Converting p53 from a killer into a healer

Food Sources of Selenium and Its Relationship with Chronic Diseases

Selenium (Se) is an essential micronutrient for mammals, and its deficiency seriously threatens human health. A series of biofortification strategies have been developed to produce Se-enriched foods for combating Se deficiency. Although there have been some inconsistent results, extensive evidence has suggested that Se supplementation is beneficial for preventing and treating several chronic diseases. Understanding the association between Se and chronic diseases is essential for guiding clinical practice, developing effective public health policies, and ultimately counteracting health issues associated with Se deficiency. The current review will discuss the food sources of Se, biofortification strategies, metabolism and biological activities, clinical disorders and dietary reference intakes, as well as the relationship between Se and health outcomes, especially cardiovascular disease, diabetes, chronic inflammation, cancer, and fertility. Additionally, some concepts were proposed, there is a non-linear U-shaped dose-responsive relationship between Se status and health effects: subjects with a low baseline Se status can benefit from Se supplementation, while Se supplementation in populations with an adequate or high status may potentially increase the risk of some diseases. In addition, at supra-nutritional levels, methylated Se compounds exerted more promising cancer chemo-preventive efficacy in preclinical trials.

Life as a Vector of Dengue Virus: The Antioxidant Strategy of Mosquito Cells to Survive Viral Infection

Dengue fever is a mosquito-borne viral disease of increasing global importance. The disease has caused heavy burdens due to frequent outbreaks in tropical and subtropical areas of the world. The dengue virus (DENV) is generally transmitted between human hosts via the bite of a mosquito vector, primarily Aedes aegypti and Ae. albopictus as a minor species. It is known that the virus needs to alternately infect mosquito and human cells. DENV-induced cell death is relevant to the pathogenesis in humans as infected cells undergo apoptosis. In contrast, mosquito cells mostly survive the infection; this allows infected mosquitoes to remain healthy enough to serve as an efficient vector in nature. Overexpression of antioxidant genes such as superoxide dismutase (SOD), catalase (CAT), glutathione peroxidase (GPx), glutathione S-transferase (GST), glutaredoxin (Grx), thioredoxin (Trx), and protein disulfide isomerase (PDI) have been detected in DENV2-infected mosquito cells. Additional antioxidants, including GST, eukaryotic translation initiation factor 5A (eIF5a), and p53 isoform 2 (p53-2), and perhaps some others, are also involved in creating an intracellular environment suitable for cell replication and viral infection. Antiapoptotic effects involving inhibitor of apoptosis (IAP) upregulation and subsequent elevation of caspase-9 and caspase-3 activities also play crucial roles in the ability of mosquito cells to survive DENV infection. This article focused on the effects of intracellular responses in mosquito cells to infection primarily by DENVs. It may provide more information to better understand virus/cell interactions that can possibly elucidate the evolutionary pathway that led to the mosquito becoming a vector.

The Tip of an Iceberg: Replication-Associated Functions of the Tumor Suppressor p53

The tumor suppressor p53 is a transcriptional factor broadly mutated in cancer. Most inactivating and gain of function mutations disrupt the sequence-specific DNA binding domain, which activates target genes. This is perhaps the main reason why most research has focused on the relevance of such transcriptional activity for the prevention or elimination of cancer cells. Notwithstanding, transcriptional regulation may not be the only mechanism underlying its role in tumor suppression and therapeutic responses. In the past, a direct role of p53 in DNA repair transactions that include the regulation of homologous recombination has been suggested. More recently, the localization of p53 at replication forks has been demonstrated and the effect of p53 on nascent DNA elongation has been explored. While some data sets indicate that the regulation of ongoing replication forks by p53 may be mediated by p53 targets such as MDM2 (murine double minute 2) and polymerase (POL) eta other evidences demonstrate that p53 is capable of controlling DNA replication by directly interacting with the replisome and altering its composition. In addition to discussing such findings, this review will also analyze the impact that p53-mediated control of ongoing DNA replication has on treatment responses and tumor suppressor abilities of this important anti-oncogene.

A novel p53 paralogue mediates antioxidant defense of mosquito cells to survive dengue virus replication

Mosquito cells allow dengue viruses (DENVs) to undergo replication without causing serious deleterious effects on the cells, leading to advantages for dissemination to other cells. Despite this, increased accumulation of reactive oxygen species (ROS) is usually detected in C6/36 cells with DENV2 infection as shown in mammalian cells. Uniquely, oxidative stress caused by the ROS is alleviated by eliciting antioxidant defense which leads to protection of mosquito cells from the infection. In the present study, a novel p53 paralogue (p53-2) was identified and proved to be regulated in C6/36 cells with DENV2 infection. With a gene-knockdown technique, p53-2 was demonstrated to transcribe catalase which plays a critical role in reducing ROS accumulation and the death rate of infected cells. Ecologically, a higher survival rate of mosquito cells is a prerequisite for prosperous production of viral progeny, allowing infected mosquitoes to remain healthy and active for virus transmission.

Age-Dependent Protective Effect of Selenium against UVA Irradiation in Primary Human Keratinocytes and the Associated DNA Repair Signature

Few studies have focused on the protective role of selenium (Se) against skin aging and photoaging even though selenoproteins are essential for keratinocyte function and skin development. To the best of our knowledge, the impact of Se supplementation on skin cells from elderly and young donors has not been reported. Therefore, the main objective of our study was to evaluate the effects of Se supplementation on skin keratinocytes at baseline and after exposure to ultraviolet A (UVA) irradiation. Low doses of Se (30 nM) were very potently protective against UVA-induced cytotoxicity in young keratinocytes, whereas the protection efficiency of Se in old keratinocytes required higher concentrations (240 nM). Additionally, the DNA repair ability of the old keratinocytes drastically decreased compared with that of the young keratinocytes at baseline and after the UVA exposure. The Se supplementation significantly enhanced the DNA repair of 8-oxoguanine (8oxoG) only in the keratinocytes isolated from young donors. Therefore, aged keratinocytes have an increased vulnerability to oxidative DNA damage, and the Se needs in the elderly should be considered. Strengthening DNA repair activities with Se supplementation may represent a new strategy to combat aging and skin photoaging.

DNA damage tolerance pathway involving DNA polymerase ι and the tumor suppressor p53 regulates DNA replication fork progression

DNA damage tolerance facilitates the progression of replication forks that have encountered obstacles on the template strands. It involves either translesion DNA synthesis initiated by proliferating cell nuclear antigen monoubiquitination or less well-characterized fork reversal and template switch mechanisms. Herein, we characterize a novel tolerance pathway requiring the tumor suppressor p53, the translesion polymerase ι (POLι), the ubiquitin ligase Rad5-related helicase-like transcription factor (HLTF), and the SWI/SNF catalytic subunit (SNF2) translocase zinc finger ran-binding domain containing 3 (ZRANB3). This novel p53 activity is lost in the exonuclease-deficient but transcriptionally active p53(H115N) mutant. Wild-type p53, but not p53(H115N), associates with POLι in vivo. Strikingly, the concerted action of p53 and POLι decelerates nascent DNA elongation and promotes HLTF/ZRANB3-dependent recombination during unperturbed DNA replication. Particularly after cross-linker-induced replication stress, p53 and POLι also act together to promote meiotic recombination enzyme 11 (MRE11)-dependent accumulation of (phospho-)replication protein A (RPA)-coated ssDNA. These results implicate a direct role of p53 in the processing of replication forks encountering obstacles on the template strand. Our findings define an unprecedented function of p53 and POLι in the DNA damage response to endogenous or exogenous replication stress.

Modulation of the poly (ADP-ribose) polymerase inhibitor response and DNA recombination in breast cancer cells by drugs affecting endogenous wild-type p53

Synthetic lethal interactions between poly (ADP-ribose) polymerase (PARP) and homologous recombination (HR) repair pathways have been exploited for the development of novel mono- and combination cancer therapies. The tumor suppressor p53 was demonstrated to exhibit indirect and direct regulatory activities in DNA repair, particularly in DNA double-strand break (DSB)-induced and replication-associated HR. In this study, we tested a potential influence of the p53 status on the response to PARP inhibition, which is known to cause replication stress. Silencing endogenous or inducibly expressing p53 we found a protective effect of p53 on PARP inhibitor (PARPi)-mediated cytotoxicities. This effect was specific for wild-type versus mutant p53 and observed in cancer but not in non-transformed cell lines. Enhanced cytotoxicities after treatment with the p53-inhibitory drug Pifithrinα further supported p53-mediated resistance to PARP inhibition. Surprisingly, we equally observed increased PARPi sensitivity in the presence of the p53-activating compound Nutlin-3. As a common denominator, both drug responses correlated with decreased HR activities: Pifithrinα downregulated spontaneous HR resulting in damage accumulation. Nutlin-3 induced a decrease of DSB-induced HR, which was accompanied by a severe drop in RAD51 protein levels. Thus, we revealed a novel link between PARPi responsiveness and p53-controlled HR activities. These data expand the concept of cell and stress type-dependent healer and killer functions of wild-type p53 in response to cancer therapeutic treatment. Our findings have implications for the individualized design of cancer therapies using PARPi and the potentially combined use of p53-modulatory drugs.

Multicenter, phase III trial comparing selenium supplementation with observation in gynecologic radiation oncology: follow-up analysis of the survival data 6 years after cessation of randomization

PURPOSE

In 2010, we reported that selenium (Se) supplementation during radiation therapy (RT) is effective for increasing blood Se levels in Se-deficient cervical and uterine cancer patients, and reduced the number of episodes and severity of RT-induced diarrhea. In the current study, we examine whether of Se supplementation during adjuvant RT affects long-term survival of these patients.

PATIENTS AND METHODS

Former patients were identified and questioned with respect to their health and well-being.

RESULTS

A total of 81 patients were randomized in the initial supplementation study, 39 of whom received Se (selenium group, SeG) and 42 of whom served as controls (control group, CG). When former patients were reidentified after a median follow-up of 70 months (range = 0-136), the actuarial 10-year disease-free survival rate in the SeG was 80.1% compared to 83.2% in the CG (P = .65), and the actuarial 10-year overall survival rate of patients in the SeG was 55.3% compared to 42.7% in the CG (P = .09).

CONCLUSIONS

Our extended follow-up analysis demonstrates that Se supplementation had no influence on the effectiveness of the anticancer irradiation therapy and did not negatively affect patients' long-term survival. In view of its positive effects on RT-induced diarrhea, we consider Se supplementation to be a meaningful and beneficial adjuvant treatment in Se-deficient cervical and uterine cancer patients while undergoing pelvic radiation therapy.

Impact of treatment planning target volumen (PTV) size on radiation induced diarrhoea following selenium supplementation in gynecologic radiation oncology--a subgroup analysis of a multicenter, phase III trial

Background

In a previous analysis (Int J Radiat Oncol Biol Phys 70:828-835,2010), we assessed whether an adjuvant supplementation with selenium (Se) improves Se status and reduces the radiation-induced side-effects of patients treated by adjuvant radiotherapy (RT) for cervical and uterine cancer. Now, a potential relation between the planning target volume (PTV) of the RT and the Se effect concerning radiation induced diarrhoea was evaluated in detail.

Methods

Whole blood Se concentrations had been measured in patients with cervical (n=11) and uterine cancer (n=70) after surgical treatment, during, and at the end of RT. Patients with initial Se concentrations of less than 84 μg/l were categorized as Se-deficient and randomized before RT to receive Se (as sodium selenite) per os on the days of RT, or to receive no supplement during RT. Diarrhoea was graded according to the Common Toxicity Criteria system (CTC, Version 2a). The evaluation of the PTV of the RT was ascertained with the help of a specialised computer-assisted treatment planning software used for radiation planning procedure.

Results

A total of 81 patients had been randomized for the initial supplementation study, 39 of which received Se [selenium group, SeG] and 42 serving as controls [control group, CG]. Mean Se levels did not differ between SeG and CG upon study initiation, but were significantly higher in the SeG compared to the CG at the end of RT. The actuarial incidence of at least CTC 2 radiation induced diarrhoea in the SeG was 20.5% compared to 44.5% in the CG (p=0.04). The median PTV in both groups was 1302 ml (916–4608). With a PTV of <= 1302 ml (n=41) the actuarial incidence of at least CTC 2 diarrhoea in the SeG was 22.3% (4 of 18 patients) compared to 34.8% (8 of 23 patients) in the CG (p=0.50). In patients with a PTV of > 1302 ml (n=40) the actuarial incidence of at least CTC 2 diarrhoea in the SeG was 19.1% (4 of 21 patients) versus 52.6% (10 of 19 patients) in the CG (p=0.046).

Conclusions

Se supplementation during RT was effective to improve blood Se status in Se-deficient cervical and uterine cancer patients, and reduces episodes and severity of RT-induced diarrhoea. This effect was most pronounced and significant in patients with large PTV (> 1302 ml).

Does a role for selenium in DNA damage repair explain apparent controversies in its use in chemoprevention?

The trace element selenium is an essential micronutrient that has received considerable attention for its potential use in the prevention of cancer. In spite of this interest, the mechanism(s) by which selenium might function as a chemopreventive remain to be determined. Considerable experimental evidence indicates that one possible mechanism by which selenium supplementation may exert its benefits is by enhancing the DNA damage repair response, and this includes data obtained using cultured cells, animal models as well as in human clinical studies. In these studies, selenium supplementation has been shown to be beneficial in reducing the frequency of DNA adducts and chromosome breaks, consequentially reducing the likelihood of detrimental mutations that ultimately contribute to carcinogenesis. The benefits of selenium can be envisioned as being due, at least in part, to it being a critical constituent of selenoproteins such as glutathione peroxidases and thioredoxin reductases, proteins that play important roles in antioxidant defence and maintaining the cellular reducing environment. Selenium, therefore, may be protective by preventing DNA damage from occurring as well as by increasing the activity of repair enzymes such as DNA glycosylases and DNA damage repair pathways that involve p53, BRCA1 and Gadd45. An improved understanding of the mechanism of selenium's impact on DNA repair processes may help to resolve the apparently contradicting data obtained from decades of animal work, human epidemiology and more recently, clinical supplementation studies.

Low doses of selenium specifically stimulate the repair of oxidative DNA damage in LNCaP prostate cancer cells

Epidemiological studies have demonstrated an inverse relationship between selenium (Se) intake and cancer incidence and/or mortality. However, the molecular mechanisms underlying the cancer chemopreventive activity of Se compounds remain largely unknown. The objective of this study was to investigate the effect of low doses of Se on the stimulation of DNA repair systems in response to four different qualities of DNA damage. P53-proficient LNCaP human prostate adenocarcinoma cells were grown either untreated or in the presence of low concentrations of two Se compounds (30° nM sodium selenite, or 10 μM selenomethionine) and exposed to UVA, H2O2, methylmethane sulfonate (MMS) or UVC. Cell viability as well as DNA damage induction and repair were evaluated by the alkaline Comet assay. Overall, Se was shown to be a very potent protector against cell toxicity and genotoxicity induced by oxidative stress (UVA or H2O2) but not from the agents that induce other types of deleterious lesions (MMS or UVC). Furthermore, Se-treated cells exhibited increased oxidative DNA repair activity, indicating a novel mechanism of Se action. Therefore, the benefits of Se could be explained by a combination of antioxidant activity, the reduction in DNA damage and the enhancement of oxidative DNA repair capacity.

Effects of acute in vivo cisplatin and selenium treatment on hematological and oxidative stress parameters in red blood cells of rats

Although cisplatin (cisPt) is one of the most often used cytotoxic drugs in the treatment of cancer, its clinical application is associated with nephrotoxicity and a cumulative anemia. In this study, we evaluated posible protective effects of selenium (Se) on hematological and oxidative stress parameters in rats, acutely treated with cisPt. Four groups of Wistar albino rats included control rats, cisPt-treated (7.5 mg/kg of body weight of cisPt, i.p.), Se-treated (6 mg/kg of body weight of Na(2)SeO(4), i.p.), and Se and cisPt co-treated rats. The rats were killed 72 h after treatment; hematological and oxidative stress parameters were followed in red blood cells. The results showed depletion in platelet number induced by high acute doses of cisPt and strong utilization of reduced glutathione, resulting in elevation of GSSG/2 GSH ratio. Se treatment was followed by stimulated erythropoiesis, increased lipid peroxidation, and GSH depletion. Se and cisPt co-treatment were followed by stimulated erythropoiesis and significant recovery of reduced glutathione status when compared with cisPt-treated rats. In conclusion, acute doses of Se and cisPt primarily act as pro-oxidants. CisPt influenced antioxidative properties of exogenous Se and their synergistic effects may partially participate in protection against cisPt-induced toxicity.

Cyclooxygenase inhibitors: scope of their use and development in cancer chemotherapy

The traditional nonsteroidal anti-inflammatory drugs (NSAIDs) exert their effect by inhibition of cyclooxygenase-1 (COX-1) as well as COX-2 enzymes. As COX-1 is responsible for maintaining normal biological functions, the nonselective inhibition of these enzymes caused side effects including gastrointestinal (GI) problems. Recently developed selective COX-2 inhibitors could reduce these adverse effects, but the evidence of cardiovascular side effects including an increased risk of myocardial infarction began to emerge, and some of the COX-2 inhibitors were eventually withdrawn from the market and this led to the downfall of this research. So, the discovery of novel COX-2 inhibitors with their safety profile became the biggest challenge in pharmaceutical research. However, recent mechanistic and clinical studies revolutionized this area by indicating the fact that COX-2 is involved in apoptosis resistance, angiogenesis, and tumor progression. Epidemiological data suggest that selective COX-2 inhibitors might prevent the development of cancers. Moreover, COX-2 is found to be overexpressed in many cancers thus making it an attractive therapeutic target for the prevention and treatment of a number of malignancies. The purpose of this review is to focus on the medicinal chemistry aspects of COX-2 inhibitors in cancer chemotherapy and recent reports on these inhibitors as anticancer agents. We attempted to cover only the COX inhibitors that showed anticancer activity, although a number of potent COX-2 inhibitors have been reported without their anticancer effects. Furthermore, structure-activity relationships (SAR) of different classes of compounds for COX-2 inhibition as well as anticancer activity, and their future applications are discussed.

Redox modulation of p53: mechanisms and functional significance

The tumor suppressor protein p53 functions as a stress-responsive transcription factor. In response to oxidative, nitrosative, and electrophilic insults, p53 undergoes post-translational modifications, such as oxidation and covalent modification of cysteines, nitration of tyrosines, acetylation of lysines, phosphorylation of serine/threonine residues, etc. Because p53 plays a vital role in the transcriptional regulation of genes encoding proteins involved in a wide spectrum of biochemical processes including DNA repair, cell-cycle regulation, and programmed cell death, the redox-modification of p53 appears to be an important determinant of cell fate. This review highlights the redox regulation of p53 and its consequences on cellular function.

Selenium or no selenium--that is the question in tumor patients: a new controversy

The essential trace element selenium, which is a crucial cofactor in the most important endogenous antioxidative systems of the human body, is attracting more attention from both laypersons and expert groups. The interest of oncologists mainly focuses on the following clinical aspects: protection of normal tissues, sensitizing in malignant tumors, antiedematous effect, prognostic impact of selenium, and effects in primary and secondary cancer prevention. Selenium is a constituent of the small group of selenocysteine-containing selenoproteins and elicits important structural and enzymatic functions. Selenium deficiency has been linked to increased infection risk and adverse mood states. It has been shown to possess cancer-preventive and cytoprotective activities in both animal models and humans. It is well established that it has a key role in redox regulation and antioxidant function, and hence in membrane integrity, energy metabolism, and protection against DNA damage. Recent clinical trials have shown the importance of selenium in clinical oncology. In 2009, a significant benefit of sodium selenite supplementation-with no protection of tumor cells, which is often suspected by oncologists- was shown in a prospective randomized trial in gynecologic cancer patients undergoing radiation therapy. More recently, concerns arose from 2 large clinical prevention trials (NPC, SELECT) that selenium may increase the risk of developing type 2 diabetes. Despite obvious flaws in both studies and good counterarguments, controversy remains on the possible advantages and risks of selenium in cancer prevention. However, in the light of the recent clinical trials the potential benefits of selenium supplementation in tumor patients are becoming obvious, even though further research is needed.

Multicenter, phase 3 trial comparing selenium supplementation with observation in gynecologic radiation oncology

PURPOSE

We assessed whether adjuvant supplementation with selenium improves the selenium status and reduces side effects of patients treated by radiotherapy (RT) for cervical and uterine cancer.

METHODS AND MATERIALS

Whole-blood selenium concentrations were measured in patients with cervical cancer (n = 11) and uterine cancer (n = 70) after surgical treatment, during RT, at the end of RT, and 6 weeks after RT. Patients with initial selenium concentrations of less than 84μg/L were randomized before RT either to receive 500 μg of selenium (in the form of sodium selenite [selenase, biosyn Arzneimittel GmbH, Fellbach, Germany]) by mouth on the days of RT and 300 μg of selenium on the days without RT or to receive no supplement during RT. The primary endpoint of this multicenter Phase 3 study was to assess the efficiency of selenium supplementation during RT; the secondary endpoint was to decrease radiation-induced diarrhea and other RT-dependent side effects.

RESULTS

A total of 81 patients were randomized. We enrolled 39 in the selenium group (SG) and 42 in the control group (CG). Selenium levels did not differ between the SG and CG upon study initiation but were significantly higher in the SG at the end of RT. The actuarial incidence of diarrhea of Grade 2 or higher according to Common Toxicity Criteria (version 2) in the SG was 20.5% compared with 44.5% in the CG (p = 0.04). Other blood parameters, Eastern Cooperative Oncology Group performance status, and self-reported quality of life were not different between the groups.

CONCLUSIONS

Selenium supplementation during RT is effective in improving blood selenium status in selenium-deficient cervical and uterine cancer patients and reduces the number of episodes and severity of RT-induced diarrhea.

p53 and chemosensitivity in bladder cancer

Urothelial carcinoma is the second most common genitourinary malignancy. Although the majority of patients present with superficial bladder tumors, there are several clinical problems, such as progression to invasive tumors, poor prognosis of invasive tumors, and chemosensitivity. Alterations in p53 represent one of the most common genetic events in patients with invasive urothelial carcinoma and are suggested to be linked to tumor progression, prognosis, and chemosensitivity. p53 possesses various functions, including induction of cell-cycle arrest, apoptosis, DNA repair, and antioxidants; it acts as a killer and a healer. In this article, we review the roles of p53 pathways in bladder carcinogenesis and findings from recent studies of ours and other groups, and we discuss the clinical significance of the abrogation of p53 pathways in the treatment of urothelial carcinoma.

Versatile functions of p53 protein in multicellular organisms

The p53 tumor suppressor plays a pivotal role in multicellular organism by enforcing benefits of the organism over those of an individual cell. The task of p53 is to control the integrity and correctness of all processes in each individual cell and in the organism as a whole. Information about the state of ongoing events in the cell is gathered through multiple signaling pathways that convey signals modifying activities of p53. Changes in the activities depend on the character of damages or deviations from optimum in processes, and the activity of p53 changes depending on the degree of the aberration, which results in either stimulation of repair processes and protective mechanisms, or the cessation of further cell divisions and the induction of programmed cell death. The strategy of p53 ensures genetic identity of cells and prevents the selection of abnormal cells. By accomplishing these strategic tasks, p53 may use a wide spectrum of activities, such as its ability to function as a transcription factor, by inducing or repressing different genes, or as an enzyme, by acting as an exonuclease during DNA reparation, or as an adaptor or a regulatory protein, intervening into functions of numerous signaling pathways. Loss of function of the p53 gene occurs in virtually every case of cancer, and deficiency in p53 is an unavoidable prerequisite to the development of malignancies. The functions of p53 play substantial roles in many other pathologies as well as in the aging process. This review is focused on strategies of the p53 gene, demonstrating individual mechanisms underlying its functions.

p53 alterations in human cancer: more questions than answers

The strongest and undisputed fact about p53 is the high frequency of p53 alterations in human cancer and that mutant p53 proteins constitute a complex family of several hundred proteins with heterogeneous properties. Beyond these observations, the p53 pathway and its regulation in a normal cell is like a desert trail, always moving with the wind of novel findings. The field is full of black boxes that are often ignored for sake of simplicity or because they do not fit with the current dominant view. Mutant p53 protein accumulation in tumours is the best example of a preconceived idea, as there is no experimental evidence to explain this observation. In this review, we will discuss several questions concerning the activity or selection of p53 mutations. The central domain of the p53 protein targeted by 80% of p53 mutations is associated with the DNA-binding activity of the p53 protein, but it is also the binding site for several proteins that play a key role in p53 regulation such as ASPP proteins or BclxL. The role of impaired DNA binding and/or protein interactions in tumour development has not been fully elucidated. Similarly, novel animal models carrying either missense p53 mutations or inducible p53 have provided abundant observations, some of which could challenge our view on p53 function as a tumour suppressor gene. Finally, the possible clinical applications of p53 will be discussed.

Reduction of gene repair by selenomethionine with the use of single-stranded oligonucleotides

Background

The repair of single base mutations in mammalian genes can be directed by single-stranded oligonucleotides in a process known as targeted gene repair. The mechanism of this reaction is currently being elucidated but likely involves a pairing step in which the oligonucleotide align in homologous register with its target sequence and a correction step in which the mutant base is replaced by endogenous repair pathways. This process is regulated by the activity of various factors and proteins that either elevate or depress the frequency at which gene repair takes place.

Results

In this report, we find that addition of selenomethionine reduces gene repair frequency in a dose-dependent fashion. A correlation between gene repair and altered cell cycle progression is observed. We also find that selenium induces expression of Ref-1 which, in turn, modifies the activity of p53 during the cell cycle.

Conclusion

We can conclude from the results that the suppression of gene repair by introduction of selenomethionine occurs through a p53-associated pathway. This result indicates that the successful application of gene repair for treatment of inherited disorders may be hampered by indirect activation of endogenous suppressor functions.

Efficacy of increasing the therapeutic index of irinotecan, plasma and tissue selenium concentrations is methylselenocysteine dose dependent

This study was designed to understand the basis for the efficacy of methylselenocysteine (MSC) in increasing the therapeutic index of irinotecan against human tumor xenografts. Nude mice bearing human head and neck squamous cells carcinoma xenografts (FaDu and A253) were treated orally with different doses of MSC and irinotecan. Plasma, tumor and normal tissue samples were collected at different times after MSC treatments and were analyzed for selenium (Se) concentration using electrothermal atomic absorption spectrophotometry. MSC is highly effective in modulating the therapeutic index of irinotecan. Enhanced irinotecan efficacy was greater in FaDu tumors (100% CR) than in A253 tumors (60% CR), and depended on MSC dose with a minimum effective dose of 0.01 mg/dx28. The highest plasma Se concentration was achieved 1h after a single dose and 28 d after daily treatments of MSC. The ability of FaDu tumors to retain Se was significantly better than A253 tumors, and the highest Se concentration in normal tissue was achieved in the liver. Peak plasma and tissue Se concentrations were functions of the dose and duration of MSC treatment. The MSC-dependent increase in Se level in normal tissues may contribute to the protective effect against irinotecan toxicity observed in those tissues. Intratumoral total Se concentration was not found to be predictive of the combination therapy response rates. There is a critical need to develop a method to measure the active metabolite of MSC, rather than total Se.

Selenium regulates cyclooxygenase-2 and extracellular signal-regulated kinase signaling pathways by activating AMP-activated protein kinase in colon cancer cells

Epidemiologic and experimental evidences indicate that selenium, an essential trace element, can reduce the risk of a variety of cancers. Protection against certain types of cancers, particularly colorectal cancers, is closely associated with pathways involving cyclooxygenase-2 (COX-2). We found that AMP-activated protein kinase (AMPK), which functions as a cellular energy sensor, mediates critical anticancer effects of selenium via a COX-2/prostaglandin E(2) signaling pathway. Selenium activated AMPK in tumor xenografts as well as in colon cancer cell lines, and this activation seemed to be essential to the decrease in COX-2 expressions. Transduction with dominant-negative AMPK into colon cancer cells or application of cox-2(-/-)-negative cells supported the evidence that AMPK is an upstream signal of COX-2 and inhibits cell proliferation. In HT-29 colon cancer cells, carcinogenic agent 12-O-tetradecanoylphorbol-13-acetate (TPA) activated extracellular signal-regulated kinase (ERK) that led to COX-2 expression and selenium blocked the TPA-induced ERK and COX-2 activation via AMPK. We also showed the role of a reactive oxygen species as an AMPK activation signal in selenium-treated cells. We propose that AMPK is a novel and critical regulatory component in selenium-induced cancer cell death, further implying AMPK as a prime target of tumorigenesis.

The thioredoxin system in cancer

Thioredoxin (Trx), NADPH and thioredoxin reductase (TrxR) comprise a thioredoxin system which exists in nearly all living cells. It functions in thiol-dependent thiol-disulfide exchange reactions crucial to control of the reduced intracellular redox environment, cellular growth, defense against oxidative stress or control of apoptosis and has multi-facetted roles in mammalian cells including implications in cancer. Eg reduced Trx activates DNA binding of transcription factors and is involved in antioxidant defense through repair of oxidatively damaged proteins or as an electron donor to peroxiredoxins. The Trx system functions in synthesis of deoxyribonucleotides for DNA synthesis, both replication and repair, by ribonucleotide reductase. Trx and truncated Trx (Trx80) act in modulation of immune cell function. TrxR isoforms in the cytosol and the mitochondria are essential selenoenzymes with a selenocysteine in the active site. These enzymes display a remarkably broad substrate specificity but are also targets for existing chemotherapeutic drugs. Mammalian TrxR enzymes are linked to selenium metabolism as a result of being selenoproteins, but can also directly reduce low molecular selenium compounds like selenite and have been implicated in the chemoprevention effects of selenium against cancer. Numerous scientific reports describe higher expression of Trx and TrxR in some, but not all tumors. Some data suggest that high Trx could be linked to resistance to chemotherapies while others suggest that high Trx and TrxR may induce apoptosis and reduce the mitotic index of certain tumors linked to the p53 dependent cell death. Recent data suggest that TrxR is essential for the carcinogenic process and invasive phenotype of cancer. Both Trx and TrxR have been regarded as interesting targets for chemotherapy.

Selective modulation of the therapeutic efficacy of anticancer drugs by selenium containing compounds against human tumor xenografts

PURPOSE

Studies were carried out in athymic nude mice bearing human squamous cell carcinoma of the head and neck (FaDu and A253) and colon carcinoma (HCT-8 and HT-29) xenografts to evaluate the potential role of selenium-containing compounds as selective modulators of the toxicity and antitumor activity of selected anticancer drugs with particular emphasis on irinotecan, a topoisomerase I poison.

EXPERIMENTAL DESIGN

Antitumor activity and toxicity were evaluated using nontoxic doses (0.2 mg/mouse/day) and schedule (14-28 days) of the selenium-containing compounds, 5-methylselenocysteine and seleno-L-methionine, administered orally to nude mice daily for 7 days before i.v. administration of anticancer drugs, with continued selenium treatment for 7-21 days, depending on anticancer drugs under evaluation. Several doses of anticancer drugs were used, including the maximum tolerated dose (MTD) and toxic doses. Although many chemotherapeutic agents were evaluated for toxicity protection by selenium, data on antitumor activity were primarily obtained using the MTD, 2 x MTD, and 3 x MTD of weekly x4 schedule of irinotecan.

RESULTS

Selenium was highly protective against toxicity induced by a variety of chemotherapeutic agents. Furthermore, selenium increased significantly the cure rate of xenografts bearing human tumors that are sensitive (HCT-8 and FaDu) and resistant (HT-29 and A253) to irinotecan. The high cure rate (100%) was achieved in nude mice bearing HCT-8 and FaDu xenografts treated with the MTD of irinotecan (100 mg/kg/week x 4) when combined with selenium. Administration of higher doses of irinotecan (200 and 300 mg/kg/week x 4) was required to achieve high cure rate for HT-29 and A253 xenografts. Administration of these higher doses was possible due to selective protection of normal tissues by selenium. Thus, the use of selenium as selective modulator of the therapeutic efficacy of anticancer drugs is new and novel.

CONCLUSIONS

We demonstrated that selenium is a highly effective modulator of the therapeutic efficacy and selectivity of anticancer drugs in nude mice bearing human tumor xenografts of colon carcinoma and squamous cell carcinoma of the head and neck. The observed in vivo synergic interaction is highly dependent on the schedule of selenium.

The tumor suppressor p53 abrogates Smad-dependent collagen gene induction in mesenchymal cells

The pleiotropic cytokine transforming growth factor-beta (TGF-beta) is a potent inducer of collagen synthesis and is implicated in the pathogenesis of fibrosis. Acting in concert with transcriptional coactivators p300/CBP, the Smads mediate TGF-beta stimulation of collagen synthesis in human dermal fibroblasts. Little information exists regarding positive and negative modulation of physiological TGF-beta responses. Because the tumor suppressor p53 is implicated in connective tissue homeostasis, here we examined the regulation of collagen gene expression by p53. Forced expression of ectopic p53 in dermal fibroblasts repressed basal and TGF-beta-stimulated collagen gene expression, whereas the absence of cellular p53 was associated with significantly enhanced transcriptional activity of the Type I collagen gene (COL1A2) and collagen synthesis. Ectopic expression of p53 also repressed TGF-beta stimulation of promoter activity driven by minimal Smad-binding elements, suggesting that p53 modulated Smad-dependent intracellular signaling. Inhibition was not due to altered levels, phosphorylation, or nuclear translocation of cellular Smads. Treatment of fibroblasts with etoposide, a potent inducer of cellular p53, abrogated TGF-beta stimulation of COL1A2 promoter activity and collagen synthesis in a p53-dependent manner. Overexpression of the transcriptional coactivator p300 rescued TGF-beta stimulation of COL1A2 promoter activity in fibroblasts overexpressing p53. Furthermore, the ligand-induced interaction of cellular Smad3 with p300 or with its cognate Smad-binding DNA element and recruitment of p300 to the DNA-protein complex assembled on the Smad-binding element were markedly reduced in p53-overexpressing fibroblasts. Collectively, these results indicate, for the first time, that p53 is a potent and selective endogenous repressor of TGF-beta-regulated collagen gene expression in dermal fibroblasts. The ligand-dependent interaction of Smad3 with p300 may be one of the targets of p53-mediated inhibition of TGF-beta responses. These findings suggest that a novel and important physiologic function for the tumor suppressor p53 is the regulation of fibrotic cellular responses.

Apoptosis - the p53 network

Exposure to cellular stress can trigger the p53 tumor suppressor, a sequence-specific transcription factor, to induce cell growth arrest or apoptosis. The choice between these cellular responses is influenced by many factors, including the type of cell and stress, and the action of p53 co-activators. p53 stimulates a wide network of signals that act through two major apoptotic pathways. The extrinsic, death receptor pathway triggers the activation of a caspase cascade, and the intrinsic, mitochondrial pathway shifts the balance in the Bcl-2 family towards the pro-apoptotic members, promoting the formation of the apoptosome, and consequently caspase-mediated apoptosis. The impact of these two apoptotic pathways may be enhanced when they converge through Bid, which is a p53 target. The majority of these apoptotic effects are mediated through the induction of specific apoptotic target genes. However, p53 can also promote apoptosis by a transcription-independent mechanism under certain conditions. Thus, a multitude of mechanisms are employed by p53 to ensure efficient induction of apoptosis in a stage-, tissue- and stress-signal-specific manner. Manipulation of the apoptotic functions of p53 constitutes an attractive target for cancer therapy.

Mechanisms of selenium-mediated protection from photocarcinogenesis and cell death are not solely p53-dependent

Recent studies published in Oncogene and Proc. Natl. Acad. Sci. USA ascribe a role for selenium, acting through wild type p53, in protecting skin cells in culture from ultraviolet radiation-induced death. While selenium clearly protects cells against ultraviolet radiation-induced death, data that we present and discuss in this letter shows that wild type p53 is not required for such protection. Moreover the non-physiologically high levels of selenium used in some studies leads us to question the relevance of such effects for selenium-induced photoprotection.

The role of p53 in determining sensitivity to radiotherapy

Ionizing radiation (IR) has proven to be a powerful medical treatment in the fight against cancer. Rational and effective use of its killing power depends on understanding IR-mediated responses at the molecular, cellular and tissue levels. Tumour cells frequently acquire defects in the molecular regulatory mechanisms of the response to IR, which sensitizes them to radiation therapy. One of the key molecules involved in a cell's response to IR is p53. Understanding these mechanisms indicates new rational approaches to improving cancer treatment by IR.