REVIEW JAK/STAT, Raf/MEK/ERK, PI3K/Akt and BCR-ABL in Cell Cycle
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Hidden Targets in RAF Signalling Pathways to Block Oncogenic RAS Signalling
G C A T T A C G G C A T genes Review Hidden Targets in RAF Signalling Pathways to Block Oncogenic RAS Signalling Aoife A. Nolan 1, Nourhan K. Aboud 1, Walter Kolch 1,2,* and David Matallanas 1,* 1 Systems Biology Ireland, School of Medicine, University College Dublin, Belfield, Dublin 4, Ireland; [email protected] (A.A.N.); [email protected] (N.K.A.) 2 Conway Institute of Biomolecular & Biomedical Research, University College Dublin, Belfield, Dublin 4, Ireland * Correspondence: [email protected] (W.K.); [email protected] (D.M.) Abstract: Oncogenic RAS (Rat sarcoma) mutations drive more than half of human cancers, and RAS inhibition is the holy grail of oncology. Thirty years of relentless efforts and harsh disappointments have taught us about the intricacies of oncogenic RAS signalling that allow us to now get a pharma- cological grip on this elusive protein. The inhibition of effector pathways, such as the RAF-MEK-ERK pathway, has largely proven disappointing. Thus far, most of these efforts were aimed at blocking the activation of ERK. Here, we discuss RAF-dependent pathways that are regulated through RAF functions independent of catalytic activity and their potential role as targets to block oncogenic RAS signalling. We focus on the now well documented roles of RAF kinase-independent functions in apoptosis, cell cycle progression and cell migration. Keywords: RAF kinase-independent; RAS; MST2; ASK; PLK; RHO-α; apoptosis; cell cycle; cancer therapy Citation: Nolan, A.A.; Aboud, N.K.; Kolch, W.; Matallanas, D. Hidden Targets in RAF Signalling Pathways to Block Oncogenic RAS Signalling. -
Profiling Data
Compound Name DiscoveRx Gene Symbol Entrez Gene Percent Compound Symbol Control Concentration (nM) JNK-IN-8 AAK1 AAK1 69 1000 JNK-IN-8 ABL1(E255K)-phosphorylated ABL1 100 1000 JNK-IN-8 ABL1(F317I)-nonphosphorylated ABL1 87 1000 JNK-IN-8 ABL1(F317I)-phosphorylated ABL1 100 1000 JNK-IN-8 ABL1(F317L)-nonphosphorylated ABL1 65 1000 JNK-IN-8 ABL1(F317L)-phosphorylated ABL1 61 1000 JNK-IN-8 ABL1(H396P)-nonphosphorylated ABL1 42 1000 JNK-IN-8 ABL1(H396P)-phosphorylated ABL1 60 1000 JNK-IN-8 ABL1(M351T)-phosphorylated ABL1 81 1000 JNK-IN-8 ABL1(Q252H)-nonphosphorylated ABL1 100 1000 JNK-IN-8 ABL1(Q252H)-phosphorylated ABL1 56 1000 JNK-IN-8 ABL1(T315I)-nonphosphorylated ABL1 100 1000 JNK-IN-8 ABL1(T315I)-phosphorylated ABL1 92 1000 JNK-IN-8 ABL1(Y253F)-phosphorylated ABL1 71 1000 JNK-IN-8 ABL1-nonphosphorylated ABL1 97 1000 JNK-IN-8 ABL1-phosphorylated ABL1 100 1000 JNK-IN-8 ABL2 ABL2 97 1000 JNK-IN-8 ACVR1 ACVR1 100 1000 JNK-IN-8 ACVR1B ACVR1B 88 1000 JNK-IN-8 ACVR2A ACVR2A 100 1000 JNK-IN-8 ACVR2B ACVR2B 100 1000 JNK-IN-8 ACVRL1 ACVRL1 96 1000 JNK-IN-8 ADCK3 CABC1 100 1000 JNK-IN-8 ADCK4 ADCK4 93 1000 JNK-IN-8 AKT1 AKT1 100 1000 JNK-IN-8 AKT2 AKT2 100 1000 JNK-IN-8 AKT3 AKT3 100 1000 JNK-IN-8 ALK ALK 85 1000 JNK-IN-8 AMPK-alpha1 PRKAA1 100 1000 JNK-IN-8 AMPK-alpha2 PRKAA2 84 1000 JNK-IN-8 ANKK1 ANKK1 75 1000 JNK-IN-8 ARK5 NUAK1 100 1000 JNK-IN-8 ASK1 MAP3K5 100 1000 JNK-IN-8 ASK2 MAP3K6 93 1000 JNK-IN-8 AURKA AURKA 100 1000 JNK-IN-8 AURKA AURKA 84 1000 JNK-IN-8 AURKB AURKB 83 1000 JNK-IN-8 AURKB AURKB 96 1000 JNK-IN-8 AURKC AURKC 95 1000 JNK-IN-8 -
RAF Protein-Serine/Threonine Kinases: Structure and Regulation
Biochemical and Biophysical Research Communications 399 (2010) 313–317 Contents lists available at ScienceDirect Biochemical and Biophysical Research Communications journal homepage: www.elsevier.com/locate/ybbrc Mini Review RAF protein-serine/threonine kinases: Structure and regulation Robert Roskoski Jr. * Blue Ridge Institute for Medical Research, 3754 Brevard Road, Suite 116, Box 19, Horse Shoe, NC 28742, USA article info abstract Article history: A-RAF, B-RAF, and C-RAF are a family of three protein-serine/threonine kinases that participate in the Received 12 July 2010 RAS-RAF-MEK-ERK signal transduction cascade. This cascade participates in the regulation of a large vari- Available online 30 July 2010 ety of processes including apoptosis, cell cycle progression, differentiation, proliferation, and transforma- tion to the cancerous state. RAS mutations occur in 15–30% of all human cancers, and B-RAF mutations Keywords: occur in 30–60% of melanomas, 30–50% of thyroid cancers, and 5–20% of colorectal cancers. Activation 14-3-3 of the RAF kinases requires their interaction with RAS-GTP along with dephosphorylation and also phos- ERK phorylation by SRC family protein-tyrosine kinases and other protein-serine/threonine kinases. The for- GDC-0879 mation of unique side-to-side RAF dimers is required for full kinase activity. RAF kinase inhibitors are MEK Melanoma effective in blocking MEK1/2 and ERK1/2 activation in cells containing the oncogenic B-RAF Val600Glu PLX4032 activating mutation. RAF kinase inhibitors lead to the paradoxical increase in RAF kinase activity in cells PLX4720 containing wild-type B-RAF and wild-type or activated mutant RAS. -
Coordinate Phosphorylation of Multiple Residues on Single AKT1 and AKT2 Molecules
Oncogene (2014) 33, 3463–3472 & 2014 Macmillan Publishers Limited All rights reserved 0950-9232/14 www.nature.com/onc ORIGINAL ARTICLE Coordinate phosphorylation of multiple residues on single AKT1 and AKT2 molecules H Guo1,6, M Gao1,6,YLu1, J Liang1, PL Lorenzi2, S Bai3, DH Hawke4,JLi1, T Dogruluk5, KL Scott5, E Jonasch3, GB Mills1 and Z Ding1 Aberrant AKT activation is prevalent across multiple human cancer lineages providing an important new target for therapy. Twenty- two independent phosphorylation sites have been identified on specific AKT isoforms likely contributing to differential isoform regulation. However, the mechanisms regulating phosphorylation of individual AKT isoform molecules have not been elucidated because of the lack of robust approaches able to assess phosphorylation of multiple sites on a single AKT molecule. Using a nanofluidic proteomic immunoassay (NIA), consisting of isoelectric focusing followed by sensitive chemiluminescence detection, we demonstrate that under basal and ligand-induced conditions that the pattern of phosphorylation events is markedly different between AKT1 and AKT2. Indeed, there are at least 12 AKT1 peaks and at least 5 AKT2 peaks consistent with complex combinations of phosphorylation of different sites on individual AKT molecules. Following insulin stimulation, AKT1 was phosphorylated at Thr308 in the T-loop and Ser473 in the hydrophobic domain. In contrast, AKT2 was only phosphorylated at the equivalent sites (Thr309 and Ser474) at low levels. Further, Thr308 and Ser473 phosphorylation occurred predominantly on the same AKT1 molecules, whereas Thr309 and Ser474 were phosphorylated primarily on different AKT2 molecules. Although basal AKT2 phosphorylation was sensitive to inhibition of phosphatidylinositol 3-kinase (PI3K), basal AKT1 phosphorylation was essentially resistant. -
Mutation-Specific RAS Oncogenicity Explains NRAS Codon 61 Selection in Melanoma
Published OnlineFirst September 24, 2014; DOI: 10.1158/2159-8290.CD-14-0729 reseArch Article Mutation-Specific RAS Oncogenicity Explains NRAS Codon 61 Selection in Melanoma Christin E. Burd1,2, Wenjin Liu3,4, Minh V. Huynh5, Meriam A. Waqas1, James E. Gillahan1,2, Kelly S. Clark3,4, Kailing Fu3,4, Brit L. Martin1, William R. Jeck3,4,. George P Souroullas3,4, David B. Darr3,4, Daniel C. Zedek6, Michael J. Miley7, Bruce C. Baguley8, Sharon L. Campbell4,5, and Norman E. Sharpless3,4 Abstrt Ac NRAS mutation at codons 12, 13, or 61 is associated with transformation; yet, in melanoma, such alterations are nearly exclusive to codon 61. Here, we compared the melanoma susceptibility of an NrasQ61R knock-in allele to similarly designed KrasG12D and NrasG12D alleles. With concomitant p16INK4a inactivation, KrasG12D or NrasQ61R expression efficiently promoted melanoma in vivo, whereas NrasG12D did not. In addition, NrasQ61R mutation potently cooperated with Lkb1/Stk11 loss to induce highly metastatic disease. Functional comparisons of NrasQ61R and NrasG12D revealed little difference in the ability of these proteins to engage PI3K or RAF. Instead, NrasQ61R showed enhanced nucleotide binding, decreased intrinsic GTPase activity, and increased stability when compared with NrasG12D. This work identifies a faithful model of human NRAS-mutant melanoma, and suggests that the increased melanomagenecity of NrasQ61R over NrasG12D is due to heightened abundance of the active, GTP-bound form rather than differences in the engagement of downstream effector pathways. SIGNIFICANCE: This work explains the curious predominance in human melanoma of mutations of codon 61 of NRAS over other oncogenic NRAS mutations. -
Interaction Between the Protein Kinase B-Raf and the A-Subunit of the IIS Proteasome Regulator1
(CANCER RESEARCH 58. 2986-2990, July 15, I998| Advances in Brief Interaction between the Protein Kinase B-Raf and the a-Subunit of the IIS Proteasome Regulator1 Andreas Kalmes,2 Garsten Hagemann,2 Christoph K. Weber, Ludmilla Wixler, Tillman Schuster, and Ulf R. Kapp* InstituÃfürMedizinische Strahlenkunde und Zettforschung (MSZ). University of Würzburg,97078 Würzburg,Germany ¡C.H.. C. K. W.. L. W.. T. S.. U. K. R.], and Department of Surgery, University of Washington, Seattle. Washington 9SI95 ¡A.KJ Abstract using the yeast two-hybrid system. In addition to the already known B-Raf-binding proteins (MEK and several 14-3-3 isoforms), we iso Protein kinases of the Raf family act as signal-transducing elements lated PA28a, the subunit of the 1IS proteasome regulator (11, 12), as downstream of activated cell surface receptors and are involved in the a novel B-Raf-binding partner. regulation of proliferation, differentiation, and cell survival. Whereas the role of c-Raf-1 as a mitogen-activated protein/extracellular signal-regu The 11S regulator is one of two known activators of the 20S lated kinase activator within the mitogenic cascade is well established, less proteasome (13). Both of these activators seem to possess different is known about the mammalian Raf isoforins A-Raf and B-Raf. Here we functions in vivo. The PA700 activator complex (19S regulator) binds report that B-Raf binds to PA28a, one of two subunits of the 1IS regulator to the 20S proteasome core complex to form the active 26S protea of proteasomes. PA28<* was isolated as a B-Raf-binding protein in a yeast some, which mediates the ATP- and ubiquitination-dependent degra two-hybrid screen of a PC 12 cDNA library. -
Divergent Regulation of Akt1 and Akt2 Isoforms in Insulin Target Tissues of Obese Zucker Rats Young-Bum Kim, Odile D
Divergent Regulation of Akt1 and Akt2 Isoforms in Insulin Target Tissues of Obese Zucker Rats Young-Bum Kim, Odile D. Peroni, Thomas F. Franke, and Barbara B. Kahn To determine whether impaired Akt (protein kinase B or rac) activation contributes to insulin resistance in vivo, we examined the expression, phosphorylation, he effect of insulin to acutely stimulate glucose and kinase activities of Akt1 and Akt2 isoforms in uptake and metabolism in peripheral tissues is insulin target tissues of insulin-resistant obese Zucker essential for normal glucose homeostasis. Resis- ؋ rats. In lean rats, insulin (10 U/kg i.v. 2.5 min) stim- tance to this effect is a major pathogenic feature ulated Akt1 activity 6.2-, 8.8-, and 4.4-fold and Akt2 T of type 2 diabetes (1,2) and contributes to the morbidity of activity 5.4-, 9.3-, and 1.8-fold in muscle, liver, and adi- pose tissue, respectively. In obese rats, insulin-stimu- obesity as well as type 1 diabetes (3). Whereas many of the lated Akt1 activity decreased 30% in muscle and 21% proximal steps in insulin signaling have been identified, the in adipose tissue but increased 37% in liver compared downstream pathways for insulin action to maintain glucose with lean littermates. Insulin-stimulated Akt2 activity homeostasis are still unknown. Insulin action involves a decreased 29% in muscle and 37% in liver but series of signaling cascades initiated by insulin binding to its increased 24% in adipose tissue. Akt2 protein levels receptor and eliciting receptor autophosphorylation and acti- were reduced 56% in muscle and 35% in liver of obese vation of receptor tyrosine kinases, which result in tyrosine rats, but Akt1 expression was unaltered. -
In Vitro and in Vivo Profile of a Selective, Allosteric Akt2 Inhibitor
In Vitro and In Vivo Profile of a Selective, Allosteric Akt2 Inhibitor Martin O'Rourke1, Caroline Boyd1, Estelle McLean1, Natalie Page1, Shane Rountree1, Steven Shepherd1, Mary McFarland1 Colin O’Dowd1, Graham Trevitt1, Iain James1, Timothy Harrison1,2, Ultan McDermott3 1Almac Discovery, Craigavon, United Kingdom; 2Centre for Cancer Research and Cell Biology, Queens University, Belfast, United Kingdom 3 Wellcome Trust Sanger Institute, Hinxton, Cambs, CB10 1SA Biochemical efficacy of ADC-0008830, an Akt2 selective Pathway biomarker Overview inhibitor engagement in vitro Figure 1: Dose dependent inhibition of a) Akt1 b) Akt2 and c) Akt3 in an FP Figure 2: Inhibition of pathway • Akt protein family members (Akt biochemical assay biomarkers by ELISA 1-3) are involved in a wide variety a b c pGSK3β pPRAS 40 of biological processes including Cell line pAkt (nM) cell proliferation, apoptosis, and (nM) (nM) tumorigenesis. PC3 60 100 15 (IC50 7,600nM) (IC 4,200nM) (IC 27nM) • We have identified a novel, 50 50 LnCaP 500 6000 3600 allosteric Akt 2 selective inhibitor, A2780 160 740 840 which exhibits single agent A549 130 129 150 efficacy in a panel of cell-lines. Panc-1 9 234 Not tested • In collaboration with the Sanger • ADC-0008830 is a potent inhibitor of the Akt2 isoform Institute, we have identified genes • ADC-0008830 inhibits key • No activity observed against PH domain null Akt isoforms confirming of sensitivity and resistance for components of the Akt pathway an allosteric mode of action subtype selective Akt inhibitors in multiple cell lines • Excellent selectivity against a kinase counterscreen panel (data not shown) Dose and time dependent inhibition of Cellular profiling of ADC-0008939, a prototypical isoform selective Akt2 pathway biomarkers (Panc-1) inhibitor, in the Sanger cell line panel Figure 3: Dose and time dependent inhibition of Akt Figure 4: Pharmacogenomic analysis in 432 cell lines of ADC-0008939, a prototypical isoform selective pathway in Panc-1 cell line Akt inhibitor (IC50 Akt1 300nM; Akt2 7nM; Akt3 1230nM). -
Supplemental Table 1: Snps Genotyped for NCO, Listed Alphabetically by Gene Name
Supplemental Table 1: SNPs genotyped for NCO, listed alphabetically by gene name. Gene Name SNP rs# ACVR1 rs10497189 ACVR1 rs10497191 ACVR1 rs10497192 ACVR1 rs10933441 ACVR1 rs1146035 ACVR1 rs1220110 ACVR1 rs17182166 ACVR1 rs17798043 ACVR1 rs4380178 ACVR1 rs6719924 ACVR2 rs1128919 ACVR2 rs10497025 ACVR2 rs1424941 ACVR2 rs1424954 ACVR2 rs17742573 ACVR2 rs2382112 ACVR2 rs4419186 AKT2 rs11671439 AKT2 rs12460555 AKT2 rs16974157 AKT2 rs2304188 AKT2 rs3730050 AKT2 rs7250897 AKT2 rs7254617 AKT2 rs874269 ALOX12B/ALOXE3 rs3809881 ALOX15B rs4792147 ALOX15B rs9898751 AMH rs10407022 AMH rs2074860 AMH rs3746158 AMH rs4806834 AMH rs757595 AMH rs886363 AMHR2 rs10876451 AMHR2 rs11170547 AMHR2 rs11170558 APC rs2229992 APC rs351771 APC rs41115 APC rs42427 APC rs459552 APC rs465899 APEX1 rs2275007 APEX1 rs3136820 15 APEX1 rs938883 APEX1 rs11160711 APEX1 rs1713459 APEX1 rs1713460 APEX1 rs1760941 APEX1 rs2275008 APEX1 rs3120073 APEX1 rs4465523 AR rs12011793 AR rs1204038 AR rs1337080 AR rs1337082 AR rs2207040 AR rs2361634 AR rs5031002 AR rs6152 AR rs962458 ATM rs1800058 ATM rs1800889 ATM rs11212570 ATM rs17503908 ATM rs227060 ATM rs228606 ATM rs3092991 ATM rs4987876 ATM rs4987886 ATM rs4987923 ATM rs4988023 ATM rs611646 ATM rs639923 ATM rs672655 ATR rs2229032 ATR rs10804682 ATR rs11920625 ATR rs13065800 ATR rs13085998 ATR rs13091637 ATR rs1802904 ATR rs6805118 ATR rs9856772 BACH1 rs388707 BACH1 rs1153276 BACH1 rs1153280 BACH1 rs1153284 BACH1 rs1153285 BACH1 rs17743655 BACH1 rs17744121 BACH1 rs2300301 BACH1 rs2832283 16 BACH1 rs411697 BACH1 rs425989 BARD1 rs1048108 -
Activation of Diverse Signalling Pathways by Oncogenic PIK3CA Mutations
ARTICLE Received 14 Feb 2014 | Accepted 12 Aug 2014 | Published 23 Sep 2014 DOI: 10.1038/ncomms5961 Activation of diverse signalling pathways by oncogenic PIK3CA mutations Xinyan Wu1, Santosh Renuse2,3, Nandini A. Sahasrabuddhe2,4, Muhammad Saddiq Zahari1, Raghothama Chaerkady1, Min-Sik Kim1, Raja S. Nirujogi2, Morassa Mohseni1, Praveen Kumar2,4, Rajesh Raju2, Jun Zhong1, Jian Yang5, Johnathan Neiswinger6, Jun-Seop Jeong6, Robert Newman6, Maureen A. Powers7, Babu Lal Somani2, Edward Gabrielson8, Saraswati Sukumar9, Vered Stearns9, Jiang Qian10, Heng Zhu6, Bert Vogelstein5, Ben Ho Park9 & Akhilesh Pandey1,8,9 The PIK3CA gene is frequently mutated in human cancers. Here we carry out a SILAC-based quantitative phosphoproteomic analysis using isogenic knockin cell lines containing ‘driver’ oncogenic mutations of PIK3CA to dissect the signalling mechanisms responsible for oncogenic phenotypes induced by mutant PIK3CA. From 8,075 unique phosphopeptides identified, we observe that aberrant activation of PI3K pathway leads to increased phosphorylation of a surprisingly wide variety of kinases and downstream signalling networks. Here, by integrating phosphoproteomic data with human protein microarray-based AKT1 kinase assays, we discover and validate six novel AKT1 substrates, including cortactin. Through mutagenesis studies, we demonstrate that phosphorylation of cortactin by AKT1 is important for mutant PI3K-enhanced cell migration and invasion. Our study describes a quantitative and global approach for identifying mutation-specific signalling events and for discovering novel signalling molecules as readouts of pathway activation or potential therapeutic targets. 1 McKusick-Nathans Institute of Genetic Medicine and Department of Biological Chemistry, Johns Hopkins University School of Medicine, 733 North Broadway, BRB 527, Baltimore, Maryland 21205, USA. -
A-Raf and Raf-1 Work Together to Influence Transient ERK Phosphorylation and Gl/S Cell Cycle Progression
Oncogene (2005) 24, 5207–5217 & 2005 Nature Publishing Group All rights reserved 0950-9232/05 $30.00 www.nature.com/onc A-Raf and Raf-1 work together to influence transient ERK phosphorylation and Gl/S cell cycle progression Kathryn Mercer1, Susan Giblett1, Anthony Oakden2, Jane Brown2, Richard Marais3 and Catrin Pritchard*,1 1Department of Biochemistry, University of Leicester, Adrian Building, University Road, Leicester LEI 7RH, UK; 2Division of Biomedical Services, University of Leicester, University Road, Leicester LEI 7RH, UK; 3Institute of Cancer Research, 237 Fulham Road, London SW3 6JB, UK The Raf/MEK/ERK (extracellular regulated kinase) Kerkhoff and Rapp, 1997, 1998; Woods et al., 1997; signal transduction pathway controls the ability of cells Marshall, 1999; Squires et al., 2002). This function to respond to proliferative, apoptotic, migratory and is mediated by the ability of ERKs to translocate to the differentiation signals. We have investigated the combined nucleus where they are able to phosphorylate transcrip- contribution of A-Raf and Raf-1 isotypes to signalling tion factors, particularly the AP-1 complex that through this pathway by generating mice with knockout comprises various heterodimers of c-fos (c-fos, FosB, mutations of both A-raf and raf-1 genes. Double knockout Fra-1, Fra-2) and c-jun (c-Jun, JunB, JunD) family (DKO) mice have a more severe phenotype than single null members (Whitmarsh and Davis, 1996; Balmanno mutations of either gene, dying in embryogenesis at E10.5. and Cook, 1999; Cook et al., 1999; Shaulian and The DKO embryos show no changes in apoptosis, but Karin, 2002). -
High-Intensity Raf Signals Convert Mitotic Cell Cycling Into Cellular Growth'
(CANCERRESEARCH58. 1636-1640. April 15. 19981 Advances in Brief High-intensity Raf Signals Convert Mitotic Cell Cycling into Cellular Growth' Eugen Kerkhoff and Ulf R. Rapp2 lnstitutfürmedizinischeStrahlenkundeund Zellforschung(MSZ). Universityof Wurzburg,97078 Wiirzburg, Germany Abstract oncogernc c-Raf-1-estrogen receptor fusion protein (c-Raf-1-BxB ERTM). Activation of the exogenous kinase induces high and sustained The selectionof NIH 3T3 cellsexpressinga hydroxytamoxifen-induci c-Raf signals and inhibits DNA synthesis and mitosis. Despite the ble c-Raf-1-estrogen receptor fusion protein (c-Raf-1-BxB-ER@) in the inhibition of cell cycle progression, cell growth remains unaffected, absence or presence of the inducer results in dramatic differences in the expression levels of the fusion protein. Hydroxytamoxifen-mediated con leading to cells with a DNA content of G1 cells and a cell size of stitutive activation of the Rafsignal favors the selection ofcells expressing G2-M-phase cells. Therefore, high-intensity Raf signals uncouple the low levels of c-R.af-1-BxB-ER@. Cells selected in the absence of hy regulation of cell growth from DNA replication and mitosis. droxytamoxifen express up to 20 tImes higher levels of the inducible Raf kinase. Activation ofthe oncogenic Rafkinase in cells expressing low levels Materials and Methods leads to a weak activation of the Raf/Mek/Erk cascade and the induction of S phase In confluentcells.The activationof cellsexpressinghigh levels Expression Vector. The pBabe-puro-BxB-ER@ vector was constructed by ofthe kinaseleadsto a strongpersIstentsignalandinhibitsDNAsynthesis releasing sequences encoding the c-Raf-l-BxB-ER@ protein from the BJ4- andmitOsisinproliferatingcells.TheinhibitionofDNAsynthesisandcell BxB-ER@ vector (7) by EcoRI digestion (2.2-kb fragment) and inserting them dIvision is presumably due to the elevated expression ofthe cyclin-depend into the unique EcoRI recognition site of the pBabe-puro vector (13).