Cellular localization and cell cycle regulation by a temperature-sensitive p53 protein

Natural convection in the cytoplasm: Theoretical predictions of buoyancy-driven flows inside a cell

The existence of temperature gradients within eukaryotic cells has been postulated as a source of natural convection in the cytoplasm, i.e. bulk fluid motion as a result of temperature-difference-induced density gradients. Recent computations have predicted that a temperature differential of ΔT ≈ 1 K between the cell nucleus and the cell membrane could be strong enough to drive significant intracellular material transport. We use numerical computations and theoretical calculations to revisit this problem in order to further understand the impact of temperature gradients on flow generation and advective transport within cells. Surprisingly, our computations yield flows that are an order of magnitude weaker than those obtained previously for the same relative size and position of the nucleus with respect to the cell membrane. To understand this discrepancy, we develop a semi-analytical solution of the convective flow inside a model cell using a bi-spherical coordinate framework, for the case of an axisymmetric cell geometry (i.e. when the displacement of the nucleus from the cell centre is aligned with gravity). We also calculate exact solutions for the flow when the nucleus is located concentrically inside the cell. The results from both theoretical analyses agree with our numerical results, thus providing a robust estimate of the strength of cytoplasmic natural convection and demonstrating that these are much weaker than previously predicted. Finally, we investigate the ability of the aforementioned flows to redistribute solute within a cell. Our calculations reveal that, in all but unrealistic cases, cytoplasmic convection has a negligible contribution toward enhancing the diffusion-dominated mass transfer of cellular material.

Osmolytes: Wonder molecules to combat protein misfolding against stress conditions

The proper functioning of any protein depends on its three dimensional conformation which is achieved by the accurate folding mechanism. Keeping away from the exposed stress conditions leads to cooperative unfolding and sometimes partial folding, forming the structures like protofibrils, fibrils, aggregates, oligomers, etc. leading to several neurodegenerative diseases like Parkinson's disease, Alzheimer's, Cystic fibrosis, Huntington, Marfan syndrome, and also cancers in some cases, too. Hydration of proteins is necessary, which may be achieved by the presence of organic solutes called osmolytes within the cell. Osmolytes belong to different classes in different organisms and play their role by preferential exclusion of osmolytes and preferential hydration of water molecules and achieves the osmotic balance in the cell otherwise it may cause problems like cellular infection, cell shrinkage leading to apoptosis and cell swelling which is also the major injury to the cell. Osmolyte interacts with protein, nucleic acids, intrinsically disordered proteins by non-covalent forces. Stabilizing osmolytes increases the Gibbs free energy of the unfolded protein and decreases that of folded protein and vice versa with denaturants (urea and guanidinium hydrochloride). The efficacy of each osmolyte with the protein is determined by the calculation of m value which reflects its efficiency with protein. Hence osmolytes can be therapeutically considered and used in drugs.

A comprehensive study of p53 protein

The protein p53 has been extensively investigated since it was found 43 years ago and has become a "guardian of the genome" that regulates the division of cells by preventing the growth of cells and dividing them, that is, inhibits the development of tumors. Initial proof of protein existence by researchers in the mid-1970s was found by altering and regulating the SV40 big T antigen termed the A protein. Researchers demonstrated how viruses play a role in cancer by employing viruses' ability to create T-antigens complex with viral tumors, which was discovered in 1979 following a viral analysis and cancer analog research. Researchers later in the year 1989 explained that in Murine Friend, a virus-caused erythroleukemia, commonly found that p53 was inactivated to suggest that p53 could be a "tumor suppressor gene." The TP53 gene, encoding p53, is one of human cancer's most frequently altered genes. The protein-regulated biological functions of all p53s include cell cycles, apoptosis, senescence, metabolism of the DNA, angiogenesis, cell differentiation, and immunological response. We tried to unfold the history of the p53 protein, which was discovered long back in 1979, that is, 43 years of research on p53, and how p53's function has been developed through time in this article.

Carbonized Silk Nanofibers in Biodegradable, Flexible Temperature Sensors for Extracellular Environments

Temperature is one of the key parameters for activity of cells. The trade-off between sensitivity and biocompatibility of cell temperature measurement is a challenge for temperature sensor development. Herein, a highly sensitive, biocompatible, and degradable temperature sensor was proposed to detect the living cell extracellular environments. Biocompatible silk materials were applied as sensing and packing layers, which endow the device with biocompatibility, biodegradability, and flexibility. The silk-based temperature sensor presented a sensitivity of 1.75%/°C and a working range of 35-63 °C with the capability to measure the extracellular environments. At the bending state, this sensor worked at promising response of cells at different temperatures. The applications of this developed silk material-based temperature sensor include biological electronic devices for cell manipulation, cell culture, and cellular metabolism.

Nanodiamond Theranostic for Light-Controlled Intracellular Heating and Nanoscale Temperature Sensing

Temperature is an essential parameter in all biological systems, but information about the actual temperature in living cells is limited. Especially, in photothermal therapy, local intracellular temperature changes induce cell death but the local temperature gradients are not known. Highly sensitive nanothermometers would be required to measure and report local temperature changes independent of the intracellular environment, including pH or ions. Fluorescent nanodiamonds (ND) enable temperature sensing at the nanoscale independent of external conditions. Herein, we prepare ND nanothermometers coated with a nanogel shell and the photothermal agent indocyanine green serves as a heat generator and sensor. Upon irradiation, programmed cell death was induced in cancer cells with high spatial control. In parallel, the increase in local temperature was recorded by the ND nanothermometers. This approach represents a great step forward to record local temperature changes in different cellular environments inside cells and correlate these with thermal biology.

p53 tetramerization: at the center of the dominant-negative effect of mutant p53

In this review, Gencel-Augusto and Lozano summarize the data on p53 mutants with a functional tetramerization domain that form mixed tetramers and in some cases have dominant-negative effects (DNE) that inactivate wild-type p53. They conclude that the DNE is mostly observed after DNA damage but fails in other contexts.

The p53 tumor suppressor functions as a tetrameric transcription factor to regulate hundreds of genes—many in a tissue-specific manner. Missense mutations in cancers in the p53 DNA-binding and tetramerization domains cement the importance of these domains in tumor suppression. p53 mutants with a functional tetramerization domain form mixed tetramers, which in some cases have dominant-negative effects (DNE) that inactivate wild-type p53. DNA damage appears necessary but not sufficient for DNE, indicating that upstream signals impact DNE. Posttranslational modifications and protein–protein interactions alter p53 tetramerization affecting transcription, stability, and localization. These regulatory components limit the dominant-negative effects of mutant p53 on wild-type p53 activity. A deeper understanding of the molecular basis for DNE may drive development of drugs that release WT p53 and allow tumor suppression.

Senescence and Apoptosis: Architects of Mammalian Development

Mammalian development involves an exquisite choreography of cell division, differentiation, locomotion, programmed cell death, and senescence that directs the transformation of a single cell zygote to a mature organism containing on the order of 40 trillion cells in humans. How a single totipotent zygote undergoes the rapid stages of embryonic development to form over 200 different cell types is complex in the extreme and remains the focus of active research. Processes such as programmed cell death or apoptosis has long been known to occur during development to help sculpt organs and tissue systems. Other processes such as cellular senescence, long thought to only occur in pathologic states such as aging and tumorigenesis have been recently reported to play a vital role in development. In this review, we focus on apoptosis and senescence; the former as an integral mechanism that plays a critical role not only in mature organisms, but that is also essential in shaping mammalian development. The latter as a well-defined feature of aging for which some reports indicate a function in development. We will dissect the dual roles of major gene families, pathways such as Hox, Rb, p53, and epigenetic regulators such as the ING proteins in both early and the late stages and how they play antagonistic roles by increasing fitness and decreasing mortality early in life but contribute to deleterious effects and pathologies later in life.

Anticancer potential of myricetin bulk and nano forms in vitro in lymphocytes from myeloma patients

Evading apoptosis and chemo-resistance are considered as very important factors which help tumour progression and metastasis. Hence, to overcome chemo-resistance, there is an urgent requirement for emergence of more effective treatment options. Myricetin, a naturally occurring flavonoid, is present in various plant-derived foods and shows antitumour potential in different cancers. In the present in vitro study, results from the comet assay demonstrated that myricetin bulk (10 µM) and nano (20 µM) forms exhibited a non-significant level of genotoxicity in lymphocytes from multiple myeloma patients when compared to those from healthy individuals. Western blot results showed a decrease in Bcl-2/Bax ratio and an increase in P53 protein levels in lymphocytes from myeloma patients, but not in lymphocytes from healthy individuals. A significant increase in intracellular reactive oxygen species level was also observed, suggesting that regulation of apoptotic proteins triggered by myricetin exposure in lymphocytes from myeloma patients occurred through P53 and oxidative stress-dependent pathways. The potency of myricetin against lymphocytes from myeloma patients marks it a potential candidate to be considered as an alternative to overcome chemo-resistance in cancer therapies.

Understanding p53 functions through p53 antibodies

TP53 is the most frequently mutated gene across all cancer types. Our understanding of its functions has evolved since its discovery four decades ago. Initially thought to be an oncogene, it was later realized to be a critical tumour suppressor. A significant amount of our knowledge about p53 functions have come from the use of antibodies against its various forms. The early anti-p53 antibodies contributed to the recognition of p53 accumulation as a common feature of cancer cells and to our understanding of p53 DNA-binding and transcription activities. They led to the concept that conformational changes can facilitate p53’s activity as a growth inhibitory protein. The ensuing p53 conformational-specific antibodies further underlined p53’s conformational flexibility, collectively forming the basis for current efforts to generate therapeutic molecules capable of altering the conformation of mutant p53. A subsequent barrage of antibodies against post-translational modifications on p53 has clarified p53’s roles further, especially with respect to the mechanistic details and context-dependence of its activity. More recently, the generation of p53 mutation-specific antibodies have highlighted the possibility to go beyond the general framework of our comprehension of mutant p53—and promises to provide insights into the specific properties of individual p53 mutants. This review summarizes our current knowledge of p53 functions derived through the major classes of anti-p53 antibodies, which could be a paradigm for understanding other molecular events in health and disease.

Benign Alveolar Ridge Keratosis: Clinical and Histopathologic Analysis of 167 Cases

Benign alveolar ridge keratosis (BARK), the intraoral counterpart of cutaneous lichen simplex chronicus, is a reactive hyperkeratosis caused by trauma or friction that presents as a poorly demarcated white papule or plaque on the keratinized mucosa of the retromolar pad or alveolar ridge mucosa (often edentulous). This is a clinical and histopathologic analysis of BARK including evaluation of p53 expression in selected cases. One hundred and sixty-seven cases of BARK were identified from 2016 to 2017 and 112 (67.1%) occurred in males with a median age of 56 years (range 15-86). The retromolar pad was affected in 107 (64.1%) cases and the edentulous alveolar mucosa in 60 (35.9%) cases, with 17.4% of the cases presenting bilaterally. BARK showed hyperkeratosis often with wedge-shaped hypergranulosis and occasional focal parakeratosis. The epithelium exhibited acanthosis and surface corrugation with tapered rete ridges often interconnected at the tips. The study for p53 performed in 12 cases showed less than 25% nuclear positivity. BARK is a distinct benign clinicopathologic entity caused by friction, which should be clearly distinguished from true leukoplakia, a potentially malignant disorder.

Cytoplasmic convection currents and intracellular temperature gradients

Intracellular thermometry has recently demonstrated temperatures in the nucleus, mitochondria, and centrosome to be significantly higher than those of the cytoplasm and cell membrane. This local thermogenesis and the resulting temperature gradient could facilitate the development of persistent, self-organizing convection currents in the cytoplasm of large eukaryotes. Using 3-dimensional computational simulations of intracellular fluid motion, we quantify the convective velocities that could result from the temperature differences observed experimentally. Based on these velocities, we identify the conditions necessary for this temperature-driven bulk flow to dominate over random thermal diffusive motion at the scale of a single eukaryotic cell. With temperature gradients of the order 1°C and diffusion coefficients comparable to those described in the literature, Péclet numbers ≥ 1 are feasible and permit comparable or greater effects of convection than diffusion in determining intracellular mass flux. In addition to the temperature gradient, the resulting flow patterns would also depend on the spatial localization of the heat source, the shape of the cell membrane, and the complex intracellular structure including the cytoskeleton. While this intracellular convection would be highly context-dependent, in certain settings, convective motion could provide a previously unrecognized mechanism for directed, bulk transport within eukaryotic cells.

Radiation Sensitization of Basal Cell and Head and Neck Squamous Cell Carcinoma by the Hedgehog Pathway Inhibitor Vismodegib

Vismodegib, an inhibitor of the Hedgehog signaling pathway, is an approved drug for monotherapy in locally advanced or metastatic basal cell carcinoma (BCC). Data on combined modality treatment by vismodegib and radiation therapy, however, are rare. In the present study, we examined the radiation sensitizing effects of vismodegib by analyzing viability, cell cycle distribution, cell death, DNA damage repair and clonogenic survival in three-dimensional cultures of a BCC and a head and neck squamous cell carcinoma (HNSCC) cell line. We found that vismodegib decreases expression of the Hedgehog target genes glioma-associated oncogene homologue (GLI1) and the inhibitor of apoptosis protein (IAP) Survivin in a cell line- and irradiation-dependent manner, most pronounced in squamous cell carcinoma (SCC) cells. Furthermore, vismodegib significantly reduced proliferation in both cell lines, while additional irradiation only slightly further impacted on viability. Analyses of cell cycle distribution and cell death induction indicated a G1 arrest in BCC and a G2 arrest in HNSCC cells and an increased fraction of cells in SubG1 phase following combined treatment. Moreover, a significant rise in the number of phosphorylated histone-2AX/p53-binding protein 1 (γH2AX/53BP1) foci in vismodegib- and radiation-treated cells was associated with a significant radiosensitization of both cell lines. In summary, these findings indicate that inhibition of the Hedgehog signaling pathway may increase cellular radiation response in BCC and HNSCC cells.

Antiproliferative effects of the CDK6 inhibitor PD0332991 and its effect on signaling networks in gastric cancer cells

PD0332991 (palbociclib/Ibrance®) is a cyclin-dependent kinase (CDK)4/6 inhibitor that has recently been approved for the treatment of estrogen receptor‑positive advanced breast cancer. The present study investigated the antiproliferative effects of PD0332991 on gastric cancer (GC) cells and the underlying molecular mechanisms. The activity of PD0332991 was tested in several GC cell lines, including AGS, KATO‑Ⅲ, NCI‑N87 and HS746T. Growth inhibitory activity of PD0332991, alone or in combination with fluorouracil (5‑FU), was measured by MTT assay. The effects of PD0332991 on cell cycle progression were analyzed by flow cytometry and western blotting. Protein pathway array and Ingenuity Pathway Analysis were used to identify signaling pathways that may mediate the antiproliferative effects of PD0332991. PD0332991 inhibited proliferation in a dose‑dependent manner and enhanced the activity of 5‑FU in all GC cell lines tested. Cells treated with PD0332991 exhibited cell cycle arrest in G1 phase of the cell cycle, whereas the number of cells in G2/M phase was decreased. PD0332991 also inhibited CDK6‑specific phosphorylation of retinoblastoma on Ser780, reduced the expression of cyclin D1, and induced expression of p53 and p27. Furthermore, 31 proteins were identified, the expression of which was significantly altered following treatment with PD0332991 in at least three cell lines. Pathway analysis indicated that the altered proteins were frequently associated with cell death, cell cycle and the molecular mechanism of cancer. The results of the present study indicated that PD0332991 may inhibit cell proliferation via modulation of the cell cycle, and may affect numerous oncogenic signaling pathways. Therefore, PD0332991 may be considered effective for the treatment of GC.

Changing relations between proteins and osmolytes: a choice of nature

The stabilization and destabilization of the protein in the presence of any additive is mainly attributed to its preferential exclusion from protein surface and its preferential binding to the protein surface, respectively.

Regulation of the Mdm2-p53 signaling axis in the DNA damage response and tumorigenesis

The p53 tumor suppressor acts as a guardian of the genome in mammalian cells undergoing DNA double strand breaks induced by a various forms of cell stress, including inappropriate growth signals or ionizing radiation. Following damage, p53 protein levels become greatly elevated in cells and p53 functions primarily as a transcription factor to regulate the expression a wide variety of genes that coordinate this DNA damage response. In cells undergoing high amounts of DNA damage, p53 can promote apoptosis, whereas in cells undergoing less damage, p53 promotes senescence or transient cell growth arrest and the expression of genes involved in DNA repair, depending upon the cell type and level of damage. Failure of the damaged cell to undergo growth arrest or apoptosis, or to respond to the DNA damage by other p53-coordinated mechanisms, can lead to inappropriate cell growth and tumorigenesis. In cells that have successfully responded to genetic damage, the amount of p53 present in the cell must return to basal levels in order for the cell to resume normal growth and function. Although regulation of p53 levels and function is coordinated by many proteins, it is now widely accepted that the master regulator of p53 is Mdm2. In this review, we discuss the role(s) of p53 in the DNA damage response and in tumor suppression, and how post-translational modification of Mdm2 regulates the Mdm2-p53 signaling axis to govern p53 activities in the cell.

Viruses as nanomedicine for cancer

Oncolytic virotherapy, a type of nanomedicine in which oncolytic viruses (OVs) are used to selectively infect and lyse cancer cells, is an emerging field in cancer therapy. Some OVs exhibit a specific tropism for cancer cells, whereas others require genetic modification to enhance their binding with and entry into cancer cells. OVs both kill tumor cells and induce the host’s immune response against tumor cells. Armed with antitumor cellular molecules, antibodies, and/or in combination with anticancer drugs, OVs can accelerate the lysis of cancer cells. Among the OVs, vaccinia virus has been the focus of preclinical and clinical research because of its many favorable properties. In this review, the basic mechanisms of action of OVs are presented, including their entry, survival, tumor lysis, and immune activation, and the latest research in vaccinia virus-based virotherapy and its status as an anticancer nanomedicine in prospective clinical trials are discussed.

p53 and Breast Cancer

Inactivation of the p53 tumor suppressor gene is one of the commonest genetic changes identified in human breast cancer. In this review, the structure and function of the p53 gene and its protein products will be discussed, with particular reference to p53 alterations that contribute to carcinogenesis. The frequency and pattern of p53 alterations in breast cancer will be outlined, laboratory methods for their detection briefly summarized, and the potential use of p53 as a prognostic and predictive marker discussed.

p53 Mutations and Tumor Progression in Well-differentiated Liposarcoma and Dermatofibrosarcoma Protuberans

Mutations of the tumor suppressor gene p53 have been identified in a wide variety of human tumors, including soft tissue sarcomas. Most missense mutations of p53 increase the half-life of the protein resulting in its accumulation in the nucleus. Immunohistochemical staining with a monoclonal antibody PABI801 (Oncogene Science, Uniondale, NY) detects the intranuclear accumulation of p53 protein in formalin-fixed tissue, and, thus, indicates the presence of missense mutations within the p53 gene. We compared p53 immunoreactivity in paraffin sections of low-grade sarcomas that progressed to high-grade lesions with low-grade sarcomas that had not progressed to high-grade lesions to determine if (1) histologic progression is associated with increasing incidence of p53 missense mutations, and (2) p53 missense mutations within low-grade areas are predictive of which lesions undergo histologic progression. To examine these questions we studied well-differentiated liposarcoma with and without dedifferentiation and dermatofibrosarcoma protuberans with and without areas of fibrosarcoma. Nuclear p53 immunoreactivity was detected in 48% (12/25) of well-differentiated liposarcoma with dedifferentiation compared to only 6% (1/17) of well-differentiated liposarcoma alone. p53 nuclear immunoreactivity was also detected in 25% (4/16) of dermatofibrosarcoma protuberans with fibrosarcoma, and in 0% (0/24) of dermatofibrosarcoma protuberans lacking fibrosarcoma. In cases of well-differentiated liposarcoma with dedifferentiation and dermatofibrosarcoma protuberans with fibrosarcoma displaying immunoreactivity, the staining occurred almost exclusively in the high-grade areas and very infrequently in the low-grade regions as well. We conclude that histologic progression of well-differentiated liposarcoma and dermatofibrosarcoma protuberans is associated with increased nuclear p53 immunoreactivity. Since p53 immunoreactivity occurs infrequently in the low-grade areas of those sarcomas that had transformed to higher grade lesions, it does not appear to be a useful predictor of tumor progression in low-grade lesions. Int J Surg Pathol 3(1):35-42, 1995

The Cell-Cycle Arrest and Apoptotic Functions of p53 in Tumor Initiation and Progression

P53 is a transcription factor highly inducible by many stress signals such as DNA damage, oncogene activation, and nutrient deprivation. Cell-cycle arrest and apoptosis are the most prominent outcomes of p53 activation. Many studies showed that p53 cell-cycle and apoptosis functions are important for preventing tumor development. p53 also regulates many cellular processes including metabolism, antioxidant response, and DNA repair. Emerging evidence suggests that these noncanonical p53 activities may also have potent antitumor effects within certain context. This review focuses on the cell-cycle arrest and apoptosis functions of p53, their roles in tumor suppression, and the regulation of cell fate decision after p53 activation.

Functional Characterization of Nupr1L, A Novel p53-Regulated Isoform of the High-Mobility Group (HMG)-Related Protumoral Protein Nupr1

We have previously demonstrated a crucial role of nuclear protein 1 (NUPR1) in tumor development and progression. In this work, we report the functional characterization of a novel Nupr1-like isoform (NUPR1L) and its functional interaction with the protumoral factor NUPR1. Through the use of primary sequence analysis, threading, and homology-based molecular modeling, as well as expression and immunolocalization, studies reveal that NUPR1L displays properties, which are similar to member of the HMG-like family of chromatin regulators, including its ability to translocate to the cell nucleus and bind to DNA. Analysis of the NUPR1L promoter showed the presence of two p53-response elements at positions -37 and -7, respectively. Experiments using reporter assays combined with site-directed mutagenesis and using cells with controllable p53 expression demonstrate that both of these sequences are responsible for the regulation of NUPR1L expression by p53. Congruently, NUPR1L gene expression is activated in response to DNA damage induced by oxaliplatin treatment or cell cycle arrest induced by serum starvation, two well-validated methods to achieve p53 activation. Interestingly, expression of NUPR1L downregulates the expression of NUPR1, its closely related protumoral isoform, by a mechanism that involves the inhibition of its promoter activity. At the cellular level, overexpression of NUPR1L induces G1 cell cycle arrest and a decrease in their cell viability, an effect that is mediated, at least in part, by downregulating NUPR1 expression. Combined, these experiments constitute the first functional characterization of NUPR1L as a new p53-induced gene, which negatively regulates the protumoral factor NUPR1.

Nano-silymarin provides protection against γ-radiation-induced oxidative stress in cultured human embryonic kidney cells

Radiation can produce biological damage, mainly oxidative stress, via production of free radicals, including reactive oxygen species (ROS). Nanoparticles are of interest as radioprotective agents, particularly due to their high solubility and bioavailability. Silymarin is a hepatoprotective agent but has poor oral bioavailability. Silymarin was formulated as a nanoemulsion with the aim of improving its bioavailability and therapeutic efficacy. In the present study, we evaluated self-nanoemulsifying drug delivery systems (SNEDDS) formulated with surfactants and co-surfactants. Nano-silymarin was characterized by estimating % transmittance, globule size, and polydispersity index, and by transmission electron microscopy (TEM). The nano-silymarin obtained was in the range of 3-8nm diameter. With regard to DNA damage, measured by a plasmid relaxation assay, maximum protection was obtained at 10μg/mL. Cytotoxicity of nano-silymarin to human embryonic kidney (HEK) cells was evaluated using the 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl-tetrazolium bromide (MTT) assay. Protective efficacy against γ-radiation was assessed by reduction in micronucleus frequency and ROS generation, using the 2',7'-dichlorodihydrofluorescein diacetate (H2DCFDA) assay. Radiation-induced apoptosis was estimated by microscopic analysis and cell-cycle estimation. Nano-silymarin was radioprotective, supporting the possibility of developing new approaches to radiation protection via nanotechnology.

SCYL1BP1 has tumor-suppressive functions in human lung squamous carcinoma cells by regulating degradation of MDM2

SCY1-like 1-binding protein 1 (SCYL1BP1) is a newly identified transcriptional activator domain containing protein with many unknown biological functions. Recently emerging evidence has revealed that it is a novel regulator of the p53 pathway, which is very important for the development of human cancer. However, the effects of SCYL1BP1 on human lung squamous carcinoma cell biological behavior remain poorly understood. In this study, we present evidence that SCYL1BP1 can promote the degradation of MDM2 protein and further inhibit the G1/S transition of lung squamous carcinoma cell lines. Functional assays found that reintroduction of SCYL1BP1 into lung squamous carcinoma cell lines significantly inhibited cell proliferation, migration, invasion and tumor formation in nude mice, suggesting strong tumor suppressive function of SCYL1BP1 in lung squamous carcinoma. Taken together, our data suggest that the interaction of SCYL1BP1/MDM2 could accelerate MDM2 degradation, and may function as an important tumor suppressor in lung squamous carcinomas.

Downregulation of p53 promotes in vitro perineural invasive activity of human salivary adenoid cystic carcinoma cells through epithelial-mesenchymal transition-like changes

Salivary adenoid cystic carcinoma (SACC) is a malignant tumor that is characterized by perineural invasion (PNI). p53 is an essential tumor-suppressor gene and p53 mutations play a critical role in tumor occurrence and progression (e.g., pancreatic, prostate and head and neck cancer). However, the regulatory role of the p53 gene in SACC and the PNI process remains unknown. In the present study, we employed RNA interference technique to downregulate p53 gene expression in SACC-83 cells to explore the role of p53 in the PNI process. Our results showed that the downregulation of the p53 gene induced significant 'epithelial-mesenchymal transition (EMT)-like changes' in SACC-83 cells, including decreased expression levels of epithelial markers (E-cadherin, EMA and CK5) and increased expression levels of mesenchymal markers (vimentin, N-cadherin and C-cadherin). The downregulation of p53 also caused a lower apoptotic index of Annexin V-FITC/PI and a lower number of SACC-83 cells in the second G0/G1 phase of the cell cycle. Furthermore, the downregulation of the p53 gene resulted in a significant increase in PNI activity in the SACC-83 cells. Thus, our findings revealed that downregulation of p53 promoted in vitro PNI activity through 'EMT-like changes' in SACC-83 cells. The present study suggests the essential regulatory role of p53 in the PNI activity of SACC cells, and implies that p53 may be a new target gene for the clinical treatment of SACC.

Involvement of p53 in the repair of DNA double strand breaks: multifaceted Roles of p53 in homologous recombination repair (HRR) and non-homologous end joining (NHEJ)

p53 is a tumor suppressor protein that prevents oncogenic transformation and maintains genomic stability by blocking proliferation of cells harboring unrepaired or misrepaired DNA. A wide range of genotoxic stresses such as DNA damaging anti-cancer drugs and ionizing radiation promote nuclear accumulation of p53 and trigger its ability to activate or repress a number of downstream target genes involved in various signaling pathways. This cascade leads to the activation of multiple cell cycle checkpoints and subsequent cell cycle arrest, allowing the cells to either repair the DNA or undergo apoptosis, depending on the intensity of DNA damage. In addition, p53 has many transcription-independent functions, including modulatory roles in DNA repair and recombination. This chapter will focus on the role of p53 in regulating or influencing the repair of DNA double-strand breaks that mainly includes homologous recombination repair (HRR) and non-homologous end joining (NHEJ). Through this discussion, we will try to establish that p53 acts as an important linchpin between upstream DNA damage signaling cues and downstream cellular events that include repair, recombination, and apoptosis.

Actin polymerization negatively regulates p53 function by impairing its nuclear import in response to DNA damage

Actin, one of the most evolutionarily conservative proteins in eukaryotes, is distributed both in the cytoplasm and the nucleus, and its dynamics plays important roles in numerous cellular processes. Previous evidence has shown that actin interacts with p53 and this interaction increases in the process of p53 responding to DNA damage, but the physiological significance of their interaction remains elusive. Here, we show that DNA damage induces both actin polymerization and p53 accumulation. To further understand the implication of actin polymerization in p53 function, cells were treated with actin aggregation agent. We find that the protein level of p53 decrease. The change in p53 is a consequence of the polymeric actin anchoring p53 in the cytoplasm, thus impairing p53 nuclear import. Analysis of phosphorylation and ubiquitination of p53 reveals that actin polymerization promotes the p53 phosphorylation at Ser315 and reduces the stabilization of p53 by recruiting Aurora kinase A. Taken together, our results suggest that the actin polymerization serves as a negative modulator leading to the impairment of nuclear import and destabilization of p53. On the basis of our results, we propose that actin polymerization might be a factor participating in the process of orchestrating p53 function in response to DNA damage.

Inhibition of p53 transcriptional activity by human cytomegalovirus UL44

Human cytomegalovirus (HCMV) stimulates cellular synthesis of DNA and proteins and induces transition of the cell cycle from G(1) to S and G(2) /M phase, in spite of increased amounts of p53 in the infected cells. The immediate early protein IE2-86  kDa (IE86) tethers a transcriptional repression domain to p53; however, its repression of p53 function is not enough to abrogate the G(1) checkpoint function of p53. Other HCMV proteins that suppress the activity of p53 were investigated in this study. Of the HCMV proteins that bind to p53 when assessed by immunoprecipitation and immunoblot analysis, HCMV UL44 was chosen as a candidate protein. It was found that reporter gene containing p53 consensus sequence was activated by transfection with wild type p53, but when plasmids of p53 with IE86 or UL44 were co-transfected, p53 transcriptional activity was decreased to 3-7% of the p53 control in a dose-dependent manner. When the deletion mutant of UL44 was co-transected with p53, the carboxyl one-third portion of UL44 had little effect on inhibition of p53 transcriptional activity. The amount of mRNA p21 was measured in H1299 by real time PCR after transfection of the combination of p53 and UL44 vectors and it was found that p21 transcription by p53 was inhibited dose-dependently by UL44. Increased G0/G1 and decreased S phases in p53 wild type-transfected H1299 cells were recovered to the level of p53 mutant type-transfected ones by the additional transfection of UL44 in a dose-dependent manner. In conclusion, the transcriptional activity of p53 is suppressed by UL44 as well as by IE86.

SCYL1 binding protein 1 promotes the ubiquitin-dependent degradation of Pirh2 and has tumor-suppressive function in the development of hepatocellular carcinoma

Pirh2 is a Ring-H2 domain containing E3 ubiquitin ligase that targets several important tumor suppressor genes for proteasomal degradation. Overexpression of Pirh2 is frequently detected in many clinical tumor tissues including hepatocellular carcinoma (HCC). However, the molecular mechanism of Pirh2 activation in tumorigenesis still remains poorly understood. In this study, we find a Pirh2-binding protein, SCYL1 binding protein 1 (SCYL1BP1), that can promote the ubiquitin-dependent degradation of Pirh2. SCYL1BP1 colocalized with Pirh2 in the cytoplasm and prevented its localization to the nucleus. Ectopic expression of SCYL1BP1 increased the expression of p53 and further inhibited the G(1)/S transition of HCC cell lines. Conversely, knock down of SCYL1BP1 restored the expression of Pirh2 and inhibited p53 at protein level. Functional assays found that reintroduction of SCYL1BP1 into HCC cell lines significantly inhibited cell proliferation, foci formation, colony formation in soft agar and tumor formation in nude mice, suggesting the strong tumor-suppressive function of SCYL1BP1 in HCC progression. Furthermore, SCYL1BP1 was found to be frequently downregulated in HCC clinical specimens compared to their paired non-tumor tissues by immunohistochemical staining. Taken together, our data suggested that the interaction of SCYL1BP1/Pirh2 could accelerate Pirh2 degradation through an ubiquitin-dependent pathway. SCYL1BP1 may function as an important tumor suppressor gene in HCC development.

ATR-p53 restricts homologous recombination in response to replicative stress but does not limit DNA interstrand crosslink repair in lung cancer cells

Homologous recombination (HR) is required for the restart of collapsed DNA replication forks and error-free repair of DNA double-strand breaks (DSB). However, unscheduled or hyperactive HR may lead to genomic instability and promote cancer development. The cellular factors that restrict HR processes in mammalian cells are only beginning to be elucidated. The tumor suppressor p53 has been implicated in the suppression of HR though it has remained unclear why p53, as the guardian of the genome, would impair an error-free repair process. Here, we show for the first time that p53 downregulates foci formation of the RAD51 recombinase in response to replicative stress in H1299 lung cancer cells in a manner that is independent of its role as a transcription factor. We find that this downregulation of HR is not only completely dependent on the binding site of p53 with replication protein A but also the ATR/ATM serine 15 phosphorylation site. Genetic analysis suggests that ATR but not ATM kinase modulates p53's function in HR. The suppression of HR by p53 can be bypassed under experimental conditions that cause DSB either directly or indirectly, in line with p53's role as a guardian of the genome. As a result, transactivation-inactive p53 does not compromise the resistance of H1299 cells to the interstrand crosslinking agent mitomycin C. Altogether, our data support a model in which p53 plays an anti-recombinogenic role in the ATR-dependent mammalian replication checkpoint but does not impair a cell's ability to use HR for the removal of DSB induced by cytotoxic agents.

E3 ubiquitin ligase Hades negatively regulates the exonuclear function of p53

Following DNA damage, p53 translocates to the cytoplasm and mitochondria, where it triggers transcription-independent apoptosis by binding to Bcl-2 family proteins. However, little is known about how this exonuclear function of p53 is regulated. Here, we identify and characterize a p53-interacting protein called Hades, an E3 ligase that interacts with p53 in the mitochondria. Hades reduces p53 stability via a mechanism that requires its RING-finger domain with ubiquitin ligase activity. Hades polyubiquitinates p53 in vitro independent of Mdm2 and targets a critical lysine residue in p53 (lysine 24) distinct from those targeted by Mdm2. Hades inhibits a p53-dependent mitochondrial cell death pathway by inhibiting p53 and Bcl-2 interactions. These findings show that Hades-mediated p53 ubiquitination is a novel mechanism for negatively regulating the exonuclear function of p53.

Phenolic fraction of tobacco smoke inhibits BPDE-induced apoptosis response and potentiates cell transformation: role of attenuation of p53 response

Polynuclear aromatic hydrocarbons (PAHs) present in tobacco smoke are regarded as chemical carcinogens. Previously, we observed that a weakly acidic phenolic fraction of tobacco smoke condensate (TSCPhFr), which is devoid of PAHs, significantly potentiates (±)-anti-BP-7,8-diol-9,10-epoxide (BPDE)-induced anchorage-independent cell growth of promotion-sensitive JB6 cell, indicating its tumor-promoting potential. In the present article, we report that further fractionation of phenolic components from TSCPhFr did not show any significant potentiation of BPDE-induced cell transformation by any of the HPLC-purified phenolic fractions, indicating several phenolic components as a whole are needed for observed activity. Although the tumor-promoting activity of weakly acidic phenolic fraction of tobacco smoke had been indicated long before, no studies have been pursued to understand the mechanism(s) underlying the tumor-promoting activity of TSCPhFr. We observed that BPDE, an ultimate carcinogenic metabolite of tobacco smoke carcinogen benzo[a]pyrene, elicits apoptosis induction, which is significantly inhibited by TSCPhFr. Increased cell transformation and decreased apoptosis by TSCPhFr were associated with attenuation of BPDE-induced p53 accumulation. JB6 cells transfected with p53 siRNA showed significantly less apoptosis induction by BPDE as compared to control cells. In p53 impaired cells (which are observed to have a faster growth rate as compared to normal cells), TSCPhFr has a practically negligible effect on apoptosis induction in response to BPDE. Also, in p53 null HCT116 p53(-/-) cells, BPDE-induced apoptosis is unresponsive to TSCPhFr. Inhibition of BPDE-induced NF-κB activation was also observed by us previously. Interestingly, treatment of cells with NF-κB-specific inhibitor IKK-NBD peptide showed no effect on BPDE-induced apoptosis, whereas TSCPhFr showed moderate inhibition of apoptosis in NF-κB inhibited cells as compared to control cells. Our observations indicate that attenuation of BPDE-induced p53 response has a role in apoptosis inhibition and increased cell transformation by TSCPhFr. These findings have implication with regard to the underlying mechanism of tumor-promoting activity of TSCPhFr in PAH-induced carcinogenesis. Although p53-mediated NF-κB activation has a role in apoptosis induction, the role of NF-κB in TSCPhFr-mediated potentiation of PAH-induced cell transformation is not clear from our studies.

Loss of HSF1 results in defective radiation-induced G(2) arrest and DNA repair

HSF1 is a transcription factor that plays a key role in the response to heat stress and was previously shown by us to also be essential for importation of p53 into the nucleus. Here we extend these studies and show that loss of HSF1 renders cells more sensitive to killing by ionizing radiation. Cells that lack a functional HSF1 were unable to arrest in G(2) after exposure to ionizing radiation, suggesting that HSF1 activity was essential for activation of this cell cycle checkpoint. In addition, cells with no HSF1 showed a reduced capacity to repair radiation-induced double-stranded DNA breaks. We found that in these cells 53BP1 did not accumulate at sites of DNA damage, suggesting that HSF1 was also essential for the functioning of this DNA damage mediator. Taken together our results indicate that HSF1 plays an important role in checkpoint activation and DNA repair and suggest that there is overlap between the heat stress response pathway and the pathway that responds to ionizing radiation.

P53 is transported into the nucleus via an Hsf1-dependent nuclear localization mechanism

Loss of p53 function can occur through disruption of its ability to localize to the nucleus. Previously we showed through characterization a set of mutant cell lines that lacked the ability to import p53 into the nucleus that nuclear translocation of p53 appeared to be mechanistically different from that of the SV40 T-antigen (SV40TAg). Here we extend that work by examining nuclear importation of p53 and SV40TAg using both in vivo and in vitro assays for nuclear localization. We show that disruption of microtubule polymerization using colchicine suppresses nuclear localization of p53 but not of SV40TAg. We also show, for the first time, that the heat shock transcription factor (Hsf1), is required for establishment of the microtubule network in cells and for nuclear localization of p53. In contrast, SV40TAg does not interact with polymerized microtubules suggesting that it is transported into the nucleus through an alternative mechanism. Interestingly, lacking of Hsf1 expression and suppressing Hsf1 by siRNA also made cells more resistant to the cytotoxic effects of paclitaxel. Hence, loss of Hsf1 activity not only suppressed p53 function, but also led to reduced sensitivity to killing by drugs that target microtubules.

p53: the attractive tumor suppressor in the cancer research field

p53 is one of the most studied tumor suppressors in the cancer research field. Of note, over 50% of human tumors carry loss of function mutations, and thus p53 has been considered to be a classical Knudson-type tumor suppressor. From the functional point of view, p53 is a nuclear transcription factor to transactivate a variety of its target genes implicated in the induction of cell cycle arrest, DNA repair, and apoptotic cell death. In response to cellular stresses such as DNA damage, p53 is activated and promotes cell cycle arrest followed by the replacement of DNA lesions and/or apoptotic cell death. Therefore, p53 is able to maintain the genomic integrity to prevent the accumulation of genetic alterations, and thus stands at a crossroad between cell survival and cell death. In this paper, we describe a variety of molecular mechanisms behind the regulation of p53.

Initial characterization of a low-molecular-weight factor enhancing the checkpoint response

In higher eukaryotes, DNA double-strand breaks (DSBs) induced by ionizing radiation activate checkpoints that delay progression through the cell cycle. Compared to delays in other phases of the cell cycle, delays induced in G(2) are longer and frequently correlate with resistance to killing by radiation. Therefore, modulation of the G(2) checkpoint offers a means to modulate cellular radiosensitivity. Although compounds are known that reduce the G(2) checkpoint and act as radiosensitizers, compounds enhancing this checkpoint have not been reported. Here we summarize evidence for a factor with such properties. We show that a highly radioresistant rat embryo fibroblast (REF) cell line displays a strong G(2) checkpoint partly as a result of a factor excreted into the growth medium by nonirradiated cells. Various tests indicate that this G(2)-arrest modulating activity (GAMA) is a small molecule showing detectable retention only after passing through filters with a molecular weight cutoff limit of less than 1,000 Da. GAMA is heat stable and resistant to treatment with proteases or nucleases. Electroelution tests show that GAMA is uncharged at neutral pH, a result that is in agreement with the observed failure to bind S- or Q-Sepharose. Investigations on the mechanism of GAMA function indicate ligand-receptor interactions and allow the classification of cells as producers, responders or both. Compounds with properties such as those of GAMA bridge intercellular communication with the DNA damage response and may function as radioprotectors.

Diagnostic utility of immunohistochemical panel in various thyroid pathologies

BACKGROUND

For management of thyroid nodules, distinction between benign and malignant tumours is essential. The study was performed to evaluate the diagnostic value of molecular markers in different thyroid tumours.

MATERIALS AND METHODS

Immunohistochemistry for CD56, HBME-1, COX-2, Ki-67, p53 and E-cadherin (E-CAD) was performed in 113 benign and 35 malignant thyroid lesions including 36 follicular adenomas (FA), 77 colloid goitres, 26 papillary thyroid carcinomas (PTC) and 9 follicular carcinomas (FC). The results were scored semiquantitatively by staining intensity (0-3 scale) and percentage of positive cells.

RESULTS

PTC was characterised by decreased E-CAD and CD56 expression in contrast to surrounding benign thyroid tissues. HBME-1 expression was absent in benign thyroid tissues but was notably high in PTC and occasionally in FC. The expression of E-CAD and CD56 in FA was significantly higher than in the surrounding thyroid tissues. No expression of p53 was found in any group. The expression of COX-2 was low in all lesions. The proliferation activity by Ki-67 was generally low; however, it was significantly higher in cancers.

CONCLUSIONS

The panel consisting of three markers, HBME-1, E-CAD and CD56, can be recommended as an adjunct to morphology criteria. HBME-1 is found in malignant lesions only and is the most sensitive and specific single marker in PTC. Decreased expression of E-CAD and CD56 distinguishes PTC from FA and FC. Both FA and FC are characterised by high expression of E-CAD and CD56. The practical use of Ki-67 is difficult due to low values. The role of adhesion factors in thyroid malignancies may be superior in comparison with cell proliferation.

Expression of bcl-2 and p53 proteins in nasopharyngeal carcinoma. Absence of correlation with the presence of EBV encoded EBER1-2 transcripts and latent membrane protein-1

Aims-To investigate the immunohistochemical expression of bcl-2 and p53 proteins in nasopharyngeal carcinomas in relation to the expression of the Epstein-Barr virus (EBV) encoded EBER messenger RNAs (mRNAs) and latent membrane protein-1 (LMP-1).Methods-Formalin fixed, paraffin wax embedded tissue from 44 nasopharyngeal carcinomas (NPCs) was stained by immunohistochemistry for p53, bcl-2 and LMP-1 proteins and by RNA in situ hybridisation for EBER mRNAs.Results-The tumours were divided histologically into 13 cases of keratinising squamous cell NPC (KNPC), 15 cases of non-keratinising squamous cell NPC (NKNPC) and 16 cases of undifferentiated NPC (UNPC). Bcl-2 expression was observed in five of 15 NKNPC cases and in six of 16 UNPC cases; p53 expression was observed in one of 13 KNPC, two of 15 NKNPC and four of 16 UNPC cases. EBER 1-2 transcripts were detected in five of 15 NKNPC and nine of 16 UNPC cases, while LMP-1 expression was observed in one of 16 UNPC cases. All 13 KNPCs were EBV and bcl-2 negative. No correlation was found between the presence of EBER 1-2 transcripts and the detection of bcl-2 or p53 proteins, or both, in NPC cells.Conclusions-The expression of bcl-2 and p53 proteins may be associated with the level of the tumour cell differentiation in NPC. In addition, in view of the important role of the bcl-2 protein in the inhibition of apoptosis, the expression of bcl-2 protein may contribute to tumour cell survival in a proportion of NPCs. Furthermore, in the light of previous findings that the p53 gene in most UNPCs is in the wild-type configuration, mechanisms other than mutation may be responsible for stabilisation of the p53 protein in UNPCs.

A small peptide mimicking the key domain of MEPE/OF45 interacting with CHK1 protects human cells from radiation-induced killing

Checkpoint activation benefits DNA homologous recombination repair and therefore protects cells from ionizing radiation (IR)-induced killing. CHK1 is one of the most important checkpoint regulators in mammalian cells. We recently reported that matrix extracellular phosphoglycoprotein/osteoblast factor 45 (MEPE/OF45) stabilizes CHK1 through interacting with CHK1, thus protecting cells from IR-induced killing. The purpose of this study is to investigate whether a small peptide that mimics the key domain of MEPE/OF45 could interact with CHK1 and protect cells from IR-induced killing. We showed here that the synthesized peptide with 18 amino acids (aa) could enter human transformed lymphoblasts when it is linked to fatty acid CH3(CH2)8CO. After the 18 aa peptide entered the human cells, it interacted with CHK1, increased the CHK1 level and induces stronger G2 arrest in the cells following IR. More importantly, the 18 aa peptide could protect the cells from IR-induced killing. Our data indicate that the 18 aa peptide, similar to MEPE/OF45, reduces CHK1 degradation and protects cells from IR-induced killing. We believe that these results provide useful information for drug development in two directions: protect cells from IR induced damage and sensitize cells to radiation therapy.

MEPE/OF45 protects cells from DNA damage induced killing via stabilizing CHK1

Matrix extracellular phosphoglycoprotein/osteoblast factor 45 (MEPE/OF45) was cloned in 2000 with functions related to bone metabolism. We identified MEPE/OF45 for the first time as a new co-factor of CHK1 in mammalian cells to protect cells from DNA damage induced killing. We demonstrate here that MEPE/OF45 directly interacts with CHK1. Knocking down MEPE/OF45 decreases CHK1 levels and sensitizes the cells to DNA damage inducers such as ionizing radiation (IR) or camptothicin (CPT)-induced killing. Over-expressing wild-type MEPE/OF45, but not the mutant MEPE/OF45 (depleted the key domain to interact with CHK1) increases CHK1 levels in the cells and increases the resistance of the cells to IR or CPT. MEPE/OF45, interacting with CHK1, increases CHK1 half-life and decreases CHK1 degradation through the ubiquitine-mediated pathway. In addition, the interaction of MEPE/OF45 with CHK1 decreases CHK1 levels in the ubiquitin E3 ligases (Cul1 and Cul4A) complex, which suggests that MEPE/OF45 competes with the ubiquitin E3 ligases binding to CHK1 and thus decreases CHK1 from ubiquitin-mediated proteolysis. These findings reveal an important role of MEPE/OF45 in protecting cells from DNA damage induced killing through stabilizing CHK1, which would provide MEPE/OF45 as a new target for sensitizing tumor cells to radiotherapy or chemotherapy.

The transcriptional regulatory function of p53 is essential for suppression of mouse skin carcinogenesis and can be dissociated from effects on TGF-beta-mediated growth regulation

Transcriptional regulation by p53 is critical for p53-mediated tumour suppression; however, p53-mediated transactivation has been dissociated from p53-mediated biological processes including apoptosis, DNA repair, and differentiation. We compared the effects of a mutant allele, p53(QS - val135), containing a double mutation in the amino-terminus abrogating transactivation activity and a modification at amino acid 135 partially affecting DNA binding, to complete loss of p53. We applied in vitro endpoints correlated with epithelial tumourigenesis and an in vivo assay of tumour phenotype to assess whether loss of p53-mediated transcriptional regulation underlies the malignant phenotype of p53(-/-)/v-ras(Ha)-overexpressing keratinocytes. Transactivation deficiency of p53QS-val135 was confirmed by reporter gene assays in fibroblasts and differentiating keratinocytes. Ras oncogene-induced senescence was lost in both p53(QS - val135/QS - val135) and p53(-/-) keratinocytes. Similarly, p53(QS - val135/QS - val135), like p53(-/-), cooperated with v-ras(Ha) to enhance malignant conversion. The tumours arising in p53(QS - val135/QS - val135) keratinocytes displayed strong nuclear p53 expression; thus, the p53(QS - val135) allele was maintained and the deficient transactivation function of the expressed p53QS mutant protein was supported by absence of p21(waf1) in these tumours. The p53(QS - val135) allele did not confer a dominant-negative phenotype, as p53(+/QS - val135) keratinocytes senesced normally in response to v-ras(Ha) expression and formed benign tumours. While p53(-/-) keratinocytes displayed diminished response to TGF-beta, p53(QS - val135/QS - val135) and p53(+/+) keratinocytes responded equivalently, indicating that the requirement for p53 in maximizing TGF-beta-mediated growth regulation is independent of its transactivation domain and that the ability of keratinocytes to respond to TGF-beta is insufficient to suppress the malignant phenotype in this model. Furthermore, TGF-beta enhances p53QS-induced activation of a dual p53-TGF-beta responsive reporter in a keratinocyte cell line. These findings support an essential role for p53-mediated transcriptional regulation in suppressing malignancies arising from ras-induced skin tumours, consistent with previous findings in spontaneous carcinogenesis in other organs, and highlight the potential importance of senescence for tumour suppression in vivo.

Human cytomegalovirus IE1-72 protein interacts with p53 and inhibits p53-dependent transactivation by a mechanism different from that of IE2-86 protein

Infection of host cells with human cytomegalovirus (HCMV) induces cell cycle dysregulation. Two HCMV immediate-early (IE) proteins, IE1-72 and IE2-86, are promiscuous transactivators that have been implicated in the dysregulatory events. Cellular p53 protein is accumulated to high levels in HCMV-infected cells, but the indicative marker of p53 transcriptional activity, p21, is markedly decreased. Both IE1-72 and IE2-86 were able to transactivate the p53 promoter and interact with p53 protein in DNA-transfected or HCMV-infected cells. HCMV UL84, a multiregulatory protein expressed in early periods of HCMV infection, also interacted with p53. HCMV IE1-72 prevented or disrupted p53 binding to p53-specific DNA sequences, while IE2-86 and/or UL84 enhanced p53 binding and induced supershift of this DNA-protein complex. Both HCMV IE1-72 and IE2-86 were able to inhibit p53-dependent transcriptional activation in plasmid-transfected cells. IE1-72, rather than IE2-86, was found to be responsible for p21 downregulation in HCMV-infected HEL cells. DNA transfection analysis using IE1-72 mutants revealed that exon 2/3 and the zinc finger region of IE1-72 are essential for IE1-72's effect on the repression of p53-dependent transcriptional activation. These data suggest that HCMV IE1-72 and/or IE2-86 transactivates the p53 promoter and induces p53 accumulation, but HCMV IE1-72 represses the p53 transactivation activity by a unique binding hindrance mechanism different from that of IE2-86. Thus, various modes of viral IE proteins and p53 interactions might result in multiple outcomes, such as stimulation of cellular DNA synthesis, cell cycle progression and cell cycle arrest, and prevention of program cell death.

Hsf1 is required for the nuclear translocation of p53 tumor suppressor

Although the p53 tumor suppressor is most frequently inactivated by genetic mutations, exclusion from the nucleus is also seen in human tumors. We have begun to examine p53 nuclear importation by isolating a series of mutant cells in which the temperature-sensitive murine p53(Val135) mutant is sequestered in the cytoplasm. We previously showed that that three of them (ALTR12, ALTR19, and ALTR25) constituted a single complementation group. Here, we found that ALTR12 cells are more sensitive to heat stress than either ALTR19 or ALTR25 and that there was a complete lack of induction of Hsp70 in response to heat shock. Western blot analysis showed no expression of the Hsf1 transcription factor, and neither heat shock nor azetidine could induce p53 nuclear localization in ALTR12 cells but did in parental A1-5 cells. Suppression of Hsf1 in A1-5 cells with quercetin or an Hsf1 siRNA reduced p53 nuclear importation and inhibited p53-mediated activation of a p21 reporter. Most convincingly, p53 nuclear importation could be restored in ALTR12 cells by introducing an exogenous Hsf1 gene. Collectively, our result suggests that Hsf1 is required for p53 nuclear importation and activation and implies that heat shock factors play a role in the regulation of p53.

Polymorphisms inGSTT1andp53and urinary transitional cell carcinoma in south-western Taiwan: A preliminary study

Little is known about the relevance of genetic polymorphisms to arsenic-related bladder cancer. A preliminary case-control study was conducted to explore the association between genetic polymorphisms of GSTT1, p53 codon 72 and bladder cancer in southern Taiwan, a former high arsenic exposure area. Fifty-nine urinary transitional cell carcinoma (TCC) patients from a referral centre in south-western Taiwan and 81 community controls matched on residence were recruited from 1996 to 1999. A questionnaire was administered to obtain arsenic exposure and general health information. Genotypes of p53 codon 72 and GSTT1 were analysed by polymerase chain reaction-restriction fragment length polymerase. The combined variant genotypes (heterozygous or homozygous variant) of p53 codon 72 and GSTT1 null were observed in 29% of cases and in 44% of controls, respectively. In this preliminary study, bladder cancer risk was slightly elevated for subjects carrying the variant genotype of p53 codon 72 or in subjects carrying the GSTT1 null genotype. Variants in p53 codon 72 increased the risk of bladder cancer among smokers. However, the results were not statistically significant and larger confirmatory studies are needed to clarify the role of candidate gene polymorphisms and bladder cancer risk in arsenic exposed populations.

Mdm2 and Mdm4 loss regulates distinct p53 activities

Mutational inactivation of p53 is a hallmark of most human tumors. Loss of p53 function also occurs by overexpression of negative regulators such as MDM2 and MDM4. Deletion of Mdm2 or Mdm4 in mice results in p53-dependent embryo lethality due to constitutive p53 activity. However, Mdm2(-/-) and Mdm4(-/-) embryos display divergent phenotypes, suggesting that Mdm2 and Mdm4 exert distinct control over p53. To explore the interaction between Mdm2 and Mdm4 in p53 regulation, we first generated mice and cells that are triple null for p53, Mdm2, and Mdm4. These mice had identical survival curves and tumor spectrum as p53(-/-) mice, substantiating the principal role of Mdm2 and Mdm4 as negative p53 regulators. We next generated mouse embryo fibroblasts null for p53 with deletions of Mdm2, Mdm4, or both; introduced a retrovirus expressing a temperature-sensitive p53 mutant, p53A135V; and examined p53 stability and activity. In this system, p53 activated distinct target genes, leading to apoptosis in cells lacking Mdm2 and a cell cycle arrest in cells lacking Mdm4. Cells lacking both Mdm2 and Mdm4 had a stable p53 that initiated apoptosis similar to Mdm2-null cells. Additionally, stabilization of p53 in cells lacking Mdm4 with the Mdm2 antagonist nutlin-3 was sufficient to induce a cell death response. These data further differentiate the roles of Mdm2 and Mdm4 in the regulation of p53 activities.