The mitochondrial type IB topoisomerase drives mitochondrial translation and carcinogenesis

Mitochondrial topoisomerase IB (TOP1MT) is a nuclear-encoded topoisomerase, exclusively localized to mitochondria, which resolves topological stress generated during mtDNA replication and transcription. Here, we report that TOP1MT is overexpressed in cancer tissues and demonstrate that TOP1MT deficiency attenuates tumor growth in human and mouse models of colon and liver cancer. Due to their mitochondrial dysfunction, TOP1MT-KO cells become addicted to glycolysis, which limits synthetic building blocks and energy supply required for the proliferation of cancer cells in a nutrient-deprived tumor microenvironment. Mechanistically, we show that TOP1MT associates with mitoribosomal subunits, ensuring optimal mitochondrial translation and assembly of oxidative phosphorylation complexes that are critical for sustaining tumor growth. The TOP1MT genomic signature profile, based on Top1mt-KO liver cancers, is correlated with enhanced survival of hepatocellular carcinoma patients. Our results highlight the importance of TOP1MT for tumor development, providing a potential rationale to develop TOP1MT-targeted drugs as anticancer therapies.

Abstract B16: Evaluation of LDH inhibition as a treatment strategy in Ewing sarcoma

Long-term outcomes for patients with relapsed, recurrent, or metastatic Ewing sarcoma (ES) remain poor despite advances in multimodal chemotherapy and local tumor control. The discovery of new targets and novel therapies is therefore critical to improving care for these patients. Recent insights into the metabolic landscape of ES have revealed that the EWS/FLI1 fusion oncoprotein regulates metabolic pathways in this disease, including shifting glucose consumption away from oxidative metabolism and towards glycolysis, a pathway that relies on LDH. Targeting this increased dependence on glycolysis presents an opportunity to inhibit the growth of ES cells through a novel therapeutic approach, while potentially limiting the toxicity delivered to normal cells. We therefore sought to evaluate the activity of NCGC-737 and NCGC-006, two novel LDHA/B inhibitors identified and validated as part of the Experimental Therapeutics Program of the National Cancer Institute (NCI-NExT), in ES.

For in vitro studies, proliferation of ES cells lines was assessed after inhibition of LDHA/B by each agent using IncuCyte and MTS assays. Protein expression of phospho- and total LDH was evaluated by Western blot. LDH activity was assessed using the pyruvate-dependent oxidation of NADH. NAD/NADH levels were determined using NAD/NADH-Glo. Analysis of glycolytic profiles was performed using the Agilent Extracellular Flux Analyzer. For in vivo studies, female SCID mice underwent orthotopic injection of ES cells from established cell lines. When tumors reached a desired size, mice were randomized and then treated on a variety of dosing schedules. Toxicity assessments included evaluation of overall appearance, weekly weights, blood sampling, and full necropsies on selected mice. Tumors were measured twice per week for assessment of efficacy. Tumors were harvested at midpoints and at study endpoint for assessments of drug level, target inhibition, and biology.

ES cell lines displayed varying sensitivity to NCGC-737 and NCGC-006, with IC-50 values ranging from 50 nM to 500 nM. While protein expression of phospho-LDH, total LDH-A, and total LDH-B were not correlated with sensitivity to either agent, glycolytic profiles were predictive of sensitivity. Cell lines that underwent a greater reduction in glycolytic capacity (the change in ECAR measured before and after oligomycin treatment) after LDHA/B inhibition experienced a greater antiproliferative effect, while cell lines that were able to maintain glycolytic capacity despite LDHA/B inhibition exhibited less of an effect on growth. In vivo studies to describe the toxicity of these agents demonstrated that hemolysis was the primary dose-limiting toxicity, and was dose dependent. Additional toxicity studies of specific tissues are ongoing and will be reported. Preliminary in vivo studies to optimize dosing regimen established that compared to oral dosing, intravenous dosing resulted in higher and more consistent tumor drug levels and improved target inhibition, with up to 93% of intratumoral LDH activity inhibited. Efficacy studies are ongoing and will be reported.

Preclinical data suggest that inhibition of LDHA/B may represent a potentially novel therapeutic strategy in the treatment of ES.

Citation Format: Christine M. Heske, Anna E. Gibson, Josh T. Baumgart, Choh Yeung, Sameer H. Issaq, A Mendoza, Michelle S. Johnson, Guiseppe L. Squadrito, Lillian Culp, Victor M. Darley-Usmar, Len M. Neckers. Evaluation of LDH inhibition as a treatment strategy in Ewing sarcoma [abstract]. In: Proceedings of the AACR Special Conference: Pediatric Cancer Research: From Basic Science to the Clinic; 2017 Dec 3-6; Atlanta, Georgia. Philadelphia (PA): AACR; Cancer Res 2018;78(19 Suppl):Abstract nr B16.

Tumor-intrinsic and tumor-extrinsic factors impacting hsp90- targeted therapy

In 1994 the first heat shock protein 90 (Hsp90) inhibitor was identified and Hsp90 was reported to be a target for anticancer therapeutics. In the past 18 years there have been 17 distinct Hsp90 inhibitors entered into clinical trial, and the small molecule Hsp90 inhibitors have been highly valuable as probes of the role of Hsp90 and its client proteins in cancer. Although no Hsp90 inhibitor has achieved regulatory approval, recently there has been significant progress in Hsp90 inhibitor clinical development, and in the past year RECIST responses have been documented in HER2-positive breast cancer and EML4-ALK-positive non-small cell lung cancer. All of the clinical Hsp90 inhibitors studied to date are specific in their target, i.e. they bind exclusively to Hsp90 and two related heat shock proteins. However, Hsp90 inhibitors are markedly pleiotropic, causing degradation of over 200 client proteins and impacting critical multiprotein complexes. Furthermore, it has only recently been appreciated that Hsp90 inhibitors can, paradoxically, cause transient activation of the protein kinase clients they are chaperoning, resulting in initiation of signal transduction and significant physiological events in both tumor and tumor microenvironment. An additional area of recent progress in Hsp90 research is in studies of the posttranslational modifications of Hsp90 itself and Hsp90 co-chaperone proteins. Together, a picture is emerging in which the impact of Hsp90 inhibitors is shaped by the tumor intracellular and extracellular milieu, and in which Hsp90 inhibitors impact tumor and host on a microenvironmental and systems level. Here we review the tumor intrinsic and extrinsic factors that impact the efficacy of small molecules engaging the Hsp90 chaperone machine.

The role of plasmacytoma growth factor in the in vitro responses of murine plasmacytoma cells

Many plasmacytomas arising in BALB/c mice are dependent upon a specific, macrophage-derived plasmacytoma growth factor for survival and proliferation in vitro. Adherent cells taken from the peritoneal oil granuloma in which the early plasmacytomas arise and proliferate produce 50 times more PCT-GF activity in vitro than do normal peritoneal cells, thus suggesting a possible in vivo role for PCT-GF. Purification and amino acid sequencing of PCT-GF derived from the murine macrophage cell line, P388D1, have identified a 23 kDa protein with a unique NH2-terminal sequence. This molecule is now known as murine IL6. As part of the characterization of murine Il-6, genomic sequences have been localized to the proximal end of mouse chromosome 5 via Southern analysis of restriction fragment length polymorphisms. The removal of IL6 from IL6-dependent PCT cell lines results in a growth arrest in early G1. This is accompanied by a rapid and specific loss of transferrin receptor expression and results in eventual cell death. It appears that the response to IL6 is at least partially dependent on Ca++ because functional Ca++ channels are necessary for the PCT cells to pass through G1 and to maintain transferrin receptor expression.

Stereospecific antitumor activity of radicicol oxime derivatives

PURPOSE

Radicicol is a novel hsp90 antagonist, distinct from the chemically unrelated benzoquinone ansamycin compounds, geldanamycin and herbimycin. Both geldanamycin and radicicol bind in the aminoterminal nucleotide-binding pocket of hsp90, destabilizing the hsp90 client proteins, many of which are essential for tumor cell growth. We describe here antitumor activity of a novel oxime derivative of radicicol, KF58333. We also investigated the mechanism of antitumor activity of KF58333 in comparison with its oxime isomer KF58332.

METHODS

Antiproliferative activities were determined in a panel of breast cancer cell lines in vitro. We also examined inhibition of hsp90 function and apoptosis induction in erbB2-overexpressing human breast carcinoma KPL-4 cells in vitro. Direct binding activity to hsp90 was assessed by hsp90-binding assays using geldanamycin or radicicol beads. In animal studies, we investigated plasma concentrations of these compounds after i.v. injection in BALB/c mice and antitumor activity against KPL-4 cells transplanted into nude mice. Inhibition of hsp90 function and induction of apoptosis in vivo were investigated using tumor specimens from drug-treated animals.

RESULTS

KF58333 showed potent antiproliferative activity against all breast cancer cell lines tested in vitro, and was more potent than its stereoisomer KF58332. These results are consistent with the ability of KF58333 to deplete hsp90 client proteins and the induction of apoptosis in KPL-4 cells in vitro. Interestingly, KF58333, but not KF58332, showed significant in vivo antitumor activity accompanied by induction of apoptosis in KPL-4 human breast cancer xenografts. Although the plasma concentrations of these compounds were equivalent, KF58333, but not KF58332, depleted hsp90 client proteins such as erbB2, raf-1 and Akt in the tumor specimen recovered from nude mice.

CONCLUSIONS

These results suggest that inhibition of hsp90 function, which causes depletion of hsp90 client proteins in tumor, contributes to the antitumor activity of KF58333, and that the stereochemistry of the oxime moiety is important for the biological activity of radicicol oxime derivatives.

The Hsp90 inhibitor geldanamycin selectively sensitizes Bcr-Abl-expressing leukemia cells to cytotoxic chemotherapy

The Bcr-Abl fusion protein drives leukemogenesis and can render leukemia cells resistant to conventional chemotherapy. Geldanamycin (GA), a drug which destabilizes Hsp90-associated proteins, depletes cells of Bcr-Abl, an Hsp90 client, but not of Abl. Both HL60 cells transfected with Bcr-Abl and naturally Ph1-positive K562 leukemia cells are resistant to most cytotoxic drugs, but were found to be sensitive to GA. Furthermore, GA sensitized Bcr-Abl-expressing cells to doxorubicin (DOX) and paclitaxel (PTX). In contrast, in parental HL60 cells, 90 nM GA inhibited PARP cleavage, nuclear fragmentation, and cell death caused by 500 ng/ml DOX. Like GA, STI 571 (an inhibitor of the Abl kinase) sensitized Bcr-Abl-expressing cells to DOX. Unlike GA, STI 571 did not antagonize the cytotoxic effects of DOX in parental HL60 cells. These results indicate that sensitization of Bcr-Abl-expressing cells, but not desensitization of HL60 cells, depends on inhibition of Bcr-Abl. Thus, GA differentially affects leukemia cells depending on their Bcr-Abl expression and selectively increases apoptosis in Bcr-Abl-expressing cells.

Requirement for HDM2 activity in the rapid degradation of p53 in neuroblastoma

The wild type p53 tumor suppressor protein is rapidly degraded in normal cells by MDM2, the ubiquitin ligase that serves as the key regulator of p53 function by modulating protein stability. Cellular exposure to genotoxic stress triggers the stabilization of p53 by multiple pathways that converge upon interference with MDM2 function. In this study, we first investigated the ability of HDM2 (MDM2 human homologue) to degrade endogenous p53 in neuroblastoma (NB). Although the p53 protein in NB has been reported to be constitutively stabilized, we find that HDM2 in NB is functional and facilitates the rapid turnover of p53 in nonstressed cells via the proteasome pathway. Second, we examined the relationship between p53 and HDM2 in the adriamycin-mediated stabilization of p53 in NB. We demonstrate that while p53 stabilization depends neither upon the phosphorylation of specific N-terminal sites nor upon dissociation from HDM2, it requires inactivation of functional HDM2. In support of this notion, p53 stabilization following adriamycin resulted in an inhibition of both p53 ubiquitination and HDM2 ligase activity. Taken together, these data implicate a requirement for enzymatic inactivation of HDM2 as a novel mechanism for p53 stabilization in the DNA damage response pathway.

Geldanamycin abrogates ErbB2 association with proteasome-resistant beta-catenin in melanoma cells, increases beta-catenin-E-cadherin association, and decreases beta-catenin-sensitive transcription

Beta-catenin undergoes both serine and tyrosine phosphorylation. Serine phosphorylation in the amino terminus targets beta-catenin for proteasome degradation, whereas tyrosine phosphorylation in the COOH terminus influences interaction with E-cadherin. We examined the tyrosine phosphorylation status of beta-catenin in melanoma cells expressing proteasome-resistant beta-catenin, as well as the effects that perturbation of beta-catenin tyrosine phosphorylation had on its association with E-cadherin and on its transcriptional activity. Beta-catenin is tyrosine phosphorylated in three melanoma cell lines and associates with both the ErbB2 receptor tyrosine kinase and the LAR receptor tyrosine phosphatase. Geldanamycin, a drug which destabilizes ErbB2, caused rapid cellular depletion of the kinase and loss of its association with beta-catenin without perturbing either LAR or beta-catenin levels or LAR/beta-catenin association. Geldanamycin also stimulated tyrosine dephosphorylation of beta-catenin and increased beta-catenin/E-cadherin association, resulting in substantially decreased cell motility. Geldanamycin also decreased the nuclear beta-catenin level and inhibited beta-catenin-driven transcription, as assessed using two different beta-catenin-sensitive reporters and the endogenous cyclin D1 gene. These findings were confirmed by transient transfection of two beta-catenin point mutants, Tyr-654Phe and Tyr-654Glu, which, respectively, mimic the dephosphorylated and phosphorylated states of Tyr-654, a tyrosine residue contained within the beta-catenin-ErbB2-binding domain. These data demonstrate that the functional activity of proteasome-resistant beta-catenin is regulated further by geldanamycin-sensitive tyrosine phosphorylation in melanoma cells.

The heat shock protein 90 antagonist novobiocin interacts with a previously unrecognized ATP-binding domain in the carboxyl terminus of the chaperone

Heat shock protein 90 (Hsp90), one of the most abundant chaperones in eukaryotes, participates in folding and stabilization of signal-transducing molecules including steroid hormone receptors and protein kinases. The amino terminus of Hsp90 contains a non-conventional nucleotide-binding site, related to the ATP-binding motif of bacterial DNA gyrase. The anti-tumor agents geldanamycin and radicicol bind specifically at this site and induce destabilization of Hsp90-dependent client proteins. We recently demonstrated that the gyrase inhibitor novobiocin also interacts with Hsp90, altering the affinity of the chaperone for geldanamycin and radicicol and causing in vitro and in vivo depletion of key regulatory Hsp90-dependent kinases including v-Src, Raf-1, and p185(ErbB2). In the present study we used deletion/mutation analysis to identify the site of interaction of novobiocin with Hsp90, and we demonstrate that the novobiocin-binding site resides in the carboxyl terminus of the chaperone. Surprisingly, this motif also recognizes ATP, and ATP and novobiocin efficiently compete with each other for binding to this region of Hsp90. Novobiocin interferes with association of the co-chaperones Hsc70 and p23 with Hsp90. These results identify a second site on Hsp90 where the binding of small molecule inhibitors can significantly impact the function of this chaperone, and they support the hypothesis that both amino- and carboxyl-terminal domains of Hsp90 interact to modulate chaperone activity.

Novel oxime derivatives of radicicol induce erythroid differentiation associated with preferential G(1) phase accumulation against chronic myelogenous leukemia cells through destabilization of Bcr-Abl with Hsp90 complex

Chronic myelogenous leukemia (CML) is a clonal disorder of a pluripotent hematopoietic stem cells characterized by a chimeric bcr-abl gene giving rise to a p210(Bcr-Abl) protein with dysregulated tyrosine kinase activity. Radicicol, a macrocyclic antifungal antibiotic, binds to the N-terminal of heat shock protein 90 (Hsp90) and destabilizes Hsp90-associated proteins such as Raf-1. This study investigated the effect of radicicol, novel oxime derivatives of radicicol (KF25706 and KF58333), and herbimycin A (HA), a benzoquinoid ansamycin antibiotic, on the growth and differentiation of human K562 CML cells. Although KF25706 and KF58333 induced the expression of glycophorin A in K562 cells, radicicol and HA caused erythroid differentiation transiently. Cell cycle analysis showed that G(1) phase accumulation was observed in K562 cells treated with KF58333. KF58333 treatment depleted p210(Bcr-Abl), Raf-1, and cellular tyrosine phosphorylated proteins in K562 cells, whereas radicicol and HA showed transient depletion of these proteins. KF58333 also down-regulated the level of cell cycle-dependent kinases 4 and 6 and up-regulated cell cycle-dependent kinase inhibitor p27(Kip1) protein without an effect on the level of Erk and Hsp90 proteins. Immunoprecipitation analysis showed that p210(Bcr-Abl) formed multiple complexes with Hsp90, some containing p23 and others Hsp70; KF58333 treatment dissociated p210(Bcr-Abl) from Hsp90/p23 chaperone complexes. Furthermore, KF58333 induced apoptosis in K562 cells and administration of KF58333 prolonged the survival time of SCID mice inoculated with K562 cells. These results suggest that KF58333 may have therapeutic potential for the treatment of CML that involves abnormal cellular proliferation induced by p210(Bcr-Abl).

The heat shock protein 90 antagonist geldanamycin alters chaperone association with p210bcr-abl and v-src proteins before their degradation by the proteasome

Several important signaling proteins including transcription factors and protein kinases depend on heat shock protein (Hsp)-90 for stability. p210bcr-abl, a protein expressed in chronic myelogenous leukemia, is functionally inhibited by the benzoquinone ansamycin herbimycin A. Benzoquinone ansamycins also bind to and inhibit the activity of Hsp90. We now demonstrate that p210bcr-abl is complexed with Hsp90 and its cochaperone p23 in K562 chronic myelogenous leukemia cells. Brief exposure to the benzoquinone ansamycin Hsp90 inhibitor geldanamycin (GA) decreases the association of p210bcr-abl with Hsp90 and p23 and increases its association with the chaperones Hsp70 and p60Hop. GA has a similar effect on chaperone association with v-src, another Hsp90-dependent oncogenic kinase. Loss of Hsp90/p23 association and acquisition of Hsp70/p60Hop association of both p210bcr-abl and v-src precede GA-induced degradation of these kinases. GA-induced degradation is mediated by the proteasome because proteasome inhibitors block the effects of GA, causing both p210bcr-abl and v-src to accumulate in a detergent-insoluble cellular fraction. Both p210bcr-abl and v-src are more susceptible to GA-induced degradation than are their normal cellular counterparts, c-abl and c-src.

Regulation of BRCA1 by protein degradation

BRCA1, a tumor suppressor protein implicated in hereditary forms of breast and ovarian cancer, is transcriptionally regulated in a proliferation-dependent manner. In this study, we demonstrate a substantial role for proteolysis in regulating the BRCA1 steady-state protein level in several cell lines. N-acetyl-leu-leu-norleucinal (ALLN), an inhibitor of the proteasome, calpain, and cathepsins, caused BRCA1 protein to accumulate in the nucleus of several human breast, prostate, and melanoma cell lines which express low or undetectable basal levels of BRCA1 protein, but not in cells with high basal expression of BRCA1. Protease inhibition did not increase BRCA1 synthesis, nor change its mRNA level, but it dramatically prolonged the protein's half-life. In contrast to ALLN, lactacystin and PS341, two specific proteasome inhibitors, as well as calpastatin peptide and PD150606, two selective calpain inhibitors, had no effect on BRCA1 stability, whereas ALLM, an effective calpain and cathepsin inhibitor but weak proteasome inhibitor, did stimulate accumulation of BRCA1. Moreover, three inhibitors of acidic cysteine proteases, chloroquine, ammonium chloride and bafilomycin, were as effective as ALLN. These results demonstrate that degradation by a cathepsin-like protease in fine balance with BRCA1 transcription is responsible for maintaining the low steady-state level of BRCA1 protein seen in many cancer cells.

Interaction of radicicol with members of the heat shock protein 90 family of molecular chaperones

The Hsp90 family of proteins in mammalian cells consists of Hsp90 alpha and beta, Grp94, and Trap-1 (Hsp75). Radicicol, an antifungal antibiotic that inhibits various signal transduction proteins such as v-src, ras, Raf-1, and mos, was found to bind to Hsp90, thus making it the prototype of a second class of Hsp90 inhibitors, distinct from the chemically unrelated benzoquinone ansamycins. We have used two novel methods to immobilize radicicol, allowing for detailed analyses of drug-protein interactions. Using these two approaches, we have studied binding of the drug to N-terminal Hsp90 point mutants expressed by in vitro translation. The results point to important drug contacts with amino acids inside the N-terminal ATP/ADP-binding pocket region and show subtle differences when compared with geldanamycin binding. Radicicol binds more strongly to Hsp90 than to Grp94, the Hsp90 homolog that resides in the endoplasmic reticulum. In contrast to Hsp90, binding of radicicol to Grp94 requires both the N-terminal ATP/ADP-binding domain as well as the adjacent negatively charged region. Radicicol also specifically binds to yeast Hsp90, Escherichia coli HtpG, and a newly described tumor necrosis factor receptor-interacting protein, Trap-1, with greater homology to bacterial HtpG than to Hsp90. Thus, the radicicol-binding site appears to be specific to and is conserved in all members of the Hsp90 family of molecular chaperones from bacteria to mammals, but is not present in other molecular chaperones with nucleotide-binding domains.

KF25706, a novel oxime derivative of radicicol, exhibits in vivo antitumor activity via selective depletion of Hsp90 binding signaling molecules

Radicicol, a macrocyclic antifungal antibiotic, has been shown to bind to the heat shock protein 90 (Hsp90) chaperone, interfering with its function. Hsp90 family chaperones have been shown to associate with several signaling molecules and play an essential role in signal transduction, which is important for tumor cell growth. Because radicicol lacks antitumor activity in vivo in experimental animal models, we examined the antitumor activity of a novel radicicol oxime derivative, radicicol 6-oxime (KF25706), on human tumor cell growth both in vitro and in vivo. KF25706 showed potent antiproliferative activities against various human tumor cell lines in vitro and inhibited v-src- and K-ras-activated signaling as well as radicicol. In addition, Hsp90 family chaperone-associated proteins, such as p185erbB2, Raf-1, cyclin-dependent kinase 4, and mutant p53, were depleted by KF25706 at a dose comparable to that required for antiproliferative activity. KF25706 was also shown to compete with geldanamycin for binding to Hsp90. KF29163, which is an inactive derivative of radicicol, was less potent both in p185erbB2 depletion and Hsp90 binding. More importantly, KF25706 showed significant growth-inhibitory activity against human breast carcinoma MX-1 cells transplanted into nude mice at a dose of 100 mg/kg twice daily for five consecutive i.v. injections. KF25706 was also shown to possess antitumor activity against human breast carcinoma MCF-7, colon carcinoma DLD-1, and vulval carcinoma A431 cell lines in vivo in an animal model. Finally, we confirmed the depletion of Hsp90-associated signaling molecules (Raf-1 and cyclin-dependent kinase 4) with ex vivo Western blotting analysis using MX-1 xenografts. In agreement with in vivo antitumor activity, KF25706 depleted Hsp90-associated molecules in vivo, whereas KF29163 and radicicol did not show this activity in vivo. Taken together, these results suggest that antitumor activity of KF25706 may be mediated, at least in part, by binding to Hsp90 family proteins and destabilization of Hsp90-associated signaling molecules.

The benzoquinone ansamycin 17-allylamino-17-demethoxygeldanamycin binds to HSP90 and shares important biologic activities with geldanamycin

PURPOSE

Benzoquinone ansamycins are antibiotics with anticancer potential. First described as tyrosine kinase inhibitors, they are now frequently used to target HSP90 chaperone function. While herbimycin A and geldanamycin (GA) have been widely used in preclinical studies, both drugs are poor candidates for clinical trials owing to their in vivo toxicity and lack of stability. We therefore examined the biologic effects of 17-allylamino-17-demethoxygeldanamycin (17-AG), an ansamycin derivative with lower in vivo toxicity than GA.

METHODS

Binding of 17-AG to HSP90 was studied in vitro using a GA-affinity beads competition assay. We analyzed the drug-induced destabilization of p185erbB2, Raf-1 and mutant p53 in SKBR3 breast cancer cells by Western blotting. The antiproliferative activities of 17-AG and GA were compared using the MTT assay.

RESULTS

We found that, in a similar manner to GA itself, 17-AG bound specifically to HSP90. It also led to degradation of the receptor tyrosine kinase p185erbB2, the serine/threonine kinase Raf-1 and mutant p53. Both GA and 17-AG displayed comparable antiproliferative effects in SKBR3 and MCF7 cells. Even though HSP90 binding by 17-AG was weaker than by GA, 17-AG and GA caused biologic effects in tumor cells at similar doses.

CONCLUSION

17-AG shares the important biologic features of its parent compound GA. Since 17-AG has a better toxicity profile than GA, it is an interesting candidate benzoquinone ansamycin for clinical development.

Antibiotic radicicol binds to the N-terminal domain of Hsp90 and shares important biologic activities with geldanamycin

The molecular chaperone Hsp90 plays an essential role in the folding and function of important cellular proteins including steroid hormone receptors, protein kinases and proteins controlling the cell cycle and apoptosis. A 15 A deep pocket region in the N-terminal domain of Hsp90 serves as an ATP/ADP-binding site and has also been shown to bind geldanamycin, the only specific inhibitor of Hsp90 function described to date. We now show that radicicol, a macrocyclic antifungal structurally unrelated to geldanamycin, also specifically binds to Hsp90. Moreover, radicicol competes with geldanamycin for binding to the N-terminal domain of the chaperone, expressed either by in vitro translation or as a purified protein, suggesting that radicicol shares the geldanamycin binding site. Radicicol, as does geldanamycin, also inhibits the binding of the accessory protein p23 to Hsp90, and interferes with assembly of the mature progesterone receptor complex. Radicicol does not deplete cells of Hsp90, but rather increases synthesis as well as the steady-state level of this protein, similar to a stress response. Finally, radicicol depletes SKBR3 cells of p185erbB2, Raf-1 and mutant p53, similar to geldanamycin. Radicicol thus represents a structurally unique antibiotic, and the first non-benzoquinone ansamycin, capable of binding to Hsp90 and interfering with its function.

Stabilization of wild-type p53 by hypoxia-inducible factor 1alpha

Although hypoxia (lack of oxygen in body tissues) is perhaps the most physiological inducer of the wild-type p53 gene, the mechanism of this induction is unknown. Cells may detect low oxygen levels through a haem-containing sensor protein. The hypoxic state can be mimicked by using cobalt chloride and the iron chelator desferrioxamine: like hypoxia, cobalt chloride and desferrioxamine activate hypoxia-inducible factor 1alpha (HIF-1alpha), which stimulates the transcription of several genes that are associated with hypoxia. Here we show that these treatments induce accumulation of wild-type p53 through HIF-1alpha-dependent stabilization of p53 protein. Induction of p53 does not occur in either a mutant hepatoma cell line that is unable to induce HIF-1alpha or embryonic stem cells derived from mice lacking HIF-1beta. HIF-1alpha is found in p53 immunoprecipitates from MCF7 cells that express wild-type p53 and are either hypoxic or have been exposed to desferrioxamine. Similarly, anti-haemagglutinin immunoprecipitates from lysates of normoxic PC3M cells that had been co-transfected with haemagglutinin-tagged HIF-1alpha and wild-type p53 also contain p53. Transfection of normoxic MCF7 cells with HIF-1alpha stimulates a co-transfected p53-dependent reporter plasmid and increases the amount of endogenous p53. Our results suggest that hypoxic induction of transcriptionally active wild-type p53 is achieved as a result of the stabilization of p53 by its association with HIF-1alpha.

In vivo degradation of N-myc in neuroblastoma cells is mediated by the 26S proteasome

N-myc is a short-lived transcription factor, frequently amplified in human neuroblastomas. The ubiquitin-proteasome system is involved in the degradation of many short-lived cellular proteins and previous studies have shown that ubiquitin-dependent proteolysis is implicated in the turn-over of N-myc in vitro. However, calpain has also been implicated in N-myc degradation in vitro. Here we report that, in vivo, N-myc is a sensitive substrate for the 26S proteasome in N-myc amplified neuroblastoma cells. We observed that inhibition of the 26S proteasome with two inhibitors, ALLnL and lactacystin, led to an elevation of the N-myc protein steady-state and increased N-myc protein polyubiquitination, as revealed by ubiquitin Western blotting. Pulse-chase experiments have shown that the increased N-myc levels resulted from stabilization of the protein. In contrast treatment with several calpain and cathepsin inhibitors failed to block N-myc degradation in vivo. Furthermore, fluorescence microscopy of ALLnL-treated cells localized N-myc exclusively to the nuclear compartment, suggesting the absence of a requirement for transport to the cytoplasm prior to degradation.

Geldanamycin-induced destabilization of Raf-1 involves the proteasome

The Raf-1-MEK-MAPK pathway plays an important role in transducing extracellular growth factor signaling into altered nuclear transcription factor function. The benzoquinone ansamycin Geldanamycin (GA) specifically binds to the heat shock protein HSP90 and alters its complex with Raf-1. This leads to a decrease in Raf-1 levels and to disruption of the Raf-1-MEK-MAPK signaling pathway. The enhanced degradation of Raf-1 protein was prevented by inhibitors of the proteasome, while inhibition of lysosomal or other proteases was ineffective. Raf-1 that was protected from GA-induced degradation was of higher molecular weight and showed a laddering pattern consistent with its polyubiquitination. Unlike Raf-1 in untreated cells, the protein was insoluble in Triton X100- or NP40-based buffers. Signaling through this pathway was inhibited by GA, concomitant with loss of Raf-1 protein, but was restored if Raf-1 was protected from GA-induced degradation by proteasome inhibitors.

The amino-terminal domain of heat shock protein 90 (hsp90) that binds geldanamycin is an ATP/ADP switch domain that regulates hsp90 conformation

Many functions of the chaperone, heat shock protein 90 (hsp90), are inhibited by the drug geldanamycin that specifically binds hsp90. We have studied an amino-terminal domain of hsp90 whose crystal structure has recently been solved and determined to contain a geldanamycin-binding site. We demonstrate that, in solution, drug binding is exclusive to this domain. This domain also binds ATP linked to Sepharose through the gamma-phosphate. Binding is specific for ATP and ADP and is inhibited by geldanamycin. Mutation of four glycine residues within two proposed ATP binding motifs diminishes both geldanamycin binding and the ATP-dependent conversion of hsp90 to a conformation capable of binding the co-chaperone p23. Since p23 binding requires regions outside the 1-221 domain of hsp90, these results indicate a common site for nucleotides and geldanamycin that regulates the conformation of other hsp90 domains.

Expression of PAX3 in Ewing's sarcoma family of tumors

The Ewing's sarcoma family of tumors (ESFT) is the second most common pediatric malignancy originating in the bone and is characterized by the t(11; 22) translocation. PAX3, a member of the paired box family of genes, is expressed during embryonal development of neural crest cells and is involved in the t(2; 13) translocation found in alveolar rhabdomyosarcoma. Since ESFTs are believed to be derived from neural crest tissue, we screened a series of Ewing's sarcoma and peripheral neuroectodermal tumor cell lines and tumor specimens for expression of PAX3. We found expression of PAX3 in most, but not all, of the specimens analyzed, including cell lines and patient material.

Destabilization of Raf-1 by geldanamycin leads to disruption of the Raf-1-MEK-mitogen-activated protein kinase signalling pathway

The serine/threonine kinase Raf-1 functions downstream of Rats in a signal transduction cascade which transmits mitogenic stimuli from the plasma membrane to the nucleus. Raf-1 integrates signals coming from extracellular factors and, in turn, activates its substrate, MEK kinase. MEK activates mitogen-activated protein kinase (MAPK), which phosphorylates other kinases as well as transcription factors. Raf-1 exists in a complex with HSP90 and other proteins. The benzoquinone ansamycin geldanamycin (GA) binds to HSP90 and disrupts the Raf-1-HSP90 multimolecular complex, leading to destabilization of Raf-1. In this study, we examined whether Raf-1 destabilization is sufficient to block the Raf-1-MEK-MAPK signalling pathway and whether GA specifically inactivates the Raf-1 component of this pathway. Using the model system of NIH 3T3 cells stimulated with phorbol 12-myristate 13-acetate (PMA), we show that GA does not affect the ability of protein kinase C alpha to be activated by phorbol esters, but it does block activation of MEK and MAPK. Further, GA does not decrease the activity of constitutively active MEK in transiently transfected cells. Finally, disruption of the Raf-1-MEK-MAPK signalling pathway by GA prevents both the PMA-induced proliferative response and PMA-induced activation of a MAPK-sensitive nuclear transcription factor. Thus, we demonstrate that interaction between HSP90 and Raf-1 is a sine qua non for Raf stability and function as a signal transducer and that the effects observed cannot be attributed to a general impairment of protein kinase function.

Taxol-induced apoptosis and phosphorylation of Bcl-2 protein involves c-Raf-1 and represents a novel c-Raf-1 signal transduction pathway

c-Raf-1 (Raf-1) is a central component of signal transduction pathways stimulated by various growth factors, protein kinase C, and other protein kinases. Raf-1 activation is thought to be initiated at the plasma membrane after its recruitment by Ras. Raf-1 activation is associated primarily with proliferation and cell survival, but it has also been implicated in apoptosis. Raf-1 has also been shown to form complexes with both R-Ras and Bcl-2, raising the possibility that this component of cellular Raf-1 plays a role in apoptosis. Recently, taxol was reported to induce Bcl-2 phosphorylation and inactivation. We have previously demonstrated Raf-1 activation following taxol in MCF7 cells. We now present evidence that taxol fails to stimulate either apoptosis or phosphorylation of Bel-2 in the absence of Raf-1. Moreover, Raf-1 activation by taxol coincided with Bel-2 phosphorylation, showing similar dose and time dependence. Thus, our data support a role for a distinct subcellular component of Raf-1, which is taxol but not phorbol myristate acetate sensitive, in mediating an apoptotic pathway involving Bc1-2.

bcl-2 protein downregulation is not required for differentiation of multidrug resistant HL60 leukemia cells

Parental and multidrug resistant HL60 leukemia cell lines were used to study coupling of expression of apoptotic/cytostatic (bcl-2, bax, bclxL, p21/Waf1, and c-myc) genes during differentiation. The multidrug resistant HL60 cell line, HL60/ADR, was less sensitive than parental cells to cytostatic activity of low (0.4-2 ng/ml) doses of PMA. However, during treatment with standard differentiating doses of PMA (10 ng/ml), no difference between the two cell lines in cytostasis and differentiation was found. Downregulation of c-myc and upregulation of p21/Waf1 proteins showed the same time-course in both cell lines. The bcl-2 mRNA was rapidly downregulated while bax and bclxL gene expression was not altered in both differentiating HL60 and HL60/ADR cells. Significant downregulation of bcl-2 protein occurred only in parental HL60 cells. In HL60/ADR, despite rapid cessation of bcl-2 protein synthesis, almost no change in steady-state bcl-2 protein level was found. The lack of bcl-2 protein downregulation was a result of the prolonged half-life of this protein in HL60/ADR cells. Thus, although downregulation of bcl-2 mRNA is coupled to differentiation, actual loss of bcl-2 protein is not required for accomplishment of the differentiation program.

Possible role for serine/threonine phosphorylation in the regulation of the heteroprotein complex between the hsp90 stress protein and the pp60v-src tyrosine kinase

The abundant, cytoplasmic 90-kDa heat-shock protein associates transiently with the Rous sarcoma virus oncogenic protein tyrosine kinase, pp60v-src, directs its cellular trafficking and negatively regulates its kinase activity. Here we report that the serine/threonine phosphatase inhibitor, okadaic acid, destabilized the heat-shock protein 90-pp60v-src chaperone complex in v-src-transfected cells. Concomitant with complex destabilization by okadaic acid, phosphoserine was doubled and phosphothreonine was increased 20-fold in the heat-shock protein 90. Although phosphorylation of the total pool of immunoprecipitable pp60v-src was unchanged, okadaic acid slightly increased phosphoserine and phosphothreonine levels specifically in pp60v-src bound to heat-shock protein 90. The low level of tyrosine phosphorylation in the pp60v-src complexed with heat-shock protein 90 was further decreased by okadaic acid. Interestingly, okadaic acid-stabilized hyperphosphorylation of the heat-shock protein 90-pp60v-src complex lowered the level of pp60v-src in cell membranes, the functional location for pp60v-src. We suggest that serine/threonine phosphorylation of heat-shock protein 90 and/or pp60v-src functions as a regulatory molecular trigger to release pp60v-src from the chaperone complex at the inner surface of cell membranes.

Inhibition of interferon-gamma-induced intercellular adhesion molecule-1 expression on human keratinocytes by phosphorothioate antisense oligodeoxynucleotides is the consequence of antisense-specific and antisense-non-specific effects

Expression of intercellular adhesion molecule-1 (ICAM-1) by keratinocytes is an important event in the pathogenesis of T-cell-mediated inflammatory skin diseases. To determine if ICAM-1 expression could be selectively modulated, two antisense phosphorothioate oligonucleotides (S-ODN) targeting the translation initiation and 3' untranslated regions of ICAM-1 mRNA were added as lipid complexes to cultures of keratinocytes. Interferon-gamma was added after 24 h to induce ICAM-1 expression, which was quantitated by flow cytometry after 48 h. The S-ODN targeting the translation initiation site did not inhibit ICAM-1 expression at 0.2-20.0 microM. In contrast, 0.2-1.0 microM of the S-ODN targeting a site in the 3' untranslated region abrogated ICAM-1 expression in up to 75% of the keratinocytes; this inhibition was reversible when complementary sense S-ODN was added. Phosphodiester ODN (PD-ODN) targeting the same sites did not inhibit ICAM-1 expression on keratinocytes, most likely as a consequence of rapid degradation. Inhibition of ICAM-1 by the antisense S-ODN was selective; expression of beta 2-microglobulin, alpha 3-integrin, and beta 1-integrin remained largely unaffected and interferon-gamma-induced HLA-DR expression was inhibited to a much lesser extent than ICAM-1. Antisense-non-specific inhibition was also noted in that two scrambled S-ODN with an identical nucleotide (14 of 20 cytosines) composition inhibited ICAM-1 expression in up to 44% of the keratinocytes, whereas a degenerate S-ODN did not. The data demonstrate the complex effects exerted by antisense S-ODN in that ICAM-1 expression was inhibited via antisense-non-specific mechanisms probably due to the intrinsic properties of the S-ODN as well as via the anticipated sequence-specific mechanisms.

The role of Bcl-2 protein and autocrine growth factors in a human follicular lymphoma-derived B cell line

We have shown that the ability of the human follicular lymphoma-derived cell line SU-DHL-6 to proliferate and survive in vitro depends on both Bcl-2 expression and multiple autocrine growth factors. Treatment with Bcl-2 antisense (AS Bcl-2) decreased Bcl-2 protein levels. However, a cytotoxic effect was seen only at very restricted cell densities. Below such densities cells underwent spontaneous death without any treatment, while above these cell densities no cytotoxic effect of AS Bcl-2 could be seen. The conditioned medium of SU-DHL cells supported the survival and growth of these cells cultivated at low cell densities and partially reversed the cytotoxicity associated with Bcl-2 depletion. RT/PCR analysis revealed autocrine expression of IL-1 beta, IL-2, IL-5 and TNF-beta in SU-DHL cells. Neutralizing antibodies against these cytokines inhibited SU-DHL proliferation. Thus, development of autocrine GF secretion may be the second step in the pathogenesis of follicular lymphomas.

Bovine serum albumin is a major oligonucleotide-binding protein found on the surface of cultured cells

To better understand the uptake of oligonucleotides into cells, we have studied the labeling of cell surface proteins by an oligonucleotide conjugated to a radiolabeled photoactivatable crosslinker (Denny-Jaffe reagent). When HL60 cells are treated with the conjugate for 2 hours in a medium containing bovine serum albumin (BSA), almost all of the cell-associated label is found in one protein, which we identify as BSA. Cells grown and treated in a serum-free medium do not show this protein, whereas it is plainly seen in cells that are grown in serum-containing medium but then treated in serum-free medium. Overall association of the oligonucleotide with cells is much higher in serum-free medium than in BSA-containing medium, but the oligonucleotide is mostly not protein-associated in the absence of BSA. We conclude that (1) BSA from the medium serves to block overall association of oligonucleotide with cells, and (2) BSA is the main cell surface protein binding oligonucleotides. We discuss the possible role of albumin in endocytic uptake of oligonucleotides in the cell and in the biodistribution of oligonucleotides in vivo.

Inhibition of heat shock protein HSP90-pp60v-src heteroprotein complex formation by benzoquinone ansamycins: essential role for stress proteins in oncogenic transformation

The molecular mechanisms by which oncogenic tyrosine kinases induce cellular transformation are unclear. Herbimycin A, geldanamycin, and certain other benzoquinone ansamycins display an unusual capacity to revert tyrosine kinase-induced oncogenic transformation. As an approach to the study of v-src-mediated transformation, we examined ansamycin action in transformed cells and found that drug-induced reversion could be achieved without direct inhibition of src phosphorylating activity. To identify mechanisms other than kinase inhibition for drug-mediated reversion, we prepared a solid phase-immobilized geldanamycin derivative and affinity precipitated the molecular targets with which the drug interacted. In a range of cell lines, immobilized geldanamycin bound elements of a major class of heat shock protein (HSP90) in a stable and pharmacologically specific manner. Consistent with these binding data, we found that soluble geldanamycin and herbimycin A inhibited specifically the formation of a previously described src-HSP90 heteroprotein complex. A related benzoquinone ansamycin that failed to revert transformed cells did not inhibit the formation of this complex. These results demonstrate that HSP participation in multimolecular complex formation is required for src-mediated transformation and can provide a target for drug modulation.

Oligonucleotides protect cells from the cytotoxicity of several anti-cancer chemotherapeutic drugs

The possibility of inhibiting gene expression with antisense oligonucleotides (AS ODNs) in combination with more conventional chemotherapy is a very attractive modality in oncology. However, possible interaction between the ODN and drug must be considered. Here we show that ODNs protect cells from the cytostatic/cytotoxic action of actinomycin D (AMD), adriamycin, daunomycin or quinacrine, but not mitomycin, camptothecin, vincristine, cisplatin, etoposide (VP-16) or cycloheximide. The cytoprotective effect depends on ODN length as well as ability to interact directly with the cytotoxic drug and is only slightly sequence selective.

Cytostatic and cytotoxic activity of sex steroids against human leukemia cell lines

We investigated effects of sex steroids and analogs (estradiol, DES, norgestrel, progesterone, medroxyprogesterone, and testosterone) on the proliferation and survival of 10 human leukemia/lymphoma cell lines (HL-60, K562, U937, CEM, KG-1, Jurkat, U266, H929, PA and SUNHL). Micromolar concentrations of sex steroids exerted cytostatic and cytotoxic effects on all cell lines tested, irrespective of their sensitivity to glucocorticoids. The order of potency of sex hormones was: DES > progesterone > or = medroxyprogesterone > testosterone > estradiol >> norgestrel. For progesterone and estradiol, cytostatic effects can be achieved at lower concentrations than cytotoxic effects. The most potent agent, DES, exerted half maximal cytotoxic activity at a median concentration of 4 microM (for 10 leukemia cell lines). Our results provide a basis for the potential therapeutic use of estrogens and progestins in glucocorticoid-resistant leukemias and lymphomas.

Sensitive and simple bioassay for human tumour necrosis factor-alpha

A simple, sensitive, and specific in vitro bioassay is described for human tumour necrosis factor alpha (TNF-alpha). The system employs the U937 cell line incubated with 10 ng/ml actinomycin D for maximum sensitivity. This method allows the detection of less than 0.6 pg/ml of TNF-alpha and the log dose-response curve is linear at concentrations 0.6-250 pg/ml TNF-alpha. Highly sensitive subclones of U937 were obtained which were capable of detecting as little as 0.1 pg/ml TNF-alpha. Finally, this bioassay demonstrates specificity by discriminating between TNF-alpha and TNF-beta.

Stability, clearance, and disposition of intraventricularly administered oligodeoxynucleotides: implications for therapeutic application within the central nervous system

We report experiments in the rat demonstrating the feasibility of intraventricular administration of oligodeoxynucleotides (ODNs) as a regional treatment approach to disorders within the central nervous system (CNS). Although we find little intrinsic nuclease activity in cerebrospinal fluid (CSF), phosphodiester ODNs are rapidly degraded by brain-associated alpha-exonuclease activity. Phosphorothioate ODNs, however, appear resistant to degradation in the CNS and, after intraventricular administration, we find they are cleared in a manner consistent with CSF bulk flow. Continuous infusion of ODN at 1.5 nmol/hr by miniosmotic pump can maintain micromolar concentrations of intact phosphorothioate ODN in CSF for at least 1 week without obvious neurologic or systemic toxicity. After infusion, extensive brain penetration and marked cellular uptake, especially by astrocytic cells, is demonstrated.

Theophylline induced differentiation provides direct evidence for the deregulation of c-myc in Burkitt's lymphoma and suggests participation of immunoglobulin enhancer sequences

Most of the evidence that supports the hypothesis that the c-myc gene is abnormally regulated in Burkitt's lymphoma (BL) is indirect. The putative abnormal expression of c-myc is likely, at least in part, to be a consequence of the usurpation of its regulatory sequences by immunoglobulin enhancer elements, which are invariably juxtaposed to c-myc by the translocations associated with this tumor (C. M. Croce, J. Erikson, A. Ar-Rushdi, D. Aden, and K. Nishikura, Proc. Natl. Acad. Sci. USA, 81: 3170-3174, 1984). We have developed a differentiation induction model system to examine this issue more directly. In a variety of non-BL cell lines, differentiation induction results in the down-regulation of c-myc (G. P. Studzinski, A. K. Bhandal, and Z. S. Brelvi, Proc. Soc. Exp. Biol. Med., 179: 288-295, 1985; Y. Matsui, R. Takahasi, K. Minara, T. Nakagawa, T. Koizumi, Y. Nakao, T. Sugiyama, and T. Fugita, Cancer Res., 49: 1366-1371, 1985; T. Mitchell, E. Sariban, and D. Kufe, Mol. Pharmacol., 30: 398-402, 1986; Z. S. Brelvi, and G. P. Studzinski, J. Cell. Physiol., 128: 171-179, 1986). Since BL is of B-cell origin, differentiation is associated with persistent or increased expression of immunoglobulin genes. Therefore, if c-myc and c-mu are coregulated in BL via immunoglobulin enhancer sequences, persistent or increased expression of the c-myc gene, rather than down-regulation, should occur in differentiated BL cells. Differentiation was induced in four BL cell lines with theophylline (7 x 10(-3) M), and mRNA was examined by Northern blot analysis. In all four BL lines studied (JD38, AG876, KK124, and Daudi), there was persistent or increased expression of both c-mu and c-myc genes (detected with a third exon c-myc probe), in contrast to the decreased expression of the c-myc gene observed in the three Epstein-Barr virus transformed lines studied (A3317, TC84, and CB34). In the BL cell line, JD38, the c-myc gene is truncated (the second and third exons are translocated to chromosome 14 while the first exon remains on chromosome 8). In this line, we demonstrated that theophylline induced differentiation results in down-regulation of the first exon while the level of expression of the translocated second and third exons remains unchanged or increases.(ABSTRACT TRUNCATED AT 400 WORDS)

A phase I study of interleukin-2 in children with cancer

Recombinant interleukin-2 (IL-2) produces clinical responses in approximately 20% of adult patients with renal cell carcinoma and melanoma, with both high-dose bolus and continuous infusion regimens. Because of the lower toxicity of continuous infusion, we elected to investigate in a Phase I trial a 5-day continuous infusion repeated for three weeks in children with malignancies refractory to standard therapy. Nineteen children with solid tumors and eight children with hematologic malignancies were entered into the study. The maximum tolerated dose was 3 x 10(6) U/m2/day, with dose-limiting toxicities occurring in five of seven patients treated at the 5 x 10(6) U/m2/day dose level. Dose-limiting toxicities included hypotension, hyperbilirubinemia, thrombocytopenia, pulmonary/pleural effusion, and nephrotoxicity. Serum IL-2 levels were detectable at the higher dose levels and were comparable to those observed in adult patients. Hematologic changes at the higher dose levels included rebound lymphocytosis occurring within 48 h of discontinuation of IL-2, eosinophilia, and decreased platelet counts. No objective responses to therapy were seen. We have identified a dose and schedule of administration for IL-2 in pediatric patients that can be given without intensive care unit support. Pediatric Phase II trials examining the anti-tumor activity of IL-2 given by this schedule are in progress.

Antisense inhibition of gene expression: a tool for studying the role of NMYC in the growth and differentiation of neuroectoderm-derived cells

To understand the role of individual genes in regulating biological processes, one must be able to interfere specifically with either their expression or function. While monoclonal antibodies have proven very useful in studying cell surface proteins, the specific inhibition of intracellular proteins in viable cells is a much more difficult problem. The goal of antisense technology is to develop small oligonucleotides, plasmids, or retroviral vectors that can be introduced easily into viable cells in order to inhibit gene products specifically. In this report, we will describe our use of antisense DNA and RNA to study the role of the NMYC proto-oncogene in neuroectodermal cell growth and differentiation.

Benzoquinonoid ansamycins possess selective tumoricidal activity unrelated to src kinase inhibition

The benzoquinonoid ansamycin antibiotics herbimycin A and geldanamycin have been shown to reverse the oncogenic phenotype of pp60v-src transformed cells as well as induce differentiation in a number of in vitro model systems, reportedly due to their inhibition of src family protein tyrosine kinases. We now report that these agents are potent cytotoxins in vitro against a panel of highly malignant human tumor cell lines possessing primitive neural features. Proliferation and/or survival of fibroblasts, primary neuronal cultures, and several leukemia cell lines are unaffected at concentrations resulting in greater than 99% cell loss in sensitive lines. The tumorigenicity in nude mice of sensitive cell lines can also be markedly reduced by either systemic or topical administration of these agents without apparent toxicity to the whole animal. The cytocidal action of these ansamycins is initiated very rapidly, is irreversible, and is clearly distinct from the delayed inhibition of src family kinases that has been reported previously. Due to their potency, relative selectivity, and novel mechanism(s) of action, these drugs could prove clinically useful in the therapy of a number of human cancers of neural derivation.

Detection of functional interferon alpha receptors in human neuroendocrine tumor cell lines using a new monoclonal antibody

While first described as antiviral agents, interferons (IFNs) exhibit significant antiproliferative and antitumor effects as well. IFN alpha has been successfully used in clinical trials to treat several malignancies, including leukemias and certain solid tumors. While many cell types have been studied for IFN alpha receptor expression, very little is known about receptor expression on human neuroendocrine cells. Using a novel anti-IFN alpha receptor monoclonal antibody, we examined IFN alpha receptor expression in 10 human cell lines derived from tumors of neuroendocrine origin, including neuroblastoma, neuroepithelioma and small cell lung carcinoma. All cell lines studied displayed a similar pattern of IFN alpha receptor expression and 5 of 8 cell lines demonstrated reduced thymidine incorporation following IFN alpha treatment. Addition of exogenous IFN alpha caused a decrease in IFN alpha receptor expression, while differentiating agents, such as phorbol esters and retinoic acid, induced an increase in receptor number without altering receptor affinity.

Analysis of oligonucleotide binding, internalization, and intracellular trafficking utilizing a novel radiolabeled crosslinker

Although antisense oligonucleotides have been widely used to inhibit gene expression, their mechanism of entry into cells and their site of action are still in some doubt. In this report, we describe a novel technique for kinetically analyzing oligonucleotide association with living cells as well as intracellular compartmentalization. The technique utilizes a photoactivatable, radiolabeled crosslinker, the Denny-Jaffe reagent. Oligonucleotides containing pendant amine groups were conjugated to this reagent, added to HL60 cells in culture, and photocrosslinked to associated proteins, which were analyzed electrophoretically. We find that several proteins are labeled, predominantly a 75 kD one that appears to be membrane-associated. Our results suggest that the majority of intracellular oligonucleotide is associated in vesicles with the same protein to which it bound on the cell surface, but only a small percentage of non-protein-bound cytosolic oligonucleotide can be detected. Additionally, oligonucleotides are readily accumulated by nuclei, and by treating whole nuclei, a unique set of nuclear binding proteins is detected.

Incorporation of radiophosphorus from labeled oligodeoxynucleotides into RNA of mycoplasma in cell cultures

We have found that various mycoplasma species quickly and efficiently incorporate radiophosphorus into their RNA from labeled oligonucleotides added to the medium. The label can be in any of several positions in an oligodeoxynucleotide, and incorporation also occurs efficiently from labeled RNA. Mycoplasmas also incorporate the radiolabel when they infect a mammalian cell culture; the host cells do not. This incorporation presumably involves uptake of the oligodeoxynucleotide followed by digestion to mononucleotides, conversion to ribonucleotides, and incorporation in new RNA. We believe that the processing of oligodeoxynucleotides by mycoplasma could be a source of artifacts in antisense work in cell culture and could have implications for the development of antisense therapeutics. We also suggest ways to exploit the incorporation phenomenon in mycoplasma testing.

Characterization of an interleukin-6-mediated autocrine growth loop in the human multiple myeloma cell line, U266

It has been reported recently that freshly isolated human myeloma cell cultures proliferate in response to added interleukin-6 (IL-6). Endogenous levels of IL-6 found in the same cultures suggested that an autocrine growth loop may contribute to cell growth. However, the lack of homogenous cell populations in primary myeloma cultures has made it difficult to distinguish between paracrine and autocrine growth mechanisms. To precisely address the autocrine growth issue we have evaluated the growth of the human myeloma cell line, U266. We have found that a neutralizing anti-IL-6 monoclonal antibody can inhibit U266 proliferation. Furthermore, the addition of IL-6 antisense oligonucleotides also inhibits U266 proliferation. These effects are reversed by adding IL-6, suggesting the presence of an autocrine loop. Using bioassays with two different IL-6-dependent cell lines, we were able to detect IL-6 in concentrated U266 supernatants. IL-6 mRNA was detected by polymerase chain reaction amplification of cDNA. Cell cycle parameter analysis shows that IL-6 acts to release a block in G1. Taken together these results present conclusive evidence for IL-6-mediated autocrine growth in the U266 human myeloma cell line.

In vivo modulation of N-myc expression by continuous perfusion with an antisense oligonucleotide

In this study we investigated the in vivo efficacy of continuous subcutaneous perfusion of unmodified phosphodiester oligodeoxynucleotides. The in vitro sequelae of antisense inhibition of the target gene, N-myc, have been documented and include moderate growth inhibition without effects on myc expression, loss of secretogranin I expression, and morphologic alterations. We chose to use N-myc as a model target to determine if antisense effects observed in vitro can be reproduced in vivo. N-myc-expressing human neuroectodermal tumors were grown as subcutaneous xenografts in athymic mice. Antisense and sense oligodeoxynucleotides directed against N-myc were delivered to the vicinity of the tumor by a subcutaneously implanted microosmotic pump. Antisense treatment led to loss of N-myc protein from the tumor, as well as to the loss of the neuroendocrine differentiation marker protein secretogranin I. Myc protein expression remained unaffected. Mean tumor mass was reduced by 50% in antisense-treated animals, and antisense-treated tumors morphologically resembled antisense-transfected in vitro cell cultures. These results demonstrate that regional, in vivo perfusion of an unmodified oligonucleotide specifically downregulates gene expression in human tumor xenografts with concomitant effects on tumor phenotype and growth rate that correlate well with in vitro observations.

Regulation of transferrin receptor expression and control of cell growth

Transferrin receptor expression is vital for the continuous growth of most cells. Although iron plays a key role in modulating transferrin receptor expression, other physiological factors are also capable of affecting receptor expression. In hematopoietic cells, these include interleukin-2, interleukin-6, calcium channels, cyclic nucleotides, phorbol esters and viruses. In this review, we will describe how these agents can alter transferrin receptor expression at the levels of transcription, translation and receptor re-cycling.

Antisense inhibition of single copy N-myc expression results in decreased cell growth without reduction of c-myc protein in a neuroepithelioma cell line

The N-myc gene is transiently expressed during normal embryonic development and abnormally expressed in several tumors of neuroendocrine origin. Little is known of the function of the N-myc gene product in either normal or neoplastic tissue. We utilized synthetic antisense oligodeoxynucleotides to specifically inhibit N-myc gene expression in the neuroepithelioma cell line CHP100. These cells contain single copy N-myc alleles but overexpress c-myc. N-myc antisense oligomer treatment was found to be growth inhibitory without affecting levels of c-myc protein. N-myc antisense oligomer-treated cells also lost the characteristic cellular heterogeneity displayed by CHP100 in vitro.

Characterization of three monoclonal antibodies that recognize the interferon alpha 2 receptor

The interferon system plays an important role in the control of viral infections and cell proliferation. These effects are mediated through the interaction of interferons with specific cell surface receptors. We report here the development of monoclonal antibodies against one of the subunits of the interferon alpha receptor. These antibodies detect a 110-kDa protein in surface-labeled cells and in Western blots, and 130- and 210-kDa bands after crosslinking to iodinated interferon alpha 2. No other subunits are disulfide-linked to the 130-kDa subunit or are coprecipitated by these antibodies. Analysis by two-dimensional gel electrophoresis revealed that the pI of this subunit is 3.5-5.0. We suggest that the protein recognized by these monoclonal antibodies be named the alpha subunit of the interferon alpha receptor.

Putative gamma-subunit of the IL-2 receptor is detected in low, intermediate, and high affinity IL-2 receptor-bearing cells

Three forms of the IL-2R have been reported: low, intermediate, and high affinity. Each form correlates with the expression of two different chains: p55 (alpha-subunit) and p75 (beta-subunit). We report here a putative new subunit of the IL-2R, termed p95-110 or gamma-subunit. This new subunit has a molecular mass of 95 to 110 kDa and is expressed in low, intermediate, and high affinity IL-2R-bearing cells. We propose that p95-110 is the partner of p75 in the formation of intermediate affinity IL-2R, inasmuch as neither p75 nor p95-110 alone can bind IL-2.

Putative gamma-subunit of the IL-2 receptor is detected in low, intermediate, and high affinity IL-2 receptor-bearing cells

Three forms of the IL-2R have been reported: low, intermediate, and high affinity. Each form correlates with the expression of two different chains: p55 (alpha-subunit) and p75 (beta-subunit). We report here a putative new subunit of the IL-2R, termed p95-110 or gamma-subunit. This new subunit has a molecular mass of 95 to 110 kDa and is expressed in low, intermediate, and high affinity IL-2R-bearing cells. We propose that p95-110 is the partner of p75 in the formation of intermediate affinity IL-2R, inasmuch as neither p75 nor p95-110 alone can bind IL-2.