhibition of myelin promoter activity by active MEK1 or dominant-negative p38MAPKalpha mutants, and phosphorylated c-Jun was detected at the MBP promoter after p38MAPK inhibition, indicating c-Jun as a negative mediator of p38MAPK action. Our findings indicate that p38MAPK activity in the brain supports myelin gene expression through distinct mechanisms via positive and negative regulatory targets. We show that oligodendrocyte differentiation involves p38-mediated Sox10 regulation and cross talk with parallel ERK and JNK pathways to repress c-Jun activity.

The PEA-15 protein regulates autophagy via activation of JNK

PEA-15/PED (phosphoprotein enriched in astrocytes 15 kDa/phosphoprotein enriched in diabetes) is a death effector domain-containing protein which is known to modulate apoptotic cell death. The mechanism by which PEA-15 inhibits caspase activation and increases ERK (extracellular-regulated kinase) activity is well characterized. Here, we demonstrate that PEA-15 is not only pivotal in the activation of the ERK pathway but also modulates JNK (c-Jun N-terminal kinase) signaling. Upon overexpression of PEA-15 in malignant glioma cells, JNK is potently activated. The PEA-15-induced JNK activation depends on the phosphorylation of PEA-15 at both phosphorylation sites (serine 104 and serine 116). The activation of JNK is substantially inhibited by siRNA-mediated down-regulation of endogenous PEA-15. Moreover, we demonstrate that glioma cells overexpressing PEA-15 show increased signs of autophagy in response to classical autophagic stimuli such as ionizing irradiation, serum deprivation, or rapamycin treatment. In contrast, the non-phosphorylatable mutants of PEA-15 are not capable of promoting autophagy. The inhibition of JNK abrogates the PEA-15-mediated increase in autophagy. In conclusion, our data show that PEA-15 promotes autophagy in glioma cells in a JNK-dependent manner. This might render glioma cells more resistant to adverse stimuli such as starvation or ionizing irradiation.

p120Ras-GAP binds the DLC1 Rho-GAP tumor suppressor protein and inhibits its RhoA GTPase and growth-suppressing activities

DLC1 (deleted in liver cancer 1), which encodes a Rho GTPase-activating protein (Rho-GAP), is a potent tumor suppressor gene that is frequently inactivated in several human cancers. DLC1 is a multidomain protein that has been shown previously to bind members of the tensin gene family. Here we show that p120Ras-GAP (Ras-GAP; also known as RASA1) interacts and extensively colocalizes with DLC1 in focal adhesions. The binding was mapped to the SH3 domain located in the N terminus of Ras-GAP and to the Rho-GAP catalytic domain located in the C terminus of the DLC1. In vitro analyses with purified proteins determined that the isolated Ras-GAP SH3 domain inhibits DLC1 Rho-GAP activity, suggesting that Ras-GAP is a negative regulator of DLC1 Rho-GAP activity. Consistent with this possibility, we found that ectopic overexpression of Ras-GAP in a Ras-GAP-insensitive tumor line impaired the growth-suppressing activity of DLC1 and increased RhoA activity in vivo. Our observations expand the complexity of proteins that regulate DLC1 function and define a novel mechanism of the cross talk between Ras and Rho GTPases.1R01CA129610

Molecular and chromosomal alterations: new therapies for relapsed acute myeloid leukemia

Acute myeloid leukemia (AML) remains the most common form of leukemia and the most common cause of leukemia death. Although conventional chemotherapy can cure between 25 and 45% of AML patients, the majority of patients die after relapse or of complications associated with treatment. Thus, more specific and less toxic treatments for AML patients are needed, especially for elderly patients. An indispensable prerequisite to investigate tailored approaches for AML is the recent progress in the understanding the molecular features that distinguish leukemia progenitors from normal hematopoietic counterparts and the identification of a variety of dysregulated molecular pathways. This in turn would allow the identification of tumor-specific characteristics that provide a rational basis for the development of more tailored, and hence potentially more effective and less toxic, therapeutic approaches. In this review, we describe some of the signaling pathways that are aberrantly regulated in AML, with a specific focus on their pathogenetic and therapeutic significance, and we examine some recent therapies directed against these targets, used in clinical trial for relapsed patients or unfit for conventional chemotherapy.

c-Jun N-terminal kinase inhibitor II (SP600125) activates Mullerian inhibiting substance type II receptor-mediated signal transduction

Müllerian inhibiting substance (MIS), the hormone required for Müllerian duct regression in fetal males, is also expressed in both adult males and females, but its physiological role in these settings is not clear. The expression of the MIS type II receptor (MISRII) in ovarian cancer cells and the ability of MIS to inhibit proliferation of these cells suggest that MIS might be a promising therapeutic for recurrent ovarian cancer. Using an MISRII-dependent activity assay in a small-molecule screen for MIS-mimetic compounds, we have identified the c-Jun N-terminal kinase inhibitor SP600125 as an activator of the MIS signal transduction pathway. SP600125 increased the activity of a bone morphogenetic protein-responsive reporter gene in a dose-dependent manner and exerted a synergistic effect when used in combination with MIS. This effect was specific for the MISRII and was not seen with other receptors of the TGFbeta family. Moreover, treatment of mouse ovarian cancer cells with a combination of SP600125 and paclitaxel, an established chemotherapeutic agent used in the treatment of ovarian cancer, or with MIS enabled inhibition of cell proliferation at a lower dose than with each treatment alone. These results offer a strong rationale for testing the therapeutic potential of SP600125, alone or in combination with already established drugs, in the treatment of recurrent ovarian cancer with a much-needed decrease in the toxic side effects of currently employed therapeutic agents.

Up-regulation of ras-GAP genes is reversed by a MEK inhibitor and doxorubicin in v-Ki-ras-transformed NIH/3T3 fibroblasts

Ras-GTPase-activating proteins (Ras-GAPs) have been implicated both as suppressors of Ras and as effectors in regulating cellular activities. To study whether Ras-GAPs have roles in tumor cell survival or not, mRNA levels of ras-related genes were measured in v-Ki-ras-transformed (DT) and the parental NIH/3T3 cells, using real-time PCR. mRNA levels of p120-Gap, Gap1(m), and PIK3CA were increased in DT cells compared with NIH/3T3 cells. p120-Gap and PIK3CA genes were induced by addition of serum or epidermal growth factor to serum-starved DT cells. Three anti-cancer drugs, an ERK kinase (MEK) inhibitor PD98059, a topoisomerase II poison doxorubicin (adriamycin), and a histone deacetylase inhibitor trichostatin A, selectively blocked the overexpression of p120-Gap and Gap1(m) genes in DT cells. These drugs also caused reversion of DT cells to the adherent shape associated with growth arrest. Our results suggest that p120-Gap and Gap1(m) genes provide important biomarkers for cancer therapies.

c-Jun NH(2)-terminal kinase 2alpha2 promotes the tumorigenicity of human glioblastoma cells

c-Jun NH(2)-terminal kinases (JNK) are members of the mitogen-activated protein kinase family and have been implicated in the formation of several human tumors, especially gliomas. We have previously shown that a 55 kDa JNK isoform is constitutively active in 86% of human brain tumors and then showed that it is specifically a JNK2 isoform and likely to be either JNK2alpha2 or JNK2beta2. Notably, we found that only JNK2 isoforms possess intrinsic autophosphorylation activity and that JNK2alpha2 has the strongest activity. In the present study, we have further explored the contribution of JNK2 isoforms to brain tumor formation. Analysis of mRNA expression by reverse transcription-PCR revealed that JNK2alpha2 is expressed in 91% (10 of 11) of glioblastoma tumors, whereas JNK2beta2 is found in only 27% (3 of 11) of tumors. Both JNK2alpha2 and JNK2beta2 mRNAs are expressed in normal brain (3 of 3). Using an antibody specific for JNK2alpha isoforms, we verified that JNK2alpha2 protein is expressed in 88.2% (15 of 17) of glioblastomas, but, interestingly, no JNK2alpha2 protein was found in six normal brain samples. To evaluate biological function, we transfected U87MG cells with green fluorescent protein-tagged versions of JNK1alpha1, JNK2alpha2, and JNK2alpha2APF (a dominant-negative mutant), and derived cell lines with stable expression. Each cell line was evaluated for various tumorigenic variables including cellular growth, soft agar colony formation, and tumor formation in athymic nude mice. In each assay, JNK2alpha2 was found to be the most effective in promoting that phenotype. To identify effectors specifically affected by JNK2alpha2, we analyzed gene expression. Gene profiling showed several genes whose expression was specifically up-regulated by JNK2alpha2 but down-regulated by JNK2alpha2APF, among which eukaryotic translation initiation factor 4E (eIF4E) shows the greatest change. Because AKT acts on eIF4E, we also examined AKT activation. Unexpectedly, we found that JNK2alpha2 could specifically activate AKT. Our data provides evidence that JNK2alpha2 is the major active JNK isoform and is involved in the promotion of proliferation and growth of human glioblastoma tumors through specific activation of AKT and overexpression of eIF4E.

Phosphorylation of neurofibromin by PKC is a possible molecular switch in EGF receptor signaling in neural cells

Children with neurofibromatosis (NF1) typically develop central nervous system (CNS) abnormalities, including aberrant proliferation of astrocytes and formation of benign astrocytomas. The NF1 gene encodes neurofibromin, a Ras-GAP, highly expressed in developing neural cells; the mechanism of regulation of neurofibromin as a Ras-GAP, remains however unknown. We now show that, in response to EGF, neurofibromin is in vivo phosphorylated on serine residues by PKC-alpha, in human, rat, and avian CNS cells and cell lines. EGF-induced PKC phosphorylation was prominent in the cysteine/serine-rich domain (CSRD) of neurofibromin, which lies in the N-terminus and upstream of the Ras-GAP domain (GRD), and this modification significantly increased the association of neurofibromin with actin in co-immunoprecipitations. In addition, we show that Ras activation in response to EGF was significantly lowered when C62B cells overexpressed a construct encoding both CSRD + GRD. Moreover, when PKC-alpha was downregulated, the Ras-GAP activity of CSRD + GRD was significantly diminished, whereas overexpressed GRD alone acted as a weaker GAP and in a PKC-independent manner. Most importantly, functional Ras inhibition and EGF signaling shifts were established at the single cell level in C6-derived cell lines stably overexpressing CSRD + GRD, when transient co-overexpression of Ras and PKC-depletion prior to stimulation with EGF-induced mitosis. Taken together, these data provide the first evidence of a functional, allosteric regulation of GRD by CSRD, which requires neurofibromin phosphorylation by PKC and association with the actin cytoskeleton. Our data may suggest a novel mechanism for regulating biological responses to EGF and provide a new aspect for the understanding of the aberrant proliferation seen in the CNS of children with NF1.

Chemopreventive effect of punicalagin, a novel tannin component isolated from Terminalia catappa, on H-ras-transformed NIH3T3 cells

Terminalia catappa and its major tannin component, punicalagin, have been characterized to possess antioxidative and anti-genotoxic activities. However, their effects on reactive oxygen species (ROS) mediated carcinogenesis are still unclear. In the present study, H-ras-transformed NIH3T3 cells were used to evaluate the chemopreventive effect of T. catappa water extract (TCE) and punicalagin. In the cell proliferation assay, TCE and punicalagin suppressed the proliferation of H-ras-transformed NIH3T3 cells with a dose-dependent manner but only partially affected non-transformed NIH3T3 cells proliferation. The differential cytotoxicity of TCE/punicalagin on the H-ras-transformed and non-transformed NIH3T3 cells indicated the selectivity of TCE/punicalagin against H-ras induced transformation. TCE or punicalagin treatment reduced anchorage-independent growth that could be due to a cell cycle arrest at G0/G1 phase. The intracellular superoxide level, known to modulate downstream signaling of Ras protein, was decreased by punicalagin treatments. The levels of phosphorylated JNK-1 and p38 were also decreased with punicalagin treatments. Thus, the chemopreventive effect of punicalagin against H-ras induced transformation could result from inhibition of the intracellular redox status and JNK-1/p38 activation.

Specificity of Inhibition of ras-p21 Signal Transduction by Peptides from GTPase Activating Protein (GAP) and the Son-of Sevenless (SOS) ras-Specific Guanine Nucleotide Exchange Protein

In previous studies, involving molecular modeling of wild-type and oncogenic forms of the ras-p21 protein bound to GTPase activating protein GAP and the ras-specific guanine nucleotide exchange-promoting protein, SOS, we identified specific domains of GAP and SOS proteins that differ in conformation when the computed average structures of the corresponding wild-type and oncogenic complexes are superimposed. Additionally, in these previous studies, we have synthesized peptides corresponding to these domains and found that all of them inhibit either or both oncogenic (Val 12-containing) p21- and insulin-activated wild-type p21-induced oocyte maturation. To document further the specificity of the inhibition of these peptides for the ras signal transduction pathway, we have now tested their effects on progesterone-induced maturation that occurs by a ras-independent pathway. None of these peptides, including a peptide corresponding to residues 980-989 of SOS that completely blocks oncogenic p21-induced maturation and also causes extensive inhibition of insulin-induced maturation, affects progesterone-induced maturation, suggesting that all of these peptides are specific for the ras pathway. Since our approach to the design of peptides that can inhibit oncogenic ras-p21 selectively is based on identifying domains that differ in conformation between oncogenic and wild-type complexes, we have now further synthesized peptides that correspond to domains of GAP (residues 903-910) and SOS (residues 792-804) that do not differ in conformation when the average structures are superimposed. These peptides do not inhibit either oncogenic p21- or insulin-induced oocyte maturation, supporting the overall strategy of using peptides from domains that change conformation as the ones most likely to inhibit oncogenic and/or wild-type ras-p21. These results further support the specificity of inhibition of the GAP and SOS peptides from the conformationally distinct domains of both proteins.

Transformation of immortalized colorectal crypt cells by microcystin involving constitutive activation of Akt and MAPK cascade

It has been shown by epidemiological and animal studies that microcystin is an important exogenous factor involved in the carcinogenesis of colorectal cancer (CRC). However, details of the mechanism remain unclear. Transformation of colorectal cells is an important initial step in carcinogenesis. Whether microcystin is capable of transforming immortalized colorectal crypt cells, and what the mechanism might be, was investigated. In the present study, we demonstrated that immortalized colorectal crypt cells could be transformed by microcystin. Transformed colorectal crypt cells showed an anchorage-independent growth phenotype, and the proliferation activities of microcystin-transformed cells were also greater than that of immortalized colorectal crypt cells. The Akt and the p38, JNK of mitogen-activated protein kinase (MAPK) pathways in microcystin-transformed cells were found to be constitutively activated. In microcystin-transformed cells, PI3K, MAPKAPK2, Akt, cyclin D1 and cyclin D3 in the Akt pathway; IQGAP-2, RabGTPase, Rap1GAP, RasGAP, R-Ras, Krev-1 and TC21 of the Ras GTP/GDP protein family; and A-Raf, B-Raf and PAK in the Ras/MAPK pathway were all markedly upregulated. However, in positive control cells, dimethylhydrazine-transformed cells, only the Akt pathway was activated by PI3K, and no evidence of alteration of any molecules of the Ras superfamily was observed. Inhibition of Akt, p38 and JNK activation led to a reduced proliferation of microcystin-transformed cells. This implies that the constitutive activation of Akt and the p38, JNK of MAPK pathways in microcystin-transformed cells may be the mechanism by which this important external factor acts in the carcinogenesis of CRC.

Vav Transformation Requires Activation of Multiple GTPases and Regulation of Gene Expression

Although Vav can act as a guanine nucleotide exchange factor for RhoA, Rac1, and Cdc42, its transforming activity has been ascribed primarily to its ability to activate Rac1. However, because activated Vav, but not Rac-specific guanine nucleotide exchange factors, exhibits very potent focus-forming transforming activity when assayed in NIH 3T3 cells, Vav transforming activity must also involve activation of Rac-independent pathways. In this study, we determined the involvement of other Rho family proteins and their signaling pathways in Vav transformation. We found that RhoA, Rac1, and Cdc42 functions are all required for Vav transforming activity. Furthermore, we determined that Vav activation of nuclear factor-κB and the Jun NH2-terminal kinase mitogen-activated protein kinase (MAPK) is necessary for full transformation by Vav, whereas p38 MAPK does not seem to play an important role. We also determined that Vav is a weak activator of Elk-1 via a Ras- and MAPK/extracellular signal-regulated kinase kinase–dependent pathway, and this activity was essential for Vav transformation. Thus, we conclude that full Vav transforming activation is mediated by the activation of multiple small GTPases and their subsequent activation of signaling pathways that regulate changes in gene expression. Because Vav is activated by the epidermal growth factor receptor and other tyrosine kinases involved in cancer development, defining the role of aberrant Vav signaling may identify activities of receptor tyrosine kinases important for human oncogenesis.

Rac1b, a tumor associated, constitutively active Rac1 splice variant, promotes cellular transformation

A novel splice variant of Rac1, designated Rac1b, is expressed in human breast and colon carcinoma cells. Rac1b contains an additional 19 amino-acid insert immediately behind the switch II domain, a region important for Rac1 interaction with regulators and effectors. Recent studies showed that Rac1b exhibited the biochemical properties of a constitutively activated GTPase, yet it showed impaired interaction with downstream effectors, suggesting that Rac1b may be defective in biological activity. Whether Rac1b is a biologically active protein was not addressed. Therefore, we evaluated the biochemical, signaling and growth-promoting properties of authentic Rac1b. Similar to previous observations, we found that Rac1b showed enhanced intrinsic guanine nucleotide exchange activity, impaired intrinsic GTPase activity, and failed to interact with RhoGDI. Surprisingly, we found that Rac1b, like the constitutively-activated and transforming Rac1(Q61L) mutant, promoted growth transformation of NIH3T3 cells. Rac1b-expressing cells also showed a loss of density-dependent and anchorage-dependent growth. Surprisingly, unlike activated Rac1(61L), Rac1b did not show enhanced activation of the nuclear factor kappaB (NF-kappaB) transcription factor or stimulate cyclin D1 expression, the signaling activities that best correlate with Rac1 transforming activity. However, Rac1b did promote activation of the AKT serine/threonine kinase. Therefore, we suggest that Rac1b selectively activates a subset of Rac1 downstream signaling pathways to facilitate cellular transformation.

Overexpression of FAK promotes Ras activity through the formation of a FAK/p120RasGAP complex in malignant astrocytoma cells

Focal adhesion kinase (FAK) signaling may be mediated through the modulation of Ras activity. We have shown previously that grade III malignant astrocytoma biopsy samples exhibit elevated levels of FAK, and that overexpression of FAK in U-251MG malignant astrocytoma cells promotes the phosphorylation of Shc, a potential upstream mediator of Ras activity. Here, we report that overexpression of FAK promotes Ras activity in U-251MG malignant astrocytoma cells cultured in aggregate suspension or as monolayers adherent to vitronectin. The overexpression of FAK also promoted the association of FAK with p120RasGAP, which is a negative regulator of Ras activity, in the U-251MG cells cultured in aggregate suspension, with this association being abrogated upon plating of the cells onto vitronectin. An association of FAK with p120RasGAP also was observed in malignant astrocytoma biopsy samples, but not in normal brain samples. As overexpression of FAK in U-251MG cells in aggregate suspension culture reduced the amount of p120RasGAP complexed with active Ras, we hypothesize that the association of FAK with p120 RasGAP may facilitate Ras activity. The overexpression of a mutated FAK in which the Y397 had been mutated to F did not result in the formation of the FAK/p120RasGAP complex and did not promote Ras activity, indicating that the Y397 residue of FAK plays a role in the formation of this complex and in the activation of Ras. Moreover, the overexpression of mutated FAK (397F) was found to inhibit anchorage-independent growth. These data provide the basis for a previously undescribed mechanism in which the elevated expression of FAK can promote Ras activity through its competitive recruitment of p120RasGAP, thereby diminishing the association of p120RasGAP with active Ras.

p38 MAPK activation selectively induces cell death in K-ras-mutated human colon cancer cells through regulation of vitamin D receptor

Ras is the most characterized oncogene in human cancer, and yet there are no effective therapeutics to selectively target this oncogene. Our previous work demonstrated the inhibitory activity of the p38 pathway in Ras proliferative signaling in experimental NIH 3T3 cells (Chen, G., Hitomi, M., Han, J., and Stacey, D. W. (2000) J. Biol. Chem. 275, 38973-38980). Here we explore the therapeutic potential of p38 kinase activation in human colon cancer cells with and without endogenous K-ras activation. p38 activation by both adenovirus-mediated gene delivery of constitutively active p38 activator MKK6 and by arsenite selectively induces cell death in K-ras-activated human colon cancer HCT116 cells but not in the K-ras-disrupted HCT116-derived sublines. The cell death-inducing effect of MKK6 is not because of its selective activation of p38 kinase or its downstream transcription factor substrates, ATF-2 or c-Jun, in K-ras-activated cells. Rather, cell death in K-ras-activated cells is linked to the down-regulation of vitamin D receptor (VDR) by an AP-1-dependent mechanism. Forced VDR expression in K-ras-activated cells inhibits p38 activation-induced cell death, and inhibition of endogenous VDR protein expression in K-ras-disrupted cells increased the arsenite-induced toxicity. Analysis of an additional two human colon cancer cell lines with and without K-ras mutation also showed a K-ras- and VDR-dependent toxicity of MKK6. Hence, p38 pathway activation selectively induces cell death in K-ras-mutated human colon cancer cells by mechanisms involving the suppression of VDR activity.

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Molecular targets in acute myelogenous leukemia

Acute myeloid leukemia (AML) remains the most common form of leukemia and the most common cause of leukemia death. Although conventional chemotherapy can cure between 25 and 45% of AML patients, most patients will either die of relapse or die from the complications associated with treatment. Thus, more specific and less toxic treatments for AML patients are needed. Recently, a small molecular inhibitor (STI571 or Gleevec) that targets the BCR-ABL gene was found to have a dramatic clinical effect in patients with chronic myelogenous leukemia (CML). These results have encouraged investigators to search for additional small molecular inhibitors and other targeted therapies that may be applicable to other forms of leukemia. In this review, we examine some of the signaling pathways that are aberrantly regulated in AML, focusing on the tyrosine kinase/RAS/MAP kinase and JAK/STAT pathways. After reviewing these two pathways, we explore some of the targeted therapies directed at these pathways that are under development for AML, many of which are already in clinical trials.

Characterization of p21Ras-mediated apoptosis induced by protein kinase C inhibition and application to human tumor cell lines

Suppression of PKC activity can selectively induce apoptosis in cells expressing a constitutively activated p21Ras protein. We demonstrate that continued expression of p21Ras activity is required in PKC-mediated apoptosis because farnesyltransferase inhibitors abrogated the loss of viability in p21Ras-transformed cells occurring following PKC inhibition. Studies utilizing gene transfer or viral vectors demonstrate that transient expression of oncogenic p21Ras activity is sufficient for induction of apoptosis by PKC inhibition, whereas physiologic activation of p21Ras by growth factor is not sufficient to induce apoptosis. Mechanistically, the p21Ras-mediated apoptosis induced by PKC inhibition is dependent upon mitochondrial dysregulation, with a concurrent loss of mitochondrial membrane potential (psim). Cyclosporine A, which prevented the loss of psim, also inhibited HMG-induced DNA fragmentation in cells expressing an activated p21Ras. Induction of apoptosis by PKC inhibition in human tumors with oncogenic p21Ras mutations was demonstrated. Inhibition of PKC caused increased apoptosis in MIA-PaCa-2, a human pancreatic tumor line containing a mutated Ki-ras allele, when compared to HS766T, a human pancreatic tumor line with normal Ki-ras alleles. Furthermore, PKC inhibition induced apoptosis in HCT116, a human colorectal tumor line containing an oncogenic Ki-ras allele but not in a subline (Hke3) in which the mutated Ki-ras allele had been disrupted. The PKC inhibitor 1-O-hexadecyl-2-O-methyl-rac-glycerol (HMG), significantly reduced p21Ras-mediated tumor growth in vivo in a nude mouse MIA-PaCa-2 xenograft model. Collectively these studies suggest the therapeutic feasibility of targeting PKC activity in tumors expressing an activated p21Ras oncoprotein.

Raf-independent deregulation of p38 and JNK mitogen-activated protein kinases are critical for Ras transformation

Activated Ras, but not Raf, causes transformation of RIE-1 epithelial cells, supporting the importance of Raf-independent pathways in mediating Ras transformation. The p38 and JNK mitogen-activated protein kinase cascades are activated by Ras via Raf-independent effector function. Therefore, we determined whether p38 and JNK activation are involved in Ras transformation of RIE-1 epithelial cells. Rather surprisingly, we found that pharmacologic inhibition of p38, together with Raf activation of ERK, was sufficient to mimic the morphologic and growth transformation caused by oncogenic Ras. p38 inhibition together with ERK activation also caused the same alterations in cyclin D1 and p21(CIP1) expression caused by Ras and induced an autocrine growth factor loop important for transformation. Finally, in contrast to p38, we found that JNK activation promoted Ras transformation, and that Ras deregulation of p38 and JNK was not mediated by activation of the Rac small GTPase. We conclude that a key action of Raf-independent effector pathways important for Ras transformation may involve inhibition of p38 and activation of JNK.

Dominant negative MEKK1 inhibits survival of pancreatic cancer cells

Human pancreatic cancers harbor mutations in the K-ras gene, and these mutations convert the gene oncogenic and constitutively active forms. However, in pancreatic cancer cells little is known about the activation of the downstream pathways of Ras, MEK-ERK and MEKK1-JNK, and their roles in cell survival and proliferation. An analysis of nine pancreatic cancer tissues revealed JNK activation in all tumor samples and ERK activation in three tumor samples. Colony formation assays by transfection of dominant negative mutants of Ras, ERK or MEKK1 into pancreatic cancer cell lines (BxPC-3, PANC-1, MIAPaCa-2 and AsPC-1) and an amnion-derived cell line (FL) revealed that DN-MEKK strongly inhibits the survival of colonies in pancreatic cancer cells, but not in FL cells. In vitro kinase assays and luciferase assays using the Gal4c-Jun system revealed that in pancreatic cancer cells DN-MEKK fails to inhibit JNK activation. In PANC-1 cells, c-Jun was found to be a major component of protein component binding to AP-1 site and CRE, but not in FL cells. The inhibitory effect of DN-MEKK in PANC-1 cells was thought to be the result of the inhibition of c-Jun DNA-binding. The difference of suppression in pancreatic cancer cells and non-pancreatic cancer cells suggested that the MEKK1 pathway mainly contributes to cell survival in pancreatic cancer cells and may provide an advantage for the gene therapy of pancreatic cancers using DN-MEKK expression vectors.

Comparison of the average structures, from molecular dynamics, of complexes of GTPase activating protein (GAP) with oncogenic and wild-type ras-p21: identification of potential effector domains

GTPase activating protein (GAP) is a known regulator of ras-p21 activity and is a likely target of ras-induced mitogenic signaling. The domains of GAP that may be involved in this signaling are unknown. In order to infer which domains of GAP may be involved, we have performed molecular dynamics calculations of GAP complexed to wild-type and oncogenic (Val 12-containing) ras-p21, both bound to GTP. We have computed and superimposed the average structures for both complexes and find that there are four domains of GAP that undergo major changes in conformation: residues 821-851, 917-924, 943-953, and 1003-1020. With the exception of the 943-953 domain, none of these domains is involved in making contacts with ras-p21, and all of them occur on the surface of the protein, making them good candidates for effector domains. In addition, three ras-p21 domains undergo major structural changes in the oncogenic p21-GAP complex: 71-76 from the switch 2 domain; 100-108, which interacts with SOS, jun and jun kinase (JNK); and residues 122-138. The change in conformation of the 71-76 domain appears to be induced by changes in conformation in the switch 1 domain (residues 32-40) and in the adjacent domain involving residues 21-31. In an accompanying paper, we present results from microinjection of peptides corresponding to each of these domains into oocytes induced to undergo maturation by oncogenic ras-p21 and by insulin-activated wild-type cellular p21 to determine whether these domain peptides may be involved in ras signaling through GAP.

Inhibition of ras-induced oocyte maturation by peptides from ras-p21 and GTPase activating protein (GAP) identified as being effector domains from molecular dynamics calculations

In the accompanying article, using molecular dynamics calculations, we found that the 66-77 and 122-138 domains in ras-p21 and the 821-827, 832-845, 917-924, 943-953, and 1003-1020 domains in GAP have different conformations in complexes of GAP with wild-type and oncogenic ras-p21. We have now synthesized peptides corresponding to each of these domains and coinjected them into oocytes with oncogenic p21, which induces oocyte maturation, or injected them into oocytes incubated with insulin that induces maturation by activating wild-type cellular ras-p21. We find that all of these peptides inhibit both agents but do not inhibit progesterone-induced maturation that occurs by a ras-independent pathway. The p21 66-77 and 122-138 peptides cause greater inhibition of oncogenic p21. On the other hand, the GAP 832-845 and 1003-1021 peptides inhibit insulin-induced maturation to a significantly greater extent. Since we have found that activated wild-type and oncogenic p21 activate downstream targets like raf differently, these GAP peptides may be useful probes for identifying elements unique to the wild-type ras-p21 pathway.

P53 is necessary for the apoptotic response mediated by a transient increase of Ras activity

The tumor suppressor p53 eliminates cancer-prone cells via multiple mechanisms, including apoptosis. Ras elicits apoptosis in cells after protein kinase C (PKC) downregulation. However, the role of p53 in Ras-mediated apoptosis has not been fully investigated. Here, we demonstrate that mouse fibroblasts that express wild-type p53 are more susceptible to apoptosis elicited by PKC inhibition if Ras is transiently expressed or upregulated as opposed to stably expressed. In the latter case, p53 is frequently mutated. Transiently increased Ras activity induces Bax, and PKC inhibition augments this induction. Overexpression of E6 inactivates p53 and thereby suppresses both Bax induction and apoptosis. In contrast, Bax is not induced in stable ras transfectants, regardless of PKC inhibition. The data suggest that short- and long-term activation of Ras use a different mechanism(s) to initiate apoptosis. The status of p53 may contribute to such differences.

Participation of small GTPases Rac1 and Cdc42Hs in myoblast transformation

We have previously shown that expression of active Rac1 and Cdc4Hs inhibits skeletal muscle cell differentiation. We show here, by bromodeoxyuridine incorporation and cyclin D1 expression, that the expression of active Rac1 and Cdc42Hs but not RhoA impairs cell cycle exit of L6 myoblasts cultured in differentiation medium. Furthermore, expression of activated forms of Rac1 and Cdc42Hs elicits the loss of cell contact inhibition and anchorage-dependent growth as measured by focus forming activity and growth in soft agar. RhoA was once again not found to have this effect. We found a constitutive Rac1 and Cdc42Hs activation in three human rhabdomyosarcoma-derived cell lines, one of the most common causes of solid tumours arising from muscle precursors during childhood. Finally, dominant negative forms of Rac1 and Cdc42Hs inhibit cell proliferation of the RD rhabdomyosarcoma cell line. These data suggest an important role for the small GTPases Rac1 and Cdc42Hs in the generation of skeletal muscle tumours.

A distinct class of dominant negative Ras mutants: cytosolic GTP-bound Ras effector domain mutants that inhibit Ras signaling and transformation and enhance cell adhesion

Cytosolic GTP-bound Ras has been shown to act as a dominant negative (DN) inhibitor of Ras by sequestering Raf in non-productive cytosolic complexes. Nevertheless, this distinct class of DN mutants has been neither well characterized nor extensively used to analyze Ras signaling. In contrast, DN Ras17N, which functions by blocking Ras guanine nucleotide exchange factors, has been well characterized and is widely used. Cytosolic GTP-bound Ras mutants could be used to inhibit particular Ras effectors by introducing additional mutations (T35S, E37G or Y40C) that permit them to associate selectively with and inhibit Raf, RalGDS, or phosphoinositide 3-kinase, respectively. When the wild-type Ras effector binding region is used, cytosolic Ras should associate with all Ras effectors, even those that are not yet identified, making these DN Ras mutants effective inhibitors of multiple Ras functions. We generated cytosolic GTP-bound H-, N-, and K-Ras, and we assessed their ability to inhibit Ras-induced phenotypes. In fibroblasts, cytosolic H-, N-, and K-Ras inhibited Ras-induced Elk-1 activation and focus formation, induced a flattened cell morphology, and increased adhesion to fibronectin through modulation of a beta(1)-subunit-containing integrin, thereby demonstrating that DN activity is not limited to a subset of Ras isoforms. We also generated cytosolic GTP-bound Ras effector domain mutants (EDMs), each of which reduced the ability of cytosolic GTP-bound Ras proteins to inhibit Elk-1 activation and to induce cell flattening, implicating multiple pathways in these phenotypes. In contrast, Ras-induced focus formation, platelet-derived growth factor (PDGF)-, or Ras-induced phospho-Akt levels and cell adhesion to fibronectin were affected by T35S and Y40C EDMs, whereas PDGF- or Ras-induced phospho-Erk levels were affected only by the T35S EDM, implying that a more limited set of Ras-mediated pathways participate in these phenotypes. These data constitute the first extensive characterization of this functionally distinct class of DN Ras inhibitor proteins.

Involvement of phosphatidylinositol 3-kinase, but not RalGDS, in TC21/R-Ras2-mediated transformation

Oncogenic Ras and activated forms of the Ras-related protein TC21/R-Ras2 share similar abilities to alter cell proliferation. However, in contrast to Ras, we found previously that TC21 fails to activate the Raf-1 serine/threonine kinase. Thus, TC21 must utilize non-Raf effectors to regulate cell function. In this study, we determined that TC21 interacts strongly with some (RalGDS, RGL, RGL2/Rlf, AF6, and the phosphatidylinositol 3-kinase (PI3K) catalytic subunit p110delta), and weakly with other Ras small middle dotGTP-binding proteins. In addition, library screening identified novel TC21-interacting proteins. We also determined that TC21, similar to Ras, mediates activation of phospholipase Cepsilon. We then examined if RalGDS, a RalA guanine nucleotide exchange factor, or PI3K are effectors for TC21-mediated signaling and cell proliferation in murine fibroblasts. We found that overexpression of full-length RalGDS reduced the focus forming activity of activated TC21. Furthermore, expression of activated Ras, but not TC21, enhanced GTP loading on RalA. In fact, TC21 attenuated insulin-stimulated RalA small middle dotGTP formation. In contrast, like Ras, expression of activated TC21 resulted in membrane translocation and an increase in the PI3K-dependent phosphorylation of Akt, and inhibition of PI3K activity interfered with TC21 focus formation. Finally, unlike Ras, TC21 did not activate the Rac small GTPase, indicating that Ras may not activate Rac by PI3K. Taken together, these results suggest that PI3K, but not RalGDS, is an important mediator of cell proliferation by TC21.

RasGAP is involved in signal transduction triggered by FGF1 inXenopusoocytes expressing FGFR1

The role of RasGAP was investigated in the model system of Xenopus oocytes expressing fibroblast growth factor receptor 1 (FGFR1) stimulated by fibroblast growth factor 1 (FGF1). The injection of the SH2-SH3-SH2 domains of RasGAP suppressed Ras activity, extracellular signal-regulated protein kinase 2 (ERK2) phosphorylation and Mos synthesis. The SH2 domain of Src, and PP2, an inhibitor of Src, also abolished Ras activity, ERK2 phosphorylation and Mos synthesis. In addition, Src activity was blocked by the SH2-SH3-SH2 domains of RasGAP. Immunoprecipitation of a chimera composed of the extracellular domain of the platelet-derived growth factor (PDGF) receptor and the intracellular domain of FGFR1 stimulated by PDGF-BB demonstrates the recruitment of phosphorylated RasGAP. This study shows that the transduction cascade induced by the FGFR1-FGF1 interaction in Xenopus oocytes involves RasGAP as a co-activator of Src to stimulate the Ras/mitogen-activated protein kinase cascade and Mos synthesis. It emphasises a new positive regulatory role for RasGAP in FGFR transduction.

G3BP is overexpressed in human tumors and promotes S phase entry

The expression of the human Ras-GTPase activating protein (GAP)-binding protein (G3BP) was studied in human tumors and cell lines of different origins. Northern blot analysis and immunoblotting experiments showed enhanced expression of G3BP in all tumor samples as compared to healthy tissue. The enhanced expression does not seem to be related to the tumor site or to the stage of development of the cancer. In light of the proposed functions of G3BP, its increased expression in tumors suggest that it plays a role in dedifferentiation and proliferation processes. We also show that G3BP promotes S phase entry in cultured fibroblasts deprived of serum and that this function is dependent on the presence of the RNA binding domain of the protein.

The p38 pathway provides negative feedback for Ras proliferative signaling

Ras activates three mitogen-activated protein kinases (MAPKs) including ERK, JNK, and p38. Whereas the essential roles of ERK and JNK in Ras signaling has been established, the contribution of p38 remains unclear. Here we demonstrate that the p38 pathway functions as a negative regulator of Ras proliferative signaling via a feedback mechanism. Oncogenic Ras activated p38 and two p38-activated protein kinases, MAPK-activated protein kinase 2 (MK2) and p38-related/activated protein kinase (PRAK). MK2 and PRAK in turn suppressed Ras-induced gene expression and cell proliferation, whereas two mutant PRAKs, unresponsive to Ras, had little effect. Moreover, the constitutive p38 activator MKK6 also suppressed Ras activity in a p38-dependent manner whereas arsenite, a potent chemical inducer of p38, inhibited proliferation only in a tumor cell line that required Ras activity. MEK was required for Ras stimulation of the p38 pathway. The p38 pathway inhibited Ras activity by blocking activation of JNK, without effect upon ERK, as evidenced by the fact that PRAK-mediated suppression of Ras-induced cell proliferation was reversed by coexpression of JNKK2 or JNK1. These studies thus establish a negative feedback mechanism by which Ras proliferative activity is regulated via signaling integrations of MAPK pathways.

c-Jun activation-dependent tumorigenic transformation induced paradoxically by overexpression or block of S-adenosylmethionine decarboxylase

All mammalian cells absolutely require polyamines (putrescine, spermidine, and spermine) for growth. Here we show that the overexpression of cDNA for S-adenosylmethionine decarboxylase (AdoMetDC), the main regulatory enzyme in the biosynthesis of higher polyamines, induces transformation of rodent fibroblasts when expressed in the sense or the antisense orientation. Both transformants were able to induce invasive tumors in nude mice. Neither transformation was associated with activation of the mitogen-activated protein kinases Erk1 and Erk2. Instead, the AdoMet DC sense, but not antisense, transformants displayed constitutive activation of the c-Jun NH(2)-terminal kinase (JNK) pathway. However, both transformations converged on persistent phosphorylation of endogenous c-Jun at Ser73. The phenotype of the AdoMetDC sense transformants was reversed by expression of dominant-negative mutants of SEK1 (MKK4), JNK1, and c-Jun (TAM-67), which were also found to impair cytokinesis. Similarly, TAM-67 reverted the morphology of the AdoMetDC-antisense expressors. This report is the first demonstration of a protein whose overexpression or block of synthesis can induce cell transformation. In addition, we show that the polyamine biosynthetic enzymes require c-Jun activation for eliciting their biological effects.

The Ras/p120 GTPase-activating protein (GAP) interaction is regulated by the p120 GAP pleckstrin homology domain

Pleckstrin homology domains are structurally conserved functional domains that can undergo both protein/protein and protein/lipid interactions. Pleckstrin homology domains can mediate inter- and intra-molecular binding events to regulate enzyme activity. They occur in numerous proteins including many that interact with Ras superfamily members, such as p120 GAP. The pleckstrin homology domain of p120 GAP is located in the NH(2)-terminal, noncatalytic region of p120 GAP. Overexpression of the noncatalytic domains of p120 GAP may modulate Ras signal transduction pathways. Here, we demonstrate that expression of the isolated pleckstrin homology domain of p120 GAP specifically inhibits Ras-mediated signaling and transformation but not normal cellular growth. Furthermore, we show that the pleckstrin homology domain binds the catalytic domain of p120 GAP and interferes with the Ras/GAP interaction. Thus, we suggest that the pleckstrin homology domain of p120 GAP may specifically regulate the interaction of Ras with p120 GAP via competitive intra-molecular binding.

c-Jun N-terminal kinase is essential for growth of human T98G glioblastoma cells

The c-Jun N-terminal kinase/stress-activated protein kinase (JNK/SAPK) pathway is activated by numerous cellular stresses. Although it has been implicated in mediating apoptosis and growth factor signaling, its role in regulating cell growth is not yet clear. Here, the influence of JNK on basal (unstimulated) growth of human tumor glioblastoma T98G cells was investigated using highly specific JNK antisense oligonucleotides to inhibit JNK expression. Transient depletion of either JNK1 or JNK2 suppressed cell growth associated with an inhibition of DNA synthesis and cell cycle arrest in S phase. The growth-inhibitory potency of JNK2 antisense ((JNK)2 IC(50) = 0.14 micrometer) was greater than that of JNK1 antisense ((JNK)1 IC(50) = 0.37 micrometer), suggesting that JNK2 plays a dominant role in regulating growth of T98G cells. Indeed, JNK2 antisense-treated populations exhibited greater inhibition of DNA synthesis and accumulation of S-phase cells than did the JNK1 antisense-treated cultures, with a significant proportion of these cells detaching from the tissue culture plate. JNK2 (but not JNK1) antisense-treated cultures exhibited marked elevation in the expression of the cyclin-dependent kinase inhibitor p21(cip1/waf1) accompanied by inhibition of Cdk2/Cdc2 kinase activities. Taken together, these results indicate that JNK is required for growth of T98G cells in nonstress conditions and that p21(cip1/waf1) may contribute to the sustained growth arrest of JNK2-depleted T98G cultures.

The importance of being K-Ras

The ras genes give rise to a family of related proteins that have strong transforming potential. Typical in vitro studies fail to discriminate between the transforming activity of the Ras proteins. Although activating mutations in ras genes are commonly found in human disease, they are not evenly distributed between the different ras members. Instead, they are concentrated in k-ras. With the absence of evidence to suggest that k-ras DNA is more prone to mutation than h-ras DNA, this imbalance in mutational frequency suggests a special biological role for the K-Ras protein in vivo.

Ectopic expression of Rsu-1 results in elevation of p21CIP and inhibits anchorage-independent growth of MCF7 breast cancer cells

Signal transduction from tyrosine kinase receptors mediates growth regulation of breast cancer cells in part through the GTPase Ras and downstream kinases. Rsu-1 is a cDNA previously identified as an inhibitor of Ras-induced transformation. An HA-epitope tagged Rsu-1 cDNA was introduced into the MCF7 breast carcinoma cell line. Stable transfectants were selected and used for analysis of Rsu-1 expression on growth control and Ras-dependent kinase pathways. Assessment of biological activity of HA-Rsu-1 transfectants revealed that HA-Rsu-1 clones showed slower anchorage dependent growth rates than control MCF7 cell lines and a significant reduction in anchorage independent growth. Analysis of cell cycle regulatory proteins required for transit through G1 revealed that HA-Rsu-1 transfectant cell lines expressed elevated levels of p21CIP CDK inhibitor. Perturbations in signal transduction pathways which can be activated by Ras were detected in the Ha-Rsu-1 transfectants. Exposure of serum-starved cells to EGF revealed that expression of HA-Rsu-1 increased ERK-2 kinase activation, decreased activation of Jun kinase and inhibited Rho-dependent Rho-alpha kinase (ROK) activity compared to control cells. While serum starvation reduced AKT activity to undetectable levels in HA-Rsu-1 transfectants but not in control MCF7 cells, activation of AKT kinase by serum was unaffected by HA-Rsu-1 expression. Finally, the level of c-myc transcription in HA-Rsu-1 transfectants reached only 60% of the MCF7 control cell line following serum stimulation of starved cells while Fos RNA levels were similar to control cells. These results demonstrate that increased Rsu-1 expression critically altered cell cycle regulation and growth of MCF7 cells as well as signaling pathways in MCF7 cells required for malignant growth.

Activation of p38 and c-Jun N-terminal kinase pathways and induction of apoptosis by chelerythrine do not require inhibition of protein kinase C

Chelerythrine, a natural benzophenanthridine alkaloid, has been reported to mediate a variety of biological activities, including inhibition of protein kinase C (PKC). Here we report that chelerythrine induced time- and dose-dependent activation of JNK1 and p38 in HeLa cells, which was mediated the upstream kinases, MEKK1 and MKK4. However, treatment with two other potent and selective PKC inhibitors, GF-109203X and Gö6983, or down-regulation of PKC activity by prolonged treatment with phorbol 12-myristate 13-acetate had no effect on JNK1 and p38 activities. Furthermore, under the conditions where JNK1 and p38 were activated, we did not observe any significant inhibitory effect of chelerythrine on the activities of PKC isozymes present in HeLa cells. Interestingly, pretreatment with the antioxidants, N-acetyl-L-cysteine, dithiothreitol, and glutathione, impaired chelerythrine-induced JNK1 and p38 activation. In addition, chelerythrine induced apoptosis that was blocked by the antioxidants and the dominant-negative mutants of MEKK1, MKK4, JNK1, and p38. Together, these results uncover a novel biochemical property of chelerythrine, i.e. activation of MEKK1- and MKK4-dependent JNK1 and p38 pathways through an oxidative stress mechanism, which mediate the induction of apoptosis, but are independent of PKC inhibition.

Suppressive effects of dominant negative ras mutant N116Y on transformed phenotypes of human bladder cancer cells

To investigate the suppressive effect of dominant negative H-ras mutant N116Y on transformed phenotypes, we established two N116Y ras mutant stable transfectant clones (C5, C13) of human bladder cancer cell line, UMUC-2. These N116Y ras mutant transfectants, especially the C5 cells, showed a dramatic change of cellular morphology and significantly reduced growth in soft agar compared to their control. Furthermore, phosphorylation of the Jun NH2-terminal kinase (JNK) was significantly decreased in these transfectants compared to the control. These results suggest that the N116Y-induced suppression of transformed phenotypes in UMUC-2 cells is associated with inhibition of JNK phosphorylation.

MAPK mediates RAS-induced chromosome instability

The generation of micronuclei is a reflection of DNA damage, defective mitosis, and loss of genetic material. The involvement of the MAPK pathway in mediating v-ras-induced micronuclei in NIH 3T3 cells was examined by inhibiting MAPK activation. Conversely, the MAPK pathway was constitutively activated by infecting cells with a v-mos retrovirus. Micronucleus formation was inhibited by the MAPK kinase inhibitors PD98059 and U0126, but not by wortmannin, an inhibitor of the Ras/phosphatidylinositol 3-kinase pathway. Transduction of cells with v-mos resulted in an increase in micronucleus formation, also consistent with the involvement of the MAPK pathway. Staining with the anti-centromeric CREST antibody revealed that instability induced by constitutive activation of MAPK is due predominantly to aberrant mitotic segregation, since most of the micronuclei were CREST-positive, reflective of lost chromosomes. A significant fraction of the micronuclei were CREST-negative, reflective of lost acentric chromosome fragments. Some of the instability observed was due to mitotic events, consistent with the increased formation of bi-nucleated cells, which result from perturbations of the mitotic spindle and failure to undergo cytokinesis. This chromosome instability, therefore, is a consequence of mitotic aberrations, mediated by the MAPK pathway, including centrosome amplification and formation of mitotic chromosome bridges.

Ras and Rap1: two highly related small GTPases with distinct function

The Ras-like family of small GTPases includes, among others, Ras, Rap1, R-ras, and Ral. The family is characterized by similarities in the effector domain. While the function of Ras is, at least in part, elucidated, little is known about other members of the family. Currently, much attention is focused on the small GTPase Rap1. Initially, this member was identified as a transformation suppressor protein able to revert the morphological phenotype of Ras-transformed fibroblasts. This has led to the hypothesis that Rap1 antagonizes Ras by interfering in Ras effector function. Recent analysis revealed that Rap1 is activated rapidly in response to activation of a variety of receptors. Rap1 activation is mediated by several second messengers, including calcium, diacylglycerol, and cAMP. Guanine nucleotide exchange factors (GEFs) have been identified that mediate these effects. The most interesting GEF is Epac, an exchange protein directly activated by cAMP, thus representing a novel cAMP-induced, protein kinase A-independent pathway. Furthermore, Rap1 is inactivated by specific GTPase-activating proteins (GAPs), one of which is regulated through an interaction with Galphai. While Ras and Rap1 may share some effector pathways, evidence is accumulating that Ras and Rap1 each regulate unique cellular processes in response to various extracellular ligands. For Rap1 these functions may include the control of cell morphology.