Current Pharmaceutical Design - Volume 12, Issue 34, 2006

Apoptosis is a highly conserved form of cell death, orchestrated by an intricate cross talk between intracellular cysteine proteases (caspases) and amplification factors released from the inter-membranous space of the mitochondria, such as cytochrome C, apoptosis inducing factor, Smac/DIABLO, HtrA2/Omi, and others. Efficient execution of the apoptotic signal is controlled by diverse intracellular mechanisms, ranging from transcriptional activation of genes involved in death signaling or the reciprocal repression of the death inhibitory genes (e.g. p53-induced transcription), to posttranslational modification of proteins and their intracellular trafficking (e.g. the Bcl-2 family). These pro-death forces are counteracted by parallel mechanisms to keep death in check, such as the anti-apoptotic members of the Bcl-2 family, transcription factors such as NF- κB, and activation of cell survival pathways such as the PI3K/Akt network. By dint of the critical role that apoptosis plays in tissue homeostasis and regulation of normal cell growth and proliferation, excessive or deficient apoptosis is an invariable finding in pathological disease states. This is particularly true during the process of cellular transformation and abnormal growth associated with the neoplastic phenotype. As a matter of fact, defect or deficiency somewhere in the apoptotic signal transduction machinery is an acquired hallmark of cancer cells. In the first article, S. Rodriguez-Nieto and B. Zhivotovsky [1] present a snapshot of the various alterations or aberrations in the apoptotic signaling circuitry, particularly in the context of carcinogenesis, and provide a logical basis for novel target-selective drug design to combat the problem of drug resistance in cancer cells. The link between genomic instability upon inactivation of essential gatekeeper genes of the p53 family or the inability to activate damage sensing systems and cancer is discussed. A synopsis of the data pertaining to the defects in receptor-mediated as well as mitochondria-dependent apoptotic signaling in cancer cells is presented as probable druggable targets for enhancing the efficacy of chemotherapy. G. Giles [2] provides an excellent commentary on the role of intracellular redox status in tailoring a milieu conducive for growth and proliferation. The author discusses the basis for the altered redox status of cancer cells and the involvement of reactive oxygen and reactive nitrogen species (ROS and RNS) in modulating the signal transduction pathways and transcription factors commonly associated with the malignant phenotype, particularly in relation to cell proliferation and apoptosis. In addition, differences in mitochondrial morphology, ROS generation, and bioenergetics, between normal and cancer cells are presented. The author proposes tumor redox status as a potential target for novel drug design. J. M. Tarr, P. Eggleton, and P.G. Winyard [3] present a comprehensive account of the role of nitric oxide (NO) in the regulation of apoptotic signaling in tumor cells. Depending upon the milieu, transduction pathways of NO may induce cytotoxicity but may also confer cell protection. The latter could be mediated via activation of signal transduction pathways involved in cell survival/proliferation and angiogenesis, or by blocking cell death signaling by inhibiting caspase activation, both of which could potentially favor the process of carcinogenesis. Alternatively, the death promoting activity of NO could be mediated via cross talk with p53 or through down-regulation of death inhibitory proteins belonging to the IAP family. Along similar lines, S. Pervaiz [4] dwells on the relationship between a prooxidant intracellular milieu and carcinogenesis. The author discusses the differential effects of intracellular superoxide and hydrogen peroxide on apoptotic signaling pathways, whereby a slight increase in intracellular superoxide favors cell survival by inhibiting apoptosis, whereas a significant increase in hydrogen peroxide creates a milieu permissive for death execution. This hypothesis is discussed in the light of recent data demonstrating the intermediary role of a pro-oxidant environment in oncogene-induced cell survival, using Bcl-2 and Rac1 as examples. In addition, the differential effect of superoxide and hydrogen peroxide is presented as a function of intracellular pH via targeting the Na+/H+ exchanger, thereby linking cytosolic acidification to an increase in sensitivity to apoptosis. The author proposes tumor intracellular redox status and pH as excellent novel targets for effective anti-cancer drug design.........

Apoptosis is a genetically controlled and evolutionarily conserved form of cell death of critical importance for normal embryonic development and for the maintenance of tissue homeostasis in the adult organism. The malfunction of the death machinery may play a primary or secondary role in various diseases, with essentially too little or too much apoptosis leading to proliferative or degenerative diseases, respectively. The machinery responsible for killing and degradation of the cell via apoptosis is expressed constitutively and become activated through various stimuli. Apoptotic mechanisms are operating during spontaneous regression of tumors as well as in response to treatment with antineoplastic drugs. The therapeutic goal in cancer treatment is to trigger tumor-selective cell death. However, resistance to treatment is a concern for many types of cancer. Since many anti-neoplastic agents induce an apoptotic type of death in susceptible cells, it is likely that defects or dysregulation of different steps of the apoptotic pathways might be an important determinant of resistance to anticancer drugs. These defects might appear at the initiation and/or execution stages of apoptosis and result in the insufficient elimination of tumor cells, which might lead either to acquired resistance to treatment, or to uncontrolled migration of cancer cells and metastasis. Hence, identification and targeting of the disabled pathway, which is most efficiently inactivated in a particular type of tumor might be the most successful approach in the future. Here we review current knowledge concerning function of apoptotic machinery in cancer cells, and how this information can be used to increase the efficiency of tumor treatment.

Reactive oxygen species (ROS) play a central role as second messengers in many signal transduction pathways, where they can post-translationally modify proteins via the oxidation of redox sensitive cysteine residues. The range of cellular processes under redox regulation is extensive and includes both the proliferative and apoptotic pathways. Control of the cellular redox environment is therefore essential for normal physiological function and perturbations to this redox balance are characteristic of many pathological states. Oxidative stress is particularly prevalent in cancer, where many malignant cell types posses an abnormal redox metabolism involving down-regulation of antioxidant enzymes and impaired mitochondrial function. This provides a major opportunity to design therapeutic strategies to selectively target cancer cells based on their redox profile. This review will provide a background to this emerging field by summarizing the known redox biochemistry of ROS signaling. The mechanisms of ROS generation by the action of oxidoreductases and nitric oxide synthases will be discussed in conjunction with the cell's major antioxidant defenses, with especial emphasis placed on the subcellular location of these redox reactions. The effect of ROS on proliferation and apoptosis will be examined by looking at interactions with transcription factors and the Akt, TNF and MAPK signaling pathways. The review will also outline the major differences in redox metabolism between cancer cells and their non-malignant counterparts. Although the full extent of the ROS regulation of signaling pathways is only beginning to be mapped, early indications are that this paradigm will provide new therapeutic targets for cancer therapy.

Nitric oxide (NO) is a small, highly reactive, diffusible free radical which has been implicated in many physiological and pathophysiological processes. It has either pro-apoptotic or anti-apoptotic effects on cells, depending upon a host of factors. This review outlines some of the regulatory molecules and organelles involved in the apoptotic pathways that can be influenced by the presence of NO, including p53, Bcl-2, caspases, mitochondria, and heat shock proteins. The effects of NO on the apoptosis of tumour cells are also examined.

Apoptosis is an essential and highly conserved process for the maintenance of tissue homeostasis and involves a programmed series of events for the removal of effete, damaged, or mutated cells. Although, activation of caspases underscores the classical signaling cascade during apoptotic execution the role of caspase-independent mechanisms in apoptosis has also gained recognition. It is now well established that apoptotic execution is an inherent tumor suppressor mechanism and failure of apoptosis invariably leads to the acquisition of the transformed phenotype. Indeed, resistance to apoptosis is an essential acquired trait that facilitates the processes of tumor initiation and progression. As a matter of fact, efficient death execution could be a critical even at an earlier stage to inhibit tumor promotion. Interestingly, there is a school of thought supported by strong data that an altered redox status and intracellular milieu of cells could provide the seeding ground for tumor promotion and initiation, and at a latter stage tumor maintenance/progression, by blunting cell death signaling. These findings have not only enhanced our understanding of the processes of carcinogenesis and drug resistance but, more importantly provide novel targets for designing strategies to overcome the problem of apoptosis resistance in tumor cells. This review focuses on the pathways of apoptotic execution, and discusses the role of intracellular redox status on cell survival and death signaling in tumor cells.

Many cancer cells exhibit a disturbed intracellular redox balance, making them distinctively different from their 'healthy' counterparts. Some tumors, such as solid lung carcinoma, are hypoxic, and its cells are therefore more reducing than normal, while others, such as the ones of breast and prostate cancer, proliferate under oxidative stress (OS). These biochemical differences between normal and tumor tissue are significant, and can be used to design effective, yet selective redox drugs. The resulting drug design can follow different avenues. The bioreductive approach is perhaps the most advanced, and uses changes in intracellular redox enzyme concentrations to activate otherwise inactive pro-drug molecules inside cancer cells by a reductive step, often followed by further chemical transformations, such as hydrolysis. Related anti-cancer compounds, such as varacin, employ an intricate combination of reduction and oxidation processes to develop their therapeutic potential inside cells. Another, just emerging approach considers the use of pro-oxidants and catalysts, taking advantage of the inherent efficiency and selectivity associated with OS-induced cell death. Even more complex tactics, such as chelator-assisted photodynamic therapy, exploit the intracellular metal homeostasis to target cancer cells. Together, all of these avenues try to endow molecules with a combination of sensor and effector properties, which might allow them to single out and selectively kill cancer cells without the need for cell-selective drug delivery systems. In the long term, such agents could be associated with high efficiency, good selectivity and dramatically reduced drug side effects.

The permeability transition pore (PTPC), a polyprotein complex, participates in the mitochondrial homeostasis as well as in the mitochondrial phase of the intrinsic pathway of apoptosis. It integrates multiple death signals including alterations of the intracellular milieu, translocation of pro-apoptotic members of the Bax/Bcl-2 family, p53, and viral proteins. As a consequence, PTPC can act as a coordinator of the pro-apoptotic mitochondrial membrane permeabilization process and the release of pro-apoptotic intermembrane space proteins into the cytosol. Moreover, the deregulation of PTPC has been involved in several major human pathologies such as cancer, neurodegeneration, ischemia/reperfusion, aging, as well as hepatotoxicity. Therefore, PTPC has emerged as a promising potential therapeutic target. Here, we will review the current knowledge concerning the two opposite functions of the PTPC and its implication in various pathologies. We will discuss the possibility to target this complex with peptides to modulate apoptosis in an innovative therapeutic perspective.

G protein-activated inwardly rectifying K + (GIRK; Kir3) channels regulate the neuronal activity and heart rate. Molecular cloning of the GIRK channel genes has led to remarkable progress in our understanding of the molecular structure, distribution and functional modulation of these channels. Furthermore, the roles of GIRK channels in vivo have been shown by studies using GIRK knockout mice and weaver mutant mice, which have a missense mutation in the GIRK2 gene. We also review the possible roles of GIRK channels in the pathophysiology of various disorders, and discuss the therapeutic potential of GIRK channel modulation.

After bacterial infection, the host reacts by signalling to the central nervous system where a cascade of physiologic, neuroendocrine and behavioural processes is orchestrated, collectively termed the acute phase response. Endotoxemia following Gram-negative bacterial infection induces a wide array of effects, including fever, loss of appetite and changes in gastrointestinal function that attempt to eliminate the challenge and restore homeostasis. Systemic administration of low doses of endotoxin (5-40 μg/kg) to rats is associated with changes in gastrointestinal motor function, inhibition of gastric acid secretion and increase in the gastric mucosal resistance to damage. These changes are rapid in onset (observed within one hour), not related to vascular dysfunction, and appear to be mediated by mechanisms that involve the peripheral and the central nervous system. Nitric oxide (NO) plays a central role in the physiology of the gastrointestinal tract and its response to illness. Accumulated evidence supports an increase of NO synthesis in the brainstem, as well as in the gastric myenteric plexus thirty minutes after endotoxin administration. Such a synthesis is due to constitutive nitric oxide synthase (NOS) and occurs before the induction of NOS takes place. In this review we provide experimental evidence supporting the hypothesis that activation of a physiologic mechanism, mediated by the autonomic and the central nervous systems as well as constitutive NOS isoforms, is involved in acute changes of gastrointestinal function during early endotoxemia.