The role of p53 in hypoxia-induced apoptosis

An RNA interference screen identifies new avenues for nephroprotection

Acute kidney injury is a major public health problem, which is commonly caused by renal ischemia and is associated with a high risk of mortality and long-term disability. Efforts to develop a treatment for this condition have met with very limited success. We used an RNA interference screen to identify genes (BCL2L14, BLOC1S2, C2ORF42, CPT1A, FBP1, GCNT3, RHOB, SCIN, TACR1, and TNFAIP6) whose suppression improves survival of kidney epithelial cells in in vitro models of oxygen and glucose deprivation. Some of the genes also modulate the toxicity of cisplatin, an anticancer agent whose use is currently limited by nephrotoxicity. Furthermore, pharmacological inhibition of TACR1 product NK1R was protective in a model of mouse renal ischemia, attesting to the in vivo relevance of our findings. These data shed new light on the mechanisms of stress response in mammalian cells, and open new avenues to reduce the morbidity and mortality associated with renal injury.

ACLY and ACC1 Regulate Hypoxia-Induced Apoptosis by Modulating ETV4 via α-ketoglutarate

In order to propagate a solid tumor, cancer cells must adapt to and survive under various tumor microenvironment (TME) stresses, such as hypoxia or lactic acidosis. To systematically identify genes that modulate cancer cell survival under stresses, we performed genome-wide shRNA screens under hypoxia or lactic acidosis. We discovered that genetic depletion of acetyl-CoA carboxylase (ACACA or ACC1) or ATP citrate lyase (ACLY) protected cancer cells from hypoxia-induced apoptosis. Additionally, the loss of ACLY or ACC1 reduced levels and activities of the oncogenic transcription factor ETV4. Silencing ETV4 also protected cells from hypoxia-induced apoptosis and led to remarkably similar transcriptional responses as with silenced ACLY or ACC1, including an anti-apoptotic program. Metabolomic analysis found that while α-ketoglutarate levels decrease under hypoxia in control cells, α-ketoglutarate is paradoxically increased under hypoxia when ACC1 or ACLY are depleted. Supplementation with α-ketoglutarate rescued the hypoxia-induced apoptosis and recapitulated the decreased expression and activity of ETV4, likely via an epigenetic mechanism. Therefore, ACC1 and ACLY regulate the levels of ETV4 under hypoxia via increased α-ketoglutarate. These results reveal that the ACC1/ACLY-α-ketoglutarate-ETV4 axis is a novel means by which metabolic states regulate transcriptional output for life vs. death decisions under hypoxia. Since many lipogenic inhibitors are under investigation as cancer therapeutics, our findings suggest that the use of these inhibitors will need to be carefully considered with respect to oncogenic drivers, tumor hypoxia, progression and dormancy. More broadly, our screen provides a framework for studying additional tumor cell stress-adaption mechanisms in the future.

During the development of most solid tumors, there are characteristic physiological differences in the tumor that result from tumor cells outgrowing their local blood supply. Two of these physiological differences, or “stresses,” that occur in the tumor are low oxygen levels (hypoxia) and an accumulation of lactic acidic (lactic acidosis). Cancer cells experiencing hypoxia and lactic acidosis tend to be more resistant to chemo- and radio-therapy and metastasize more readily. Therefore, it is important to understand how tumor cells adapt to and survive these stresses. We used a large scale screening experiment in order to find which genes and proteins are involved in tumor cell adaptation and survival under hypoxia or lactic acidosis. We found that inhibiting either of two genes involved in lipid synthesis allowed tumor cells to survive hypoxia. This occurred because silencing these genes led to an increase in the metabolite α-ketoglutarate, which repressed a transcription factor that contributed to cell death under hypoxia. This research specifically advances our understanding of how tumor cells survive hypoxia and lactic acidosis and more broadly enhances our understanding of the cellular biology of solid tumors.

Hypoxia-induced p53 modulates both apoptosis and radiosensitivity via AKT

Restoration of hypoxia-induced apoptosis in tumors harboring p53 mutations has been proposed as a potential therapeutic strategy; however, the transcriptional targets that mediate hypoxia-induced p53-dependent apoptosis remain elusive. Here, we demonstrated that hypoxia-induced p53-dependent apoptosis is reliant on the DNA-binding and transactivation domains of p53 but not on the acetylation sites K120 and K164, which, in contrast, are essential for DNA damage-induced, p53-dependent apoptosis. Evaluation of hypoxia-induced transcripts in multiple cell lines identified a group of genes that are hypoxia-inducible proapoptotic targets of p53, including inositol polyphosphate-5-phosphatase (INPP5D), pleckstrin domain-containing A3 (PHLDA3), sulfatase 2 (SULF2), B cell translocation gene 2 (BTG2), cytoplasmic FMR1-interacting protein 2 (CYFIP2), and KN motif and ankyrin repeat domains 3 (KANK3). These targets were also regulated by p53 in human cancers, including breast, brain, colorectal, kidney, bladder, and melanoma cancers. Downregulation of these hypoxia-inducible targets associated with poor prognosis, suggesting that hypoxia-induced apoptosis contributes to p53-mediated tumor suppression and treatment response. Induction of p53 targets, PHLDA3, and a specific INPP5D transcript mediated apoptosis in response to hypoxia through AKT inhibition. Moreover, pharmacological inhibition of AKT led to apoptosis in the hypoxic regions of p53-deficient tumors and consequently increased radiosensitivity. Together, these results identify mediators of hypoxia-induced p53-dependent apoptosis and suggest AKT inhibition may improve radiotherapy response in p53-deficient tumors.

Staphylococcus aureus induces hypoxia and cellular damage in porcine dermal explants

We developed a porcine dermal explant model to determine the extent to which Staphylococcus aureus biofilm communities deplete oxygen, change pH, and produce damage in underlying tissue. Microelectrode measurements demonstrated that dissolved oxygen (DO) in biofilm-free dermal tissue was 4.45 ± 1.17 mg/liter, while DO levels for biofilm-infected tissue declined sharply from the surface, with no measurable oxygen detectable in the underlying dermal tissue. Magnetic resonance imaging demonstrated that biofilm-free dermal tissue had a significantly lower relative effective diffusion coefficient (0.26 ± 0.09 to 0.30 ± 0.12) than biofilm-infected dermal tissue (0.40 ± 0.12 to 0.48 ± 0.12; P < 0.0001). Thus, the difference in DO level was attributable to biofilm-induced oxygen demand rather than changes in oxygen diffusivity. Microelectrode measures showed that pH within biofilm-infected explants was more alkaline than in biofilm-free explants (8.0 ± 0.17 versus 7.5 ± 0.15, respectively; P < 0.002). Cellular and nuclear details were lost in the infected explants, consistent with cell death. Quantitative label-free shotgun proteomics demonstrated that both proapoptotic programmed cell death protein 5 and antiapoptotic macrophage migration inhibitory factor accumulated in the infected-explant spent medium, compared with uninfected-explant spent media (1,351-fold and 58-fold, respectively), consistent with the cooccurrence of apoptosis and necrosis in the explants. Biofilm-origin proteins reflected an extracellular matrix-adapted lifestyle of S. aureus. S. aureus biofilms deplete oxygen, increase pH, and induce cell death, all factors that contribute to impede wound healing.

The fate of chemoresistance in triple negative breast cancer (TNBC)

BACKGROUND

Treatment options for women presenting with triple negative breast cancer (TNBC) are limited due to the lack of a therapeutic target and as a result, are managed with standard chemotherapy such as paclitaxel (Taxol®). Following chemotherapy, the ideal tumour response is apoptotic cell death. Post-chemotherapy, cells can maintain viability by undergoing viable cellular responses such as cellular senescence, generating secretomes which can directly enhance the malignant phenotype.

SCOPE OF REVIEW

How tumour cells retain viability in response to chemotherapeutic engagement is discussed. In addition we discuss the implications of this retained tumour cell viability in the context of the development of recurrent and metastatic TNBC disease. Current adjuvant and neo-adjuvant treatments available and the novel potential therapies that are being researched are also reviewed.

MAJOR CONCLUSIONS

Cellular senescence and cytoprotective autophagy are potential mechanisms of chemoresistance in TNBC. These two non-apoptotic outcomes in response to chemotherapy are inextricably linked and are neglected outcomes of investigation in the chemotherapeutic arena. Cellular fate assessments may therefore have the potential to predict TNBC patient outcome.

GENERAL SIGNIFICANCE

Focusing on the fact that cancer cells can bypass the desired cellular apoptotic response to chemotherapy through cellular senescence and cytoprotective autophagy will highlight the importance of targeting non-apoptotic survival pathways to enhance chemotherapeutic efficacy.

Upregulation of ATG3 contributes to autophagy induced by the detachment of intestinal epithelial cells from the extracellular matrix, but promotes autophagy-independent apoptosis of the attached cells

Detachment of nonmalignant intestinal epithelial cells from the extracellular matrix (ECM) triggers their growth arrest and, ultimately, apoptosis. In contrast, colorectal cancer cells can grow without attachment to the ECM. This ability is critical for their malignant potential. We found previously that detachment-induced growth arrest of nonmalignant intestinal epithelial cells is driven by their detachment-triggered autophagy, and that RAS, a major oncogene, promotes growth of detached cells by blocking such autophagy. In an effort to identify the mechanisms of detachment-induced autophagy and growth arrest of nonmalignant cells we found here that detachment of these cells causes upregulation of ATG3 and that ATG3 upregulation contributes to autophagy and growth arrest of detached cells. We also observed that when ATG3 expression is artificially increased in the attached cells, ATG3 promotes neither autophagy nor growth arrest but triggers their apoptosis. ATG3 upregulation likely promotes autophagy of the detached but not that of the attached cells because detachment-dependent autophagy requires other detachment-induced events, such as the upregulation of ATG7. We further observed that those few adherent cells that do not die by apoptosis induced by ATG3 become resistant to apoptosis caused by cell detachment, a property that is critical for the ability of normal epithelial cells to become malignant. We conclude that cell-ECM adhesion can switch ATG3 functions: when upregulated in detached cells in the context of other autophagy-promoting events, ATG3 contributes to autophagy. However, when overexpressed in the adherent cells, in the circumstances not favoring autophagy, ATG3 triggers apoptosis.

A comparison of the imaging characteristics and microregional distribution of 4 hypoxia PET tracers

We compared the imaging characteristics and hypoxia selectivity of 4 hypoxia PET radiotracers ((18)F-fluoromisonidazole [(18)F-FMISO], (18)F-flortanidazole [(18)F-HX4], (18)F-fluoroazomycin arabinoside [(18)F-FAZA], and (64)Cu-diacetyl-bis(N4-methylsemicarbazone) [(64)Cu-ATSM]) in a single murine xenograft tumor model condition using small-animal PET imaging and combined ex vivo autoradiography and fluorescence immunohistochemistry.

METHODS

Nude mice bearing SQ20b xenograft tumors were administered 1 of 4 hypoxia PET tracers and images acquired 80-90 min after injection. Frozen sections from excised tumors were then evaluated for tracer distribution using digital autoradiography and compared with histologic markers of tumor hypoxia (pimonidazole, carbonic anydrase 9 [CA9]) and vascular perfusion (Hoechst 33342).

RESULTS

The highest tumor uptake was observed with (64)Cu-ATSM (maximum standardized uptake values [SUV(max)], 1.26 ± 0.13) and the lowest with (18)F-FAZA (SUVmax, 0.41 ± 0.24). (18)F-FMISO and (18)F-HX4 had similar intermediate tumor uptake (SUV(max), 0.76 ± 0.38 and 0.65 ± 0.19, respectively). Digital autoradiographs of hypoxia tracer distribution were compared pixel by pixel with images of immunohistochemistry stains. The fluorinated nitroimidazoles all showed radiotracer uptake increasing with pimonidazole and CA9 staining. (64)Cu-ATSM showed the opposite pattern, with highest radiotracer uptake observed in regions with the lowest pimonidazole and CA9 staining.

CONCLUSION

The fluorinated nitroimidazoles showed similar tumor distributions when compared with immunohistochemistry markers of hypoxia. Variations in tumor standardized uptake value and normal tissue distribution may determine the most appropriate clinical setting for each tracer. (64)Cu-ATSM showed the highest tumor accumulation and little renal clearance. However, the lack of correlation between (64)Cu-ATSM distribution and immunohistochemistry hypoxia markers casts some doubt on the hypoxia selectivity of (64)Cu-ATSM.

Preeclampsia is associated with alterations in the p53-pathway in villous trophoblast

BACKGROUND

Preeclampsia (PE) is characterized by exaggerated apoptosis of the villous trophoblast of placental villi. Since p53 is a critical regulator of apoptosis we hypothesized that excessive apoptosis in PE is mediated by abnormal expression of proteins participating in the p53 pathway and that modulation of the p53 pathway alters trophoblast apoptosis in vitro.

METHODS

Fresh placental villous tissue was collected from normal pregnancies and pregnancies complicated by PE; Western blotting and real-time PCR were performed on tissue lysate for protein and mRNA expression of p53 and downstream effector proteins, p21, Bax and caspases 3 and 8. To further assess the ability of p53 to modulate apoptosis within trophoblast, BeWo cells and placental villous tissue were exposed to the p53-activator, Nutlin-3, alone or in combination with the p53-inhibitor, Pifithrin-α (PFT-α). Equally, Mdm2 was knocked-down with siRNA.

RESULTS

Protein expression of p53, p21 and Bax was significantly increased in pregnancies complicated by PE. Conversely, Mdm2 protein levels were significantly depleted in PE; immunohistochemistry showed these changes to be confined to trophoblast. Reduction in the negative feedback of p53 by Mdm2, using siRNA and Nutlin-3, caused an imbalance between p53 and Mdm2 that triggered apoptosis in term villous explants. In the case of Nutlin, this was attenuated by Pifithrin-α.

CONCLUSIONS

These data illustrate the potential for an imbalance in p53 and Mdm2 expression to promote excessive apoptosis in villous trophoblast. The upstream regulation of p53 and Mdm2, with regard to exaggerated apoptosis and autophagy in PE, merits further investigation.

Codon 104 variation of p53 gene provides adaptive apoptotic responses to extreme environments in mammals of the Tibet plateau

Mutational changes in p53 correlate well with tumorigenesis. Remarkably, however, relatively little is known about the role that p53 variations may play in environmental adaptation. Here we report that codon asparagine-104 (104N) and glutamic acid-104 (104E), respectively, of the p53 gene in the wild zokor (Myospalax baileyi) and root vole (Microtus oeconomus) are adaptively variable, meeting the environmental stresses of the Tibetan plateau. They differ from serine-104 (104S) seen in other rodents, including the lowland subterranean zokor Myospalax cansus, and from serine 106 (106S) in humans. Based on site-directed mutational analysis in human cell lines, the codon 104N variation in M. baileyi is responsible for the adaptive balance of the transactivation of apoptotic genes under hypoxia, cold, and acidic stresses. The 104E p53 variant in Microtus oeconomus suppresses apoptotic gene transactivation and cell apoptosis. Neither 104N nor 104E affects the cell-cycle genes. We propose that these variations in p53 codon 104 are an outcome of environmental adaptation and evolutionary selection that enhance cellular strategies for surviving the environmental stresses of hypoxia and cold (in M. baileyi and M. oeconomus) and hypercapnia (in M. baileyi) in the stressful environments of the Qinghai-Tibet plateau.

Hypoxia-induced cytotoxic drug resistance in osteosarcoma is independent of HIF-1Alpha

Survival rates from childhood cancer have improved dramatically in the last 40 years, such that over 80% of children are now cured. However in certain subgroups, including metastatic osteosarcoma, survival has remained stubbornly poor, despite dose intensive multi-agent chemotherapy regimens, and new therapeutic approaches are needed. Hypoxia is common in adult solid tumours and is associated with treatment resistance and poorer outcome. Hypoxia induces chemotherapy resistance in paediatric tumours including neuroblastoma, rhabdomyosarcoma and Ewing’s sarcoma, in vitro, and this drug resistance is dependent on the oxygen-regulated transcription factor hypoxia inducible factor-1 (HIF-1). In this study the effects of hypoxia on the response of the osteosarcoma cell lines 791T, HOS and U2OS to the clinically relevant cytotoxics cisplatin, doxorubicin and etoposide were evaluated. Significant hypoxia-induced resistance to all three agents was seen in all three cell lines and hypoxia significantly reduced drug-induced apoptosis. Hypoxia also attenuated drug-induced activation of p53 in the p53 wild-type U2OS osteosarcoma cells. Drug resistance was not induced by HIF-1α stabilisation in normoxia by cobalt chloride nor reversed by the suppression of HIF-1α in hypoxia by shRNAi, siRNA, dominant negative HIF or inhibition with the small molecule NSC-134754, strongly suggesting that hypoxia-induced drug resistance in osteosarcoma cells is independent of HIF-1α. Inhibition of the phosphoinositide 3-kinase (PI3K) pathway using the inhibitor PI-103 did not reverse hypoxia-induced drug resistance, suggesting the hypoxic activation of Akt in osteosarcoma cells does not play a significant role in hypoxia-induced drug resistance. Targeting hypoxia is an exciting prospect to improve current anti-cancer therapy and combat drug resistance. Significant hypoxia-induced drug resistance in osteosarcoma cells highlights the potential importance of hypoxia as a target to reverse drug resistance in paediatric osteosarcoma. The novel finding of HIF-1α independent drug resistance suggests however other hypoxia related targets may be more relevant in paediatric osteosarcoma.

CRHR1 mediates p53 transcription induced by high altitude hypoxia through ERK 1/2 signaling in rat hepatic cells

We have previously reported that hypoxia activates corticotrophin-releasing hormone (CRH) and the expression of its type-1 receptor (CRHR1) and induces disorders of the brain-endocrine-immune network. p53 is activated by hypoxia and involved in tumorigenesis and apoptosis. Whether CRHR1 regulates p53 transactivation to further influence apoptotic genes remains unclear. Here, we showed that hypoxia at a simulated altitude of 5km or 7km for 8 and 24h increased p53 protein and mRNA, and reduced apoptotic bax and IGFBP3 gene expression while upregulating the cell-arrest gene p21 for 8h in rat liver cells. The upregulation of p53 mRNA and downregulation of bax mRNA induced by hypoxia were blocked by pretreatment with the specific CRHR1 antagonist CP-154,526, but the downregulation of IGFBP3 and upregulation of p21 mRNA were not. Furthermore, CRH stimulated p53 mRNA via the ERK 1/2 pathway in the BRL-3A cell line and this was blocked by the ERK 1/2 antagonist U0126. These data provide novel evidence that the CRHR1-triggered ERK 1/2 pathway is involved in the activation of p53 and suppression of the apoptotic bax gene by hypoxia in rat liver.

Cotransfection of survivin and CD44v3 short hairpin RNAs affects proliferation, apoptosis, and invasiveness of colorectal cancer

INTRODUCTION

Colorectal cancer is one of the common malignant tumors in humans, and the incidence rate is gradually increasing year by year. Survivin and CD44v3 are ideal targets for gene therapy due to their overexpression in colorectal cells. Studies show that downregulation of survivin could promote apoptosis and depress proliferation, and reduction of CD44v3 expression could inhibit tumor invasive capacity. It is difficult to achieve satisfactory curative effect.

OBJECTIVE

In this study, we use survivin and CD44v3 short hairpin RNAs (shRNA) combined transfection into colorectal cancer cell line SW480 to investigate its effects on the cell apoptosis, proliferation and invasiveness.

METHODS

ShRNA plasmids targeting survivin and CD44v3 were singly or co-transfected into SW480 cells.

RESULTS

The co-transfection group exhibited the most significant inhibitory effect on cell growth (P < 0.05) and the highest apoptosis rate (P < 0.05). In addition, the invasive capacity in the co-transfected group was the least. The tumor inhibition rate of the cotransfected group in xenograft tumor mice was significantly higher than other groups (P < 0.05). Moreover, the microvessel density of the co-transfected group was significantly decreased compared with other groups (P < 0.05).

CONCLUSION

These results suggest combined transfection of survivin shRNA and CD44v3 shRNA may produce a synergistic effect on gene therapy in colorectal cancer.

p53 and cell cycle dependent transcription of kinesin family member 23 (KIF23) is controlled via a CHR promoter element bound by DREAM and MMB complexes

The microtubule-dependent molecular motor KIF23 (Kinesin family member 23) is one of two components of the centralspindlin complex assembled during late stages of mitosis. Formation of this complex is known as an essential step for cytokinesis. Here, we identified KIF23 as a new transcriptional target gene of the tumor suppressor protein p53. We showed that p53 reduces expression of KIF23 on the mRNA as well as the protein level in different cell types. Promoter reporter assays revealed that this repression results from downregulation of KIF23 promoter activity. CDK inhibitor p21^WAF1/CIP1 was shown to be necessary to mediate p53-dependent repression. Furthermore, we identified the highly conserved cell cycle genes homology region (CHR) in the KIF23 promoter to be strictly required for p53-dependent repression as well as for cell cycle-dependent expression of KIF23. Cell cycle- and p53-dependent regulation of KIF23 appeared to be controlled by differential binding of DREAM and MMB complexes to the CHR element. With this study, we describe a new mechanism for transcriptional regulation of KIF23. Considering the strongly supporting function of KIF23 in cytokinesis, its p53-dependent repression may contribute to the prevention of uncontrolled cell growth.

The contribution of N₂O₃ to the cytotoxicity of the nitric oxide donor DETA/NO: an emerging role for S-nitrosylation

The relationship between the biological activity of NO and its chemistry is complex. The objectives of this study were to investigate the influence of oxygen tension on the cytotoxicity of the NO^• donor DETA/NO and to determine the effects of oxygen tension on the key RNS (reactive nitrogen species) responsible for any subsequent toxicity. The findings presented in this study indicate that the DETA/NO-mediated cytotoxic effects were enhanced under hypoxic conditions. Further investigations revealed that neither ONOO⁻ (peroxynitrite) nor nitroxyl was generated. Fluorimetric analysis in the presence of scavengers suggest for the first time that another RNS, dinitrogen trioxide may be responsible for the cytotoxicity with DETA/NO. Results showed destabilization of HIF (hypoxia inducible factor)-1α and depletion of GSH levels following the treatment with DETA/NO under hypoxia, which renders cells more susceptible to DETA/NO cytotoxicity, and could account for another mechanism of DETA/NO cytotoxicity under hypoxia. In addition, there was significant accumulation of nuclear p53, which showed that p53 itself might be a target for S-nitrosylation following the treatment with DETA/NO. Both the intrinsic apoptotic pathway and the Fas extrinsic apoptotic pathway were also activated. Finally, GAPDH (glyceraldehyde-3-phosphate dehydrogenase) is another important S-nitrosylated protein that may possibly play a key role in DETA/NO-mediated apoptosis and cytotoxicity. Therefore this study elucidates further mechanisms of DETA/NO mediated cytotoxicity with respect to S-nitrosylation that is emerging as a key player in the signalling and detection of DETA/NO-modified proteins in the tumour microenvironment.

Effects of HDM2 antagonism on sunitinib resistance, p53 activation, SDF-1 induction, and tumor infiltration by CD11b+/Gr-1+ myeloid derived suppressor cells

Background

The studies reported herein were undertaken to determine if the angiostatic function of p53 could be exploited as an adjunct to VEGF-targeted therapy in the treatment of renal cell carcinoma (RCC).

Methods

Nude/beige mice bearing human RCC xenografts were treated with various combinations of sunitinib and the HDM2 antagonist MI-319. Tumors were excised at various time points before and during treatment and analyzed by western blot and IHC for evidence of p53 activation and function.

Results

Sunitinib treatment increased p53 levels in RCC xenografts and transiently induced the expression of p21^waf1, Noxa, and HDM2, the levels of which subsequently declined to baseline (or undetectable) with the emergence of sunitinib resistance. The development of resistance and the suppression of p53-dependent gene expression temporally correlated with the induction of the p53 antagonist HDMX. The concurrent administration of MI-319 markedly increased the antitumor and anti-angiogenic activities of sunitinib and led to sustained p53-dependent gene expression. It also suppressed the expression of the chemokine SDF-1 (CXCL12) and the influx of CD11b⁺/Gr-1⁺ myeloid-derived suppressor cells (MDSC) otherwise induced by sunitinib. Although p53 knockdown markedly reduced the production of the angiostatic peptide endostatin, the production of endostatin was not augmented by MI-319 treatment.

Conclusions

The evasion of p53 function (possibly through the expression of HDMX) is an essential element in the development of resistance to VEGF-targeted therapy in RCC. The maintenance of p53 function through the concurrent administration of an HDM2 antagonist is an effective means of delaying or preventing the development of resistance.

Rate of environmental change determines stress response specificity

Cells use general stress response pathways to activate diverse target genes in response to a variety of stresses. However, general stress responses coexist with more specific pathways that are activated by individual stresses, provoking the fundamental question of whether and how cells control the generality or specificity of their response to a particular stress. Here we address this issue using quantitative time-lapse microscopy of the Bacillus subtilis environmental stress response, mediated by σ(B). We analyzed σ(B) activation in response to stresses such as salt and ethanol imposed at varying rates of increase. Dynamically, σ(B) responded to these stresses with a single adaptive activity pulse, whose amplitude depended on the rate at which the stress increased. This rate-responsive behavior can be understood from mathematical modeling of a key negative feedback loop in the underlying regulatory circuit. Using RNAseq we analyzed the effects of both rapid and gradual increases of ethanol and salt stress across the genome. Because of the rate responsiveness of σ(B) activation, salt and ethanol regulons overlap under rapid, but not gradual, increases in stress. Thus, the cell responds specifically to individual stresses that appear gradually, while using σ(B) to broaden the cellular response under more rapidly deteriorating conditions. Such dynamic control of specificity could be a critical function of other general stress response pathways.

MicroRNAs as putative mediators of treatment response in prostate cancer

MicroRNAs (miRNAs) are an abundant class of noncoding RNAs that function to regulate post-transcriptional gene expression, predominantly by translational repression. In addition to their role in prostate cancer initiation and progression, recent evidence suggests that miRNAs might also participate in treatment response across a range of therapies including radiation treatment, chemotherapy and androgen suppression. The mechanism of this regulation is thought to be multifactorial and is currently poorly understood. To date, only a small number of studies have examined the functional role of miRNAs in response to prostate cancer treatment. Elucidating the role of miRNAs in treatment response following radiotherapy, chemotherapy and androgen suppression will provide new avenues of investigation for the development of novel therapies for the treatment of prostate cancer.

Hypoxia activates tumor suppressor p53 by inducing ATR-Chk1 kinase cascade-mediated phosphorylation and consequent 14-3-3γ inactivation of MDMX protein

It has been known that p53 can be induced and activated by hypoxia, an abnormal condition that often occurs in rapidly growing solid tumors or when normal tissues undergo ischemia. Although the ATR-Chk1 kinase cascade was associated with hypoxia-induced p53 activation, molecules that directly link this hypoxia-ATR-Chk1 pathway to p53 activation have been elusive. Here, we showed that hypoxia could induce phosphorylation of MDMX at Ser-367 and enhance the binding of this phosphorylated MDMX to 14-3-3γ, consequently leading to p53 activation. A Chk1 inhibitor or knockdown of ATR and Chk1 inhibited the phosphorylation of MDMX at Ser-367 and impaired the binding of MDMX to 14-3-3γ in addition to p53 activation in response to hypoxia. In primary mouse embryonic fibroblast cells that harbor a mutant MDMX, including the S367A mutation, hypoxia also failed to induce the binding of this mutant MDMX to 14-3-3γ and to activate p53 and its direct targets. These results demonstrate that hypoxia can activate p53 through inactivation of MDMX by the ATR-Chk1-MDMX-14-3-3γ pathway.

Metabolic reprogramming of the tumor

Cancer is classically considered as a genetic and, more recently, epigenetic multistep disease. Despite seminal studies in the 1920s by Warburg showing a characteristic metabolic pattern for tumors, cancer bioenergetics has often been relegated to the backwaters of cancer biology. This review aims to provide a historical account on cancer metabolism research, and to try to integrate and systematize the metabolic strategies in which cancer cells engage to overcome selective pressures during their inception and evolution. Implications of this renovated view on some common concepts and in therapeutics are also discussed.

Overexpression of Bcl2 in osteoblasts inhibits osteoblast differentiation and induces osteocyte apoptosis

Bcl2 subfamily proteins, including Bcl2 and Bcl-X_L, inhibit apoptosis. As osteoblast apoptosis is in part responsible for osteoporosis in sex steroid deficiency, glucocorticoid excess, and aging, bone loss might be inhibited by the upregulation of Bcl2; however, the effects of Bcl2 overexpression on osteoblast differentiation and bone development and maintenance have not been fully investigated. To investigate these issues, we established two lines of osteoblast-specific BCL2 transgenic mice. In BCL2 transgenic mice, bone volume was increased at 6 weeks of age but not at 10 weeks of age compared with wild-type mice. The numbers of osteoblasts and osteocytes increased, but osteoid thickness and the bone formation rate were reduced in BCL2 transgenic mice with high expression at 10 weeks of age. The number of BrdU-positive cells was increased but that of TUNEL-positive cells was unaltered at 2 and 6 weeks of age. Osteoblast differentiation was inhibited, as shown by reduced Col1a1 and osteocalcin expression. Osteoblast differentiation of calvarial cells from BCL2 transgenic mice also fell in vitro. Overexpression of BCL2 in primary osteoblasts had no effect on osteoclastogenesis in co-culture with bone marrow cells. Unexpectedly, overexpression of BCL2 in osteoblasts eventually caused osteocyte apoptosis. Osteocytes, which had a reduced number of processes, gradually died with apoptotic structural alterations and the expression of apoptosis-related molecules, and dead osteocytes accumulated in cortical bone. These findings indicate that overexpression of BCL2 in osteoblasts inhibits osteoblast differentiation, reduces osteocyte processes, and causes osteocyte apoptosis.

The three-layer concentric model of glioblastoma: cancer stem cells, microenvironmental regulation, and therapeutic implications

Tumors arising in the central nervous system are thought to originate from a sub-population of cells named cancer stem cells (CSCs) or tumor initiating cells (TICs) that possess an immature phenotype, combined with self-renewal and chemotherapy resistance capacity. Moreover, in the last years, these cells have been identified in particular brain tumor niches fundamental for supporting their characteristics. In this paper, we report studies from many authors demonstrating that hypoxia or the so called “hypoxic niche” plays a crucial role in controlling CSC molecular and phenotypic profile. We recently investigated the relationship existing between Glioblastoma (GBM) stem cells and their niche, defining the theory of three-concentric layers model for GBM mass. According to this model, GBM stem cells reside preferentially within the hypoxic core of the tumour mass, while more differentiated cells are mainly localized along the peripheral and vascularized part of the tumour. This GBM model provides explanation of the effects mediated by the tumour microenvironment on the phenotypic and molecular regulation of GBM stem cells, describing their spatial distribution in the tumor bulk. Moreover, we discuss the possible clinical implications of the creation of this model for future GBM patient management and novel therapeutic strategies development.

Differential modulatory effects of GSK-3β and HDM2 on sorafenib-induced AIF nuclear translocation (programmed necrosis) in melanoma

Background

GSK-3β phosphorylates numerous substrates that govern cell survival. It phosphorylates p53, for example, and induces its nuclear export, HDM2-dependent ubiquitination, and proteasomal degradation. GSK-3β can either enhance or inhibit programmed cell death, depending on the nature of the pro-apoptotic stimulus. We previously showed that the multikinase inhibitor sorafenib activated GSK-3β and that this activation attenuated the cytotoxic effects of the drug in various BRAF-mutant melanoma cell lines. In this report, we describe the results of studies exploring the effects of GSK-3β on the cytotoxicity and antitumor activity of sorafenib combined with the HDM2 antagonist MI-319.

Results

MI-319 alone increased p53 levels and p53-dependent gene expression in melanoma cells but did not induce programmed cell death. Its cytotoxicity, however, was augmented in some melanoma cell lines by the addition of sorafenib. In responsive cell lines, the MI-319/sorafenib combination induced the disappearance of p53 from the nucleus, the down modulation of Bcl-2 and Bcl-x_L, the translocation of p53 to the mitochondria and that of AIF to the nuclei. These events were all GSK-3β-dependent in that they were blocked with a GSK-3β shRNA and facilitated in otherwise unresponsive melanoma cell lines by the introduction of a constitutively active form of the kinase (GSK-3β-S9A). These modulatory effects of GSK-3β on the activities of the sorafenib/MI-319 combination were the exact reverse of its effects on the activities of sorafenib alone, which induced the down modulation of Bcl-2 and Bcl-x_L and the nuclear translocation of AIF only in cells in which GSK-3β activity was either down modulated or constitutively low. In A375 xenografts, the antitumor effects of sorafenib and MI-319 were additive and associated with the down modulation of Bcl-2 and Bcl-x_L, the nuclear translocation of AIF, and increased suppression of tumor angiogenesis.

Conclusions

Our data demonstrate a complex partnership between GSK-3β and HDM2 in the regulation of p53 function in the nucleus and mitochondria. The data suggest that the ability of sorafenib to activate GSK-3β and alter the intracellular distribution of p53 may be exploitable as an adjunct to agents that prevent the HDM2-dependent degradation of p53 in the treatment of melanoma.

Interaction of p53 with tumor suppressive and oncogenic signaling pathways to control cellular reactive oxygen species production

p53 is a crucial transcription factor with tumor suppressive properties that elicits its function through specific target genes. It constitutes a pivotal system that integrates information received by many signaling pathways and subsequently orchestrates cell fate decisions, namely, growth-arrest, senescence, or apoptosis. Reactive oxygen species (ROS) production in cells can play a key role in signal transduction, being able to trigger different processes as cell death or cell proliferation. Sustained oxidative stress can induce genomic instability and collaborates with cancer development, whereas acute enhancement of high ROS levels leads to toxic oxidative cell damage and cell death. Here, it has been considered p53 broad potential contribution through its ability to regulate selected key cancer signaling pathways, where ROS participate as inductors or effectors of the final biological outcome. Further, we have discussed how p53 could play a role in preventing potentially harmful oxidative state and cell proliferation by pro-oncogenic pathways such as PI3K/AKT/mTOR and WNT/β-catenin or under hypoxia state. In addition, we have considered potential mechanisms by which p53 could collaborate with signal transduction pathways such as transforming growth factor-β (TGF-β) and stress-activated protein kinases (SAPK) that produce ROS, to stop or eliminate uncontrolled proliferating cells.

microRNA-210 is upregulated in hypoxic cardiomyocytes through Akt- and p53-dependent pathways and exerts cytoprotective effects

microRNA-210 (miR-210) is upregulated in hypoxia, but its function in cardiomyocytes and its regulation in response to hypoxia are not well characterized. The purpose of this study was to identify upstream regulators of miR-210, as well as to characterize miR-210's function in cardiomyocytes. We first showed miR-210 is upregulated through both hypoxia-inducible factor (HIF)-dependent and -independent pathways, since aryl hydrocarbon nuclear translocator (ARNT) knockout mouse embryonic fibroblasts (MEF), lacking intact HIF signaling, still displayed increased miR-210 levels in hypoxia. To determine the mechanism for HIF-independent regulation of miR-210, we focused on p53 and protein kinase B (Akt). Overexpression of p53 in wild-type MEFs induced miR-210, whereas p53 overexpression in ARNT knockout MEFs did not, suggesting p53 regulates miR-210 in a HIF-dependent mechanism. Akt inhibition reduced miR-210 induction by hypoxia, whereas Akt overexpression increased miR-210 levels in both wild-type and ARNT knockout MEFs, indicating Akt regulation of miR-210 is HIF-independent. We then studied the effects of miR-210 in cardiomyocytes. Overexpression of miR-210 reduced cell death in response to oxidative stress and reduced reactive oxygen species (ROS) production both at baseline and after treatment with antimycin A. Furthermore, downregulation of miR-210 increased ROS after hypoxia-reoxygenation. To determine a mechanism for the cytoprotective effects of miR-210, we focused on the predicted target, apoptosis-inducing factor, mitochondrion-associated 3 (AIFM3), known to induce cell death. Although miR-210 reduced AIFM3 levels, overexpression of AIFM3 in the presence of miR-210 overexpression did not reduce cellular viability either at baseline or after hydrogen peroxide treatment, suggesting AIFM3 does not mediate miR-210's cytoprotective effects. Furthermore, HIF-3α, a negative regulator of HIF signaling, is targeted by miR-210, but miR-210 does not modulate HIF activity. In conclusion, we demonstrate a novel role for p53 and Akt in regulating miR-210 and demonstrate that, in cardiomyocytes, miR-210 exerts cytoprotective effects, potentially by reducing mitochondrial ROS production.

Reactive oxygen species mediate arsenic induced cell transformation and tumorigenesis through Wnt/β-catenin pathway in human colorectal adenocarcinoma DLD1 cells

Long term exposure to arsenic can increase incidence of human cancers, such as skin, lung, and colon rectum. The mechanism of arsenic induced carcinogenesis is still unclear. It is generally believed that reactive oxygen species (ROS) may play an important role in this process. In the present study, we investigate the possible linkage between ROS, β-catenin and arsenic induced transformation and tumorigenesis in human colorectal adenocarcinoma cell line, DLD1 cells. Our results show that arsenic was able to activate p47(phox) and p67(phox), two key proteins for activation of NADPH oxidase. Arsenic was also able to generate ROS in DLD1 cells. Arsenic increased β-catenin expression level and its promoter activity. ROS played a major role in arsenic-induced β-catenin activation. Treatment of DLD1 cells by arsenic enhanced both transformation and tumorigenesis of these cells. The tumor volumes of arsenic treated group were much larger than those without arsenic treatment. Addition of either superoxide dismutase (SOD) or catalase reduced arsenic induced cell transformation and tumor formation. The results indicate that ROS are involved in arsenic induced cell transformation and tumor formation possible through Wnt/β-catenin pathway in human colorectal adenocarcinoma cell line DLD1 cells.

p53 directly suppresses BNIP3 expression to protect against hypoxia-induced cell death

Hypoxia stabilizes the tumour suppressor p53, allowing it to function primarily as a transrepressor; however, the function of p53 during hypoxia remains unclear. In this study, we showed that p53 suppressed BNIP3 expression by directly binding to the p53-response element motif and recruiting corepressor mSin3a to the BNIP3 promoter. The DNA-binding site of p53 must remain intact for the protein to suppress the BNIP3 promoter. In addition, taking advantage of zebrafish as an in vivo model, we confirmed that zebrafish nip3a, a homologous gene of mammalian BNIP3, was indeed induced by hypoxia and p53 mutation/knockdown enhanced nip3a expression under hypoxia resulted in cell death enhancement in p53 mutant embryos. Furthermore, p53 protected against hypoxia-induced cell death mediated by p53 suppression of BNIP3 as illustrated by p53 knockdown/loss assays in both human cell lines and zebrafish model, which is in contrast to the traditional pro-apoptotic role of p53. Our results suggest a novel function of p53 in hypoxia-induced cell death, leading to the development of new treatments for ischaemic heart disease and cerebral stroke.

Reciprocal influence of the p53 and the hypoxic pathways

When cells sense a decrease in oxygen availability (hypoxia), they develop adaptive responses in order to sustain this condition and survive. If hypoxia lasts too long or is too severe, the cells eventually die. Hypoxia is also known to modulate the p53 pathway, in a manner dependent or not of HIF-1 (hypoxia-inducible factor-1), the main transcription factor activated by hypoxia. The p53 protein is a transcription factor, which is rapidly stabilised by cellular stresses and which has a major role in the cell responses to these stresses. The aim of this review is to compile what has been reported until now about the interconnection between these two important pathways. Indeed, according to the cell line, the severity and the duration of hypoxia, oxygen deficiency influences very differently p53 protein level and activity. Conversely, p53 is also described to affect HIF-1α stability, one of the two subunits of HIF-1, and HIF-1 activity. The direct and indirect interactions between HIF-1α and p53 are described as well as the involvement in this complex network of their respective ubiquitin ligases von Hippel Lindau protein and murine double minute 2. Finally, the synergistic or antagonistic effects of p53 and HIF-1 on some important cellular pathways are discussed.

Targeting survivin in cancer: the cell-signalling perspective

Survivin, a prominent anticancer target, is ubiquitously expressed in a plethora of cancers and the evolving complexity in functional regulation of survivin is yet to be deciphered. However, pertaining to the recent studies, therapeutic modulation of survivin is critically regulated by interaction with prominent cell-signalling pathways [HIF-1α, HSP90, PI3K/AKT, mTOR, ERK, tumour suppressor genes (p53, PTEN), oncogenes (Bcl-2, Ras)] and a wide range of growth factors (EGFR, VEGF, among others). In our article we discuss, in detail, an overview of the recent developments in the pharmacological modulation of survivin via cell-signalling paradigms and antisurvivin therapeutics, along with an outlook on therapeutic management of survivin in drug-resistant cancers.

Graded enhancement of p53 binding to CREB-binding protein (CBP) by multisite phosphorylation

The transcriptional activity of p53 is regulated by a cascade of posttranslational modifications. Although acetylation of p53 by CREB-binding protein (CBP)/p300 is known to be indispensable for p53 activation, the role of phosphorylation, and in particular multisite phosphorylation, in activation of CBP/p300-dependent p53 transcriptional pathways remains unclear. We investigated the role of single site and multiple site phosphorylation of the p53 transactivation domain in mediating its interaction with CBP and with the ubiquitin ligase HDM2. Phosphorylation at Thr18 functions as an on/off switch to regulate binding to the N-terminal domain of HDM2. In contrast, binding to CBP is modulated by the extent of p53 phosphorylation; addition of successive phosphoryl groups enhances the affinity for the TAZ1, TAZ2, and KIX domains of CBP in an additive manner. Activation of p53-dependent transcriptional pathways requires that p53 compete with numerous cellular transcription factors for binding to limiting amounts of CBP/p300. Multisite phosphorylation represents a mechanism for a graded p53 response, with each successive phosphorylation event resulting in increasingly efficient recruitment of CBP/p300 to p53-regulated transcriptional programs, in the face of competition from cellular transcription factors. Multisite phosphorylation thus acts as a rheostat to enhance binding to CBP/p300 and provides a plausible mechanistic explanation for the gradually increasing p53 response observed following prolonged or severe genotoxic stress.

Improved diabetic wound healing through topical silencing of p53 is associated with augmented vasculogenic mediators

Diabetes is characterized by several poorly understood phenomena including dysfunctional wound healing and impaired vasculogenesis. p53, a master cell cycle regulator, is upregulated in diabetic wounds and has recently been shown to play a regulatory roles in vasculogenic pathways. We have previously described a novel method to topically silence target genes in a wound bed with small interfering (si)RNA. We hypothesized that silencing p53 results in improved diabetic wound healing and augmentation of vasculogenic mediators. Paired 4-mm stented wounds were created on diabetic db/db mice. Topically applied p53 siRNA, evenly distributed in an agarose matrix, was applied to wounds at postwound day 1 and 7 (matrix alone and nonsense siRNA served as controls). Animals were sacrificed at postwound days 10 and 24. Wound time to closure was photometrically assessed, and wounds were harvested for histology, immunohistochemistry, and immunofluorescence. Vasculogenic cytokine expression was evaluated via Western blot, reverse transcription-polymerase chain reaction, and enzyme-linked immunosorbent assay. The ANOVA/t-test was used to determine significance (p≤ 0.05). Local p53 silencing resulted in faster wound healing with wound closure at 18±1.3 d in the treated group vs. 28±1.0 d in controls. The treated group demonstrated improved wound architecture at each time point while demonstrating near-complete local p53 knockdown. Moreover, treated wounds showed a 1.92-fold increase in CD31 endothelial cell staining over controls. Western blot analysis confirmed near-complete p53 knockdown in treated wounds. At day 10, VEGF secretion (enzyme-linked immunosorbent assay) was significantly increased in treated wounds (109.3±13.9 pg/mL) vs. controls (33.0±3.8 pg/mL) while reverse transcription-polymerase chain reaction demonstrated a 1.86-fold increase in SDF-1 expression in treated wounds vs. controls. This profile was reversed after the treated wounds healed and before closure of controls (day 24). Augmented vasculogenic cytokine profile and endothelial cell markers are associated with improved diabetic wound healing in topical gene therapy with p53 siRNA.

The relevance of protein-protein interactions for p53 function: the CPE contribution

The relevance of p53 as a tumour suppressor is evident from the fact that more than 50% of the human cancers hold mutations in the gene coding for p53, and of the remaining cancers a considerable number have alterations in the p53 pathway. From its discovery 30 years ago, the importance of p53 as an essential transcription factor for tumour suppression has become clear. More recently, new and seemingly diverse roles of p53 have been discovered. It soon became clear that protein-protein interactions play an important role in the regulation of the p53 function at different levels. Here we review the contribution by Prof. Fersht and his group towards understanding the basis and functional relevance of p53 protein-protein interactions, and the important role that protein science, biophysics and structural biology have played in the science produced in the Centre for Protein Engineering over the years.

Dietary hyperglycemia, glycemic index and metabolic retinal diseases

The glycemic index (GI) indicates how fast blood glucose is raised after consuming a carbohydrate-containing food. Human metabolic studies indicate that GI is related to patho-physiological responses after meals. Compared with a low-GI meal, a high-GI meal is characterized with hyperglycemia during the early postprandial stage (0-2h) and a compensatory hyperlipidemia associated with counter-regulatory hormone responses during late postprandial stage (4-6h). Over the past three decades, several human health disorders have been related to GI. The strongest relationship suggests that consuming low-GI foods prevents diabetic complications. Diabetic retinopathy (DR) is a complication of diabetes. In this aspect, GI appears to be useful as a practical guideline to help diabetic people choose foods. Abundant epidemiological evidence also indicates positive associations between GI and risk for type 2 diabetes, cardiovascular disease, and more recently, age-related macular degeneration (AMD) in people without diabetes. Although data from randomized controlled intervention trials are scanty, these observations are strongly supported by evolving molecular mechanisms which explain the pathogenesis of hyperglycemia. This wide range of evidence implies that dietary hyperglycemia is etiologically related to human aging and diseases, including DR and AMD. In this context, these diseases can be considered as metabolic retinal diseases. Molecular theories that explain hyperglycemic pathogenesis involve a mitochondria-associated pathway and four glycolysis-associated pathways, including advanced glycation end products formation, protein kinase C activation, polyol pathway, and hexosamine pathway. While the four glycolysis-associated pathways appear to be universal for both normoxic and hypoxic conditions, the mitochondria-associated mechanism appears to be most relevant to the hyperglycemic, normoxic pathogenesis. For diseases that affect tissues with highly active metabolism and that frequently face challenge from low oxygen tension, such as retina in which metabolism is determined by both glucose and oxygen homeostases, these theories appear to be insufficient. Several lines of evidence indicate that the retina is particularly vulnerable when hypoxia coincides with hyperglycemia. We propose a novel hyperglycemic, hypoxia-inducible factor (HIF) pathway, to complement the current theories regarding hyperglycemic pathogenesis. HIF is a transcription complex that responds to decrease oxygen in the cellular environment. In addition to playing a significant role in the regulation of glucose metabolism, under hyperglycemia HIF has been shown to increase the expression of HIF-inducible genes, such as vascular endothelial growth factor (VEGF) leading to angiogenesis. To this extent, we suggest that HIF can also be described as a hyperglycemia-inducible factor. In summary, while management of dietary GI appears to be an effective intervention for the prevention of metabolic diseases, specifically AMD and DR, more interventional data is needed to evaluate the efficacy of GI management. There is an urgent need to develop reliable biomarkers of exposure, surrogate endpoints, as well as susceptibility for GI. These insights would also be helpful in deciphering the detailed hyperglycemia-related biochemical mechanisms for the development of new therapeutic agents.

Hypoxia-regulated angiogenic inhibitors

The regulation of angiogenesis by hypoxia is an essential homeostatic mechanism that depends on a precise balance between positive and negative angiogenic regulatory molecules. Proangiogenic factors are well characterized; however, several in vivo and in vitro studies indicate that there are feedback mechanisms in place to inhibit angiogenesis during hypoxia. Understanding the signaling pathways leading to the negative feedback of angiogenesis will undoubtedly provide important tools to develop novel therapeutic strategies not only to enhance the angiogenic response in coronary artery disease but also to hinder deregulated angiogenesis in tumorigenesis.

Stress-specific response of the p53-Mdm2 feedback loop

Background

The p53 signalling pathway has hundreds of inputs and outputs. It can trigger cellular senescence, cell-cycle arrest and apoptosis in response to diverse stress conditions, including DNA damage, hypoxia and nutrient deprivation. Signals from all these inputs are channeled through a single node, the transcription factor p53. Yet, the pathway is flexible enough to produce different downstream gene expression patterns in response to different stresses.

Results

We construct a mathematical model of the negative feedback loop involving p53 and its inhibitor, Mdm2, at the core of this pathway, and use it to examine the effect of different stresses that trigger p53. In response to DNA damage, hypoxia, etc., the model exhibits a wide variety of specific output behaviour - steady states with low or high levels of p53 and Mdm2, as well as spiky oscillations with low or high average p53 levels.

Conclusions

We show that even a simple negative feedback loop is capable of exhibiting the kind of flexible stress-specific response observed in the p53 system. Further, our model provides a framework for predicting the differences in p53 response to different stresses and single nucleotide polymorphisms.

Novel dyskerin-mediated mechanism of p53 inactivation through defective mRNA translation

In up to 60% of human cancers, p53 gene mutations are responsible for direct inactivation of the tumor suppressor function of p53. Alternative mechanisms of p53 inactivation described thus far mainly affect its posttranslational regulation. In X-linked dyskeratosis congenita, a multisystemic syndrome characterized by increased cancer susceptibility, mutations of the DKC1 gene encoding dyskerin cause a selective defect in the translation of a subgroup of internal ribosome entry site (IRES)-containing cellular mRNAs. In this study, we show that impairment of dyskerin function can cause p53 inactivation due to a defect in p53 mRNA translation. siRNA-mediated reduction of dyskerin levels caused a decrease of p53 mRNA translation, protein levels, and functional activity, both in human breast cancer cells and in primary mammary epithelial progenitor cells. These effects seemed to be independent of the known role of dyskerin in telomerase function, and they were associated with a specific impairment of translation initiation mediated by IRES elements present in p53 mRNA. In a series of human primary breast cancers retaining wild-type p53, we found that low levels of dyskerin expression were associated with reduced expression of p53-positive target genes. Our findings suggest that a dyskerin-mediated mechanism of p53 inactivation may occur in a subset of human tumors.

Single nucleotide polymorphisms in Wnt signaling and cell death pathway genes and susceptibility to colorectal cancer

It is well known that approximately 90% of colorectal cancer (CRC) cases originate from the constitutive activation of the canonical Wnt signaling pathway. There is increasing evidence that genetic variation both in Wnt and apoptotic pathway genes affects CRC susceptibility and progression. This population-based case-control study, including 1795 CRC cases and 1805 controls, investigates the association between common, putative functional polymorphisms in DNFA5, HIF1A, NDRG1, PYGO1, SFRP2, SFRP4, WISP1 and WISP3 genes and CRC risk. We found no evidence for an association between the selected allelic variants and risk of CRC. Subsite analyses, however, revealed a significant association of HIF1A c.*191T>C with rectal cancer risk [odds ratio (OR) = 1.25, 95% confidence interval (CI), 1.03-1.51, P = 0.03] comparing minor allele carriers with major allele homozygotes. In addition, homozygosity for the minor allele of SFRP4 P320T was significantly associated with rectal cancer risk (OR = 1.37, 95% CI, 1.06-1.79, P = 0.02) and early-stage CRC (OR = 1.33, 95% CI, 1.05-1.69, P = 0.02). This study does not support the hypothesis that Wnt signaling- and apoptosis-related polymorphisms contribute to CRC risk. However, our results provide evidence that CRC subsets may be affected. If confirmed, this knowledge may be used to assess individual susceptibility and to target potential measures of cancer prevention.

Protection of cells in physiological oxygen tensions against DNA damage-induced apoptosis

Oxygen availability has important effects on cell physiology. Although hyperoxic and hypoxic stresses have been well characterized, little is known about cellular functions in the oxygen levels commonly found in vivo. Here, we show that p53-dependent apoptosis in response to different DNA-damaging agents was reduced when normal and cancer cells were cultured at physiological oxygen tensions instead of the usual atmospheric levels. Different from what has been described in hypoxia, this was neither determined by decreases in p53 induction or its transactivation activity, nor by differences in the intracellular accumulation of reactive oxygen species. At these physiological oxygen levels, we found a constitutive activation of the ERK1/2 MAPK in all the models studied. Inhibition of this signaling pathway reversed the protective effect in some but not all cell lines. We conclude that a stress-independent constitutive activation of prosurvival pathways, including but probably not limited to MAPK, can protect cells in physiological oxygen tensions against genotoxic stress. Our results underscore the need of considering the impact of oxygen levels present in the tissue microenvironment when studying cell sensitivity to treatments such as chemotherapy and radiotherapy.

Tumour hypoxia induces a metabolic shift causing acidosis: a common feature in cancer

Maintenance of cellular pH homeostasis is fundamental to life. A number of key intracellular pH (pHi) regulating systems including the Na⁺/H⁺ exchangers, the proton pump, the monocarboxylate transporters, the HCO₃⁻ transporters and exchangers and the membrane-associated and cytosolic carbonic anhydrases cooperate in maintaining a pHi that is permissive for cell survival. A common feature of tumours is acidosis caused by hypoxia (low oxygen tension). In addition to oncogene activation and transformation, hypoxia is responsible for inducing acidosis through a shift in cellular metabolism that generates a high acid load in the tumour microenvironment. However, hypoxia and oncogene activation also allow cells to adapt to the potentially toxic effects of an excess in acidosis. Hypoxia does so by inducing the activity of a transcription factor the hypoxia-inducible factor (HIF), and particularly HIF-1, that in turn enhances the expression of a number of pHi-regulating systems that cope with acidosis. In this review, we will focus on the characterization and function of some of the hypoxia-inducible pH-regulating systems and their induction by hypoxic stress. It is essential to understand the fundamentals of pH regulation to meet the challenge consisting in targeting tumour metabolism and acidosis as an anti-tumour approach. We will summarize strategies that take advantage of intracellular and extracellular pH regulation to target the primary tumour and metastatic growth, and to turn around resistance to chemotherapy and radiotherapy.

p53 regulation of metabolic pathways

During the course of tumorigenesis, cells acquire a number of alterations that contribute to the acquisition of the malignant phenotype, allowing them to survive and flourish in increasingly hostile environments. Cancer cells can be characterized by perturbations in the control of cell proliferation and growth, resistance to death, and alterations in their interactions with the microenvironment. Underpinning many of these changes are shifts in metabolism that allow cancer cells to use alternative pathways for energy production and building the macromolecules necessary for growth, as well as regulating the generation of signaling molecules such as reactive oxygen species (ROS). In the past few years, it became clear that p53, the most studied, if not most important, tumor suppressor protein, can also directly control metabolic traits of cells.

Tid1 is a new regulator of p53 mitochondrial translocation and apoptosis in cancer

The p53 tumor suppressor protein induces apoptosis in response to genotoxic and environmental stresses. Recent studies have revealed the existence of a transcription-independent mitochondrial p53 apoptotic pathway; however, the mechanism that regulates its translocation to the mitochondria has been unknown. In this study, we show that the tumor suppressor Tid1 forms a complex with p53 under hypoxic conditions that directs p53 translocation to the mitochondria and the subsequent initiation of the mitochondrial apoptosis pathway. Loss of Tid1 expression abrogated p53 translocation to the mitochondria and inhibited apoptosis, whereas the over-expression of Tid1 promoted p53 mitochondrial localization and apoptosis. Tid1's mitochondrial signal sequence and DnaJ domain were both required for the movement of the p53-Tid1 complex from the cytosol to the mitochondria. When Tid is over-expressed in cancer cell lines expressing mutant p53 isoforms defective in transcriptional activity, mitochondrial localization and pro-apoptotic activities of the mutant p53 proteins was restored. Our results establish Tid1 as a novel regulator of p53-mediated apoptosis, and suggest that therapies designed to enhance Tid1's function in promoting mitochondrial localization of p53 and apoptosis could be an effective therapy in many cancers.

Radiobiological effects of hypoxia-dependent uptake of 64Cu-ATSM: enhanced DNA damage and cytotoxicity in hypoxic cells

PURPOSE

Hypoxia occurs frequently in cancers and can lead to therapeutic resistance due to poor perfusion and loss of the oxygen enhancement effect. (64)Cu-ATSM has shown promise as a hypoxia diagnostic agent due to its selective uptake and retention in hypoxic cells and its emission of positrons for PET imaging. (64)Cu also emits radiotoxic Auger electrons and beta(-) particles and may therefore exhibit therapeutic potential when concentrated in hypoxic tissue.

METHODS

MCF-7 cells were treated with 0-10 MBq/ml (64)Cu-ATSM under differing oxygen conditions ranging from normoxia to severe hypoxia. Intracellular response to hypoxia was measured using Western blotting for expression of HIF-1alpha, while cellular accumulation of (64)Cu was measured by gamma counting. DNA damage and cytotoxicity were measured with, respectively, the Comet assay and clonogenic survival.

RESULTS

(64)Cu-ATSM uptake in MCF-7 cells increased as atmospheric oxygen decreased (up to 5.6 Bq/cell at 20.9% oxygen, 10.4 Bq/cell at 0.1% oxygen and 26.0 Bq/cell at anoxia). Toxicity of (64)Cu-ATSM in MCF-7 cells also increased as atmospheric oxygen decreased, with survival of 9.8, 1.5 and 0% in cells exposed to 10 MBq/ml at 20.9, 0.1 and 0% oxygen. The Comet assay revealed a statistically significant increase in (64)Cu-ATSM-induced DNA damage under hypoxic conditions.

CONCLUSION

The results support a model in which hypoxia-enhanced uptake of radiotoxic (64)Cu induces sufficient DNA damage and toxicity to overcome the documented radioresistance in hypoxic MCF-7 cells. This suggests that (64)Cu-ATSM and related complexes have potential for targeted radionuclide therapy of hypoxic tumours.

Repression of the miR-17-92 cluster by p53 has an important function in hypoxia-induced apoptosis

We here report that miR-17-92 cluster is a novel target for p53-mediated transcriptional repression under hypoxia. We found the expression levels of miR-17-92 cluster were reduced in hypoxia-treated cells containing wild-type p53, but were unchanged in hypoxia-treated p53-deficient cells. The repression of miR-17-92 cluster under hypoxia is independent of c-Myc. Luciferase reporter assays mapped the region responding to p53-mediated repression to a p53-binding site in the proximal region of the miR-17-92 promoter. Chromatin immunoprecipitation (ChIP), Re-ChIP and gel retardation assays revealed that the binding sites for p53- and the TATA-binding protein (TBP) overlap within the miR-17-92 promoter; these proteins were found to compete for binding. Finally, we show that pri-miR-17-92 expression correlated well with p53 status in colorectal carcinomas. Over-express miR-17-92 cluster markedly inhibits hypoxia-induced apoptosis, whereas blocked miR-17-5p and miR-20a sensitize the cells to hypoxia-induced apoptosis. These data indicated that p53-mediated repression of miR-17-92 expression likely has an important function in hypoxia-induced apoptosis, and thus further our understanding of the tumour suppressive function of p53.