Cell growth and metastasis in pancreatic cancer: is Vav the Rho'd to activation?

The best-known family of low molecular weight GTP-binding proteins is Ras, owing to their high incidence of gain of function mutations in a variety of human cancers including pancreatic cancer. Unlike Ras, no activating mutations have been observed thus far for Rho family GTP-binding proteins in cancer, yet there is increasing evidence that overexpression of Rho family members and/or dysregulation of the GDP-->GTP cycle play an important role in cancer development and progression. The activation of Rho family GTPases downstream of cell surface receptors results in the induction of several intracellular signaling cascades that have been shown to impact on such diverse cellular responses as reorganization of the actin cytoskeleton, gene transcription, cell survival, and cell proliferation. One family of guanine nucleotide exchange factors (GEFs) that have the potential to couple the activation of Rho family members to upstream growth factor receptor tyrosine kinases (RTKs) is the Vav family of proto-oncogenes. Recent experimental evidence has implicated Vav in the regulation of numerous Rho-mediated pathways downstream of RTKs and other cell surface receptors. In this review, we will discuss our current understanding of how Vav proteins are regulated, and how Vav and their target GTP-binding proteins participate in tumorigenesis.

Mouse lipocalin as an enhancer of spermatozoa motility

The 24p3 protein is a 25 kDa glycoprotein that is secreted into the uterine fluid during the proestrous phase of mice. We assessed the effects on spermatozoa motility and on the functions of mouse spermatozoa using the computer-assisted sperm analysis method, cytochemical staining and detection of the protein tyrosine phosphorylation pattern. Compared with the control cells, sperm motility was stimulated by the addition of 24p3 protein into the medium. Introducing 24p3 protein enhanced progressive motility but did not promote the appearance of hyperactivated movement. The presence of 24p3 protein in the medium did not allow the cells to undergo the capacitated protein tyrosine phosphorylation pattern and acrosome reaction. The tyrosine phosphorylation pattern shows phosphoproteins in the range of Mr 50,000-106,000 correlated with the sperm progressive motility after the addition of 24p3 protein into the medium. Using flow cytometry, we assessed the changes in the intracellular pH and measured the intracellular cAMP concentration with an immunodetection kit. The results indicated that the elevation in intracellular pH from 6.67 to 6.89, increase of intracellular cAMP accumulation, and protein tyrosine phosphorylation might be the factors in enhancement of sperm motility as the 24p3 protein bound to the spermatozoa. The 24p3 protein may have a role in regulating flagellar motility.

The LIM-only protein, LMO4, and the LIM domain-binding protein, LDB1, expression in squamous cell carcinomas of the oral cavity

Carcinoma cells can lose their epithelial cell characteristics and dedifferentiate into a fibroblast-like cell during progression of a neoplasm. Aberrant expression of oligomeric transcriptional complexes contributes to progression of carcinomas. Although individual transcription factors initiating progression remain unknown, LIM-only protein (LMO) and LIM-domain binding protein (LDB) negatively regulate breast carcinoma cell differentiation. In this study, we investigated the expression of LMO4 and LDB in squamous cell carcinomas of the oral cavity. LMO4 mRNA was amplified in four of six carcinoma tissues and eight of 12 carcinoma cell lines, and LDB1 in three carcinoma tissues and 11 cell lines examined. Immunoprecipitation studies revealed that LMO4 and LDB1 interact with each other in the nuclear milieu of the carcinoma cells indicating the presence of an LMO4-LDB1-mediated transcription complex. Both LMO4 and LDB1 proteins were preferentially localised in the nuclei of carcinoma cells at the invasive front and the immunoreactivity was increased in less-differentiated carcinoma tissues (P<0.01). Carcinoma cells metastasised to the cervical lymph nodes with increased immunoreactivity compared to the primary site of neoplasm (P<0.05). These data suggest that the LMO4-LDB1 complexes may be involved in carcinoma progression possibly through dedifferentiation of squamous carcinoma cells of the oral cavity.

Peptide mimotopes of oncoproteins as therapeutic agents in breast cancer

Generation of an immune response to oncoproteins can lead to a cancer specific protective immunity. Several such oncoproteins are being examined as tumor targets with mixed results. We are evaluating the clinical utility of synthetic peptides that would mimic the antigen immunologically and elicit a tumor specific immune response. HER-2/neu, an oncoprotein whose expression in breast cancer is associated with poor prognosis, lower disease free-survival and a propensity for metastases was chosen as a model. Antibodies, Ab2, Ab4 and Ab5 directed towards the extracellular domain of HER-2/neu were reacted to peptides from two synthetic phage display peptide libraries, LX-8 (12-mer peptide library containing disulfide bridge) and X-15 (linear 15-mer). The isolated peptides were sequenced and characterized for ability to produce high titer antibodies and cross-reactivity. The peptides isolated did not show any sequence homology to protein databases but did show a hierarchy of immunogenic epitopes. Antibodies generated against peptides selected against the same antibody Ab2 or Ab4 showed affinity variation. Phages selected against Ab2 were also able to compete with binding of Ab2 to HER-2/neu. These results validate our hypothesis that synthetic peptides that mimic the antigenic epitope of oncoprotein can be generated and their clinical utility rests on devising a screening mechanism to identify peptides that can elicit an immune response directed to the oncoprotein and if possible its antigenic variants.

Transcriptional activation by AP-2alpha is modulated by the oncogene DEK

Cell differentiation and development are highly regulated processes at the transcriptional level. One of the main transcription factors that regulate these processes is AP-2alpha, a cell-type specific protein required for vertebrate development and embryogenesis. AP-2alpha also regulates apoptosis and cell-cycle specific events by interacting with the oncogene c-Myc. In searching for novel AP-2alpha- interacting factors, using an affinity chromatography approach, we have observed that oncoprotein DEK interacts with AP-2alpha in vitro. The existence of an interaction between AP-2alpha and DEK in cellular cultures was demonstrated by expression of a tagged AP-2alpha form followed by immunodetection. By transient co-expression experiments using a reporter for APOE promoter activity we have found that DEK stimulates the transactivation activity of AP-2alpha over APOE promoter. Finally, electrophoretic mobility shift assays suggested that DEK enhances the DNA-binding activity of AP-2alpha. Our data suggest a novel cellular function of DEK as a transcriptional co-activator.

[Lymphocytes: what is "Vav"]

Guanine exchange factors (GEF) of the Vav family are critical activators of Rho GTPases, which control actin cytoskeletal reorganization and gene transcription. Among all GEFs identified, Vav proteins are the only GEFs regulated by tyrosine phosphorylation. Moreover, their structure contains several protein-protein or protein-lipid interaction domains. These domains are involved in the formation of multimolecular signalling complexes, highlighting the adaptor role of Vav proteins. The unique combination of these properties makes Vav proteins privileged integrators of multiple signalling pathways in a broad range of tissues and cells. Lymphocyte function during inflammatory and immune responses requires a dynamic remodeling of cellular architecture. Thus, it is not surprising that Vav proteins have been found to play a central role in the regulation of physiologic and pathologic lymphocyte responses.

The neutrophil lipocalin NGAL is a bacteriostatic agent that interferes with siderophore-mediated iron acquisition

First identified as a neutrophil granule component, neutrophil gelatinase-associated lipocalin (NGAL; also called human neutrophil lipocalin, 24p3, uterocalin, or neu-related lipocalin) is a member of the lipocalin family of binding proteins. Putative NGAL ligands, including neutrophil chemotactic agents such as N-formylated tripeptides, have all been refuted by recent biochemical and structural results. NGAL has subsequently been implicated in diverse cellular processes, but without a characterized ligand, the molecular basis of these functions remained mysterious. Here we report that NGAL tightly binds bacterial catecholate-type ferric siderophores through a cyclically permuted, hybrid electrostatic/cation-pi interaction and is a potent bacteriostatic agent in iron-limiting conditions. We therefore propose that NGAL participates in the antibacterial iron depletion strategy of the innate immune system.

Inactivation of the retinoblastoma protein family can bypass the HCF-1 defect in tsBN67 cell proliferation and cytokinesis

Owing to a single missense mutation in the cell proliferation factor HCF-1, the temperature-sensitive tsBN67 hamster cell line arrests proliferation at nonpermissive temperatures, primarily in a G(0)/G(1) state, and displays temperature-sensitive cytokinesis defects. The HCF-1 mutation in tsBN67 cells also causes a temperature-sensitive dissociation of HCF-1 from chromatin prior to cell proliferation arrest, suggesting that HCF-1-chromatin association is important for mammalian-cell proliferation. Here, we report that the simian virus 40 (SV40) early region, in particular, large T antigen (Tag), and the adenovirus oncoprotein E1A can rescue the tsBN67 cell proliferation defect at nonpermissive temperatures. The SV40 early region rescues the tsBN67 cell proliferation defect without restoring the HCF-1-chromatin association, indicating that these oncoproteins bypass a requirement for HCF-1 function. The SV40 early region also rescues the tsBN67 cytokinesis defect, suggesting that the roles of HCF-1 in cell proliferation and proper cytokinesis are intimately linked. The ability of SV40 Tag and adenovirus E1A to inactivate members of the pRb protein family-pRb, p107, and p130-is important for the bypass of HCF-1 function. These results suggest that HCF-1 regulates mammalian-cell proliferation and cytokinesis, at least in part, by either directly or indirectly opposing pRb family member function.

Inhibition of apoptosis by ATFx: a novel role for a member of the ATF/CREB family of mammalian bZIP transcription factors

The mammalian ATF/CREB family of transcription factors comprises a large group of basic-region leucine zipper (bZIP) proteins whose members mediate diverse transcriptional regulatory functions. Here we report that expression of a specific mouse ATF gene, ATFx, is down-regulated in a variety of cells undergoing apoptosis following growth factor deprivation. When stably expressed in an interleukin 3 (IL-3)-dependent cell line, ATFx suppresses apoptosis resulting from cytokine deprivation. Conversely, a dominant-negative ATFx mutant induces apoptosis of cells cultured in the presence of growth factors. We also show that 24p3, a secreted lipocalin that induces apoptosis when added to hematopoietic cells, represses ATFx expression. However, constitutive expression of ATFx renders cells resistant to 24p3-mediated apoptosis. Collectively, our results indicate that ATFx is an anti-apoptotic factor, a novel role for an ATF protein.

The coiled-coil domain and Tyr177 of bcr are required to induce a murine chronic myelogenous leukemia-like disease by bcr/abl

The bcr/abl fusion in chronic myelogenous leukemia (CML) creates a chimeric tyrosine kinase with dramatically different properties than intact c-abl. In P210 bcr/abl, the bcr portion includes a coiled-coil oligomerization domain (amino acids 1-63) and a grb2-binding site at tyrosine 177 (Tyr177) that are critical for fibroblast transformation, but give variable results in other cell lines. To investigate the role of the coiled-coil domain and Tyr177 in promoting CML, 4 P210 bcr/abl-derived mutants containing different bcr domains fused to abl were constructed. All 4 mutants, Delta(1-63) bcr/abl, (1-63) bcr/abl, Tyr177Phe bcr/abl, and (1-210) bcr/abl exhibited elevated tyrosine kinase activity and conferred factor-independent growth in cell lines. In contrast, differences in the transforming potential of the 4 mutants occurred in our mouse model, in which all mice receiving P210 bcr/abl-expressing bone marrow cells exclusively develop a myeloproliferative disease (MPD) resembling human CML. Of the 4 mutants assayed, only 1-210 bcr/abl, containing both the coiled-coil domain and Tyr177, induced MPD. Unlike full-length P210, this mutant also caused a simultaneous B-cell acute lymphocytic leukemia (ALL). The other 3 mutants, (1-63) bcr/abl, Tyr177Phe bcr/abl, and Delta(1-63) bcr/abl, failed to induce an MPD but instead caused T-cell ALL. These results show that both the bcr coiled-coil domain and Tyr177 are required for MPD induction by bcr/abl and provide the basis for investigating downstream signaling pathways that lead to CML.

Vav-induced activation of the human IFN-gamma gene promoter is mediated by upregulation of AP-1 activity

The role of Vav in the transcriptional regulation of the human interferon-gamma (IFN-gamma) promoter was investigated. Overexpression of Vav in Jurkat-TAg cells enhanced T cell receptor (TCR)-induced activation of a luciferase (Luc) reporter gene construct driven by cis-regulatory element of the IFN-gamma gene (-346 to +7). Electrophoresis mobility shift and Luc reporter assays demonstrated that the DNA-binding and transcriptional activity of the proximal AP-1-dependent NFAT site (positions -172 to -138), the AP-1/Ying-Yang 1 (YY1)-binding site (-209 to -184), and a consensus AP-1-binding site were upregulated by Vav. Vav enhanced TCR-induced activation of c-Jun N-terminal kinase (JNK) and its upstream regulator, Rho family GTPases. Finally, coexpression of a dominant-negative Rac1 mutant suppressed Vav-mediated upregulation of the transcriptional and DNA-binding activity of the proximal NFAT/AP-1 site and the AP-1/YY1 site, as well as the complete IFN-gamma promoter activity. Vav activates the IFN-gamma promoter via upregulation of AP-1-binding through a Rac1/JNK pathway.

Nck and phosphatidylinositol 4,5-bisphosphate synergistically activate actin polymerization through the N-WASP-Arp2/3 pathway

The Wiskott-Aldrich syndrome protein (WASP) and its relative neural WASP (N-WASP) regulate the nucleation of actin filaments through their interaction with the Arp2/3 complex and are regulated in turn by binding to GTP-bound Cdc42 and phosphatidylinositol 4,5-bisphosphate. The Nck Src homology (SH) 2/3 adaptor binds via its SH3 domains to a proline-rich region on WASP and N-WASP and has been implicated in recruitment of these proteins to sites of tyrosine phosphorylation. We show here that Nck SH3 domains dramatically stimulate the rate of nucleation of actin filaments by purified N-WASP in the presence of Arp2/3 in vitro. All three Nck SH3 domains are required for maximal activation. Nck-stimulated actin nucleation by N-WASP.Arp2/3 complexes is further stimulated by phosphatidylinositol 4,5-bisphosphate, but not by GTP-Cdc42, suggesting that Nck and Cdc42 activate N-WASP by redundant mechanisms. These results suggest the existence of an Nck-dependent, Cdc42-independent mechanism to induce actin polymerization at tyrosine-phosphorylated Nck binding sites.

Characterization of human bone morphogenetic protein (BMP)-4 and -7 gene promoters: activation of BMP promoters by Gli, a sonic hedgehog mediator

Among the bone morphogenetic protein (BMP) family, which plays a crucial role not only in bone formation but also in development, BMP-2, -4, and -7 participate predominantly in various aspects. To undertake complex tasks, their expression is strictly controlled. In this study we isolated and analyzed the 5'-flanking regions of the human BMP-4 and -7 genes to elucidate the mechanism of their temporally and spatially specific expression. As for BMP-4 expression, a reverse transcription-polymerase chain reaction (RT-PCR) assay with specially designed sets of primers demonstrated that osteoblastic SaOS-2 and Hos cells expressed two types of transcripts comprising one of the 5'-untranslated first exons, whereas MG63 cells displayed only the transcript with the BMP-4 proximal first exon. Likewise, RT-PCR revealed that Hos and MG63 cells expressed BMP-7. Subsequent 5'-RACE confirmed an alternative usage of the BMP-4 first exons with clustered multiple transcription start sites in the distal exon and the sole start site in the proximal exon. The transcription start site of the BMP-7 gene was found to be far upstream (764 bp) of the initiation ATG codon. We constructed a series of deletion mutants of fusions between these BMP promoters and the luciferase gene and examined their activity by transient transfection into osteoblastic Hos and renal COS-7 cells. The degree of distal and proximal BMP-4 promoter activity was in accordance with the expression level of the corresponding transcripts. Both distal and proximal BMP-4 promoters possessed suppressor elements that are operative only in Hos cells. The positive and negative elements identified in the BMP-7 promoter were more remarkably effective in Hos cells. The activities of the respective BMP-4 promoters and BMP-7 promoter were all stimulated upon the cotransfection of a potential sonic hedgehog (SHH) mediator, Gli1 or Gli3 into COS-7 cells, providing direct evidence that the Gli proteins are capable of inducing the BMP expression. Our systems are helpful for assessment of the complicated interactions of molecules involved in the skeletogenesis and developmental processes.

Characterization of Ggrb4, an adapter protein interacting with Bcr-Abl

We report here the characterization of an adapter protein identified in a yeast 2-hybrid screen with the use of Bcr-Abl as the bait. Grb4 bound to Bcr-Abl in a variety of systems, both in vitro and in vivo, and is an excellent substrate of the Bcr-Abl tyrosine kinase. The association of Grb4 and Bcr-Abl in intact cells was mediated by an src homology (SH)2-mediated phosphotyrosine-dependent interaction as well as an SH3-mediated phosphotyrosine-independent interaction. Grb4 has 68% homology to the adapter protein Nck and has similar but distinct binding specificities in K562 lysates. Subcellular localization studies indicate that Grb4 localizes to both the nucleus and the cytoplasm. Coexpression of kinase-active Bcr-Abl with Grb4 resulted in the translocation of Grb4 from the cytoplasm and the nucleus to the cytoskeleton to colocalize with Bcr-Abl. In addition, expression of Grb4 with kinase-active Bcr-Abl resulted in a redistribution of actin-associated Bcr-Abl. Finally, coexpression of Grb4 and oncogenic v-Abl strongly inhibited v-Abl-induced AP-1 activation. Together, these data indicate that Grb4 in conjunction with Bcr-Abl may be capable of modulating the cytoskeletal structure and negatively interfering with the signaling of oncogenic Abl kinases. Grb4 may therefore play a role in the molecular pathogenesis of chronic myelogenous leukemia. (Blood. 2000;96:618-624) (Blood. 2000;96:618-624)

Cyclin D2 is essential for BCR-mediated proliferation and CD5 B cell development

Progression into G(1) in B lymphocytes is regulated by cyclins D2 and D3, components of the cell cycle machinery currently believed to have overlapping and potentially redundant roles in cell cycle control. To study the specific role of cyclin D2 in B lymphocyte proliferation, we examined B cells from cyclin D2(-/-) mice and demonstrate a specific requirement for cyclin D2 in BCR- but not CD40- or lipopolysaccharide-induced proliferation. Furthermore, conventional B cell development proceeds normally in the mutant mice; however, the CD5 B cell compartment is dramatically reduced, suggesting that cyclin D2 is important in CD5 B cell development as well as antigen-dependent B cell clonal expansion.

The Saccharomyces cerevisiae Cdc14 phosphatase is implicated in the structural organization of the nucleolus

Cdc14, a dual-specificity protein phosphatase, has been previously implicated in triggering exit from mitosis in the yeast Saccharomyces cerevisiae. Using immunofluorescence microscopy and immunogold labeling, we demonstrate that a functional HA-tagged version of the phosphatase Cdc14 localizes to the nucleolus. Moreover, Cdc14-HA co-localized with the nucleolar NOP2 and GAR1 proteins. By immunofluorescence, Cdc14-HA was found in the nucleolus during most of the mitotic cell cycle, except during anaphase-telophase when it redistributed along the mitotic spindle. While this work was in progress, the same pattern of Cdc14 localization was described by others (Visintin et al, Nature 398 (1999) 818). Constitutive overexpression of CDC14 was toxic and led to cell cycle arrest of cells, mainly in G1. This correlated with the appearance of abnormal nuclear structures. A genetic search for suppressors of the lethality associated with CDC14 overexpression identified YJL076W. Because overproduction of Yj1076w buffered the toxic effect of Cdc14 overproduction, this suggested that it might be a substrate of Cdc14. This has indeed been found to be the case by others who recently described Yj1076w/Netl as a nucleolar protein that physically associates with Cdc14 (Shou et al, Cell 97 (1999) 233). The present data confirm several recently uncovered aspects of the regulation of Cdc14 localization and activity and suggest that the level of expression of CDC14 influences the structural organization of the nucleolus.

Integrin-dependent tyrosine phosphorylation and growth regulation by Vav

The proto-oncogene product p95Vav (Vav) undergoes rapid phosphorylation on tyrosine following stimulation of the T or B cell antigen receptor, and in response to a variety of other cell surface stimuli. Vav contains, among other, a guanine nucleotide exchange factor domain with homology to the Rho/Rac/CDC42 exchange protein Db1. It has been recently shown that Vav is functionally linked to small GTPases of the Rho family, suggesting that it is an activator of Rho GTPases and may participate in regulation of cytoskeletal organization. The present study shows that cell adhesion to fibronectin triggers rapid phosphorylation of Vav on tyrosine in Vav-transfected CHO cells and in Jurkat T cells. Vav phosphorylation is strongly dependent on adhesion and is mediated by beta 1 integrins. Furthermore, Vav overexpression enhances the adhesion-dependent increase in the rate and extent of phosphorylation on focal adhesion kinase and paxillin, and the formation of stress fibers and lamellipodia. In addition, there is a marked increase in the amount of Vav localized to the triton-insoluble fraction following 1 h of incubation on FN. Finally, Vav increases the growth rate of the cells in an adhesion-dependent manner. Our results strongly implicate Vav as a mediator of integrin signal transduction.

Shc and Enigma are both required for mitogenic signaling by Ret/ptc2

Ret/ptc2 is a constitutively active, oncogenic form of the c-Ret receptor tyrosine kinase. Like the other papillary thyroid carcinoma forms of Ret, Ret/ptc2 is activated through fusion of the Ret tyrosine kinase domain to the dimerization domain of another protein. Investigation of requirements for Ret/ptc2 mitogenic activity, using coexpression with dominant negative forms of Ras and Raf, indicated that these proteins are required for mitogenic signaling by Ret/ptc2. Because activation of Ras requires recruitment of Grb2 and SOS to the plasma membrane, the subcellular distribution of Ret/ptc2 was investigated, and it was found to localize to the cell periphery. This localization was mediated by association with Enigma via the Ret/ptc2 sequence containing tyrosine 586. Because Shc interacts with MEN2 forms of Ret, and because phosphorylation of Shc results in Grb2 recruitment and subsequent signaling through Ras and Raf, the potential interaction between Ret/ptc2 and Shc was investigated. The PTB domain of Shc also interacted with Ret/ptc2 at tyrosine 586, and this association resulted in tyrosine phosphorylation of Shc. Coexpression of chimeric proteins demonstrated that mitogenic signaling from Ret/ptc2 required both recruitment of Shc and subcellular localization by Enigma. Because Shc and Enigma interact with the same site on a Ret/ptc2 monomer, dimerization of Ret/ptc2 allows assembly of molecular complexes that are properly localized via Enigma and transmit mitogenic signals via Shc.

Human chondrocyte expression of growth-arrest-specific gene 6 and the tyrosine kinase receptor axl: potential role in autocrine signaling in cartilage

OBJECTIVE

To determine if human articular chondrocytes express the axl tyrosine kinase receptor and its ligand Gas-6, a protein product of growth-arrest-specific gene 6, and to determine if Gas-6 and axl function in the regulation of chondrocyte growth and survival.

METHODS

The presence of Gas-6 and axl was examined in situ in human articular cartilage by immunohistochemistry and in vitro in cell culture studies using primary human chondrocytes and immortalized human chondrocytes. The ability of recombinant Gas-6 to mediate adhesion of chondrocytes and to stimulate chondrocyte axl phosphorylation was determined. Studies of the role of Gas-6 and axl in cell proliferation and survival were also performed.

RESULTS

Both Gas-6 and axl were detected in cartilage by immunohistochemical staining. Gas-6 and axl messenger RNA (mRNA) and protein were also detected in cultures of primary and immortalized human chondrocytes. Compared with cells cultured in medium containing 10% serum, Gas-6 mRNA levels were increased in immortalized chondrocytes cultured in serum-free medium, while axl expression decreased. Chondrocytes attached to Gas-6-coated plastic, and the attachment was blocked by a soluble Ig fusion protein containing the axl extracellular domain. Recombinant human Gas-6 and serum-free conditioned medium from primary and immortalized human chondrocyte cultures stimulated chondrocyte axl tyrosine phosphorylation. A mitogenic effect was noted both when immortalized chondrocytes were stimulated with recombinant Gas-6 or when they were made to overexpress axl by transfection. Addition of recombinant Gas-6 to serum-free medium resulted in increased survival of primary chondrocytes cultured at low density in agarose.

CONCLUSION

These findings present evidence for an autocrine signaling pathway in cartilage involving Gas-6 and the axl tyrosine kinase adhesion receptor. Stimulation of axl by Gas-6 may play an important role in the control of chondrocyte growth and survival.

Juxtamembrane tyrosine residues couple the Eph family receptor EphB2/Nuk to specific SH2 domain proteins in neuronal cells

Eph-related receptor tyrosine kinases have been implicated in the control of axonal navigation and fasciculation. To investigate the biochemical mechanisms underlying such functions, we have expressed the EphB2 receptor (formerly Nuk/Cek5/Sek3) in neuronal NG108-15 cells, and have observed the tyrosine phosphorylation of multiple cellular proteins upon activation of EphB2 by its ligand, ephrin-B1 (formerly Elk-L/Lerk2). The activated EphB2 receptor induced the tyrosine phosphorylation of a 62-64 kDa protein (p62[dok]), which in turn formed a complex with the Ras GTPase-activating protein (RasGAP) and SH2/SH3 domain adaptor protein Nck. RasGAP also bound through its SH2 domains to tyrosine-phosphorylated EphB2 in vitro, and complexed with activated EphB2 in vivo. We have localized an in vitro RasGAP-binding site to conserved tyrosine residues Y604 and Y610 in the juxtamembrane region of EphB2, and demonstrated that substitution of these amino acids abolishes ephrin-B1-induced signalling events in EphB2-expressing NG108-15 cells. These tyrosine residues are followed by proline at the + 3 position, consistent with the binding specificity of RasGAP SH2 domains determined using a degenerate phosphopeptide library. These results identify an EphB2-activated signalling cascade involving proteins that potentially play a role in axonal guidance and control of cytoskeletal architecture.

CDK-independent activation of estrogen receptor by cyclin D1

Both cyclin D1 and estrogens have an essential role in regulating proliferation of breast epithelial cells. We show here a novel role for cyclin D1 in growth regulation of estrogen-responsive tissues by potentiating transcription of estrogen receptor-regulated genes. Cyclin D1 mediates this activation independent of complex formation to a CDK partner. Cyclin D1 activates estrogen receptor-mediated transcription in the absence of estrogen and enhances transcription in its presence. The activation of estrogen receptor by cyclin D1 is not inhibited by anti-estrogens. A direct physical binding of cyclin D1 to the hormone binding domain of the estrogen receptor results in an increased binding of the receptor to estrogen response element sequences, and upregulates estrogen receptor-mediated transcription. These results highlight a novel role for cyclin D1 as a CDK-independent activator of the estrogen receptor.

Inhibition of cell proliferation by the Mad1 transcriptional repressor

Mad1 is a basic helix-loop-helix-leucine zipper protein that is induced upon differentiation of a number of distinct cell types. Mad1 dimerizes with Max and recognizes the same DNA sequences as do Myc:Max dimers. However, Mad1 and Myc appear to have opposing functions. Myc:Max heterodimers activate transcription while Mad:Max heterodimers repress transcription from the same promoter. In addition Mad1 has been shown to block the oncogenic activity of Myc. Here we show that ectopic expression of Mad1 inhibits the proliferative response of 3T3 cells to signaling through the colony-stimulating factor-1 (CSF-1) receptor. The ability of over-expressed Myc and cyclin D1 to complement the mutant CSF-1 receptor Y809F (containing a Y-to-F mutation at position 809) is also inhibited by Mad1. Cell cycle analysis of proliferating 3T3 cells transfected with Mad1 demonstrates a significant decrease in the fraction of cells in the S and G2/M phases and a concomitant increase in the fraction of G1 phase cells, indicating that Mad1 negatively influences cell cycle progression from the G1 to the S phase. Mutations in Mad1 which inhibit its activity as a transcription repressor also result in loss of Mad1 cell cycle inhibitory activity. Thus, the ability of Mad1 to inhibit cell cycle progression is tightly coupled to its function as a transcriptional repressor.

Different regulation of the human thymidine kinase promoter in normal human diploid IMR-90 fibroblasts and HeLa cells

Transcriptional activation of the human thymidine kinase (hTK) promoter plays an important role in the cell cycle control of thymidine kinase expression. Using the luciferase reporter cotransfection assay, we found that the activity of the hTK promoter in IMR-90 normal human diploid fibroblasts was increased by the constitutively over-expressed cyclin A or cyclin E but not by cyclin D, suggesting that the former two cyclins may act as positive regulators for the hTK promoter. The sequence responsible for the transcriptional activation by cyclin E was identified to be located between -133 and -92 of the hTK promoter. Regulation of the hTK promoter in HeLa cells appeared to be different from that in IMR-90 fibroblasts. Firstly, the hTK promoter in HeLa was already highly activated and could not be further activated by ectopically expressed cyclin A or E. Secondly, the -133 to -92 region of the hTK promoter was important for the promoter strength in HeLa cells but not in IMR-90 cells. The steady-state levels of cyclins A and E were readily detected in HeLa cells but not in normal IMR-90 fibroblasts. Based on these results, we propose that the cellular environment of the HeLa cell allows the hTK promoter to stay fully activated for transcription regardless of ectopically expressed cyclin A or E and that transcriptional activation of thymidine kinase gene is deregulated in these tumor cells.

Transformation assays of the cell mitotic and proliferation activities of purified oncogene products

Using two direct introduction methods, DNA synthesis or cell proliferation activities of three purified proteins from E. coli, namely, human papillomavirus (HPV) E7 proteins of type 16, a mutant type 16 (24 C-G) (transformation defective) and type 6b, were measured in mouse fibroblast, C127 cells. By a microinjection method, the order of the cell mitotic indexes for the three E7 proteins as determined by 5-bromo-2'-deoxy-uridine (BrdU) staining was type 16, 6b and 16 (24 C-G). By the osmotic shock method, the 3H-TdR incorporation and coloration by (3-carboxymethoxy phenyl)-2-(4-sulfophenyl)-2H-tetolazorium (MTS) for the three proteins correlated with the pRb binding and focus forming activities previously reported (Munger et al. 1991). These results indicate that the simple osmotic shock method for direct protein introduction may be generally useful for transformation assays of oncoproteins.

Thyroid hormone receptor/c-erbA: control of commitment and differentiation in the neuronal/chromaffin progenitor line PC12

The c-erbA proto-oncogenes encode nuclear receptors for thyroid hormone (T3), a hormone intimately involved in mammalian brain maturation. To study thyroid hormone receptor (TR) action on neuronal cells in vitro, we expressed the chicken c-erbA/TR alpha-1 as well as its oncogenic variant v-erbA in the adrenal medulla progenitor cell line PC12. In the absence of T3, exogenous TR alpha-1 inhibits NGF-induced neuronal differentiation and represses neuron-specific gene expression. In contrast, TR alpha-1 allows normal differentiation and neuronal gene expression to occur in the presence of T3. Finally, TR alpha-1-expressing cells become NGF-responsive for proliferation when T3 is absent, but NGF-dependent for survival in presence of T3. A similar differentiation induction by NGF plus T3 was observed in a central nervous system-derived neuronal cell line (E 18) expressing exogenous TR alpha-1. Together with the finding that TR alpha-1 constitutively blocked dexamethasone-induced differentiation of PC12 cells into the chromaffin pathway, these results suggest that TR alpha-1 plays an important role in regulating commitment and maturation of neuronal progenitors. In contrast, the v-erbA oncogene, a mutated, oncogenic version of TR alpha-1, partially but constitutively inhibited NGF-induced neuronal differentiation of PC12 cells and potentiated dexamethasone-induced chromaffin differentiation, giving rise to an aberrant "interlineage" cell phenotype.