Current Medicinal Chemistry - Anti-Cancer Agents, Vol. 3, No. 4, 2003

Apoptosis, the cell’s intrinsic death program, is a key regulator of tissue homeostasis. An imbalance between cell death and proliferation may result in tumor formation. Also, killing of cancer cells by cytotoxic therapies, such as chemotherapy, g-irradiation or ligation of death receptors is predominantly mediated by triggering apoptosis in target cells. Death receptor signaling pathways have been implied to contribute to the efficacy of cancer therapy. Failure to undergo apoptosis in response to anticancer therapy may lead to resistance. Understanding the molecular events that regulate apoptosis induced by anticancer therapy and how cancer cells evade apoptosis may provide new opportunities for drug development. Thus, novel strategies targeting tumor cell resistance will be based on insights into the molecular mechanisms of cell death.

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Implication of Raft Microdomains in Drug Induced Apoptosis

Christine Bezombes, Guy Laurent, Jean-Pierre Jaffrezou

DNA damaging agents such as 1-b-D-arabinofuranosylcytosine (Ara-C) and daunorubicin (DNR) are widely used in the treatment of acute nonlymphocytic leukemia. These drugs have, of course, been the objects of intense basic research, as well as preclinical and clinical study. Although specific biochemical lesions (DNA damage) have been associated with Ara-C- and DNR-mediated cytotoxicity, the pathways leading to the induction of apoptosis remain ill defined. This standpoint has forced investigators to explore a new concept in cell response to cytotoxic stress: apoptosis signaling. The recent identification of a ceramide (CER) mediated apoptotic signaling pathway triggered by antitumor agents offers a new perspective for the treatment of neoplastic cells. Indeed, these agents have been shown to induce apoptosis through the activation of a sphingomyelinase (SMase) responsible for the hydrolysis of sphingomyelin (SM) and the generation of CER. The latter acts as a potent apoptosis mediator, triggering several downstream signaling pathways among which the stress-activated protein kinase cascade (MEKK1-SEK1-SAP/JNK) plays a critical role in apoptosis induction. However, the spacio-temporal organization of the key early signaling events is unclear. The present review delineates what appears to be a critical factor in apoptosis signaling: sphingomyelin enriched plasma membrane rafts. The apparent topological partitioning between DNA damage and apoptosis signaling (integrated into specialized plasma membrane domains) is discussed.

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Apoptosis Induced by Topoisomerase Inhibitors

Olivier Sordet, Qasim A. Khan, Kurt W. Kohn, Yves Pommier

Topoisomerase inhibitors are among the most efficient inducers of apoptosis. The main pathways leading from topoisomerase-mediated DNA damage to cell death involve activation of caspases in the cytoplasm by proapoptotic molecules released from mitochondria. In some cells, apoptotic response also involves the death receptor Fas (APO-1/CD95). The engagement of these apoptotic effector pathways is tightly controlled by upstream regulatory pathways that respond to DNA lesions-induced by topoisomerase inhibitors in cells undergoing apoptosis. These include the proapoptotic Chk2, c-Abl and SAPK/JNK pathways, the survival PI(3)kinase-Akt-dependent pathway and the transcription factors p53 and NF-kB. Initiation of cellular responses to DNA lesions-induced by topoisomerase inhibitors is ensured by the protein kinases DNA-PK, ATM and ATR, which bind to DNA breaks. These kinases commonly called “DNA sensors” mediate their effects (DNA repair, cell cycle arrest and/or apoptosis) by phosphorylating a large number of substrates, including several downstream kinases such as c-Abl and the checkpoint protein Chk2. c-Abl induces apoptosis by activating cell death pathways (e.g., SAPK, p53 and p73) and inhibiting cell survival pathways [e.g., PI(3)kinase]. The DNA-damage regulating kinase Chk2, in addition to its role in cell cycle arrest and/or DNA repair, can induce apoptosis by phosphorylation/activation of the promyelocytic leukemia (PML) protein and p53. Finally, we will review the recent observations that support a role for topoisomerases in chromatin fragmentation during the execution phase of apoptosis.

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Signal Transduction Pathways of Taxanes-Induced Apoptosis

Valerie Ganansia-Leymarie , Pierre Bischoff , Jean-Pierre Bergerat , Vincent Holl

Docetaxel (Taxotere) is a member of the taxane class of anticancer agents to reach clinical use. This semisynthetic analog of paclitaxel (Taxol) is one of the newer potent anti-neoplastic agents now undergoing extensive laboratory and clinical investigations. Several studies indicate that antimicrotubule agents are potent promoters of apoptosis in cancer cells. Cytotoxic mechanisms of antimitotic taxoids are not yet fully understood, but it has been demonstrated that docetaxel increases tubulin polymerisation, promotes microtubule assembly and also inhibits tubulin depolymerisation. Disruption of microtubules results also in the induction of tumor suppressor gene p53 and inhibitor of cyclin-dependent kinases and activation/inactivation of several protein kinases. As a consequence cells are arrested in the G2-M phase of the cell cycle, after which they may either undergo cell death by apoptosis or necrosis or overcome the G2-M stop and continue in the division cycle (often toward a post-mitotic cell death) depending on the tumor cell type. Nevertheless, how docetaxel induces apoptotic cell death or caspases activation is not yet defined. One may assume that taxanes are able to induce the phosphorylation of Bcl-X_L/Bcl-2 members and thus inactivate their anti-apoptotic capacities. The down-regulation of Bcl-2 and/or the upregulation of p53 and p21/WAF-1 are certainly one of the important modes of apoptosis induction by taxanes. The aim of this framework is to summarize the effects of microtubuline targeting agents on apoptotic signal transduction and new molecular pathways. Finally, we will also discuss the potential therapeutic interest in the association of docetaxel and ionizing radiation.

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Bcl-2 Proteins: Targets and Tools for Chemosensitisation of Tumor Cells

Ali Bettaieb , Laurence Dubrez-Daloz , Sophie Launay , Stephanie Plenchette , Cedric Rebe , Severine Cathelin , Eric Solary

Proteins of the Bcl-2 family share one or several Bcl-2 homology (BH) regions and behave as pro- or antiapoptotic proteins. Prosurvival members such as Bcl-2 and Bcl-X_L are supposed to preserve mitochondrial outer membrane integrity, thus preventing the release of soluble apoptogenic molecules. Pro-apoptotic members include BH3- only proteins that act as sensors of cellular damage and initiate the death process and Bax-like proteins that act downstream of BH3-only proteins to permeabilise the mitochondrial outer membrane. Whether BH3-only proteins directly activate Bax-like proteins or prevent prosurvival members of the family from inhibiting Bax-like proteins or both remains a matter of controversy. Expression of these proteins is altered in various human tumours and this abnormal expression may contribute to oncogenesis and tumour cell resistance to anticancer drug-induced cell death. Based on these observations, prosurvival proteins are attractive intracellular targets for inducing tumour cell death or sensitising tumour cells to death induced by chemotherapeutic drugs. The use of 18-mer antisense oligonucleotides (G3139 or Genasense) targeting the first six codons of bcl-2 mRNA is currently developed in clinics with phase I studies demonstrating that thrombocytopenia may be the main dose-limiting side effect. This strategy, that efficiently decreases Bcl-2 protein expression in some tumour cells, is currently tested in phase II and phase III trials. Alternative approaches to achieve the functional knock-out of Bcl-2 include the use of either peptides mimicking the BH3 domain of Bcl-2-related proteins or more stable, non peptidic BH3 mimetics and the pharmacological modulation of the post-translational modifications of the protein.