Nuclear beta-catenin displays GSK-3beta- and APC-independent proteasome sensitivity in melanoma cells

Colon carcinoma and melanoma cells containing either a deletion of the adenomatous polyposis coli tumor suppressor protein (APC) or mutation of the site in beta-catenin phosphorylated by glycogen synthase kinase-3beta (GSK-3beta) display elevated levels of detergent-soluble beta-catenin due to insensitivity of the cytosolic protein to proteasome-dependent degradation. In this study, we have examined the effect of beta-catenin mutation (S37F) or APC loss on the proteasome sensitivity of additional subcellular beta-catenin pools in melanoma cells. In contrast to detergent-soluble beta-catenin, the detergent-insoluble protein remains proteasome-sensitive irrespective of S37F mutation or APC status. This insoluble component appears associated primarily with nuclear cytoskeletal elements. In addition, DNase I treatment solubilized a portion of detergent-insoluble beta-catenin, suggesting that this fraction also contains chromatin-associated protein, and correlating with a proteasome-sensitive elevation in beta-catenin-stimulated reporter activity. Since the detergent-insoluble nuclear component of beta-catenin displays GSK-3beta- and APC-independent proteasome sensitivity, distinct from the soluble nuclear and cytosolic pools of this protein, regulation of beta-catenin proteasome sensitivity and the contribution of this process to beta-catenin function may be more complex than previously appreciated.

Novobiocin and related coumarins and depletion of heat shock protein 90-dependent signaling proteins

BACKGROUND

Heat shock protein 90 (Hsp90) interacts with and stabilizes several oncogenic protein kinases (e.g., p185(erbB2), p60(v-src), and Raf-1) and is required for the stability and dominant-negative function of mutated p53 protein. Two unrelated antibiotics, geldanamycin and radicicol, bind specifically to an atypical nucleotide-binding pocket of Hsp90, a site that shares homology with the adenosine triphosphate (ATP)-binding domain of bacterial DNA gyrase B. This interaction leads to destabilization of proteins that interact with Hsp90. Since the nucleotide-binding site of gyrase B is targeted by coumarin antibiotics (e.g., novobiocin), we investigated whether these drugs can also interact with Hsp90 and affect its activity.

METHODS

We used immobilized novobiocin, geldanamycin, or radicicol to isolate either endogenous Hsp90 from cell lysates or Hsp90 deletion fragments translated in vitro. Effects of the coumarin antibiotics novobiocin, chlorobiocin, and coumermycin A1 on several proteins interacting with Hsp90 were assessed in vitro and in vivo.

RESULTS

Hsp90 binding to immobilized novobiocin was competed by soluble coumarins and ATP but not by geldanamycin or radicicol. A carboxy-terminal Hsp90 fragment bound immobilized novobiocin but not immobilized geldanamycin, while a geldanamycin-binding amino-terminal fragment did not bind novobiocin. All three coumarins markedly reduced cellular levels of p185(erbB2), p60(v-src), Raf-1, and mutated p53. Furthermore, novobiocin reduced Raf-1 levels in the spleens of mice treated with the drug.

CONCLUSIONS

These coumarin antibiotics, particularly novobiocin, represent a first-generation alternative to other Hsp90-targeting drugs that are not as well tolerated. Novobiocin's unique interaction with Hsp90 identifies an additional site on this protein amenable to pharmacologic interference with small molecules.

Effects of geldanamycin and other naturally occurring small molecule antagonists of heat shock protein 90 on HER2 protein expression

Several natural product antibiotics, including herbimycin, geldanamycin, and radicicol, bind to an amino terminal nucleotide binding pocket in the heat shock protein Hsp90. Drug binding alters the conformation of Hsp90 and interferes with its ability to chaperone a distinct group of "client" proteins, including a number of transmembrane and soluble tyrosine and serine/threonine kinases. Prominent among the kinases dependent on Hsp90 is the ErbB family member HER2, which is frequently overexpressed in adenocarcinoma and is associated with a poor prognosis and resistance to chemotherapy. Disruption of Hsp90 function promotes the proteasome-dependent and ubiquitin-mediated degradation of HER2, making small molecule chaperone antagonists exciting candidates for clinical development.

The measurement of ubiquitin and ubiquitinated proteins

Ubiquitination of key cellular proteins involved in signal transduction, gene transcription and cell-cycle regulation usually condemns those proteins to proteasomal or lysosomal degradation. Additionally, cycles of reversible ubiquitination regulate the function of certain proteins in a manner analogous to phosphorylation. In this short review we describe the current methodology for measuring ubiquitin and ubiquitination, provide examples which illustrate how various techniques have been used to study protein ubiquination, alert the readers of pitfalls to avoid, and offer guidelines to investigators newly interested in this novel post-translational protein modification.

The benzoquinone ansamycin geldanamycin stimulates proteolytic degradation of focal adhesion kinase

FAK is a nonreceptor tyrosine kinase involved in adhesion-mediated signal transduction whose level of expression is related to the invasiveness of malignant tumors. In seeking strategies to downregulate FAK, we treated various cell lines in vitro with the benzoquinone ansamycin geldanamycin (GA) which was previously described as a tyrosine kinase inhibitor, but recently has been shown to exert its effects by interfering with the chaperone function of members of the hsp90 family of heat-shock proteins. We evaluated the effects of benzoquinone ansamycins on FAK steady-state protein level and FAK half-life in breast and prostate carcinoma, Ewing's sarcoma, and 3T3 fibroblasts. Our data demonstrate that GA stimulates the proteolytic degradation of FAK in all cell lines examined and markedly reduces the half-life of newly synthesized FAK protein without significantly altering the level of FAK mRNA. These data demonstrate FAK to be another tyrosine kinase sensitive to the destabilizing effects of benzoquinone ansamycins and further show that small molecule-mediated pharmacologic modulation of FAK protein level is a feasible approach to the interdiction of FAK function.

p53 inhibits hypoxia-inducible factor-stimulated transcription

p53 is required for hypoxia-induced apoptosis in vivo, although the mechanism by which this occurs is not known. Conversely, induction of the hypoxia-inducible factor-1 (HIF-1) transactivator stimulates transcription of a number of genes crucial to survival of the hypoxic state. Here we demonstrate that p53 represses HIF-1-stimulated transcription. Although higher levels of p53 are required to inhibit HIF than are necessary to transcriptionally activate p53 target genes, these levels of p53 are similar to those that stimulate cleavage of poly(ADP-ribose) polymerase, an early event in apoptosis. Transfection of full-length p300 stimulates both p53-dependent and HIF-dependent transcription but does not relieve p53-mediated inhibition of HIF. In contrast, a p300 fragment, which binds to p53 but not to HIF-1, prevents p53-dependent repression of HIF activity. Transcriptionally inactive p53, mutated in its DNA binding domain, retains the ability to block HIF transactivating activity, whereas a transcriptionally inactive double point mutant defective for p300 binding does not inhibit HIF. Finally, depletion of doxorubicin-induced endogenous p53 by E6 protein attenuates doxorubicin-stimulated inhibition of HIF, suggesting that a p53 level sufficient for HIF inhibition can be achieved in vivo. These data support a model in which stoichiometric binding of p53 to a HIF/p300 transcriptional complex mediates inhibition of HIF activity.

Rapid deubiquitination of nucleosomal histones in human tumor cells caused by proteasome inhibitors and stress response inducers: effects on replication, transcription, translation, and the cellular stress response

The proteasome inhibitors, lactacystin and N-acetyl-leucyl-leucyl-norlucinal, caused a rapid and near-complete loss of approximately 22-23-kDa ubiquitinated nucleoproteins, which we have identified as monoubiquitinated nucleosomal histones H2A and H2B by immunological and two-dimensional electrophoretic techniques. In human SKBr3 breast tumor cells, depletion of monoubiquitinated histones by the proteasome inhibitors coincided with the accumulation of high molecular weight ubiquitinated proteins in both nucleoprotein and cytosolic fractions and decreased unconjugated ubiquitin in the cytosol, without changes in the nonubiquitinated core histones. Unconjugated ubiquitin was not detected in isolated tumor cell nuclei. A similar loss in monoubiquitinated histones occurred in cells harboring a defective, temperature-sensitive mutation of the ubiquitin-activating E1 enzyme, after these cells were elevated from 33 degrees C to the non-permissive temperature of 39 degrees C. DNA replication and RNA transcription were decreased by the proteasome inhibitors most strongly after 90% of the ubiquitin had been removed from ubiquitinated histones H2A and H2B, suggesting a relationship between the nucleosomal histone ubiquitin status and the processing of genetic information. Interestingly, although both proteasome inhibitors caused a generalized decrease in methionine incorporation into proteins, they strongly induced the synthesis of the hsp72 and hsp90 stress proteins. Finally, treating cells with heat-shock at 43 degrees C, with stress response-provoking chemicals or with several other proteasome inhibitors caused ubiquitinated proteins to accumulate, depleted free ubiquitin, and concomitantly decreased nucleosomal monoubiquitinated histones. These results suggest that deubiquitination of nucleosomal histones H2A and H2B may play a previously unrecognized role in the cellular stress response, as well as in the processing of chromatin, and emphasize the important role of the proteasome in cellular homeostasis.

Geldanamycin-stimulated destabilization of mutated p53 is mediated by the proteasome in vivo

Mutation of the tumor suppressor gene p53 is the most common genetic abnormality detected in human cancers. Wild type p53 is a short-lived protein with very low basal intracellular levels. Most mutated forms of the protein, however, display markedly increased intracellular levels as an essential feature of their positive transforming activity. In this report, we have used selective inhibitors of the 20S proteasome to demonstrate that processing of p53 by ubiquitination and proteasome-mediated degradation is impaired by commonly occuring mutations of the protein. We found that this impairment of p53 turnover can be reversed by treatment of tumor cells with the benzoquinone ansamycin, geldanamycin, leading to a marked reduction in intracellular p53 levels. Finally, using cells which over-express a mutant p53 protein, we were able to demonstrate that restoration of proteasome-mediated degradation by geldanamycin is accompanied by p53 polyubiquitination. Although much remains to be learned about the mechanisms involved, our data demonstrate that selective de-stabilization of mutant transforming proteins such as p53 can be achieved pharmacologically with agents such as geldanamycin which modify the function of molecular chaperone proteins within tumor cells.

Genistein-induced apoptosis of prostate cancer cells is preceded by a specific decrease in focal adhesion kinase activity

Genistein (5,7,4'-trihydroxyisoflavone), an isoflavinoid found in soy beans, has been identified as potentially causal for the low incidence of metastatic prostate cancer (PCa) in certain countries. Although genistein-induced PCa cell adhesion has been identified as a possible causative mechanism, direct growth inhibition by genistein has been reported and also could be causal. If in vivo growth inhibition was significant, then growth inhibition should occur at concentrations attained with dietary consumption, the mechanism of growth inhibition should be relevant to PCa, and genistein (a broad-spectrum in vitro protein-tyrosine kinase inhibitor) should have relatively specific kinase inhibitory effects in vivo. These considerations were investigated by measuring growth inhibitory activity in a variety of PCa cell lines. Growth inhibitory effects were shown not to occur with concentrations below the low micromolar range (i.e., 3 logs above that attained in serum). In-depth mechanistic studies with the PC3-M metastatic variant cell line demonstrated that growth inhibition was independent of genistein's estrogenic effects. Genistein was shown to decrease the viability of nonadherent cells, suggesting a lack of dependence on cell adhesion for growth inhibition. However, important molecular and kinetic differences between genistein's effects on growth in adherent versus nonadherent cells were identified. Specific suppression of focal adhesion kinase activity (without global decreases in phosphotyrosine) was shown to precede induction of apoptosis, which was responsible for growth inhibition in adherent cells. These findings do not support an in vivo growth inhibitory role by genistein consumed in quantities associated with a soy-based diet. They do, however, identify genistein as a potential therapeutic agent for PCa and as a tool with which to study the control of apoptosis in PCa.

Inhibition of EWS-FLI-1 fusion protein with antisense oligodeoxynucleotides

Ewing's sarcoma family of tumors (EFT) contain reciprocal translocations, of which approximately 90% occur between the long arm of chromosomes 11 and 22,t(11;22)(q24;q12) resulting in the formation of chimeric proteins generated by a fusion of the EWS and FLI-1 genes. To determine if EWS-FLI-1 protein is responsible for the Ewing sarcoma phenotype we have used sequence-specific antisense oligodeoxynucleotides (ODN) to block its expression. We have evaluated a series of antisense ODN directed toward the breakpoint region in an effort to prevent translation of the fusion messenger RNA. ODN were first evaluated in a cell-free in vitro translation system. Exogenously added RNase H was found to be required for translation inhibition. ODN that showed complete inhibition of translation were electroporated into TC-32 cells, a EFT cell line. Fusion protein and EWS protein levels were evaluated by Western blot analysis. A 40-60% decrease in the fusion protein was observed in TC-32 cells with antisense ODN directed toward the breakpoint region. Cell viability was reduced with antisense sequences in TC-32 cells but not in a prostate cancer cell line. Since inhibition of t(11:22) gene product is correlated to effects on cell viability reduction of the fusion protein may thus offer insight into the biology of EFT.

Depletion of p185erbB2, Raf-1 and mutant p53 proteins by geldanamycin derivatives correlates with antiproliferative activity

PURPOSE

Recently, it has been shown that geldanamycin (GA), a benzoquinone ansamycin, is able to deplete mutant p53, p185erbB2 and Raf-1 proteins in cancer cells. However, the relationship between these activities of GA and its antiproliferative activity is not clear. Here we investigated the effects of 28 GA derivatives in SKBr3, a human breast cancer cell line.

METHODS

We performed Western blot analysis of Raf-1, p185erbB2 and mutant p53 proteins following drug treatment and correlated these findings with the cytotoxicity of the various GA derivatives.

RESULTS

We found that downregulation of Raf-1, p185erbB2 and mutant p53 proteins was correlated. Thus, a drug that was active against one oncoprotein was equally active against the two others. Inactive derivatives were identified by their inability to downregulate these oncoproteins, even at a high dose (2 microM). These inactive drugs also had no or minimal antiproliferative activity (IC50 > 3 microM). All other analogs (at a concentration of 2 microM) downregulated p53, p185erbB2, and Raf-1, and also displayed cytotoxicity (IC50 in the range 6-600 nM). This category of drugs was further divided into more- and less-active agents by testing at lower doses (40 nM). The drugs that remained active against their molecular targets had an IC50 for antiproliferative activity of less than 40 nM. Maximal effects on mutant p53, p185erbB2 and Raf-1 were observed at doses that were 4-5 times greater than the cytotoxic IC50.

CONCLUSIONS

These findings suggest that GA and its derivatives are cytostatic/cytotoxic at concentrations that also downregulate Raf-1, p185erbB2 and mutant p53, and raise the possibility that depletion of these proteins and the antiproliferative activities of GA have a common mechanism.

Raf-1/bcl-2 phosphorylation: a step from microtubule damage to cell death

Recent studies have shown that paclitaxel leads to activation of Raf-1 kinase and have suggested that this activation is essential for bcl-2 phosphorylation and apoptosis. In the present study, we demonstrate that, in addition to paclitaxel, other agents that interact with tubulin and microtubules also induce Raf-1/bcl-2 phosphorylation, whereas DNA-damaging drugs, antimetabolites, and alkylating agents do not. Activation of Raf-1 kinase by paclitaxel is linked to tubulin polymerization; the effect is blunted in paclitaxel-resistant cells, the tubulin of which does not polymerize following the addition of paclitaxel. In contrast, vincristine and vinblastine, drugs to which the paclitaxel-resistant cells retain sensitivity were able to bring about Raf-1 phosphorylation. The requirement for disruption of microtubules in this signaling cascade was strengthened further using paclitaxel analogues by demonstrating a correlation between tubulin polymerization, Raf-1/bcl-2 phosphorylation, and cytotoxicity. Inhibition of RNA or protein synthesis prevents Raf-1 activation and bcl-2 phosphorylation, suggesting that an intermediate protein(s) acts upstream of Raf-1 in this microtubule damage-activating pathway. A model is proposed that envisions a pathway of Raf-1 activation and bcl-2 phosphorylation following disruption of microtubular architecture, serving a role similar to p53 induction following DNA damage.

Polyubiquitination and proteasomal degradation of the p185c-erbB-2 receptor protein-tyrosine kinase induced by geldanamycin

Treatment of SKBr3 human breast carcinoma cells with the benzoquinoid ansamycin, geldanamycin, rapidly depletes p185c-erbB-2 protein-tyrosine kinase. Loss of p185c-erbB-2 is initiated by disruption of a heteromeric complex between p185c-erbB-2 and the 94-kDa glucose-regulated protein, GRP94, to which geldanamycin binds avidly. Here we report that within minutes of exposure to geldanamycin, mature p185c-erbB-2 becomes polyubiquitinated. Treatment of cells with the specific proteasome proteolytic inhibitor, lactacystin, blocked geldanamycin-induced degradation of p185c-erbB-2 and enhanced the accumulation of polyubiquitinated p185c-erbB-2. Following geldanamycin and lactacystin treatment, a higher molecular weight form of p185c-erbB-2, which likely represents ubiquitin-p185c-erbB-2 conjugates, was detected by anti-p185c-erbB-2 immunoblotting. Nascent p185c-erbB-2 synthesized in the presence of geldanamycin is incompletely glycosylated and remains sequestered in the endoplasmic reticulum. While this immature form of the protein is not ubiquitinated in the presence of geldanamycin, its marked, drug-induced instability is nonetheless antagonized by lactacystin. Thus, the rapid depletion of mature p185c-erbB-2 caused by geldanamycin and the marked, drug-stimulated decrease in half-life of the newly synthesized protein are both mediated by the proteasome, although only the former phenomenon involves polyubiquitination.

Genistein-stimulated adherence of prostate cancer cells is associated with the binding of focal adhesion kinase to beta-1-integrin

The isoflavinoid genistein is a protein-tyrosine kinase inhibitor which has been identified as a putative cancer prevention agent. Its consumption is associated with a low incidence of clinical metastatic prostate cancer in the face of a sustained high incidence of organ-confined prostate cancer. We therefore undertook studies to examine genistein's effect upon cell adhesion as one possible mechanism by which it could be acting as an antimetastatic agent. A morphogenic analysis revealed that genistein caused cell flattening in a variety of cell lines: PC3-M, PC3, and DU-145 prostate carcinoma cells, as well as MCF-7 breast carcinoma cells. Mechanistic studies focused on the highly metastatic PC3-M cell line, and revealed that cell flattening was accompanied by an increase in cell adhesion. Further investigations demonstrated that focal adhesion kinase (FAK) accumulated in areas of focal cell attachment, and that this accumulation occurred only when cells were actively undergoing genistein-mediated morphologic change. Concurrent formation of a complex between the cell attachment molecule, beta-1-integrin, and FAK was shown to occur, and to correlate with transient activation of FAK activity. Genistein is presented as a novel investigative tool for use in the study of molecular events involved in the process of cell adhesion.

Mutant conformation of p53 translated in vitro or in vivo requires functional HSP90

The p53 mutant, 143ala, was translated in vitro in either rabbit reticulocyte lysate (RRL) or wheat germ extract (WGE). In RRL, p53-143ala protein of both mutant and wild-type conformation, as detected immunologically with conformation-specific antibodies, was translated. The chaperone protein HSP90, present in RRL, was found to coprecipitate only with the mutated conformation of p53. Geldanamycin, shown previously to bind to HSP90 and destabilize its association with other proteins, decreased the amount of immunologically detectable mutated p53 and increased the amount of detectable wild-type protein, without affecting the total translation of p53. When translated in WGE, known to contain functionally deficient HSP90, p53-143ala produced p53 protein, which was not recognized by a mutated conformation-specific antibody. In contrast, the synthesis of conformationally detectable wild-type p53 in this system was not compromised. Reconstitution of HSP90 function in WGE permitted synthesis of conformationally detectable mutated p53, and this was abrogated by geldanamycin. Finally, when p53-143ala was stably tansfected into yeast engineered to be defective for HSP90 function, conformational recognition of mutated p53 was impaired. When stable transfectants of p53-143ala were prepared in yeast expressing wild-type HSP90, conformational recognition of mutated p53 was antagonized by macbecin I, a geldanamycin analog also known to bind HSP90. Taken together, these data demonstrate a role for HSP90 in the achievement and/or stabilization of the mutated conformation of p53-143ala. Furthermore, we show that the mutated conformation of p53 can be pharmacologically antagonized by drugs targeting HSP90.

Electroporation of synthetic oligodeoxynucleotides: a novel technique for ex vivo bone marrow purging

Recent data suggest that tumor cells contaminating reinfused bone marrow may contribute to relapse in patients undergoing autologous bone marrow transplantation. Purging strategies that are able to remove these contaminating tumor cells need to be developed. This study describes how electroporation (EP) can be used to improve intracellular delivery of synthetic antisense oligodeoxynucleotides (ODNs), thereby enhancing their ability to suppress a target protein. Antisense ODNs that were introduced into cells by EP led to immediate suppression of targeted c-myc protein; this was associated with rapid cell death in the diffuse histiocytic lymphoma, U937; Burkitt's lymphoma, ST486; breast carcinoma, MCF-7; and Ewing's sarcoma, CHP-100, cell lines. Electroporation was found to have little or no detrimental effect on cells responsible for murine hematopoietic long-term reconstitution as determined from in vivo competitive repopulation studies. Using human c-myc-directed antisense ODNs as a model for the application of this approach to bone marrow purging, selective killing of human lymphoma U937 cells relative to normal human bone marrow cells was shown in cell mixing studies. In vivo studies were performed in which a survival advantage was shown for athymic mice that were inoculated with antisense-treated U937 cells as opposed to control cells. These studies suggest that EP of bone marrow may be of use in enhancing intracellular delivery of a variety of molecular/pharmaceutical agents. Taken together, these data suggest that the use of electroporation to enhance delivery of antisense ODNs is a promising new approach towards ex vivo bone marrow purging.

p185erbB2 binds to GRP94 in vivo. Dissociation of the p185erbB2/GRP94 heterocomplex by benzoquinone ansamycins precedes depletion of p185erbB2

Treatment of SKBr3 cells with benzoquinone ansamycins, such as geldanamycin (GA), depletes p185erbB2, the receptor tyrosine kinase encoded by the erbB2 gene. In the same cells, a biologically active benzoquinone photoaffinity label specifically binds a protein of about 100 kDa, and the ability of various GA derivatives to reduce the intracellular level of p185erbB2 correlates with their ability to compete with the photoaffinity label for binding to this protein. In this report, we present evidence that the approximately 100-kDa ansamycin-binding protein is GRP94. Membrane-associated p185erbB2 exists in a stable complex with GRP94. GA binding to GRP94 disrupts this complex, leading to degradation of pre-existing p185erbB2 protein, and resulting in an altered subcellular distribution of newly synthesized p185erbB2.

Taxol induction of p21WAF1 and p53 requires c-raf-1

Taxol stabilizes microtubules, prevents tubulin depolymerization, and promotes tubulin bundling and is one of the most effective drugs for the treatment of metastatic breast and ovarian cancer. Although its interaction with tubulin has been well characterized, the mechanism by which taxol induces growth arrest and cytotoxicity is not well understood. Herein, we show that taxol induced dose- and time-dependent accumulation of the cyclin inhibitor p21WAF1 in both p53 wild-type and p53-null cells, although the degree of induction was greater in cells expressing wild-type p53. In MCF7 cells, wild-type p53 protein was also induced after taxol treatment, and this induction was mediated primarily by increased protein stability. Taxol induced both p21WAF1 and wild-type p53 optimally in MCF7 cells after 20-24-h exposure with an EC50(3) of 5 nM. In p53-null PC3M cells, p21WAF1 was similarly induced after 24-h exposure to taxol. Coincident with these biochemical effects, taxol altered the electrophoretic mobility of c-raf-1 and stimulated mitogen activated protein kinase. Previous depletion of c-raf-1 inhibited both the p21WAF1- and p53-inducing properties of taxol, as well as the activation of MAP kinase. These data suggest that induction of p21WAF1 by taxol requires c-raf-1 activity, but that it is not strictly dependent on wild-type p53. Furthermore, the ability of taxol to both induce wild-type p53 in MCF7 cells and activate MAP kinase is also dependent on c-raf-1 expression.

Disruption of the Raf-1-Hsp90 molecular complex results in destabilization of Raf-1 and loss of Raf-1-Ras association

Cytosolic Raf-1 exists in a high molecular weight complex with the heat shock protein Hsp90, the purpose of which is unknown. The benzoquinone ansamycin, geldanamycin, specifically binds to Hsp90 and disrupts certain multimolecular complexes containing this protein. Using this drug, we are able to demonstrate rapid dissociation of both Raf-1-Hsp90 and Raf-1-Ras multimolecular complexes, concomitant with a markedly decreased half-life of the Raf-1 protein. Continued disruption of the Raf-1-Hsp90 complex results in apparent loss of Raf-1 protein from the cell, although Raf-1 synthesis is actually increased. Prevention of Raf-1-Hsp90 complex formation interferes with trafficking of newly synthesized Raf-1 from cytosol to plasma membrane. These data indicate that association with Hsp90 is essential for both Raf-1 protein stability and its proper localization in the cell.

Contribution of sequence and phosphorothioate content to inhibition of cell growth and adhesion caused by c-myc antisense oligomers

c-myc is overexpressed in glioblastoma multiforme, the most common form of brain tumor. To find a suitable target for in vivo antisense therapy of gliomas, we investigated the biological effects on the human glioma cell line, U87MG, of antisense oligonucleotides targeted against the translation start site of c-myc mRNA. Parameters examined included c-myc protein level, cell proliferation, and cell adhesion to substratum. Oligonucleotides were administered by electroporation as capped phosphorothioates. Antisense oligomers caused a reduction in c-myc protein expression, loss of cell adhesion to plastic, and complete growth inhibition. Various control sequences, including sense, scrambled, and three-base mismatched oligomers, were also tested. Some of the controls retained a dG quartet found in the antisense sequence. Reduction in c-myc protein and cell growth and loss of cell adhesion were specific to the antisense sequence. Surprisingly, fully thioated antisense and scrambled sequences, either containing or lacking a dG quartet, were equally inhibitory to both cell growth and adhesion. Loss of cell adhesion was observed with only phosphorothioate-containing oligomers, not with either their phosphodiester or nuclease-resistant PA congeners, and was completely reversed when cells were plated onto fibronectin. These results demonstrate that a commonly used c-myc antisense oligomer also displays dramatic, sequence- but not antisense-specific effects on cell proliferation and cellular adhesion, depending on the backbone.

Geldanamycin selectively destabilizes and conformationally alters mutated p53

Mutated p53 proteins interfere in the function of wild type p53 and may also serve as a dominant oncogene. The vast majority of p53 mutations result in a protein of altered conformation and prolonged half-life. We sought to examine whether geldanamycin, a drug capable of destabilizing several oncogene and proto-oncogene products, could alter the stability and DNA binding characteristics of several mutated p53 proteins. Brief exposure to GA destabilized the p53 protein of several breast, prostate and leukemic cell lines harboring mutated p53 alleles, resulting in a significant reduction in p53 steady state level and half-life. In contrast to its effects on mutated p53, GA altered neither steady state level nor inducibility of the wild type protein. In addition to its effects on protein stability, GA also altered the conformation of mutated p53, so that it was no longer detectable with a mutant conformation-specific antibody. Finally, mutated p53 protein isolated from GA-treated cells regained partial ability to bind a wild type-specific p53 DNA consensus sequence. These data indicate the feasibility of pharmacologic intervention for altering the mutated p53 phenotype.

Inhibition of protein-tyrosine kinase activity in intact cells by the aptameric action of oligodeoxynucleotides

Direct interaction of oligodeoxynucleotides (ODNs) with proteins represents one of the nonantisense-mediated effects of ODNs. Phosphorothioate-capped ODNs have been shown to inhibit directly the in vitro kinase activity of the chronic myelogenous leukemia-associated protein-tyrosine kinase p210bcr-abl. In this study we have determined the efficacy of this aptameric ODN in a cellular system using the K562 chronic myelogenous leukemia-derived cell line. Significant effects upon cellular phosphotyrosine content, as well as cellular growth in soft agar, are observed. These effects are sequence specific and are not mediated through changes in p210bcr-abl protein levels. Additional ODNs are described that also reduce cellular phosphotyrosine levels and inhibit growth in soft agar but do not inhibit p210bcr-abl kinase activity in vitro.

Episome generated C-myc antisense RNA inhibits growth and tumorigenicity of a human neuroendocrine tumor-cell line

The neuroepithelioma cell line CHP100 expresses low but detectable amounts of N-myc protein together with large amounts of c-myc protein. We have recently demonstrated that antisense inhibition of N-myc expression in CHP100 cells leads to decreased in vitro growth and alterations in cellular morphology without affecting tumorigenicity in nude mice. In this study we report the construction of an episomally replicating vector designed to generate RNA antisense to part of the human c-myc gene. Such a Vector is able to inhibit c-myc expression in cell lines carrying multiple copies of the gene. Inhibition of c-myc expression leads to a decrease of in vitro growth and cloning efficiency and in vivo tumorigenicity of CHP100 cells. Our findings suggest that N-myc and c-myc subserve different functions in regulating the biology of CHP100 cells.