MDM2--arbiter of p53's destruction

The Emerging Role of Ubiquitin-Specific Protease 36 (USP36) in Cancer and Beyond

The balance between ubiquitination and deubiquitination is instrumental in the regulation of protein stability and maintenance of cellular homeostasis. The deubiquitinating enzyme, ubiquitin-specific protease 36 (USP36), a member of the USP family, plays a crucial role in this dynamic equilibrium by hydrolyzing and removing ubiquitin chains from target proteins and facilitating their proteasome-dependent degradation. The multifaceted functions of USP36 have been implicated in various disease processes, including cancer, infections, and inflammation, via the modulation of numerous cellular events, including gene transcription regulation, cell cycle regulation, immune responses, signal transduction, tumor growth, and inflammatory processes. The objective of this review is to provide a comprehensive summary of the current state of research on the roles of USP36 in different pathological conditions. By synthesizing the findings from previous studies, we have aimed to increase our understanding of the mechanisms underlying these diseases and identify potential therapeutic targets for their treatment.

Conformational Stability of the N-Terminal Region of MDM2

MDM2 is an E3 ubiquitin ligase which is crucial for the degradation and inhibition of the key tumor-suppressor protein p53. In this work, we explored the stability and the conformational features of the N-terminal region of MDM2 (N-MDM2), through which it binds to the p53 protein as well as other protein partners. The isolated domain possessed a native-like conformational stability in a narrow pH range (7.0 to 10.0), as shown by intrinsic and 8-anilinonapthalene-1-sulfonic acid (ANS) fluorescence, far-UV circular dichroism (CD), and size exclusion chromatography (SEC). Guanidinium chloride (GdmCl) denaturation followed by intrinsic and ANS fluorescence, far-UV CD and SEC at physiological pH, and differential scanning calorimetry (DSC) and thermo-fluorescence experiments showed that (i) the conformational stability of isolated N-MDM2 was very low; and (ii) unfolding occurred through the presence of several intermediates. The presence of a hierarchy in the unfolding intermediates was also evidenced through DSC and by simulating the unfolding process with the help of computational techniques based on constraint network analysis (CNA). We propose that the low stability of this protein is related to its inherent flexibility and its ability to interact with several molecular partners through different routes.

Synthesis and Biological Evaluation of Novel Dispiro-Indolinones with Anticancer Activity

Novel variously substituted thiohydantoin-based dispiro-indolinones were prepared using a regio- and diastereoselective synthetic route from 5-arylidene-2-thiohydantoins, isatines, and sarcosine. The obtained molecules were subsequently evaluated in vitro against the cancer cell lines LNCaP, PC3, HCT^wt, and HCT^(-/-). Several compounds demonstrated a relatively high cytotoxic activity vs. LNCaP cells (IC₅₀ = 1.2-3.5 µM) and a reasonable selectivity index (SI = 3-10). Confocal microscopy revealed that the conjugate of propargyl-substituted dispiro-indolinone with the fluorescent dye Sulfo-Cy5-azide was mainly localized in the cytoplasm of HEK293 cells. P388-inoculated mice and HCT116-xenograft BALB/c nude mice were used to evaluate the anticancer activity of compound 29 in vivo. Particularly, the TGRI value for the P388 model was 93% at the final control timepoint. No mortality was registered among the population up to day 31 of the study. In the HCT116 xenograft model, the compound (170 mg/kg, i.p., o.d., 10 days) provided a T/C ratio close to 60% on day 8 after the treatment was completed. The therapeutic index-estimated as LD₅₀/ED₅₀-for compound 29 in mice was ≥2.5. Molecular docking studies were carried out to predict the possible binding modes of the examined molecules towards MDM2 as the feasible biological target. However, such a mechanism was not confirmed by Western blot data and, apparently, the synthesized compounds have a different mechanism of cytotoxic action.

Imbalance in ALR ubiquitination accelerates the progression of nonalcoholic steatohepatitis to hepatocellular carcinoma

Hepatocellular carcinoma (HCC) is the most common form of primary liver cancer. Accumulating evidence indicates that non-alcoholic steatohepatitis (NASH) is a key predisposing factor for HCC occurrence. However, the precise mechanisms driving NASH transition to HCC remain largely obscure. Augmenter of liver regeneration (ALR) is a sulfhydryl oxidase and cytochrome c reductase that functions as an important regulator of mitochondrial dynamics. In this study, we focused on ALR ubiquitination-mediated degradation and its potential contribution to NASH-driven HCC progression at the mitochondrial level. Hepatic ALR expression in HCC patients was determined using immunohistochemical analysis. Mice with liver-specific deletion of ALR (ALR^CKO) and ALR^WT mice were fed a western diet (WD) and high-sugar solution for induction of NASH. HCC in animals was induced via peritoneal administration of CCl₄. ALR expression was markedly decreased in liver tissues of patients with NASH and HCC compared with non-NASH and non-tumor tissues. Similarly, in ALR^WT mice, the ALR level in tumor tissue was reduced relative to that in para-tumor tissue. In the ALR^CKO group, mice fed WD plus CCl₄ developed HCC starting at week 12 while ALR^WT mice fed WD plus CCl₄ developed HCC at week 24. Analysis of protein posttranslational modifications revealed ubiquitylation (Ub) and deubiquitination (DUb) of ALR by murine double minute 2 (MDM2) and ubiquitin-specific protease 36 (USP36), respectively. Imbalance between Ub and DUb of ALR resulted in profound ALR degradation, which appeared to be reversibly associated with Edmondson-Steiner tumor grade. Rescue of ALR levels via gene transfection abolished tumor malignant features to a certain extent in vitro. Notably, ALR deletion substantially enhanced mitochondrial fission by activating Drp1 phosphorylation at Ser616, thus disrupting the balance of mitochondrial dynamics between fission and fusion and severely impairing oxidative phosphorylation (OXPHOS) and ATP synthesis, instead enhancing anaerobic metabolism, which might be attributed to steatotic hepatocyte transition into the malignant HCC phenotype. Hepatic ALR depletion via dysregulation of ubiquitination is a critical aggravator of NASH-HCC progression and represents a promising therapeutic target for related liver diseases.

Features of p53 protein distribution in the corneal epithelium and corneal tear film

Tumor suppressor protein p53 is the key factor in the regulation of cell proliferation. Its concentration is low in the cytoplasm of most cell types. However, in corneal epithelium cells, abnormally high p53 content is detected. The aim of the present study was to characterize p53 distribution in the corneal epithelium. For this purpose, immunohistochemistry, western blot analysis and electronic microscope examinations were performed. A low level of p53 was identified in the lens, iris and retina; by contrast, a significantly high concentration of this protein was observed in the corneal epithelium. In opposite, MDM2 was identified in the lens, iris and retina while it is completely absent in the corneal epithelium. In addition, we found a significant amount of exosomes and other microvesicles containing p53 in the corneal mucin layer. We thus hypothesize that a significantly high level of p53 was caused by a combination of absents of MDM2 in parallel with p53 microvesicles storage.

JAB1/COPS5 is a putative oncogene that controls critical oncoproteins deregulated in prostate cancer

Recent evidence support that the c-Jun activation domain-binding protein 1 (JAB1)/COPS5 has an oncogenic function in various tissues. We show that JAB1 amplification in human prostate cancer (PCa) correlates with reduced overall survival and disease-free progression. Immunohistochemical staining shows enhanced expression of JAB1 in the cytoplasmic compartment of PCa cells compared to the normal prostate epithelium, indicating the activity/function of JAB1 is altered in PCa. To test the function of JAB1 in PCa, we efficiently silenced JAB1 expression using four unique shRNAs in three PCa cell lines (LNCaP, C4-2, and PC-3) and an immortalized prostate epithelial cell line, RWPE-1. Our data clearly show that silencing JAB1 robustly suppresses the growth of PCa cells, but not RWPE-1 cells, suggesting that PCa cells become addicted to JAB1. To study the potential mechanism by which JAB1 controls PCa growth, we profiled gene expression changes by whole transcriptome microarray analysis of C4-2 cells silenced for JAB1 using a pool of 3 shRNAs compared to scrambled shRNA control. We identified 1268 gene changes ≥1.5 fold by silencing JAB1 in C4-2. Western blot confirmation and bioinformatics pathway analyses support that PCa cells become addicted to JAB1 through controlling the following signaling pathways: cell cycle, p53 signaling, DNA replication, TGF-β/BMP, MAPK, TNF, and steroid hormone biosynthesis. We propose that UGT2B28, UGT2B10, UGT2B11, Skp2, EZH2, MDM2, BIRC5 (Survivin), UBE2C, and Smads 1/5/8, which are all associated with the abovementioned key oncogenic pathways, may play critical roles in the putative oncogenic function of JAB1 in PCa.

Therapeutics targeting Bcl-2 in hematological malignancies

Members of the B-cell lymphoma 2 (BCL-2) gene family are attractive targets for cancer therapy as they play a key role in promoting cell survival, a long-since established hallmark of cancer. Clinical utility for selective inhibition of specific anti-apoptotic Bcl-2 family proteins has recently been realized with the Food and Drug Administration (FDA) approval of venetoclax (formerly ABT-199/GDC-0199) in relapsed chronic lymphocytic leukemia (CLL) with 17p deletion. Despite the impressive monotherapy activity in CLL, such responses have rarely been observed in other B-cell malignancies, and preclinical data suggest that combination therapies will be needed in other indications. Additional selective antagonists of Bcl-2 family members, including Bcl-X_L and Mcl-1, are in various stages of preclinical and clinical development and hold the promise of extending clinical utility beyond CLL and overcoming resistance to venetoclax. In addition to direct targeting of Bcl-2 family proteins with BH3 mimetics, combination therapies that aim at down-regulating expression of anti-apoptotic BCL-2 family members or restoring expression of pro-apoptotic BH3 family proteins may provide a means to deepen responses to venetoclax and extend the utility to additional indications. Here, we review recent progress in direct and selective targeting of Bcl-2 family proteins for cancer therapy and the search for rationale combinations.

Correlation of TP53 Overexpression and Clinical Parameters with Five-Year Survival in Oral Squamous Cell Carcinoma Patients

Introduction

TP53 mutation and overexpression have been correlated with poor survival in many cancers including oral squamous cell carcinoma (OSCC). We aim to understand the role of TP53 overexpression in OSCC in our population and correlate it with five-year survival to test its viability as a prognostic marker for OSCC patients.

Materials and methods

Patients with biopsy proven OSCC at Aga Khan University Hospital from January 2000 to January 2008 were recruited. Immunohistochemistry was used to establish TP53 status and the results were published. Following up on these patients, five-year data were collected and correlated with TP53 status and other clinicopathologic parameters.

Results

Overexpression of TP53 was not significantly associated with five-year survival (hazard ratio [HR]: 1.543; 95% CI: 0.911-2.612; p = 0.107).

Conclusion

Although we had proven statistical relevance when correlated with overall survival in our previous study, we were unable to extend the same relevance to TP53 overexpression when it comes to five-year survival.

The overexpression of SOX2 affects the migration of human teratocarcinoma cell line NT2/D1

The altered expression of the SOX2 transcription factor is associated with oncogenic or tumor suppressor functions in human cancers. This factor regulates the migration and invasion of different cancer cells. In this study we investigated the effect of constitutive SOX2 overexpression on the migration and adhesion capacity of embryonal teratocarcinoma NT2/D1 cells derived from a metastasis of a human testicular germ cell tumor. We detected that increased SOX2 expression changed the speed, mode and path of cell migration, but not the adhesion ability of NT2/D1 cells. Additionally, we demonstrated that SOX2 overexpression increased the expression of the tumor suppressor protein p53 and the HDM2 oncogene. Our results contribute to the better understanding of the effect of SOX2 on the behavior of tumor cells originating from a human testicular germ cell tumor. Considering that NT2/D1 cells resemble cancer stem cells in many features, our results could contribute to the elucidation of the role of SOX2 in cancer stem cells behavior and the process of metastasis.

Structure-activity relationship and antitumor activity of thio-benzodiazepines as p53-MDM2 protein-protein interaction inhibitors

In order to discuss the structure-activity relationship (SAR) of the thio-benzodiazepine compounds which showed excellent activity against p53-MDM2 protein-protein interaction, we designed and synthesized twenty compounds with electrophilic and nucleophilic groups on the benzene ring. Among them, compounds 8i (K(i) = 91 nM) and 8n (K(i) = 89 nM) showed better binding activity than that of the reference drug Nutlin-3a (K(i) = 121 nM). In addition, in vitro antitumor activity against Saos-2, U-2 OS, A549 and NCI-H1299 cell-lines were assayed by the MTT method. Especially, compounds 8i and 8n possessed excellent biological activity and good selectivity comparable to Nutlin-3a, which were promising candidates for further evaluation.

Molecular dynamic simulation insights into the normal state and restoration of p53 function

As a tumor suppressor protein, p53 plays a crucial role in the cell cycle and in cancer prevention. Almost 50 percent of all human malignant tumors are closely related to a deletion or mutation in p53. The activity of p53 is inhibited by over-active celluar antagonists, especially by the over-expression of the negative regulators MDM2 and MDMX. Protein-protein interactions, or post-translational modifications of the C-terminal negative regulatory domain of p53, also regulate its tumor suppressor activity. Restoration of p53 function through peptide and small molecular inhibitors has become a promising strategy for novel anti-cancer drug design and development. Molecular dynamics simulations have been extensively applied to investigate the conformation changes of p53 induced by protein-protein interactions and protein-ligand interactions, including peptide and small molecular inhibitors. This review focuses on the latest MD simulation research, to provide an overview of the current understanding of interactions between p53 and its partners at an atomic level.

Udu deficiency activates DNA damage checkpoint

Udu has been shown to play an essential role during blood cell development; however, its roles in other cellular processes remain largely unexplored. In addition, ugly duckling (udu) mutants exhibited somite and myotome boundary defects. Our fluorescence-activated cell sorting analysis also showed that the loss of udu function resulted in defective cell cycle progression and comet assay indicated the presence of increased DNA damage in udu(tu24) mutants. We further showed that the extensive p53-dependent apoptosis in udu(tu24) mutants is a consequence of activation in the Atm-Chk2 pathway. Udu seems not to be required for DNA repair, because both wild-type and udu embryos similarly respond to and recover from UV treatment. Yeast two-hybrid and coimmunoprecipitation data demonstrated that PAH-L repeats and SANT-L domain of Udu interacts with MCM3 and MCM4. Furthermore, Udu is colocalized with 5-bromo-2'-deoxyuridine and heterochromatin during DNA replication, suggesting a role in maintaining genome integrity.

Clinicopathological correlations of cyclooxygenase-2, MDM2, and p53 expressions in surgically resectable pancreatic invasive ductal adenocarcinoma

OBJECTIVES

Cyclooxygenase-2 (COX-2) and p53 represent molecules linked to oncogenesis of pancreatic cancer, and there is also a known regulatory loop between mouse double minute 2 (MDM2) and p53. The complex cross talks between p53 and COX-2 and scenarios explaining patterns of p53 and COX-2 expressions in precursor and cancer lesions have been recently reported.

METHODS

The expressions of COX-2, p53, and MDM2 were examined using immunohistochemistry in 85 resection specimens of pancreatic ductal adenocarcinoma.

RESULTS

The positive tumor expression rates of COX-2, p53, and MDM2 were 69.4%, 60.0%, and 41.2%, respectively. Significant correlations between COX-2 and p53 expressions and between p53 and MDM2 expressions were revealed. In the Kaplan-Meier analysis, no statistically significant correlations were found among the levels of COX-2, p53, and MDM2 expressions and survival rates. In the multivariate Cox proportional hazards regression model, grade and nodal status showed to be a valuable predictor of a worse overall survival.

CONCLUSIONS

The reported findings confirmed the relationship of p53, MDM2, and COX-2 with the biological process of pancreatic cancer. The expression of none of the examined proteins showed to be a valuable independent prognostic factor. On the contrary, grade and nodal status showed to be a valuable predictor of a worse survival.

Epstein-Barr virus nuclear antigen 3C augments Mdm2-mediated p53 ubiquitination and degradation by deubiquitinating Mdm2

Epstein-Barr virus (EBV) nuclear antigen 3C (EBNA3C) is one of the essential latent antigens for primary B-cell transformation. Previous studies established that EBNA3C facilitates degradation of several vital cell cycle regulators, including the retinoblastoma (pRb) and p27(KIP) proteins, by recruitment of the SCF(Skp2) E3 ubiquitin ligase complex. EBNA3C was also shown to be ubiquitinated at its N-terminal residues. Furthermore, EBNA3C can bind to and be degraded in vitro by purified 20S proteasomes. Surprisingly, in lymphoblastoid cell lines, EBNA3C is extremely stable, and the mechanism for this stability is unknown. In this report we show that EBNA3C can function as a deubiquitination enzyme capable of deubiquitinating itself in vitro as well as in vivo. Functional mapping using deletion and point mutational analysis showed that both the N- and C-terminal domains of EBNA3C contribute to the deubiquitination activity. We also show that EBNA3C efficiently deubiquitinates Mdm2, an important cellular proto-oncogene, which is known to be overexpressed in several human cancers. The data presented here further demonstrate that the N-terminal domain of EBNA3C can bind to the acidic domain of Mdm2. Additionally, the N-terminal domain of EBNA3C strongly stabilizes Mdm2. Importantly, EBNA3C simultaneously binds to both Mdm2 and p53 and can form a stable ternary complex; however, in the presence of p53 the binding affinity of Mdm2 toward EBNA3C was significantly reduced, suggesting that p53 and Mdm2 might share a common overlapping domain of EBNA3C. We also showed that EBNA3C enhances the intrinsic ubiquitin ligase activity of Mdm2 toward p53, which in turn facilitated p53 ubiquitination and degradation. Thus, manipulation of the oncoprotein Mdm2 by EBNA3C potentially provides a favorable environment for transformation and proliferation of EBV-infected cells.

Updates on p53: modulation of p53 degradation as a therapeutic approach

The p53 pathway is aberrant in most human tumours with over 50% expressing mutant p53 proteins. The pathway is critically controlled by protein degradation. Here, we discuss the latest developments in the search for small molecules that can modulate p53 pathway protein stability and restore p53 activity for cancer therapy.

Gene expression and development of mouse zygotes following droplet vitrification

The concept of ultra-rapid vitrification has emerged in recent years; the accelerated cooling rate reduced injury attributed to cryopreservation and improved post-freezing developmental competence of vitrified oocytes and embryos. The objectives of the present study were to develop a simple and effective ultra-rapid vitrification method (droplet vitrification) and evaluate its effects on post-thaw development and apoptosis-related gene expression in mouse zygotes. Presumptive zygotes were equilibrated for 3 min in equilibration medium and washed 3 times in vitrification solution. A drop (5 microL) of vitrification solution containing 10-12 embryos was placed directly onto surface of liquid nitrogen, with additional liquid nitrogen poured over the drop. For thawing and cryoprotectant removal, vitrified drops were put into dilution medium for 3 min, followed by M2 medium for 5 min. Although cleavage rate did not differ significantly among the control (90.8+/-2.8%; mean+/-S.E.M.), toxicity control (83.5+/-3.2%), and vitrified (86.2+/-3.1%) zygotes, rates of blastocyst and hatched blastocyst formation were lower (P<0.01) in vitrified zygotes (49.7+/-4.7% and 36.0+/-4.7%) and toxicity controls (47.3+/-4.6% and 40.3+/-4.6%) compared with controls (65.5+/-4.1% and 54.2+/-4.3%). Exposure of zygotes to vitrification solution, as well as the vitrification process, down-regulated the expression of Bax, Bcl2, and p53 genes in blastocysts. Although droplet vitrification was efficient and easy, it altered the transcriptional activities of Bax, Bcl2, and p53 genes in vitrified embryos, indicating a strong relationship between reduced developmental competence and the altered transcriptional activities of these genes.

TAT-RasGAP317-326 requires p53 and PUMA to sensitize tumor cells to genotoxins

Although chemotherapy has revolutionized cancer treatment, the associated side effects induced by lack of specificity to tumor cells remain a challenging problem. We have previously shown that TAT-RasGAP(317-326),a cell-permeable peptide derived from RasGAP, specifically sensitizes cancer cells to the action of genotoxins. The underlying mechanisms of this sensitization were not defined however. Here, we report that TAT-RasGAP(317-326) requires p53, but not the Ras effectors Akt and extracellular signal-regulated kinase, to mediate its tumor sensitization abilities. The TAT-RasGAP(317-326) peptide, although not modulating the transcriptional activity of p53 or its phosphorylation and acetylation status, nevertheless requires a functional p53 cellular status to increase the sensitivity of tumor cells to genotoxins. Genes regulated by p53 encode proapoptotic proteins, such as PUMA, and cell cycle control proteins, such as p21. The ability of TAT-RasGAP(317-326) to sensitize cancer cells was found to require PUMA but not p21. TAT-RasGAP(317-326) did not affect PUMA levels, however, but increased genotoxin-induced mitochondrial depolarization and caspase-3 activation. These results indicate that TAT-RasGAP(317-326) sensitizes tumor cells by activating signals that intersect with the p53 pathway downstream of, or at the level of, proapoptotic p53 target gene products to increase the activation of the mitochondrial death pathway.

Ubiquitination of MDM2 modulated by Epstein-Barr virus encoded latent membrane protein 1

Epstein-Barr virus encoded latent membrane protein 1 (LMP1), an oncogenic protein, plays an important role in the carcinogenesis of nasopharyngeal carcinoma. The MDM2 gene is a cellular pro-oncogene that is abnormally up-regulated in human tumors. MDM2 is overexpressed in nasopharyngeal carcinoma, which is associated with the presence of EBV and cervical lymph node metastasis. Because MDM2 is capable of self-ubiquitination, and the ubiquitin proteasome pathway-dependent degradation is an important mechanism for regulating MDM2 levels in cells. Here we show that LMP1 augment MDM2 protein expression in dose-dependent level, and also lead to a drastic accumulation of ubiquitinated MDM2 species, this effect is associated with the stability of MDM2 modulated by LMP1. This is the first time to explain LMP1-regulated MDM2 through a post-ubiquitination mechanism.

Ribosomal protein S7 as a novel modulator of p53-MDM2 interaction: binding to MDM2, stabilization of p53 protein, and activation of p53 function

As a major negative regulator of p53, the MDM2 oncogene plays an important role in carcinogenesis and tumor progression. MDM2 promotes p53 proteasomal degradation and negatively regulates p53 function. The mechanisms by which the MDM2-p53 interaction is regulated are not fully understood, although several MDM2-interacting molecules have recently been identified. To search for novel MDM2-binding partners, we screened a human prostate cDNA library by the yeast two-hybrid assay using full-length MDM2 protein as the bait. Among the candidate proteins, ribosomal protein S7 was identified and confirmed as a novel MDM2-interacting protein. Herein, we demonstrate that S7 binds to MDM2, in vitro and in vivo, and that the interaction between MDM2 and S7 leads to modulation of MDM2-p53 binding by forming a ternary complex among MDM2, p53 and S7. This results in the stabilization of p53 protein through abrogation of MDM2-mediated p53 ubiquitination. Consequently, S7 overexpression increases p53 transactivational activities, induces apoptosis, and inhibits cell proliferation. The identification of S7 as a novel MDM2-interacting partner contributes to elucidation of the complex regulation of the MDM2-p53 interaction and has implications in cancer prevention and therapy.

Protoporphyrin IX Interacts with Wild-type p53 Protein in Vitro and Induces Cell Death of Human Colon Cancer Cells in a p53-dependent and -independent Manner

Photodynamic therapy (PDT) of cancer is an alternative treatment for tumors resistant to chemo- and radiotherapy. It induces cancer cell death mainly through generation of reactive oxygen species by a laser light-activated photosensitizer. It has been suggested that the p53 tumor suppressor protein sensitizes some human cancer cells to PDT. However, there is still no direct evidence for this. We have demonstrated here for the first time that the photosensitizer protoporphyrin IX (PpIX) binds to p53 and disrupts the interaction between p53 tumor suppressor protein and its negative regulator HDM2 in vitro and in cells. Moreover, HCT116 colon cancer cells exhibited a p53-dependent sensitivity to PpIX in a dose-dependent manner, as was demonstrated using 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide assay and fluorescence-activated cell sorter (FACS) analysis of cell cycle profiles. We have also observed induction of p53 target pro-apoptotic genes, e.g. puma (p53-up-regulated modulator of apoptosis), and bak in PpIX-treated cells. In addition, p53-independent growth suppression by PpIX was detected in p53-negative cells. PDT treatment (2 J/cm2) of HCT116 cells induced p53-dependent activation of pro-apoptotic gene expression followed by growth suppression and induction of apoptosis.

Effects of in vitro fertilization and embryo culture on TRP53 and Bax expression in B6 mouse embryos

In the mouse, embryo culture results in a characteristic phenotype of retarded embryo preimplantation development and reduced numbers of cells within embryos. The expression of TRP53 is central to the regulation of the cell's capacity to proliferate and survive. In this study we found that Trp53 mRNA is expressed throughout the preimplantation stage of development. Levels of TRP53 protein expression were low during the cleavage stages and increased at the morula and blastocyst stages in B6 embryos collected from the reproductive tract. Embryos collected at the zygote stage and cultured for 96 h also showed low levels of TRP53 expression at precompaction stages. There were higher levels of TRP53 in cultured morula and the level in cultured blastocysts was clearly increased above blastocysts collected directly from the uterus. Immunolocalization of TRP53 showed that its increased expression in cultured blastocysts corresponded with a marked accumulation of TRP53 within the nuclei of embryonic cells. This pattern of expression was enhanced in embryos produced by in vitro fertilization and subjected to culture. The TRP53 was transcriptionally active since culture also induced increased expression of Bax, yet this did not occur in embryos lacking Trp53 (Trp53-/-). The rate of development of Trp53-/- zygotes to the blastocyst stage was not different to wildtype controls when embryos were cultured in groups of ten but was significantly faster when cultured individually. The results show that zygote culture resulted in the accumulation of transcription activity of TRP53 in the resulting blastocysts. This accounts for the adverse effects of culture of embryos individually, but does not appear to be the sole cause of the retarded preimplantation stage growth phenotype associated with culture in vitro.

Culture of zygotes increases TRP53 [corrected] expression in B6 mouse embryos, which reduces embryo viability

The expression of TRP53 in blastocysts that had been cultured from the zygote stage in vitro for 90 h was compared with that in blastocysts collected from the uterus in C57BL6 (B6) and in F1 hybrid (B6CBF1) strain mice. In both strains, there was little TRP53 detected in blastocysts collected from the uterus. There was some increased expression in cultured embryos from B6CBF1 mice and marked increased expression in cultured B6 blastocysts. In cultured B6 embryos, there was obvious accumulation of TRP53 within the nuclear region of embryonic cells. Cultured B6 zygotes had significantly poorer rates of blastocyst formation and of capacity to undergo implantation or form viable fetuses than cultured zygotes from B6CBF1 mice or B6 blastocysts collected from the uterus. Trp53-/- zygotes (B6 background) were significantly more likely to form blastocysts than sibling wild-type embryos, with Trp53+/- embryos having an intermediate level of viability (P<0.01). On transfer of blastocysts to recipient females, Trp53-/- blastocysts were more likely to form viable fetuses than wild-type or heterozygous sibling blastocysts when the embryos resulted from culture of zygotes (P<0.001). This shift in viability did not occur when embryos were only subjected to 24 h of culture from the compacted embryo stage. Culture in vitro in the B6 strain caused a marked increase in the expression and nuclear accumulation of TRP53. This expression was a significant cause of the loss of viability that occurs on culture of zygotes from this strain in vitro.

Conformational changes of the p53-binding cleft of MDM2 revealed by molecular dynamics simulations

Two 35-ns molecular dynamics simulations of both ligated [mouse double minute protein 2 (MDM2(p53))] and unligated (MDM2(apo)) structures of human MDM2 bound to the N-terminal domain of the tumor suppressor p53 have been performed. Analysis of the dynamics revealed that the most flexible region of MDM2 was the p53-binding cleft. When MDM2 was bound to p53, a wider and more stable topology of the cleft was obtained, while unligated MDM2 showed a narrower and highly flexible cleft. It was also found that the dynamics involved in the opening/closing motions were due to the movement of different domains of the protein, which is in agreement with recent experimental data. Considering our results, a mechanism in which p53 might be recognized and attached to MDM2 is proposed, and some implications on future directions for in silico anticancer drug design efforts are discussed. In summary, the observations made here would be very useful not only for better understanding of the biological implications of the MDM2 dynamics, but also for future efforts in anticancer drug design and discovery.

KAP1 dictates p53 response induced by chemotherapeutic agents via Mdm2 interaction

KAP1 recruits many proteins involved in gene silencing and functions as an integral part of co-repressor complex. KAP1 was identified as Mdm2-binding protein and shown to form a complex with Mdm2 and p53 in vivo. We examined the role of KAP1 in p53 activation after the treatment of cells with different types of external stresses. KAP1 reduction markedly enhanced the induction of p21, a product of the p53 target gene, after treatment with actinomycin D or gamma-irradiation, but not with camptothecin. Treatment with actinomycin D, but not with camptothecin, augmented the interaction of p53 with Mdm2 and KAP1. Further, KAP1 reduction in actinomycin D-treated cells facilitated cell cycle arrest and negatively affected clonal cell growth. Thus, the reduction of KAP1 levels promotes p53-dependent p21 induction and inhibits cell proliferation in actinomycin D-treated cells. KAP1 may serve as a therapeutic target against cancer in combination with actinomycin D.

Increased frequencies of CD8+ T lymphocytes recognizing wild-type p53-derived epitopes in peripheral blood correlate with presence of epitope loss tumor variants in patients with hepatocellular carcinoma

Wild-type (WT) sequence p53 peptides are attractive candidates for broadly applicable cancer vaccines. The aim of this study was to evaluate the potential of a WT p53-based immunotherapeutic approach for patients with hepatocellular carcinoma (HCC). Circulating CD8+ T cells specific for WT p53(149-157) and WT p53(264-272) HLA-A*0201 restricted epitopes were directly identified in the peripheral blood by the use of peptide/HLA-A2.1 tetramers in 24 HCC patients. Cytotoxic T lymphocyte (CTL) activity after WT p53 peptide-specific stimulation was assessed by analysis of granzyme B and interferon-gamma mRNA transcription, using a quantitative real-time polymerase chain reaction assay. Tumor immunophenotyping was performed to evaluate the p53 status, the expression of major histocompatibility complex (MHC) and costimulatory molecules in freshly isolated tumor cells. HCC patients exhibited significantly higher frequencies of WT p53-specific memory CD8+ T cells and stronger WT p53-specific CTL activity, when compared with healthy controls. Increased frequencies of p53-specific CD8+ T cells and their activity correlated with selective HLA-A2 allele loss and reduced costimulatory molecule expression of tumor cells. Moreover, augmented numbers of p53-specific T cells coincided with high MHC class II expression in tumor cells but were inversely related to the T status of the tumor node metastasis staging system. Our results indicate the existence of natural immunosurveillance and tumor immune evasion, involving a T cell response against WT p53 tumor antigen in patients with HCC. These findings may have important implications for the future development of cancer vaccines.

DNA damage-induced phosphorylation of MdmX at serine 367 activates p53 by targeting MdmX for Mdm2-dependent degradation

Understanding how p53 activity is regulated is crucial in elucidating mechanisms of cellular defense against cancer. Genetic data indicate that Mdmx as well as Mdm2 plays a major role in maintaining p53 activity at low levels in nonstressed cells. However, biochemical mechanisms of how Mdmx regulates p53 activity are not well understood. Through identification of Mdmx-binding proteins, we found that 14-3-3 proteins are associated with Mdmx. Mdmx harbors a consensus sequence for binding of 14-3-3. Serine 367 (S367) is located within the putative binding sequence for 14-3-3, and its substitution with alanine (S367A) abolishes binding of Mdmx to 14-3-3. Transfection assays indicated that the S367A mutation, in cooperation with Mdm2, enhances the ability of Mdmx to repress the transcriptional activity of p53. The S367A mutant is more resistant to Mdm2-dependent ubiquitination and degradation than wild-type Mdmx, and Mdmx phosphorylated at S367 is preferentially degraded by Mdm2. Several types of DNA damage markedly enhance S367 phosphorylation, coinciding with increased binding of Mdmx to 14-3-3 and accelerated Mdmx degradation. Furthermore, promotion of growth of normal human fibroblasts after introduction of Mdmx is enhanced by the S367 mutation. We propose that Mdmx phosphorylation at S367 plays an important role in p53 activation after DNA damage by triggering Mdm2-dependent degradation of Mdmx.

Structure of free MDM2 N-terminal domain reveals conformational adjustments that accompany p53-binding

Critical to the inhibitory action of the oncogene product, MDM2, on the tumour suppressor, p53, is association of the N-terminal domain of MDM2 (MDM2N) with the transactivation domain of p53. The structure of MDM2N was previously solved with a p53-derived peptide, or small-molecule ligands, occupying its binding cleft, but no structure of the non-liganded MDM2N (i.e. the apo-form) has been reported. Here, we describe the solution structure and dynamics of apo-MDM2N and thus reveal the nature of the conformational changes in MDM2N that accompany binding of p53. The new structure suggests that p53 effects displacement of an N-terminal segment of apo-MDM2N that occludes access to the shallow end of the p53-binding cleft. MDM2N must also undergo an expansion upon binding, achieved through a rearrangement of its two pseudosymetrically related sub-domains resulting in outward displacements of the secondary structural elements that comprise the walls and floor of the p53-binding cleft. MDM2N becomes more rigid and stable upon binding p53. Conformational plasticity of the binding cleft of apo-MDM2N could allow the parent protein to bind specifically to several different partners, although, to date, all the known liganded structures of MDM2N are highly similar to one another. The results indicate that the more open conformation of the binding cleft of MDM2N observed in structures of complexes with small molecules and peptides is a more suitable one for ligand discovery and optimisation.

Quinone reductases multitasking in the metabolic world

The multiple functions of NAD(P)H:quinone oxidoreductase 1 (NQO1, DT-diaphorase) in the cell are reviewed. NQO1 has long been viewed as a chemoprotective enzyme involved in cellular defense against the electrophilic and oxidizing metabolites of xenobiotic quinones. It also participates in reduction of endogenous quinones, such as vitamin E quinone and ubiquinone, generating antioxidant forms of these molecules. NQO1 has recently been shown to interact with superoxide and may be involved in scavenging superoxide within the cell. In addition, the possible role of NQO1 in p53 stabilization and consequently in contributing to p53-dependent stress responses is summarized. Such protein multitasking is a good strategy in terms of cellular economy. NQO1 can also be exploited in the design of NQO1-directed antitumor agents such as the new aziridinylbenzoquinone RH1 and Hsp90 inhibitors such as 17AAG. Polymorphisms in NQO1 which have profound influence on phenotype such as the NQO1*2 polymorphism may influence the chemoprotective actions of NQO1, and should be considered when NQO1-directed antitumor quinones are used for therapy in patients.

Transcription - guarding the genome by sensing DNA damage

Cells induce the expression of DNA-repair enzymes, activate cell-cycle checkpoints and, under some circumstances, undergo apoptosis in response to DNA-damaging agents. The mechanisms by which these cellular responses are triggered are not well understood, but there is recent evidence that the transcription machinery might be used in DNA-damage surveillance and in triggering DNA-damage responses to suppress mutagenesis. Transcription might also act as a DNA-damage dosimeter where the severity of blockage determines whether or not to induce cell death. Could transcription therefore be a potential therapeutic target for anticancer strategies?

Role of cell cycle in mediating sensitivity to radiotherapy

Multiple pathways are involved in maintaining the genetic integrity of a cell after its exposure to ionizing radiation. Although repair mechanisms such as homologous recombination and nonhomologous end-joining are important mammalian responses to double-strand DNA damage, cell cycle regulation is perhaps the most important determinant of ionizing radiation sensitivity. A common cellular response to DNA-damaging agents is the activation of cell cycle checkpoints. The DNA damage induced by ionizing radiation initiates signals that can ultimately activate either temporary checkpoints that permit time for genetic repair or irreversible growth arrest that results in cell death (necrosis or apoptosis). Such checkpoint activation constitutes an integrated response that involves sensor (RAD, BRCA, NBS1), transducer (ATM, CHK), and effector (p53, p21, CDK) genes. One of the key proteins in the checkpoint pathways is the tumor suppressor gene p53, which coordinates DNA repair with cell cycle progression and apoptosis. Specifically, in addition to other mediators of the checkpoint response (CHK kinases, p21), p53 mediates the two major DNA damage-dependent cellular checkpoints, one at the G(1)-S transition and the other at the G(2)-M transition, although the influence on the former process is more direct and significant. The cell cycle phase also determines a cell's relative radiosensitivity, with cells being most radiosensitive in the G(2)-M phase, less sensitive in the G(1) phase, and least sensitive during the latter part of the S phase. This understanding has, therefore, led to the realization that one way in which chemotherapy and fractionated radiotherapy may work better is by partial synchronization of cells in the most radiosensitive phase of the cell cycle. We describe how cell cycle and DNA damage checkpoint control relates to exposure to ionizing radiation.

Reversible aggregation plays a crucial role on the folding landscape of p53 core domain

The role of tumor suppressor protein p53 in cell cycle control depends on its flexible and partially unstructured conformation, which makes it crucial to understand its folding landscape. Here we report an intermediate structure of the core domain of the tumor suppressor protein p53 (p53C) during equilibrium and kinetic folding/unfolding transitions induced by guanidinium chloride. This partially folded structure was undetectable when investigated by intrinsic fluorescence. Indeed, the fluorescence data showed a simple two-state transition. On the other hand, analysis of far ultraviolet circular dichroism in 1.0 M guanidinium chloride demonstrated a high content of secondary structure, and the use of an extrinsic fluorescent probe, 4,4'-dianilino-1,1' binaphthyl-5,5'-disulfonic acid, indicated an increase in exposure of the hydrophobic core at 1 M guanidinium chloride. This partially folded conformation of p53C was plagued by aggregation, as suggested by one-dimensional NMR and demonstrated by light-scattering and gel-filtration chromatography. Dissociation by high pressure of these aggregates reveals the reversibility of the process and that the aggregates have water-excluded cavities. Kinetic measurements show that the intermediate formed in a parallel reaction between unfolded and folded structures and that it is under fine energetic control. They are not only crucial to the folding pathway of p53C but may explain as well the vulnerability of p53C to undergo departure of the native to an inactive state, which makes the cell susceptible to malignant transformation.

Regulation of p53 by the ubiquitin-conjugating enzymes UbcH5B/C in vivo

p53 levels are regulated by ubiquitination and 26 S proteasome-mediated degradation. p53 is a substrate for the E3 ligase Mdm2, however, the ubiquitin-conjugating enzymes (E2s) involved in p53 ubiquitination in intact cells have not been defined previously. To investigate the E2 specificity of Mdm2 we carried out an in vitro screen using a panel of ubiquitin E2s. Of the E2s tested only UbcH5A, -B, and -C and E2-25K support Mdm2-mediated ubiquitination of p53. The same E2s also support Mdm2 auto-ubiquitination. Small interfering RNA-mediated knockdown of UbcH5B/C causes accumulation of Mdm2 and p53 in unstressed cells. We show that suppression of UbcH5B/C inhibits p53 ubiquitination and degradation. Despite up-regulating the level of nuclear p53, UbcH5B/C knockdown does not on its own result in an increase in p53 transcriptional activity or sensitize p53 to activation by the therapeutic drugs doxorubicin and actinomycin D. We provide evidence that Mdm2 is responsible, at least in part, for repression of the transcriptional activity of the accumulated p53. In MCF7 cells levels of UbcH5B/C are reduced by doxorubicin and actinomycin D. This observation and the sensitivity of p53 expression to levels of UbcH5B/C raise the possibility that E2 regulation could be involved in signaling pathways that control the stability of p53. Our data indicate that UbcH5B/C are physiological E2s for Mdm2, which make a significant contribution to the maintenance of low levels of p53 and Mdm2 in unstressed cells and that inhibition of p53 ubiquitination and degradation by targeting UbcH5B/C is not sufficient to up-regulate p53 transcriptional activity.

Phosphorylation of the histone deacetylase 7 modulates its stability and association with 14-3-3 proteins

Class II histone deacetylases (HDACs) play a role in myogenesis and inhibit transcriptional activation by myocyte enhancer factors 2. A distinct feature of class II HDACs is their ability to shuttle between the nucleus and the cytoplasm in a cell type- and signal-dependent manner. We demonstrate here that treatment with the 26 S proteosome inhibitors, MG132 and ALLN, leads to detection of ubiquitinated HDAC7 and causes accumulation of cytoplasmic HDAC7. We also show that treatment with calyculin A, a protein phosphatase inhibitor, leads to a marked increase of HDAC7 but not HDAC5. The increase in HDAC7 is accompanied by enhanced interaction between 14-3-3 proteins and HDAC7. HDAC7 mutations that prevent the interaction with 14-3-3 proteins also block calyculin A-mediated stabilization. Expression of constitutively active calcium/calmodulin-dependent kinase I stabilizes HDAC7 and causes an increased association between HDAC7 and 14-3-3. Together, our results suggest that calcium/calmodulin-dependent kinase I-mediated phosphorylation of HDAC7 acts, in part, to promote association of HDAC7 with 14-3-3 and stabilizes HDAC7.

Overexpression of leukocyte marker CD43 causes activation of the tumor suppressor proteins p53 and ARF

CD43 or leukosialin is a transmembrane sialoglycoprotein, whose extracellular domain participates in cell adhesiveness and the cytoplasmic tail regulates a variety of intracellular signal transduction pathways involved in cell proliferation. CD43 is abundantly expressed on the surface of hematopoietic cells, but CD43 expression is also frequently found in the tumor cells of nonhematopoietic origin. In the early stages of some tumors, the accumulation of tumor suppressor protein p53 has been described. Here, we show that the expression of CD43 causes the induction of functionally active p53 protein. Moreover, we found that the activation of p53 by CD43 is mediated by tumor suppressor protein ARF. The coexpression of CD43 and ARF in ARF-null mouse embryonic fibroblasts resulted in programmed cell death, but that was not the case when CD43 alone was expressed in these cells. These data provide the first evidence of the connection between p53- and CD43-dependent pathways.

Binding of p53-derived ligands to MDM2 induces a variety of long range conformational changes

We have used NMR to study the effects of peptide binding on the N-terminal p53-binding domain of human MDM2 (residues 25-109). There were changes in HSQC-chemical shifts throughout the domain on binding four different p53-derived peptide ligands that were significantly large to be indicative of global conformational changes. Large changes in chemical shift were observed in two main regions: the peptide-binding cleft that directly binds the p53 ligands; and the hinge regions connecting the beta-sheet and alpha-helical structures that form the binding cleft. These conformational changes reflect the adaptation of the cleft on binding peptide ligands that differ in length and amino acid composition. Different ligands may induce different conformational transitions in MDM2 that could be responsible for its function. The dynamic nature of MDM2 might be important in the design of anti-cancer drugs that are targeted to its p53-binding site.

Distinct MDM2 and P14ARF expression and centrosome amplification in well-differentiated liposarcomas

Well-differentiated liposarcomas (WDLs) are common soft-tissue tumors in adults. They are characterized by large marker chromosomes and/or ring chromosomes containing 12q-derived sequences in which MDM2 is consistently amplified. WDLs are subdivided into two subtypes according to their karyotype. Type D cells exhibit a near-diploid karyotype, with very few or no chromosome changes. Type H cells exhibit a near-tetraploid karyotype and many structural changes. Expression of P14ARF, MDM2, and TP53 proteins was assayed in the two WDL subtypes to establish whether distinct expression profiles correlated with cell ploidy. Although a transcriptionally functional TP53 was present in most tumors independent of their karyotype, type H cells were characterized by high levels of P14ARF and MDM2 proteins. Although amplified within similar chromosome markers in type D tumors, MDM2 did not appear to be overexpressed. In addition, it was present as a C-terminal truncated protein, indicative of alternatively spliced variants of MDM2 mRNA. As the existence of karyotypically distinct tumors could result from alterations of the mitotic machinery, we investigated the centrosome behavior in the two WDL subtypes. Centrosome amplification occurred in WDL tumors types H and D independent of their ploidy status. Moreover, no functional centrosome difference was found between the two tumor subtypes.

Prognostic and predictive factors in oral squamous cell cancer: important tools for planning individual therapy?

An escalation in the incidence of oral cancer and its attributable mortality has been observed in recent decades in Europe; oral cancer is expected to become a public health problem in the foreseeable future. However, survival rates have remained at a disappointingly stable level despite significant development in the multimodality treatment of the disease. Additionally, due to the limited prognostic value of conventional prognostic factors and the uniformity of treatment strategies, several patients are still over- or under-treated with significant personal and socio-economical impact. Here we review some promising prognostic and predictive markers that can help the clinician to improve prognostic accuracy and define the most appropriate management for the individual patient with oral cancer.

Functional role of Mdm2 phosphorylation by ATR in attenuation of p53 nuclear export

Mdm2 oncoprotein plays a major role in inhibiting the p53 tumor suppressor protein. Here, we investigate phosphorylation of Mdm2 at serine 407 (S407). S407 is phosphorylated in cells after treatment with camptothecin (CPT) or hydroxyurea, inhibitors of DNA replication. S407 phosphorylation after CPT treatment is induced upon cell cycle arrest during S phase and prevented if entry into S phase of cell cycle is blocked. We found that a major kinase responsible for S407 phosphorylation is ATR, a DNA damage checkpoint protein that induces cell cycle arrest and promotes DNA repair in response to impaired DNA replication; induction of S407 phosphorylation is enhanced after expression of wild-type ATR, while it is inhibited by a dominant-negative form of ATR. Further, S407 is specifically phosphorylated by ATR in vitro. Substitution of S407 with aspartate (S407D), but not with alanine (S407A), promotes nuclear localization of p53. Taken together, our data indicate that S407 phosphorylation of Mdm2 by ATR reduces Mdm2-dependent export of p53 from nuclei to cytoplasm.

Enhanced degradation of MDM2 by a nuclear envelope component, mouse germ cell-less

A mouse homologue of Drosophila germ cell less, mouse germ cell less-1 (mgcl-1), encodes a nuclear envelope component essential for nuclear integrity. To analyze the molecular function of mGCL-1, we carried out two hybrid screening and found that mGCL-1 bound to the gene product of tumor susceptibility gene 101 (tsg101). Effects of mGCL-1 on the expression of MDM2-p53 axis were examined, since TSG101 has been shown to elevate the amount of MDM2 by inhibiting the ubiquitination. mGCL-1 significantly reduced the amount of MDM2 probably by changing the sub-cellular localization of the MDM2 and facilitating the ubiquitination of MDM2. In addition, the amount of p53 was increased and transactivation by p53 was enhanced by mGCL-1. Thus, mGCL-1 turned out to be a factor modulating MDM2-p53 axis by enhanced degradation of MDM2.

Molecular responses to photogenotoxic stress induced by the antibiotic lomefloxacin in human skin cells: from DNA damage to apoptosis

Photo-unstable chemicals sometimes behave as phototoxins in skin, inducing untoward clinical side-effects when exposed to sunlight. Some drugs, such as psoralens or fluoroquinolones, can damage genomic DNA, thus increasing the risk of photocarcinogenesis. Here, lomefloxacin, an antibiotic from the fluoroquinolone family known to be involved in skin tumor development in photoexposed mice, was studied using normal human skin cells in culture: fibroblasts, keratinocytes, and Caucasian melanocytes. When treated cells were exposed to simulated solar ultraviolet A (320-400 nm), lomefloxacin induced damage such as strand breaks and pyrimidine dimers in genomic DNA. Lomefloxacin also triggered various stress responses: heme-oxygenase-1 expression in fibroblasts, changes in p53 status as shown by the accumulation of p53 and p21 proteins or the induction of MDM2 and GADD45 genes, and stimulation of melanogenesis by increasing the tyrosinase activity in melanocytes. Lomefloxacin could also lead to apoptosis in keratinocytes exposed to ultraviolet A: caspase-3 was activated and FAS-L gene was induced. Moreover, keratinocytes were shown to be the most sensitive cell type to lomefloxacin phototoxic effects, in spite of the well-established effectiveness of their antioxidant equipment. These data show that the phototoxicity of a given drug can be driven by different mechanisms and that its biologic impact varies according to cell type.

Peptides with anticancer use or potential

This review is an attempt to illustrate the diversity of peptides reported for a potential or an established use in cancer therapy. With 612 references, this work aims at covering the patents and publications up to year 2000 with many inroads in years 2001-2002. The peptides are classed according to four categories of effective (or plausible) biological mechanisms of action: receptor-interacting compounds; inhibitors of protein-protein interaction; enzymes inhibitors; nucleic acid-interacting compounds. The fifth group is made of the peptides for which no mechanism of action has been found yet. Incidentally this work provides an overview of many of the modern targets of anticancer research.

The initial evaluation of non-peptidic small-molecule HDM2 inhibitors based on p53-HDM2 complex structure

Peptidic Mouse Double Minute (MDM2) inhibitors have been demonstrated to effectively inhibit the interaction between p53 and MDM2, thus providing a therapeutic strategy for some tumors. However, there is no report on non-peptidic inhibitors. In this study non-peptidic HDM2 (the human homologue of MDM2) inhibitors were obtained by computer-aided design and subsequently synthesized by chemical method. Bio-evaluation showed that some of these inhibitors have affinity with HDM2, and can cause death of some tumor cells which express wild-type p53. Cellular assays showed that one of these compounds, syc-7, can activate the p53 pathway in some of these tumor cell lines, and further induce apoptosis. The results suggest that developing non-peptidic small-molecule HDM2 inhibitors is a promising way for new antitumor drug discovery.

Have p53 gene mutations and protein expression a different biological significance in colorectal cancer?

p53 alterations are considered the most common genetic events in many types of neoplasms, including colorectal carcinoma (CRC). These alterations include mutations of the gene and/or overexpression of the protein. The aim of our study was to assess whether in 160 patients undergoing resective surgery for primary operable CRC there was an association between p53 mutations and protein overexpression and between these and other biological variables, such as cell DNA content (DNA-ploidy) and S-phase fraction (SPF), and the traditional clinicopathological variables. p53 mutations, identified by PCR-SSCP-sequencing analysis, were found in 68/160 patients (43%) and positive staining for p53 protein, detected with the monoclonal antibody DO-7, was present in 48% (77/160) of the cases, with agreement of 57% (91/160). In particular, a significant association was found between increased p53 expression and genetic alterations localized in the conserved regions of the gene or in the L3 DNA-binding domain and the specific type of mutation. Furthermore, both overexpression of p53 and mutations in the conserved areas of the gene were found more frequently in distal than in proximal CRCs, suggesting that they might be "biologically different diseases." Although p53 mutations in conserved areas were associated with flow cytometric variables, overexpression of p53 and mutations in its L3 domain were only related respectively to DNA-aneuploidy and high SPF. These data may reflect the complex involvement of p53 in the different pathways regulating cell-cycle progression. In conclusion, the combination of the mutational status and immunohistochemistry of p53, and flow cytometric data may provide an important insight into the biological features of CRCs.

P53: an ubiquitous target of anticancer drugs

The p53 tumor suppressor can induce growth arrest, apoptosis and cell senescence. Not surprisingly, p53 is an appealing target for therapeutic intervention. Although current anticancer agents do not directly interact with p53, these agents (including DNA damaging drugs, antimetabolites, microtubule-active drugs and inhibitors of the proteasome) cause accumulation of wt p53. Depending on the p53 status of cancer cells, diverse therapeutic strategies are under development. These include pharmacological rescue of mutant p53 function and reactivation of wt p53 in E6-expressing cells. For protection of normal cells, strategies range from abrogation of wt p53 induction, thereby decreasing the toxicity of DNA damaging agents, to activation of wt p53-dependent checkpoints, thereby protecting cells against cell cycle-dependent therapeutics.

A direct interaction between the survival motor neuron protein and p53 and its relationship to spinal muscular atrophy

Mutations in the SMN1 (survival motor neuron 1) gene cause spinal muscular atrophy (SMA). We now show that SMN protein, the SMN1 gene product, interacts directly with the tumor suppressor protein, p53. Pathogenic missense mutations in SMN reduce both self-association and p53 binding by SMN, and the extent of the reductions correlate with disease severity. The inactive, truncated form of SMN produced by the SMN2 gene in SMA patients fails to bind p53 efficiently. SMN and p53 co-localize in nuclear Cajal bodies, but p53 redistributes to the nucleolus in fibroblasts from SMA patients. These results suggest a functional interaction between SMN and p53, and the potential for apoptosis when this interaction is impaired may explain motor neuron death in SMA.

Genotoxic and non-genotoxic pathways of p53 induction

Since the initial concept of p53 as a sensor of DNA-damage, the picture of the role of p53 has widened to include the sensing of much more diverse forms of stress, including hypoxia and constitutive activation of growth-promoting cascades. The pathways by which these processes regulate p53 are partially overlapping, but imply different patterns of post-translational modifications. In this review, we summarize current knowledge on post-translational modifications of p53, and we discuss how hypoxia and oncogene activation stresses may induce p53 independently of DNA damage.

Cocompartmentalization of p53 and Mdm2 is a major determinant for Mdm2-mediated degradation of p53

The product of the Mdm2 oncogene directly interacts with p53 and promotes its ubiquitination and proteasomal degradation. Initial biological studies identified nuclear export sequences (NES), similar to that of the Rev protein from the human immunodeficiency virus, both in Mdm2 and p53. The reported phenotypes resulting from mutation of these NESs, together with results obtained using the nuclear export inhibitor leptomycin B (LMB), have led to a model according to which nuclear export of p53 (via either the NES of Mdm2 or its own NES) is required for efficient p53 degradation. In this study we demonstrate that Mdm2 can promote degradation of p53 in the nucleus or in the cytoplasm, provided both proteins are colocalized. We also investigated if nuclear export is an obligate step on the p53 degradation pathway. We find that (1) when proteasome activity is inhibited, ubiquitinated p53 accumulates in the nucleus and not in the cytoplasm; (2) Mdm2 with a mutated NES can efficiently mediate degradation of wild type p53 or p53 with a mutated NES; (3) the nuclear export inhibitor LMB can increase the steady-state level of p53 by inhibiting Mdm2-mediated ubiquitination of p53; and (4) LMB fails to inhibit Mdm2-mediated degradation of the p53NES mutant, demonstrating that Mdm2-dependent proteolysis of p53 is feasible in the nucleus in the absence of any nuclear export. Therefore, given cocompartmentalization, Mdm2 can promote ubiquitination and proteasomal degradation of p53 with no absolute requirement for nuclear to cytoplasmic transport.