The catalytic DNA topoisomerase II inhibitor ICRF-193 and all-trans retinoic acid cooperatively induce granulocytic differentiation of acute promyelocytic leukemia cells: candidate drugs for chemo-differentiation therapy against acute promyelocytic leukemia

Synergistic combination therapy with cotylenin A and vincristine in multiple myeloma models

Multiple myeloma is a malignant proliferative disease of plasma cells in the bone marrow and remains largely incurable. Cotylenin A, a fusicoccane diterpene glycoside with a complex sugar moiety, was isolated as a plant-growth regulator. Cotylenin A has been shown to inhibit the growth of various cancer cells. Herein, we examined the anti-myeloma effects of cotylenin A using five human myeloma cell lines (RPMI-8226, KMS-11, KMS-26, KMS-12 PE and KMS-12 BM) and xenografts in immunodeficient mice. Cotylenin A and vincristine synergistically inhibited the growth and induced apoptosis in myeloma cells. While other microtubule-disturbing agents also showed co-operative effects with cotylenin A, other anticancer agents, such as doxorubicin, cisplatin, camptothecin, methotrexate, gemcitabine and 5-fluorouracil, did not show such co-operation with cotylenin A. These differences might be attributed to the effects on autophagic responses. Combined treatment with cotylenin A and vincristine induced autophagy (formation of LC3-II and degradation of p62 protein). However, doxorubicin did not enhance the autophagy induced by cotylenin A. A colony-forming assay indicated that the combined treatment with cotylenin A and vincristine more effectively suppressed the formation of large colonies, which have higher self-renewal activity than vincristine alone. Expression of pluripotency-associated transcription factor Sox2 mRNA in RPMI-8226 myeloma cells was significantly suppressed by treatment with cotylenin A. Combined treatment with cotylenin A and vincristine significantly inhibited the growth of KMS-26 myeloma cells as xenografts. Our results suggest that the combination of cotylenin A and vincristine may have therapeutic value. Recently, it was reported that cotylenin A modulates the 14-3-3 intracellular signaling pathway. The 14-3-3 proteins may be novel targets in treating myeloma. However, our study could not explain how the sensitization to vincristine is related to the effects of cotylenin A on the 14-3-3 signaling pathway and further studies will be needed.

Induction of acute lymphocytic leukemia differentiation by maintenance therapy

Despite extensive study in many malignancies, maintenance therapy has clinically benefited only two diseases: acute lymphocytic leukemia (ALL) and acute promyelocytic leukemia (APL). ALL maintenance therapy utilizes low-dose 6-mercaptopurine (6MP) and methotrexate (MTX), while maintenance in APL primarily consists of all-trans-retinoic acid (ATRA). 6MP and MTX as used in ALL are also now usually added to maintenance ATRA for APL, based on data suggesting an improved disease-free survival. Although the mechanism of action of MTX and 6MP as maintenance is unknown, low-dose cytotoxic agents are potent inducers of differentiation in vitro. Thus, we studied whether maintenance therapy in ALL, like ATRA in APL, may be inducing terminal differentiation of ALL progenitors. The APL cell line NB4, the ALL cell lines REH and RS4;11, and patients' ALL blasts were incubated with ATRA, 6MP, and MTX in vitro. All three drugs inhibited the clonogenic growth of the APL and ALL cell lines without inducing immediate apoptosis, but associated with induction of phenotypic differentiation. The three drugs similarly upregulated lymphoid antigen expression, while decreasing CD34 expression, on patients' ALL blasts. These data suggest that induction of leukemia progenitor differentiation plays an important role in the mechanism of action of maintenance therapy in ALL.

The design of selective and non-selective combination therapy for acute promyelocytic leukemia

Acute promyelocytic leukemia (APL) is an unique subtype of acute myeloid leukemia typically carrying a specific reciprocal chromosome translocation, t(15;17), leading to the expression of a leukemia-generating fusion protein, PML-RARalpha. APL patients are responsive to APL-selective reagents such as all-trans retinoic acid (ATRA) or arsenic trioxide and non-selective cytotoxic chemotherapy. Nearly all de novo APL patients undergo clinical remission when treated with ATRA plus chemotherapy or with the combinational selective therapy, ATRA plus As2O3. Combining ATRA with As2O3 as an induction followed by chemotherapy consolidation results in more profound clinical remissions compared to treatment with any agent alone or any of the other possible combinations. The mechanism of action of each of these agents differs. ATRA induces APL cell differentiation and PML-RARalpha proteolysis. As2O3 induces APL cell partial differentiation, PML-RARalpha proteolysis, and apoptosis. Chemotherapy, mainly using anthracyclines, induces APL cell death. The combined effects of selective APL therapy (ATRA and As2O3) and/or non-selective chemotherapy in APL cells in vitro and their mechanisms in relation to clinical protocol design are discussed.

Topoisomerase II-DNA complexes trapped by ICRF-193 perturb chromatin structure

DNA topoisomerase II (topo II) is involved in unlinking replicating DNA and organizing mitotic chromosomes. Topo II is the target of many antitumour drugs. Topo II inhibition results in extensive catenation of newly replicated DNA and may potentially perturb chromatin assembly. Here, we show that the topo II inhibitor ICRF-193 does not prevent the bulk of nucleosome deposition, but perturbs nucleosome spacing in Xenopus egg extracts. This is due to the trapping of topo II-closed clamps on the DNA rather than increased DNA catenation. Inhibition of replicative DNA decatenation has in itself little or no effect on nucleosome deposition and spacing, suggesting that DNA can easily accommodate the sharp bending constraints imposed by the co-habitation of nucleosomes and catenane nodes. Chromatin perturbation by topo II clamps may explain some dominant cellular effects of ICRF-193. Nucleosome-driven bending of precatenane nodes may facilitate their unlinking by topo II during unperturbed replication.

Requirement for myeloid growth factors in the differentiation of acute promyelocytic leukaemia

It is well known that the differentiation of acute promyelocytic leukaemia (APL) cells by all-trans-retinoic acid (ATRA) may be enhanced by myeloid growth factors, but the requirement for growth factors in this process is unclear. Our previous studies in multiple myeloma and non-APL acute myeloid leukaemia demonstrated that lineage-specific growth factors are required for the maximal activity of many pharmacologic differentiating agents in vitro. Thus, we studied whether the differentiation of APL is similarly dependent on growth factors. We found that the myeloid growth factors granulocyte colony-stimulating factor or granulocyte-macrophage colony-stimulating factor markedly increased the differentiation of NB4 cells or APL blasts from clinical samples treated with ATRA, arsenic trioxide (ATO), or bryostatin-1 as evidenced by the enhanced expression of myeloid surface antigens and the inhibition of clonogenic growth. Furthermore, myeloid growth factors were necessary for the differentiation of APL cells since the activity of each pharmacologic agent could be blocked by specific growth factor-neutralizing antibodies. Each differentiating agent was active only at concentrations that inhibited cell cycling, suggesting that this property is also required for differentiation. These data demonstrate that both pharmacologic differentiating agents and myeloid growth factors are required, but neither sufficient, for the differentiation of APL cells. The combined use of pharmacologic differentiating agents and growth factors may improve the clinical efficacy of differentiation therapy in APL.

Cancer therapy: new targets for chemotherapy

The number two cause of mortality in developed countries is cancer. Despite the enormous effort put into cancer prevention, early diagnosis and treatment, it is likely that the incidence of the cancer morbidity and mortality will increase for the foreseeable future. This is due to various factors such as increased life expectancy, changes in environment and also the socio-economic situation around the world. Some cancer attracts more attention than others and increasingly epidemiological information is reaching the general public and is beginning to influence behavior. It is now well recognized that, for example, 1 of 8 women in the industrialized world will be diagnosed with breast cancer. Additionally, a strong correlation was established between lung cancer incidence and smoking and it is broadly accepted that the incidence of colon cancer is directly related to age and diet, and has been increasing over time. The current failure of preventive measures to significantly reduce the increasing incidence of these common tumors illustrates the importance of effective cancer treatment strategies, including chemotherapy. The combination of various anticancer drugs, given together with surgery and radiotherapy, gives hope to many patients. There has been recent evidence of improved therapeutic outcome with recent approaches and newer agents but for continuing effective chemotherapeutic treatment there is a need for a detailed understanding of their mechanisms of action and on the rationale of their application. This review attempts to provide up-to-date information regarding the development of new and innovative treatment strategies for cancer chemotherapy. Virtually, every year several of new targets for cancer therapy on both, cellular and molecular levels, are identified and new drugs enter not only clinical trials but also are included in well accepted and documented therapeutic protocols. As this review is in addition to our review published previously (Medical Principles and Practice 11, 2002, 117-125), we have tried to include new and innovative targets and drugs that attract attention at present. Although it is not possible to provide a complete list of all achievements and cover all work done in this field, we hope to be able to give some insight into this rapidly developing area.