Leukemia treatment in severe combined immunodeficiency mice by antisense oligodeoxynucleotides targeting cooperating oncogenes

MYC Oncogene Contributions to Release of Cell Cycle Brakes

Promotion of the cell cycle is a major oncogenic mechanism of the oncogene c-MYC (MYC). MYC promotes the cell cycle by not only activating or inducing cyclins and CDKs but also through the downregulation or the impairment of the activity of a set of proteins that act as cell-cycle brakes. This review is focused on the role of MYC as a cell-cycle brake releaser i.e., how MYC stimulates the cell cycle mainly through the functional inactivation of cell cycle inhibitors. MYC antagonizes the activities and/or the expression levels of p15, ARF, p21, and p27. The mechanism involved differs for each protein. p15 (encoded by CDKN2B) and p21 (CDKN1A) are repressed by MYC at the transcriptional level. In contrast, MYC activates ARF, which contributes to the apoptosis induced by high MYC levels. At least in some cells types, MYC inhibits the transcription of the p27 gene (CDKN1B) but also enhances p27's degradation through the upregulation of components of ubiquitin ligases complexes. The effect of MYC on cell-cycle brakes also opens the possibility of antitumoral therapies based on synthetic lethal interactions involving MYC and CDKs, for which a series of inhibitors are being developed and tested in clinical trials.

c-Myc depletion inhibits proliferation of human tumor cells at various stages of the cell cycle

A major role for c-Myc in the proliferation of normal cells is attributed to its ability to promote progression through G(1) and into S phase of the cell cycle. The absolute requirement of c-Myc for cell cycle progression in human tumor cells has not been comprehensively addressed. In the present work, we used a lentiviral-based short hairpin RNA (shRNA) expression vector to stably reduce c-Myc expression in a large number of human tumor cell lines and in three different types of normal human cells. In all cases, cell proliferation was severely inhibited, with normal cells ultimately undergoing G(0)/G(1) growth arrest. In contrast, tumor cells demonstrated a much more variable cell cycle response with cells from several lines accumulating in S or G(2)/M phases. Moreover, in some tumor lines, the phase of cell cycle arrest caused by inhibition of c-Myc could be altered by depleting tumor suppressor protein p53 or its transcriptional target p21(CIP/WAF). Our data suggest that, as in the case of normal cells, c-Myc is essential for sustaining proliferation of human tumor cells. However its rate-limiting role in cell cycle control is variable and is reliant upon the status of other cell cycle regulators.

Interplay between the retinoblastoma related pRb2/p130 and E2F-4 and -5 in relation to JCV-TAg

Human polyomaviruses, which include JC virus (JCV) and BK virus (BKV), as well as the simian virus 40 (SV40), have been associated with human tumors and have been shown to be highly tumorigenic in experimental animal models. Although the mechanism by which JCV induces tumorigenesis is not entirely clear, earlier studies point to the involvement of the viral early protein T-antigen which has the ability to bind and inactivate tumor suppressors and cell cycle regulatory proteins, such as the retinoblastoma family proteins and p53. We investigated if the distribution between nucleus and cytoplasm of the transcription factors E2F4 and E2F5 is mediated by pRb2/p130 and if the presence of JCV T-antigen may impair this shuttling by sequestering pRb2/p130. The results showed that E2F4 was prevalently localized in the nucleus of both T-antigen positive and -negative R503 cells independently of the cell cycle phase. E2F5 instead was prevalently localized in the cytoplasmic fraction in G(0)/G(1), S-phase synchronized, and asynchronous R503 and R503 T-Ag positive cells. The presence of T-antigen did not influence the subcellular localization of these transcription factors E2F4 and E2F5, at least in this murine cellular model. Moreover, Small interference RNA experiments directed toward silencing the Rb2/p130 gene demonstrated that pRb2/p130 does not play a predominant role in the nuclear transportation of E2F4 and E2F5.

Novel triiodophenol derivatives induce caspase-independent mitochondrial cell death in leukemia cells inhibited by Myc

2,4,6-Triiodophenol (Bobel-24, AM-24) was originally described as a nonsteroid antiinflammatory molecule. We have synthesized three derivatives of Bobel-24 (Bobel-4, Bobel-16, and Bobel-30) and tested their activities as putative antileukemic agents. We have found that Bobel-24 and Bobel-16 were dual inhibitors of cyclooxygenase and 5-lipoxygenase, whereas Bobel-4 and Bobel-30 were selective against 5-lipoxygenase. We have tested the antiproliferative activity of these compounds on a panel of cell lines derived from myeloid and lymphoid leukemias (K562, Raji, HL-60, and Molt4). The cytotoxic IC(50) in these cell lines ranged between 14 and 50 micromol/L, but it was higher for nontransformed cells such as 32D, NIH3T3, or human leukocytes. All compounds showed cytotoxic activity on all tested cell lines, accompanied by DNA synthesis inhibition and arrest in the G(0)/G(1) phase. Bobel-16, Bobel-4, and Bobel-24 induced a caspase-independent cell death in K562 and Raji cells, accompanied by chromatin condensation, cytochrome c release, and dissipation of mitochondrial membrane potential in a concentration-dependent manner and production of reactive oxygen species. As the proto-oncogene MYC is involved in mitochondrial biogenesis and survival of leukemia cells, we tested its effect on bobel activity. Bobel-24 induced down-regulation of MYC in K562 and, consistently, ectopic expression of MYC results in partial protection towards the cytotoxic effect of Bobel-24. In conclusion, Bobel derivatives induce a caspase- and Bcl-2-independent cell death in which mitochondrial permeabilization and MYC down-regulation are involved. Bobels may serve as prototypes for the development of new agents for the therapy of leukemia.

T-antigen of the human polyomavirus JC attenuates faithful DNA repair by forcing nuclear interaction between IRS-1 and Rad51

JC polyomavirus (JCV), which infects 90% of the human population, is detectable in human tumors. Its early protein, JCV T-antigen, transforms cells in vitro and is tumorigenic in experimental animals. Although T-antigen-mediated transformation involves genetic alterations of the affected cells, the mechanism underlying this genomic instability is not known. We show that JCV T-antigen inhibits homologous recombination DNA repair (HRR), which results in an accumulation of mutations. T-antigen does not operate directly but utilizes a cytosolic molecule, insulin receptor substrate 1 (IRS-1). Following T-antigen-mediated nuclear translocation, IRS-1 binds Rad51 at the site of damaged DNA. This T-antigen-mediated inhibition of HRR does not function in cells lacking IRS-1, and can be reproduced in the absence of T-antigen by IRS-1 with artificial nuclear localization signal. Our observations define a new mechanism by which viral protein utilizes cytosolic molecule to inhibit faithful DNA repair, and suggest how polyomaviruses could compromise stability of the genome. (c) 2005 Wiley-Liss, Inc.

Intrinsic regulation of the interactions between the SH3 domain of p85 subunit of phosphatidylinositol-3 kinase and the protein network of BCR/ABL oncogenic tyrosine kinase

OBJECTIVE

BCR/ABL fusion tyrosine kinase is responsible for the initiation and maintenance of the Philadelphia chromosome-positive chronic myelogenous leukemia (CML) and a cohort of acute lymphocytic leukemias. We show that a signaling protein, phosphatidylinositol-3 kinase (PI-3k), is essential for growth of CML cells, but not of normal hematopoietic cells, and that p85alpha subunit of PI-3k co-immunoprecipitates with BCR/ABL. Therefore, we made an attempt to better characterize p85alpha-BCR/ABL interactions.

MATERIALS AND METHODS

The mutants of p85alpha-SH3 domain were generated by in vitro site-directed mutagenesis system. Protein lysates were obtained from p210BCR/ABL-transformed murine 32Dcl3 myeloid cells, and in vitro transcription/translation was used to produce BCR/ABL protein. Pull-down and Western analyses were performed to detect the interaction between BCR/ABL and p85alpha-SH3. BCR/ABL-transformed 32Dcl3 cells were infected with internal ribosome entry site-green fluorescent protein retroviral construct encoding p85alpha-SH3 mutants to assess their biological effects.

RESULTS

We show here that the SH3 domain of p85alpha (p85alpha-SH3) pulls down the p210BCR/ABL kinase from hematopoietic cell lysates. The interaction between p85alpha-SH3 and BCR/ABL may be intermediated by proteins such as c-Cbl, Shc, Grb2, and/or Gab2. Mutations in the p85alpha-SH3 region responsible for proline-rich motif binding either abrogate or enhance these interactions. These mutants exert a modest inhibitory effect on growth factor-independent proliferation of BCR/ABL-positive 32Dcl3 cells.

CONCLUSIONS

Based on this information we speculate on the capability of p85alpha-SH3 to interact with the protein network of BCR/ABL oncoprotein.

Activated IGF-1R inhibits hyperglycemia-induced DNA damage and promotes DNA repair by homologous recombination

The IGF-1R is a genetic determinant of oxidative stress and longevity. Hyperglycemia induces an exponential increase in the production of a key danger signal, reactive oxygen intermediates, which target genomic DNA. Here, we report for the first time that ligand activation of the IGF-1R prevents hyperglycemia-induced genotoxic stress and enhances DNA repair, maintaining genomic integrity and cell viability. We performed single gel electrophoresis (comet assay) to evaluate DNA damage in serum-starved SV40 murine mesangial cells (MMC) and normal human mesangial cells (NHMC), maintained at high ambient glucose concentration. Hyperglycemia inflicted an impressive array of DNA damage in the form of single-strand breaks (SSBs) and double-strand breaks (DSBs). The inclusion of IGF-1 to culture media of MMC and NHMC prevented hyperglycemia-induced DNA damage. To determine whether DNA damage was mediated by reactive oxygen species (ROS), ROS generation was evaluated, in the presence of IGF-1, or the free radical scavenger n-acetyl-cysteine (NAC). IGF-1 and NAC inhibited hyperglycemic-induced ROS production and hyperglycemia-induced DNA damage. We next asked whether IGF-1 promotes the repair of DSB under hyperglycemic conditions, by homologous recombination (HRR) or nonhomologous end joining (NHEJ). Repair of DSB by NHEJ and HRR was operative in MMC maintained under hyperglycemic conditions. IGF-1 increased HRR by nearly twofold, whereas IGF-1 did not affect DNA repair by NHEJ. IGF-1R enhancement of HRR correlated with the translocation of Rad51 to foci of DNA damage. Inhibition of Rad51 expression by short interfering RNA experiments markedly decreased percentage of MMC positive for Rad51 nuclear foci and increased hyperglycemic DNA damage. We conclude that the activated IGF-1R rescues mesangial cells from hyperglycemia-induced danger signals that target genomic DNA by suppressing ROS and enhancing DNA repair by HRR.

Impaired homologous recombination DNA repair and enhanced sensitivity to DNA damage in prostate cancer cells exposed to anchorage-independence

During metastases, cancer cells are temporarily exposed to the condition in which interactions with extracellular environment can be restricted (anchorage-independence). We demonstrate that the sensitivity of prostate cancer cell lines, DU145 and PC-3, to genotoxic treatment (cisplatin and gamma-irradiation) increased several folds when cells were forced to grow in anchorage-independence. This enhanced drug sensitivity was associated with a severe impairment of homologous recombination-directed DNA repair (HRR). The mechanism involves Rad51, which is the major enzymatic component of HRR. The protein level of Rad51 and its recruitment to DNA double-strand breaks (DSBs) were both attenuated. Rad51 deficiency in anchorage-independence was not associated with Rad51 promoter activity, and was not compensated by a constitutive overexpression of Rad51 cDNA. Instead, Rad51 protein level and its ability to colocalize with DSBs were restored in the presence of proteosome inhibitors, or when cells from the suspension cultures were allowed reattachment. Presented results indicate that anchorage-independence sensitizes prostate cancer cells to genotoxic agents; however, it also attenuates faithful component of DNA repair by targeting stability of Rad51. This temporal attenuation of HRR may contribute to the accumulation mutations after DNA damage, and possibly the selection of new adaptations in cells, which survived genotoxic treatment.

DNA binding and antigene activity of a daunomycin-conjugated triplex-forming oligonucleotide targeting the P2 promoter of the human c-myc gene

Triplex-forming oligonucleotides (TFO) that bind DNA in a sequence-specific manner might be used as selective repressors of gene expression and gene-targeted therapeutics. However, many factors, including instability of triple helical complexes in cells, limit the efficacy of this approach. In the present study, we tested whether covalent linkage of a TFO to daunomycin, which is a potent DNA-intercalating agent and anticancer drug, could increase stability of the triple helix and activity of the oligonucleotide in cells. The 11mer daunomycin-conjugated GT (dauno-GT11) TFO targeted a sequence upstream of the P2 promoter, a site known to be critical for transcription of the c-myc gene. Band-shift assays showed that the dauno-GT11 formed triplex DNA with enhanced stability compared to the unmodified TFO. Band shift and footprinting experiments demonstrated that binding of dauno-GT11 was highly sequence-specific with exclusive binding to the 11 bp target site in the c-myc promoter. The daunomycin-conjugated TFO inhibited transcription in vitro and reduced c-myc promoter activity in prostate and breast cancer cells. The daunomycin-conjugated TFO was taken up by cells with a distinctive intracellular distribution compared to free daunomycin. However, cationic lipid-mediated delivery was required for enhanced cellular uptake, nuclear localization and biological activity of the TFO in cells. Dauno-GT11 reduced transcription of the endogenous c-myc gene in cells, but did not affect expression of non-target genes, such as ets-1 and ets-2, which contained very similar target sequences in their promoters. Daunomycin-conjugated control oligonucleotides unable to form triplex DNA with the target sequence did not have any effect in these assays, indicating that daunomycin was not directly responsible for the activity of daunomycin-conjugated TFO. Thus, attachment of daunomycin resulted in increased triplex stability and biological activity of the 11mer GT-rich TFO without compromising its specificity. These results encourage further testing of this approach to develop novel antigene therapeutics.

Role of the insulin-like growth factor I/insulin receptor substrate 1 axis in Rad51 trafficking and DNA repair by homologous recombination

The receptor for insulin-like growth factor I (IGF-IR) controls normal and pathological growth of cells. DNA repair pathways represent an unexplored target through which the IGF-IR signaling system might support pathological growth leading to cellular transformation. However, this study demonstrates that IGF-I stimulation supports homologous recombination-directed DNA repair (HRR). This effect involves an interaction between Rad51 and the major IGF-IR signaling molecule, insulin receptor substrate 1 (IRS-1). The binding occurs within the cytoplasm, engages the N-terminal domain of IRS-1, and is attenuated by IGF-I-mediated IRS-1 tyrosine phosphorylation. In the absence of IGF-I stimulation, or if mutated IGF-IR fails to phosphorylate IRS-1, localization of Rad51 to the sites of damaged DNA is diminished. These results point to a direct role of IRS-1 in HRR and suggest a novel role for the IGF-IR/IRS-1 axis in supporting the stability of the genome.

The future of antisense therapy: combination with anticancer treatments

The current direction in cancer research is rational drug design, which is based on the evidence that transformed cells are characterized by alterations of genes devoted to the regulation of both cell proliferation and apoptosis. A variety of approaches have been carried out to develop new agents selective for cancer cells. Among these, antisense oligonucleotides (ASOs) are one of such class of new agents able to inhibit specifically the synthesis of a particular cancer-associated protein by binding to protein-encoding RNA, thereby preventing RNA function. In the past decade, several ASOs have been developed and tested in preclinical and clinical studies. Many have shown convincing in vitro reduction in target gene expression and promising activity against a wide variety of tumors. However, because of the multigenic alterations of tumors, the use of ASOs as single agents does not seem to be effective in the treatment of malignancies. Antisense therapy that interferes with signaling pathways involved in cell proliferation and apoptosis are particularly promising in combination with conventional anticancer treatment. An overview of the progress of ASOs used in combination therapy is provided.

A "combination oligonucleotide" antisense strategy to downregulate thymidylate synthase and decrease tumor cell growth and drug resistance

Thymidylate synthase (TS) catalyzes de novo production of thymidylate for DNA synthesis and cell proliferation. As such, TS has been a target of antitumor chemotherapy for many years. Our laboratory has identified several antisense oligodeoxynucleotides (ODNs) that downregulate TS mRNA and protein, inhibit cell proliferation, and sensitize cells to TS-directed chemotherapeutic drugs. Based on our observation that targeting distinct regions of the TS mRNA with a variety of antisense molecules resulted in differential effects on TS mRNA levels, it was hypothesized that use of multiple ODNs targeting distinct noncontiguous regions would result in synergistic or antagonistic interactions. In this study, we report that some combinations of TS antisense ODNs were more effective at reducing TS mRNA abundance and inhibiting cell proliferation than the individual ODNs used alone. However, in contrast to the effects on cell proliferation, the enhanced sensitivity to anti-TS chemotherapeutic drugs (i.e., raltitrexed and 5-fluorodeoxyuridine) that is achieved by treatment with individual ODNs was not further augmented by combined ODN treatment. This suggests that ODNs targeting TS mRNA inhibit an alternative function of TS mRNA or protein, distinct from thymidylate production. The results are evidence that the novel use of multiple antisense ODNs that target different regions of the same mRNA represents a general strategy to improve antisense effectiveness.

Glutathione influences c-Myc-induced apoptosis in M14 human melanoma cells

The objective of this article is to dissect the mechanisms by which the down-regulation of c-Myc induces programmed cell death in melanoma cells. In stable and doxycycline-inducible M14 melanoma cells, down-regulation of c-Myc induced apoptosis subsequent to a decrease in the intracellular reduced glutathione content and a concomitant accumulation of its oxidized form. This redox alteration was associated with a decrease of the enzyme activities of gamma-glutamyl-cysteine synthetase and NADPH-dependent GSSG reductase, as well as a consequent glutathione release in the extracellular medium. Cytochrome c was released into the cytosol at very early stages of apoptosis induction, long before detectable production of reactive oxygen species and activation of caspase-9 and -3. Macroarray analysis revealed that down-regulation of c-Myc produced striking changes in gene expression in the section related to metabolism, where the expression of gamma-glutamyl-cysteine synthetase and GSSG reductase was found to be significantly reduced. The addition of N-acetyl-l-cysteine or glutathione ethyl ester inhibited the apoptotic process, thus confirming the key role of glutathione in programmed cell death induced by c-Myc.

Phosphatidylinositol-3 kinase inhibitors enhance the anti-leukemia effect of STI571

BCR/ABL fusion tyrosine kinase is responsible for the initiation and maintenance of the Philadelphia chromosome (Ph(1))-positive chronic myelogenous leukemia (CML) and a cohort of acute lymphocytic leukemias (ALL). STI571 (Gleevec), a novel anti-leukemia drug targeting BCR/ABL kinase can induce remissions of the Ph(1)-positive leukemias. STI571 was recently combined with the standard cytostatic drugs to achieve better therapeutic results and to overcome emerging drug resistance mechanisms. We decided to search for a more specific partner compound for STI571. Our previous studies showed that a signaling protein phosphatidylinositol-3 kinase (PI-3k) is essential for the growth of CML cells, but not of normal hematopoietic cells (Blood, 86:726,1995). Therefore the anti- Ph(1)-leukemia effect of the combination of BCR/ABL kinase inhibitor STI571 and PI-3k inhibitor wortmannin (WT) or LY294002 (LY) was tested. We showed that STI571+WT exerted a synergistic effect against the Ph(1)-positive cell lines, but did not affect the growth of Ph(1)-negative cell line. Moreover, the combinations of STI571+WT or STI571+LY were effective in the inhibition of clonogenic growth of CML-chronic phase and CML-blast crisis patient cells, while sparing normal bone marrow cells. Single colony RT-PCR assay showed that colonies arising from the mixture of CML cells and normal bone marrow cells after treatment with STI571+WT were selectively depleted of BCR/ABL-positive cells. Biochemical analysis of the CML cells after the treatment revealed that combination of STI571+WT caused a more pronounced activation of caspase-3 and induced massive apoptosis, in comparison to STI571 and WT alone. In conclusion, combination of STI571+WT or STI571+LY may represent a novel approach against the Ph(1)-positive leukemias.

Fusion tyrosine kinases induce drug resistance by stimulation of homology-dependent recombination repair, prolongation of G(2)/M phase, and protection from apoptosis

Fusion tyrosine kinases (FTKs) such as BCR/ABL, TEL/ABL, TEL/JAK2, TEL/PDGF beta R, TEL/TRKC(L), and NPM/ALK arise from reciprocal chromosomal translocations and cause acute and chronic leukemias and non-Hodgkin's lymphoma. FTK-transformed cells displayed drug resistance against the cytostatic drugs cisplatin and mitomycin C. These cells were not protected from drug-mediated DNA damage, implicating activation of the mechanisms preventing DNA damage-induced apoptosis. Various FTKs, except TEL/TRKC(L), can activate STAT5, which may be required to induce drug resistance. We show that STAT5 is essential for FTK-dependent upregulation of RAD51, which plays a central role in homology-dependent recombinational repair (HRR) of DNA double-strand breaks (DSBs). Elevated levels of Rad51 contributed to the induction of drug resistance and facilitation of the HRR in FTK-transformed cells. In addition, expression of antiapoptotic protein Bcl-xL was enhanced in cells transformed by the FTKs able to activate STAT5. Moreover, cells transformed by all examined FTKs displayed G(2)/M delay upon drug treatment. Individually, elevated levels of Rad51, Bcl-xL, or G(2)/M delay were responsible for induction of a modest drug resistance. Interestingly, combination of these three factors in nontransformed cells induced drug resistance of a magnitude similar to that observed in cells expressing FTKs activating STAT5. Thus, we postulate that RAD51-dependent facilitation of DSB repair, antiapoptotic activity of Bcl-xL, and delay in progression through the G(2)/M phase work in concert to induce drug resistance in FTK-positive leukemias and lymphomas.

Acridine-modified, clamp-forming antisense oligonucleotides synergize with cisplatin to inhibit c-Myc expression and B16-F0 tumor progression

The c-myc protooncogene plays a key role in the abnormal growth regulation of melanoma cells. We have targeted three polypurine sequences within the mouse myc mRNA with acridine-modified, clamp-forming antisense oligonucleotides (AS ODNs) in an effort to inhibit growth of murine melanoma cells. These ODNs are unique in that they hybridize to the target mRNA by both Watson-Crick and Hoogsteen hydrogen bond interactions, forming a triple-stranded structure. At a concentration of 3 microM E1C, E2C and E3C inhibit B16-F0 proliferation by 76, 66 and 78%, respectively. Both immunofluorescent staining and western blot analysis corroborate a proportional reduction in c-Myc expression by all three ODNs. There were clear distinctions in the ability of these ODNs to inhibit tumor progression in C57BL/6 mice as a function of Myc expression. There was no synergy demonstrated between ODN E1C with cisplatin (DDP), which inhibited tumor growth by 77% alone and 82% in combination. Although E2C inhibited growth by 54%, its effect was decreased to 32% with DDP, when compared with controls. E3C, on the other hand, demonstrated a synergistic effect with DDP, inhibiting growth by 72% in combination, but only by 1% as a single agent. Immunofluorescence analysis of tumors for each group revealed a concomitant reduction in c-Myc expression in tumors from mice treated with the most active clamp ODN alone (E1C) or clamp ODN + DDP (E1C/E3C + DDP). Western blot analysis confirmed this decrease in target protein expression. Our results document the growth-inhibitory activity of two myc-targeting antisense clamp ODNs; E1C, which has activity as a single agent, and E3C, which has in vivo synergy with DDP pretreatment. These data confirm the antiproliferative effects of these novel ODNs and document an interesting synergy with the chemotherapeutic agent DDP.

[Contribution of antineoplastic biotherapy in the treatment of leukemia in children]

Improvements in the chemotherapeutic and transplant regimens have had a significant impact in improving survival rates for pediatric leukemia. However, there are still major problems to address including what options are available for patients with chemoresistant disease and what strategies are available to avoid toxicity associated with highly cytotoxic treatment regimens. Gene and immunotherapy protocols hold great promise. Using gene transfer of a marker gene, a number of biologic issues in the therapy of leukemia have been addressed. For example, by gene marking autologous bone marrow grafts it has been possible to demonstrate that infused marrow contributes to relapse in acute and chronic myeloid leukemias. In the allogeneic transplant setting, genetically modified T-cells have proven valuable for the prophylaxis and treatment of viral diseases and may have an important role in preventing or treating disease relapse. Gene transfer is also being used to modify tumor function, enhance immunogenicity, and confer drug-resistance to normal hematopoietic stem cells. With the continued scientific advancements in this field, gene therapy will almost certainly have a major impact on the treatment of pediatric leukemia in the future.

Myc down-regulation induces apoptosis in M14 melanoma cells by increasing p27(kip1) levels

In recent years, increasing evidence indicated the importance of a deregulated c-myc gene in the melanoma pathogenesis. We have previously demonstrated that treatment of melanoma cells with c-myc antisense oligodeoxynucleotides can inhibit cell proliferation and activate apoptosis. To gain insight into the mechanisms activated by Myc down-regulation, we have now developed an experimental model that allows modulating Myc protein expression in melanoma cells. This was achieved by originating stable melanoma cell clones expressing ecdysone-inducible c-myc antisense RNA. We show that the induction of c-myc antisense RNA in M14 melanoma cells leads to an inhibition of cell proliferation characterized by accumulation of cells in the G(1) phase of the cell cycle (up to 80%) and activation of apoptosis (50%). These data are associated with an increase of p27(kip1) levels and a significant reduction of the cdk2-associated kinase activity. In addition, we show that an ectopic overexpression of p27(kip1) in this experimental model can enhance the apoptotic rate. Our results indicate that down-regulation of Myc protein induces a G(1) arrest and activates apoptosis by increasing p27(kip1) content in melanoma cells, that are known to be defective for the p16-cyclinD/cdk4-pRb G(1) checkpoint. This is particularly relevant for identifying new therapeutic strategies based on the re-establishment of the apoptotic pathways in cancer cells.

Progressive changes in the leukemogenic signaling in BCR/ABL-transformed cells

Our previous study indicated that BCR/ABL SH2 domain and BCR/ABL SH3 domain+SH2 domain complex are required for immediate activation of the phosphatidylinositol-3 kinase PI-3k)--> Akt serine/threonine kinase pathway and of the signal transducer and activator of transcription 5 (STAT5), respectively, in hematopoietic cells. We show here that the defect in activation of PI-3k/Akt by BCR/ABL DeltaSH2 mutant (SH2 domain deleted) and of STAT5 by BCR/ABL DeltaSH3+DeltaSH2 mutant (SH3 and SH2 domains deleted) is not permanent and both Akt and STAT5 could be 're-activated' by in vitro culture. This phenomenon was responsible for increased resistance to apoptosis, growth factor-independent proliferation and leukemogenesis in SCID mice. Incubation of cells with BCR/ABL tyrosine kinase inhibitor STI571 abrogated the 're-activation' of Akt or STAT5 by BCR/ABL SH3+SH2 mutants in some clones, in the others Akt and STAT5 activation became independent on BCR/ABL kinase activity. The immediate upstream activators of Akt and STAT5 such as PI-3k and Jak-2 were also activated. In addition, the common beta subunit of IL-3/IL-5/GM-CSF receptor was tyrosine phosphorylated in the clones in which 're-activation' was dependent on the BCR/ABL kinase activity. These results suggested that 're-activation' of Akt and STAT5, in the absence of functional BCR/ABL SH3+SH2 domains, may be achieved by two different mechanisms: (i) BCR/ABL kinase-dependent activation of alternative pathway(s) and (ii) additional genetic changes stimulating Akt and STAT5 independently of BCR/ABL. Oncogene (2000) 19, 4117 - 4124

A combination treatment of c-myc antisense DNA with all-trans-retinoic acid inhibits cell proliferation by downregulating c-myc expression in small cell lung cancer

The dysregulation of both c-myc expression and retinoid signaling pathways commonly occurs in small cell lung cancers (SCLC), frequently accompanying tumor relapse, and contributing to the poor prognosis of patients with SCLC. In this study, we investigated whether c-myc antisense oligodeoxynucleoside phosphorothioate (OPT) covering the translational initiation site of c-myc mRNA used in combination with all-trans-retinoic acid (RA) would be more effective than either agent alone in inhibiting the growth of an SCLC cell line, NCI-H82, overexpressing c-myc with amplification of this gene, and whether this combination could be an experimental therapeutic tool against SCLC. c-myc antisense OPT decreased c-myc expression in Northern and Western blot analyses, thus inducing 40% and 20% cell growth inhibition compared with scrambled and sense OPT and with scrambled four guanosine-containing OPT (p < 0.01, and p < 0.01, respectively). All-trans-RA also inhibited cell proliferation at the rate of 40% by downregulating c-myc expression. Having obtained these results, we tested the hymothesis that c-myc antisense OPT in combination with all-trans-RA may further reduce c-myc expression and lead to improved cell growth control. This combination showed a greater inhibition of cell proliferation than either agent given alone (p < 0.01) (60% inhibition of cell growth compared with treatment of control scrambled or sense OPT alone, p < 0.01) through enhanced downregulation of c-myc expression. In conclusion, c-myc antisense DNA in combination with other modalities for c-myc downregulation may represent an attractive gene regulation-based therapy of SCLC in the future. Further efforts, however, using new oligodeoxynucleotide analogs, specific interventions for DNA delivery into cells, and more potent therapeutic agents are required to increase the potentiation of c-myc downregulation and cell growth inhibition.

Inhibition of gene expression and cell proliferation by triple helix-forming oligonucleotides directed to the c-myc gene

Triple helix-forming oligonucleotides (TFOs) bind with high affinity and specificity to homopurine-homopyrimidine sequences in DNA and have been shown to inhibit transcription of target genes in various experimental systems. In the present study, we evaluated the ability of 3'-amino-modified phosphodiester TFOs directed to four sites in the c-myc gene to inhibit gene expression and proliferation of human leukemia (CEM, KG-1, and HL-60) and lymphoma (Raji and ST486) cells. GT-rich TFOs were designed to target sequences located either upstream (myc1 and -2) or downstream (myc3 and -4) of the P2 promoter, which is the major c-myc promoter. Myc2, which was directed to a site immediately upstream of this promoter, inhibited c-myc expression and proliferation of CEM cells. The effects of this TFO were sequence- and target-specific, since control oligonucleotides and TFOs directed to other sites were less or not active. Myc2 was also effective in KG-1, HL-60, and Raji cells. In contrast, ST486 cells were more sensitive to myc3, which targets a sequence in intron 1 upstream of the P3 promoter, than myc2. As result of a chromosomal translocation, P3 is the active promoter in ST486 cells. This study demonstrates the activity and specificity of TFOs designed to act as repressors of c-myc gene expression in human leukemia and lymphoma cells. Our results suggest that this is a valid approach to selectively inhibit gene expression and cancer cell growth, and encourage further investigation of its potential applications in cancer therapy.

Limited synergistic effect of antisense oligonucleotides against bcr-abl and transferrin receptor mRNA in leukemic cells in culture

The synergistic use of antisense oligonucleotides (ASOs) towards the bcr-abl and the transferrin receptor (TfR) mRNA was studied in a chronic myeloid leukemia (CML) cell line, aiming to improve the efficiency of individual ASO treatment. At 20 microM concentration, bcr-abl ASOs reduced cell growth by 40% and was specific for cells that have the translocation: there was a 34% reduction of BCR-ABL protein. The TfR ASO reduced cell growth by 20% and decreased TfR protein by 24%. The ASOs were more potent at reducing cell growth when used in combination (respectively, -20 and -17% than bcr-abl ASO and TfR ASO when used individually at the 10 microM concentration), thus we postulate that there is synergism of action. Cell cycle analysis also revealed that the sub-G1 peak was bigger in the synergistic treatment.

Enhanced anti-tumor effects with microencapsulated c-myc antisense oligonucleotide

A phosphorothioate c-myc antisense oligonucleotide was complexed with zinc and encapsulated into injectable biodegradable microspheres. The efficacy of this novel formulation was compared with intravenous administration of the unencapsulated drug in human melanoma and leukemia xenografts in immunocompromised mice. The microencapsulated formulation was more effective as shown by reduced tumor growth, a decreased number of metastases, reduced c-myc expression, and increased survival in the melanoma model, and decreased metastatic potential and increased survival in the leukemia model. These results show that, as has been demonstrated previously with protein and peptide drugs, greater therapeutic efficacy can be obtained when antisense oligonucleotides are delivered from sustained-release formulations.

Signal transducer and activator of transcription (STAT)5 activation by BCR/ABL is dependent on intact Src homology (SH)3 and SH2 domains of BCR/ABL and is required for leukemogenesis

Signal transducer and activator of transcription (STAT)5 is constitutively activated in BCR/ ABL-expressing cells, but the mechanisms and functional consequences of such activation are unknown. We show here that BCR/ABL induces phosphorylation and activation of STAT5 by a mechanism that requires the BCR/ABL Src homology (SH)2 domain and the proline-rich binding site of the SH3 domain. Upon expression in 32Dcl3 growth factor-dependent myeloid precursor cells, STAT5 activation-deficient BCR/ABL SH3+SH2 domain mutants functioned as tyrosine kinase and activated Ras, but failed to protect from apoptosis induced by withdrawal of interleukin 3 and/or serum and did not induce leukemia in severe combined immunodeficiency mice. In complementation assays, expression of a dominant-active STAT5B mutant (STAT5B-DAM), but not wild-type STAT5B (STAT5B-WT), in 32Dcl3 cells transfected with STAT5 activation-deficient BCR/ABL SH3+SH2 mutants restored protection from apoptosis, stimulated growth factor-independent cell cycle progression, and rescued the leukemogenic potential in mice. Moreover, expression of a dominant-negative STAT5B mutant (STAT5B-DNM) in 32Dcl3 cells transfected with wild-type BCR/ABL inhibited apoptosis resistance, growth factor-independent proliferation, and the leukemogenic potential of these cells. In retrovirally infected mouse bone marrow cells, expression of STAT5B-DNM inhibited BCR/ABL-dependent transformation. Moreover, STAT5B-DAM, but not STAT5B-WT, markedly enhanced the ability of STAT5 activation-defective BCR/ABL SH3+SH2 mutants to induce growth factor-independent colony formation of primary mouse bone marrow progenitor cells. However, STAT5B-DAM did not rescue the growth factor-independent colony formation of kinase-deficient K1172R BCR/ABL or the triple mutant Y177F+R522L+ Y793F BCR/ABL, both of which also fail to activate STAT5. Together, these data demonstrate that STAT5 activation by BCR/ABL is dependent on signaling from more than one domain and document the important role of STAT5-regulated pathways in BCR/ABL leukemogenesis.

2′,5′-Oligoadenylate-Antisense Chimeras Cause RNase L to Selectively Degrade bcr/abl mRNA in Chronic Myelogenous Leukemia Cells

We report an RNA targeting strategy, which selectively degrades bcr/abl mRNA in chronic myelogenous leukemia (CML) cells. A 2′,5′-tetraadenylate activator (2-5A) of RNase L was chemically linked to oligonucleotide antisense directed against either the fusion site or against the translation start sequence in bcr/abl mRNA. Selective degradation of the targeted RNA sequences was demonstrated in assays with purified RNase L and decreases of p210bcr/abl kinase activity levels were obtained in the CML cell line, K562. Furthermore, the 2-5A-antisense chimeras suppressed growth of K562, while having substantially reduced effects on the promyelocytic leukemia cell line, HL60. Findings were extended to primary CML cells isolated from bone marrow of patients. The 2-5A-antisense treatments both suppressed proliferation of the leukemia cells and selectively depleted levels of bcr/abl mRNA without affecting levels of β-actin mRNA, determined by reverse transcriptase-polymerase chain reaction (RT-PCR). The specificity of this approach was further shown with control oligonucleotides, such as chimeras containing an inactive dimeric form of 2-5A, antisense lacking 2-5A, or chimeras with altered sequences including several mismatched nucleotides. The control oligonucleotides had either reduced or no effect on CML cell growth and bcr/abl mRNA levels. These findings show that CML cell growth can be selectively suppressed by targeting bcr/abl mRNA with 2-5A-antisense for decay by RNase L and suggest that these compounds should be further explored for their potential as ex vivo purging agents of autologous hematopoietic stem cell transplants from CML patients.

2′,5′-Oligoadenylate-Antisense Chimeras Cause RNase L to Selectively Degrade bcr/abl mRNA in Chronic Myelogenous Leukemia Cells

We report an RNA targeting strategy, which selectively degrades bcr/abl mRNA in chronic myelogenous leukemia (CML) cells. A 2′,5′-tetraadenylate activator (2-5A) of RNase L was chemically linked to oligonucleotide antisense directed against either the fusion site or against the translation start sequence in bcr/abl mRNA. Selective degradation of the targeted RNA sequences was demonstrated in assays with purified RNase L and decreases of p210bcr/abl kinase activity levels were obtained in the CML cell line, K562. Furthermore, the 2-5A-antisense chimeras suppressed growth of K562, while having substantially reduced effects on the promyelocytic leukemia cell line, HL60. Findings were extended to primary CML cells isolated from bone marrow of patients. The 2-5A-antisense treatments both suppressed proliferation of the leukemia cells and selectively depleted levels of bcr/abl mRNA without affecting levels of β-actin mRNA, determined by reverse transcriptase-polymerase chain reaction (RT-PCR). The specificity of this approach was further shown with control oligonucleotides, such as chimeras containing an inactive dimeric form of 2-5A, antisense lacking 2-5A, or chimeras with altered sequences including several mismatched nucleotides. The control oligonucleotides had either reduced or no effect on CML cell growth and bcr/abl mRNA levels. These findings show that CML cell growth can be selectively suppressed by targeting bcr/abl mRNA with 2-5A-antisense for decay by RNase L and suggest that these compounds should be further explored for their potential as ex vivo purging agents of autologous hematopoietic stem cell transplants from CML patients.

Gene therapy for leukemia and lymphoma

Gene therapy of malignant diseases can be divided into four basic approaches: gene interference, gene insertion, immunopotentiation, and suicide gene approaches. This article reviews the application of these approaches in the therapy of leukemias and lymphomas.

Antileukemia effect of c-myc N3'-->P5' phosphoramidate antisense oligonucleotides in vivo

In vitro, uniformly modified oligonucleotide N3'-->P5' phosphoramidates are apparently more potent antisense agents than phosphorothioate derivatives. To determine whether such compounds are also effective in vivo, severe combined immunodeficiency mice injected with HL-60 myeloid leukemia cells were treated systemically with equal doses of either phosphoramidate or phosphorothioate c-myc antisense or mismatched oligonucleotides. Compared with mice treated with mismatched oligodeoxynucleotides, the peripheral blood leukemic load of mice treated with the antisense sequences was markedly reduced, and such effects were associated with significantly prolonged survival of the antisense-treated mice. Moreover, with each of three different treatment schedules (100, 300, or 900 microg/day for 6 consecutive days), survival of the phosphoramidate-treated mice was significantly longer than that of the phosphorothioate-treated mice. Both phosphoramidate and phosphorothioate oligonucleotides were efficiently taken up by leukemic cells in vivo and were capable of specifically down-regulating c-Myc expression. Moreover, tissue distribution of the phosphoramidate derivatives was undistinguishable from that of the phosphorothioate derivatives. Collectively, these studies suggest that phosphoramidate oligonucleotides can serve as potent and specific antisense agents in the treatment of human leukemia and probably of other malignancies.

Treatment of Philadelphia leukemia in severe combined immunodeficient mice by combination of cyclophosphamide and bcr/abl antisense oligodeoxynucleotides

BACKGROUND

Philadelphia cells are human chronic myelogenous leukemia (CML) cells that contain the BCR/ABL oncogene (a fusion of the BCR and ABL genes). Selective eradication of these cells in vitro can be achieved by combined treatment with antisense phosphorothioate oligodeoxynucleotides ([S]ODNs) specifically targeted to this oncogene (bcr/abl [S]ODNs) and a suboptimal (for use as a single agent) dose of mafosfamide (the in vitro active form of cyclophosphamide).

PURPOSE

We evaluated the ability of bcr/abl antisense [S]ODNs, alone or subsequent to treatment with a single injection of cyclophosphamide, to suppress the leukemic process induced in severe combined immunodeficient (SCID) mice by Philadelphia cells (i.e., primary CML-blast crisis [CML-BC] cells). In addition, we studied potential mechanisms that might explain the efficacy of the bcr/abl antisense [S]ODN-mafosfamide combination against Philadelphia cells in vitro.

METHODS

The effects of treating leukemic mice with cyclophosphamide (25 mg/kg body weight; 25% of the dose required to eradicate evidence of leukemia in SCID mice) and/or bcr/abl antisense [S]ODNs were assessed by analysis of survival, by examination of bone marrow for the presence of leukemia cells (using a colony formation assay or using coupled reverse transcription and the polymerase chain reaction to screen for bcr/abl messenger RNA), and by examination of a variety of tissues for the presence of infiltrating leukemia cells. The induction of apoptosis (a cell death program) in vitro in primary CML-BC cells following treatment with bcr/abl antisense [S]ODNs plus or minus prior treatment with mafosfamide was monitored by use of a commercial assay. Relative cellular uptake of [S]ODNs by CML-BC cells treated in vitro with or without prior treatment with mafosfamide was determined by use of confocal microscopy and flow cytometry (for fluorescent [S]ODNs) or by use of blotting techniques that employed radioactively labeled probes (for extracted, unlabeled [S]ODNs). Levels of specific proteins in treated and untreated cells were determined by use of western blotting methods. Reported P values are two-sided.

RESULTS

The disease process in leukemic mice was retarded substantially by combination treatment with cyclophosphamide and specific bcr/abl antisense [S]ODNs (P < .001, relative to treatment with specific antisense [S]ODNs alone, cyclophosphamide alone, or cyclophosphamide plus nonspecific [i.e., control] antisense [S]ODNs); 50% of the mice treated with cyclophosphamide and specific antisense [S]ODNs appeared to be cured of leukemia. The combination treatment was associated with increased induction of apoptosis. In addition, cellular uptake of bcr/abl antisense [S]ODNs appeared to be increased twofold to sixfold by prior treatment with mafosfamide. This increased uptake of [S]ODNs was associated with enhanced suppression of p210bcr/abl protein levels.

CONCLUSIONS AND IMPLICATIONS

Combination therapy with antisense [S]ODNs targeted to specific oncogenes and less toxic doses of anticancer drugs may represent a rational strategy to purpose for the treatment of human leukemias.

Contiguous four-guanosine sequence in c-myc antisense phosphorothioate oligonucleotides inhibits cell growth on human lung cancer cells: possible involvement of cell adhesion inhibition

A contiguous four-guanosine (4G) sequence in c-myc antisense phosphorothioate oligonucleotides caused an antiproliferative effect in smooth muscle cells. To investigate the antiproliferative effect of c-myc antisense oligonucleotides on human lung cancer cell lines, we synthesized oligonucleotides of various lengths and sequences, focusing on the contiguous four-guanosine (4G) sequence. While a c-myc antisense oligonucleotide (20AS1 (4G)) targeted to the translation initiation codon of c-myc mRNA inhibited cell growth of A549 cells by 69% at 10 microM, a scrambled oligonucleotide (20SCR1 (4G)) containing the contiguous four-guanosine (4G) sequence also inhibited cell growth by 72% at the same dose. Although treatment with either 20AS1 (4G) or 20SCR1 (4G) inhibited cell adhesion by 70% at 10 microM, expression of c-myc protein was significantly suppressed only by 20AS1 (4G) (62%), and was only weakly inhibited by 20SCR1 (4G) (32%). Furthermore, a small cell lung carcinoma cell line, Lu65, which can grow in suspension form, was highly resistant to 20AS1 (4G) treatment (IC50>20 microM). These results suggest that the cell growth inhibition by c-myc antisense oligonucleotides containing the contiguous four-guanosine (4G) sequence was possibly correlated with inhibition of cell adhesion, but not with inhibition of c-myc protein expression, via a sequence-specific non-antisense mechanism.

Blastic transformation of p53-deficient bone marrow cells by p210bcr/abl tyrosine kinase

Blastic transformation of chronic myelogenous leukemia (CML) is characterized by the presence of nonrandom, secondary genetic abnormalities in the majority of Philadelphia1 clones, and loss of p53 tumor suppressor gene function is a consistent finding in 25-30% of CML blast crisis patients. To test whether the functional loss of p53 plays a direct role in the transition of chronic phase to blast crisis, bone marrow cells from p53+/+ or p53-/- mice were infected with a retrovirus carrying either the wild-type BCR/ABL or the inactive kinase-deficient mutant, and were assessed for colony-forming ability. Infection of p53-/- marrow cells with wild-type BCR/ABL, but not with the kinase-deficient mutant, enhanced formation of hematopoietic colonies and induced growth factor independence at high frequency, as compared with p53+/+ marrow cells. These effects were suppressed when p53-/- marrow cells were coinfected with BCR/ ABL and wild-type p53. p53-deficient BCR/ABL-infected marrow cells had a proliferative advantage, as reflected by an increase in the fraction of S+G2 phase cells and a decrease in the number of apoptotic cells. Immunophenotyping and morphological analysis revealed that BCR/ABL-positive p53-/- cells were much less differentiated than their BCR/ABL-positive p53+/+ counterparts. Injection of immunodeficient mice with BCR/ABL-positive p53-/- cells produced a transplantable, highly aggressive, poorly differentiated acute myelogenous leukemia. In marked contrast, the disease process in mice injected with BCR/ABL-positive p53+/+ marrow cells was characterized by cell infiltrates with a more differentiated phenotype and was significantly retarded, as indicated by a much longer survival of leukemic mice. Together, these findings directly demonstrate that loss of p53 function plays an important role in blast transformation in CML.

BCR/ABL regulation of PI-3 kinase activity

The ability of BCR-ABL oncoproteins to induce leukemic transformation of hematopoietic cells depends on their tyrosine kinase activity, which is essential for recruitment and activation of multiple pathways that transduce oncogenic signals. Although it is unknown yet whether activation of PI 3-kinase is required for transformation, the colony-forming ability of Philadelphia cells is dependent on PI 3-kinase activity, as indicated by the results of studies using a number of strategies to interfere with the synthesis and/or the function of the regulatory and catalytic subunits of this kinase. In particular, wortmannin, a specific PI 3-kinase inhibitor, preferentially affected colony formation of Philadelphia cells over that of normal marrow hematopoietic progenitors. The mechanism(s) of such effects are unknown, but PI 3-kinase inhibitors may represent a novel class of therapeutic agents for the ex vivo and/or in vivo treatment of Philadelphia leukemias.