Dosage-dependent dominance over wild-type p53 of a mutant p53 isolated from nasopharyngeal carcinoma

Cell fate regulation governed by p53: Friends or reversible foes in cancer therapy

Cancer is a leading cause of death worldwide. Targeted therapies aimed at key oncogenic driver mutations in combination with chemotherapy and radiotherapy as well as immunotherapy have benefited cancer patients considerably. Tumor protein p53 (TP53), a crucial tumor suppressor gene encoding p53, regulates numerous downstream genes and cellular phenotypes in response to various stressors. The affected genes are involved in diverse processes, including cell cycle arrest, DNA repair, cellular senescence, metabolic homeostasis, apoptosis, and autophagy. However, accumulating recent studies have continued to reveal novel and unexpected functions of p53 in governing the fate of tumors, for example, functions in ferroptosis, immunity, the tumor microenvironment and microbiome metabolism. Among the possibilities, the evolutionary plasticity of p53 is the most controversial, partially due to the dizzying array of biological functions that have been attributed to different regulatory mechanisms of p53 signaling. Nearly 40 years after its discovery, this key tumor suppressor remains somewhat enigmatic. The intricate and diverse functions of p53 in regulating cell fate during cancer treatment are only the tip of the iceberg with respect to its equally complicated structural biology, which has been painstakingly revealed. Additionally, TP53 mutation is one of the most significant genetic alterations in cancer, contributing to rapid cancer cell growth and tumor progression. Here, we summarized recent advances that implicate altered p53 in modulating the response to various cancer therapies, including chemotherapy, radiotherapy, and immunotherapy. Furthermore, we also discussed potential strategies for targeting p53 as a therapeutic option for cancer.

E3 Ubiquitin Ligases in Gammaherpesviruses and HIV: A Review of Virus Adaptation and Exploitation

For productive infection and replication to occur, viruses must control cellular machinery and counteract restriction factors and antiviral proteins. Viruses can accomplish this, in part, via the regulation of cellular gene expression and post-transcriptional and post-translational control. Many viruses co-opt and counteract cellular processes via modulation of the host post-translational modification machinery and encoding or hijacking kinases, SUMO ligases, deubiquitinases, and ubiquitin ligases, in addition to other modifiers. In this review, we focus on three oncoviruses, Epstein-Barr virus (EBV), Kaposi's sarcoma herpesvirus (KSHV), and human immunodeficiency virus (HIV) and their interactions with the ubiquitin-proteasome system via viral-encoded or cellular E3 ubiquitin ligase activity.

Should mutant TP53 be targeted for cancer therapy?

Mutations in the TP53 tumour suppressor gene are found in ~50% of human cancers [1-6]. TP53 functions as a transcription factor that directly regulates the expression of ~500 genes, some of them involved in cell cycle arrest/cell senescence, apoptotic cell death or DNA damage repair, i.e. the cellular responses that together prevent tumorigenesis [1-6]. Defects in TP53 function not only cause tumour development but also impair the response of malignant cells to anti-cancer drugs, particularly those that induce DNA damage [1-6]. Most mutations in TP53 in human cancers cause a single amino acid substitution, usually within the DNA binding domain of the TP53 protein. These mutant TP53 proteins are often expressed at high levels in the malignant cells. Three cancer causing attributes have been postulated for mutant TP53 proteins: the inability to activate target genes controlled by wt TP53 (loss-of-function, LOF) that are critical for tumour suppression, dominant negative effects (DNE), i.e. blocking the function of wt TP53 in cells during early stages of transformation when mutant and wt TP53 proteins are co-expressed, and gain-of-function (GOF) effects whereby mutant TP53 impacts diverse cellular pathways by interacting with proteins that are not normally engaged by wt TP53 [1-6]. The GOF effects of mutant TP53 were reported to be essential for the sustained proliferation and survival of malignant cells and it was therefore proposed that agents that can remove mutant TP53 protein would have substantial therapeutic impact [7-9]. In this review article we discuss evidence for and against the value of targeting mutant TP53 protein for cancer therapy.

p53, A Victim of the Prion Fashion

Identified in the late 1970s as an oncogene, a driving force leading to tumor development, p53 turned out to be a key tumor suppressor gene. Now p53 is considered a master gene regulating the transcription of over 3000 target genes and controlling a remarkable number of cellular functions. The elevated prevalence of p53 mutations in human cancers has led to a recurring questioning about the roles of mutant p53 proteins and their functional consequences. Both mutants and isoforms of p53 have been attributed dominant-negative and gain of function properties among which is the ability to form amyloid aggregates and behave in a prion-like manner. This report challenges the ongoing "prion p53" hypothesis by reviewing evidence of p53 behavior in light of our current knowledge regarding amyloid proteins, prionoids and prions.

How mutations shape p53 interactions with the genome to promote tumorigenesis and drug resistance

The tumor suppressive transcription factor p53 regulates a wide array of cellular processes that confer upon cells an essential protection against cancer development. Wild-type p53 regulates gene expression by directly binding to DNA in a sequence-specific manner. p53 missense mutations are the most common mutations in malignant cells and can be regarded as synonymous with anticancer drug resistance and poor prognosis. The current review provides an overview of how the extraordinary variety of more than 2000 different mutant p53 proteins, known as the p53 mutome, affect the interaction of p53 with DNA. We discuss how the classification of p53 mutations to loss of function (LOF), gain of function (GOF), and dominant-negative (DN) inhibition of a remaining wild-type allele, hides a complex p53 mutation spectrum that depends on the distinctive nature of each mutant protein, requiring different therapeutic strategies for each mutant p53 protein. We propose to regard the different mutant p53 categories as continuous variables, that may not be independent of each other. In particular, we suggest here to consider GOF mutations as a special subset of LOF mutations, especially when mutant p53 binds to DNA through cooperation with other transcription factors, and we present a model for GOF mechanism that consolidates many observations on the GOF phenomenon. We review how novel mutant p53 targeting approaches aim to restore a wild-type-like DNA interaction and to overcome resistance to cancer therapy.

Cdk3-promoted epithelial-mesenchymal transition through activating AP-1 is involved in colorectal cancer metastasis

Cyclin dependent kinase-3 (Cdk3) is a positive regulator of the G1 mammalian cell cycle phase. Cdk3 is involved in cancer progression, but very little is known about its mechanism in cancer development and progression. Herein, we found that Cdk3 increased colorectal cancer metastasis through promoting epithelial-mesenchymal transition (EMT) shift. Cdk3 was found to highly express in metastatic cancer and induce cell motility and invasion. Cdk3 was shown to phosphorylate c-Jun at Ser 63 and Ser 73 in vitro and ex vivo. Cdk3-phosphorylated c-Jun at Ser 63 and Ser 73 resulted in an increased AP-1 activity. Ectopic expression of Cdk3 promoted colorectal cancer from epithelial to mesenchymal transition conjugating AP-1 activation, while AP-1 inhibition dramatically decreased Cdk3-increased EMT shift. These results showed that the Cdk3/c-Jun signaling axis mediating epithelial-mesenchymal transition plays an important role in colorectal cancer metastasis.

The Contrived Mutant p53 Oncogene - Beyond Loss of Functions

Mutations in p53 are almost synonymous with cancer – be it susceptibility to the disease or response to treatment – and therefore, are a critical determinant of overall survival. As most of these mutations occur in the DNA-binding domain of p53, many of the clinical correlations with mutant p53 have been initially relegated to the loss of its transcription-dependent activities as a tumor suppressor. However, significant efforts over the last two decades have led to the vast knowledge on the potential functions of the mutated p53 protein, which have been attributed to the physical presence of the mutant protein rather than the loss of its wild-type (WT) functions. Beyond the inhibitory effects of mutant p53 on the remaining WT protein that leads to the dominant-negative effect in the heterozygous state, mutant p53’s presence has also been significantly attributed to novel gain-of-functions that lead to addiction of cancer cells to its presence for survival, as well as for their ability to invade and metastasize, elevating it to a contrived oncogene that drives the cancer cells forward. This review will summarize the functional consequences of the presence of mutant p53 protein on cellular and organismal physiology.

Viral oncoprotein LMP1 disrupts p53-induced cell cycle arrest and apoptosis through modulating K63-linked ubiquitination of p53

Disruption of the gatekeeper p53 tumor suppressor is involved in various virus-associated tumorigeneses, with aberrant ubiquitination as the major cause of p53 abnormalities in virus-associated tumors. Of note, wild-type p53 is accumulated in Epstein-Barr virus (EBV)-associated tumors, especially in nasopharyngeal carcinoma (NPC). We have previously identified that p53 is accumulated and phosphorylated by EBV oncoprotein latent membrane protein 1 (LMP1) in NPC. Here, we further found that LMP1 promoted p53 accumulation via two distinct ubiquitin modifications. LMP1 promoted p53 stability and accumulation by suppressing K48-linked ubiquitination of p53 mediated by E3 ligase MDM2, which is associated with its phosphorylation at Ser20, while increasing the levels of total cellular ubiquitinated p53. LMP1 also induced K63-linked ubiquitination of p53 by interacting with tumor necrosis factor receptor-associated factor 2 (TRAF2), thus contributing to p53 accumulation. Furthermore, LMP1 rescued tumor cell apoptosis and cell cycle arrest mediated by K63-linked ubiquitination of p53. Collectively, these results demonstrate aberrant ubiquitin modifications of p53 and its biological functions by viral protein LMP1, which has broad implications to the pathogenesis of multiple EBV-associated tumors.

Restoring expression of wild-type p53 suppresses tumor growth but does not cause tumor regression in mice with a p53 missense mutation

The transcription factor p53 is a tumor suppressor. As such, the P53 gene is frequently altered in human cancers. However, over 80% of the P53 mutations found in human cancers are missense mutations that lead to expression of mutant proteins that not only lack p53 transcriptional activity but exhibit new functions as well. Recent studies show that restoration of p53 expression leads to tumor regression in mice carrying p53 deletions. However, the therapeutic efficacy of restoring p53 expression in tumors containing p53 missense mutations has not been evaluated. Here we demonstrate that restoring wild-type p53 expression halted tumor growth in mice inheriting a p53(R172H) missense mutation that is equivalent to a P53 missense mutation detected in approximately 6% of human cancers. However, it did not lead to tumor regression, as was observed in mice lacking p53. We further showed that the dominant-negative effect of the mutant p53 encoded by p53(R172H) dampened the activity of the restored wild-type p53. We therefore conclude that in a mutant p53 background, p53 restoration has the therapeutic potential to suppress tumor progression. Our findings support using p53 restoration as a strategy to treat human cancers with P53 missense mutations and provide direction for optimizing p53 restoration in cancer therapy.

The Accessory SecA2 System of Mycobacteria Requires ATP Binding and the Canonical SecA1

In bacteria, the majority of exported proteins are transported by the general Sec pathway from their site of synthesis in the cytoplasm across the cytoplasmic membrane. The essential SecA ATPase powers this Sec-mediated export. Mycobacteria possess two nonredundant SecA homologs: SecA1 and SecA2. In pathogenic Mycobacterium tuberculosis and the nonpathogenic model mycobacterium Mycobacterium smegmatis, SecA1 is essential for protein export and is the "housekeeping" SecA, whereas SecA2 is an accessory SecA that exports a specific subset of proteins. In M. tuberculosis the accessory SecA2 pathway plays a role in virulence. In this study, we uncovered basic properties of the mycobacterial SecA2 protein and its pathway for exporting select proteins. By constructing secA2 mutant alleles that encode proteins defective in ATP binding, we showed that ATP binding is required for SecA2 function. SecA2 mutant proteins unable to bind ATP were nonfunctional and dominant negative. By evaluating the subcellular distribution of each SecA, SecA1 was shown to be equally divided between cytosolic and cell envelope fractions, whereas SecA2 was predominantly localized to the cytosol. Finally, we showed that the canonical SecA1 has a role in the process of SecA2-dependent export. The accessory SecA2 export system is important to the physiology and virulence of mycobacteria. These studies help establish the mechanism of this new type of specialized protein export pathway.

Elimination of wild-type P53 mRNA in glioblastomas showing heterozygous mutations of P53

We screened 50 glioblastomas for P53 mutations. Five glioblastomas showed heterozygous mutations, while three were putatively heterozygous. Six of these eight glioblastomas showed elimination of wild-type P53 mRNA. These results strongly suggest that some sort of mechanism(s) favouring mutated over wild-type P53 mRNA exists in glioblastoma cells with heterozygous mutations of this gene.

Functional interactions between Sae2 and the Mre11 complex

The Mre11 complex functions in double-strand break (DSB) repair, meiotic recombination, and DNA damage checkpoint pathways. Sae2 deficiency has opposing effects on the Mre11 complex. On one hand, it appears to impair Mre11 nuclease function in DNA repair and meiotic DSB processing, and on the other, Sae2 deficiency activates Mre11-complex-dependent DNA-damage-signaling via the Tel1-Mre11 complex (TM) pathway. We demonstrate that SAE2 overexpression blocks the TM pathway, suggesting that Sae2 antagonizes Mre11-complex checkpoint functions. To understand how Sae2 regulates the Mre11 complex, we screened for sae2 alleles that behaved as the null with respect to Mre11-complex checkpoint functions, but left nuclease function intact. Phenotypic characterization of these sae2 alleles suggests that Sae2 functions as a multimer and influences the substrate specificity of the Mre11 nuclease. We show that Sae2 oligomerizes independently of DNA damage and that oligomerization is required for its regulatory influence on the Mre11 nuclease and checkpoint functions.

Arsenite inhibits p53 phosphorylation, DNA binding activity, and p53 target gene p21 expression in mouse epidermal JB6 cells

Epidemiologic investigations demonstrated that arsenite exposure increases the risk of various human cancers, including skin, lung, bladder, and kidney cancers. However, oral administration of arsenite alone has failed to induce tumors in animal models, suggesting that arsenic may act to enhance mutagenicity induced by other carcinogens. Arsenite may function as a co-carcinogen, acting by inhibiting repair of carcinogen-induced DNA damage mediated by p53 and p21, a p53 target gene. To elucidate the interaction between arsenite and p53 tumor suppressor protein, we studied the effect of arsenite on ultraviolet B (UVB)-induced p53 phosphorylation, p53 DNA binding activity, and p53-induced target gene transactivation in the JB6 Cl41 mouse epidermal skin cell model. Our results indicated that arsenite suppressed UVB-induced p53 phosphorylation and p53 DNA binding activity. Arsenite also inhibited casein kinase 2 (CK2) activity and decreased p53-regulated p21 protein expression. These data suggest that the direct inhibition of p53 functional activation is one of the mechanisms through which arsenite interferes with p53 function, and thus may be a significant mechanism for the co-carcinogenic effects of arsenite.

Ribosomal protein S27L is a direct p53 target that regulates apoptosis

Ribosomal proteins were recently shown to regulate p53 activity by abrogating Mdm2-induced p53 degradation (L23, L11, L5) or by enhancing p53 translation (L26). Here, we report that a novel ribosomal protein, RPS27L (S27-like protein), is a direct p53 target. RPS27L, but not its family member RPS27, was identified as a p53 inducible gene in a genome-wide chip-profiling study. Further characterization revealed a p53-dependent induction of RPS27L in multiple cancer cell models. Indeed, a consensus p53-binding site was identified in the first intron of the RPS27L gene and a direct binding of p53 to this site was demonstrated both in vitro and in vivo. Characterization of a luciferase reporter driven by the RPS27L intron fragment revealed a p53-binding site-dependent transaction by wild-type p53, but not by several transactivating-deficient p53 mutants. This transactivation was enhanced by etoposide, a DNA damaging agent that activates p53 and was completely blocked by a dominant-negative p53 mutant. Functionally, overexpression of RPS27L within the physiological inducible levels promoted, whereas siRNA silencing of RPS27L inhibited, apoptosis induced by etoposide. This is the first report, to our knowledge, that p53 directly induces the expression of a ribosomal protein, RPS27L, which in turn promotes apoptosis.

Complementation of two mutant p53: Implications for loss of heterozygosity in cancer

Remarkably, a cancer cell rarely possesses two mutant p53 proteins. Instead, mutation of one allele is usually associated with loss of the second p53 allele. Why do not two mutant p53 co-exist? We hypothesize that two different p53 may complement each other, when expressed at equal levels. By titrating trans-deficient and DNA-binding-deficient p53 in cells with mutant p53 and by co-transfecting distinct mutant p53 in p53-null cells, we demonstrated activation of p53-dependent transcription. We suggest that, due to complementation of two mutant p53, cancer cells need to delete the second p53 allele rather than mutate it.

Cooperative interactions among p53, bcl-2 and Epstein-Barr virus latent membrane protein 1 in nasopharyngeal carcinoma cells

Interactions among p53, bcl-2 and Epstein-Barr virus (EBV) latent membrane protein 1 (LMP1) in nasopharyngeal carcinoma (NPC) cells were evaluated by gene cotransfections. The data showed that bcl-2 expression was not only able to prevent the growth suppression induced by wild-type p53 but was also paradoxically able to inhibit the growth enhancement induced by mutant p53. Latent membrane protein 1 was shown to be capable of overcoming the growth inhibition induced by wild-type p53 and the synergistic cooperation with bcl-2 to enhance cellular growth. Latent membrane protein 1 could also cooperate with mutant p53 to provide a growth advantage for NPC cells. Most NPC revealed detectable overexpression of p53, and the majority of those were a wild type possibly responding to EBV infection. The coexpression of bcl-2 and LMP1 was thought to inhibit the growth suppression induced by wild-type p53 in NPC. But there was no associated expression between LMP1 and bcl-2 because we demonstrated that transfected LMP1 failed to induce bcl-2 expression in NPC cells in contrast to the findings in B cells. It is theorized that the cooperative expression of bcl-2 and LMP1 exists in the majority of NPC, while a minority of NPC have cooperative expression of LMP1 and mutant p53. Each cooperative interaction could play an important role in the development and progression of NPC.

Mutation of p53 in head and neck squamous cell carcinoma correlates with Bcl-2 expression and increased susceptibility to cisplatin-induced apoptosis

BACKGROUND

The p53 protein, a well-known tumor suppressor that functions primarily as a transcription factor, initiates cell cycle arrest and apoptosis after genotoxic stress. The antiapoptotic regulator Bcl-2 is a downstream modulator of p53-induced apoptosis. Loss of function of the p53 tumor suppressor through mutation is an important event that contributes to cellular transformation. Mutation of p53 is one of the most common genetic alterations in squamous cell carcinomas of the head and neck (SCCHN). We hypothesized that p53 mutation is associated with Bcl-2 expression and susceptibility to apoptosis in SCCHN.

METHODS

Exons 5 to 8 of the p53 gene were sequenced in 22 SCCHN tumor samples and correlated with the Bcl-2 expression and apoptosis rates in these tumors. In addition, a Bcl-2-expressing SCCHN cell line, UMSCC74B, was stably transfected with a temperature-sensitive mutant p53 construct, and Bcl-2 expression levels were examined at the mutant and the wild-type temperatures.

RESULTS

Bcl-2 expression was inversely correlated with wild-type p53 status in SCCHN tumors (p = .05). Furthermore, there was a modest increase (1.7-fold) in apoptosis in the wild-type p53 tumors compared with mutant p53 SCCHN. Immunoblotting of UMSCC74B cells stably transfected with the temperature-sensitive mutant p53 construct demonstrated that shifting these cells to the mutant p53 temperature (39.5 degrees C) resulted in decreased expression of Bcl-2 compared with levels in cells grown at the wild-type p53 temperature (32.5 degrees C). Further investigation showed that SCCHN cells expressing predominantly mutant p53 and decreased Bcl-2 were more susceptible to cisplatin-induced apoptosis than vector-transfected controls (p < .0001).

CONCLUSIONS

These results suggest that p53 mutation directly modulates Bcl-2 expression and therefore susceptibility to chemotherapy-induced apoptosis in SCCHN cells in vitro.

p53 activation in chronic radiation-treated breast cancer cells: regulation of MDM2/p14ARF

Mammalian cells chronically exposed to ionizing radiation (IR) induce stress response with a tolerance to the subsequent cytotoxicity of IR. Although p53 is well documented in IR response, the signaling network causing p53 activation in chronic IR remains to be identified. Using breast carcinoma MCF+FIR cells that showed a transient radioresistance after exposure chronically to fractionated IR (FIR), the present study shows that the basal DNA binding and transcriptional activity of p53 was elevated by FIR. p53-controlled luciferase activity was strikingly induced ( approximately 7.9-fold) with little enhancement of p53/DNA binding activity ( approximately 1.3-fold). The phosphorylated p53 (Thr 55) was increased in the cytoplasm and nucleus of MCF+FIR but not in the sham-FIR control cells. On the contrary, the sham-FIR control MCF-7 cells showed a low p53 luciferase transcription ( approximately 3-fold) but a striking enhancement of p53/DNA binding (12-fold) after 5 Gy of IR. To determine the signaling elements regulating p53 activity, DNA microarray of MCF+FIR using sham-FIR MCF-7 cells as a reference demonstrated that the mRNA of p21, MDM2, and p14ARF was up-regulated. Time course Western blot analysis, however, showed no difference in p21 induction. In contrast, MDM2 that was absent in control cells and was predominantly induced by IR was not induced in MCF+FIR cells. In agreement with MDM2 inhibition, MDM2-inhibitory protein p14ARF was increased in MCF+FIR cells. In summary, these results demonstrate that up-regulation of p14ARF paralleled with MDM2 inhibition contributes to p53 accumulation in the nucleus and causes a high responsiveness of p53 in chronic IR-treated breast cancer cells.

Conversion of wild-type p53 core domain into a conformation that mimics a hot-spot mutant

The wild-type p53 protein can be driven into a conformation corresponding to that adopted by structural mutant forms by heterodimerization with a mutant subunit. To seek partially folded states of the wild-type p53 core domain (p53C) we used high hydrostatic pressure (HP) and subzero temperatures. Aggregation of the protein was observed in parallel with its pressure denaturation at 25 and 37 degrees C. However, when HP experiments were performed at 4 degrees C, the extent of denaturation and aggregation was significantly less pronounced. On the other hand, subzero temperatures under pressure led to cold denaturation and yielded a non-aggregated, alternative conformation of p53C. Nuclear magnetic resonance (1H15N-NMR) data showed that the alternative p53C conformation resembled that of the hot-spot oncogenic mutant R248Q. This alternative state was as susceptible to denaturation and aggregation as the mutant R248Q when subjected to HP at 25 degrees C. Together these data demonstrate that wild-type p53C adopts an alternative conformation with a mutant-like stability, consistent with the dominant-negative effect caused by many mutants. This alternative conformation is likely related to inactive forms that appear in vivo, usually driven by interaction with mutant proteins. Therefore, it can be a valuable target in the search for ways to interfere with protein misfolding and hence to prevent tumor development.

ATF3 represses 72-kDa type IV collagenase (MMP-2) expression by antagonizing p53-dependent trans-activation of the collagenase promoter

The murine homologue of the ATF3 transcription factor increases tumor metastases but, surprisingly, represses 72-kDa type IV metalloproteinase (MMP-2) expression. The current study describes a novel mechanism by which ATF3 regulates transcription. Progressive deletions of the MMP-2 promoter indicated a 38-base pair region (-1659/-1622) necessary for the ATF3-mediated repression. This region lacked CREB/AP-1 motifs but contained a consensus p53 motif shown previously to regulate MMP-2 expression. The activity of a p53 response element-driven luciferase reporter was reduced in ATF3-expressing HT1080 clones. Although MMP-2 promoter activity was not repressed by ATF3 in p53-deficient Saos-2 cells, p53 re-expression increased MMP-2 promoter activity and restored the sensitivity to ATF3. The activity of a GAL4-driven reporter in HT1080 cells co-expressing the full-length p53 sequence fused to the GAL4 DNA binding domain was diminished by ATF3. p53-ATF3 protein-protein interactions were demonstrated both in vivo and in vitro. Cell cycle analysis, performed as an independent assay of p53 function, revealed that gamma-irradiation-induced slowed G(2)/M cell cycle progression (attributable to p53) was countered by ATF3. Thus, ATF3 represses MMP-2 expression by decreasing the trans-activation of this gene by p53.

Proteinase inhibitors I and II from potatoes block UVB-induced AP-1 activity by regulating the AP-1 protein compositional patterns in JB6 cells

Proteinase inhibitor I (Inh I) and proteinase inhibitor II (Inh II) from potato tubers are effective proteinase inhibitors of chymotrypsin and trypsin. Inh I and Inh II were shown to suppress irradiation-induced transformation in mouse embryo fibroblasts suggesting that they possess anticarcinogenic characteristics. We have previously demonstrated that Inh I and Inh II could effectively block UV irradiation-induced activation of transcription activator protein 1 (AP-1) in mouse JB6 epidermal cells, which mechanistically may explain their anticarcinogenic actions. In the present study, we investigated the effects of Inh I and Inh II on the expression and composition pattern of the AP-1 complex following stimulation by UV B (UVB) irradiation in the JB6 model. We found that Inh I and Inh II specifically inhibited UVB-induced AP-1, but not NFkappaB, activity in JB6 cells. Both Inh I and Inh II up-regulated AP-1 constituent proteins, JunD and Fra-2, and suppressed c-Jun and c-Fos expression and composition in bound AP-1 in response to UVB stimulation. This regulation of the AP-1 protein compositional pattern in response to Inh I or Inh II may be critical for the inhibition of UVB-induced AP-1 activity by these agents found in potatoes.

Effector genes altered in MCF-7 human breast cancer cells after exposure to fractionated ionizing radiation

Understanding the molecular mechanisms involved in the response of tumors to fractionated exposures to ionizing radiation is important for improving radiotherapy and/or radiochemotherapy. In the present study, we examined the expression of stress-related genes in an MCF-7 cell population (MCF-IR20) that has been derived through treatment with fractionated irradiation (2 Gy per fraction with a total dose of 40 Gy). MCF-IR20 cells showed a 1.6-fold increase in sensitization with dose at 10% isosurvival in a clonogenic assay, and a reduced growth delay ( approximately 15 h compared to approximately 27 h), compared to the parental MCF-7 cells treated with a single dose of 5 Gy. To determine which effector genes were altered in the MCF-IR20 cells, the expression of stress-related effector genes was measured using a filter with 588 genes (Clontech) that included major elements involved in cell cycle control, DNA repair, and apoptosis. Compared to MCF-7 cells that were not exposed to fractionated radiation, 19 genes were up- regulated (2.2-5.1-fold) and 4 were down-regulated (2.7-3.4- fold) in the MCF-IR20 cells. In agreement with the array results, 6 up-regulated genes tested by RT-PCR showed elevated expression. Also, activities of the stress-related transcription factors NFKB, TP53 and AP1 showed a 1.2-4.5-fold increase after a single dose of 5 Gy in MCF-IR20 cells compared with parental MCF-7 cells. However, when the radioresistant MCF-IR20 cell were cultured for more than 12 passages after fractionated irradiation (MCF-RV), radioresistance was lost, with the radiosensitivity being the same as the parental MCF- 7 cells. Interestingly, expression levels of CCNB1, CD9 and CDKN1A in MCF-RV cells returned to levels expressed by the parental cells, whereas the expression levels of three other genes, MSH2, MSH6 and RPA remained elevated. To determine if any of the changes in gene expression could be responsible for the induced radioresistance, CCNB1 and CDKN1A, both of which were up-regulated in MCF-IR20 cells and down-regulated in MCF-RV cells, were studied further by transfection with antisense oligonucleotides. Antisense of CCNB1 significantly reduced the clonogenic survival of MCF- IR20 cells at doses of 5 and 10 Gy, from 42% to 26% and from 5.7% to 1.0%, respectively. Antisense of CDKN1A, however, had no effect on radiation survival of MCF-IR20 cells. In summary, these results suggest that stress-related effector genes are altered in cells after treatment with fractionated irradiation, and that up-regulation of CCNB1 is responsible, at least in part, for radioresistance after fractionated irradiation.

Induced expression of dominant-negative c-jun downregulates NFkappaB and AP-1 target genes and suppresses tumor phenotype in human keratinocytes

Neoplastically transformed mouse and human keratinocytes elevate transactivation of both activator protein 1 (AP-1) and nuclear factor kappaB (NFkappaB) transcription factors. The present study addresses the question of whether elevated NFkappaB in addition to elevated AP-1-dependent gene expression is necessary for maintaining the tumor cell phenotype. When a tetracycline-regulatable dominant-negative c-jun (TAM67, having a truncated transactivation domain) was expressed in tumorigenic human keratinocytes, AP-1- and NFkappaB- but not p53-dependent reporter activity was inhibited by 40-60%. Tumor phenotype, as measured by anchorage-independent growth, was inhibited by 90%. Neither AP-1/NFkappaB activation nor expression of tumor phenotype was inhibited in TAM67-harboring keratinocytes under noninducing conditions. Electrophoretic mobility shift analysis showed that induction of TAM67 expression slightly increased AP-1- but reduced NFkappaB DNA-binding activity. Immunoprecipitation showed that TAM67 interacted in keratinocyte nuclei with NFkappaB p65, suggesting that inhibition of NFkappaB by TAM67 is mediated by direct protein-protein interactions, possibly producing decreased binding to DNA or inactivating p65. To analyze the putative effector genes that may be targeted by TAM67, expression of genes responsive to AP-1 or NFkappaB was measured by reverse transcriptase-polymerase chain reaction in TAM67 transfectants with or without TAM67 induction. Induction of TAM67 inhibited or reduced the expression of collagenase I, stromelysin I (AP-1 responsive), and interleukins 1 and 6 (NFkappaB responsive). These results indicate that genes controlled by NFkappaB and by AP-1 may be transformation-relevant targets of TAM67 and that TAM67 may inhibit NFkappaB activation through direct interaction with NFkappaB p65. Moreover, the findings provide proof for the principle of using inducible TAM67 as a gene therapy to suppress tumor phenotype in human carcinoma cells.

PTGF-beta, a type beta transforming growth factor (TGF-beta) superfamily member, is a p53 target gene that inhibits tumor cell growth via TGF-beta signaling pathway

Identification and characterization of p53 target genes would lead to a better understanding of p53 functions and p53-mediated signaling pathways. Two putative p53 binding sites were identified in the promoter of a gene encoding PTGF-beta, a type beta transforming growth factor (TGF-beta) superfamily member. Gel shift assay showed that p53 bound to both sites. Luciferase-coupled transactivation assay revealed that the gene promoter was activated in a p53 dose- as well as p53 binding site-dependent manner by wild-type p53 but not by several p53 mutants. The p53 binding and transactivation of the PTGF-beta promoter was enhanced by etoposide, a p53 activator, and was largely blocked by a dominant negative p53 mutant. Furthermore, expression of endogenous PTGF-beta was remarkably induced by etoposide in p53-positive, but not in p53-negative, cell lines. Finally, the conditioned medium collected from PTGF-beta-overexpressing cells, but not from the control cells, suppressed tumor cell growth. Growth suppression was not, however, seen in cells that lack functional TGF-beta receptors or Smad4, suggesting that PTGF-beta acts through the TGF-beta signaling pathway. Thus, PTGF-beta, a secretory protein, is a p53 target that could mediate p53-induced growth suppression in autocrinal as well as paracrinal fashions. The finding made a vertical connection between p53 and TGF-beta signaling pathways in controlling cell growth and implied a potential important role of p53 in inflammation regulation via PTGF-beta.

Complete loss of wild-type TP53 in a nontransformed human epithelial cell line is preceded by a phase during which a heterozygous TP53 mutant effectively outgrows the homozygous wild-type cells

HMT-3522 is a spontaneously immortalized cell line derived from a fibrocystic breast lesion. After continuous accumulation of genetic changes, the cell line was transformed from a nontumorigenic to a malignant phenotype. One of the earliest genetic aberrations is a missense mutation of codon 179 (His179Asn) in the tumor suppressor gene TP53 leading to outgrowth of a cell type expressing only the mutant form of TP53. In this report, we extend earlier investigations to reveal the genetic background for the evolution from homozygous wild type to hemizygous mutated cells. The status of the TP53 alleles was followed at different stages by fluorescence in situ hybridization (FISH) and allele-specific PCR (ASPCR) on total DNA, as well as flow-sorted chromosomes--taking advantage of a size difference between the two homologues of chromosome 17 that harbor TP53 on 17p. This further allowed us to determine on which of the two chromosomes the mutated allele was located. The results presented here show that the cells have undergone an evolution from homozygous wild type for TP53 to heterozygous (His179Asn mutation in one allele), and finally to a hemizygous mutated state (deletion of the remaining wild-type allele). The finding of a transient period in which heterozygous cells dominate the population before the eventual outgrowth of hemizygous cells strongly indicates that the His179Asn mutation results in a tp53 protein with a dominant negative effect that does not totally abrogate the function of wild type TP53 in vitro.

Expression, purification, and biochemical characterization of SAG, a ring finger redox-sensitive protein

We recently reported the cloning and characterization of SAG (sensitive to apoptosis gene), a novel zinc RING finger protein, that is redox responsive and protects mammalian cells from apoptosis. Here we report the expression, purification, and biochemical characterization of SAG. Bacterially expressed SAG is brown in color and dithiothreitol (DTT)-sensitive. SAG forms large oligomers without DTT that can be reduced into a monomer in the presence of DTT. These features help us to purify SAG using the chromatography with or without DTT. Likewise, purified SAG is redox sensitive. Upon H2O2 exposure, SAG forms oligomers as well as monomer doublets due to the formation of the inter- or intramolecular disulfide bonds, respectively. This process can be reversed by DTT or prevented by pretreatment with the alkylating reagent, N-ethylmaleimide (NEM). Although SAG contains two putative heme-binding sites and a RING finger domain, the protein appears not to bind with heme and to lack transcription factor activity as determined in a Gal4-fusion/transactivation assay. Wildtype, but not RING finger domain-disrupted SAG mutants, prevents copper-induced lipid peroxidation. These results, along with our previous observations, suggest that SAG is an intracellular antioxidant molecule that may act as a redox sensor to buffer oxidative-stress induced damage.

p53 down-regulates human matrix metalloproteinase-1 (Collagenase-1) gene expression

Recent studies show that the p53 tumor suppressor protein is overexpressed in rheumatoid arthritis (RA) synovium and that somatic mutations previously identified in human tumors are present in RA synovium (Firestein, G. S., Echeverri, F., Yeo, M., Zvaifler, N. J., and Green, D. R. (1997) Proc. Natl. Acad. Sci. U. S. A. 94, 10895-10900; Firestein, G. S., Nguyen, K., Aupperle, K. R., Yeo, M., Boyle, D. L., and Zvaifler, N. J. (1996) Am. J. Pathol. 149, 2143-2151; Reme, T., Travaglio, A., Gueydon, E., Adla, L., Jorgensen, C., and Sany, J. (1998) Clin. Exp. Immunol. 111, 353-3581). We hypothesize that the abnormality of p53 seen in RA synovium may contribute to joint degeneration through the regulation of human matrix metalloproteinase-1 (hMMP-1, collagenase-1) gene expression. Transcription assays were performed with luciferase reporters driven by the promoter of the hMMP-1 gene or by a minimal promoter containing tandem repeats of the consensus binding sequence for activator protein-1, cotransfected with p53-expressing plasmids. The results revealed that (i) wild-type (wt) p53 down-regulated the promoter activity of hMMP-1 in a dose-dependent fashion; (ii) four of six p53 mutants (commonly found in human cancers) lost this repression activity; and (iii) this p53 repression activity was mediated at least in part by the activator protein-1 sites found in the hMMP-1 promoter. These findings were further confirmed by Northern analysis. The down-regulation of hMMP-1 gene expression by endogenous wt-p53 was shown by treatment of U2-OS cells, a wt-p53-containing osteogenic sarcoma line, and Saos-2 cells, a p53-negative osteogenic sarcoma line, with etoposide, a potent inducer of p53 expression. p53, activated by etoposide, appears to block hMMP-1 promoter activity induced by etoposide in U2-OS cells. In summary, we have shown for the first time that the hMMP-1 gene is a p53 target gene, subject to p53 repression. Because MMP-1 is principally responsible for the irreversible destruction of collagen in articular tissue in RA, abnormality of p53 may contribute to joint degeneration through the regulation of MMP-1 expression.

Transcriptional activation of the human glutathione peroxidase promoter by p53

Glutathione peroxidase (GPX) is a primary antioxidant enzyme that scavenges hydrogen peroxide or organic hydroperoxides. We have recently found that GPX is induced by etoposide, a topoisomerase II inhibitor and a p53 activator. In a search for a cis-element that confers potential p53 regulation of GPX, we identified a p53 binding site in the promoter of the GPX gene. This site bound to purified p53 as well as p53 in nuclear extract activated by etoposide. A luciferase reporter driven by a 262-base pair GPX promoter fragment was transcriptionally activated by wild type p53 in a p53 binding site-dependent manner. The same reporter was also activated in a p53 binding site-independent manner by several p53 mutants. The p53 binding and transactivation of the GPX promoter were enhanced by etoposide in p53-positive U2-OS cells. Etoposide-induced transactivation was blocked by a dominant negative p53 mutant, indicating that endogenous wild type p53, upon activation by etoposide, transactivated the GPX promoter. Furthermore, expression of endogenous GPX was induced significantly at both mRNA and enzyme activity levels by etoposide in U2-OS cells but not in p53-negative Saos-2 cells. This is the first report demonstrating that GPX is a novel p53 target gene. The finding links the p53 tumor suppressor to an antioxidant enzyme and will facilitate study of the p53 signaling pathway and antioxidant enzyme regulation.

Transcriptional activation of the human S100A2 promoter by wild-type p53

S100A2, a calcium binding protein of the EF-hand family, was recently identified to be inducible by etoposide, a p53 activator. A potential p53 binding site was identified in the promoter of the S100A2 gene, which binds to purified p53 as well as p53 in nuclear extract activated by etoposide. Transactivation assays using the promoter driven luciferase reporters revealed that the S100A2 promoter was transcriptionally activated by wild-type p53, but not by p53 mutants, in a dose-dependent as well as a p53 binding site-dependent manner. The p53-induced transactivation of the S100A2 promoter was enhanced by etoposide and blocked by a dominant negative p53 mutant. Furthermore, endogenous S100A2 mRNA expression is induced by etoposide in p53 positive, but not in p53 negative cells. Thus, p53 appears to positively regulate S100A2 expression.

The effects of combining ionizing radiation and adenoviral p53 therapy in nasopharyngeal carcinoma

PURPOSE

Nasopharyngeal carcinoma (NPC) is a malignant disease of the head/neck region, with a 5-year survival level of approximately 65%. To explore gene therapy as a novel approach which might improve outcome, we have shown previously that introduction of human recombinant wild-type p53 mediated by the adenoviral vector (Ad5CMV-p53) was cytotoxic in two human nasopharyngeal carcinoma (NPC) cell lines (CNE-1 and CNE-2Z). The current work was designed to determine whether this strategy, combined with ionizing radiation (XRT), was more effective than either treatment alone.

METHODS AND MATERIALS

CNE-1, CNE-2Z, and a normal human nasopharyngeal fibroblast strain, KS1, were infected with 2- and 6-plaque-forming units (pfu)/cell of Ad5CMV-p53, respectively. These doses were isoeffective for beta-galactosidase activity in the CNE-1 and CNE-2Z cells. XRT was administered 24 h post-infection, and Western blot analyses were conducted for p53, p21WAF1/CIP1, bax, and bcl-2 2 days after XRT. Cell survival was assessed using a clonogenic assay. Presence of DNA ladders reflecting apoptosis was detected using DNA agarose gel electrophoresis, and cell cycle was analyzed using flow cytometry.

RESULTS

The combination of Ad5CMV-p53 plus XRT (2, 4, and 6 Gy) resulted in an approximately 1-log greater level of cytotoxicity compared to that observed with XRT alone for both NPC cell lines. The two modalities appear to be interacting in a synergistic manner in cancer cells, but not in KS1 fibroblasts. XRT alone stimulated minimal p53 expression in control cells; Ad5CMV-p53 alone induced significant recombinant p53 expression, which was not further enhanced by the addition of XRT. Similar observations were made for p21WAF1/CIP1 expression. No changes were observed for bax or bcl-2 expression with any of these treatments. Apoptosis was induced following 4 Gy of XRT alone, but was observed after only 2 Gy when combined with Ad5CMV-p53. Cell cycle analysis indicated that Ad5CMV-p53 infection did not perturb the cell cycle beyond that observed with XRT alone.

CONCLUSION

p53 gene therapy and XRT appears to interact in a synergistic manner; underscoring the significant potential of this novel strategy in the treatment of NPC.

Transcriptional activation by p53 of the human type IV collagenase (gelatinase A or matrix metalloproteinase 2) promoter

p53, a tumor suppressor and a transcription factor, has been shown to transcriptionally activate the expression of a number of important genes involved in the regulation of cell growth, DNA damage, angiogenesis, and apoptosis. In a computer search for other potential p53 target genes, we identified a perfect p53 binding site in the promoter of the human type IV collagenase (also called 72-kDa gelatinase or matrix metalloproteinase 2 [MMP-2]) gene. This p53 binding site was found to specifically bind to p53 protein in a gel shift assay. Transcription assays with luciferase reporters driven by the promoter or enhancer of the type IV collagenase gene revealed that (i) activation of the promoter activity is p53 binding site dependent in p53-positive cells but not in p53-negative cells and (ii) wild-type p53, but not p53 mutants commonly found in human cancers, transactivates luciferase expression driven by the type IV collagenase promoter as well as by a p53 site-containing enhancer element in the promoter. Significantly, expression of the endogenous type IV collagenase is also under the control of p53. Treatment of U2-OS cells, a wild-type p53-containing osteogenic sarcoma line, with a common p53 inducer, etoposide, induced p53 DNA binding and transactivation activities in a time-dependent manner. Induction of type IV collagenase expression followed the p53 activation pattern. No induction of type IV collagenase expression can be detected under the same experimental conditions in p53-negative Saos-2 cells. All these in vitro and in vivo assays strongly suggest that the type IV collagenase gene is a p53 target gene and that its expression is subject to p53 regulation. Our finding links p53 to a member of the MMP genes, a family of genes implicated in trophoblast implantation, wound healing, angiogenesis, arthritis, and tumor cell invasion. p53 may regulate these processes by upregulating expression of type IV collagenase.

Proteinase inhibitors I and II from potatoes specifically block UV-induced activator protein-1 activation through a pathway that is independent of extracellular signal-regulated kinases, c-Jun N-terminal kinases, and P38 kinase

Solar UV irradiation is the causal factor for the increasing incidence of human skin carcinomas. The activation of the transcription factor activator protein-1 (AP-1) has been shown to be responsible for the tumor promoter action of UV light in mammalian cells. We demonstrate that proteinase inhibitor I (Inh I) and II (Inh II) from potato tubers, when applied to mouse epidermal JB6 cells, block UV-induced AP-1 activation. The inhibition appears to be specific for UV-induced signal transduction for AP-1 activation, because these inhibitors did not block UV-induced p53 activation nor did they exhibit any significant influence on epidermal growth factor-induced AP-1 transactivation. Furthermore, the inhibition of UV-induced AP-1 activity occurs through a pathway that is independent of extracellular signal-regulated kinases and c-Jun N-terminal kinases as well as P38 kinases. Considering the important role of AP-1 in tumor promotion, it is possible that blocking UV-induced AP-1 activity by Inh I or Inh II may be functionally linked to irradiation-induced cell transformation.

p53 activity is essential for normal development in Xenopus

BACKGROUND

The tumor suppressor p53 plays a key role in regulating the cell cycle and apoptosis in differentiated cells. Mutant mice lacking functional p53 develop normally but die from multiple neoplasms shortly after birth. There have been hints that p53 is involved in morphogenesis, but given the relatively normal development of p53 null mice, the significance of these data has been difficult to evaluate. To examine the role of p53 in vertebrate development, we have determined the results of blocking its activity in embryos of the frog Xenopus laevis.

RESULTS

Two different methods have been used to block p53 protein activity in developing Xenopus embryos--ectopic expression of dominant-negative forms of human p53 and ectopic expression of the p53 negative regulator, Xenopus dm-2. In both instances, inhibition of p53 activity blocked the ability of Xenopus early blastomeres to undergo differentiation and resulted in the formation of large cellular masses reminiscent of tumors. The ability of mutant p53 to induce such developmental tumors was suppressed by co-injection with wild-type human or wild-type Xenopus p53. Cells expressing mutant p53 activated zygotic gene expression and underwent the mid-blastula transition normally. Such cells continued to divide at approximately normal rates but did not form normal embryonic tissues and never underwent terminal differentiation, remaining as large, yolk-filled cell masses that were often associated with the neural tube or epidermis.

CONCLUSIONS

In Xenopus, the maternal stockpile of p53 mRNA and protein seems to be essential for normal development. Inhibiting p53 function results in an early block to differentiation. Although it is possible that mutant human p53 proteins have a dominant gain-of-function or neomorphic activity in Xenopus, and that this is responsible for the development of tumors, most of the evidence indicates that this is not the case. Whatever the basis of the block to differentiation, these results indicate that Xenopus embryos are a sensitive system in which to explore the role of p53 in normal development and in developmental tumors.

Co-localization of endogenous and exogenous p53 proteins in nasopharyngeal carcinoma cells

Recently, we have established nine nasopharyngeal carcinoma (NPC) cell lines in which only one cell line showed the p53 mutation. For investigation of the p53 mutation in this line, immunostaining using anti-p53 antibody was applied and showed the presence of p53 protein in the cytoplasm but not in the nucleus. Single strand conformation polymorphism analysis of the p53 gene showed one normal and one additional DNA band. Cloning and sequencing of PCR-amplified DNA showed an AGA (arginine) to ACA (threonine) heterozygous point mutation at codon 280. Transfection of the p53 DNA binding sequence and chloramphenicol acetyltransferase assay revealed loss of transcriptional activation function of endogenous p53 protein. Co-localization of the endogenous and the transfected exogenous p53 protein by polyclonal antibodies to anti-p53 protein revealed strong exogenous p53 staining in the transfected nuclei and weak staining of endogenous p53 protein in the cytoplasm. We concluded that (a) a heterozygous point mutation at codon 280 was identified in the NPC-TW 06 cell line; (b) the point mutation may cause the stagnation of mutant p53 protein in the cytoplasm, and loss of its transcriptional activation function; (c) endogenous and exogenous p53 protein can be co-localized at the same time in the transfected cells; and (d) 280 mutant p53 protein in NPC cells does not cause a decrease or increase in sensitivity to chemotherapy.

Functional characterization of p53 molecules expressed in human squamous cell carcinomas of the head and neck

Mutation of the p53 tumor suppressor gene has been demonstrated in a large proportion of human head and neck squamous cell carcinomas (HNSCCs) and has been assumed to play a role in the pathogenesis of these tumors, although no formal evidence of functional aberration has been demonstrated. In this study, we isolated cDNA clones encoding the entire p53 coding region from six human HNSCC cell lines that showed aberrant patterns of p53 expression in the parental cells, analyzed their nucleotide sequences, and characterized their function in vivo. cDNAs cloned from four cell lines harbored alterations within the p53 coding sequence (one missense mutation, one missense mutation plus in-frame deletion, one splice donor mutation, and a 1-nt insertion). HN30 cells, which contained wild-type p53 nucleotide sequences, showed a high constitutive level of protein expression. HN26 cells contained wild-type coding sequences but did not express the 53-kDa protein, although the mRNA was transcribed and a molecule of increased molecular mass (70 kDa) was observed by western blotting. Functional studies revealed that none of the four proteins encoded by mutant cDNAs were able to transactivate expression of a reporter plasmid containing a wild-type p53 consensus binding site when cotransfected into p53-null cells, whereas molecules encoded by wild-type p53 cDNAs increased reporter gene expression about a hundredfold over uninduced levels. Co-expression of each mutant cDNA with wild-type p53 cDNA and a wild-type p53-responsive reporter gene demonstrated that each of the proteins encoded by mutant cDNAs harbored some degree of inhibitory activity that varied depending on the mutation present. Thus, aberrant p53 function as a result of mutation or altered expression characterizes oral squamous cell carcinomas. The inhibitory activity of these molecules may be a mechanism for deregulation of the function of co-expressed wild-type p53 that may be of importance during the early stages of tumor development.

Redox regulation of transcriptional activators

Transcription factors/activators are a group of proteins that bind to specific consensus sequences (cis elements) in the promoter regions of downstream target/effector genes and transactivate or repress effector gene expression. The up- or downregulation of effector genes will ultimately lead to many biological changes such as proliferation, growth suppression, differentiation, or senescence. Transcription factors are subject to transcriptional and posttranslational regulation. This review will focus on the redox (reduction/oxidation) regulation of transcription factors/activators with emphasis on p53, AP-1, and NF-kappa B. The redox regulation of transcriptional activators occurs through highly conserved cysteine residues in the DNA binding domains of these proteins. In vitro studies have shown that reducing environments increase, while oxidizing conditions inhibit sequence-specific DNA binding of these transcriptional activators. When intact cells have been used for study, a more complex regulation has been observed. Reduction/oxidation can either up- or downregulate DNA binding and/or transactivation activities in transcriptional activator-dependent as well as cell type-dependent manners. In general, reductants decrease p53 and NF-kappa B activities but dramatically activate AP-1 activity. Oxidants, on the other hand, greatly activate NF-kappa B activity. Furthermore, redox-induced biochemical alterations sometimes lead to change in the biological functions of these proteins. Therefore, differential regulation of these transcriptional activators, which in turn, regulate many target/effector genes, may provide an additional mechanism by which small antioxidant molecules play protective roles in anticancer and antiaging processes. Better understanding of the mechanism of redox regulation, particularly in vivo, will have an important impact on drug discovery for chemoprevention and therapy of human disease such as cancer.

Alterations of the p53 tumor-suppressor gene in transformed mouse liver cells

Mutational inactivation of p53, a potential tumor-suppressor gene, has been found in many tumors of humans as well as rodents. The p53 status in normal and transformed mouse liver cell lines has, however, not been investigated. We examined possible point mutations and compared mRNA and protein expression of the p53 gene in normal vs. transformed mouse liver cells. The transformed cells studied included lines spontaneously transformed by sub-culture, virally transformed by simian virus 40 (SV40), and chemically transformed by N-methyl-N-nitro-N-nitrosoguanidine (MNNG) or methylcholanthrene epoxide (MC). A heterozygous G-->A point mutation at codon 241, position 1, of p53 was detected in MNNG-transformed cells after screening of 5 evolutionarily conserved regions where mutation hot-spots are clustered. The mutation causes a gly-->arg substitution. No mutations were found in normal or other transformed cells. The steady-state levels of p53 mRNA were decreased in chemically transformed (both MNNG- and MC-transformed) cells. Elevated levels of p53 protein were found in spontaneously transformed and SV40-transformed cells, an observation that may reflect a longer half-life of the protein, as has been shown in other transformed lines. The low level of the p53 protein in MC-transformed cells may result from transcriptional depression of the p53 gene. We conclude from these data that abnormal p53 status, such as point mutation or altered expression, may play a role during the malignant transformation of mouse liver cells.