Liposomal Bcl-2 antisense oligonucleotides enhance proliferation, sensitize acute myeloid leukemia to cytosine-arabinoside, and induce apoptosis independent of other antiapoptotic proteins

The roles of phosphorylation of signaling proteins in the prognosis of acute myeloid leukemia

Signaling pathways of Retinoblastoma (Rb) protein, Akt-kinase, and Erk-kinase (extracellular signal-regulated kinase) have an important role in the pathogenesis of acute myeloid leukemia. Constitutive activation of these proteins by phosphorylation contributes to cell survival by regulation of cell cycle, proliferation and proapoptotic signaling processes. According to previous data phosphorylated forms of these proteins represent a worse outcome for cancer patients. We investigated the presence of phosphorylated Rb (P-Rb), Akt (P-Akt) and Erk (P-Erk) proteins by Western blot technique using phospho-specific antibodies in bone marrow or peripheral blood samples of 69 AML patients, 36 patients with myelodysplastic syndrome (MDS) and 10 healthy volunteers. Expression level of PTEN (Phosphatase and tensin homolog) and PHLPP (PH domain and leucine-rich repeat Protein Phosphatase) phosphatases, the negative regulators of Akt kinase pathway were also examined. We tested the effect of these proteins on survival and on the correlation with known prognostic features in AML. We found 46.3% of AML patients had detectable P-Rb, 34.7% had P-Akt and 28.9% had P-Erk protein. 66.1% of patients expressing PTEN, 38.9% PHLPP, 37.2% both PTEN and PHLPP and 32.2% neither PTEN nor PHLPP phosphatases. Compared to nucleophosmin mutation (NPMc) negative samples P-Erk was significantly less in nucleophosmin mutated patients, P-Rb was significantly less in patients' group with more than 30 G/L peripheral leukocyte count by diagnosis. PHLPP was significantly present in FAB type M5. The expression of P-Rb represented significant better overall survival (OS), while P-Akt represented significantly worse event-free survival (EFS) in unfavorable cytogenetics patients. The presence of both PHLPP and PTEN phosphatases contributes to better OS and EFS, although the differences were not statistically significant. We confirmed significant positive correlation between P-Akt and PHLPP. Assessing the phosphorylation of Rb, Akt and Erk may define a subgroup of AML patients who would benefit especially from new targeted treatment options complemented the standard chemotherapy, and it may contribute to monitoring remission, relapse or progression of AML.

Gambogic acid is an antagonist of antiapoptotic Bcl-2 family proteins

The natural product gambogic acid (GA) has been reported to have cytotoxic activity against tumor cells in culture and was identified as an active compound in a cell-based high-throughput screening assay for activators of caspases, proteases involved in apoptosis. Using the antiapoptotic Bcl-2 family protein, Bfl-1, as a target for screening of a library of natural products, we identified GA as a competitive inhibitor that displaced BH3 peptides from Bfl-1 in a fluorescence polarization assay. Analysis of competition for BH3 peptide binding revealed that GA inhibits all six human Bcl-2 family proteins to various extents, with Mcl-1 and Bcl-B the most potently inhibited [concentrations required for 50% inhibition (IC(50)), < 1 micromol/L]. Competition for BH3 peptide binding was also confirmed using a time-resolved fluorescence resonance energy transfer assay. GA functionally inhibited the antiapoptotic Bcl-2 family proteins as shown by experiments using isolated mitochondria in which recombinant purified Bcl-2 family proteins suppress SMAC release in vitro, showing that GA neutralizes their suppressive effects on mitochondria in a concentration-dependent manner. GA killed tumor cell lines via an apoptotic mechanism, whereas analogues of GA with greatly reduced potency at BH3 peptide displacement showed little or no cytotoxic activity. However, GA retained cytotoxic activity against bax-/-bak-/- cells in which antiapoptotic Bcl-2 family proteins lack a cytoprotective phenotype, implying that GA also has additional targets that contribute to its cytotoxic mechanism. Altogether, the findings suggest that suppression of antiapoptotic Bcl-2 family proteins may be among the cytotoxic mechanisms by which GA kills tumor cells.

Novel therapies targeting the apoptosis pathway for the treatment of acute myeloid leukemia

Defects in the core regulators of the apoptosis pathway contribute to chemoresistance and poor outcomes in patients with acute myeloid leukemia (AML). To overcome these defects, novel molecules that target key proteins in the mitochondrial, death receptor, and convergence pathways of caspase activation are being developed. This review will highlight selected molecules including Bcl-2 and XIAP inhibitors that are advanced stages of development and have entered clinical trial for AML. In addition, this review will discuss how these novel therapies are being used as chemical probes to better understand the biology of the apoptosis pathway.

Breast Cancer Cells Can Evade Apoptosis-Mediated Selective Killing by a Novel Small Molecule Inhibitor of Bcl-2

Proteins of the Bcl-2 family are key regulators of caspase activation and apoptosis. Some members of this family, notably Bcl-2 and Bcl-x(L), are overexpressed in cancer cells, which have been associated with chemoresistance. We have designed and synthesized a small molecule inhibitor of Bcl-2, named YC137, and studied its role in cancer cells. In vitro studies showed that YC137 inhibits the binding of the Bid BH3 peptide to Bcl-2, thus disrupting an interaction essential for the antiapoptotic activity of Bcl-2. This inhibitor induces apoptosis of hematopoietic progenitors overexpressing Bcl-2 but not Bcl-x(L) and breast cancer cells that express high levels of Bcl-2. On the contrary, a variety of normal primary cells, including CD34(+) progenitors, myoblasts, and peripheral blood mononuclear cells, do not respond to the inhibitor. A breast cancer cell line resistant to YC137 was generated. Analysis of resistant cells revealed a reduced expression of Bcl-2, which correlated with low activation of signal transducer and activator of transcription-3 (Stat3) and reduced expression of the human epidermal growth factor receptor-2 (HER2). Of note, YC137-resistant cells were more sensitive to apoptosis induced by chemotherapy. Because HER2 has not been linked previously to the Stat3-Bcl-2 transcriptional pathway, we additionally confirmed that specific blockade of HER2 in breast cancer cells resulted in down-regulation of Stat3 activity and reduced levels of Bcl-2. Consistently, HER2 blockade led to YC137 resistance. These data provide evidence for the selective killing of tumor cells by YC137 and represent the first example of in vitro selection of cancer cells refractory to a Bcl-2 inhibitor.

Concepts of human leukemic development

Two fundamental problems in cancer research are identification of the normal cell within which cancer initiates and identification of the cell type capable of sustaining the growth of the neoplastic clone. There is overwhelming evidence that virtually all cancers are clonal and represent the progeny of a single cell. What is less clear for most cancers is which cells within the tumor clone possess tumorigenic or 'cancer stem cell' (CSC) properties and are capable of maintaining tumor growth. The concept that only a subpopulation of rare CSC is responsible for maintenance of the neoplasm emerged nearly 50 years ago. Testing of this hypothesis is most advanced for the hematopoietic system due to the establishment of functional in vitro and in vivo assays for stem and progenitor cells at all stages of development. This body of work led to conclusive proof for CSC with the identification and purification of leukemic stem cells capable of repopulating NOD/SCID mice. This review will focus on the historical development of the CSC hypothesis, the mechanisms necessary to subvert normal developmental programs, and the identification of the cell in which these leukemogenic events first occur.

Enhancement of the efficacy of chemotherapy for lung cancer by simultaneous suppression of multidrug resistance and antiapoptotic cellular defense: novel multicomponent delivery system

The efficacy of chemotherapy of lung cancer is limited by the development of resistance in cancer cells during treatment. In most lung cancers, this resistance is associated with the overexpression of (a) multidrug resistance-associated protein (MRP) responsible for drug efflux from the cancer cells (pump resistance) and (b) BCL2 protein that activates antiapoptotic cellular defense (nonpump resistance). A novel liposomal proapoptotic anticancer drug delivery system was developed to enhance anticancer efficacy of the well-established drug doxorubicin (DOX). This multicomponent drug delivery system was tested on multidrug-sensitive and -resistant human small-cell lung cancer cells. The drug delivery system includes four components: (a) liposome as a carrier, (b) DOX as an inductor of apoptosis, (c) antisense oligonucleotides (ASOs) targeted to MRP1 mRNA as a suppressor of pump resistance, and (d) ASOs targeted to BCL2 mRNA as a suppressor of nonpump resistance. Intracellular internalization of ASOs and DOX; the influence of the proposed system on the expression of genes and proteins involved in the multidrug resistance, cytotoxicity, and apoptosis induction and antiapoptotic defense; and the activity of caspases were studied. It was found that the proposed liposomal delivery system successfully delivered ASOs and DOX to cell nuclei, inhibited MRP1 and BCL2 protein synthesis, and substantially increased the anticancer action of DOX by stimulating the caspase-dependent pathway of apoptosis in multidrug-resistant human lung cancer cells.

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.

All- trans-retinoic acid increases cytotoxicity of 1-beta-D-arabinofuranosylcytosine in NB4 cells

PURPOSE

Clinically, the benefits of combining all- trans-retinoic acid (ATRA) with chemotherapy have been well documented in the treatment of acute promyelocytic leukemia (APL). Changes in nucleoside transporter expression and activity have been shown to occur in NB4 cells in vitro following treatment with ATRA. In this study we investigated whether ATRA treatment increases sensitivity to ara-C in NB4 cells. Specifically, we examined the role of ATRA-associated changes in nucleoside transporter expression and activity in eliciting ara-C cytotoxicity.

METHODS

Cellular uptake of [(3)H]-ara-C and nucleoside transporter abundance were determined in untreated cells and cells treated with 1 microM ATRA for 12-72 h using an inhibitor and oil stop procedure, and an equilibrium [(3)H]-NBMPR binding assay, respectively. Cytotoxicity of ara-C and the apoptotic response prior to and following ATRA treatment were determined using the MTT viability assay and the TUNEL assay, respectively.

RESULTS

ATRA treatment increased ara-C cytotoxicity and potency, ara-C transport, and augmented ara-C-induced apoptosis. The combination effect was supraadditive under some conditions and sequence-dependent whereby the maximum effect was seen when the addition of ATRA preceded the addition of ara-C, and when ara-C administration closely followed ATRA administration.

CONCLUSIONS

The ATRA-induced increase in cytotoxicity of ara-C was, in part, the result of an increase in the functional expression of nucleoside transporters, and a role for bcl-2 was also indicated. Our results would suggest that timing of ara-C therapy should be tied to maximal es transporter expression, which is likely to be 24 h after ATRA treatment begins. It remains to be seen whether the response in the clinic can be further enhanced in APL by taking advantage of ara-C transporter regulation by ATRA.

NF-kappaB activates Bcl-2 expression in t(14;18) lymphoma cells

The t(14;18) translocation, which is characteristic of follicular lymphoma, results in the overexpression of the bcl-2 gene dependent upon regulatory elements within the bcl-2 5' flanking region and the immunoglobulin heavy chain gene enhancers. Conflicting evidence exists on the effects of NF-kappaB expression on Bcl-2 levels in different cell types. Lymphoma cells with the t(14;18) translocation show high levels of nuclear NF-kappaB proteins. We observed decreased levels of endogenous Bcl-2 when the IkappaBalpha-super-repressor was expressed in a t(14;18) cell line. Deletion analysis of the bcl-2 promoter indicated that the repressive effect of the IkappaBalpha-super-repressor occurred through a region that contained no NF-kappaB consensus sequences. This highly active region contained a c-AMP response element (CRE) and several Sp1 binding sites. Chromatin immunoprecipitation assays with antibodies specific for the NF-kappaB and CREB/ATF family members, as well as Sp1, resulted in the isolation of this IkappaBalpha-super-repressor responsive region of the bcl-2 promoter. Mutation of the CRE and the two Sp1 sites in different combinations in bcl-2 reporter constructs resulted in the loss of bcl-2 promoter repression by the IkappaBalpha-super-repressor. We therefore conclude that the activation of bcl-2 by NF-kappaB in t(14;18) lymphoma cells is mediated through the CRE and Sp1 binding sites.

Caspases: more than just killers?

Proteases of the caspase family constitute the central executioners of apoptosis. Several recent observations suggest that caspases and apoptosis-regulatory molecules exert important functions beyond that of cell death, including the control of T-cell proliferation and cell-cycle progression. Here, Los and colleagues propose a model that directly connects cell suicide mechanisms to the regulation of cell-cycle progression.