Energy requirement for degradation of tumor-associated protein p53

Understanding p53 functions through p53 antibodies

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

Immunohistochemical analysis of beta-catenin, E-cadherin and p53 in canine gastrointestinal epithelial tumors

Wnt/beta-catenin signaling, E-cadherin and p53 reportedly play important roles in the development and/or progression of human gastrointestinal cancer. The present study evaluated the roles of beta-catenin, E-cadherin and p53 in canine gastrointestinal tumors. Endoscopic biopsy or surgically resected samples, a total of 131, including 38 gastric, 13 small intestinal and 80 large intestinal tumors, were obtained from 95 dogs. Those specimens were examined pathologically. Immunohistochemically, nuclear beta-catenin expression was found in 88% (42/48) of polypoid type adenocarcinomas. Most cases of non-polypoid type adenocarcinomas lacked nuclear expression of beta-catenin with the exception of one case (6%, 1/17). Nuclear beta-catenin expression was not observed in signet ring cell carcinomas (0/15), mucinous adenocarcinomas (0/7) and undifferentiated carcinomas (0/4). The findings indicate that nuclear translocation of beta-catenin is closely related to the development of polypoid type adenocarcinomas but not that of non-polypoid type malignant tumors. The immunoreactivity of E-cadherin for tumor cells tended to decline overall in most of cases including benign tumors. Significant immunoreactivity for p53 was not found in 61% of tumors examined (80/131), including malignant tumors (63%, 57/91), while intense p53-immunoreactivity was rarely found in a few cases of malignant tumors (8%, 7/91). We could not conclude clearly significant correlations between histopathological tumor types and immunohistochemical results of E-cadherin or p53. This paper indicates the importance of the nuclear translocation of beta-catenin for the tumorigenesis of canine intestinal polypoid type adenocarcinomas, especially in the colorectum.

PD-L1 expression correlated with p53 expression in oral squamous cell carcinoma

Background

Programmed cell death ligand 1 (PD-L1) is an immune checkpoint molecule that attenuates the immune response. PD-L1 contributes to failed antitumor immunity; thereby, blockade of PD-L1 with monoclonal antibody enhances the immune response. Recently, it was reported that PD-L1 was regulated by protein 53 (p53). Besides, cytokeratin 17 (CK17) is thought to be a diagnostic marker of oral squamous cell carcinoma (OSCC). Our aim was to evaluate the correlation between the immunohistochemical expression of PD-L1, p53 and CK17 with clinicopathological characteristics and disease-specific survival in patients with OSCC.

Methods

A total of 48 patients with OSCC were included in this study. Immunohistochemical staining was performed to evaluate the correlation among the expressions of PD-L1, p53 and CK17, and furthermore the correlation among various clinicopathological factors, PD-L1, p53 and CK17.

Results

The positive rate of p53, CK17, PD-L1 (tumor cells) and PD-L1 (tumor-infiltrating lymphocytes) was 63.2%, 91.7%, 48.9% and 57.1%. A statistically significant correlation between p53 expression and T stage and TNM stage (p = 0.049, p = 0.03, respectively) was observed. Also, a statistically significant correlation between p53 and PD-L1 (TCs) expression (p = 0.0009) was observed. Five-year disease-specific survival rate was not significantly correlated with gender, TNM stage, p53 expression, PD-L1 expression and CK17 expression.

Conclusion

The expression of p53 and PD-L1 shows significantly positive correlation in oral squamous cell carcinoma in tumor cells. Also, a significant correlation between p53 expression and T stage and TNM stage was observed. No other significant correlation between PD-L1 staining or CK17 and clinical or pathologic characteristics was identified.

Multifunctional protein: cardiac ankyrin repeat protein

Cardiac ankyrin repeat protein (CARP) not only serves as an important component of muscle sarcomere in the cytoplasm, but also acts as a transcription co-factor in the nucleus. Previous studies have demonstrated that CARP is up-regulated in some cardiovascular disorders and muscle diseases; however, its role in these diseases remains controversial now. In this review, we will discuss the continued progress in the research related to CARP, including its discovery, structure, and the role it plays in cardiac development and heart diseases.

A comparative analysis of human papillomavirus types 16 and 18 and expression of p53 gene and Ki-67 in cervical, vaginal, and vulvar carcinomas

This study aimed to investigate the correlation between HPV positivity, p53 overexpression, and cell proliferative activity in cervical, vaginal, and vulvar squamous cell carcinoma.

METHODS

Sixteen vaginal and 31 vulvar squamous cell carcinomas were examined retrospectively for overexpression of p53 gene and Ki67 antigen by immunohistochemistry and for the presence of HPV types 16 and 18 DNA using a polymerase chain reaction (PCR) method. The results were compared with those obtained from 40 cervical squamous cell carcinomas.

RESULTS

HPV type 16 or 18 DNA was detected in 21 (52.8%) of 40 cases of cervical carcinomas and p53 overexpression in one (2.5%), while HPV DNA sequences were detected in seven (43.7%) of 16 cases of vaginal carcinoma and p53 overexpression in three (18.7%). With regard to vulvar carcinoma, HPV was harbored in four (12.8%) of 31 cases and p53 overexpression in 19 (61.2%). These results indicated statistically significant inverse correlations between HPV positivity and p53 overexpression (R = -0.999, P < 0.0001). Overexpression of Ki-67 was detected in 28 (70.0%) of 40, 12 (75.0%) of 16, and 21 (67.7%) of 31, cervical, vaginal, and vulvar carcinomas, respectively. There was no significant difference among the three groups.

CONCLUSIONS

In cervical carcinoma, HPV types 16 and 18 might play a common causal role, and in vulvar carcinoma, p53 gene mutations might be a main causal factor for carcinogenesis. Vaginal carcinoma, on the other hand, is considered to have transitional characteristics between cervical and vulvar carcinoma.

p53 protein expression in squamous cell carcinoma of the vulva

BACKGROUND

The objective of the study was to evaluate the pathogenetic and prognostic value of p53 protein expression in squamous cell carcinoma of the vulva.

METHODS

The clinical data in charts of 167 patients with International Federation of Gynecology and Obstetrics (FIGO) Stages I-III primary tumors who were treated by surgery were reviewed. Samples from the primary tumor were immunostained for p53 protein. p53 overexpression was defined as immunoreactivity in > 5% of nuclei.

RESULTS

p53 overexpression was observed in 92 tumors (55%). p53 overexpression did not correlate with age at diagnosis, FIGO stage, histologic grade, vessel invasion, tumor thickness, tumor greatest dimension, DNA ploidy, or inguinal lymph node metastasis. In the whole group a significantly reduced 5-year corrected survival was observed in patients with p53 overexpression compared with p53 negative patients (P = 0.04). In the different FIGO stages, disease-related survival was not influenced by p53 overexpression in 37 patients with Stage I disease (P = 0.60) or in 86 patients with Stage II disease (P = 0.96). In 44 patients with Stage III disease, p53 overexpression was significantly associated with poorer prognosis (P = 0.004). Independent prognostic factors for corrected survival in the entire group of 167 patients were: vascular invasion, groin metastasis, tumor greatest dimension, and p53 overexpression. In patients with FIGO Stage III disease p53 overexpression was not an independent prognostic factor.

CONCLUSIONS

p53 protein overexpression appears to be involved in the pathogenesis of vulvar squamous cell carcinoma. p53 protein overexpression was significantly associated with disease-related survival. p53 prognostic impact was observed only in patients with advanced disease.

Immunohistochemical examination of proliferative potentials and the expression of cell cycle-related proteins of intracranial chordomas

The difference in biological features between recurrent and nonrecurrent intracranial chordomas has not been studied. In this study, proliferative potentials of chordomas were studied with an immunohistochemical staining method, mainly using anti-Ki-67 antibody, MIB-1, which is known to be available for archival paraffin sections, together with immunohistochemical studies on the expression of cell cycle or apoptosis-related proteins, including p53, cyclin D1, and bcl-2 proteins. The correlation among MIB-1 staining indices, the immunoreactivities of these proteins, and clinical courses of intracranial chordomas were analyzed retrospectively, and the statistically significant correlation between MIB-1 staining index (SI) and recurrence has been clarified. The mean MIB-1 SI of recurrent tumors was 10.2%, being shown to be higher than that of nonrecurrent tumors (2.8%). The immunohistochemically positive staining of cell cycle-related protein, especially p53 and cyclin D1 proteins, correlated well with recurrence and high MIB-1 SI. In conclusion, both the examination of proliferative potentials of chordomas using MIB-1 SI and the study of the immunoreactivity of p53 and cyclin D1 proteins are important for their biological and histopathological analyses and the prediction of future recurrence.

On the involvement of calpains in the degradation of the tumor suppressor protein p53

A crude fraction that contains ubiquitin-protein ligases contains also a proteolytic activity of approximately 100 kDa that cleaves p53 to several fragments. The protease does not require ATP and is inhibited in the crude extract by an endogenous approximately 250 kDa inhibitor. The proteinase can be inhibited by chelating the Ca2+ ions, by specific cysteine proteinase inhibitors and by peptide aldehyde derivatives that inhibit calpains. Purified calpain demonstrates an identical activity that can be inhibited by calpastatin, the specific protein inhibitor of the enzyme. Thus, it appears that the activity we have identified in the extract is catalyzed by calpain. The calpain in the extract degrades also N-myc, c-Fos and c-Jun, but not lysozyme. In crude extract, the calpain activity can be demonstrated only when the molar ratio of the calpain exceeds that of its native inhibitor. Recent experimental evidence implicates both the ubiquitin proteasome pathway and calpain in the degradation of the tumor suppressor, and it was proposed that the two pathways may play a role in targeting the protein under various conditions. The potential role of the two systems in this important metabolic process is discussed.

Detection of circulating anti-p53 antibodies in patients with colorectal carcinoma and the antibody's relation to clinical factors

PURPOSE

Mutation of the p53 tumor suppressor gene is the most common genetic alternation in colorectal carcinoma and is assessed mainly by molecular analysis of the gene sequence or by immunohistochemical analysis of p53 protein accumulation. Purpose of this study was to detect circulating antibody against p53 proteins in serum of patients with colorectal carcinoma and to evaluate its clinical significance.

METHODS

We used immunoblotting techniques to detect circulating anti-p53 antibodies. Relationship among staging, immunohistochemical expression of p53 in the primary tumor, and serum carcinoembryonic antigen level was investigated.

RESULTS

Circulating anti-p53 antibodies were detected in 32 (68 percent) of 47 patients. Antibody was found in patients at all clinical stages of disease. In 33 patients whose sera and primary tumor tissues were available for testing, 16 (48 percent) were positive for both circulating anti-p53 antibody and p53 expression in the tumor. The anti-p53 antibody test was positive in 78 and 56 percent of patients with high and normal serum carcinoembryonic antigen levels, respectively.

CONCLUSION

Detection of anti-p53 antibodies may become a new diagnostic indicator of colorectal carcinomas.

TP53 alterations in atypical ductal hyperplasia and ductal carcinoma in situ of the breast

Abnormalities in the TP53 tumour suppressor gene in 75 atypical ductal hyperplasias and 62 ductalcarcinomas in situ (DCIS) of the breast were studied using immunohistochemistry and mutation analysis. Accumulation of p53 protein was detected in 10 out of 62 (16%) DCIS, whereas no cases of positive staining was observed in the atypical lesions. TP53 mutations were identified in four out of 30 (13%) DCIS by constant denaturant gel electrophoresis (CDGE). Two of these cases were positive and two negative for p53 protein. A total of 12 out of 62 DCIS (19%) carried TP53 mutation and/or p53 protein over-expression. The present results suggest that TP53 alterations may be important in the development of a subset of DCIS.

Modulation of expression of p53 and cell proliferation in locally advanced cervical carcinoma after neoadjuvant combination chemotherapy

OBJECTIVE

In the present study, we investigated changes of p53 expression and the cell proliferation index detected with MIB 1 in tumors before and after neoadjuvant combination chemotherapy with respect to the outcome of the disease. Our aim was to define more appropriately the significance of chemotherapy in locally advanced cervical carcinoma.

MATERIALS AND METHODS

Our study included 17 women with locally advanced squamous cervical carcinoma who had been admitted to the Institute of Gynecology and Obstetrics Ancona University, between January 1990 and December 1994. The patients received neoadjuvant combination chemotherapy consisting of three cycles of cisplatin (80 mg/m2) and bleomycin (30 mg/m2). After chemotherapy, radical surgery was performed. Bioptic specimens were obtained from cervical tumors before and after chemotherapy and processed for immunohistochemical staining with a monoclonal antibody against p53 and with the monoclonal antibody MIB 1.

RESULTS

Thirteen patients (76.5%) showed a clinical response (4 complete and 9 partial), while of the remaining 4 cases (23.5%) 3 had no change and 1 showed progression after neoadjuvant combination chemotherapy. A significant relationship was observed between the overexpression of p53 and the sensitivity to chemotherapy; responder patients showed a higher frequency of p53 positive cells than non-responders (P = 0.03). No significant relationship with MIB 1 index was observed. Both expression of p53 protein (P < 0.001) and reaction with MIB 1 (P = 0.003) significantly decreased after chemotherapy. The decrease in expression of p53 protein and staining with antibody MIB 1 was particularly evident in patients who responded to chemotherapy.

DISCUSSION

In tumors, p53 protein and index of proliferating cells as determined with MIB 1 showed a significant modulation after treatment, suggesting an association with sensitivity to chemotherapy. However, the limited number of our series of patients does not permit a statement on the clinical implication of expression of p53 and cell proliferation in patients undergoing neoadjuvant combination chemotherapy for locally advanced cervical carcinoma.

p53 expression and DNA ploidy of cartilage lesions

The aim of this study was to investigate the expression of a tumor suppressor gene (p53) in cartilage lesions of bone and its relationship to their histological grade and DNA ploidy. An immunohistochemical assay for p53 and Feulgen-stained DNA preparations was subjected to computerized image analysis. Enchondromas, synovial chondromatosis, and low grade (grade I and II) chondrosarcomas were diploid. High grade (grade III) chondrosarcomas and high grade sarcomatous components of dedifferentiated chondrosarcomas were aneuploid. Well differentiated cartilaginous components of dedifferentiated chondrosarcomas were diploid. Microscopic examination showed weak focal positivity for p53 in one of 10 enchondromas one of six examples of synovial chondromatosis, and three of four low grade (grade I and II) chondrosarcomas. All three high grade (grade III) chondrosarcomas were strongly positive for p53. The high grade sarcomatous component of all four dedifferentiated chondrosarcomas was strongly positive for p53, whereas only focal weak positivity was noted in the well differentiated cartilaginous areas. These results were confirmed by quantitative computer-assisted image analysis, which showed that high grade aneuploid cartilage tumors demonstrated strikingly higher levels of p53 than did diploid low grade malignant tumors or benign cartilage lesions.

Tumor suppressor p53 mutations and breast cancer: a critical analysis

Alterations in the tumor suppressor gene p53 are the most commonly identified changes in cancer, including neoplasia of the breast. The activity of p53 is regulated post-translationally. Phosphorylation state, subcellular localization, and interaction with any of a number of cellular proteins are likely to influence the function of p53. The exact effect of p53-mediated growth suppression seems to be cell-type specific but appears to be directly related to the ability of p53 to act as a specific transcriptional activator. The role that transcriptional repression plays in the function of WT p53 is less clear. It is also possible that p53 has a more direct activity in DNA replication and repair. Most documented p53 mutations result in single amino acid substitutions which may confer one or more of a spectrum of transforming abilities on the protein. Mutation may lead to nuclear accumulation of p53 protein; however, inactivation of p53 by nuclear exclusion and interaction with the mdm2 protein also appear to be important in tumorigenesis. Used in conjunction with other established factors, accumulation of cellular p53 may be a useful prognostic indicator in breast cancer. A syngeneic mouse model system yielded evidence that p53 mutations are important in the early, preneoplastic stages of mammary tumorigenesis. This murine system may provide the ability to investigate the functions of p53 in the early stages of breast cancer which are technically difficult to examine in the human system.

Retinoblastoma and p53 tumour suppressor gene protein expression in carcinomas of the thyroid gland

One hundred and thirty-one thyroid tumours were examined immunohistochemically for expression of retinoblastoma (RB) and p53 protein. The results demonstrate that RB protein is not lost in any cases, indicating that inactivation of the RB gene is unlikely to play a central role in the pathogenesis of thyroid tumours. Eighteen of 24 (75 per cent) undifferentiated carcinomas, 6 of 32 (19 per cent) papillary carcinomas, 5 of 29 (17 per cent) follicular carcinomas, and 6 of 46 (13 per cent) medullary carcinomas showed p53 protein nuclear staining. In 46 per cent of the undifferentiated carcinomas many of the tumour cells had accumulated p53 protein, whereas in the other positive cases less than 5 per cent of the cells had increased p53 protein levels. Our results strongly suggest that p53 protein abnormalities play a crucial role in the progression of well-differentiated to undifferentiated thyroid carcinomas.

Genetic alterations of the TP53 gene, p53 protein expression and HPV infection in primary cervical carcinomas

Primary cervical carcinomas from 92 patients were investigated for genetic alterations in the tumour suppressor gene TP53. Studies of allelic imbalance (AI) were performed by Southern blot analysis and by using two PCR (polymerase chain reaction) polymorphisms within the TP53 gene. AI in the tumour was observed in 22 per cent (11 of 52 informative patients) and was significantly associated with recurrence both in a univariate (P = 0.013) and in a multivariate (P = 0.045) analysis. The DNA samples were subjected to mutation analysis of four of the conserved domains in the TP53 gene, using PCR followed by constant denaturant gel electrophoresis (CDGE). Mutations were observed in 2 of 92 tumours (2 per cent), of which one was a silent mutation and the other a frameshift. Overexpression of the p53 protein was found by immunostaining of sections from formalin-fixed, paraffin-embedded material in 55 per cent (51/92) of the tumours. In 88 per cent (45/51) of these, overexpression was present in less than 5 per cent of the tumour cells. Overexpression was significantly associated with relapse-free survival only in a univariate analysis (P = 0.045). AI of the TP53 locus did not correlate with p53 expression or mutation. The important gene on 17p, responsible for the shorter disease-free survival for patients with AI of TP53, may therefore be another gene closely linked to TP53. In addition, the 92 tumour samples were tested for the presence of human papillomavirus (HPV) types 16 and 18. Fifty-four per cent (50/92) of the samples were positive for HPV 16 using in situ hybridization, and 93 per cent (86/92) using the PCR technique. The numbers for HPV 18 were 15 per cent (14/92) and 23 per cent (21/92), respectively. Twenty-one per cent (19/92) were positive for both HPV 16 and HPV 18, while 4 per cent (4/92) were negative for both HPV 16 and 18. The tumour with the frameshift TP53 mutation was HPV 16-positive, and the four samples negative for HPV 16 and 18 did not contain TP53 mutations within the conserved domains but had elevated p53 protein expression.

Immunohistochemical analysis of p53 protein overexpression in normal, premalignant, and malignant tissues of the cervix uteri

Two hundred and thirty-eight cervical lesions ranging from normal to malignant were examined for overexpression of p53 protein. Whereas p53 protein was identified in 62 per cent of invasive squamous cell carcinomas, 11 per cent of invasive adenocarcinomas, and 7 per cent of squamous cell carcinomas in situ, no staining was found in adenocarcinoma in situ, dysplastic tissue, condyloma, and normal tissue. In 9 per cent of the positive cases of invasive squamous cell carcinomas, 5-50 per cent of the tumour cells were immunoreactive for p53 protein, whereas the other positive specimens were characterized by only rare p53-positive cells. We conclude that in invasive cervical carcinomas widespread overexpression of p53 protein is unusual, but occasional positive nuclei can be found frequently. Furthermore, our results indicate that altered expression of p53 protein may be involved in the progression of cervical carcinomas.

The ubiquitin-mediated proteolytic pathway

Ubiquitin modification of a variety of protein targets within the cell plays important roles in many cellular processes. Among these are regulation of gene expression, regulation of cell cycle and division, involvement in the cellular stress response, modification of cell surface receptors, DNA repair, and biogenesis of mitochondria and ribosomes. The best studied modification occurs in the ubiquitin-dependent proteolytic pathway. Degradation of a protein by the ubiquitin system involves two discrete steps. Initially, multiple ubiquitin molecules are covalently linked in an ATP-dependent mode to the protein substrate. The protein moiety of the conjugate is then degraded by a specific protease into free amino acids with the release of free and reutilizable ubiquitin. This process also requires energy. In addition, stable mono-ubiquitin adducts are also found intracellularly, for example, those involving nucleosomal histones. Despite the considerable progress that has been made in elucidating the mode of action and roles of the ubiquitin system, many problems remain unsolved. For example, very little is known about the cellular substrates of the system and the signals that target them for conjugation and degradation. The scope of this review is to summarize briefly what is currently known on the role of the ubiquitin system in protein turnover, and to discuss in detail the mechanisms involved in selection of substrates for conjugation and in degradation of ubiquitin-conjugated proteins.

The mdm-2 oncogene product forms a complex with the p53 protein and inhibits p53-mediated transactivation

A cellular phosphoprotein with an apparent molecular mass of 90 kd (p90) that forms a complex with both mutant and wild-type p53 protein has been characterized, purified, and identified. The protein was identified as a product of the murine double minute 2 gene (mdm-2). The mdm-2 gene enhances the tumorigenic potential of cells when it is overexpressed and encodes a putative transcription factor. To determine if mdm-2 could modulate p53 transactivation, a p53-responsive element from the muscle creatine kinase gene was employed. A wild-type p53-expressing plasmid enhanced the expression of the p53-responsive element when cotransfected into cells that contain no endogenous p53. When a cosmid expressing mdm-2 was transfected with this p53-expressing plasmid, the transactivation of the p53-responsive element was inhibited. Thus, a product of the mdm-2 oncogene forms a tight complex with the p53 protein, and the mdm-2 oncogene can inhibit p53-mediated transactivation.

Stability of p53 protein in rat cells transformed by various viral transforming genes

To clarify the correlation between metabolic stabilization of p53 protein and cellular transformation, we transformed the normal rat cell line F2408 with various viral transforming genes and examined the expression and stability of p53 protein in these transformed cells by pulse-chase immunoprecipitation experiments. As expected, the level of p53 in the SV40-transformed rat cell line was higher than that in the normal cell line and its stability was also increased. In contrast, in cells transformed with the E7 and E6 genes of human papillomavirus type 16, the level and stability of p53 were similar to those in the normal cells. In cells transformed by the middle T or large T of polyomavirus, v-K-ras, and v-src, the levels and stabilities of p53 were also not elevated, although the level of p53 was increased in activated c-H-ras-transformed cells without an increase in its stability. These results show that increased stability and expression of the p53 protein are not common events in viral transformation of the rat cell line. In addition, we demonstrated with a monoclonal antibody specific for the mutant form of p53 that cellular transformation by viral transforming genes does not involve mutational activation of p53 to an oncogenic form.

The role of p53 in the normal control of cell proliferation

The regulation of cell proliferation involves p53. A mutant allele of p53, p53-Val135, has been found to be temperature-sensitive for function with separable suppressor and promoter effects on cell proliferation. These opposing suppressor and promoter functions of p53 correlate with two alternative, temperature-sensitive conformations of the p53-Val135 polypeptide.

Correlation of metabolic stability and altered quaternary structure of oncoprotein p53 with cell transformation

The phosphoprotein p53 seems to be implicated in various processes connected with cell transformation and in particular with the regulation of cell cycle and probably DNA replication. In the present paper we have analyzed two sets of closely related cell lines expressing the same p53 which exhibited either a nontransformed or a transformed phenotype. These cell lines were used to study biochemical properties of the p53 protein which might be correlated with cell transformation. We found a positive correlation among an elevated stability of p53, the formation of high-molecular-weight forms of p53, and the transformed phenotype of the corresponding cell lines. Furthermore, these data indicate that self-aggregation prevents p53 from rapid degradation. By a comparative analysis of the stability and oligomerization properties of mutant p53 and wild-type p53, we could demonstrate that elevated stability and self-aggregation of p53 are correlated with the transformed phenotype of the cells and independent of a particular mutation in the p53 gene.

Expression of p53 in human leukemic cell lines

The cell-encoded p53 antigen seems to be tightly associated with various human malignancies. We have analyzed biochemical properties of p53 in two different cell lines derived from patients with ALL or ANLL. p53 was found in elevated levels in both leukemic cell lines compared to unstimulated or stimulated normal lymphocytes. High levels of p53 in these cell lines are due to an extended stability of p53 protein rather than to different rates of synthesis. p53 from both cell lines formed low- and high-molecular weight oligomers which revealed that p53 exists in a heterogenous population in these tumor cells. The presence of immunologically different subsets of p53 was demonstrated by sequential immunoprecipitation experiments with different p53 specific monoclonal antibodies. Our results showed structural and immunological variabilities of p53 in cell lines derived from human tumors and may thus provide an insight into the role p53 may play in human malignancies.

Degradation of ornithine decarboxylase in mammalian cells is ATP dependent but ubiquitin independent

Ornithine decarboxylase (ODC), a key enzyme in the biosynthesis of polyamines in mammalian cells is characterized by an extremely short half-life. In the present study, ODC degradation was investigated in 653-1 mouse myeloma cells that overproduce ODC and in ts85 cells that are thermosensitive for conjunction of ubiquitin to target proteins. Addition of 2-deoxyglucose and dinitrophenol (agents that efficiently deplete cellular ATP) to the growth medium of these cells inhibited ODC degradation. In contrast, chloroquine and leupeptin, inhibitors of intralysosomal proteolysis, did not affect ODC degradation. Shifting ts85 cells to 42 degrees C (a non-permissive temperature that inhibited conjugation of ubiquitin to target proteins) did not prevent ODC degradation. The ATP-dependent degradation of ODC in 653-1 cells was inhibited substantially by N alpha-tosyl-L-lysine chloromethane (TosPheMeCl), iodoacetamide and o-phenanthroline. These results suggest that ODC degradation occurs via a non-lysosomal. ATP-requiring and ubiquitin-independent cellular proteolytic mechanism, and that serine proteases and enzymes containing sulphydryl groups and metalloenzyme(s) may be involved in this process.

Evolutionary conservation of the biochemical properties of p53: specific interaction of Xenopus laevis p53 with simian virus 40 large T antigen and mammalian heat shock proteins 70

We have investigated the biochemical properties of Xenopus laevis p53. With an in vitro binding assay, we can detect a specific association between X. laevis p53 and simian virus 40 large T antigen. Furthermore, X. laevis p53 expressed in monkey COS cells is stably associated with this viral antigen. Like mammalian p53, X. laevis p53 in complex with simian virus 40 large T antigen exhibits a 20-fold increase of its half-life. On the other hand, X. laevis p53 is unable to associate either in vivo or in vitro with adenovirus type 5 E1B 55-kilodalton protein. We show by an immunological technique that X. laevis p53 forms specific complexes with mammalian hsp72 and hsp73 heat shock proteins only at a temperature well above the optimal growth temperature for X. laevis. Our results suggest that the protein-binding properties of p53 are closely related to the functional activity of the protein.

The transformation-related protein p53 is not bound to the SV40 T antigen in BALB 3T12 cells expressing T antigen

In most murine cells transformed by the SV40 virus, virtually all of the cellular phosphoprotein p53 is in a complex with the SV40 T antigen. Here, we report that, in SV40-infected T-antigen-positive Balb 3T12 mouse cells, most (approximately 80%) of the p53 is not in complex. Complex formation was determined by measuring the amounts of [35S]methionine-labeled p53 which coprecipitated with T antigen when using monoclonal antibody to T antigen. The amount of complex formation was expressed as a percentage of total p53 present, measured by the amount of p53 precipitated with the monoclonal antibody to the p53. The values were confirmed by Western blotting procedure, in which the steady-state levels of the proteins were measured. In these measurements after complete precipitation with antibody to T antigen, the residual p53 in the supernatant was precipitated by antibody to p53, and this amount was denoted as free p53. There was no significant difference seen between the [35S]methionine-labeled tryptic peptides of complexed and the free p53 (or between complexed and free T antigens) as determined by two-dimensional gel electrophoresis and chromatography. Virus rescue experiments and retransformation by the rescued virus showed that there was no mutation in the SV40 DNA coding for the T antigen which could account for the lack of complex formation. Both p53 and T antigen were underphosphorylated in cells which exhibited reduced complex formation. Tumorigenicity in syngeneic mice and anchorage-independent cell growth in culture of various cloned mouse cells with or without T antigen expression was compared. The changes in the biologic properties were explainable solely on the basis of known or expected effects of expression of the T antigen and were independent of complex formation or of absence of complex formation between p53 and T antigen.

The cellular tumour antigen p53: evidence for transformation-related, immunological variants of p53

The protein p53 is involved in the control of normal cell proliferation. However, when expressed abnormally, p53 also contributes to the process of cell transformation. The possibility that multifunctional forms of p53 exist was realised when two cell cycle dependent forms of the protein were identified in normal cells (lymphocytes: J. Milner, 1984, Nature (London) 310, 143-145). Interestingly these two forms of p53 in normal cells lack some of the epitopes recognised by monoclonal antibodies developed against p53 in transformed cells. This suggests that p53 may bear additional transformation-related epitopes in transformed cells. We have now investigated this possibility by screening a variety of cell lines for immunological variants of p53. The monoclonal antibodies PAb421, PAb122, RA3.2C2, PAb248, PAb200.47, and PAb246 were used. Our results indicate the existence of two immunologically distinct variants of p53 expressed in transformed cells. The two p53 variants are distinguished by the presence or absence of the epitope recognised by PAb246. A given cell line generally expressed a single immunological variant of p53. However, 3T6 cells expressed two subsets of p53 and appeared to modulate p53 synthesis in response to cell density.

Identification of the p53 protein domain involved in formation of the simian virus 40 large T-antigen-p53 protein complex

An expression vector utilizing the enhancer and promoter region of the simian virus 40 (SV40) DNA regulating a murine p53 cDNA clone was constructed. The vector produced murine p53 protein in monkey cells identified by five different monoclonal antibodies, three of which were specific for the murine form of p53. The murine p53 produced in monkey cells formed an oligomeric protein complex with the SV40 large tumor antigen. A large number of deletion mutations, in-frame linker insertion mutations, and linker insertion mutations resulting in a frameshift mutation were constructed in the cDNA coding portion of the p53 protein expression vector. The wild-type and mutant p53 cDNA vectors were expressed in monkey cells producing the SV40 large T antigen. The conformation and levels of p53 protein and its ability to form protein complexes with the SV40 T antigen were determined by using five different monoclonal antibodies with quite distinct epitope recognition sites. Insertion mutations between amino acid residues 123 and 215 (of a total of 390 amino acids) eliminated the ability of murine p53 to bind to the SV40 large T antigen. Deletion (at amino acids 11 through 33) and insertion mutations (amino acids 222 through 344) located on either side of this T-antigen-binding protein domain produced a murine p53 protein that bound to the SV40 large T antigen. The same five insertion mutations that failed to bind with the SV40 large T antigen also failed to react with a specific monoclonal antibody, PAb246. In contrast, six additional deletion and insertion mutations that produced p53 protein that did bind with T antigen were each recognized by PAb246. The proposed epitope for PAb246 has been mapped adjacent (amino acids 88 through 109) to the T-antigen-binding domain (amino acids 123 through 215) localized by the mutations mapped in this study. Finally, some insertion mutations that produced a protein that failed to bind to the SV40 T antigen appeared to have an enhanced ability to complex with a 68-kilodalton cellular protein in monkey cells.

Specific interaction between the p53 cellular tumour antigen and major heat shock proteins

The protein p53 is capable of participating in neoplastic transformation and can form specific complexes with the large-T antigen of simian virus 40 (SV40). This interaction probably results in the stabilization of p53 (refs 7,8) and may contribute to SV40-mediated transformation. Several non-SV40-transformed cells also exhibit a stabilized p53 which is present in elevated levels. Recently, this stabilization was shown to coincide with the ability to precipitate a polypeptide (p68) of relative molecular mass (Mr) 68,000-70,000 by anti-p53 monoclonal antibodies. We now report that this co-precipitation indeed represents a specific complex between the two proteins; the complex sediments on a sucrose gradient as a relatively broad peak of 10-14S and can be dissociated in vitro. Furthermore, p68 is the HSP70 heat shock protein cognate, found in elevated levels in a p53-overproducing cell line. On heat-shock treatment of such overproducers, p53 also forms a complex with the related highly inducible HSP68.

Ginseng extract inhibits protein degradation and stimulates protein synthesis in human fibroblasts

Aqueous extracts of Panax ginseng inhibit intracellular protein degradation in confluent cultures of IMR-90 human diploid fibroblasts. The magnitude of the inhibition is similar to that observed with insulin and polypeptide growth factors. Furthermore, the inhibition of proteolysis by ginseng, like that produced by insulin and growth factors, is selective in that it applies to long-lived proteins but not to short-lived proteins. Ginseng also stimulates protein synthesis in human fibroblasts indicating that components of ginseng extract are capable of acting directly on human cells to promote protein accumulation.