Anti-apoptotic activity of low levels of wild-type p53

Metabolic Roles of HIF1, c-Myc, and p53 in Glioma Cells

The metabolic reprogramming that promotes tumorigenesis in glioblastoma is induced by dynamic alterations in the hypoxic tumor microenvironment, as well as in transcriptional and signaling networks, which result in changes in global genetic expression. The signaling pathways PI3K/AKT/mTOR and RAS/RAF/MEK/ERK stimulate cell metabolism, either directly or indirectly, by modulating the transcriptional factors p53, HIF1, and c-Myc. The overexpression of HIF1 and c-Myc, master regulators of cellular metabolism, is a key contributor to the synthesis of bioenergetic molecules that mediate glioma cell transformation, proliferation, survival, migration, and invasion by modifying the transcription levels of key gene groups involved in metabolism. Meanwhile, the tumor-suppressing protein p53, which negatively regulates HIF1 and c-Myc, is often lost in glioblastoma. Alterations in this triad of transcriptional factors induce a metabolic shift in glioma cells that allows them to adapt and survive changes such as mutations, hypoxia, acidosis, the presence of reactive oxygen species, and nutrient deprivation, by modulating the activity and expression of signaling molecules, enzymes, metabolites, transporters, and regulators involved in glycolysis and glutamine metabolism, the pentose phosphate cycle, the tricarboxylic acid cycle, and oxidative phosphorylation, as well as the synthesis and degradation of fatty acids and nucleic acids. This review summarizes our current knowledge on the role of HIF1, c-Myc, and p53 in the genic regulatory network for metabolism in glioma cells, as well as potential therapeutic inhibitors of these factors.

VesiX cetylpyridinium chloride is rapidly bactericidal and reduces uropathogenic Escherichia coli bladder epithelial cell invasion in vitro

Management of urinary tract infection (UTI) in postmenopausal women can be challenging. The recent rise in resistance to most of the available oral antibiotic options together with high recurrence rate in postmenopausal women has further complicated treatment of UTI. As such, intravesical instillations of antibiotics like gentamicin are being investigated as an alternative to oral antibiotic therapies. This study evaluates the efficacy of the candidate intravesical therapeutic VesiX, a solution containing the cationic detergent Cetylpyridinium chloride, against a broad range of uropathogenic bacterial species clinically isolated from postmenopausal women with recurrent UTI (rUTI). We also evaluate the cytotoxicity of VesiX against cultured bladder epithelial cells and find that low concentrations of 0.0063% and 0.0125% provide significant bactericidal effect toward diverse bacterial species including uropathogenic Escherichia coli (UPEC), Klebsiella pneumoniae, Enterococcus faecalis, Pseudomonas aeruginosa, and Proteus mirabilis while minimizing cytotoxic effects against cultured 5637 bladder epithelial cells. Lastly, to begin to evaluate the potential utility of using VesiX in combination therapy with existing intravesical therapies for rUTI, we investigate the combined effects of VesiX and the intravesical antibiotic gentamicin. We find that VesiX and gentamicin are not antagonistic and are able to reduce levels of intracellular UPEC in cultured bladder epithelial cells.

IMPORTANCE

When urinary tract infections (UTIs), which affect over 50% of women, become resistant to available antibiotic therapies dangerous complications like kidney infection and lethal sepsis can occur. New therapeutic paradigms are needed to expand our arsenal against these difficult to manage infections. Our study investigates VesiX, a Cetylpyridinium chloride (CPC)-based therapeutic, as a candidate broad-spectrum antimicrobial agent for use in bladder instillation therapy for antibiotic-resistant UTI. CPC is a cationic surfactant that is FDA-approved for use in mouthwashes and is used as a food additive but has not been extensively evaluated as a UTI therapeutic. Our study is the first to investigate its rapid bactericidal kinetics against diverse uropathogenic bacterial species isolated from postmenopausal women with recurrent UTI and host cytotoxicity. We also report that together with the FDA-approved bladder-instillation agent gentamicin, VesiX was able to significantly reduce intracellular populations of uropathogenic bacteria in cultured bladder epithelial cells.

Inhibition of CX3C receptor 1-mediated autophagy in macrophages alleviates pulmonary fibrosis in hyperoxic lung injury

AIMS

To investigate the role of CX3CR1 in hyperoxic lung injury induced pulmonary fibrosis.

MATERIALS AND METHODS

Hyperoxic lung injured mice were used as the disease model. Pulmonary fibrosis was determined by H&E and Masson's staining. Autophagy was investigated by western blot, immunofluorescence staining, and transmission electron microscopy.

KEY FINDINGS

We observed that increased CX3CR1 expression corresponded with increased pulmonary fibrosis. Additionally, silencing of CX3CR1 significantly alleviated the fibrosis when compared to the control. We observed that exposure of mouse to hyperoxic environment increased macrophage levels along with an increased CD11b expression in the lung tissues. Subsequently, we also observed an increased expression of LC3-II and decreased p62 expression in hyperoxic mice models, suggesting the potential role of hyperoxia induced autophagy. CD11b and LC3/CX3CR1 were expressed and co-localized in a manner indicating CX3CR1 indeed does regulate macrophage autophagy in the hyperoxic lung injury model. We observed a decrease in hyperoxia-associated fibrosis, along with a decrease in autophagy when we used 3-MA (autophagy inhibitor) in our hyperoxic lung injury model. To elucidate the pathway through which CX3CR1 regulated autophagy, we further analyzed the Akt1 pathway. Our experimental results indicated that the Akt1 inhibitor (A-674563) did significantly decrease macrophage autophagy and fibrosis in hyperoxic mice models.

SIGNIFICANCE

Thus, our data indicates a novel role of CX3CR1 in regulation of macrophage autophagy and promotion of pulmonary fibrosis in hyperoxic lung injured mice.

The role of Trp53 in the mouse embryonic response to DNA damage

Apoptosis occurs primarily in the blastocyst inner cell mass, cells of which go on to form the foetus. Apoptosis is likely to play a role in ensuring the genetic integrity of the foetus, yet little is known about its regulation. In this study, the role of the mouse gene, transformation-related protein 53 (Trp53) in the response of embryos to in vitro culture and environmentally induced DNA damage was investigated using embryos from a Trp53 knockout mouse model. In vivo-derived blastocysts were compared to control embryos X-irradiated at the two-cell stage and cultured to Day 5. An analysis of DNA by comet assay demonstrated that 1.5 Gy X-irradiation directly induced damage in cultured two-cell mouse embryos; this was correlated with retarded development to blastocyst stage and increased apoptosis at the blastocyst stage but not prior to this. Trp53 null embryos developed to blastocysts at a higher frequency and with higher cell numbers than wild-type embryos. Trp53 also mediates apoptosis in conditions of low levels of DNA damage, in vivo or in vitro in the absence of irradiation. However, following DNA damage induced by X-irradiation, apoptosis is induced by Trp53 independent as well as dependent mechanisms. These data suggest that Trp53 and apoptosis play important roles in normal mouse embryonic development both in vitro and in vivo and in response to DNA damage. Therefore, clinical ART practices that alter apoptosis in human embryos and/or select embryos for transfer, which potentially lack a functional Trp53 gene, need to be carefully considered.

Exercise enhances skeletal muscle regeneration by promoting senescence in fibro-adipogenic progenitors

Idiopathic inflammatory myopathies cause progressive muscle weakness and degeneration. Since high-dose glucocorticoids might not lead to full recovery of muscle function, physical exercise is also an important intervention, but some exercises exacerbate chronic inflammation and muscle fibrosis. It is unknown how physical exercise can have both beneficial and detrimental effects in chronic myopathy. Here we show that senescence of fibro-adipogenic progenitors (FAPs) in response to exercise-induced muscle damage is needed to establish a state of regenerative inflammation that induces muscle regeneration. In chronic inflammatory myopathy model mice, exercise does not promote FAP senescence or resistance against tumor necrosis factor–mediated apoptosis. Pro-senescent intervention combining exercise and pharmacological AMPK activation reverses FAP apoptosis resistance and improves muscle function and regeneration. Our results demonstrate that the absence of FAP senescence after exercise leads to muscle degeneration with FAP accumulation. FAP-targeted pro-senescent interventions with exercise and pharmacological AMPK activation may constitute a therapeutic strategy for chronic inflammatory myopathy.

Some exercises exacerbate chronic inflammation and muscle fibrosis in chronic myopathy. Here, the authors show that senescence of fibro-adipogenic progenitors (FAPs) in response to exercise induces muscle regeneration, and impaired FAP senescence worsens inflammation and fibrosis in chronic myopathy in mice.

Reduced P53 levels ameliorate neuromuscular junction loss without affecting motor neuron pathology in a mouse model of spinal muscular atrophy

Spinal Muscular Atrophy (SMA) is a childhood motor neuron disease caused by mutations or deletions within the SMN1 gene. At endstages of disease there is profound loss of motor neurons, loss of axons within ventral roots and defects at the neuromuscular junctions (NMJ), as evidenced by pathological features such as pre-synaptic loss and swelling and post-synaptic shrinkage. Although these motor unit defects have been widely described, the time course and interdependancy of these aspects of motor unit degeneration are unclear. Recent reports have also revealed an early upregulation of transcripts associated with the P53 signalling pathway. The relationship between the upregulation of these transcripts and pathology within the motor unit is also unclear. In this study, we exploit the prolonged disease timecourse and defined pre-symptomatic period in the Smn^2B/- mouse model to perform a temporal analysis of the different elements of motor unit pathology. We demonstrate that NMJ loss occurs prior to cell body loss, and coincides with the onset of symptoms. The onset of NMJ pathology also coincides with an increase in P53-related transcripts at the cell body. Finally, using a tamoxifen inducible P53 knockout, we demonstrate that post-natal reduction in P53 levels can reduce NMJ loss, but does not affect other aspects of NMJ pathology, motor neuron loss or the phenotype of the Smn^2B/- mouse model. Together this work provides a detailed temporal description of pathology within motor units of an SMA mouse model, and demonstrates that NMJ loss is a P53-dependant process. This work supports the role for P53 as an effector of synaptic and axonal degeneration in a die-back neuropathy.

Inhibition of p53 and/or AKT as a new therapeutic approach specifically targeting ALT cancers

While the majority of all human cancers counteract telomere shortening by expressing telomerase, ~15% of all cancers maintain telomere length by a telomerase-independent mechanism known as alternative lengthening of telomeres (ALT). Here, we show that high load of intrinsic DNA damage is present in ALT cancer cells, leading to apoptosis stress by activating p53-independent, but JNK/c-Myc-dependent apoptotic pathway. Notably, ALT cells expressing wild-type p53 show much lower apoptosis than p53-deficient ALT cells. Mechanistically, we find that intrinsic DNA damage in ALT cells induces low level of p53 that is insufficient to initiate the transcription of apoptosis-related genes, but is sufficient to stimulate the expression of key components of mTORC2 (mTOR and Rictor), which in turn leads to phosphorylation of AKT. Activated AKT (p-AKT) thereby stimulates downstream anti-apoptotic events. Therefore, p53 and AKT are the key factors that suppress spontaneous apoptosis in ALT cells. Indeed, inhibition of p53 or AKT selectively induces rapid death of ALT cells in vitro, and p53 inhibitor severely suppresses the growth of ALT-cell xenograft tumors in mice. These findings reveal a previously unrecognized function of p53 in anti-apoptosis and identify that the inhibition of p53 or AKT has a potential as therapeutics for specifically targeting ALT cancers.

BBC3 in macrophages promoted pulmonary fibrosis development through inducing autophagy during silicosis

Following inhalation into the lungs, silica particles are engulfed by alveolar macrophages, which triggers endogenous or exogenous apoptosis signaling pathways. As an inducer of apoptosis, the role of BBC3/PUMA (BCL2-binding component 3) in macrophages during silicosis remains unknown. Here, we exposed U937 cell-derived macrophages (UDMs) to SiO₂in vitro to explore the function of BBC3 in SiO₂-induced disease. We found that SiO₂ induced increased BBC3 expression, as well as macrophage activation and apoptosis. Knockdown of Bbc3 with specific siRNA significantly mitigated the SiO₂-induced effects. In addition, our results clearly showed increased levels of autophagy in macrophages exposed to SiO₂. However, inhibition of BBC3 decreased the occurrence of autophagy. Furthermore, we observed that the blockade of autophagy with 3-MA, an autophagy inhibitor, inhibited SiO₂-induced macrophage activation and apoptosis. In contrast, rapamycin, an autophagy inducer, further enhanced the effects induced by SiO₂. The conditioned medium from macrophages exposed to SiO₂ promoted the proliferation and migration of fibroblasts, and the inhibition of BBC3/autophagy reduced the effects of the conditioned medium on fibroblasts. In the mouse model of silicosis, Bbc3 knockout mice clearly exhibited decreased levels of autophagy and fibrosis progression. These results suggest that downregulation of BBC3 expression may become a novel therapeutic strategy for the treatment of silicosis.

p53 and the Carcinogenicity of Chronic Inflammation

Chronic inflammation is a major cancer predisposition factor. Constitutive activation of the inflammation-driving NF-κB pathway commonly observed in cancer or developed in normal tissues because of persistent infections or endogenous tissue irritating factors, including products of secretion by senescent cells accumulating with age, markedly represses p53 functions. In its turn, p53 acts as a suppressor of inflammation helping to keep it within safe limits. The antagonistic relationship between p53 and NF-κB is controlled by multiple mechanisms and reflects cardinal differences in organismal responses to intrinsic and extrinsic cell stresses driven by these two transcription factors, respectively. This provides an opportunity for developing drugs to treat diseases associated with inappropriate activity of either p53 or NF-κB through targeting the opposing pathway. Several drug candidates of this kind are currently in clinical testing. These include anticancer small molecules capable of simultaneous suppression of p53 and activation of NF-κB and NF-κB-activating biologics that counteract p53-mediated pathologies associated with systemic genotoxic stresses such as acute radiation syndrome and side effects of cancer treatment.

The Receptor for Advanced Glycation Endproducts Drives T Cell Survival and Inflammation in Type 1 Diabetes Mellitus

The ways in which environmental factors participate in the progression of autoimmune diseases are not known. After initiation, it takes years before hyperglycemia develops in patients at risk for type 1 diabetes (T1D). The receptor for advanced glycation endproducts (RAGE) is a scavenger receptor of the Ig family that binds damage-associated molecular patterns and advanced glycated endproducts and can trigger cell activation. We previously found constitutive intracellular RAGE expression in lymphocytes from patients with T1D. In this article, we show that there is increased RAGE expression in T cells from at-risk euglycemic relatives who progress to T1D compared with healthy control subjects, and in the CD8⁺ T cells in the at-risk relatives who do versus those who do not progress to T1D. Detectable levels of the RAGE ligand high mobility group box 1 were present in serum from at-risk subjects and patients with T1D. Transcriptome analysis of RAGE⁺ versus RAGE^- T cells from patients with T1D showed differences in signaling pathways associated with increased cell activation and survival. Additional markers for effector memory cells and inflammatory function were elevated in the RAGE⁺ CD8⁺ cells of T1D patients and at-risk relatives of patients before disease onset. These studies suggest that expression of RAGE in T cells of subjects progressing to disease predates dysglycemia. These findings imply that RAGE expression enhances the inflammatory function of T cells, and its increased levels observed in T1D patients may account for the chronic autoimmune response when damage-associated molecular patterns are released after cell injury and killing.

p53 signaling is involved in leptin-induced growth of hepatic and breast cancer cells

Leptin, an adipokine predominantly produced from adipose tissue, is well known to induce tumor growth. However, underlying molecular mechanisms are not established yet. While p53 has long been well recognized as a potent tumor suppressor gene, accumulating evidence has also indicated its potential role in growth and survival of cancer cells depending on experimental environments. In the present study, we examined if p53 signaling is implicated in leptin-induced growth of cancer cells. Herein, we demonstrated that leptin treatment significantly increased p53 protein expression in both hepatic (HepG2) and breast (MCF-7) cancer cells without significant effect on mRNA expression. Enhanced p53 expression by leptin was mediated via modulation of ubiquitination, in particular ubiquitin specific protease 2 (USP2)-dependent manner. Furthermore, gene silencing of p53 by small interfering RNA (siRNA) suppressed leptin-induced growth of hepatic and breast cancer cells, indicating the role of p53 signaling in tumor growth by leptin. In addition, we also showed that knockdown of p53 restored suppression of caspase-3 activity by leptin through modulating Bax expression and prevented leptin-induced cell cycle progression, implying the involvement of p53 signaling in the regulation of both apoptosis and cell cycle progression in cancer cells treated with leptin. Taken together, the results in the present study demonstrated the potential role of p53 signaling in leptin-induced tumor growth.

HIPK family kinases bind and regulate the function of the CCR4-NOT complex

Down-regulation of the HIPK interactor CNOT2 leads to reduced HIPK2 protein levels, identifying the CCR4-NOT complex as a new regulator of HIPK2 abundance. Functional assays reveal that HIPK2 and HIPK1 restrict CNOT2-dependent mRNA decay, thus extending the regulatory potential of these kinases to the level of posttranscriptional gene regulation.

The serine/threonine kinase HIPK2 functions as a regulator of developmental processes and as a signal integrator of a wide variety of stress signals, such as DNA damage, hypoxia, and reactive oxygen intermediates. Because the kinase is generated in a constitutively active form, its expression levels are restricted by a variety of different mechanisms. Here we identify the CCR4-NOT complex as a new regulator of HIPK2 abundance. Down-regulation or knockout of the CCR4-NOT complex member CNOT2 leads to reduced HIPK2 protein levels without affecting the expression level of HIPK1 or HIPK3. A fraction of all HIPK family members associates with the CCR4-NOT components CNOT2 and CNOT3. HIPKs also phosphorylate the CCR4-NOT complex, a feature that is shared with their yeast progenitor kinase, YAK1. Functional assays reveal that HIPK2 and HIPK1 restrict CNOT2-dependent mRNA decay. HIPKs are well known regulators of transcription, but the mutual regulation between CCR4-NOT and HIPKs extends the regulatory potential of these kinases by enabling posttranscriptional gene regulation.

p53-mediated autophagic regulation: A prospective strategy for cancer therapy

Autophagy is a major catabolic process that degrades and recycles cytosolic components in autophagosomes, which fuse with lysosomes. This process enables starving cells to sustain their energy requirements and metabolic states, thus facilitating their survival, especially in cancer pathogenesis. The regulation of autophagy is quite intricate. It involves a series of signaling cascades including p53, known as the best-characterized tumor suppressor protein. Recent reports have indicated that p53 plays dual roles in regulating autophagy depending on its subcellular localization. Nuclear p53 facilitates autophagy by transactivating its target genes, whereas cytoplasmic p53 mainly inhibits autophagy through extranuclear, transcription-independent mechanisms. The relationship between autophagy and neoplasia is complicated. It may be intrinsically associated with the functional status of p53, but this is not clearly elucidated. This review focuses on the role of p53 as a master regulator of autophagy. We conclude that the contextual role of autophagy in cancer, which could be switched by p53 status, is expected to be developed into a new anticancer therapeutic approach.

Caught in the cross fire: p53 in inflammation

The p53 transcription factor is a major tumor suppressor, whose diverse activities serve to ensure genome stability and inhibit neoplastic processes. In recent years, it is becoming increasingly clear that p53 also plays a broader role in maintaining cellular homeostasis, as well as contributing to tissue homeostasis in a non-cell-autonomous fashion. Chronic inflammation is a potential cancer-promoting condition, and as such is also within the radar of p53, which mounts a multifaceted attempt to prevent the escalation of chronic tissue imbalance into neoplasia. Recent understanding of the p53 pathway and other family members reveals a broad interaction with inflammatory elements such as reactive oxygen and nitrogen species, cytokines, infectious agents and major immune-regulatory pathways like nuclear factor-kappaB. This complex cross talk is highly dependent on p53 status, as different p53 isoforms and p53 mutants can mediate different responses and even promote chronic inflammation and associated cancer, acting in the tumor cells as well as in the stromal and immune compartments.

Molecular processes that drive cigarette smoke-induced epithelial cell fate of the lung

Cigarette smoke contains numerous chemical compounds, including abundant reactive oxygen/nitrogen species and aldehydes, and many other carcinogens. Long-term cigarette smoking significantly increases the risk of various lung diseases, including chronic obstructive pulmonary disease and lung cancer, and contributes to premature death. Many in vitro and in vivo studies have elucidated mechanisms involved in cigarette smoke-induced inflammation, DNA damage, and autophagy, and the subsequent cell fates, including cell death, cellular senescence, and transformation. In this Translational Review, we summarize the known pathways underlying these processes in airway epithelial cells to help reveal future challenges and describe possible directions of research that could lead to better management and treatment of these diseases.

Mitochondrial liaisons of p53

Mitochondria play a central role in cell survival and cell death. While producing the bulk of intracellular ATP, mitochondrial respiration represents the most prominent source of harmful reactive oxygen species. Mitochondria participate in many anabolic pathways, including cholesterol and nucleotide biosynthesis, yet also control multiple biochemical cascades that contribute to the programmed demise of cells. The tumor suppressor protein p53 is best known for its ability to orchestrate a transcriptional response to stress that can have multiple outcomes, including cell cycle arrest and cell death. p53-mediated tumor suppression, however, also involves transcription-independent mechanisms. Cytoplasmic p53 can physically interact with members of the BCL-2 protein family, thereby promoting mitochondrial membrane permeabilization. Moreover, extranuclear p53 can suppress autophagy, a major prosurvival mechanism that is activated in response to multiple stress conditions. Thirty years have passed since its discovery, and p53 has been ascribed with an ever-increasing number of functions. For instance, p53 has turned out to influence the cell's redox status, by transactivating either anti- or pro-oxidant factors, and to regulate the metabolic switch between glycolysis and aerobic respiration. In this review, we will analyze the mechanisms by which p53 affects the balance between the vital and lethal functions of mitochondria.

The p53 tumor suppressor protein regulates hematopoietic stem cell fate

The p53 tumor suppressor protein is a key transcription factor that regulates several signaling pathways involved in the cell's response to stress. Through stress-induced activation, p53 accumulates and triggers the expression of target genes that protect the genetic integrity of all cells including hematopoietic stem cells (HSCs). These protective mechanisms include cell-cycle arrest, DNA repair, induction of apoptosis, or initiation of senescence. In addition to its function under stress conditions, p53 has important functions during steady-state hematopoiesis, regulating HSC quiescence and self-renewal. In addition, it appears that p53 levels affect HSC competition for the hematopoietic niche, with the less p53 activated HSCs preferentially surviving. The specific genes and precise mechanisms underlying p53's effects on normal HSCs are slowly being clarified. p53 also plays an important role in leukemia stem cell (LSC) behavior, with p53 loss affecting drug resistance and disease progression. Pharmacologic activation of p53 function could overcome the adverse impact of p53 inactivation in LSCs. Thus, understanding the p53 regulatory mechanisms active in HSCs and LSCs may promote the development of new therapeutic strategies that could eliminate the population of largely quiescent LSCs.

p53 and Notch signaling in chronic lymphocytic leukemia: clues to identifying novel therapeutic strategies

The p53 tumor suppressor protein has a key role in the induction of apoptosis of chronic lymphocytic leukemia (CLL) cells. Abnormalities within the p53 pathway identify a subset of patients with a poor prognosis. This review describes recent advances in understanding the mechanisms that regulate p53 levels and the role of p53 in the control of the cell cycle and of apoptosis. The classical model of p53-mediated apoptosis emphasizes the transcriptional activation of proapoptotic genes. In contrast, a novel model emphasizes p53's non-transcriptional actions as the major route of apoptosis induction, whereas its transcriptional arm predominantly upregulates antiapoptotic genes, thus providing a negative feedback mechanism that limits apoptosis. Further studies have identified the Notch pathway as a candidate p53-induced antiapoptotic mechanism. In contrast to the classical model, the novel model predicts that pharmacological inhibition of p53's transcriptional function or of the Notch signaling pathway will augment apoptosis induction by cytotoxic agents. Therapeutic strategies based on the novel model, which we review here for the first time, may significantly augment the antitumor actions of cytotoxic agents in CLL and in other malignancies.

p53 promotes cellular survival in a context-dependent manner by directly inducing the expression of haeme-oxygenase-1

A variety of cellular insults activate the tumour suppressor p53, leading generally to cell-cycle arrest or apoptosis. However, it is not inconceivable that cellular protective mechanisms may be required to keep cells alive while cell-fate decisions are made. In this respect, p53 has been suggested to perform functions that allow cells to survive, by halting of the cell-cycle, and thus preventing immediate cell death. Nonetheless, the existence of direct pro-survival p53 target genes regulating cellular survival is lacking. We show here evidence for p53-dependent cellular survival in a context-dependent manner. Both mouse and human cells lacking p53 are hypersensitive to hydrogen peroxide (H(2)O(2))-induced cell death compared with their isogenic wild-type counterparts. By contrast, p53(-/-) cells are expectedly resistant to cell death upon exposure to DNA-damaging agents such as cisplatin (CDDP) and etoposide. Although p53 and its classical targets such as p21 and Mdm2 are activated by both H(2)O(2) and CDDP, we found that the expression of haeme-oxygenase-1 (HO-1)-an antioxidant and antiapoptotic protein-was directly induced only upon H(2)O(2) treatment in a p53-dependent manner. Consistently, p53, but not its homologue p73, activated HO-1 expression and was bound to the HO-1 promoter specifically only upon H(2)O(2) treatment. Moreover, silencing HO-1 expression enhanced cell death upon H(2)O(2) treatment only in p53-proficient cells. Finally, H(2)O(2)-mediated cell death was rescued significantly in p53-deficient cells by antioxidant treatment, as well as by bilirubin, a by-product of HO-1 metabolism. Taken together, these data demonstrate a direct role for p53 in promoting cellular survival in a context-specific manner through the activation of a direct transcriptional target, HO-1.

p53-dependent regulation of autophagy protein LC3 supports cancer cell survival under prolonged starvation

The p53 tumor suppressor is mutated in a high percentage of human tumors. However, many other tumors retain wild-type (wt) p53 expression, raising the intriguing possibility that they actually benefit from it. Recent studies imply a role for p53 in regulation of autophagy, a catabolic pathway by which eukaryotic cells degrade and recycle macromolecules and organelles, particularly under conditions of nutrient deprivation. Here, we show that, in many cell types, p53 confers increased survival in the face of chronic starvation. We implicate regulation of autophagy in this effect. In HCT116 human colorectal cancer cells exposed to prolonged nutrient deprivation, the endogenous wt p53 posttranscriptionally down-regulates LC3, a pivotal component of the autophagic machinery. This enables reduced, yet sustainable autophagic flux. Loss of p53 impairs autophagic flux and causes excessive LC3 accumulation upon starvation, culminating in apoptosis. Thus, p53 increases cell fitness by maintaining better autophagic homeostasis, adjusting the rate of autophagy to changing circumstances. We propose that some cancer cells retain wt p53 to benefit from the resultant increased fitness under limited nutrient supply.

Involvement of stromal p53 in tumor-stroma interactions

p53 is a major tumor-suppressor gene, inactivated by mutations in about half of all human cancer cases, and probably incapacitated by other means in most other cases. Most research regarding the role of p53 in cancer has focused on its ability to elicit apoptosis or growth arrest of cells that are prone to become malignant owing to DNA damage or oncogene activation, i.e. cell-autonomous activities of p53. However, p53 activation within a cell can also exert a variety of effects upon neighboring cells, through secreted factors and paracrine and endocrine mechanisms. Of note, p53 within cancer stromal cells can inhibit tumor growth and malignant progression. Cancer cells that evolve under this inhibitory influence acquire mechanisms to silence stromal p53, either by direct inhibition of p53 within stromal cells, or through pressure for selection of stromal cells with compromised p53 function. Hence, activation of stromal p53 by chemotherapy or radiotherapy might be part of the mechanisms by which these treatments cause cancer regression. However, in certain circumstances, activation of stromal p53 by cytotoxic anti-cancer agents might actually promote treatment resistance, probably through stromal p53-mediated growth arrest of the cancer cells or through protection of the tumor vasculature. Better understanding of the underlying molecular mechanisms is thus required. Hopefully, this will allow their manipulation towards better inhibition of cancer initiation, progression and metastasis.

Targeted repression of bone morphogenetic protein 7, a novel target of the p53 family, triggers proliferative defect in p53-deficient breast cancer cells

p53 tumor suppressor and its family members, p63 and p73, are known to play a role in the survival of cells exposed to stress signals. As a transcription factor, the p53 family proteins induce a plethora of target genes that mediate their functions in the cell cycle, apoptosis, and other biological activities. However, the mechanism by which the p53 family proteins regulate their cell survival functions is still not clear. Here, we showed that bone morphogenetic protein 7 (BMP7) is a novel target gene regulated by the p53 family and mediates the cell survival function of the basal physiologically relevant level of p53. Specifically, we found that knockdown of BMP7 markedly inhibits the proliferation of p53-deficient, but not p21-knockdown, breast cancer cells compared with the ones with wild-type p53. In addition, we found that inhibitor of differentiation or DNA binding 2 (Id2), a transcription factor implicated for cell survival, is regulated by the BMP7 and p53 pathways. Interestingly, whereas a functional BMP7 or p53 pathway is sufficient to maintain the basal level of Id2 expression, loss of both pathways abrogates Id2 expression. Furthermore, we showed that overexpression of Id2 can restore p53-deficient cells to survive in the absence of BMP7. As a result, we identified a previously unrecognized role for BMP7 in the maintenance of cell survival for p53-deficient cells, at least in part, through Id2. Together, we hypothesize that breast cancer patients with mutant p53 might benefit from targeted repression of BMP7 expression and/or targeted inhibition of the BMP7 pathway.

Mechanisms underlying the pro-survival pathway of p53 in suppressing mitotic death induced by adriamycin

The p53 tumor suppressor responds to chemotherapeutic stress by triggering apoptosis or eliciting pro-survival pathway through arresting cell cycle progression for DNA damage repair. Here we examined the pro-survival activity of p53 on the adriamycin-induced stress using H1299 cells stably expressing tsp53 V143A, a temperature-sensitive mutant activating only the subset of p53 target genes related to growth arrest and DNA repair, but not apoptosis. At 38 degrees C, cells evaded from adriamycin-induced G2 arrest and died of apoptosis and mitotic catastrophe, which could be inhibited by Cdk inhibitors. Activation of functional tsp53 V143A at 32 degrees C led to suppression of Cdk1/2 activities and Cyclin B1/Cdk1 expression, cells exhibited prolonged G2 arrest, regained reproductive potential and were protected from mitotic catastrophe induced by adriamycin. Inhibition of mitotic catastrophe and Cyclin B1/Cdk1 expression was ablated upon silencing p21 Waf1 expression in tsp53 V143A-H1299 cells or in HCT116 cells. Together we show that p21 Waf1 is a key component of G2 checkpoint necessary and sufficient for protecting tumor cells against adriamycin-induced mitotic catastrophe.

p53 in health and disease

As a component of the response to acute stress, p53 has a well established role in protecting against cancer development. However, it is now becoming clear that p53 can have a much broader role and can contribute to the development, life expectancy and overall fitness of an organism. Although the function of p53 as a tumour suppressor ensures that we can't live without it, an integrated view of p53 suggests that not all of its functions are conducive to a long and healthy life.

Outcomes of p53 activation--spoilt for choice

The p53 tumour suppressor protein can efficiently inhibit tumour development. This activity reflects its ability to induce a number of different responses, including cell cycle arrest and apoptosis. Recent studies have revealed some interesting insights into how the choice of response to p53 is regulated, highlighting a correlation between the activation of cell cycle arrest and survival with the ability of p53 to reduce oxidative stress and protect cells from genotoxic damage. Understanding the molecular mechanisms that determine which response is selected may allow us to modulate these pathways so that therapeutic reactivation of p53 favours apoptotic cell death in tumour cells, but a reversible--and therefore far less toxic--induction of cell cycle arrest in normal cells.

The UVB-induced gene expression profile of human epidermis in vivo is different from that of cultured keratinocytes

In order to obtain a comprehensive picture of the molecular events regulating cutaneous photodamage of intact human epidermis, suction blister roofs obtained after a single dose of in vivo ultraviolet (UV)B exposure were used for microarray profiling. We found a changed expression of 619 genes. Half of the UVB-regulated genes had returned to pre-exposure baseline levels at 72 h, underscoring the transient character of the molecular cutaneous UVB response. Of special interest was our finding that several of the central p53 target genes remained unaffected following UVB exposure in spite of p53 protein accumulation. We next compared the in vivo expression profiles of epidermal sheets to that of cultured human epidermal keratinocytes exposed to UVB in vitro. We found 1931 genes that differed in their expression profiles between the two groups. The expression profile in intact epidemis was geared mainly towards DNA repair, whereas cultured keratinocytes responded predominantly by activating genes associated with cell-cycle arrest and apoptosis. These differences in expression profiles might reflect differences between mature differentiating keratinocytes in the suprabasal epidermal layers versus exponentially proliferating keratinocytes in cell culture. Our findings show that extreme care should be taken when extrapolating from findings based on keratinocyte cultures to changes in intact epidermis.

Overexpression of Sp1 transcription factor induces apoptosis

Transcription factor Sp1 has recently been shown to be overexpressed in a number of human cancers and its overexpression contributes to malignant transformation. Sp1 regulates the expression of a number of genes participating in multiple aspects of tumorigenesis such as angiogenesis, cell growth and apoptosis resistance. To better understand the role of increased Sp1 levels on apoptosis regulation we have used retroviruses to overexpress this protein in haematopoietic Baf-3 cells and in 3T3 fibroblasts. We have also used inducible expression systems to control ectopic Sp1 levels in different cell types. Surprisingly, Sp1 overexpression on its own induces apoptosis in all the cellular models tested. The apoptotic pathways induced by Sp1 overexpression are cell type specific. Finally, using a truncated form of Sp1, we show that Sp1-induced apoptosis requires its DNA-binding domain. Our results highlight that Sp1 levels in untransformed cells must be tightly regulated as Sp1 overexpression leads to the induction of apoptosis. Our results also suggest that cancer cells overexpressing Sp1 can avoid Sp1-induced apoptosis.

Cellular UV damage responses--functions of tumor suppressor p53

DNA damage, provoked by ultraviolet (UV) radiation, evokes a cellular damage response composed of activation of stress signaling and DNA checkpoint functions. These are translated to responses of replicative arrest, damage repair, and apoptosis aimed at cellular recovery from the damage. p53 tumor suppressor is a central stress response protein, activated by multiple endogenous and environmental insults, including UV radiation. The significance of p53 in the DNA damage responses has frequently been reviewed in the context of ionizing radiation or other double strand break (DSB)-inducing agents. Despite partly similar patterns, the molecular events following UV radiation are, however, distinct from the responses induced by DSBs and are profoundly coupled with transcriptional stress. These are illustrated, e.g., by the UV damage-specific translocations of Mdm2, promyelocytic leukemia protein, and nucleophosmin and their interactions with p53. In this review, we discuss UV damage-provoked cellular responses and the functions of p53 in damage recovery and cell death.

Differential splicing of E6 within human papillomavirus type 18 variants and functional consequences

Persistent infections of the uterine cervix with ‘high-risk’ human papillomavirus (HPV) are now recognized as necessary for the development of cervical cancer. Among them, HPV types 16 and 18 exhibit numerous variants associated with different risks for cervical cancer development. In this study, the questions of whether different HPV type 18 variants exhibit changes in early gene transcription and the molecular mechanisms underlying these differences were investigated. It was shown that, indeed, type 18 variants exhibited singular differences in E6 transcripts in vivo. Higher levels of the E6*I transcript were detected regularly in clones harbouring the African variant, as opposed to low levels of this transcript detected in clones containing the reference clone (Asian–Amerindian), where significantly higher levels of full-length E6 transcript were usually observed. As a direct consequence, higher levels of p53 protein were found in the presence of African E6, as opposed to the low levels of p53 observed with the Asian–Amerindian E6. These variations in consequence affected the levels of cellular proteins regulated by p53, such as Bax. Similar changes in the relative levels of E6 transcripts were observed when tumours containing type 18 E6 variants were analysed. The different ability of cells containing variant E6 genes to form tumours in nude mice was suggested by the fact that tumour volumes were considerably higher when cells expressed the Asian–Amerindian E6. Mutagenesis analysis of the reference clone showed that a C491A change reverts the phenotype. These results suggest that different splicing patterns of E6 within HPV type 18 variants may possibly have biological implications in viral tumorigenesis.

topors, a p53 and topoisomerase I-binding RING finger protein, is a coactivator of p53 in growth suppression induced by DNA damage

The RING family zinc-finger protein topors (topoisomerase I-binding protein) binds not only topoisomerase I, but also p53 and the AAV-2 Rep78/68 proteins. topors maps to human chromosome 9p21, which contains candidate tumor suppressor genes implicated in small cell lung cancers. In this study, we isolated the murine counterpart of topors and investigated its impact on p53 function. The deduced amino-acid sequence of mouse topors exhibits extensive similarity to human topors. Overexpressed myc-tagged topors associates with and stabilizes p53, and enhances the p53-dependent transcriptional activities of p21(Waf1), MDM2 and Bax promoters and elevates endogenous p21(Waf1) mRNA levels. Overexpression of topors consequently results in the suppression of cell growth by cell cycle arrest and/or by the induction of apoptosis. Taken together, these studies identify topors as a positive regulator of p53. The expression of topors is induced by exposure to the genotoxic reagents cisplatin and camptothecin, a DNA topoisomerase I inhibitor. We therefore postulate that topors mediates p53-dependent cellular responses induced by DNA damage, suggesting its physiological role as a tumor suppressor.

Endogenous p53 protects vascular smooth muscle cells from apoptosis and reduces atherosclerosis in ApoE knockout mice

Recent studies have indicated that the tumor suppressor gene p53 limits atherosclerosis in animal models; p53 expression is also increased in advanced human plaques compared with normal vessels, where it may induce growth arrest and apoptosis. However, controversy exists as to the role of endogenous levels of p53 in different cell types that comprise plaques. We examined atherosclerotic plaque development and composition in brachiocephalic arteries and aortas of p53-/-/ApoE-/- mice versus wild type p53 controls. p53-/- mice demonstrated increased aortic plaque formation, with increased rates of cell proliferation and reduced rates of apoptosis in brachiocephalic arteries. Although most proliferating cells were monocyte/macrophages, apoptotic cells were both vascular smooth muscle cells (VSMCs) and macrophages. Transplant of p53 bone marrow to p53-/-/ApoE-/- mice reduced aortic plaque formation and cell proliferation in brachiocephalic plaques, but also markedly reduced apoptosis. To examine p53 regulation of these processes, we studied proliferation and apoptosis in macrophages, bone marrow stromal cells and VSMCs cultured from these mice. Although endogenous p53 promoted apoptosis in macrophages, it protected VSMCs and stromal cells from death, a hitherto unknown function in these cells, in part by inhibiting DNA damage response enzymes. p53 also inhibited stromal cell expression of VSMC markers. We conclude that endogenous levels of p53 protect VSMCs and stromal cells against apoptosis, while promoting apoptosis in macrophages, and protect against atherosclerosis development.

p53 and cell-cycle-regulated protein expression in small intestinal cells after fast-neutron irradiation in mice

The involvement of the p53 gene in apoptosis of many cell types towards y-radiation is well established. However, little information is available on the relationship between p53 status and cells' ability to undergo apoptosis following exposure to fast neutrons. The aim of this study was to characterize the apoptotic pathway traveled by neutrons in mouse intestinal crypt cells. Each mouse received whole body doses of 0.25-8 Gy fast neutrons and were sacrificed 0, 4, 6, 12, 24, 48, and 72 h, respectively, after irradiation. Apoptosis of crypt cells and expression of p53, cyclin A, cyclin B, cyclin D, and cyclin E were measured. The apoptosis in crypt cells was maximal at 4 and 6 h after irradiation, showing a gradual decline at 24 h. The highest frequency of apoptosis was seen at a 1 Gy dose and then declined gradually beyond a 2 Gy dose with high levels of damage. In immunoblot analysis, apoptosis was confirmed to be dependent on p53 function after fast-neutron irradiation. In addition, cyclin B1, cyclin D, and cyclin E were overexpressed in intestinal cells after fast-neutron irradiation and their immunoreactivities were increased strongly in round and oval cells of laminar propria in villi core and crypts. The results of the current study suggest that apoptosis in crypt cells shows a time- and dose-dependent increase after fast-neutron irradiation. In addition, fast-neutron-induced apoptosis in mouse intestinal crypt cells appears to be related to the increase in functional p53 proteins to a level sufficient to initiate apoptosis and up-regulation of cell-cycle-regulated proteins, which may lead to resistance to DNA damage through cell cycle arrest, is involved deeply in protection of gastrointestinal cells after low doses of fast-neutron irradiation.

Cell and tissue responses to genotoxic stress

Cancers arise as a consequence of the accumulation of multiple genetic mutations in a susceptible cell, resulting in perturbation of regulatory networks that control proliferation, survival, and cellular function. Here, the sources of cellular stress that can cause oncogenic mutations and the responses of cells to DNA damage are reviewed. The role of different repair pathways and the potential for cell- and tissue-specific reliance on individual repair mechanisms are discussed. Evidence for cell- and tissue-specific activation of p53-mediated growth arrest and apoptosis after exposure to an individual genotoxin is assessed and some of the potential mediators of these different responses are provided. These cell- and tissue-specific responses to particular forms of DNA damage are likely to be key determinants of tissue-specific tumour susceptibility, and there is good evidence for genetic variations in these responses. The role that genotoxic agents play in altering the microenvironment to produce indirect effects on tumourigenesis through altered production of free radicals and cytokines that are characteristic of inflammatory-type processes is also evaluated. Changes to the microenvironment as direct or indirect effects of genotoxic stress can be involved in both tumour initiation and progression and may even be a prerequisite for tumourigenesis. Therefore, tumour susceptibility after endogenous or exogenous genotoxic stress represents a balance between cell-intrinsic responses of target cells and changes to the microenvironment. A fuller understanding of cell- and tissue-specific responses, alterations to the microenvironment, and genetic modifiers of these responses could lead to novel prevention and therapeutic strategies for common forms of human malignancy.

The switch from survival responses to apoptosis after chromosomal breaks

Eukaryotic cells have evolved highly sophisticated cellular responses to DNA double strand breaks (DSBs) that increase the likelihood of survival. However, cells can also respond to DSBs by undergoing programmed cell death. The mechanisms underlying the cellular decision on whether to repair and survive or to die are not well understood but may be related to the efficiency of repair or the extent of the damage. Presumably, a few easily reparable DSBs will not result in cell death in most cell types. However, abundant complex DSBs will present a severe challenge to the repair machineries with repeated attempts at repair likely to result in genome instability. For multicellular eukaryotes at least, struggling to complete repair is folly, whereas removal of severely damaged cells is a more sensible strategy. Here we discuss recent evidence linking DSBs to a highly regulated form of cell death termed, apoptosis. In particular, we focus on the roles of the tumour suppressor, p53 and a recently discovered role for an isotype of the linker histone H1. We present a hypothesis that the elevated levels of ssDNA produced during ongoing attempts at DSB repair may be involved in the switch from repair to apoptosis.

Attenuation of the p53 response to DNA damage by high cell density

The p53 tumor suppressor is critical for preventing cancer progression. Numerous observations suggest that p53 function can be modulated by the cells' microenvironment. We addressed specifically the impact of cell crowding on the induction of p53 by DNA damage. We report that cell crowding attenuates markedly p53 upregulation, transcriptional activation and subsequent p53-dependent apoptosis following exposure to genotoxic stress. The p53 protein remains short-lived in confluent cultures regardless of the extent of DNA damage, even though it undergoes efficient phosphorylation on the mouse equivalent of human p53 serine 15. This inhibitory effect of cell crowding is not a secondary consequence of density-dependent cell cycle arrest (contact inhibition). Microscopic examination indicates that dense cultures display prominent cadherin-mediated cell-cell junctions, and only poor cell-matrix focal adhesions, whereas sparse cells possess conspicuous matrix adhesions and essentially no cell-cell contacts. High-density cell culture might recapitulate the microenvironment of cells in a living organism, where the response of p53 to DNA damage is reported to be low in some organs and ages. The impact of cell density on p53 activation may have important bearings on the involvement of p53 in tumor suppression and the cellular response to anticancer therapy.

Response of Ewing tumor cells to forced and activated p53 expression

The EWS-FLI1 transcription factor is consistently expressed in 85% of Ewing tumors (EFT). In heterologous cells, EWS-FLI1 induces p53-dependent cell cycle arrest or apoptosis. It has been speculated that the p53 tumor suppressor pathway may be generally compromised in EFT despite only rare p53 mutations. In order to test for functional integrity of this pathway, we have investigated a series of EFT cell lines that differ from each other with respect to their endogenous p53 and INK4A gene status for their response to ectopic p53 expression and to stimulation of endogenous p53 activity by X-ray treatment. Significant interindividual and intratumoral variations in the apoptotic propensity of EFT cell lines to transient expression of ectopic p53 were observed, which was independent of the level of p53 expression. In cell lines with a low apoptotic incidence, apoptosis was delayed and the surviving fraction showed a prolonged growth arrest. Complete resistance to p53-induced apoptosis in two cell lines established from the same patient was associated with a high BCL2/BAX ratio and low levels of APAF1. Sensitivity to X-rays showed a trend towards a higher apoptotic rate in wild-type (wt) p53 expressing than in p53 mutant cells. However, one wt p53-expressing EFT cell line was completely refractory to irradiation-stimulated cell death despite high apoptotic responsiveness to ectopic p53. No difference in Ser15 phosphorylation and the transcriptional activation of p53 targets was observed in wt p53 EFT cell lines irrespective of the induction of cell death or growth arrest. All together, our results demonstrate that despite significant variability in the outcome, cell death or cell cycle arrest, the p53 downstream pathway and the DNA damage signaling pathway are functionally intact in EFT.

The p53-Mdm2 module and the ubiquitin system

The p53 tumor suppressor protein is a short-lived protein, which is stabilized in response to cellular stress. The ubiquitination and degradation of p53 are largely controlled by Mdm2, an oncogenic E3 ligase. Stress signals lead to p53 stabilization either by induction of covalent modifications in Mdm2 and p53, or through altered protein-protein interactions. Mdm2 also harbors a post-ubiquitination function, probably enabling efficient targeting of ubiquitinated p53 to the proteasome. p53 ubiquitination is associated with its export from the nucleus into the cytoplasm. However, the exact site of degradation of p53 is presently under debate. p53 may be targeted by other E3 ligases besides Mdm2, as well as by non-proteasomal mechanisms. Despite extensive information about p53 degradation, many important aspects remain unresolved.

Tissue-specific p53 responses to ionizing radiation and their genetic modification: the key to tissue-specific tumour susceptibility?

Although little is understood of the underlying mechanisms, there are tissue-specific responses to tumourigenic and therapeutic agents and these responses are influenced by genetic factors. Ionizing radiation is an important tumourigenic and therapeutic agent for which there is substantial evidence for such tissue-dependent and genotype-dependent responses. Because the p53 tumour suppressor protein is a major determinant of cellular responses to radiation, the present study has investigated whether modification of the p53 pathway contributes to tissue-dependent and genotype-dependent responses using inbred strains of mice. Comparison of responses in haemopoietic and epithelial cells in irradiated C57BL/6 and DBA/2 mice revealed significant differences in p53 and apoptotic responses in different cell types and in different cells of the same type, reflecting the complexity of damage responses operating in the whole organism. The data suggest that p53-mediated up-regulation of Bax is a major determinant of apoptosis in the spleen, but not in the intestine, whereas p53-mediated induction of p21(waf1) plays an anti-apoptotic role in the spleen, but not in the intestine. It is also shown that p53 stabilization and differential transactivational activities towards Bax or p21(waf1) are influenced by genetic factors that act in a tissue-specific manner. Analysis of ATM, a potential mediator of differential p53 activation, indicates that this key regulator of radiation responses is preferentially induced in epithelial cells, but is unlikely to account for genetic modification of p53 or apoptotic responses in the mouse strains studied. Polymorphisms in the p53 or DNA-PKcs genes are also unlikely to account for the genetic modifications that are reported here. There are numerous further potential modifiers of the p53 pathway, but analysis of backcross and inter-cross mice demonstrates that genes responsible for the complex modification of these in vivo responses can be identified by linkage analysis. This approach has the potential to reveal new or unexpected interactions involving the p53 pathway that determine both short-term and long-term effects of radiation exposure and the basis of tissue-specific responses and tumour susceptibility.

Apoptosis induced by progesterone in human ovarian cancer cell line SNU-840

Progesterone has been used as an ingredient of anticancer drug for patients with ovarian carcinoma. However, the mechanism of anticancer effects by progesterone has not been understood. In this study, the effects of progesterone on ovarian cancer cells, SNU-840, were investigated. After the incubation with progesterone, the viability of the cells was evaluated by MTT assay. As a result, 45% of the cells were viable after 48 h of incubation with 100 microM progesterone. In addition, [(3)H]thymidine incorporation assay showed that the proliferation of the cells was completely inhibited by progesterone after 48 h of incubation at 100 microM concentration. Colorimetric TUNEL assay revealed the fragmentation of the chromosomal DNA, suggesting that the process of the cell death was apoptosis. The level of the p53 mRNA was determined by northern blotting assay, since many apoptosis processes are mediated by up-regulation of the p53 expression. The level of the p53 mRNA reached its maximum at 12 h and decreased after 24 h of incubation with progesterone. In conclusion, progesterone inhibits the proliferation and elicits apoptosis of SNU-840 cells. Also, it up-regulates the p53 mRNA transiently.

Caffeine sensitizes human H358 cell line to p53-mediated apoptosis by inducing mitochondrial translocation and conformational change of BAX protein

The mechanisms involved in p53-mediated cell death remain controversial. In the present study, we investigated this cell death pathway by stably transfecting the p53-null H358 cell line with a tetracycline-dependent wild type p53-expressing vector. Restoration of p53 triggered a G(2)/M cell cycle arrest and enhanced BAX protein expression, without inducing apoptosis or potentiating the cytotoxic effect of etoposide, vincristine, and cis-platinum. Accordingly, overexpression of BAX in H358 cells, through stable transfection of a tetracycline-regulated expression vector, did not induce cell death. Interestingly, the methylxanthine caffeine (4 mm) promoted the translocation of BAX from the cytosol to the mitochondria. In the setting of an overexpression of BAX, caffeine induced a conformational change of the protein and apoptosis. The consequences of caffeine were independent of its cell cycle-related activities. All together, caffeine synergizes with p53 for inducing cell death through a cell cycle-independent mechanism, involving mitochondrial translocation and conformational change of BAX protein.

The C-terminus of mutant p53 is necessary for its ability to interfere with growth arrest or apoptosis

The ability to suppress wild type p53-independent apoptosis may play an important role in the oncogenicity of p53 mutant proteins. However, structural elements necessary for this activity are unknown. Furthermore, it is unclear whether this mutant p53 mediated inhibition is specific to the apoptotic pathway or a more general suppression of the cellular response to stress. We observed that an unmodified C-terminus was required for the suppression of apoptosis by the p53 135(Ala to Val) oncogenic p53 mutant. It was also required for the novel activity of G2 arrest suppression, the predominant response at low levels of genotoxic stress. These observations are consistent with a model whereby mutant p53 suppressive activity is not specific to the apoptotic pathway, but rather increases the threshold of genotoxic stress needed for a DNA damage response to occur. Furthermore, these observations indicate that it may be possible to selectively kill mutant p53 expressing cells based on the lower sensitivity of their growth arrest response.

Malignancy: Tumor Suppressor Gene Aberrations in Acute Myelogenous Leukemia

Acute myelogenous leukemia is a heterogeneous disease that appears to evade the normal regulatory controls of tumor suppressor genes. Studies in AML have documented mutations in both p53 and Retinoblastoma (Rb) genes, but these mutations are relatively uncommon, especially compared to their mutational frequency in solid tumors. In addition, expression abnormalities have now been documented in several tumor suppressor genes or related genes including MDM2, p73, Rb, p14(ARF), p15(INK4B), and p16(INK4A). We review the current literature regarding tumor suppressor genes in AML and suggest how these genes may be involved in the development of the disease.

Induction of apoptosis in head-and-neck squamous carcinoma cells by gamma-irradiation and bleomycin is p53-independent

We have examined the ability of gamma-irradiation and bleomycin to induce apoptosis in a model system consisting of cell lines derived from naturally occurring human head-and-neck squamous-cell carcinomas with contrasting p53 status and expression levels of pro- and anti-apoptotic molecules. Following exposure to gamma-irradiation (20 Gy) or bleomycin (3.5 microM) for 0 to 96 hr, cells expressing either transcriptionally inactive mutant p53 (HN6) or a truncated p53 molecule (HN19) underwent apoptosis, as assessed by fluorescence-activated cell sorting and terminal deoxynucleotidyl transferase-mediated dUTP nick end-labeling, in contrast to cells that express wild-type p53 (HN30), suggesting that apoptosis induced by these agents occurs by p53-independent mechanisms. Apoptosis in HN6 and HN19 cells was preceded by a G(2)/M cell-cycle block, as analyzed by DNA content and BrdU staining. In contrast, HN30 cells remained blocked in both G(1) and G(2)/M and failed to re-enter the cell cycle. Levels of Bcl-2 were elevated in 3 of 10 cell lines, and only marginal differences were observed for Bcl-x(L). Pro-apoptotic proteins bax and Bcl-x(S) were detectable in normal keratinocytes and 4 tumor cell lines. Bax-delta (16 kDa) was highly represented in normal keratinocytes, and levels of bak were variable between cell lines. Elevated expression of Bcl-2 failed to protect HN19 cells from either gamma-irradiation or bleomycin-induced apoptosis. Our data support the existence of p53- and Bcl-2-independent pathways regulating apoptosis in keratinocytes and suggest that efficacy of either radiotherapy or bleomycin treatment for oral squamous-cell neoplasms may not, therefore, be influenced solely by endogenous p53 status.

Metalloregulation of the tumor suppressor protein p53: zinc mediates the renaturation of p53 after exposure to metal chelators in vitro and in intact cells

The tumor suppressor p53 is a transcription factor which binds DNA through a structurally complex domain stabilized by a zinc atom. Zinc chelation disrupts the architecture of this domain, inducing the protein to adopt an immunological phenotype identical to that of many mutant forms of p53. In this report, we used 65Zn to show that incorporation of zinc within the protein was required for folding in the 'wild-type' conformation capable of specific DNA-binding. Using a cellular assay, we show that addition of extracellular zinc at concentrations within the physiological range (5 microM) was required for renaturation and reactivation of wild-type p53. Among other divalent metals tested (Cd2+, Cu2+, Co2+, Fe2+ and Ni2+), only Co2+ at 125 microM had a similar effect. Recombinant metallothionein (MT), a metal chelator protein, was found to modulate p53 conformation in vitro. In cultured cells, overexpression of MT by transfection could modulate p53 transcriptional activity. Taken together, these results suggest that zinc binding plays a regulatory role in the control of p53 folding and DNA-binding activity.

Prohibitins, stomatins, and plant disease response genes compose a protein superfamily that controls cell proliferation, ion channel regulation, and death

Prohibitins, stomatins, and a group of plant defense response genes are demonstrated to belong to a novel protein superfamily. This superfamily is bound by similar primary and secondary predicted protein structures and hydropathy profiles. A PROSITE-formatted regular expression was generated that is highly predictive for identifying members of this superfamily using PHI-BLAST. The superfamily is named PID (proliferation, ion, and death) because prohibitins are involved in proliferation and cell cycle control, stomatins are involved in ion channel regulation, and the plant defense-related genes are involved in cell death. The plant defense gene family is named HIR (hypersensitive induced reaction) because its members are associated with hypersensitive reactions involving cell death and pathogen resistance. For this study, eight novel maize genes were introduced: four closely related to prohibitins (Zm-phb1, Zm-phb2, Zm-phb3, and Zm-phb4), one to stomatins (Zm-stm1), and three to a gene implicated in plant disease responses (Zm-hir1, Zm-hir2, and Zm-hir3). The maize Zm-hir3 gene transcript is up-regulated in a disease lesion mimic mutation (Les9), supporting a role in maize defense responses. Members of this gene superfamily are involved in diverse functions, but their structural similarity suggests a conserved molecular mechanism, which we postulate to be ion channel regulation.

Nitric oxide-dependent activation of p53 suppresses bleomycin-induced apoptosis in the lung

Chronic inflammation leading to pulmonary fibrosis develops in response to environmental pollutants, radiotherapy, or certain cancer chemotherapeutic agents. We speculated that lung injury might be mediated by p53, a proapoptotic transcription factor widely implicated in the response of cells to DNA damage. Intratracheal administration of bleomycin led to caspase-mediated DNA fragmentation characteristic of apoptosis. The effects of bleomycin were associated with translocation of p53 from the cytosol to the nucleus only in alveolar macrophages that had been exposed to the drug in vivo, suggesting that the lung microenvironment regulated p53 activation. Experiments with a thiol antioxidant (N-acetylcysteine) in vivo and nitric oxide (NO) donors in vitro confirmed that reactive oxygen species were required for p53 activation. A specific role for NO was demonstrated in experiments with inducible nitric oxide synthase (iNOS)(-/)- macrophages, which failed to demonstrate nuclear p53 localization after in vivo bleomycin exposure. Strikingly, rates of bleomycin-induced apoptosis were at least twofold higher in p53(-/)- C57BL/6 mice compared with heterozygous or wild-type littermates. Similarly, levels of apoptosis were also twofold higher in the lungs of iNOS(-/)- mice than were observed in wild-type controls. Consistent with a role for apoptosis in chronic lung injury, levels of bleomycin-induced inflammation were substantially higher in iNOS(-/)- and p53(-/)- mice compared with wild-type controls. Together, our results demonstrate that iNOS and p53 mediate a novel apoptosis-suppressing pathway in the lung.

Cell death beyond apoptosis

Though the term apoptosis was originated in pathology and developmental biology as an alternative to necrosis, the tissue necrosis with inflammation is irrelevant to cell culture conditions where apoptosis is mostly studied. Furthermore, no one single morphological feature is either necessary or sufficient to define apoptosis. The emerging biochemical definition, a cell death with caspase activation, allows the distinction of alternative forms of cell death. Thus, inhibition of caspases delays but does not prevent cell death. Slow cell death without caspase activation may nevertheless be associated with DNA fragmentation. Oncogenic Ras, Raf, and mitogen-activated kinases inhibit apoptosis by affecting the cytochrome C/caspase-9 pathway but may arrest growth and cause slow cell death with delayed DNA fragmentation. Such 'slow' cell death without caspase activation is often caused by chemotherapeutic drugs. Whether a cell will undergo apoptosis or slow death depends not only on a chemotherapeutic agent but also on the readiness of cellular caspases. Therefore, one can distinguish apoptosis-prone (eg leukemia) vs apoptosis-resistant cells. Cell susceptibilities to spontaneous, starvation-induced and drug-induced apoptosis are correlated and characterize an apoptosis-prone phenotype. Finally, distinction of slow cell death allows rephrasing of a question regarding the goal of cancer therapy: apoptosis vs slow cell death, or cancer cell-selectivity regardless of the mode of cell death.