Ras signals to the cell cycle machinery via multiple pathways to induce anchorage-independent growth

PKC is an indispensable factor in promoting environmental toxin chromium-mediated transformation and drug resistance

Hexavalent chromium [Cr(VI)] pollution is a serious environmental problem, due to not only its toxicity but also carcinogenesis. Although studies reveal several features of Cr(VI)-induced carcinogenesis, the underlying mechanisms of how Cr(VI) orchestrates multiple mitogenic pathways to promote tumor initiation and progression remain not fully understood. Src/Ras and other growth-related pathways are shown to be key players in Cr(VI)-initiated tumor prone actions. The role of protein kinase C (PKC, an important signal transducer) in Cr(VI)-mediated carcinogenesis has not been thoroughly investigated. In this study, using human bronchial/lung epithelial cells and keratinocytes, we demonstrate that PKC activity is increased by transient or chronic Cr(VI) exposure, which plays no role in the activation of Src/Ras signaling and ROS upregulation by this metal toxin. PKC in chronic Cr(VI)-treated cells stabilizes Bcl-2 to mitigate doxorubicin (an anti-cancer drug)-mediated apoptosis. After the suppression of this kinase by GO6976 (a PKC inhibitor), the cells chronically exposed to Cr(VI) partially regain the sensitivity to doxorubicin. However, when co-suppressed PKC and Ras, the chronic Cr(VI)-treated cells become fully responsive to doxorubicin and are unable to be transformed. Taken together, our study provides a new insight into the mechanisms, in which PKC is an indispensable player and cooperates with other mitogenic pathways to achieve Cr(VI)-induced carcinogenesis as well as to establish drug resistance. The data also suggest that active PKC can serve as a potential biomarker for early detection of health damages by Cr(VI) and therapeutic target for developing new treatments for diseases caused by Cr(VI).

Oncolytic Virotherapy: The Cancer Cell Side

Simple Summary

Oncolytic viruses (OVs) are a promising immunotherapy that specifically target and kill cancer cells and stimulate anti-tumor immunity. While different OVs are endowed with distinct features, which enhance their specificity towards tumor cells; attributes of the cancer cell also critically contribute to this specificity. Such features comprise defects in innate immunity, including antiviral responses, and the metabolic reprogramming of the malignant cell. The tumorigenic features which support OV replication can be intrinsic to the transformation process (e.g., a direct consequence of the activity of a given oncogene), or acquired in the course of tumor immunoediting—the selection process applied by antitumor immunity. Oncogene-induced epigenetic silencing plays an important role in negative regulation of immunostimulatory antiviral responses in the cancer cells. Reversal of such silencing may also provide a strong immunostimulant in the form of viral mimicry by activation of endogenous retroelements. Here we review features of the cancer cell that support viral replication, tumor immunoediting and the connection between oncogenic signaling, DNA methylation and viral oncolysis. As such, this review concentrates on the malignant cell, while detailed description of different OVs can be found in the accompanied reviews of this issue.

Abstract

Cell autonomous immunity genes mediate the multiple stages of anti-viral defenses, including recognition of invading pathogens, inhibition of viral replication, reprogramming of cellular metabolism, programmed-cell-death, paracrine induction of antiviral state, and activation of immunostimulatory inflammation. In tumor development and/or immunotherapy settings, selective pressure applied by the immune system results in tumor immunoediting, a reduction in the immunostimulatory potential of the cancer cell. This editing process comprises the reduced expression and/or function of cell autonomous immunity genes, allowing for immune-evasion of the tumor while concomitantly attenuating anti-viral defenses. Combined with the oncogene-enhanced anabolic nature of cancer-cell metabolism, this attenuation of antiviral defenses contributes to viral replication and to the selectivity of oncolytic viruses (OVs) towards malignant cells. Here, we review the manners by which oncogene-mediated transformation and tumor immunoediting combine to alter the intracellular milieu of tumor cells, for the benefit of OV replication. We also explore the functional connection between oncogenic signaling and epigenetic silencing, and the way by which restriction of such silencing results in immune activation. Together, the picture that emerges is one in which OVs and epigenetic modifiers are part of a growing therapeutic toolbox that employs activation of anti-tumor immunity for cancer therapy.

Linking FOXO3, NCOA3, and TCF7L2 to Ras pathway phenotypes through a genome-wide forward genetic screen in human colorectal cancer cells

Background

The Ras pathway genes KRAS, BRAF, or ERBBs have somatic mutations in ~ 60% of human colorectal carcinomas. At present, it is unknown whether the remaining cases lack mutations activating the Ras pathway or whether they have acquired mutations in genes hitherto unknown to belong to the pathway.

Methods

To address the second possibility and extend the compendium of Ras pathway genes, we used genome-wide transposon mutagenesis of two human colorectal cancer cell systems deprived of their activating KRAS or BRAF allele to identify genes enabling growth in low glucose, a Ras pathway phenotype, when targeted.

Results

Of the 163 recurrently targeted genes in the two different genetic backgrounds, one-third were known cancer genes and one-fifth had links to the EGFR/Ras/MAPK pathway. When compared to cancer genome sequencing datasets, nine genes also mutated in human colorectal cancers were identified. Among these, stable knockdown of FOXO3, NCOA3, and TCF7L2 restored growth in low glucose but reduced MEK/MAPK phosphorylation, reduced anchorage-independent growth, and modulated expressions of GLUT1 and Ras pathway related proteins. Knockdown of NCOA3 and FOXO3 significantly decreased the sensitivity to cetuximab of KRAS mutant but not wild-type cells.

Conclusions

This work establishes a proof-of-concept that human cell-based genome-wide forward genetic screens can assign genes to pathways with clinical importance in human colorectal cancer.

Electronic supplementary material

The online version of this article (doi:10.1186/s13073-017-0511-4) contains supplementary material, which is available to authorized users.

Geminin functions downstream of p53 in K-ras-induced gene amplification of dihydrofolate reductase

DNA strand breakage and perturbation of cell-cycle progression contribute to gene amplification events that can drive cancer. In cells lacking p53, DNA damage does not trigger an effective cell-cycle arrest and in this setting promotes gene amplification. This is also increased in cells harboring oncogenic Ras, in which cell-cycle arrest is perturbed and ROS levels that cause DNA single strand breaks are elevated. This study focused on the effects of v-K-ras and p53 on Methotrexate (MTX)-mediated DHFR amplification. Rat lung epithelial cells expressing v-K-ras or murine lung cancer LKR cells harboring active K-ras continued cell-cycle progression when treated with MTX. However, upon loss of p53, amplification of DHFR and formation of MTX-resistant colonies occurred. Expression levels of cyclin A, Geminin, and Cdt1 were increased in v-K-ras transfectants. Geminin was sufficient to prevent the occurrence of multiple replications via interaction with Cdt1 after MTX treatment, and DHFR amplification proceeded in v-K-ras transfectants that possess a functional p53 in the absence of geminin. Taken together, our findings indicate that p53 not only regulates cell-cycle progression, but also functions through geminin to prevent DHFR amplification and protect genomic integrity.

Activation of the aryl-hydrocarbon receptor inhibits invasive and metastatic features of human breast cancer cells and promotes breast cancer cell differentiation

The current statistics associated with breast cancer continue to show a relatively high recurrence rate together with a poor survival for aggressive metastatic disease. These findings reflect, in part, the pharmaceutical intractability of processes involved in the metastatic process and highlight the need to identify additional drug targets for the treatment of late-stage disease. In the current study, we report that ligand activation of the aryl-hydrocarbon receptor (AhR) inhibits multiple aspects of the metastatic process in a panel of breast cancer cell lines that represent the major breast cancer subtypes. Specifically, it was observed that treatment with exogenous AhR agonists significantly inhibited cell invasiveness and motility in the Boyden chamber assay and inhibited colony formation in soft agar regardless of estrogen receptor (ER), progesterone receptor, or human epidermal growth factor receptor 2 status. Knockdown of the AhR using small interfering RNA duplexes demonstrated that the inhibition of invasiveness was receptor dependent and that endogenous receptor activity was protective in each cell type examined. The inhibition of invasiveness and anchorage-independent growth correlated with the ability of exogenous AhR agonists to promote differentiation. Finally, exogenous AhR agonists were able to promote differentiation in a putative mammary cancer stem cell line. Cumulatively, these results suggest that the AhR plays an important role in mammary epithelial differentiation and, as such, represent a promising therapeutic target for a range of phenotypically distinct human breast cancers.

Competitive nuclear export of cyclin D1 and Hic-5 regulates anchorage dependence of cell growth and survival

Anchorage dependence of cell growth and survival is a critical trait that distinguishes nontransformed cells from transformed cells. We demonstrate that anchorage dependence is determined by anchorage-dependent nuclear retention of cyclin D1, which is regulated by the focal adhesion protein, Hic-5, whose CRM1-dependent nuclear export counteracts that of cyclin D1. An adaptor protein, PINCH, interacts with cyclin D1 and Hic-5 and potentially serves as an interface for the competition between cyclin D1 and Hic-5 for CRM1. In nonadherent cells, the nuclear export of Hic-5, which is redox-sensitive, was interrupted due to elevated production of reactive oxygen species, and cyclin D1 was exported from the nucleus. When an Hic-5 mutant that was continuously exported in a reactive oxygen species-insensitive manner was introduced into the cells, cyclin D1 was retained in the nucleus under nonadherent conditions, and a significant population of cells escaped from growth arrest or apoptosis. Interestingly, activated ras achieved predominant cyclin D1 nuclear localization and thus, growth in nonadherent cells. We report a failsafe system for anchorage dependence of cell growth and survival.

p204 protein is a novel modulator of ras activity

The murine p200 family protein, p204, modulates cell proliferation and tissue differentiation. Many of its activities are exerted in the nucleus. However, in cardiac myocytes, p204 accumulated in the cytoplasm. A yeast two-hybrid assay revealed a p204-cytoplasmic Ras protein interaction. This was confirmed (i) by coimmunoprecipitation of p204 with Ras in mouse heart extract and with endogenous or ectopic H-Ras and K-Ras in cell lysates as well as (ii) by binding of purified H-Ras-GTP to purified p204 in vitro. p204 inhibited (i) the cleavage of RasGTP to RasGDP by RasGAP; (ii) the binding to RasGTP of Raf-1, phosphatidylinositol 3-kinase, and Ral-GDS, effectors of Ras signaling; and (iii) activation by the Ras pathway of the phosphorylation and thus activation of downstream targets (e.g. MEK, Akt, and p38 MAPK). Oncogenic Ras expression triggered the phosphorylation and translocation of p204 from the nucleus to the cytoplasm. This is expected to increase the interaction between the two proteins. Translocation triggered by Ras oncoprotein was blocked by the LY294002 inhibitor of phosphatidylinositol 3-kinase. Ras did not promote phosphorylation or translocation to the cytoplasm of mutated p204 in which serine 179 was replaced by alanine. p204 overexpression inhibited the anchorage-independent proliferation of cells expressing Ras(Q61L) oncoprotein. Ras oncoprotein triggered in MEF3T3 cells the rearrangement of the actin cytoskeleton and the enhancement of cell migration through a membrane. Overexpression of p204 inhibited both. Ras oncoprotein or activated, wild-type Ras was described to increase Egr-1 transcription factor expression. We report that a sequence in the gene encoding p204 bound Egr-1, and Egr-1 activated p204 expression. Ras oncoprotein or activated wild-type Ras increased the expression in 3T3 cells of p204 together with that of Egr-1. Furthermore, the activation of expression of a single copy of K-ras oncogene in cultured murine embryonic cells induced the expression of a high level of p204 as well as its distribution between the nuclei and the cytoplasm. Thus, p204 may serve as a negative feedback inhibitor of Ras activity.

Ras effector pathways modulate scatter factor-stimulated NF-kappaB signaling and protection against DNA damage

Scatter factor (SF) (hepatocyte growth factor) is a pleiotrophic cytokine that accumulates within tumors in vivo and protects tumor cells against cytotoxicity and apoptosis due to DNA damaging agents in vitro. Previous studies have established that SF-mediated cell protection involves antiapoptotic signaling from its receptor (c-Met) to PI3 kinase --> c-Akt --> Pak1 (p21-activated kinase -1) --> NF-kappaB (nuclear factor-kappa B). Here, we found that Ras proteins (H-Ras and R-Ras) enhance SF-mediated activation of NF-kappaB and protection of DU-145 and MDCK (Madin-Darby canine kidney) cells against the topoisomerase IIalpha inhibitor adriamycin. Studies of Ras effector loop mutants and their downstream effectors suggest that Ras/PI3 kinase and Ras/Raf1 pathways contribute to SF stimulation of NF-kappaB signaling and cell protection. Further studies revealed that Raf1 positively regulates the ability of SF to stimulate NF-kappaB activity and cell protection. The ability of Raf1 to stimulate NF-kappaB activity was not due to the classical Raf1 --> MEK1/2 --> ERK1/2 pathway. However, we found that a MEK3/6 --> p38 pathway contributes to SF-mediated activation of NF-kappaB. In contrast, RalA, a target of the Ras/RalGDS pathway negatively regulated the ability of SF to stimulate NF-kappaB activity and cell protection. Ras, Raf1 and RalA modulate SF stimulation of NF-kappaB activity, in part, by regulating IkappaB kinase (IKK)-beta kinase activity. These findings suggest that Ras/Raf1/RalA pathways may converge to modulate NF-kappaB activation and SF-mediated survival signaling at the IKK complex and/or a kinase upstream of this complex.

A secondary disruption of the dmpA gene encoding a large membrane protein allows aggregation defective Dictyostelium rasC- cells to form multicellular structures

The disruption of the gene encoding the Dictyostelium Ras subfamily protein, RasC, results in a strain that does not aggregate and has defects in both cAMP signal relay and cAMP chemotaxis. Disruption of a second gene in the rasC(-) strain by Restriction Enzyme Mediated Integration produced cells that were capable of forming multicellular structures in plaques on bacterial lawns. The disrupted gene (dmpA) encoded a novel membrane protein that was designated Dmp1. Although the rasC(-)/dmpA(-) cells progressed through early development, they did not form aggregation streams on a plastic surface under submerged starvation conditions. Phosphorylation of PKB in response to cAMP, which is significantly reduced in rasC(-) cells, remained low in the rasC(-)/dmpA(-) cells. However, in spite of this low PKB phosphorylation, the rasC(-)/dmpA(-) cells underwent efficient chemotaxis to cAMP in a spatial gradient. Cyclic AMP accumulation, which was greatly reduced in the rasC(-) cells, was restored in the rasC(-)/dmpA(-) strain, but cAMP relay in these cells was not apparent. These data indicate that although the rasC(-)/dmpA(-) cells were capable of associating to form multicellular structures, normal aggregative cell signaling was clearly not restored. Disruption of the dmpA gene in a wild-type background resulted in cells that exhibited a slight defect in aggregation and a more substantial defect in late development. These results indicate that, in addition to the role played by Dmp1 in aggregation, it is also involved in late development.

The effect of the disruption of a gene encoding a PI4 kinase on the developmental defect exhibited by Dictyostelium rasC(-) cells

The disruption of the gene encoding the Dictyostelium Ras subfamily protein, RasC results in a strain that fails to aggregate with defects in both cAMP signal relay and chemotaxis. Restriction enzyme mediated integration disruption of a second gene in the rasC(-) strain resulted in cells that were capable of forming multicellular structures in plaques on bacterial lawns. The disrupted gene, designated pikD(1), encodes a member of the phosphatidyl-inositol-4-kinase beta subfamily. Although the rasC(-)/pikD(1) cells were capable of progressing through early development, when starved on a plastic surface under submerged conditions, they did not form aggregation streams or exhibit pulsatile motion. The rasC(-)/pikD(1) cells were extremely efficient in their ability to chemotax to cAMP in a spatial gradient, although the reduced phosphorylation of PKB in response to cAMP observed in rasC(-) cells, was unchanged. In addition, the activation of adenylyl cyclase, which was greatly reduced in the rasC(-) cells, was only minimally increased in the rasC(-)/pikD(1) strain. Thus, although the rasC(-)/pikD(-) cells were capable of associating to form multicellular structures, normal cell signaling was clearly not restored. The disruption of the pikD gene in a wild type background resulted in a strain that was delayed in aggregation and formed large aggregation streams, when starved on a plastic surface under submerged conditions. This strain also exhibited a slight defect in terminal development. In conclusion, disruption of the pikD gene in a rasC(-) strain resulted in cells that were capable of forming multicellular structures, but which did so in the absence of normal signaling and aggregation stream formation.

Krüppel-like factor 5 promotes mitosis by activating the cyclin B1/Cdc2 complex during oncogenic Ras-mediated transformation

We previously showed that the zinc finger-containing transcription factor Krüppel-like factor 5 (KLF5) is important in mediating transformation by oncogenic H-Ras through induction of cyclin D1 expression and acceleration of the G1/S transition of the cell cycle. Here we present evidence of a role for KLF5 in accelerating mitotic entry in H-Ras-transformed NIH3T3 fibroblasts. When compared with non-transformed parental NIH3T3 cells, H-Ras-transformed fibroblasts exhibit an increase in mitotic index, levels of cyclin B1 and Cdc2, and cyclin B1/Cdc2 kinase activity. Inhibition of KLF5 expression in H-Ras-transformed cells with KLF5-specific small interfering RNA (siRNA) results in a decrease in each of the aforementioned parameters, with a concomitant reduction in the transforming potential of the cells. Conversely, over-expression of KLF5 in NIH3T3 cells leads to an increase in the promoter activity of the genes encoding cyclin B1 and Cdc2. These results indicate that KLF5 accelerates mitotic entry in H-Ras-transformed cells by transcriptionally activating cyclin B1 and Cdc2, which leads to an increase in cyclin B1/Cdc2 kinase activity. Extending our previous observation that KLF5 activates cyclin D1 transcription to promote G1/S transition, our current results further support a crucial function for KLF5 in mediating cellular transformation caused by oncogenic H-Ras.

Effects of HRAS oncogene on cell cycle progression in a cervical cancer-derived cell line

BACKGROUND

Human papillomavirus (HPV) infection is the most prevalent factor in anogenital cancers. However, epidemiological surveys and molecular data indicate that viral presence is not enough to induce cervical cancer, suggesting that cellular factors could play a key role. One of the most important genes involved in cancer development is the RAS oncogene, and activating mutations in this gene have been associated with HPV infection and cervical neoplasia. Thus, we determined the effect of HRAS oncogene expression on cell proliferation in a cell line immortalized by E6 and E7 oncogenes.

METHODS

HPV positive human cervical carcinoma-derived cell lines (HeLa), previously transfected with the HRAS oncogene or the empty vector, were used. We first determined the proliferation rate and cell cycle profile of these cells by using flow cytometry and BrdU incorporation assays. In order to determine the signaling pathway regulated by HRAS and implicated in the alteration of proliferation of these cells, we used specific chemical inhibitors to inactivate the Raf and PI3K pathways.

RESULTS

We observed that HeLa cells stably transfected with oncogenic HRAS progressed faster than control cells on the cell cycle by reducing their G1 phase. Additionally, HRAS overexpression accelerated the G1/S transition. Specific chemical inhibitors for PI3K and MEK activities indicated that both PI3K/AKT and RAF/MEK/ERK pathways are involved in the HRAS oncogene-induced reduction of the G1 phase.

CONCLUSIONS

Our results suggest that the HRAS oncogene could play an important role in the development of cervical cancer, in addition to the presence of HPV, by reducing the G1 phase and accelerating the G1/S transition of infected cells.

Cyclin A is a c-Jun target gene and is necessary for c-Jun-induced anchorage-independent growth in RAT1a cells

Overexpression of c-Jun enables Rat1a cells to grow in an anchorage-independent manner. We used an inducible c-Jun system under the regulation of doxycycline in Rat1a cells to identify potential c-Jun target genes necessary for c-Jun-induced anchorage-independent growth. Induction of c-Jun results in sustained expression of cyclin A in the nonadherent state with only minimal expression in the absence of c-Jun. The promoter activity of cyclin A2 was 4-fold higher in Rat1a cells in which c-Jun expression was induced compared with the control cells. Chromatin immunoprecipitation demonstrated that c-Jun bound directly to the cyclin A2 promoter. Mutation analysis of the cyclin A2 promoter mapped the c-Jun regulatory site to an ATF site at position -80. c-Jun was able to bind to this site both in vitro and in vivo, and mutation of this site completely abolished promoter activity. Cyclin A1 was also elevated in c-Jun-overexpressing Rat1a cells; however, c-Jun did not regulate this gene directly, since it did not bind directly to the cyclin A1 promoter. Suppression of cyclin A expression via the introduction of a cyclin A antisense sequences significantly reduced the ability of c-Jun-overexpressing Rat1a cells to grow in an anchorage-independent fashion. Taken together, these results suggest that cyclin A is a target of c-Jun and is necessary but not sufficient for c-Jun-induced anchorage-independent growth. In addition, we demonstrated that the cytoplasmic oncogenes Ras and Src transcriptionally activated the cyclin A2 promoter via the ATF site at position -80. Using a dominant negative c-Jun mutant, TAM67, we showed that this transcriptional activation of cyclin A2 requires c-Jun. Thus, our results suggest that c-Jun is a mediator of the aberrant cyclin A2 expression associated with Ras/Src-induced transformation.

Long-term exposure to nicotine, via ras pathway, induces cyclin D1 to stimulate G1 cell cycle transition

Nicotine, a major component in tobacco, has been implicated as a potential factor that promotes the development of lung cancer. However, the molecular mechanism of its action is still unclear. In this study, we have shown that, via nicotinic acetylcholine receptors, persistent exposure of mouse epithelial cells to nicotine elicits Ras signaling and subsequent Raf/MAP kinase activity, accompanied by a significant increase in cyclin D1 promoter activity and its protein expression. AP-1 is required for activation of the cyclin D1 promoter. The induction of cyclin D1 expression and its promoter activity by nicotine is abolished by the suppression of Raf/MAP kinase signaling. Furthermore, upon nicotine treatment, the cells do not arrest in the G(1) phase of the cell cycle following serum starvation. The perturbation of the G(1) cell cycle checkpoint is caused by the deregulation of retinoblastoma/E2F activity. Therefore, our data indicated that by targeting the Ras pathway, long-term exposure to nicotine disrupts cell cycle restriction machinery and thus potentiates tumor development.

Krüppel-like factor 5 mediates the transforming activity of oncogenic H-Ras

Previous studies indicate that Krüppel-like factor 5 (KLF5), also known as intestinal-enriched Krüppel-like factor (IKLF), is a positive regulator of cell proliferation and gives rise to a transformed phenotype when overexpressed. Here we demonstrate that levels of KLF5 transcript and protein are significantly elevated in oncogenic H-Ras-transformed NIH3T3 cells. These cells display an accelerated rate of proliferation in both serum-containing and serum-deprived media and form anchorage-independent colonies in soft agar assays. H-Ras-transformed cells also contain elevated mitogen-activated protein kinase (MAPK) activity. When treated with inhibitors of MEK (MAPK kinase), H-Ras-transformed cells lose their growth advantage and no longer form colonies. Significantly, levels of KLF5 transcript and protein are substantially reduced in H-Ras-transformed cells treated with MEK inhibitors. Moreover, inhibition of KLF5 expression in H-Ras-transformed cells with KLF5-specific small interfering RNA (siRNA) leads to a decreased rate of proliferation and a significant reduction in colony formation. H-Ras-transformed cells also contain elevated levels of Egr1 that are diminished by MEK inhibitors. Inhibition of Egr1 by siRNA results in a reduced level of KLF5, indicating that Egr1 mediates the inductive action of MAPK on KLF5. Lastly, KLF5 activates expression of cyclin D1. These findings indicate that the increased expression of KLF5 in H-Ras-transformed cells is secondary to increased MAPK activity from H-Ras overexpression and that the elevated level of KLF5 is in part responsible for the proproliferative and transforming activities of oncogenic H-Ras.

C3G-mediated suppression of oncogene-induced focus formation in fibroblasts involves inhibition of ERK activation, cyclin A expression and alterations of anchorage-independent growth

We showed previously that exogenous overexpression of C3G, a guanine nucleotide releasing factor (GEF) for Rap1 and R-Ras proteins, blocks the focus-forming activity of cotransfected, activated, sis, ras and v-raf oncogenes in NIH 3T3 cells. In this report, we show that C3G also interferes with dbl and R-Ras focus-forming activity and demonstrate that the transformation suppressor ability of C3G maps to its Crk-binding region (SH3-b domain). Using full-length C3G and C3GDeltaCat mutant, lacking catalytic domain, we showed here that overexpression of cotransfected C3G or C3GDeltaCat inhibited oncogenic Hraslys12-mediated phosphorylation of ERK, without altering Ras and Raf-1 kinase activation. We also showed that, overexpressed C3G and C3GdeltaCat inhibited the viability of oncogenic Ras-induced colonies in soft agar, indicating that C3G interferes with the anchorage-independent growth of Ras-transformed cells in a Rap1-independent manner. Consistent with both observations, overexpression of exogenous C3G and C3GDeltaCat also caused downregulation of Ras-induced cyclin A expression. Altogether, our results indicate that C3G interferes with at least two separate aspects of oncogenic transformation - cell cycle progression and loss of contact inhibition - and that these inhibitory effects probably account for its transformation suppressor activity.

The molecular scaffold KSR1 regulates the proliferative and oncogenic potential of cells

The specificity of signaling through mitogen-activated protein kinase pathways has been attributed to both the control of intensity and duration of signaling and the actions of protein scaffolds. Here we demonstrate that the molecular scaffold KSR1 regulates the intensity and duration of ERK activation to modulate a cell's proliferative and oncogenic potential. Deletion of KSR1 eliminates the prolonged phase of ERK activation induced by platelet-derived growth factor and blocks Ras(V12)-induced transformation. The introduction of KSR1 into KSR1(-/-) mouse embryo fibroblasts causes a concentration-dependent increase in signaling and transformation, to a maximum at 14 times the wild-type KSR1 expression levels, but inhibits these responses at higher expression levels. An increase in KSR1 expression to levels that are optimal for signaling leads to a threefold increase in proliferative capacity and is coincident with the level of KSR1 expression that maximally associates with all members of the Raf/MEK/ERK cascade. These data reveal that cells contain a reserve proliferative capacity that is accessible by the optimal expression of a noncatalytic signaling component and that altering the expression level of a molecular scaffold can modulate the actions of growth factors and oncogenes.

Chemoattractant-induced Ras activation during Dictyostelium aggregation

Ras proteins are highly conserved molecular switches that regulate cellular response to external stimuli. Dictyostelium discoideum contains an extensive family of Ras proteins that function in regulation of mitosis, cytoskeletal function and motility, and the onset of development. Little is known about the events that lead to the activation of Ras proteins in Dictyostelium, primarily owing to a lack of a biochemical assay to measure the levels of activated Ras. We have adapted an assay, used successfully to measure activated Ras in mammalian cells, to monitor activation of two Dictyostelium Ras proteins, RasC and RasG. We have found that the Ras-binding domain (RBD) of mammalian Raf1 was capable of binding to the activated form of RasG, but not to the activated form of RasC; however, the RBD of Schizosaccharomyces pombe Byr2 was capable of binding preferentially to the activated forms of both RasC and RasG. Using this assay, we discovered that RasC and RasG showed a rapid and transient activation when aggregation-competent cells were stimulated with the chemoattractant cAMP, and this activation did not occur in a number of cAMP signalling mutants. These data provide further evidence of a role for both RasC and RasG in the early development of Dictyostelium.

Molecular regulation of angiogenesis and tumorigenesis by signal transduction pathways: evidence of predictable and reproducible patterns of synergy in diverse neoplasms

A large number of oncogenes, tumor suppressor genes, and signal transduction pathways have been described. Currently, a framework that allows prediction of tumor behavior based upon oncogenes, tumor suppressors, and signal transduction pathways is lacking. In 1869, Mendeleev published a periodic table of elements which allowed prediction of properties of elements based upon atomic weights that allowed prediction of chemical and physical properties of elements yet to be discovered. In this paper, I will discuss recurrent patterns of synergy found in the literature and our laboratory between tumor suppressor genes, oncogenes, and signaling pathways that allows one to predict the signaling pathway in a given tumor based upon the inactivation of a tumor suppressor gene. These patterns can be found in multiple different human neoplasms. Conversely, one can predict the inactivation of a tumor suppressor based upon the activation status of a signaling pathway. This knowledge can be used by a clinician or pathologist with access to immunohistochemistry to make predictions based upon simple technologies and determine the signaling pathways involved in a patient's tumor. These strategies may be useful in the design of prevention and treatment strategies for cancer.

Identifying the factors and signal pathways necessary for anchorage-independent growth of Ha-ras oncogene-transformed NIH/3T3 cells

Ha-ras(Val 12) overexpression was positively correlated with colony formation by NIH/3T3 derivative "2-12" cells harboring an inducible Ha-ras(Val 12) transgene. The ras-farnesylation inhibitor, Lovastatin, completely suppressed colony formation at higher dosages. However, Ha-ras oncogene overexpression alone could not stimulate colony formation under serum-deprived conditions, suggesting that ras is required but not sufficient for supporting colony formation. Substituting cow colostrum (AC-2) for serum did not result in colony formation from 2-12 cells in soft agar, suggesting the colostrum lacked or contained insufficient amounts of factors that stimulate colony formation. Supplementation of AC-2-containing medium with growth factors, such as insulin-like growth factor-1 (IGF-1), partially restored the capability of anchorage-independent cell growth induced by Ha-ras overexpression. Consistently, antibodies specific for IGF-1 receptors only partially blocked colony formation from 2-12 cells. The data indicate that multiple factors, including IGF-1, are required for Ha-ras-dependent colony formation. Signal transduction studies revealed that, under Ha-ras overexpression conditions, IGF-1 utilizes phosphatidyl inositol 3-kinase and NF-kappaB to transduce colony formation-related signaling.

fau and its ubiquitin-like domain (FUBI) transforms human osteogenic sarcoma (HOS) cells to anchorage-independence

Arsenite is the most likely carcinogenic form of arsenic in the environment. Previously, expression cloning for cDNAs whose overexpression confers arsenite-resistance in Chinese hamster V79 cells identified two genes: fau and a novel gene, asr2. The fau gene encodes a ubiquitin-like protein (here called FUBI) fused to the ribosomal S30 protein. Since the expression of the fox sequence (antisense to fau) increased the tumorigenicity of a mouse sarcoma virus, it was proposed that fau might be a tumor suppressor gene. We intended to test its ability to block arsenite-induced transformation of human osteogenic sarcoma (HOS) cells to anchorage-independence. Instead, we found that overexpressing fau itself was able to transform HOS cells. When the two domains were expressed separately, only FUBI was transforming and only the S30 domain conferred arsenite resistance. An incidental finding was the transforming activity of the selectable marker, hyg. FUBI belongs to the ubiquitin-like protein group that is capable of forming conjugates to other proteins, although none have so far been identified. Alternatively, FUBI may act as a substitute or inhibitor of ubiquitin, to which it is most closely related, or to close ubiquitin-like relatives UCRP, FAT10, and/or Nedd8.

A novel zinc finger protein, ZZaPK, interacts with ZAK and stimulates the ZAK-expressing cells re-entering the cell cycle

ZAK has been implicated in cell cycle arrest regulation through its function on decreasing cyclin E expression. To explore the mechanistic basis for this regulation, the yeast two-hybrid system was used with a novel Krüppel-type C2H2 zinc finger member cloned. This cloned cDNA encodes a novel protein with Krüppel-type zinc fingers designed as ZZaPK (zinc finger and ZAK associated protein with KRAB domain) and is widely expressed. ZZaPK, when it is expressed in cells, is growth promoted and might lead to increasing E2F expression and induce cyclin E/CDK2 activity, which counteracts the ZAK function. The model proposed here is that ZAK might play a role as an upstream signal to suppress the ZZaPK function and decrease E2F expression.

Arsenite induces delayed mutagenesis and transformation in human osteosarcoma cells at extremely low concentrations

Arsenite is a human multisite carcinogen, but its mechanism of action is not known. We recently found that extremely low concentrations (</=0.1 microM) of arsenite transform human osteosarcoma TE85 (HOS) cells to anchorage-independence. In contrast to other carcinogens which transform these cells within days of exposure, almost 8 weeks of arsenite exposure are required for transformation. We decided to reexamine the question of arsenite mutagenicity using chronic exposure in a spontaneous mutagenesis assay we previously developed. Arsenite was able to cause a delayed increase in mutagenesis at extremely low concentrations (</=0.1 microM) in a dose-dependent manner. The increase in mutant frequency occurred after almost 20 generations of growth in arsenite. Transformation required more than 30 generations of continuous exposure. We also found that arsenite induced gene amplification of the dihydrofolate reductase (DHFR) gene in a dose-dependent manner. Since HOS cells are able to methylate arsenite at a very low rate, it was possible that active metabolites such as monomethylarsonous acid (MMA(III)) contributed to the delayed mutagenesis and transformation in these cells. However, when the assay was repeated with MMA(III), we found no significant increase in mutagenesis or transformation, suggesting that arsenite-induced delayed mutagenesis and transformation are not caused by arsenite's metabolites, but by arsenite itself. Our results suggest that long-term exposure to low concentrations of arsenite may affect signaling pathways that result in a progressive genomic instability.

Mixed lineage kinase ZAK utilizing MKK7 and not MKK4 to activate the c-Jun N-terminal kinase and playing a role in the cell arrest

The leucine-zipper (LZ) and sterile-alpha motif (SAM) kinase (ZAK) belongs to the MAP kinase kinase kinase (MAP3K) when upon over-expression in mammalian cells activates the JNK/SAPK pathway. The mechanisms by which ZAK activity is regulated are not well understood. Co-expression of dominant-negative MKK7 but not MKK4 and ZAK significantly attenuates JNK/SAPK activation. This result suggests that ZAK activates JNK/SAPK mediated by downstream target, MKK7. Expression of ZAK but not kinase-dead ZAK in 10T1/2 cells results in the disruption of actin stress fibers and morphological changes. Therefore, ZAK activity may be involved in actin organization regulation. Expression of wild-type ZAK increases the cell population in the G(2)/M phase of the cell cycle, which may indicate G(2) arrest. Western blot analysis shows that the decreased cyclin E level correlated strongly with the low proliferative capacity of ZAK-expressed cells.

High expression of Met/hepatocyte growth factor receptor suppresses tumorigenicity in NCI-H1264 lung carcinoma cells

The protein product of c-met proto-oncogene, Met, is a tyrosine kinase receptor for the hepatocyte growth factor (HGF). Met receptor is expressed in normal human bronchial epithelium. In comparison, its expression in squamous cell carcinoma (SQCC) of the lung is markedly decreased in a great majority of cases. To understand further the role of Met receptor overexpression in non-small-cell lung carcinoma, we forced-expressed the full-length met cDNA in the NCI-H1264 (H1264) lung carcinoma cell line with low constitutive expression of this receptor. In vitro studies demonstrated that increased Met expression in H1264 cells resulted in strong inhibition of their ability to form soft agar colonies and in marked suppression of tumorigenicity in the subcutaneous tissue of immune-deficient mice. This is despite inconsistent alteration in the proliferation rate on plastic surfaces. Tumor cells explanted from occasional xenograft tumors formed by the Met-overexpressing H1264 cells also demonstrated marked down-regulation of the receptor protein levels as compared to the transplanted cells. The results suggest that constitutive overexpression of Met receptor may negatively regulate the malignancy of certain human lung cancer cells.

Ras and Rho regulation of the cell cycle and oncogenesis

The important contribution of aberrant Ras activation in oncogenesis is well established. Our knowledge of the signaling pathways that are regulated by Ras is considerable. However, the number of downstream effectors of Ras continues to increase and our understanding of the role of these effector signaling pathways in mediating oncogenesis is far from complete and continues to evolve. Similarly, our understanding of the components that control mitogen-stimulated cell cycle progression is also very advanced. Where our understanding has lagged has been the delineation of the mechanism by which Ras causes a deregulation of cell cycle progression to promote the uncontrolled proliferation of the cancer cell. In this review, we summarize our current knowledge of how deregulated Ras activation alters the function of cyclin D1, p21(Cip1), and p27(Kip1). The two themes that we have emphasized are the involvement of Rho small GTPases in cell cycle regulation and the cell-type differences in how Ras signaling interfaces with the cell cycle machinery.

Ras adenoviruses modulate cyclin E protein expression and DNA synthesis after partial hepatectomy

Ras-genes encode for proteins important for transmitting extracellular signals from the cytoplasm to the nucleus. In this study we investigated the impact of Ras on cell cycle progression after hepatectomy by using adenoviral vectors (adv) expressing beta-galactosidase (beta-gal), a dominant-negative (Ras N17) or a dominant-active (Ras 61L) form of H-Ras. Partial hepatectomy was performed in mice treated with the different adenoviruses and cell cycle progression was studied by analysing factors involved in cell cycle control during liver regeneration. After hepatectomy, adv Ras 61L increases DNA synthesis significantly in comparison to the other treatment groups. Higher Ras activity results in an early increase of transcriptional active E2F-3, which is associated with higher cyclin E, but almost unchanged cyclin D protein expression. However, Northern blot analysis and cyclin E promoter experiments indicate that, besides transcriptional mechanisms also post-transcriptional mechanisms are involved in regulating cyclin E protein expression after partial hepatectomy in mice treated with adv Ras 61L. Cyclin E phosphorylation studies demonstrate that adv Ras 61L results in hypophosphorylation of cyclin E compared to the control group at early time points after hepatectomy, when cyclin E protein expression strongly increases and there is only a minor effect on cyclin E mRNA levels. Our experiments indicate adv Ras 61L in vivo increases Cyclin E expression by higher transcription via E2F and a post-transcriptional mechanism. These mechanisms result in an earlier activation of an active CDK2/Cyclin E complex which, in turn, triggers DNA synthesis.

The small-GTPase RalA activates transcription of the urokinase plasminogen activator receptor (uPAR) gene via an AP1-dependent mechanism

The urokinase plasminogen activator receptor (uPAR) focuses extracellular protease activity to the cell surface, modulates cell adhesion and activates intracellular signal transduction pathways. In a range of cancers uPAR expression often has a negative correlation with prognosis. Here we show that uPAR transcription is stimulated by V12 H-Ras, the effector loop mutant V12 H-Ras G37 and constitutively-active RalA 72L. RalA-dependent transcription required the presence of the ATF2-like AP1-site at -70 bp and the c-Jun binding motif at -184 bp in the uPAR promoter. Consistent with this, both Gal4-c-Jun- and Gal4-ATF2-fusion proteins were activated by RalA signalling through phosphorylation of their activation domains at Ser63 and Ser73 of c-Jun or Thr69 and Thr71 of ATF2. A transdominant inhibitory mutant of c-Jun N-terminal kinase (JNK) failed to inhibit uPAR transcription demonstrating that JNK activation is not a prerequisite for RalA-dependent uPAR transcription. A dominant negative inhibitor of c-Src effectively inhibited RalA-dependent uPAR transcription identifying it as a downstream effector in the RalA signalling pathway. These data provide evidence for the existence of a novel signalling pathway that links RalA to the activation of uPAR transcription via a c-Src intermediate and activation of AP1.

Suppression of ras-mediated transformation and inhibition of tumor growth and angiogenesis by anthrax lethal factor, a proteolytic inhibitor of multiple MEK pathways

Lethal factor is a protease, one component of Bacillus anthracis exotoxin, which cleaves many of the mitogen-activated protein kinase kinases (MEKs). Given the importance of MEK signaling in tumorigenesis, we assessed the effects of anthrax lethal toxin (LeTx) on tumor cells. LeTx was very effective in inhibiting mitogen-activated protein kinase activation in V12 H-ras-transformed NIH 3T3 cells. In vitro, treatment of transformed cells with LeTx caused them to revert to a nontransformed morphology, and inhibited their abilities to form colonies in soft agar and to invade Matrigel without markedly affecting cell proliferation. In vivo, LeTx inhibited growth of ras-transformed cells implanted in athymic nude mice (in some cases causing tumor regression) at concentrations that caused no apparent animal toxicity. Unexpectedly, LeTx also greatly decreased tumor neovascularization. These results demonstrate that LeTx potently inhibits ras-mediated tumor growth and is a potential antitumor therapeutic.

Ral promotes anchorage-independent growth of a human fibrosarcoma, HT1080

Ral has been shown to act downstream of Ras oncoprotein. However, the role of Ral in Ras-induced cellular transformation has not been fully understood. To test the involvement of Ral in Ras-induced anchorage-independent growth, we ectopically expressed Ral mutants in HT1080 cells, whose ability to grow in the absence of anchorage depends on the oncogenic mutation of N-ras. Expression of an activated mutant of Ral resulted in enhanced growth of HT1080 cells in soft agar, whereas a dominant-negative mutant of Ral inhibited their anchorage-independent growth. Moreover, the activated Ral mutant decreased the amount of p27(Kip1) in the absence of adhesion, while the dominant-negative mutant increased it. These results suggest that Ral is involved in the Ras-dependent anchorage-independent growth of HT1080 cells by regulating p27(Kip1).

Ras inactivation of the retinoblastoma pathway by distinct mechanisms in NIH 3T3 fibroblast and RIE-1 epithelial cells

Although Ras and Raf cause transformation of NIH 3T3 fibroblasts, only Ras causes transformation of RIE-1 intestinal epithelial cells. To determine if the inability of Raf to transform RIE-1 cells is due to a failure to deregulate cell cycle progression, we evaluated the consequences of sustained Ras and Raf activation on steady state levels of cyclin D1, p21(CIP/WAF), and p27(KIP1). Both Ras- and Raf-transformed NIH 3T3 cells showed up-regulated expression of cyclin D1, p21, and p27 protein, increased retinoblastoma (Rb) hyperphosphorylation, and increased activation of E2F-mediated transcription. Similarly, Ras-transformed RIE-1 cells also showed up-regulation of cyclin D1, p21, and hyperphosphorylated Rb. In contrast, Ras-mediated down-regulation of p27 was seen in RIE-1 cells. Conversely, stable expression of activated Raf alone caused only a partial up-regulation of p21 and Rb hyperphosphorylation but no activation of E2F-responsive transcription or down-regulation of p27 in RIE-1 cells. Moreover, we found that the AP-1 site was dispensable for Ras-mediated stimulation of the cyclin-D1 promoter in NIH 3T3 cells but was essential for Ras-mediated stimulation in RIE-1 cells. Thus, up-regulation of p21, rather than the down-regulation seen in previous transient expression studies, is seen with sustained Ras activation. Additionally, p27 may serve a positive (NIH 3T3) or negative (RIE-1) regulatory role in Ras transformation that is cell type-dependent. The involvement of Raf and phosphatidylinositol 3-kinase in mediating Ras changes in cyclin D1, p21, and p27 was also very distinct in NIH 3T3 and RIE-1 cells. Taken together, these results demonstrate the importance of Raf-independent pathways in mediating oncogenic Ras deregulation of cell cycle progression in epithelial cells.

Reversal of the Ras-induced transformed phenotype by HR12, a novel ras farnesylation inhibitor, is mediated by the Mek/Erk pathway

We have used the selective farnesylation inhibitor HR12 [cysteine-N(methyl)valine-N(cyclohexyl) glycine-methionine-O-methyl-ester] to study the role of oncogenic Ras in cytoskeletal reorganization in Ha-ras(V12)-transformed Rat1 cells (Rat1/ras). Application of HR12 resulted in complete restoration of the cytoskeleton and associated cell adhesions disrupted by oncogenic Ras. This included an increase in the number and size of focal adhesions, accompanied by massive stress fiber formation and enhanced tyrosine phosphorylation. Furthermore, HR12 induced assembly of adherens junctions and dramatically elevated the level of the junctional components, cadherin and beta-catenin. HR12 was unable to restore the nontransformed phenotype in cells expressing farnesylation-independent, myristylated Ras. Examination of the main Ras-regulated signaling pathways revealed that HR12 induced a dose- and time-dependent decline in Erk1&2 activation (t(1/2) approximately 6 h), which correlated with the accumulation of nonfarnesylated oncogenic-Ras. Inhibition of the Mek/Erk pathway in Rat1/ras cells, using the Mek inhibitor, PD98059, resulted in complete cytoskeletal recovery, indistinguishable from that induced by HR12. Moreover, a constitutively active Mek mimicked the effect of ras transformation in Rat1 cells, and prevented HR12-induced cytoskeletal effects in Rat1/ras cells. No such effects were observed after treatment of Rat1/ras cells with the phosphatidylinositol 3-kinase inhibitor LY294002. These findings establish the Mek/Erk pathway as the dominant pathway involved in conferring the cytoskeletal and junctional manifestations of the Ras-induced transformed phenotype.

Ral GTPases contribute to regulation of cyclin D1 through activation of NF-kappaB

Ral GTPases have been implicated as mediators of Ras-induced signal transduction from observations that Ral-specific guanine nucleotide exchange factors associate with Ras and are activated by Ras. The cellular role of Ral family proteins is unclear, as is the contribution that Ral may make to Ras-dependent signaling. Here we show that expression of activated Ral in quiescent rodent fibroblasts is sufficient to induce activation of NF-kappaB-dependent gene expression and cyclin D1 transcription, two key convergence points for mitogenic and survival signaling. The regulation of cyclin D1 transcription by Ral is dependent on NF-kappaB activation and is mediated through an NF-kappaB binding site in the cyclin D1 promoter. Ral activation of these responses is likely through an as yet uncharacterized effector pathway, as we find activation of NF-kappaB and the cyclin D1 promoter by Ral is independent of association of Ral with active phospholipase D1 or Ral-binding protein 1, two proteins proposed to mediate Ral function in cells.

Cyclin A2 transcriptional regulation: modulation of cell cycle control at the G1/S transition by peripheral cues

Several types of cyclins have been identified and among these, cyclin A2 is synthesized in somatic cells at the onset of DNA synthesis as well as during the G2/M transition associated with cyclin-dependent protein kinases 1 and 2. Modulation of cyclin A transcription is due to the interplay between a cell cycle-dependent periodic relief of a transcriptional repression and signals transduced through adenosine 3',5'-cyclic monophosphate, transforming growth factor-beta, and the integrin-mediated pathways. Using primary mouse embryonic fibroblasts from embryos where the genes coding for the protein responsible for susceptibility to retinoblastoma (pRB) and the related p107 and p130 proteins had been individually inactivated, we showed that cyclin A is a functional target of pRB-mediated cell cycle arrest. The factors involved are discussed.

Induction of postmitotic neuroretina cell proliferation by distinct Ras downstream signaling pathways

Ras-induced cell transformation is mediated through distinct downstream signaling pathways, including Raf, Ral-GEFs-, and phosphatidylinositol 3-kinase (PI 3-kinase)-dependent pathways. In some cell types, strong activation of the Ras-Raf-MEK-extracellular signal-regulated kinase (ERK) cascade leads to cell cycle arrest rather than cell division. We previously reported that constitutive activation of this pathway induces sustained proliferation of primary cultures of postmitotic chicken neuroretina (NR) cells. We used this model system to investigate the respective contributions of Ras downstream signaling pathways in Ras-induced cell proliferation. Three RasV12 mutants (S35, G37, and C40) which differ by their ability to bind to Ras effectors (Raf, Ral-GEFs, and the p110 subunit of PI 3-kinase, respectively) were able to induce sustained NR cell proliferation, although none of these mutants was reported to transform NIH 3T3 cells. Furthermore, they all repressed the promoter of QR1, a neuroretina growth arrest-specific gene. Overexpression of B-Raf or activated versions of Ras effectors Rlf-CAAX and p110-CAAX also induced NR cell division. The mitogenic effect of the RasC40-PI 3-kinase pathway appears to involve Rac and RhoA GTPases but not the antiapoptotic Akt (protein kinase B) signaling. Division induced by RasG37-Rlf appears to be independent of Ral GTPase activation and presumably requires an unidentified mechanism. Activation of either Ras downstream pathway resulted in ERK activation, and coexpression of a dominant negative MEK mutant or mKsr-1 kinase domain strongly inhibited proliferation induced by the three Ras mutants or by their effectors. Similar effects were observed with dominant negative mutants of Rac and Rho. Thus, both the Raf-MEK-ERK and Rac-Rho pathways are absolutely required for Ras-induced NR cell division. Activation of these two pathways by the three distinct Ras downstream effectors possibly relies on an autocrine or paracrine loop, implicating endogenous Ras, since the mitogenic effect of each Ras effector mutant was inhibited by RasN17.

Cloning and expression of ZAK, a mixed lineage kinase-like protein containing a leucine-zipper and a sterile-alpha motif

A novel mixed lineage kinase-like protein ZAK, containing a leucine-zipper (LZ) and a sterile-alpha motif (SAM), was cloned. This cDNA has 2456 bp and encodes a protein of 800 amino acids that contains a kinase catalytic domain, a leucine-zipper and a SAM. The molecular weight of this protein is 91kDa. Northern blot analysis revealed that the expression of this ZAK gene is found in various parts of human tissues. We also found that ZAK proteins might form homodimers or oligomers in mammalian cells. MLKs have been proposed to function as mitogen-activated protein kinase kinase kinase in pathways leading to MAPK cascade. The expression of ZAK in mammalian cells specifically leads to the activation of the JNK/SAPK pathway as well as the activation of transcription factor, NF-kappaB. Overexpression of the ZAK gene induces the apoptosis of a hepatoma cell line.

Full oncogenic activities of v-Src are mediated by multiple signaling pathways. Ras as an essential mediator for cell survival

Tyrosine kinase oncoproteins cause simultaneous activation of multiple intracellular signaling pathways. However, the precise mechanisms by which individual pathways induce oncogenesis are not well understood. We have investigated the roles of individual signaling pathways in v-Src-dependent cell growth and survival by inhibiting one particular pathway. v-Src induced constitutive activation of signal transducers and activators of transcription 3 (STAT3), phosphatidylinositol 3-kinase, and Ras in murine Ba/F3 cells and led to factor-independent proliferation. Dominant-negative mutants of STAT3 (STAT3D) and phosphatidylinositol 3-kinase (Deltap85) inhibited v-Src-dependent growth by approximately 60 and approximately 40%, respectively. Moreover, dominant-negative Ras (N17) induced severe apoptosis, which was accompanied by down-regulation of Bcl-2 and activation of caspase-3. Although cells overexpressing Bcl-2 or caspase-3 inhibitors remained viable even when N17 was expressed, the growth was reduced by approximately 85%. During N17- and STAT3D-induced growth suppression, expression of cyclin D2, cyclin D3, c-myc, and c-fos was suppressed by N17, whereas that of cyclin D2, cyclin E, and c-myc was suppressed by STAT3D. Thus, v-Src-activated Ras and STAT3 are involved in distinct but partly overlapping transcriptional regulation of cell cycle regulatory molecules. These results suggest that the full oncogenic activity of v-Src requires simultaneous activation of multiple signalings, in which Ras is particularly required for survival.

Regulation of cyclin-dependent kinase (Cdk) 2 Thr-160 phosphorylation and activity by mitogen-activated protein kinase in late G1 phase

Mitogen-activated protein (MAP) kinases, p42(MAPK) and p44(MAPK), are central components of growth-promoting signalling pathways. However, how stimulation of MAP kinases culminates in cell-cycle progression is still poorly understood. Here we show that mitogenic stimulation of NIH 3T3 cells causes a sustained activation of MAP kinases, which lasts until cells begin progressing through the G(1)/S boundary. Furthermore, we observed that disruption of the MAP-kinase pathway with a selective MEK (MAP kinase/extracellular-signal-regulated protein kinase kinase) inhibitor, PD98059, prevents the activation of cyclin-dependent kinase (Cdk) 2 and DNA synthesis, even when added during late G(1) phase, once the known mechanisms by which MAP kinase controls G(1) progression, accumulation of G(1) cyclins and degradation of Cdk inhibitors have already taken place. Moreover, we provide evidence indicating that MAP kinases control Cdk2 Thr-160 activating phosphorylation and function, possibly by regulating the activity of a Cdk-activating kinase, thus promoting the re-initiation of DNA synthesis. These findings suggest the existence of a novel mechanism whereby signal-transducing pathways converging on MAP kinases can affect the cell-cycle machinery and, ultimately, participate in cell-growth control.

PI-3-kinase is an essential anti-apoptotic effector in the proliferative response of primary human epithelial cells to mutant RAS

In contrast to its growth-inhibitory effect on primary mesenchymal cells, RAS oncogene activation induces a proliferative phenotype in normal human thyroid epithelial cells in vitro, consistent with its putative role in tumour initiation. Using this model, we previously showed that activation of the MAP kinase (MAPK) pathway is necessary, but not sufficient for the proliferative response to mutant (V12) H-RAS. Here we extend this work to show that another major RAS effector-- phosphatidylinositol-3-kinase (PI-3-K)--while also insufficient alone, is able to synergize with MAPK activation to mimic the effect of mutant RAS, albeit at reduced efficiency. Furthermore we show that PI-3-K is an absolute requirement for the proliferative response to RAS in these cells, acting via suppression of RAS-induced apoptosis. These data extend our understanding of RAS signalling in a clinically-relevant cell context and point to the use of PI-3-K inhibitors as potential therapeutic agents for targetting human cancers induced by RAS mutation.

Regulation of urokinase receptor transcription by Ras- and Rho-family GTPases

How cell adhesion is coordinated with extracellular proteolysis is a key question in understanding cell migration. Potentially, the small GTP-binding proteins that affect actin organisation and signal transduction may also regulate the expression of genes associated with extracellular proteolysis. We investigated the ability of Ras, Rac-1, Cdc42Hs, and RhoA to regulate transcription from the1.55-kb promoter region of the human urokinase plasminogen activator receptor (uPAR) gene. Constitutively active V12 H-Ras and Rho-A stimulated uPAR transcription while Cdc42Hs and Rac-1 did not. The use of Ras effector-loop mutants indicated that signalling via multiple Ras-effectors is necessary for the maximum activation of transcription.

Integration of positive and negative growth signals during ras pathway activation in vivo

Expression of RAS proteins can have either positive or negative effects on cell growth, differentiation and death. New technologies are being developed for the generation of animal models to address the questions of where, when and how much Ras is expressed during tumorigenesis, and how these disparate signals are integrated during multistage carcinogenesis.

Tumor necrosis factor-alpha promotes human papillomavirus (HPV) E6/E7 RNA expression and cyclin-dependent kinase activity in HPV-immortalized keratinocytes by a ras-dependent pathway

Tumor necrosis factor-alpha (TNF-alpha) inhibits growth of normal cervical keratinocytes but stimulates proliferation of human papillomavirus (HPV)-immortalized and cervical carcinoma-derived cell lines when mitogens such as epidermal growth factor (EGF) or serum are depleted. Current work identifies the mechanism of growth stimulation. TNF-alpha promoted cell cycle progression by increasing expression of HPV-16 E6/E7 RNAs and enhancing activity of cyclin-dependent kinase (cdk)2 and cdc2 after 3 d. Increased kinase activity was mediated by upregulation of cyclins A and B and decreases in cdk inhibitors p21(waf) and p27(kip). TNF-alpha stimulated these changes in part by increasing transcription and stabilization of RNA for amphiregulin, an EGF receptor ligand, and amphiregulin directly increased HPV-16 E6/E7 and cyclin A RNAs. To define which components of the EGF receptor signaling pathway were important, HPV-immortalized cells were transfected with activated or dominant negative mutants of Ha-ras, raf, or MAPKK. Expression of activated Ha-ras maintained HPV-16 and cyclin gene expression and promoted rapid growth in the absence of EGF. Furthermore, ras activation was necessary for TNF-alpha mitogenesis as transfection with a dominant negative ras mutant (Asn-17) strongly inhibited growth. Thus, activation of ras promotes expression of HPV-16 E6/E7 RNAs, induces cyclins A and B, and mediates growth stimulation of immortal keratinocytes by TNF-alpha. These studies define a pathway by which ras mutations, which occur in a subset of cervical cancers, may contribute to pathogenesis. Mol. Carcinog. 27:97-109, 2000. Published by Wiley-Liss, Inc.

Genetic pathways in colorectal and other cancers

Cells from cancers show aberrant behaviour such as unrestrained growth, invasion into adjacent tissue and metastasis. All these features of cancer cell behaviour can be explained in terms of genetic changes and the functional impact of these changes. In this review, colorectal cancer (CRC) is examined as a classical example of multistep carcinogenesis. First there is an overview which shows that cancers develop by a process of somatic evolution. This gives rise to preferred genetic pathways of tumorigenesis. The factors which may influence the development and ultimate choice of genetic pathways are then examined. Next, CRC is studied as a specific disease and the putative genetic pathways are described. The mutations that comprise these pathways and the possible functional sequelae of these are explored. The review concludes with a look at those avenues which may further elucidate the natural history of CRC and lead to improved therapy.

The structural and mechanical complexity of cell-growth control

Tight control of cell proliferation is required to ensure normal tissue patterning and prevent cancer formation. The analysis of cultured cells has led to an explosion in our understanding of the molecules that trigger growth and mediate cell-cycle progression. However, the mechanism by which the local growth differentials that drive morphogenesis are established and maintained still remains unknown. Here we review recent work that reveals the importance of cell binding to the extracellular matrix, and associated changes in cell shape and cytoskeletal tension, to the spatial control of cell-cycle progression. These findings change the paradigm of cell-growth control, by placing our understanding of molecular signalling cascades in the context of the structural and mechanical complexity of living tissues.

Differential roles of Akt, Rac, and Ral in R-Ras-mediated cellular transformation, adhesion, and survival

Multiple biological functions have been ascribed to the Ras-related G protein R-Ras. These include the ability to transform NIH 3T3 fibroblasts, the promotion of cell adhesion, and the regulation of apoptotic responses in hematopoietic cells. To investigate the signaling mechanisms responsible for these biological phenotypes, we compared three R-Ras effector loop mutants (S61, G63, and C66) for their relative biological and biochemical properties. While the S61 mutant retained the ability to cause transformation, both the G63 and the C66 mutants were defective in this biological activity. On the other hand, while both the S61 and the C66 mutants failed to promote cell adhesion and survival in 32D cells, the G63 mutant retained the ability to induce these biological activities. Thus, the ability of R-Ras to transform cells could be dissociated from its propensity to promote cell adhesion and survival. Although the transformation-competent S61 mutant bound preferentially to c-Raf, it only weakly stimulated the mitogen-activated protein kinase (MAPK) activity, and a dominant negative mutant of MEK did not significantly perturb R-Ras oncogenicity. Instead, a dominant negative mutant of phosphatidylinositol 3-kinase (PI3-K) drastically inhibited the oncogenic potential of R-Ras. Interestingly, the ability of the G63 mutant to induce cell adhesion and survival was closely associated with the PI3-K-dependent signaling cascades. To further delineate R-Ras downstream signaling events, we observed that while a dominant negative mutant of Akt/protein kinase inhibited the ability of R-Ras to promote cell survival, both dominant negative mutants of Rac and Ral suppressed cell adhesion stimulated by R-Ras. Thus, the biological actions of R-Ras are mediated by multiple effectors, with PI3-K-dependent signaling cascades being critical to its functions.

The Ras mutant D119N is both dominant negative and activated

The introduction of mutation D119N (or its homolog) in the NKxD nucleotide binding motif of various Ras-like proteins produces constitutively activated or dominant-negative effects, depending on the system and assay. Here we show that Ras(D119N) has an inhibitory effect at a cell-specific concentration in PC12 and NIH 3T3 cells. Biochemical data strongly suggest that the predominant effect of mutation D119N in Ras-a strong decrease in nucleotide affinity-enables this mutant (i) to sequester its guanine nucleotide exchange factor, as well as (ii) to rapidly bind GTP, independent of the regulatory action of the exchange factor. Since mutation D119N does not affect the interaction between Ras and effector molecules, the latter effect causes Ras(D119N) to act as an activated Ras protein at concentrations higher than that of the exchange factor. In comparison, Ras(S17N), which also shows a strongly decreased nucleotide affinity, does not bind to effector molecules. These results point to two important prerequisites of dominant-negative Ras mutants: an increased relative affinity of the mutated Ras for the exchange factor over that for the nucleotide and an inability to interact with the effector or effectors. Remarkably, the introduction of a second, partial-loss-of-function, mutation turns Ras(D119N) into a strong dominant-negative mutant even at high concentrations, as demonstrated by the inhibitory effects of Ras(E37G/D119N) on nerve growth factor-mediated neurite outgrowth in PC12 cells and Ras(T35S/D119N) on fetal calf serum-mediated DNA synthesis in NIH 3T3 cells. Interpretations of these results are discussed.

Activation of mitogen-activated protein kinase is necessary but not sufficient for proliferation of human thyroid epithelial cells induced by mutant Ras

Given the high frequency of ras oncogene activation in several common human cancers, its signal pathways are an important target for novel therapy. For practical reasons, however, these have been studied mainly in the context of transformation of established fibroblast cell lines, whereas ras acts at an earlier stage in human tumorigenesis and predominantly on epithelial cells. Here we have developed a more directly relevant model - human primary thyroid epithelial cells - which are a major target of naturally-occurring Ras mutation, and in which expression of mutant Ras in culture induces clonal expansion without morphological transformation, closely reproducing the phenotype of the corresponding tumour in vivo. Transient or stable expression of mutant H-ras (by scrapeloading or retroviral infection) at levels which stimulated proliferation induced sustained activation and translocation of MAP kinase (MAPK) in these cells. Inhibition of the MAPK pathway at the level of MAPKK, by expression of a dominant-negative mutant or by the pharmacological inhibitor PD98059, efficiently blocked the proliferative response. Conversely, selective activation of MAPK by a constitutively-active MAPKK1 mutant failed to mimic the action of Ras and, although this was achievable with activated Raf, micro-injection of anti-ras antibodies showed that this still required endogenous wild-type Ras function. In contrast to recent results obtained with a rodent thyroid cell line (WRT), therefore, activation of the MAPK pathway is necessary, but not sufficient, for the proliferogenic action of mutant Ras on primary human thyroid cells. These data emphasize the unreliability of extrapolation from cell lines and establish the feasibility of using a more representative human epithelial model for Ras signalling studies.

Multiple ras effector pathways contribute to G(1) cell cycle progression

The involvement of Ras in the activation of multiple early signaling pathways is well understood, but it is less clear how the various Ras effectors interact with the cell cycle machinery to cause G(1) progression. Ras-mediated activation of extracellular-regulated kinase/mitogen-activated protein kinase has been implicated in cyclin D(1) up-regulation, but there is little extracellular-regulated kinase activity during the later stages of G(1), when cyclin D(1) expression becomes maximal, implying that other effector pathways may also be important in cyclin D(1) induction. We have addressed the involvement of Ras effectors from the phosphatidylinositol (PI) 3-kinase and Ral-GDS families in G(1) progression and compared it to that of the Raf/mitogen-activated protein kinase pathway. PI 3-kinase activity is required for the expression of endogenous cyclin D(1) and for S phase entry following serum stimulation of quiescent NIH 3T3 fibroblasts. Activated PI 3-kinase induces cyclin D(1) transcription and E2F activity, at least in part mediated by the serine/threonine kinase Akt/PKB, and to a lesser extent the Rho family GTPase Rac. In addition, both activated Ral-GDS-like factor and Raf stimulate cyclin D(1) transcription and E2F activity and act in synergy with PI 3-kinase. Therefore, multiple cooperating pathways mediate the effects of Ras on progression through the cell cycle.