Cancer genetics. Is p53 the only real tumor suppressor gene?

The Paradox of p53: What, How, and Why?

Unlike the rather stereotypic image by which it was portrayed until not too many years ago, p53 is now increasingly emerging as a multifaceted transcription factor that can sometimes exert opposing effects on biological processes. This includes pro-survival activities that seem to contradict p53's canonical proapoptotic features, as well as opposing effects on cell migration, metabolism, and differentiation. Such antagonistic bifunctionality (balancing both positive and negative signals) bestows p53 with an ideal attribute to govern homeostasis. The molecular mechanisms underpinning the paradoxical activities of p53 may be related to a protein conformational spectrum (from canonical wild-type to "pseudomutant"), diversity of DNA response elements, and/or higher-order chromatin configuration. Altogether, this functional flexibility positions p53 as a transcriptional "super hub" that dictates cell homeostasis, and ultimately cell fate, by governing a hierarchy of other functional hubs. Deciphering the mechanisms by which p53 determines which hubs to engage, and how one might modulate the preferences of p53, remains a major challenge for both basic science and translational cancer medicine.

Implications of p53 gene mutations on patient survival in transitional cell carcinoma of the bladder: a long-term study

OBJECTIVE

To determine the prognostic value of p53 gene mutations and P53 overexpression for predicting the incidence of recurrence, progression and long-term survival of patients with transitional cell carcinoma (TCC) of the bladder.

METHODS

Prospective cohort study with 94 consecutive patients diagnosed and treated for TCC. DNA was obtained from tumor tissue to perform PCR-SSCP of p53 exons 5-9, with automatic sequencing of any mutated samples. Immunohistochemistry using anti-human P53 monoclonal antibody was also performed. Survival was analyzed and the survival curves compared (Mantel-Haenszel). Lastly, a Cox proportional hazards model was constructed.

RESULTS

Mutations were found in 46.8% of samples, with 61.8% in infiltrating tumors. Exon 8 was involved in 42.3%. P53 overexpression (cutoff > or =20%) was found in 52.1%. Mean follow-up was 44.1 months; 43.6% had died by the end of this period. Mean survival was lower in patients with exon 8 mutations (38.4 months), compared with patients without this exon mutated (P = 0.016). There were no differences in patient survival based on positive or negative immunohistochemistry (cutoff > or =20%), although survival was lower in patients with a percentage higher than 50% of antibody-stained cells (P = 0.02). In the Cox analysis, tumor stage, pM stage, and interaction between stage > or =pT2 and mutated p53 gene were independent risk factors, with a 6.13-fold risk of death in these patients (P = 0.019). The number of tumors, nuclear grade, pTa stage, and the interaction between GI degree and nonmutated p53 gene remained in the Cox model for superficial tumors, such that these patients had a lower risk of recurrence or progression (P = 0.008).

CONCLUSIONS

Alterations in the p53 gene may be indicative of poorer prognosis and greater recurrence in patients with urothelial bladder tumor, in particular, the presence of mutations in exon 8 and a greater percentage of stained cells in the immunohistochemistry. Nevertheless, the classic prognostic factors (primarily, pTNM stage) should still be considered the most useful factors for follow-up of these patients.

New molecular targets and novel agents in the treatment of advanced urothelial cancer

Standard cytotoxic regimens for metastatic bladder cancer, such as gemcitabine/cisplatin or methotrexate, vinblastine, doxorubicin, and cipslatin (M-VAC), yield impressive overall response rates of 45% to 70%. Despite this, long-term, disease-free, overall survival is rare, and most patients eventually succumb to the disease. Much work has been undertaken evaluating the clinical and molecular factors associated with progressive bladder cancer, and this has, in turn, led to the development of both novel targets and agents. These include standard cytotoxics such as pemetrexed, an antifolate and antimetabolite agent that has demonstrated an overall response rate of 30% in early studies, and small-molecule tyrosine kinase inhibitors such as sunitinib, which will be studied as maintenance therapy for patients who respond to first-line chemotherapy. The evaluation of new targets and new agents in the midst of limited patient, logistical, and financial resources will be one of the more difficult challenges for investigators over the next several years.

UCN-01-mediated G1 arrest in normal but not tumor breast cells is pRb-dependent and p53-independent

In this study we investigated the growth inhibitory effects of UCN-01 in several normal and tumor-derived human breast epithelial cells. We found that while normal mammary epithelial cells w were very sensitive to UCN-01 with an IC(50) of 10nM tumor cells displayed little to no inhibition of growth with any measurable IC(50) at low UCN-01 concentrations (i.e. 0-80 nM). The UCN-01 treated normal cells arrested in G1 phase and displayed decreased expression of most key cell cycle regulators examined, resulting in inhibition of CDK2 activity due to increased binding of p27 to CDK2. Tumor cells on the other hand displayed no change in any cell cycle distribution or expression of cell cycle regulators. Examination of E6- and E7-derived strains of normal cells revealed that pRb and not p53 function is essential for UCN-01-mediated G1 arrest. Lastly, treatment of normal and tumor cells with high doses of UCN-01 (i.e. 300 nM) revealed a necessary role for a functional G1 checkpoint in mediating growth arrest. Normal cells, which have a functional G1 checkpoint, always arrest in G1 even at very high concentrations of UCN-01. Tumor cells on the other hand have a defective G1 checkpoint and only arrest in S phase with high concentrations of UCN-01. The effect of UCN-01 on the cell cycle is thus quite different from staurosporine, a structural analogue of UCN-01, which arrests normal cells in both G1 and G2, while tumor cells arrest only in the G2 phase of the cell cycle. Our results show the different sensitivity to UCN-01 of normal compared to tumor cells is dependent on a functional pRb and a regulated G1 checkpoint.

Cell cycle regulation and apoptosis

Tissue homeostasis requires a balance between cell proliferation and death. Apoptosis and proliferation are linked by cell cycle regulators, and apoptotic stimuli affect both cell proliferation and death. Glucocorticoids induce G1 arrest and apoptosis in transformed lymphoid cells. Decreased expression of the cell cycle components c-myc and cyclin D3 is essential for glucocorticoid-induced growth arrest and death in dividing cells. Other G1 regulators, such as p53, pRb, and E2F, have also been implicated in apoptosis. Mice lacking either p53 or E2F display aberrant cell proliferation and tumor formation, suggesting that these proteins are involved in the elimination of abnormal cells through apoptosis. In contrast, pRb induces G1 arrest and suppresses apoptosis in cultured cells. Mice that lack pRb are nonviable and show ectopic mitosis and massive cell death, suggesting that pRb is an apoptotic suppressor. Further analysis of common components of apoptotic and cell cycle machinery may provide insight into the coordinated regulation of these antagonistic processes.

Transcription factors: an overview

This special issue of the International Journal of Biochemistry and Cell Biology contains a series of review articles and original papers dealing with the topic of transcription factors. The purpose of this introductory article is to provide an overview of these factors, their mechanism of action, their regulation and the manner in which alterations in them can result in disease.

Inhibitory transcription factors

Although the majority of the transcription factors that were initially characterized had a stimulatory effect on gene expression, it is now clear that transcription factors that inhibit gene transcription are at least as important in regulating a wide variety of processes, including development. These factors can act either by interfering with the action of a positively acting factor (indirect repression) or by directly interfering with transcription by interacting with the basal transcriptional complex of RNA polymerase and associated factors (direct repression). Indirect repression often operates by the negative factor preventing the positively acting factor binding to DNA. This can involve reorganization of chromatin structure, blockage of the binding site in the DNA by binding of the inhibitory factor or formation of a non-DNA binding protein-protein complex. Indirect repression can also occur via quenching of the activity of a positive factor that remains bound to DNA. Direct repression can be produced by factors that interact with the basal transcriptional complex to reduce its activity or stability. This can be achieved either by factors that interact with the complex following binding to DNA or by those which bind directly to it. Factors that act by each of these means, and their mechanisms of action, are discussed.

Negative feedback regulation of wild-type p53 biosynthesis

When growth-arrested mouse fibroblasts re-entered the cell-cycle, the rise in tumour suppressor p53 mRNA level markedly preceded the rise in expression of the p53 protein. Furthermore, gamma-irradiation of such cells led to a rapid increase in p53 protein biosynthesis even in the presence of the transcription inhibitor actinomycin D. Both findings strongly suggest that p53 biosynthesis in these cells is regulated at the translational level. We present evidence for an autoregulatory control of p53 expression by a negative feed-back loop: p53 mRNA has a predicted tendency to form a stable stem-loop structure that involves the 5'-untranslated region (5'-UTR) plus some 280 nucleotides of the coding sequence. p53 binds tightly to the 5'-UTR region and inhibits the translation of its own mRNA, most likely mediated by the p53-intrinsic RNA re-annealing activity. The inhibition of p53 biosynthesis requires wild-type p53, as it is not observed with MethA mutant p53, p53-catalysed translational inhibition is selective; it might be restricted to p53 mRNA and a few other mRNAs that are able to form extensive stem-loop structures. Release from negative feed-back regulation of p53 biosynthesis, e.g. after damage-induced nuclear transport of p53, might provide a means for rapidly increasing p53 protein levels when p53 is required to act as a cell-cycle checkpoint determinant after DNA damage.

Induction of apoptosis in HeLa cells by trans-activation-deficient p53

The p53 tumor suppressor protein is a transcriptional activator, which can mediate apoptotic cell death in a variety of cell types. To determine whether sequence-specific trans-activation is a prerequisite for the induction of apoptosis by p53, the apoptotic effects of various p53 deletion mutants were monitored in an assay based on the transient transfection of HeLa cells. A truncated protein (p53dl214), containing only the first 214 amino-terminal residues of murine p53, induced extensive apoptosis, albeit at a slower rate than trans-activation-competent wild-type p53. p53dl214 also suppressed the transformation of rat fibroblasts by several oncogene combinations and particularly by myc plus ras and HPV E7 plus ras. p53dl214 lacks a major portion of the DNA-binding domain and cannot activate p53-responsive promoters. Moreover, a human p53 protein carrying mutations in residues 22 and 23 also triggered HeLa cell apoptosis, despite failing to induce significant activation of relevant p53 target promoters. These data suggest the existence of two p53-dependent apoptotic pathways--one requiring activation of specific target genes, and the other independent of sequence-specific trans-activation. The latter pathway may actually be totally uncoupled from the binding of p53 to its consensus DNA sites. The relative contribution of trans-activation-independent apoptosis to tumor suppression by p53 may be dictated by the specific genetic lesions present in the particular tumor.