Current Cancer Drug Targets - Volume 8, Issue 2, 2008

The evolutionarily conserved process of apoptosis plays an important role in removing cells during development and in maintaining tissue homeostasis. However, dysregulation of apoptosis, leading to too much or too little cell death, contributes to a number of diseases including cancer. Designing anti-cancer therapeutic solutions requires the understanding of the interplay between positive and negative regulators of the apoptotic program. Indeed, much has been learned about the molecular mechanisms of programmed cell death and the escape of tumor cells from apoptotic demise in recent years. These findings provide a foundation for novel drug design efforts that are poised to utilize newly acquired knowledge. Several of these strategies have already advanced to human clinical trials while many more are still tested in pre-clinical settings. Meulmeester and Jochemsen illustrate in great detail the role of p53 as a master regulator of apoptosis. Often called the guardian of the genome, the p53 tumor suppressor protein commands cell fate by controlling initiation of apoptosis, cell cycle arrest, senescence, and other seminal cellular processes. These authors examine complex mechanistic regulation of p53-induced apoptosis, and present evidence for both transcription independent and dependent pathways. Given the strong association of mutations in the p53 gene locus with occurrence of human tumors it is not surprising that p53 is one of the primary targets for anti-tumor therapeutic intervention. Increased understanding of its mechanistic and functional properties should enable development of p53-targeting anti-cancer agents. In their review Sarah MacKenzie and Clay Clarke dissect the role of caspases in apoptotic pathways. Caspases represent a convergent point of apoptotic pathways. As such, they are irreplaceable components of cell death machinery. The authors note that caspase activity is often diminished in tumor cells due to their decreased expression or inadequate activation. Thus, triggering the activation of caspases, especially effector caspases like caspase-3 that are the ultimate executors of cell death, might reduce tumor resistance to cytotoxic drug treatments. Dimerization of caspases is discussed as one of the targeting modalities since dimerization is essential for proper active site formation of all caspases. Hence, a small molecule that binds the dimer interface of procaspases and drives their activation could be useful in cancer treatment. In my review I discuss inhibitor of apoptosis (IAP) proteins and their role in cancer. IAP proteins are a family of anti-apoptotic regulators that block cell death in response to diverse stimuli. They are expressed in the majority of human malignancies at elevated levels and play an active role in promoting tumor maintenance through the inhibition of cellular death and participation in signaling pathways associated with malignancies. These features make them attractive targets for therapeutic intervention. Several IAP-targeting strategies are discussed, including small molecule IAP antagonists (SMAC mimetics), anti-sense RNAs, and immunotherapy. Maria Miasari, Hamsa Puthalakath and John Silke report on ubiquitylation and cancer development in their review. The Ubiquitin Proteasome System has garnered a lot of attention recently, and there is increased awareness of its importance in various cellular processes, including cell growth and death. The authors discuss the relevance of ubiquitylation and proteasomal degradation for tumor maintenance. Furthermore, they analyze in detail the proteasomal regulation of p53 and the Bcl-2 family of apoptotic regulators, as well as the proteasomal involvement in unfolded proteins-triggered ER stress in multiple myelomas. Tumor Necrosis Factor (TNF) and the TNF super-family as cancer therapeutics is the subject of the review by Dylan Daniel and Nick Wilson. Several ligands of the TNF super-family, including TNF-alpha, lymphotoxin, FAS ligand (FasL), and Apo2 ligand/TNF-related apoptosis-inducing ligand (Apo2L/TRAIL) have been tested in various stages of clinical research for their anti-tumor efficacy. Moreover, several antibodies to TNF receptor (TNFR) super-family members are now being explored as cancer therapeutics. The authors integrate the results of pre-clinical and clinical trials with a concise synopsis of the TNF signaling network, and attempt to reconcile our understanding of how the cell biology and tumor biology relate mechanistically. In her review, Simone Fulda focuses on the modulation of Apo2L/TRAIL-induced apoptosis by HDAC inhibitors. Apo2L/TRAIL, a member of the TNF super-family of death inducing ligands, is of special interest for cancer therapy as it predominantly kills cancer cells while sparing normal cells. Many cancers, however, fail to undergo apoptosis in response to Apo2L/TRAIL treatment. Thus, combination therapies are needed for cancer-specific sensitization towards Apo2L/TRAIL. In recent years, HDAC inhibitors (HDACI) that reverse aberrant epigenetic changes have emerged as a potential strategy to sensitize cancer cells to Apo2L/TRAIL-induced apoptosis. Simone discusses recent advances in the understanding of the molecular events that underlie the synergistic interaction of HDACI and Apo2L/TRAIL as well as ways of translating this knowledge into the design of novel cancer-selective therapeutics. In conclusion, the review articles in this issue of Current Cancer Drug Targets are intended to provide instructive and thought-provoking analyses of apoptotic pathways, together with an in-depth examination of their usability as targets for therapeutic intervention in human malignancies.

Approximately 50% of sporadic human tumors harbor somatic mutations in the p53 gene locus, while germ line mutations confer a high familial risk and are associated with Li-Fraumeni Syndrome patients. The p53 tumor suppressor protein is often referred to as the “guardian of the genome” since its response to DNA-damage or checkpoint failure gives rise to a series of anti-proliferative responses. One of the most important functions of p53 is its ability to induce apoptosis, while disruption of this route can promote tumor progression and chemo resistance. Besides its ability to promote apoptosis through transcription dependent mechanisms, p53 may also be able to activate apoptosis independent of transcriptional regulation. Therefore, to ensure normal cell growth, p53 levels and activity are tightly regulated. Upon diverse forms of cellular stress the steady state levels and transcriptional activity of p53 are considerably increased. The stabilization and activation of p53 are a result of hindered inhibition by its negative regulators, e.g. Mdmx (also known as Mdm4) and Mdm2, while on the other hand activators such as HIPK2 and DYRK2 enhance the p53 response. The continually increasing understanding of the mechanisms of regulation of p53 may provide the basis for new drug designs that could eventually lead to therapeutics to reactivate p53 in cancers.

Cytotoxic approaches to killing tumor cells, such as chemotherapeutic agents, γ-irradiation, suicide genes or immunotherapy, have been shown to induce cell death through apoptosis. The intrinsic apoptotic pathway is activated following treatment with cytotoxic drugs, and these reactions ultimately lead to the activation of caspases, which promote cell death in tumor cells. In addition, activation of the extrinsic apoptotic pathway with death-inducing ligands leads to an increased sensitivity of tumor cells toward cytotoxic stimuli, illustrating the interplay between the two cell death pathways. In contrast, tumor resistance to cytotoxic stimuli may be due to defects in apoptotic signaling. As a result of their importance in killing cancer cells, a number of apoptotic molecules are implicated in cancer therapy. The knowledge gleaned from basic research into apoptotic pathways from cell biological, structural, biochemical, and biophysical approaches can be used in strategies to develop novel compounds that eradicate tumor cells. In addition to current drug targets, research into molecules that activate procaspase-3 directly may show the direct activation of the executioner caspase to be a powerful therapeutic strategy in the treatment of many cancers.

Apoptosis, or programmed cell death, is a cell suicide process with a major role in development and homeostasis in vertebrates and invertebrates. Dysregulation of apoptosis leading to early cell death or the absence of normal cell death contributes to a number of disease conditions including neurodegenerative diseases and cancer. Inhibition of apoptosis enhances the survival of cancer cells and facilitates their escape from immune surveillance and cytotoxic therapies. Inhibitor of apoptosis (IAP) proteins, a family of anti-apoptotic regulators that block cell death in response to diverse stimuli through interactions with inducers and effectors of apoptosis are among the principal molecules contributing to this phenomenon. IAP proteins are expressed in the majority of human malignancies at elevated levels and play an active role in promoting tumor maintenance through the inhibition of cellular death and participation in signaling pathways associated with malignancies. Herein, the role of IAP proteins in cancer and strategies toward targeting IAP proteins for therapeutic intervention will be discussed.

Ubiquitylation is an essential cellular process, and yet many cancer cells appear to be more reliant upon it than normal cells as they are surprisingly sensitive to proteasome inhibitors (PI) and proteasome inhibitor drugs are well tolerated in vivo. Several reviews have suggested that specific protein targets account for PI induced cell death, but fail to adequately explain why cancer cells are more sensitive than normal cells to PIs. We review the evidence for these models, focusing primarily on inducers of cell death including p53 and the pro-apoptotic Bcl-2 Homology proteins (BH3 proteins) and propose an additional hypothesis; that a tumour cell's abnormal physiology makes it particularly reliant upon the proteasome. This hypothesis is well supported in the case of Multiple Myelomas, that may produce large amounts of antibodies and are therefore under considerable ER strain and in turn particularly reliant upon the proteasome to clear the large numbers of misfolded proteins. We propose that other cell types, tumor or non tumour, that are already under ER stress, or its equivalent, maybe particularly susceptible to proteasome inhibitors.

Since the discovery of tumor necrosis factor (TNF)-alpha, researchers have pursued many approaches to harness the potency of TNF-alpha and TNF superfamily members to treat human cancers. Several ligands of the TNF superfamily, including TNF-alpha, lymphotoxin, FAS ligand (FasL), and APO2 ligand/TNF-related apoptosis-inducing ligand (Apo2L/TRAIL) have been tested in various stages of clinical research for their anti-tumor efficacy. Moreover, several antibodies to TNF receptor (TNFR) superfamily members are now being explored as cancer therapeutics. Due to the toxicity associated with delivering TNF-alpha systemically at clinically relevant doses, more targeted methods are now seen as a likely alternative to provide a localized therapeutically effective dose of TNF-alpha. In this review we revisit historical attempts to use TNF-alpha to treat human cancer, and put this into the context of more recent targeted strategies to circumvent TNF-alpha's systemic toxicity. We will attempt to integrate the results of pre-clinical and clinical trials with a concise synopsis of the TNF-alpha signaling network, with the goal of reconciling our understanding of how the cell biology and tumor biology mechanistically relate.

Triggering apoptosis, the cell's intrinsic death program, is a promising approach for cancer therapy. TNF-related apoptosisinducing ligand (TRAIL), a member of the TNF superfamily of death inducing ligands, is of special interest for cancer therapy, since TRAIL has been shown to predominantly kill cancer cells, while sparing normal cells. However, since many cancers fail to undergo apoptosis in response to TRAIL treatment, TRAIL-based combination therapies have been developed for cancer-cell specific sensitization towards TRAIL. Chromatin remodelling plays an important role in gene regulation and aberrant architecture of the chromatin has been implicated in tumor formation and progression. In recent years, HDAC inhibitors (HDACI) that reverse aberrant epigenetic changes have emerged as a potential strategy to sensitize cancer cells for TRAIL-induced apoptosis. Synergistic tumor cell death has been reported in a variety of human cancers using different HDACI together with TRAIL. Here, recent advances in the understanding of the molecular events that underlie the synergistic interaction of HDACI and TRAIL are discussed as well as how this knowledge can be translated into the design of cancer-selective novel therapeutics.

In Europe more than 3 million individuals develop a malignancy annually. Despite recent progress in screening, diagnosis and therapy of most cancers, prognosis remains poor and only a minority of patients are cured. This owes to the fact that most cancers are diagnosed in advanced stages and due to the fact that treatment options for most cancers are limited. While there has been a substantial improvement in systemic therapy for many cancers it remains difficult to assess the potential responsiveness of the cancers towards these newly developed drugs, which include small molecules and monoclonal antibodies. Therefore various strategies have been developed in order to assess the prognosis and predict the expected tumor response in order to individualize the treatment, thereby offering the patient a tailored therapy which accounts for the tumor- and patient-specific morphological and molecular characteristics of the disease. This review summarizes recent efforts, challenges and limitations of proteome analysis in the assessment of prognosis and response-prediction in human cancers.

Prostate cancer is the most common cancer in men and one of the leading causes of cancer-related deaths in Western countries. The extraordinary biological heterogeneity, the increasing incidence of this disease, and the presence of putative premalignant conditions make prostate cancer a crucial pathology to study and test pharmacological or nutritional chemopreventive strategies. It has been demonstrated that the incidence of prostate cancer is lower in Asian people, and that it increases in Asian men living in Western countries; these data point to a pivotal role of diet in the onset of prostate cancer. A large amount of work has been done in investigating chemopreventive properties of dietary compounds widely used in Asian countries (i.e. soy, soybeans, green tea, fish) in respect of the oxidants- and meatrich diet typical of Western people, particularly of central and northern Europe. Some dietary products appear promising as chemopreventive agents for prostate cancer, because they display both anti-oxidant and anti-inflammatory activity - and inflammation is crucial for the aetiology of adeno-carcinoma of the prostate. There is increasing evidence for close correlation between inflammation, the microenvironment and tumour-associated neo-angiogenesis causing the adverse outcomes of prostate cancer. It may thus be useful to develop new strategies to couple the treatment of inflammation-related prostate cancer and the generation of angiopreventive or antiinflammatory molecules to prevent this disease. The search for compounds with few or no adverse effects - particularly cardiovascular - as compared with the agents currently in use is therefore of greatest relevance. This paper reviews the beneficial effects in this context of the most promising compounds: β-carotene, capsaicin, curcumin, daidzein, EGCG, genistein, hyperforin, lycopene, N-acetyl-L-cysteine, reductase inhibitors, resveratrol, selenium, silybinin, quercetin, vitamin-D and vitamin-E.

For many years, alkylating agents and purine nucleoside analogs (PNA) have been considered the drug of choice for treatment of chronic lymphocytic leukemia (CLL). More recently the introduction of monoclonal antibodies (mAb), especially rituximab directed against CD20 and alemtuzumab directed against CD52, has renewed interest in CLL therapy. Over the last few years, several new mAbs directed against lymphoid cells have been developed and investigated in preclinical studies and clinical trials. Some of them are highly active in CLL. New mAbs directed against CD20 include human mAb ofatumumab (HuMax CD20) , IMMU-106 (hA20) which has a >90% humanized framework and GA-101, a novel third - generation fully humanized and optimized mAb. These agents are highly cytotoxic against B-cell lymphoid cells and are evaluated in CLL. Lumiliximab (anti-CD23 mAb) is a genetically engineered macaque-human immunoglobulin (Ig) A1. This antibody showed high activity and good tolerability in phase I clinical trial and is evaluated in phase I/II clinical trials as a single agent and in combination. Epratuzumab is a humanized anti-CD22 mAb currently used in clinical trials for treatment of non-Hodgkin lymphoma and autoimmune disorders. Further studies are needed to elucidate the role of this agent in CLL. Apolizumab (HU1D10) is a humanized IgG1 antibody specific for a polymorphic determinant found on the HLA-DRβ chain. Preclinical and early clinical studies suggest that this mAb has some activity in CLL. HCD122 (CHIR-12.12) and SGN-40 are anti-CD40 mAbs which induce cytotoxicity against CLL cells. Phase I study has shown a favorable safety profile and some activity of HCD122 in pretreated CLL patients. Immunotoxins, especially BL22, LMP-2 and denileukin diftitox, are also being evaluated in lymphoid malignancies and seem to be active in CLL. Finally, antiangiogenic mAbs, especially bevacimzumab, have a potential therapeutic role in this disease. In this review, new mAbs, potentially useful in CLL are presented.