Current Pharmaceutical Design - Volume 16, Issue 12, 2010

Cell death and differentiation represent different types of cell response to environmental stimuli. The focus of the present issue is to outline physical and chemical conditions responsible for inducing various modes of cell damage and death useful for successful anti-cancer treatment. In recent years new demanding fields of research have developed with the aim of studying human exposure to environmental chemical, physical or biological agents to increase the knowledge of the mechanisms and dynamics of events causing adverse health effects [1]. This topic deserves particular attention in light of the impact that climate changes, global warming and pollution are having on human health and habitat. When compared to the chemical induction of apoptosis, relatively little is known about cell treatment with physical agents. According to epidemiological findings and compelling experimental evidences, ultraviolet radiation (UVR) appears to be the most important environmental carcinogen, whose harmful effects can be prevented through education and accurate photo-protective strategies at skin and ocular level [2]. UV, ionizing radiation (IR), reactive oxygen species (ROS) and a number of chemicals, including anti-neoplastic drugs, induce DNA damage, which can be repaired by the co-ordinated action of the DNA repair system and cell cycle checkpoint controls or, if not, can result in cell cytotoxicity or senescence [2-7]. Though direct as well as indirect, non-targeted and delayed effects (bystander and adaptive responses, genomic instability) of radiation or chemical exposure need to be further evaluated in terms of cancer risk assessment [8], it is evident that cell detrimental effects induced by both physical and chemical agents have been greatly exploited in cancer therapy. Interestingly, in light of its capability of inducing apoptosis, hyperthermia has become the fifth strategy, after surgery, chemotherapy, radiation and biological therapy, in the fight against cancer [9]. Moreover, during the last decades, a better understanding of cancer biology has led to the development of new promising therapeutic approaches, based on “molecular targeted” drugs, directed against specific “target” molecules playing a key role in tumour maintenance [10]. Among these molecules, protein kinases (e.g. tyrosine kinase, serine/threonine kinase) play a key role in tumorigenesis, cancer progression and metastasis and, thus, represent ideal anti-cancer therapeutic targets [11]. Small kinase inhibitors or monoclonal antibodies have been used as monotherapy or in combination with conventional treatments (chemotherapy or radiotherapy) to reduce the common emergence of drug or radiation resistance. In this respect, the design of PI3K or multi-target inhibitors appears to be a promising tool for the treatment of tumours even after development of resistance to traditional chemotherapy [12]. Besides oncogenes over-expression and cell cycle control mechanisms disruption, mutations in apoptotic regulators (namely p53) are very frequent in cancer cells and represent for them a way to escape toxic effects inducible with chemo/radiotherapy. As an alternative strategy to restoring transcriptional activation to mutant p53 proteins in solid tumours, small molecule selective inhibitors of p53/MDM2 interaction (Nutlins) are emerging as an innovative tool in the treatment of malignancies expressing wild-type p53 including haematological disorders [5]. Based on these findings, the topic of this issue was chosen to improve our understanding of therapeutic potential of physical and chemical anti-cancer strategies. In the opening paper “Signalling Pathways Activated by Ultraviolet Radiation: Role in Ocular and Cutaneous Health” [2], Dr. N. Di Girolamo provides clinical and experimental findings highlighting the importance of ultraviolet radiation (UVR) as the key trigger for ocular and cutaneous disorders. An overview of the effects of different UV wavelengths (UVA, UVB, UVC) on the eye and skin is reported with a particular emphasis on a UV-induced ocular disease known as pterygium. The article provides a valuable insight into the processes and signal transduction pathways activated by UVR at cellular level and mediating both cell death (apoptosis) and survival (autophagy) responses, indicating the molecular basis of chemopreventive potential of retinoids in UVR-associated epidermal and ocular damage. The second review “Morpho-Functional Features of In-Vitro Cell Death Induced by Physical Agents” by Dr. S. Burattini and co-authors [9] summarizes the present knowledge on the apoptogenic effect of some physical agents (hypo- or hyperthermia, UVR, mild static magnetic fields) on several cell types, mostly in culture systems. The authors outline the usefulness of hyperthermia in the treatment of advanced cancer and demonstrate with nice pictures, obtained with transmission and scanning electron microscopy as well as with TUNEL technique, the morphological features of different types of cell damage and death induced by the different agents. The third review “Cell Responses to Oxidative Stressors” by Prof. A. Cataldi [6] takes into consideration stimuli acting as potential stressors for cell homeostasis among which ionizing radiation, hypoxia, hyperoxia and chemotherapeutics. The article focuses the attention to the role played by mitochondria in the physiological and non-physiological signalling responses of eukaryotic cells to oxidative stressors and gives evidence to detrimental but also beneficial effects of ROS, up to recent years considered harmful but currently assigned a role as mediators of physiological processes, like cell differentiation, proliferation and migration. The next three reviews deal with new avenues in cancer treatment based on “molecular targeted” drugs.

The eye and the skin are protected by a multi-layered epithelial barrier that provides the first line of defence against invading pathogens and environmental stresses such as damaging solar ultraviolet radiation. Extensive epidemiological findings supported by compelling experimental evidence in culture and animal models suggest that ultraviolet radiation is the most important environmental carcinogen leading to the development of a variety of benign and malignant ocular and cutaneous conditions. Epithelial cells have evolved several key defence mechanisms to prevent ultraviolet radiation-induced DNA mutations from stably entering the genome and ultraviolet radiation-damaged cells from establishing themselves in an otherwise normal microenvironment. Firstly, through the process excision repair, cells have the ability to correct carcinogen-induced DNA damage. Secondly, severely damaged cells are eliminated from healthy tissues by molecular programs that trigger cell death. These processes are activated through complex intracellular signalling circuits that ensure that organs such as the eyes and skin are maintained in a disease-free state. Finally, abnormal cells can be recognized, targeted and destroyed by surveillance from the immune system, however this is complicated further by the immunosuppressive effects of ultraviolet radiation that promote tumour growth. This review will discuss in greater detail some of the processes and pathways that are activated in response to ultraviolet radiation and their effect on ocular and cutaneous health.

Based on the study of the recent literature, the aim of this report is to present and discuss in vitro cell death, and, more specifically, apoptosis appearing after exposure to physical conditions such as hyperthermia, hypothermia, UVB radiation and static magnetic fields. Hyperthermia (i.e. variable exposures to 42 ° C) as well as hypothermia conditions (i.e. variable exposures to 0 ° C) were considered mild and were both followed by incubation at physiological conditions. Heat exposure can be considered as a powerful apoptotic inducer in a variety of cells, where it induces classical apoptotic changes and well known biochemical pathways. The effect of hyperthermia has been described in adherent human tumour cells that undergo cell rounding and progressively detach from the substrate, due to the concomitant down-regulation of adhesion molecules. Hypothermia instead, as a cold treatment, only occasionally triggers apoptosis, but more frequently induces cell necrosis. UVB radiation induces oxidative stress, leading, in most experimental models, to apoptotic cell death, generally through the intrinsic apoptotic pathway, even if the extrinsic pathway cannot be excluded. Interestingly, UVB radiation appears effective also on cell systems that are normally apoptosis-resistant, such as muscle cells. Most cell alterations induced by static magnetic fields result from alterations at the plasma membrane and microvilli; in this case, apoptotic cell death is rarely detected. Therefore, cell death can be induced with physical agents in dependence on the treatment and cell model employed. In particular,hyperthermia and UVB can be considered a reliable and reproducible apoptotic trigger.

Stress is a stimulus or a succession of stimuli tending to disrupt the homeostasis of an organism. An organism is consisting of a multitude of cells that singly undergo the effects of external factors that disturb or upset their homeostatic regulation. Stimuli acting as potential stressors are numerous, and include physical agents (ionizing radiation), non-physiological oxygen levels (hypoxia, hyperoxia) and chemotherapeutics. Lastly, also senescence, a physiological process occurring in all organisms, can be considered as a potential stressor. The cell response to multiple oxidative stresses involves mitochondria, since these organelles represent the major source of Reactive Oxygen Species (ROS) that drive the occurrence of pathological conditions and ageing by activating specific signalling pathways. Nevertheless, under physiological conditions the cells are able to exert an antioxidant response which, controlling ROS/Reactive Nitrogen Species (RNS) homeostasis, is involved in mediating cell differentiation, proliferation and migration. Thus, this review focuses the attention to the role played by mitochondria in the physiological and non-physiological signalling responses of eukaryotic cells to some oxidative stresses, in order to identify potential therapeutic targets to counteract oxidative stress effects and mitochondrial-related pathologies.

During the last decades, the improvement of our knowledge of the mechanisms responsible for cancer development has led to the introduction of new promising strategies of treatment, based on “molecular targeted” drugs. These drugs are designed to act on specific molecules, identified as major players in the maintenance of the malignant status. The development of inhibitors, mainly monoclonal antibodies and small-molecules, directed against activated oncogenes has been the most widely used approach for this kind of treatment. Among the oncogenes implicated in human cancers, tyrosine kinases play a critical role. This observation, together with the discovery that cancer cells can be dependent for their survival from the continuous expression of activated oncogenes (a concept defined as “oncogene addiction”), has made protein kinases ideal targets for targeted therapy in cancer. As the field of targeted therapies is now rapidly growing and a comprehensive survey would be too wide, this review will thus mainly focus on strategies aimed at inhibiting tyrosine kinases and their signal transduction pathways.

The resistance of many types of cancer to chemotherapies represents the major hurdle in successful cancer treatment. Cancer cells can escape the toxic effect of most commonly used drugs despite their different chemical structure and intracellular targets. The mechanisms underlying the failure of chemotherapeutic drugs have been well studied. Here I review the role of a signalling pathway activated by the lipid kinase phosphoinositide 3-kinase (PI3K) and the serine/threonine kinase, protein kinase B (PKB) or Akt, in chemotherapeutic resistance. Activation of this pathway plays a key role in different cellular functions such as growth, migration, survival and differentiation. Data accumulated in the last decade have established that this pathway plays a key role in cancer development and progression. More recently it has been shown that this pathway plays also a key role in resistance to chemotherapy. Therefore drugs designed to specifically target this pathway are under development to be used as single agent and in combination to chemotherapy to overcome therapeutic resistance.

The cell cycle consists of a number of complex biochemical pathways that ensure that the start of a particular event depends on the successful and right end of previous steps in the pathway. An important role is played by cyclin/cyclin-dependent kinase (cdk) complexes which are critical regulators of cell cycle progression and RNA transcription. To ensure proper progression through each phase, cells have developed a series of orchestrated checkpoints that govern the different cellular kinases required for distinct cell cycle events. In particular, several cell cycle protein kinases, including members of the Aurora family and the Polo-like kinases, play critical roles in mitotic entry and chromosome segregation that ensure the correct formation of daughter cells. Tumour cell proliferation is frequently associated to both genetic and epigenetic mechanisms commonly affecting the expression of cell cycle regulatory proteins or causing an incompetent checkpoint control, resulting in aberrant responses to cellular damage. These alterations result not only in proliferative advantages but also in an increased susceptibility to the accumulation of additional genetic alterations that contribute to tumour progression and acquisition of more aggressive phenotypes. In the last years, the identification of anticancer drugs directed against critical cell cycle regulators has received particular attention. Specifically, several preclinical and clinical trials are addressing cdks or cell cycle protein kinase inhibitors. Starting from a description of cell cycle, this review summarizes the most recent studies on drugs targeting cell cycle regulators that are being used in cancer therapy.

Radioresistance stands as a fundamental barrier that limits the effectiveness of radiotherapy in cancer treatment. Recent evidences suggest that radioresistance is due to tumour repopulation and involves several signalling pathways, including p53/MDM2 interaction. Ionizing radiation induces p53-dependent MDM2 gene transcription that, in turn, inhibits p53 transcriptional activity, favouring its nuclear export and stimulating its degradation. In light of the observation that in many human tumours the inadequate function of p53 is the result of MDM2 over-expression, several authors have considered as an attractive therapeutic strategy to activate p53 signalling in tumours by inhibiting MDM2 activities or p53/MDM2 interaction. We retain that, by preventing the interaction p53/MDM2 with Nutlin, a small molecule that binds at the interface between these two proteins, the effectiveness of ionizing radiation treatment could be improved. Promising results have recently emerged from in vitro studies performed on laryngeal, prostate and lung cancer cell lines treated with Nutlin in combination with ionizing radiation. Based on these findings, we believe that the combined approach Nutlin/ionizing radiation should be further investigated for efficacy on both solid tumours and lymphoproliferative disorders as well as for side effects on normal cells and tissues. Therefore, the purpose of this review is to report the first results obtained by using Nutlins alone or in combination with other therapeutic agents on primary tumour cells, in vitro cell lines or tumour xenografts and to present the most recent advances in the understanding of the molecular mechanisms underlining ionizing radiation cytotoxicity and resistance.