Current Cancer Drug Targets - Volume 10, Issue 6, 2010

The form of programmed cell death known as apoptosis has become an intense focus of investigation in various fields including carcinogenesis and cancer therapy. Defects in apoptosis disturb tissue homeostatsis and contribute to the multistep process of carcinogenesis. Since cancer therapy-induced cytotoxicity largely depends on the intact apoptosis signaling cascades, the blockade of apoptotic pathways may enable malignant cells to evade therapy-induced cell death. Treatment failure due to clinical resistance to cancer treatment is common in many types of cancers. With more understanding and accumulation of knowledge in the regulatory and effecter molecules of apoptosis, new strategies and anti-cancer agents have been developed to manipulate the apoptotic balance in cancer cells in favor of apoptosis. In this special issue of Current Cancer Drug Targets (CCDT), five manuscripts summarize recent development in apoptosisrelated carcinogenesis and cancer therapy from different aspects. Cresce and Koropatnick discussed the application of antisense oligonucleotides (ASOs) to arrest tumor growth via targeting Bcl-2 and clusterin. While Bcl-2 is a well-known anti-apoptotic molecule, clusterin is bi-functional, with a nuclear form of clusterin being pro-apoptotic and a secretory form of clusterin being pro-survival [1]. Several protocols of ASOs are currently under clinical trials for several types of cancers, particularly metastatic melanoma and hormone-refractory prostate cancer, both of which are the focus of the discussion in this review. Bcl-2 ASOs have also been used to treat other tumors, for example, lung cancer and ovarian carcinoma, which have been mentioned in the articles by Han and Roman, and Pennington et al. respectively, in this special issue. Han and Roman have systemically described how lung cancer evades apoptotic death followed by analyzing the implications for treatment with various protocols. Pennington et al. nicely summarized how suicide prevention promotes chemoresistance in ovarian carcinoma. Ovarian cancer is one of malignant epithelial cell types. It appears that cross-talk between the tumor microenvironment and malignant epithelial cells may determine apoptotic response. Traditionally, Bid is a typical pro-apoptotic molecule [2]. Recent development has shown that Bid has double roles with respect to stress-response, which is intricately involved in the carcinogenesis and cancer treatment. Song et al. reviewed the recent development in this area, particularly how Bid is activated and how it contributes to the regulation of the cross-talk of cell cycle arrest and apoptosis. Inflammation is associated with the most of, if not all, cancers [3]. Inflammatory molecules frequently affect the apoptotic pathway, and contribute to carcinogenesis and resistance to multiple drug therapy. Among various inflammatory molecules, nuclear factor kappaB (NF-κB), a transcriptional factor, appears to be an important one. Wang and Cho extensively analyzed the role of NF-κB signaling on the apoptotic effect in inflammation-associated carcinogenesis. Increasing evidence has indicated that targeting NF-κB can improve the sensitization of cancer cells to anti-tumor treatment [4]. We believe that this special issue is an essential compilation of up-to-date research on an emerging topic of central concern in the field of apoptosis in carcinogenesis and cancer therapy, which will be useful for anyone interested in this area.

Antisense reagents and technology have developed as extraordinarily useful tools for analysis of gene function. The capacity of antisense to reduce expression of RNA (including protein-encoding mRNA and non-coding RNA) important in a multitude of diseases (including cancer) has led to the concept of using antisense molecules as drugs to treat those diseases. Both antisense RNA (RNAi) and antisense oligonucleotides (ASOs) are being developed for this purpose, with ASOs currently the most advanced in clinical testing. ASOs inhibit translation or induce degradation of complementary target RNA, and both Phase I and Phase II trials are either completed or in progress for a number of diseases. In this review, we focus on antisense approaches to treatment of two cancers (melanoma and hormone-resistant prostate cancer) where the early application of ASOs has provided important information revealing both potential for success and lessons for future preclinical and clinical investigation of ASOs as anti-cancer drugs. The progress of clinical application of two ASOs showing promise in treatment of human cancers –– Oblimersen(G3139), targeting BCL2 for the treatment of metastatic melanoma, and Custirsen (OGX-11), targeting clusterin for the treatment of hormone refractory prostate cancer (HRPC) - is examined.

Lung carcinoma is the leading cause of carcinoma death in the world. Despite recent advances in understanding the molecular biology of lung cancer and the introduction of new therapeutic agents for its treatment, its dismal 5-year survival rate has not changed substantially. Clinical approaches have not significantly improved the survival of patients with advanced lung cancer. However, recent discoveries about the molecular mechanisms responsible for lung cancer initiation and proliferation have unveiled new targets for therapy. One of the hallmark features of cancer cells is their ability to evade programmed cell death or apoptosis. Alterations in pro- and anti-apoptotic pathways are common in cancer cells and defects in regulation of apoptosis have been implicated in both lung tumorigenesis and drug resistance. Thus, targeting apoptosis through the direct or indirect manipulation of the pro-apoptotic machinery offers a novel strategy for treatment. This mini review summaries the molecular events that contribute to drug-induced apoptosis and how lung tumors evade apoptotic death followed by an analysis of the implications for treatment.

Ovarian cancer is the most lethal of gynecologic malignancies. Currently, standard treatment for epithelial ovarian cancer consists of surgical debulking followed by adjuvant chemotherapy with a platinum-based drug coupled with paclitaxel. While initial response to chemotherapy is high, the majority of patients develop recurrent disease which is characterized by chemoresistance. The primary cytotoxic effect of many chemotherapy drugs is mediated by apoptotic response in tumor cells. Recent data indicate that cross talk between the tumor microenvironment and malignant epithelial cells can influence apoptotic response as well. The identification of molecules involved in the regulation and execution of apoptosis, and their alterations in ovarian carcinoma have provided new insights into the mechanism behind the development of chemoresistance in this disease. Our challenge now is to devise strategies to circumvent cell death defects and ultimately improve response to treatment in ovarian carcinoma patients.

Bid, a BH3-only Bcl-2 family member, is proven to be a pivotal molecule for the regulation of tumorigenesis by its multiple functions in promoting apoptosis, survival and proliferation. Growing evidence supports that Bid has double roles with respect to stress-response. In most cases it functions in a truncated form, but the cleavage of Bid may not be an absolute requirement for Bid to be pro-apoptotic. Full-length Bid can also translocate to and activate the mitochondria without cleavage. Bid has emerged as a central player linking death signals through surface death receptors to the core apoptotic mitochondrial pathway. Bid is also involved in DNA damage response, and the phosphorylated Bid may negatively regulate its pro-apoptotic function independent of the BH3 domain. This review surveys recent developments in understanding the molecular mechanisms of Bid activation and its roles in regulating the cross-talk of cell cycle arrest and apoptosis.

The causal relationship between inflammation and cancer has been documented for sometime, but its molecular nature remains ill defined. Increasing evidence suggested that inflammatory microenvironment in and around tumors is an indispensable participant in the neoplastic process. High level of free radicals produced during inflammation significantly induces DNA damage while evading apoptosis, a hallmark of cancer, reduces the capability of tissues to eliminate damaged cells. Therefore, the mechanism by which inflammation affects the apoptosis pathway is crucial to understand inflammation- associated carcinogenesis. Nuclear factor-κB (NF-κB), a transcriptional factor, plays an important role in the regulation of inflammatory responses. NF-κB signaling, which can be activated by diverse stimuli including proinflammatory cytokines, infectious agents and cellular stresses, has been shown to be involved in carcinogenesis and resistance to multiple drug therapy. In this review, we focus on the role of NF-κB signaling on the apoptotic effect in inflammationassociated carcinogenesis. These insights may help us to consider the role of NF-κB in inflammation and cancer and further on as a target of drugs for the prevention and treatment of these diseases.

The response of pancreatic cancer to treatments remains unsatisfactory, highlighting the need for more effective therapeutic regimens. Sorafenib, an orally available multikinase inhibitor, is active against different tumors, including pancreatic cancer. We studied the antitumor efficacy of sorafenib in combination with different antitumor drugs currently used in clinical practice in in vitro and in vivo experimental models of human pancreatic cancer. The cytotoxic effect of sorafenib and conventional antitumor drug combinations was evaluated in vitro in human pancreatic cancer cell lines and the efficacy of the most active combination was tested on tumor-bearing mice. Flow cytometric, Western blot and immunohistochemistry analyses were performed to investigate the mechanisms involved in the activity of single drugs and in their interaction when used in combination. Sorafenib showed a strong sequence-dependent synergistic interaction in vitro with docetaxel, which was highly dependent on the drug sequence employed. In vivo, human pancreatic cancer-xenografted mice treated with docetaxel followed by sorafenib reduced and delayed tumor growth, with complete tumor regression observed in half of the mice. This marked antitumor effect resulted in an overall increase in mouse survival of about 70% and in a complete cure in 3 of the 8 treated mice. The strong activity was also accompanied by marked apoptosis induction, inhibition of tumor angiogenesis and downregulation of ERK signalling. Our results show that the docetaxel and sorafenib combination exerts high therapeutic efficacy in experimental models of human pancreatic cancer, indicating a promising antitumor strategy for clinical use.

Purpose: To investigate the capacity for ZD6474, a small molecule tyrosine kinase inhibitor, to enhance antitumor and anti-metastasis effects of radiation on human nasopharyngeal carcinoma (NPC). Experimental Design: NPC cell lines and xenograft models were evaluated following treatment with ZD6474 and radiation alone and in combination compared with untreated control mice. Results: Treatment with ZD6474 enhanced the anti-proliferative effect of radiation on NPC cell lines as detected by cell proliferation and apoptosis assays. ZD6474 also induced a significant increase in the radiosensitivity of NPC cells, with radiation enhancement ratios (RERs) ranging from 1.2 to 1.6. Despite the cytotoxicity exhibited by NPC cells following radiotherapy, the invasion and migration of NPC cells were found to be unaffected. In contrast, treatment with ZD6474 strongly inhibited the invasion and migration of NPC cells. When the administration of radiation and ZD6474 was investigated in vitro, the ability of ZD6474 to inhibit activation of the pro-survival signaling pathways induced by radiation was demonstrated. In vivo, ZD6474 significantly enhanced the anti-metastasis effects of radiation, while treatment with radiation and ZD6474 was found to be well tolerated and resulted in a strong inhibition of tumor growth. Conclusions: Our results suggest the combination of radiation and ZD6474 represents a promising strategy for the treatment of human NPC.

Spindle poisons/anti-microtubule drugs are established chemotherapy drugs. These drugs primarily target microtubules and mitotic spindles, activate spindle assembly checkpoint (SAC), resulting in caspase-mediated cell death. However, the terminal phenotypes of drug-treated cells are surprisingly heterogeneous ranging from mitotic catastrophe to apparent senescence, suggesting that input from a variety of signaling pathways influences the cell death process. In recent years, studies revealed several signaling pathways that modulate the efficacy of spindle poisons. In this review, we discuss the genes and pathways whose inhibition or overexpression modulates spindle poison sensitivity. These genes cluster to (i) microtubule, microtubule associated proteins (MAPS) and actin cytoskeleton regulators, (ii) the SAC components, (iii) signaling proteins, (iv) chaperones, (v) cell cycle regulators, (vi) proteasome components, (vii) transcription factors and nuclear receptors, and (viii) apoptotic factors. These gene products would be potential targets for drugs to be combined with spindle poisons. Expression status of these genes would also serve as a prognostic marker for spindle poison- mediated chemotherapy. Understanding signaling pathways involved in drug efficacy will aid to rationally develop synergistic chemotherapy strategy.

Polyisoprenylated proteins (PPs) methylation by polyisoprenylated protein methyl transferase (PPMTase) is counteracted by polyisoprenylated methylated protein methyl esterase (PMPMEase). This is the only reversible step of the polyisoprenylation pathway as the relative amounts of the acid and ester forms are determined by the two competing reactions. Since PMPMEase and PPMTase may influence both the structural/functional conformations of PPs, a thorough study of these enzymes is essential to our understanding of the structural/functional features of PPs. PMPMEase has been reported under such pseudonyms as human carboxylesterase 1 (hCE1) because of its apparent broad substrate spectrum. The current study aimed to show the complementarity between its active site and the polyisoprenylated substrates that it metabolizes. Kinetics analysis was conducted with N-, S- and O-substituted substrates using porcine liver PMPMEase and docking analysis using Arguslab. Consistent with the biochemical analysis, the S-ethyl analog yielded an AScore binding energy of -11.32 compared to -13.48, -14.88, -16.15, and -16.81 kcal/mol for S-prenyl (C-5), S-trans-geranyl (C-10), Strans, trans-farnesyl (C-15) and S-all trans-geranylgeranyl (C-20), respectively. The all trans-geranylgeranyl moiety provides the optimal size for active site interactions. The data reveal that the trans,trans-farnesyl and all trans-geranylgeranyl groups, which are reminiscent of endogenous PPs modifications, have the highest affinity for PMPMEase. Since PPs such as monomeric G-proteins are oncogenic, PMPMEase may be viewed as a viable target for anticancer drug development. The analyses reveal the important structural elements for the design of specific PMPMEase inhibitors to serve in the modulation of oncogenic PPs activities. The results also show that hCE1's repertoire of substrates extends beyond xenobiotics to include PPs as its endogenous substrates.