Chemoprotection from p53-dependent apoptosis: potential clinical applications of the p53 inhibitors

Pifithrin-alpha suppresses p53 and protects cochlear and vestibular hair cells from cisplatin-induced apoptosis

Cisplatin, a commonly used antineoplastic agent, destroys the sensory hair cells in the cochlear and vestibular system leading to irreversible hearing loss and balance problems. Cisplatin-induced hair cell damage presumably occurs by apoptosis. Recent studies suggest that p53 may play an important role initiating cisplatin-induced apoptosis in some cell types. To determine if p53 plays a role in cisplatin-mediated hair cell loss, cochlear and utricular organotypic cultures were prepared from postnatal day 3-4 rats and treated with cisplatin or cisplatin plus pifithrin-alpha (PFT), a p53 inhibitor. Control cultures were devoid of p53 immunolabeling, caspase-1 and caspase-3 labeling and p53 protein was absent from Western blots. Cisplatin (1-10 microg/ml) caused a dose-dependent loss of hair cells in cochlear and utricular cultures, up-regulated phospho-p53 serine 15 immunolabeling, increased the expression of phospho-p53 serine 15 in Western blots from 6 to 48 h after the onset of cisplatin-treatment, and increased caspase-1 and caspase-3 labeling in cochlear and vestibular cultures. Addition of PFT (20-100 microM) to cisplatin-treated cochlear and utricular cultures resulted in a dose-dependent increase in hair cell survival; suppressed the expression of p53 in Western blots and eliminated caspase-1 and caspase-3 labeling in cultures. These results suggest that the tumor suppressor protein, p53, plays a critical role in initiating apoptosis in cochlear and vestibular hair cells. Temporary suppression of p53 with PFT provides significant protection against cisplatin-induced hair cell loss and offers the potential for reducing the ototoxic, vestibulotoxic and neurotoxic side effects of cisplatin.

p53 inhibitor pifithrin alpha can suppress heat shock and glucocorticoid signaling pathways

Pifithrin alpha (PFTalpha) is a chemical compound isolated for its ability to suppress p53-mediated transactivation. It can protect cells from p53-mediated apoptosis induced by various stimuli and reduce sensitivity of mice to gamma radiation. Identification of molecular targets of PFTalpha is likely to provide new insights into mechanisms of regulation of p53 pathway and is important for predicting potential risks associated with administration of PFTalpha-like p53 inhibitors in vivo. We found that PFTalpha, in addition to p53, can suppress heat shock and glucocorticoid receptor signaling but has no effect on nuclear factor-kappaB signaling. PFTalpha reduces activation of heat shock transcription factor (HSF1) and increases cell sensitivity to heat. Moreover, it reduces activation of glucocorticoid receptor and rescues mouse thymocytes in vitro and in vivo from apoptotic death after dexamethasone treatment. PFTalpha affected both signaling pathways in a p53-independent manner. These observations suggest that PFTalpha targets some unknown factor that is common for three major signal transduction pathways.

Novel p53 inactivators with neuroprotective action: syntheses and pharmacological evaluation of 2-imino-2,3,4,5,6,7-hexahydrobenzothiazole and 2-imino-2,3,4,5,6,7-hexahydrobenzoxazole derivatives

Tumor suppressor protein, p53, is an intracellular protein that is critical within the biochemical cascade that leads to cell death via apoptosis. Recent studies identified the tetrahydrobenzothiazole analogue, pifithrin-alpha (2), as a p53 inhibitor that was effective in protecting neuronal cells against a variety of lethal insults and reducing the side effects of anticancer drugs. As up-regulation of p53 has been described as a common feature of several neurodegenerative disorders, including Alzheimer's disease, 2 and novel analogues (3-16) were synthesized to (i) assess the value of tetrahydrobenzothiazole analogues as neuroprotective agents and (ii) define the structural requirements for p53 inactivation. Not only did 2 exhibit neuroprotective activity in both tissue culture and in vivo stroke models but also compounds 6, 7, 10, 13, 15, and 16 proved to be highly potent in protecting PC12 cells and compounds 3, 4, and 6 were highly potent in protecting primary hippocampal cells against death induced by the DNA-damaging agent, camptothecin.

Radioprotection: the non-steroidal anti-inflammatory drugs (NSAIDs) and prostaglandins

Clinical and experimental studies of the acute and late effects of radiation on cells have enhanced our knowledge of radiotherapy and have led to the optimisation of radiation treatment schedules and to more precise modes of radiation delivery. However, as both normal and cancerous tissues have similar response to radiation exposure, radiation-induced injury on normal tissues may present either during, or after the completion of, the radiotherapy treatment. Studies on both NSAIDs and prostaglandins have indeed shown some evidence of radioprotection. Both have the potential to increase the survival of cells but by entirely different mechanisms. Studies of cell kinetics reveal that cells in the mitotic (M) and late G2 phases of the cell cycle are generally most sensitive to radiation compared with cells in the early S and G1/G0 phases. Furthermore, radiation leads to a mitotic delay in the cell cycle. Thus, chemical agents that either limit the proportion of cells in the M and G2 phases of the cell cycle or enhance rapid cell growth could in principle be exploited for their potential use as radioprotectors to normal tissue during irradiation. NSAIDs have been shown to exert anti-cancer effects by causing cell-cycle arrest, shifting cells towards a quiescence state (G0/G1). The same mechanism of action was observed in radioprotection of normal tissues. An increase in arachidonic acid concentrations after exposure to NSAIDs also leads to the production of an apoptosis-inducer ceramide. NSAIDs also elevate the level of superoxide dismutase in cells. Activation of heat shock proteins by NSAIDs increases cell survival by alteration of cytokine expression. A role for NSAIDs with respect to inhibition of cellular proliferation possibly by an anti-angiogenesis mechanism has also been suggested. Several in-vivo studies have provided evidence suggesting that NSAIDs may protect normal tissues from radiation injury. Prostaglandins do not regulate the cell cycle, but they do have a variety of effects on cell growth and differentiation. PGE(2) mediates angiogenesis, increasing the supply of oxygen and nutrients, essential for cellular survival and growth. Accordingly, PGE(2) at sufficiently high plasma concentrations enhances cellular survival by inhibiting pro-inflammatory cytokines such as TNF-alpha and IL-1beta. Thus, PGE(2) acts as a modulator, rather than a mediator, of inflammation. Prospective studies have suggested the potential use of misoprostol, a PGE(1) analogue, before irradiation, in prevention of radiation-induced side effects. The current understanding of the pharmacology of NSAIDs and prostaglandins shows great potential to minimise the adverse effects of radiotherapy on normal tissue.

Death-associated protein kinase as a potential therapeutic target

Death associated protein kinase (DAPK) is a calmodulin (CaM)-regulated serine/threonine protein kinase implicated in diverse apoptosis pathways, including those involved in neuronal cell death and tumour suppression. The requirement of DAPK catalytic activity for its proposed cell functions and the validation of protein kinases as therapeutic targets demand that DAPK be examined as a potential therapeutic target in human disease. The relevant placement of DAPK activity in apoptosis pathways is at an early stage of investigation, making its study as a therapeutic target tenuous. However, the current body of knowledge raises the possibility of DAPK as a therapeutic target for diseases characterised by rapid neurodegeneration, such as stroke or traumatic brain injury. The unmet need in these diseases is for an acute treatment schedule that might reduce neuronal loss. Bioavailable inhibitors of DAPK catalytic activity that target the central nervous system have a potential to fill this need. The development of such DAPK inhibitors is now feasible based on the recent emergence of enabling technology and knowledge. These include a quantitative and selective enzyme assay, a high resolution structure of the active catalytic domain and discovery of cell-permeable, low molecular weight inhibitors of CaM kinases that cross the blood-brain barrier. DAPK as a potential therapeutic target for cancer is less attractive due to the incomplete state of knowledge about DAPK and inherent limitations in drug development for the discovery of specific activators of genes downregulated by promoter hypermethylation. This article provides a brief summary of relevant research and the rationale that is at the foundation of this opinion.

Apoptosis: a link between cancer genetics and chemotherapy

Defects in apoptosis underpin both tumorigenesis and drug resistance, and because of these defects chemotherapy often fails. Understanding the molecular events that contribute to drug-induced apoptosis, and how tumors evade apoptotic death, provides a paradigm to explain the relationship between cancer genetics and treatment sensitivity and should enable a more rational approach to anticancer drug design and therapy.