Biological assays for cellular transformation

Critical and distinct roles of amino- and carboxyl-terminal sequences in regulation of the biological activity of the Chp atypical Rho GTPase

Chp (Cdc42 homologous protein) shares significant sequence and functional identity with the human Cdc42 small GTPase, and like Cdc42, promotes formation of filopodia and activates the p21-activated kinase serine/threonine kinase. However, unlike Cdc42, Chp contains unique amino- and carboxyl-terminal extensions. Here we determined whether Chp, like Cdc42, can promote growth transformation and evaluated the role of the amino- and carboxyl-terminal sequences in Chp function. Surprisingly, we found that a GTPase-deficient mutant of Chp exhibited low transforming activity but that deletion of the amino terminus of Chp greatly enhanced its transforming activity. Thus, the amino terminus may serve as a negative regulator of Chp function. The carboxyl terminus of Cdc42 contains a CAAX (where C is cysteine, A is aliphatic amino acid, X is terminal amino acid) tetrapeptide sequence that signals for the posttranslational modification critical for Cdc42 membrane association and biological function. Although Chp lacks aCAAXmotif, we found that Chp showed carboxyl terminus-dependent localization to the plasma membrane and to endosomes. Furthermore, an intact carboxyl terminus was required for Chp transforming activity. However, treatment with inhibitors of protein palmitoylation, but not prenylation, caused Chp to mislocalize to the cytoplasm. Thus, Chp depends on palmitoylation, rather than isoprenylation, for membrane association and function. In summary, Chp is implicated in cell transformation, and the unique amino and carboxyl termini of Chp represent atypical mechanisms of regulation of Rho GTPase function.

Metastasis-associated protein 1 interacts with NRIF3, an estrogen-inducible nuclear receptor coregulator

The transcriptional activity of estrogen receptor alpha (ER-alpha) is modified by regulatory action and interactions of coactivators and corepressors. Recent studies have shown that the metastasis-associated protein 1 (MTA1) represses estrogen receptor element (ERE)-driven transcription in breast cancer cells. With a yeast two-hybrid screen to clone MTA1-interacting proteins, we identified a known nuclear receptor coregulator (NRIF3) as an MTA1-binding protein. NRIF3 interacted with MTA1 both in vitro and in vivo. NRIF3 bound to the C-terminal region of MTA1, while MTA1 bound to the N-terminal region of NRIF3, containing one nuclear receptor interaction LXXLL motif. We showed that NRIF3 is an ER coactivator, hyperstimulated ER transactivation functions, and associated with the endogenous ER and its target gene promoter. MTA1 repressed NRIF3-mediated stimulation of ERE-driven transcription and interfered with NRIF3's association with the ER target gene chromatin. In addition, NRIF3 deregulation enhanced the responsiveness of breast cancer cells to estrogen-induced stimulation of growth and anchorage independence. Furthermore, we found that NRIF3 is an estrogen-inducible gene and activated ER associated with the ER response element in the NRIF3 gene promoter. These findings suggest that NRIF3, an MTA1-interacting protein, is an estrogen-inducible gene and that regulatory interactions between MTA1 and NRIF3 might be important in modulating the sensitivity of breast cancer cells to estrogen.

12-O-Tetradecanoylphorbol-13-acetate and UV Radiation-induced Nucleoside Diphosphate Protein Kinase B Mediates Neoplastic Transformation of Epidermal Cells

The molecular changes associated with early skin carcinogenesis are largely unknown. We have previously identified 11 genes whose expression was up- or down-regulated by 12-O-tetradecanoylphorbol-13-acetate (TPA) in mouse skin keratinocyte progenitor cells (Wei, S.-J., Trempus, C. S., Cannon, R. E., Bortner, C. D., and Tennant, R. W. (2003) J. Biol. Chem. 278, 1758-1768). Here, we show an induction of a nucleoside diphosphate protein kinase B (NDPK-B) gene in response to TPA or UV radiation (UVR). TPA or UVR significantly induced the expression of NDPK-B both in vivo hyperplastic mouse skin and in vitro mouse JB6 Cl 41-5a epidermal cells. Indeed, this gene was also up-regulated in TPA or UVR-mediated skin tumors including papillomas, spindle cell tumors, and squamous cell carcinomas, relative to adjacent normal skins. Functional studies by constitutive expression of nm23-M2/NDPK-B in TPA susceptible JB6 Cl 41-5a and TPA-resistant JB6 Cl 30-7b preneoplastic epidermal cell lines showed a remarkable gene dosage-dependent increase in foci-forming activity, as well as an enhancement in the efficiency of neoplastic transformation of these cells in soft agar but no effect on proliferation in monolayer cultures. Interestingly, stable transfection of the nm23-M2/NDPK-B del-RGD or G106A mutant gene in JB6 Cl 41-5a cells selectively abrogated NDPK-B-induced cellular transformation, implicating a possible Arg105-Gly106-Asp107 regulatory role in early skin carcinogenesis.

NF1 modulates the effects of ras oncogenes: Evidence of other NF1 function besides its GAP activity

Neurofibromin (NF1) (the product of Nf1 gene) is a large cytosolic protein known as a negative regulator of Ras. A fragment of some 400 residues located at the center of the NF1 GAP-Related Domain (NF1-GRD) has strong identity with other molecules of the GAP family, which comprises, among others, the mammalian proteins NF1 and p120GAP, and the yeast proteins IRA1 and IRA2. GAP family members are known by their ability to promote the GTPase activity of Ras proteins, facilitating the transit of those proteins to their inactive state. Recent findings (Tong et al., 2002, Nat Neurosci 5:95-96) indicate that NF1 may be involved in the regulation of adenyl cyclase activity. Our results show that NF1-GRD cooperates with Ras in the anchorage-independent growth capacity of Ras-expressing fibroblasts, without affecting: (i) their ability to grow in low serum, (ii) their cellular adhesion capability, or (iii) the expression of key proteins involved in cell-cell and cell-matrix interactions. On the other hand, NF1 overexpression induces an increase in the expression levels of the focal adhesion kinase (FAK), and specific changes in the activation status of the mitogen-activated protein kinases (MAPKs). These results suggest the existence of a Ras-independent NF1-dependent pathway able to modify the levels of expression of FAK and the levels of activation of MAPKs. Because FAK and many proteins recently found to bind NF1 have a role in the cytoskeleton, this pathway may involve rearrangement of cytoskeletal components that facilitate anchorage independence.

Identification of Dss1 as a 12-O-tetradecanoylphorbol-13-acetate-responsive gene expressed in keratinocyte progenitor cells, with possible involvement in early skin tumorigenesis

This study identifies genes expressed early in 12-O-tetradecanoylphorbol-13-acetate (TPA)-induced skin carcinogenesis in genetically initiated Tg.AC v-Ha-ras transgenic mice. Keratinocyte progenitor cells from TPA-treated Tg.AC mice were isolated with fluorescence-activated cell sorting and expression was analyzed using cDNA microarray technology. Eleven genes were identified whose expression changed significantly in response to carcinogen treatment. Deleted in split hand/split foot 1 (Dss1) is a gene associated with a heterogeneous limb developmental disorder called split hand/split foot malformation. cDNA microarray expression analysis showed that the mouse homologue of Dss1 is induced by TPA. Dss1 overexpression was detected by Northern blot analysis in early TPA-treated hyperplastic skins and in JB6 Cl 41-5a epidermal cells. Interestingly, Dss1 expression was also shown to be elevated in skin papillomas relative to normal skins, and further increased in squamous cell malignancies. Functional studies by ectopically constitutive expression of Dss1 in JB6 Cl 41-5a preneoplastic cells strongly increased focus formation and proliferation of these cells and enhanced efficiency of neoplastic transformation of the cells in soft agar. These results strongly suggest that Dss1 is a TPA-inducible gene that may play an important role in the early stages of skin carcinogenesis.

In vivo effects of the Epstein-Barr virus small RNA EBER-1 on protein synthesis and cell growth regulation

Recent studies have suggested a role for the Epstein-Barr virus-encoded RNA EBER-1 in malignant transformation. EBER-1 inhibits the activity of the protein kinase PKR, an inhibitor of protein synthesis with tumour suppressor properties. In human 293 cells and murine embryonic fibroblasts, transient expression of EBER-1 promoted total protein synthesis and enhanced the expression of cotransfected reporter genes. However reporter gene expression was stimulated equally well in cells from control and PKR knockout mice. NIH 3T3 cells stably expressing EBER-1 exhibited a greatly increased frequency of colony formation in soft agar, and protein synthesis in these cells was relatively resistant to inhibition by the calcium ionophore A23187. Nevertheless clones containing a high concentration of EBER-1 were not invariably tumourigenic. We conclude that EBER-1 can enhance protein synthesis by a PKR-independent mechanism and that, although this RNA may contribute to the oncogenic potential of Epstein-Barr virus, its expression is not always sufficient for malignant transformation.

Ras signalling on the endoplasmic reticulum and the Golgi

Current models evoke the plasma membrane (PM) as the exclusive platform from which Ras regulates signalling. We developed a fluorescent probe that reports where and when Ras is activated in living cells. We show that oncogenic H-Ras and N-Ras engage Raf-1 on the Golgi and that endogenous Ras and unpalmitoylated H-Ras are activated in response to mitogens on the Golgi and endoplasmic reticulum (ER), respectively. We also demonstrate that H-Ras that is restricted to the ER can activate the Erk pathway and transform fibroblasts, and that Ras localized on different membrane compartments differentially engages various signalling pathways. Thus, Ras signalling is not limited to the PM, but also proceeds on the endomembrane.

The GTP binding proteins Gem and Rad are negative regulators of the Rho-Rho kinase pathway

The cytoskeletal changes that alter cellular morphogenesis and motility depend upon a complex interplay among molecules that regulate actin, myosin, and other cytoskeletal components. The Rho family of GTP binding proteins are important upstream mediators of cytoskeletal organization. Gem and Rad are members of another family of small GTP binding proteins (the Rad, Gem, and Kir family) for which biochemical functions have been mostly unknown. Here we show that Gem and Rad interface with the Rho pathway through association with the Rho effectors, Rho kinase (ROK) alpha and beta. Gem binds ROKbeta independently of RhoA in the ROKbeta coiled-coil region adjacent to the Rho binding domain. Expression of Gem inhibited ROKbeta-mediated phosphorylation of myosin light chain and myosin phosphatase, but not LIM kinase, suggesting that Gem acts by modifying the substrate specificity of ROKbeta. Gem or Rad expression led to cell flattening and neurite extension in N1E-115 neuroblastoma cells. In interference assays, Gem opposed ROKbeta- and Rad opposed ROKalpha-mediated cell rounding and neurite retraction. Gem did not oppose cell rounding initiated by ROKbeta containing a deletion of the Gem binding region, demonstrating that Gem binding to ROKbeta is required for the effects observed. In epithelial or fibroblastic cells, Gem or Rad expression resulted in stress fiber and focal adhesion disassembly. In addition, Gem reverted the anchorage-independent growth and invasiveness of Dbl-transformed fibroblasts. These results identify physiological roles for Gem and Rad in cytoskeletal regulation mediated by ROK.

Immortal activated human hepatic stellate cells generated by ectopic telomerase expression

Telomere shortening controls the entry of cells into senescence. Functional expression of the telomerase catalytic subunit (human telomerase reverse transcriptase or hTERT) stabilizes telomere length and extends the life span of various normal human cells. Our aim was to assess the role of telomerase activity and telomere maintenance in regulating the proliferation of activated human hepatic stellate cells (HSCs), to establish an immortal human HSC cell line. Human HSCs were isolated from surgical specimens of normal liver and infected with a retrovirus expressing hTERT. Ectopic expression of hTERT reconstituted telomerase activity and maintained telomere length in human HSCs. Control human HSCs, which were either not infected or infected with a retroviral vector containing only the neomycin resistance gene, showed no detectable telomerase activity and had slightly shortened telomeres. These telomerase-negative HSCs entered a nondividing state after about 9 to 15 passages and senesced. In contrast, telomerase-positive HSCs to date have undergone 69 passages. Telomerase-positive HSCs did not undergo oncogenic transformation and exhibit morphologic and functional characteristics of activated HSCs. Microarray and RT-PCR analysis showed that mRNA expression patterns in telomerase-positive HSCs are very similar to those in activated human HSCs. Plating telomerase-positive HSCs on a basement membrane-like matrix reverts them toward a more quiescent phenotype. In conclusion, introduction of hTERT into activated human HSCs immortalizes them and maintains their activated phenotype. This newly developed cell line will be a useful tool to study the cell biology of human HSCs in culture.

Inhibition of cell transformation by sulindac sulfide is confined to specific oncogenic pathways

Non-steroidal anti-inflammatory drugs (NSAIDs) have been shown to reduce the risk of colorectal cancer (CRC). They are also known to induce the regression of colorectal adenomas, which are precursors to CRC. Despite these evidences, the exact mechanism by which NSAIDs exerts its anti-oncogenic effect is not completely understood. Using a focus formation assay, here we show that sulindac sulfide, a NSAID, specifically inhibits cell transformation mediated by oncogenic Ha-Ras, but not by other established oncogene products such as v-Src, Galpha12, and Galpha13. Our results suggest that the ability of sulindac sulfide to suppress transformation is confined to specific oncogenic pathways. Further studies of the sulindac-resistant oncogenic pathways may lead to identification of novel therapeutic agents that are effective in the prevention or treatment of CRC.

Chondrocyte-derived ezrin-like domain containing protein (CDEP), a rho guanine nucleotide exchange factor, is inducible in chondrocytes by parathyroid hormone and cyclic AMP and has transforming activity in NIH3T3 cells

OBJECTIVE

The purpose of this study is to investigate stage- and hormone-dependent expression of chondrocyte-derived ezrin-like domain containing protein (CDEP), a putative guanine nucleotide exchange factor (GEF) for Rho in chondrocytes, and demonstrate the guanine nucleotide exchange activity of this protein in vitro, as well as the transforming activity in NIH3T3 cells.

METHODS

The expression of CDEP mRNA in growth plate chondrocytes in vivo and in vitro was examined by RT-PCR Southern analysis. The guanine nucleotide exchange activity was determined using a recombinant CDEP peptide containing the DH and PH domains in Sf9 cell lysates. The transforming activity was examined using NIH3T3 cells transiently transfected with a truncated CDEP cDNA.

RESULTS

CDEP mRNA was expressed at the highest level in the hypertrophic (terminal) stage of chondrocytes in vivoand in vitro. Parathyroid hormone (PTH) elicited a biphasic increase of CDEP mRNA in chondrocytes. The CDEP mRNA level increased within 1 h, then decreased nearly to the control level at 3 h. Thereafter the mRNA level started to increase at 6 h, reaching a plateau at 24 h. Dibutyryl cyclic AMP had a similar effect on CDEP expression in chondrocytes. The dissociation of [3H]GDP from RhoA was stimulated dose-dependently by Sf9 cell lysates containing the CDEP peptide. Furthermore, transfection of a truncated CDEP cDNA induced focus formation in NIH3T3 cultures.

CONCLUSIONS

CDEP is a novel GEF for Rho family GTPases with the transforming activity. CDEP may play a role in mediating or modulating the action of cAMP-elevating hormones on maturing chondrocytes.

Signal pathways which promote invasion and metastasis: critical and distinct contributions of extracellular signal-regulated kinase and Ral-specific guanine exchange factor pathways

Approximately 50% of metastatic tumors contain Ras mutations. Ras proteins can activate at least three downstream signaling cascades mediated by the Raf-MEK-extracellular signal-regulated kinase family, phosphatidylinositol-3 (PI3) kinase, and Ral-specific guanine nucleotide exchange factors (RalGEFs). Here we investigated the contribution of RalGEF and ERK activation to the development of experimental metastasis in vivo and associated invasive properties in vitro. Each pathway contributes distinct properties to the metastatic phenotype. Following lateral tail vein injection, 3T3 cells transformed by constitutively active Raf or MEK produced lung metastasis that displayed circumscribed, noninfiltrating borders. In contrast, 3T3 cells transformed by Ras(12V,37G), a Ras effector mutant that activates RalGEF but not Raf or P13 kinase, formed aggressive, infiltrative metastasis. Dominant negative RalB inhibited Ras(12V,37G)-activated invasion and metastasis, demonstrating the necessity of the RalGEF pathway for a fully transformed phenotype. Moreover, 3T3 cells constitutively expressing a membrane-associated form of RalGEF (RalGDS-CAAX) formed invasive tumors as well, demonstrating that activation of a RalGEF pathway is sufficient to initiate the invasive phenotype. Despite the fact that Ras(12V,37G) expression does not elevate ERK activity, inhibition of this kinase by a conditionally expressed ERK phosphatase demonstrated that ERK activity was necessary for Ras(12V,37G)-transformed cells to express matrix-degrading activity in vitro and tissue invasiveness in vivo. Therefore, these experiments have revealed a hitherto-unknown but essential interaction of the RalGEF and ERK pathways to produce a malignant phenotype. The generality of the role of the RalGEF pathway in metastasis is supported by the finding that Ras(12V,37G) increased the invasiveness of epithelial cells as well as fibroblasts.

Intestinal-enriched Krüppel-like factor (Krüppel-like factor 5) is a positive regulator of cellular proliferation

Intestinal-enriched Krüppel-like factor (IKLF or KLF5) belongs to the family of mammalian Krüppel-like transcription factors. Previous studies indicate that expression of IKLF is enriched in the proliferating crypt epithelial cells of the intestinal tract. However, the biological function of IKLF is unknown. In the current study, we have shown that the level of IKLF mRNA was nearly undetectable in serum-deprived NIH3T3 fibroblasts but became acutely and significantly increased upon the addition of fetal bovine serum or the phorbol ester, PMA. This induction required protein synthesis because it was prevented by cycloheximide. Transfection of IKLF into NIH3T3 cells resulted in the formation of foci in a manner similar to that caused by the activated Ha-ras oncogene. Constitutive expression of IKLF in transfected NIH3T3 cells significantly increased the rate of proliferation when compared with cells transfected with an empty vector. The growth of IKLF-transfected cells was no longer inhibited by cell-cell contact or by low serum content. Moreover, these cells proliferated in an anchorage-independent fashion. We conclude that IKLF encodes a delayed early response gene product that positively regulates cellular proliferation and may give rise to a transformed phenotype when overexpressed.

Regulatable expression of p21-activated kinase-1 promotes anchorage-independent growth and abnormal organization of mitotic spindles in human epithelial breast cancer cells

Stimulation of growth factor signaling has been implicated in the development of invasive phenotypes and the activation of p21-activated kinase (Pak1) in human breast cancer cells (Adam, L., Vadlamudi, R., Kondapaka, S. B., Chernoff, J., Mendelsohn, J., and Kumar, R. (1998) J. Biol. Chem. 273, 28238-28246; Adam, L., Vadlamudi, R., Mandal, M., Chernoff, J., and Kumar, R. (2000) J. Biol. Chem. 275, 12041-12050). To study the role of Pak1 in the regulation of motility and growth of breast epithelial cells, we developed human epithelial MCF-7 clones that overexpressed the kinase-active T423E Pak1 mutant under an inducible tetracycline promoter or that stably expressed the kinase-active H83L,H86L Pak1 mutant, which is deficient in small GTPase binding sites. The expression of both T423E and H83L,H86L Pak1 mutants in breast epithelial cells was accompanied by increased cell motility without any apparent effect on the growth rate of cells. The T423E Pak1 mutant was primarily localized to filopodia, and the H83L,H86L Pak1 mutant was primarily localized to ruffles. Cells expressing T423E Pak1 exhibited a regulatable stimulation of mitogen-activated protein kinase and Jun N-terminal kinase activities. The expression of kinase-active Pak1 mutants significantly stimulated anchorage-independent growth of cells in soft agar in a preferential mitogen-activated protein kinase-sensitive manner. In addition, regulatable expression of kinase-active Pak1 resulted in an abnormal organization of mitotic spindles characterized by appearance of multiple spindle orientations. We also provide evidence to suggest a close correlation between the status of Pak1 kinase activity and base-line invasiveness of human breast cancer cells and breast tumor grades. This study is the first demonstration of Pak1 regulation of anchorage-independent growth, potential Pak1 regulation of invasiveness, and abnormal organization of mitotic spindles of human epithelial breast cancer cells.

Rit, a non-lipid-modified Ras-related protein, transforms NIH3T3 cells without activating the ERK, JNK, p38 MAPK or PI3K/Akt pathways

The biological functions of Rit (Ras-like protein in tissues) and Rin (Ras-like protein in neurons), members of a novel branch of Ras-related GTP-binding proteins that are approximately 50% identical to Ras, have not been characterized. Therefore, we assessed their activity in growth control, transformation and signaling. NIH cells stably expressing a constitutively activated mutant of Rit [Rit(79L)] (analogous to the oncogenic mutant H-Ras(61L)) demonstrated strong growth transformation, proliferating rapidly in low serum and forming colonies in soft agar and tumors in nude mice. Although Rit(79L) alone did not promote morphologically transformed foci, it cooperated with both Raf and Rho A to form Rac/Rho-like foci. Rin [Rin(78L)] cooperated only with Raf. Rit(79L) but not Rin(78L) stimulated transcription from luciferase reporter constructs regulated by SRF, NF-kappaB, Elk-1 and Jun. However, neither activated ERK, JNK or p38, or PI3-K/Akt kinases in immune complex kinase assays. Interestingly, although Rit lacks any known recognition signal for C-terminal lipidation, Rit-transformed cell growth and survival in low serum is dependent on a farnesylated protein, as treatment with farnesyltransferase inhibitors caused apoptosis. Rin cooperated with Raf in focus assays but did not otherwise function in these assays, perhaps due to a lack of appropriate effector pathways in NIH3T3 fibroblasts for this neural-specific Ras family member. In summary, although Rit shares most core effector domain residues with Ras, our results suggest that Rit uses novel effector pathways to regulate proliferation and transformation.

Cell cycle arrest and reversion of Ras-induced transformation by a conditionally activated form of mitogen-activated protein kinase kinase kinase 3

Signal-induced proliferation, differentiation, or stress responses of cells depend on mitogen-activated protein kinase (MAPK) cascades, the core modules of which consist of members of three successively acting kinase families (MAPK kinase kinase [MAP3K], MAPK kinase, and MAPK). It is demonstrated here that the MEKK3 kinase inhibits cell proliferation, a biologic response not commonly associated with members of the MAP3K family of kinases. A conditionally activated form of MEKK3 stably expressed in fibroblasts arrests these cells in early G1. MEKK3 critically blocks mitogen-driven expression of cyclin D1, a cyclin which is essential for progression of fibroblasts through G1. The MEKK3-induced block of cyclin D1 expression and of cell cycle progression may be mediated via p38 MAPK, a downstream effector of MEKK3. The MEKK3-mediated block of proliferation also reverses Ras-induced cellular transformation, suggesting possible tumor-suppressing functions for this kinase. Together, these results suggest an involvement of the MEKK3 kinase in negative regulation of cell cycle progression, and they provide the first insights into biologic activities of this kinase.

The tmp gene, encoding a membrane protein, is a c-Myc target with a tumorigenic activity

The c-Myc oncoprotein induces cell proliferation and transformation through its activity as a transcription factor. Uncovering the genes regulated by c-Myc is an essential step for understanding these processes. We recently isolated the tumor-associated membrane protein gene, Tmp, from a c-myc-induced mouse brain tumor. Here we show that Tmp is specifically highly expressed in mammary tumors and T-cell lymphomas which develop in c-myc transgenic mice, suggesting that Tmp expression is a general characteristic of c-Myc-induced tumors. In addition, Tmp expression is induced upon serum stimulation of fibroblasts as shown in a time course closely correlated with c-myc expression. We have isolated the Tmp promoter region and identified a putative c-Myc binding element, CACGTG, located in the first intron of the gene. We show here that constructs containing the Tmp regulatory region fused to a reporter gene are activated by c-Myc through this CACGTG element and that the c-Myc-Max protein complex can bind to this element. Moreover, an inducible form of c-Myc, the MycER fusion protein, can activate the endogenous Tmp gene. We also show that Tmp-overexpressing fibroblasts induce rapidly growing tumors when injected into nude mice, suggesting that Tmp may possess a tumorigenic activity. Thus, TMP, a member of a novel family of membrane glycoproteins with a suggested role in cellular contact, is a c-Myc target and is possibly involved in c-Myc-induced transformation.

Regulation of actin organisation by TGF-beta in H-ras-transformed fibroblasts

The actin cytoskeleton undergoes architectural changes during the processes of cell transformation and tumourigenesis. Transforming growth factors beta arrest cell cycle progression, regulate differentiation and modulate the onset of oncogenesis and tumourigenesis. Here, we investigated the direct role of transforming growth factor beta-1 in altering the transformed phenotype and regulating the actin organisation of oncogenic fibroblasts that constitutively or inducibly express the H-ras oncogene. Following transforming growth factor beta-1 treatment, these transformed fibroblasts undergo a dramatic morphological alteration that includes a discrete reorganisation of their actin cytoskeleton and focal adhesions. Quantitative biochemical analysis demonstrated that transforming growth factor beta-1 potently induced polymerisation of globular to filamentous actin, thus corroborating the morphological analysis. The effect of transforming growth factor beta-1 on the cytoskeleton correlates with the ability of this cytokine to suppress anchorage-independent growth of the transformed fibroblasts. Furthermore, transforming growth factor beta-1 upregulates considerably the levels of the RhoB small GTPase and less the RhoA levels. Finally, The beta GTPase inhibitor, C3 exotransferase, blocks the ability of TGF-beta1 to induce cytoskeletal reorganisation. These findings indicate that transforming growth factor beta can regulate cell morphology and growth in a concerted manner possibly via mechanisms that control the actin cytoskeleton.

Signals from the Ras, Rac, and Rho GTPases converge on the Pak protein kinase in Rat-1 fibroblasts

Ras plays a key role in regulating cellular proliferation, differentiation, and transformation. Raf is the major effector of Ras in the Ras > Raf > Mek > extracellular signal-activated kinase (ERK) cascade. A second effector is phosphoinositide 3-OH kinase (PI 3-kinase), which, in turn, activates the small G protein Rac. Rac also has multiple effectors, one of which is the serine threonine kinase Pak (p65(Pak)). Here we show that Ras, but not Raf, activates Pak1 in cotransfection assays of Rat-1 cells but not NIH 3T3 cells. We tested agents that activate or block specific components downstream of Ras and demonstrate a Ras > PI 3-kinase > Rac/Cdc42 > Pak signal. Although these studies suggest that the signal from Ras through PI 3-kinase is sufficient to activate Pak, additional studies suggested that other effectors contribute to Pak activation. RasV12S35 and RasV12G37, two effector mutant proteins which fail to activate PI 3-kinase, did not activate Pak when tested alone but activated Pak when they were cotransfected. Similarly, RacV12H40, an effector mutant that does not bind Pak, and Rho both cooperated with Raf to activate Pak. A dominant negative Rho mutant also inhibited Ras activation of Pak. All combinations of Rac/Raf and Ras/Raf and Rho/Raf effector mutants that transform cells cooperatively stimulated ERK. Cooperation was Pak dependent, since all combinations were inhibited by kinase-deficient Pak mutants in both transformation assays and ERK activation assays. These data suggest that other Ras effectors can collaborate with PI 3-kinase and with each other to activate Pak. Furthermore, the strong correlation between Pak activation and cooperative transformation suggests that Pak activation is necessary, although not sufficient, for cooperative transformation of Rat-1 fibroblasts by Ras, Rac, and Rho.

Differential immediate early gene expression in conditional hepatitis B virus pX-transforming versus nontransforming hepatocyte cell lines

We report construction and characterization of tetracycline-controlled hepatitis B virus pX-expressing hepatocyte (AML12) cell lines. These cell lines were constructed in AML12 clonal isolates (clones 3 and 4), which express constitutively the tetracycline-controlled transactivator. Since pX is implicated in HCC, this immortalized hepatocyte model system was used to investigate the mechanism of pX in transformation. Clonal isolates of 3pX and 4pX lineages display conditional synthesis of pX mRNA and protein and a 2-fold increase in growth saturation density following tetracycline removal, implicating pX in monolayer overgrowth. Interestingly, only 3pX clones display pX-dependent anchorage independence. Clone 3 lineages express hepatocyte nuclear factor-1alpha and hepatocyte-specific marker genes; clone 4 lineages express hepatocyte nuclear factor-1beta and reduced levels of hepatocyte-specific marker genes, suggesting the importance of the differentiated hepatocyte in pX-mediated oncogenic transformation. Importantly, 3pX and 4pX lineages display differential expression of immediate early genes c-fos and ATF3. The pX-transforming 3pX lineage displays early, pX-dependent induction of ATF3 and prolonged induction of c-fos. The nontransforming 4pX cells display an absence of pX-dependent ATF3 induction and transient induction of c-fos. Our results support the direct link of pX expression to oncogenic transformation in 3pX lineage clones and underscore the advantage of this conditional cellular model system for studying mechanisms of pX-mediated oncogenesis.

KRAB-independent suppression of neoplastic cell growth by the novel zinc finger transcription factor KS1

The study of zinc finger proteins has revealed their potential to act as oncogenes or tumor suppressors. Here we report the molecular, biochemical, and functional characterization of KS1 (KRAB/zinc finger suppressor protein 1), a novel, ubiquitously expressed zinc finger gene initially isolated from a rat pancreas library. KS1 contains 10 C2H2 zinc fingers, a KRAB-A/B motif, and an ID sequence that has been shown previously to participate in growth factor-regulated gene expression. Northern blot analysis using pancreatic cell lines demonstrates that KS1 mRNA is inducible by serum and epidermal growth factor, suggesting a role for this gene in cell growth regulation. Biochemical analysis reveals that KS1 is a nuclear protein containing two transcriptional repressor domains, R1 and R2. R1 corresponds to the KRAB-A motif, whereas R2 represents a novel sequence. Transformation assays using NIH3T3 cells demonstrate that KS1 suppresses transformation by the potent oncogenes Ha-ras, Galpha12, and Galpha13. Deletion of the R1/ KRAB-A domain does not modify the transformation suppressive activity of KS1, whereas deletion of R2 abolishes this function. Thus, KS1 is a novel growth factor-inducible zinc finger transcriptional repressor protein with the potential to protect against neoplastic transformation induced by several oncogenes.

A role for Pak protein kinases in Schwann cell transformation

Neurofibromatosis type 1 (NF1), a common autosomal dominant disorder caused by loss of the NF1 gene, is characterized clinically by neurofibromas and more rarely by neurofibrosarcomas. Neurofibromin, the protein encoded by NF1, possesses an intrinsic GTPase accelerating activity for the Ras proto-oncogene. Through this activity, it is a negative regulator of Ras. The Pak protein kinase is a candidate for a downstream signaling protein that may mediate Ras signals because it is activated by Rac and Cdc42, two small G proteins required for Ras signaling. Here, we use Pak mutants to explore the role of Pak in Ras signaling in Schwann cells, the cells affected in NF1. Whereas an activated Pak mutant does not transform cells, dominant negative Pak mutants are potent inhibitors of Ras transformation of rat Schwann cells and of a neurofibrosarcoma cell line from an NF1 patient. Although activated Pak stimulated jun-N-terminal kinase, inhibition of Ras transformation by dominant negative Pak did not require inhibition of jun-N-terminal kinase. Instead, the Pak mutants appeared to inhibit transformation by preventing Ras activation of the ERK/mitogen-activated protein kinase cascade. These results have implications for our understanding of NF1 because a neurofibrosarcoma cell line derived from a patient with NF1 was reverted by stable expression of the Pak dominant negative mutants.

Kinase-deficient Pak1 mutants inhibit Ras transformation of Rat-1 fibroblasts

Among the mechanisms by which the Ras oncogene induces cellular transformation, Ras activates the mitogen-activated protein kinase (MAPK or ERK) cascade and a related cascade leading to activation of Jun kinase (JNK or SAPK). JNK is additionally regulated by the Ras-related G proteins Rac and Cdc42. Ras also regulates the actin cytoskeleton through an incompletely elucidated Rac-dependent mechanism. A candidate for the physiological effector for both JNK and actin regulation by Rac and Cdc42 is the serine/threonine kinase Pak (p65pak). We show here that expression of a catalytically inactive mutant Pak, Pak1(R299), inhibits Ras transformation of Rat-1 fibroblasts but not of NIH 3T3 cells. Typically, 90 to 95% fewer transformed colonies were observed in cotransfection assays with Rat-1 cells. Pak1(R299) did not inhibit transformation by the Raf oncogene, indicating that inhibition was specific for Ras. Furthermore, Rat-1 cell lines expressing Pak1(R299) were highly resistant to Ras transformation, while cells expressing wild-type Pak1 were efficiently transformed by Ras. Pak1(L83,L86,R299), a mutant that fails to bind either Rac or Cdc42, also inhibited Ras transformation. Rac and Ras activation of JNK was inhibited by Pak1(R299) but not by Pak1(L83,L86,R299). Ras activation of ERK was inhibited by both Pak1(R299) and Pak1(L83,L86,R299), while neither mutant inhibited Raf activation of ERK. These results suggest that Pak1 interacts with components essential for Ras transformation and that inhibition can be uncoupled from JNK but not ERK signaling.

Protein binding and signaling properties of RIN1 suggest a unique effector function

Human RIN1 was first characterized as a RAS binding protein based on the properties of its carboxyl-terminal domain. We now show that full-length RIN1 interacts with activated RAS in mammalian cells and defines a minimum region of 434 aa required for efficient RAS binding. RIN1 interacts with the "effector domain" of RAS and employs some RAS determinants that are common to, and others that are distinct from, those required for the binding of RAF1, a known RAS effector. The same domain of RIN1 that binds RAS also interacts with 14-3-3 proteins, extending the similarity between RIN1 and other RAS effectors. When expressed in mammalian cells, the RAS binding domain of RIN1 can act as a dominant negative signal transduction blocker. The amino-terminal domain of RIN1 contains a proline-rich sequence similar to consensus Src homology 3 (SH3) binding regions. This RIN1 sequence shows preferential binding to the ABL-SH3 domain in vitro. Moreover, the amino-terminal domain of RIN1 directly associates with, and is tyrosine phosphorylated by, c-ABL. In addition, RIN1 encodes a functional SH2 domain that has the potential to activate downstream signals. These data suggest that RIN1 is able to mediate multiple signals. A differential pattern of expression and alternate splicing indicate several levels of RIN1 regulation.

Hormone-defined cell system for studying G-protein coupled receptor agonist-activated growth modulation: delta-opioid and serotonin-5HT2C receptor activation show opposite mitogenic effects

G-protein-coupled receptor (GPCR) agonist-activated transformation of NIH/3T3 fibroblast cells has been documented by many workers. Our present interest is in the growth control exerted by these agonists. The mechanisms involved in GPCR agonist-activated growth regulation are not known and investigations using existing cell lines are complicated by the endogenous expression of numerous different GPCRs as well as by the fact that these cell lines are cultured in serum that contains naturally occurring agonists for these receptors. To study the agonist induced growth response of cells transfected with either delta-opioid or serotonin-5HT2C neurotransmitter receptor genes, we have developed new clonal cell lines derived from NIH/3T3 mouse fibroblast cells. These new cell lines, designated with the suffix 3T3DA, can be cultured stably in serum-free, hormone-defined medium: insulin is the only exogenous growth factor added to the culture medium of proliferating 3T3DA cell lines, and their proliferation can be stopped and started by the respective removal or addition of insulin. Micromolar concentrations of agonists were used to activate the corresponding opioid and serotonin receptors over periods extending to 6 days. We observed distinct patterns of GPCR-specific, agonist-activated growth regulation in serum-free cultures, but not in serum-supplemented cultures. At concentrations > 10 microM, morphine inhibits growth of delta-opioid receptor-expressing cells by 40% with respect to normal 3T3DA cells. Opioid agonist induced inhibition of cyclic AMP (cAMP) production as well as growth down-regulation are pertussis toxin sensitive indicating that the exogenously expressed delta-opioid receptors demonstrate classical opioid receptor signaling. The presence of 1 microM serotonin stimulates growth of serotonin-5HT2C receptor- expressing cells by approximately 100% with respect to normal 3T3DA cells. Neither the untreated nor the agonist-treated cells form colonies in soft agar, indicating that they retain anchorage-dependent growth control. These cell lines provide a simple system that could be used as a tool for probing the complex molecular mechanisms associated with GPCR agonist-activated growth control.

Characterization of downstream Ras signals that induce alternative protease-dependent invasive phenotypes

Invasive and metastatic cells require protease expression for migration through the extracellular matrix. Metastatic NIH 3T3 fibroblasts transformed by different activated ras genes showed two different protease phenotypes, rasuPA+/CL- and rasCL+/uPA- (Zhang, J-Y., and Schultz, R. M. (1992) Cancer Research 52, 6682-6689). Phenotype rasuPA+/CL- is dependent on expression of the serine-type protease urokinase plasminogen activator (uPA) and the phenotype rasCL+/uPA- on the cystine-type protease cathepsin L (CL) for lung colonization in experimental metastasis. The existence of multiple invasive phenotypes on ras-isoform transformation implied the activation of alternative pathways downstream from Ras. We now show that c-Raf-1, extracellular signal-regulated protein kinase (ERK)-1, and ERK-2 are hyperphosphorylated, and the ERK activity is high in both the uPA- and CL-dependent ras-transformed invasive phenotypes. Levels of c-Jun and c-Jun NH2-terminal kinase (JNK) activity are also high in the uPA-dependent phenotype, but they are almost undetectable in the CL-dependent phenotype. The uPA Ras-response element is a PEA3/URTF element, and mobility shift assays show a strong PEA3/URTF protein band in the uPA-dependent phenotype. This band is competed by a consensus AP-1 DNA sequence and by antibodies to PEA3 and c-Jun. Thus, the uPA-invasive phenotype appears to require the activation of Ets/PEA3 and c-Jun transcription factors activated by the ERK and JNK pathways, while the CL-invasive phenotype appears to require ERK activity with suppression of JNK and c-Jun activities. These postulates are supported by the introduction of a dominant negative c-Jun, TAM67, into cells of phenotype rasuPA+/CL-, which down-regulated the high uPA mRNA levels characteristic of this phenotype to basal levels and up-regulated basal levels of CL mRNA to levels similar to those observed in cells of phenotype rasCL+/uPA-. We conclude that the JNK pathway acts as a switch between two distinct protease phenotypes that are redundant in their abilities to grow tumors and metastasize.

Identification of a novel RalGDS-related protein as a candidate effector for Ras and Rap1

Although Ras and Rap1 share interaction with common candidate effector proteins, Rap1 lacks the transforming activity exhibited by Ras proteins. It has been speculated that Rap antagonizes Ras transformation through the formation of nonproductive complexes with critical Ras effector targets. To understand further the distinct biological functions of these two closely related proteins, we searched for Rap1b-binding proteins by yeast two-hybrid screening. We identified multiple clones that encode the COOH-terminal sequences of a protein that shares sequence identity with RalGDS and RGL, which we have designated RGL2. A 158-amino acid COOH-terminal fragment of RGL2 (RGL2 C-158) bound to Ras superfamily proteins which shared identical effector domain sequences with Rap1 (Ha-Ras, R-Ras, and TC21). RGL2 C-158 binding was impaired by effector domain mutations in Rap1b and Ha-Ras. Furthermore, RGL2 C-158 bound exclusively to the GTP-, but not the GDP-bound form of Ha-Ras. Finally, coexpression of RGL2 C-158 impaired oncogenic Ras activation of transcription from a Ras-responsive promoter element and focus-forming activity in NIH 3T3 cells. We conclude that RGL2 may be an effector for Ras and/or Rap proteins.

Evidence that farnesyltransferase inhibitors suppress Ras transformation by interfering with Rho activity

Small-molecule inhibitors of the housekeeping enzyme farnesyltransferase (FT) suppress the malignant growth of Ras-transformed cells. Previous work suggested that the activity of these compounds reflected effects on actin stress fiber regulation rather than Ras inhibition. Rho proteins regulate stress fiber formation, and one member of this family, RhoB, is farnesylated in vivo. Therefore, we tested the hypothesis that interference with RhoB was the principal basis by which the peptidomimetic FT inhibitor L-739,749 suppressed Ras transformation. The half-life of RhoB was found to be approximately 2 h, supporting the possibility that it could be functionally depleted within the 18-h period required by L-739,749 to induce reversion. Cell treatment with L-739,749 disrupted the vesicular localization of RhoB but did not effect the localization of the closely related RhoA protein. Ras-transformed Rat1 cells ectopically expressing N-myristylated forms of RhoB (Myr-rhoB), whose vesicular localization was unaffected by L-739,749, were resistant to drug treatment. The protective effect of Myr-rhoB required the integrity of the RhoB effector domain and was not due to a gain-of-function effect of myristylation on cell growth. In contrast, Rat1 cells transformed by a myristylated Ras construct remained susceptible to growth inhibition by L-739,749. We concluded that Rho is necessary for Ras transformation and that FT inhibitors suppress the transformed phenotype at least in part by direct or indirect interference with Rho, possibly with RhoB itself.

Two distinct Raf domains mediate interaction with Ras

A key event for Ras transformation involves the direct physical association between Ras and the Raf-1 kinase. This interaction promotes both Raf translocation to the plasma membrane and activation of Raf kinase activity. Although substantial experimental evidence has demonstrated that Raf residues 51-131 alone are sufficient for Ras binding, conflicting observations have suggested that the Raf cysteine-rich domain (residues 139-184) may also be important for interaction with Ras. To clarify the role of the Raf cysteine-rich domain in Ras-Raf binding, we have compared the ability of two distinct Raf fragments to interact with Ras using both in vitro Ras binding and in vivo Ras inhibition assays. First, we determined that both Raf sequences 2-140 and 139-186 (designated Raf-Cys) showed preferential binding to active, GTP-bound Ras in vitro. Second, we observed that Raf-Cys antagonized oncogenic Ras(Q61L)-mediated transactivation of Ras-responsive elements and focus-forming activity in NIH 3T3 cells and insulin-induced germinal vesicle breakdown in Xenopus laevis oocytes in vivo. This inhibitory activity suggests that Raf-Cys can interact with Ras in vivo. Taken together, these results suggest that Ras interaction with two distinct domains of Raf-1 may be important in Ras-mediated activation of Raf kinase activity.

Aberrant function of the Ras-related protein TC21/R-Ras2 triggers malignant transformation

Although the human Ras proteins are members of a large superfamily of Ras-related proteins, to date, only the proteins encoded by the three mammalian ras genes have been found to possess oncogenic potential. Among the known Ras-related proteins, TC21/R-Ras2 exhibits the most significant amino acid identity (55%) to Ras proteins. We have generated mutant forms of TC21 that possess amino acid substitutions analogous to those that activate Ras oncogenic potential [designated TC21(22V) and TC21(71L)] and compared the biological properties of TC21 with those of Ras proteins in NIH 3T3 and Rat-1 transformation assays. Whereas wild-type TC21 did not show any transforming potential in vitro, both TC21(22V) and TC21(71L) displayed surprisingly potent transforming activities that were comparable to the strong transforming activity of oncogenic Ras proteins. Like Ras-transformed cells, NIH 3T3 cells expressing mutant TC21 proteins formed foci of morphologically transformed cells in monolayer cultures, proliferated in low serum, formed colonies in soft agar, and developed progressive tumors in nude mice. Thus, TC21 is the first Ras-related protein to exhibit potent transforming activity equivalent to that of Ras. Furthermore, mutant TC21 proteins also stimulated constitutive activation of mitogen-activated protein kinases as well as transcriptional activation from Ras-responsive promoter elements (Ets/AP-1 and NF-kappa B). We conclude that aberrant TC21 function may trigger cellular transformation via a signal transduction pathway similar to that of oncogenic Ras and suggest that deregulated TC21 activity may contribute significantly to human oncogenesis.

Aberrant function of the Ras-related protein TC21/R-Ras2 triggers malignant transformation

Although the human Ras proteins are members of a large superfamily of Ras-related proteins, to date, only the proteins encoded by the three mammalian ras genes have been found to possess oncogenic potential. Among the known Ras-related proteins, TC21/R-Ras2 exhibits the most significant amino acid identity (55%) to Ras proteins. We have generated mutant forms of TC21 that possess amino acid substitutions analogous to those that activate Ras oncogenic potential [designated TC21(22V) and TC21(71L)] and compared the biological properties of TC21 with those of Ras proteins in NIH 3T3 and Rat-1 transformation assays. Whereas wild-type TC21 did not show any transforming potential in vitro, both TC21(22V) and TC21(71L) displayed surprisingly potent transforming activities that were comparable to the strong transforming activity of oncogenic Ras proteins. Like Ras-transformed cells, NIH 3T3 cells expressing mutant TC21 proteins formed foci of morphologically transformed cells in monolayer cultures, proliferated in low serum, formed colonies in soft agar, and developed progressive tumors in nude mice. Thus, TC21 is the first Ras-related protein to exhibit potent transforming activity equivalent to that of Ras. Furthermore, mutant TC21 proteins also stimulated constitutive activation of mitogen-activated protein kinases as well as transcriptional activation from Ras-responsive promoter elements (Ets/AP-1 and NF-kappa B). We conclude that aberrant TC21 function may trigger cellular transformation via a signal transduction pathway similar to that of oncogenic Ras and suggest that deregulated TC21 activity may contribute significantly to human oncogenesis.