Current Cancer Drug Targets - Volume 4, Issue 1, 2004

Several epigenetic alterations leading to constitutively active mitogenic and cell-survival signaling, and loss of apoptotic response are causally involved in self-sufficiency of prostate cancer (PCA) cells toward uncontrolled growth, and increased secretion of pro-angiogenic factors. Therefore, one targeted approach for PCA prevention, growth control and / or treatment could be inhibition of epigenetic molecular events involved in PCA growth, progression and angiogenesis. In this regard, silibinin / silymarin (silibinin is the major active compound in silymarin) has shown promising efficacy. Our extensive studies with silibinin / silymarin and PCA cells have shown the pleiotropic anticancer effects leading to cell growth inhibition in culture and nude mice. The underlying mechanisms of silibinin / silymarin efficacy against PCA involve alteration in cell cycle progression, and inhibition of mitogenic and cell survival signaling, such as epidermal growth factor receptor, insulin-like growth factor receptor type I and nuclear factor kappa B signaling. Silibinin also synergizes the therapeutic effects of doxorubicin in PCA cells, making it a strong candidate for combination chemotherapy. Silibinin / silymarin also inhibits the secretion of proangiogenic factors from tumor cells, and causes growth inhibition and apoptotic death of endothelial cells accompanied by disruption of capillary tube formation on Matrigel. More importantly, silibinin inhibits the growth of in vivo advanced human prostate tumor xenograft in nude mice. Recently, due to its non-toxic and mechanismbased strong preventive / therapeutic efficacy, silibinin has entered in phase I clinical trial in prostate cancer patients.

Epidemiological studies, preclinical investigations and clinical intervention trials support the role of selenium compounds as potent cancer chemopreventive agents; the dose and the form of selenium are critical factors in cancer prevention. Induction of apoptosis and inhibition of cell proliferation are considered important cellular events that can account for the cancer preventive effects of selenium. Toxicity should always be considered a determining factor in the selection of potential chemopreventive agents. Prior to induction of apoptosis, selenium compounds alter the expression and / or activities of a number of cell cycle regulatory proteins, signaling molecules, proteases, mitochondrial associated factors, transcriptional factors, tumor suppressor genes, polyamine and glutathione levels. Depending on the form, selenium compounds can target separate pathways but more efforts are needed to learn about disrupting different pathways converging to apoptosis. Numerous selenium compounds are known to inhibit carcinogenesis in several animal models but not all of these have been examined for their efficacy to induce apoptosis or vice versa in the corresponding target organ. Studies aimed at investigating the effects of selenium compounds on apoptosis in the target organ in vivo and in vitro are limited. On the basis of information provided in this review, we recommend that additional molecular markers should be added to those proposed in the Selenium and Vitamin E Cancer Prevention Trial (SELECT) on prostate cancer. Apart from the selenium compounds reviewed here, several novel synthetic organoselenium compounds need to be examined both in vitro and in vivo for their potential to induce apoptosis; such an investigation may provide better and mechanism-based cancer chemoprevention as well as chemotherapeutic agents.

Several epidemiological, clinical and experimental studies established nonsteroidal anti-inflammatory drugs (NSAIDs) as promising cancer chemopreventive agents. Long-term use of aspirin and other NSAIDs has been shown to reduce the risk of cancer of the colon and other gastrointestinal organs as well as of cancer of the breast, prostate, lung, and skin. Understanding the action of NSAIDs provides substantial insights into the mechanisms by which these unique agents regulate tumor cell growth and enable better strategies for prevention and treatment. NSAIDs restore normal apoptosis and reduce cell proliferation in human adenomatous colorectal polyps, experimental colonic tumors, and in various cancer cell lines that have lost critical genes required for normal function. NSAIDs, particularly selective cyclooxygenase-2 (COX-2) inhibitors such as celecoxib, have been shown to inhibit angiogenesis in cell culture and in rodent models of angiogenesis. Exploration of the multistep process of carcinogenesis has provided substantial insights into the mechanisms by which NSAIDs modulate these events. However, unresolved questions with regard to safety, efficacy, optimal treatment regimen, and mechanism of action currently limit the clinical application of NSAIDs to the prevention of polyposis in FAP patients. Moreover, the development of safe and effective NSAIDs for chemoprevention is complicated by the potential that rare, serious toxicity may offset the benefit of treatment with these drugs given to healthy individuals who have a low risk of developing the disease. Growing knowledge in this area has brought about innovative approaches using combine actions of NSAIDs with other agents that have different modes of action. It has also led to the development of nitric oxide-releasing NSAIDs, that induce tumor cell apoptosis and compensate for COX function, as a means of increasing efficacy and minimizing toxicity. There is growing optimism for the view that full exploration of the role of NSAIDs in the prevention and treatment of epithelial cancers will serve towards reducing of mortality and morbidity from various cancers.

Ionizing radiation exposure is associated with activation of certain immediate-early genes that function as transcription factors. These include members of jun or fos and early growth response (EGR) gene families. In particular, the functional role of EGR-1 in radiation-induced signaling is pivotal since the promoter of EGR-1 contains radiation inducible CArG DNA sequences. The Egr-1 gene belongs to a family of Egr genes that includes EGR-1, EGR-2, EGR-3, EGR-4, EGR-α and the tumor suppressor, Wilms' tumor gene product, WT1. The Egr-1 gene product, EGR-1, is a nuclear protein that contains three zinc fingers of the C2H2 subtype. The EGR-1 GC-rich consensus target sequence, 5'-GCGT / GGGGCG-3' or 5'-TCCT / ACCTCCTCC-3', has been identifiedin the promoter regions of transcription factors, growth factors, receptors, cell cycle regulators and proapoptotic genes. The gene targets mediated by Egr-1 in response to ionizing radiation include TNF-α, p53, Rb and Bax, all these are effectors of apoptosis. Based on these targets, Egr-1 is a pivotal gene that initiates early signal transduction events in response to ionizing radiation leading to either growth arrest or cell death in tumor cells. There are two potential application of Egr-1 gene in therapy of cancer. First, the Egr-1 promoter contains information for appropriate spatial and temporal expression in-vivo that can be regulated by ionizing radiation to control transcription of genes that have pro-apoptotic and suicidal function. Secondly, EGR-1 protein can eliminate ‘induced-radiation resistance’ by inhibiting the functions of radiation-induced prosurvival genes (NFκB activity and bcl-2 expression) and activate proapoptotic genes (such as bax) to confer a significant radio-sensitizing effect. Together, the review of reported findings demonstrate clearly that EGR-1 is an early central gene that confers radiation sensitivity and its pro-apoptotic functions are synergized by abrogation of induced radiation resistance.

Evidence accumulated over the past two decades has indicated that exposure of cell populations to ionizing radiation results in significant biological effects occurring in both the irradiated and non-irradiated cells in the population. This phenomenon, termed the ‘bystander response’, has been shown to occur both in vitro and in vivo. Experiments have indicated that genetic alterations, changes in gene expression and lethality occur in bystander cells that neighbor directly irradiated cells. Furthermore, cells recipient of growth medium harvested from irradiated cultures exhibit responses similar to those of the irradiated cells. Several mechanisms involving secreted soluble factors, gap-junction intercellular communication and oxidative metabolism have been proposed to regulate the radiation-induced bystander effect. In this review, our current knowledge of this phenomenon and its potential impact both on the estimation of risks of exposure to low doses / low fluences of ionizing radiation and on radiotherapy is discussed.

Cyclin E is essential for progression through the G1-phase of the cell cycle and initiation of DNA replication by interacting with and activating its catalytic partner, the cyclin dependent kinase 2 (Cdk2). Rb, as well as Cdc6, NPAT, and nucleophosmin, critical components of cell proliferation and DNA replication, respectively, are targets of Cyclin E / Cdk2 phosphorylation. There are a number of putative binding sites for E2F in the cyclin E promoter region, suggesting an E2F-dependent regulation. Skp2 and Fbw7 are novel proteins, responsible for ubiquitin-dependent proteolysis of Cyclin E. The tight regulation of cyclin E expression, both at the transcriptional level and by ubiquitin-mediated proteolysis, indicates that it has a major role in the control of the G1- and S-phase transitions. Cyclin E is also transcriptionally regulated during radiation-induced apoptosis of hematopoietic cells. In addition to its biological roles, deregulated cyclin E expression has an established role in tumorigenesis. Cell cycle regulatory molecules, such as cyclin E, are frequently deregulated in different types of cancers, where overexpressed native or low molecular weight forms of Cyclin E have a significant role in oncogenesis. During apoptosis of hematopoietic cells, caspase-dependent proteolysis of Cyclin E generates a p18-Cyclin E variant. Understanding the role of Cyclin E in apoptosis may provide a novel target, which may be effective in cancer therapy. This review summarizes what is known about the biological role of cyclin E, its deregulation in cancer, and the opportunities it may provide as a target in clinical therapy.

Recent studies have generated sufficient information to warrant a consideration of protein kinase CK2 as a potential target for cancer therapy. CK2 is a ubiquitous and highly conserved protein serine / threonine kinase that has long been considered to play a role in cell growth and proliferation. It is essential for cell survival, and considerable evidence suggests that it can also exert potent suppression of apoptosis in cells. This is important since the cancer phenotype is characterized by deregulation of not only proliferation but also of apoptosis. In normal cells, the level of CK2 appears to be tightly regulated, and cells resist a change in their intrinsic level of CK2. However, in all the cancers that have been examined an elevation of CK2 has been observed. Further, it appears that modest deregulation in the CK2 expression imparts a potent oncogenic potential to the cells. Disruption of CK2 by treatment of cells with antisense CK2 results in induction of apoptosis in a time and dose-dependent manner. Thus, we propose that down-regulation of CK2 by employing specific strategies to deliver antisense CK2 in vivo could have a potential role in cancer therapy.

Selection of treatment options for clinically localized prostate cancer is based on a host of factors including the patient's age, overall health status, potential complications, clinical tumor stage and Gleason score. It is widely acknowledged that androgen independent disease remains the main obstacle to improving the survival and quality of life in patients with advanced prostate cancer. Apoptosis as a genetically regulated process has a critical endpoint that coincides with the therapeutic goal of successful treatment of androgen-dependent and androgen-independent prostate cancer. Opportunities to alter the apoptotic threshold of prostate cancer cells using antisense technology and gene therapy certainly exist, but the scope and extent of their applicability and action depends upon research delineating the many subtleties within the apoptotic pathway. Most epithelial and endothelial cells undergo apoptosis when they loose contact with the extracellular matrix (ECM), via the phenomenon of anoikis. Signaling interaction between growth factor apoptosis-signaling pathways and cellular effectors of anoikis potential and tumor vascularity provides a new molecular basis for optimizing combination approaches for the effective treatment of advanced prostate cancer. Agents that induce epithelial or endothelial cell apoptosis by antagonizing integrin binding are considered for cancer therapy via their ability to inhibit tumor vascularization. This review summarizes the current knowledge of the therapeutic benefit of apoptosis induction within the context of tumor neovascularization inhibition, and provides an insight into the consequences of anoikis induction (by different agents) in targeting angiogenesis in prostate cancer cells.

To date six bona fide death receptors (DRs) have been discovered and include tumor necrosis factor receptor 1 (TNF-R1), Fas, DR3, DR4, DR5 and DR6. Each receptor contains an extracellular region and an intracellular region; the intracellular region harbors a death domain that is critical for transduction of apoptotic signals. These death receptors are activated by their respective ligands. For example, TNFα activates TNF-R1 while FasL and TL1A activate Fas and DR3 respectively. TNF-related apoptosis inducing ligand (TRAIL), also known as Apo2L, activates DR4 and DR5. The ligand for DR6 has yet to be identified. These death receptors are also believed to be activated in a ligand-independent manner. A large body of recent evidence suggests that death receptors could be utilized as key molecular targets to develop novel therapeutics. This review discuses the pros and cons of targeting death receptors in the development of novel cancer therapeutic agents.