Species- and tissue-specific expression of the C-terminal alternatively spliced form of the tumor suppressor p53

Application of nanomaterials in the bioanalytical detection of disease-related genes

In the diagnosis of genetic diseases and disorders, nanomaterials-based gene detection systems have significant advantages over conventional diagnostic systems in terms of simplicity, sensitivity, specificity, and portability. In this review, we describe the application of nanomaterials for disease-related genes detection in different methods excluding PCR-related method, such as colorimetry, fluorescence-based methods, electrochemistry, microarray methods, surface-enhanced Raman spectroscopy (SERS), quartz crystal microbalance (QCM) methods, and dynamic light scattering (DLS). The most commonly used nanomaterials are gold, silver, carbon and semiconducting nanoparticles. Various nanomaterials-based gene detection methods are introduced, their respective advantages are discussed, and selected examples are provided to illustrate the properties of these nanomaterials and their emerging applications for the detection of specific nucleic acid sequences.

Novel electrochemical biosensor based on functional composite nanofibers for sensitive detection of p53 tumor suppressor gene

A novel electrochemical biosensor based on functional composite nanofibers for sensitive hybridization detection of p53 tumor suppressor using methylene blue (MB) as an electrochemical indicator is developed. The carboxylated multi-walled carbon nanotubes (MWNTs) doped nylon 6 (PA6) composite nanofibers (MWNTs-PA6) was prepared using electrospinning, which served as the nanosized backbone for pyrrole (Py) electropolymerization. The functional composite nanofibers (MWNTs-PA6-PPy) used as supporting scaffolds for ssDNA immobilization can dramatically increase the amount of DNA attachment and the hybridization sensitivity. The biosensor displayed good sensitivity and specificity. The target wild type p53 sequence (wtp53) can be detected as low as 50 fM and the discrimination is up to 57.5% between the wtp53 and the mutant type p53 sequence (mtp53). It holds promise for the early diagnosis of cancer development and monitoring of patient therapy.

p53 Isoforms: An Intracellular Microprocessor?

Normal function of the p53 pathway is ubiquitously lost in cancers either through mutation or inactivating interaction with viral or cellular proteins. However, it is difficult in clinical studies to link p53 mutation status to cancer treatment and clinical outcome, suggesting that the p53 pathway is not fully understood. We have recently reported that the human p53 gene expresses not only 1 but 12 different p53 proteins (isoforms) due to alternative splicing, alternative initiation of translation, and alternative promoter usage. p53 isoform proteins thus contain distinct protein domains. They are expressed in normal human tissues but are abnormally expressed in a wide range of cancer types. We have recently reported that p53 isoform expression is associated with breast cancer prognosis, suggesting that they play a role in carcinogenesis. Indeed, the cellular response to damages can be switched from cell cycle arrest to apoptosis by only manipulating p53 isoform expression. This may provide an explanation to the hitherto inconsistent relationship between p53 mutation, treatment response, and outcome in breast cancer. However, the molecular mechanism is still unknown. Recent reports suggest that it involves modulation of gene expression in a p53-dependent and -independent manner. In this review, we summarize our current knowledge about the biological activities of p53 isoforms and propose a molecular mechanism conciliating our current knowledge on p53 and integrating p63 and p73 isoforms in the p53 pathway.

Reduced bone formation in alcohol-induced osteopenia is associated with elevated p21 expression in bone marrow cells in aldehyde dehydrogenase 2-disrupted mice

INTRODUCTION

High consumption of alcohol is one of the risk factors for osteoporosis. Approximately 45% of Chinese and Japanese individuals have the inactive aldehyde dehydrogenase 2 (Aldh2) phenotype. The absence of the ALDH2*2 allele is found to adversely influence the risk of osteoporosis. The aim of this study is to clarify the effects of alcohol consumption on osteoblast differentiation in bone marrow and trabecular bone formation in Aldh2-disrupted mice.

MATERIALS AND METHODS

Seven-week-old male Aldh2 knockout mice (Aldh2(-/-)) and wild-type (Aldh2(+/+)) mice were fed with water (groups Aldh2(-/-)/Wa and Aldh2(+/+)/Wa) or with 5% ethanol (groups Aldh2(-/-)/Al and Aldh2(+/+)/Al) for 4 weeks. At the age of 12 weeks, bone histomorphometry was performed at the secondary spongiosa of the tibias. Bone marrow cells from the bilateral femurs and tibias were used for mRNA expression analysis.

RESULTS

Histomorphometrical study revealed that trabecular bone was significantly reduced in the Aldh2(-/-)/Al group compared with that in the Aldh2(-/-)/Wa and Aldh2(+/+)/Wa groups. Bone formation rate was significantly decreased in Aldh2(-/-)/Al compared with the other three groups. Quantitative RT-PCR revealed a significant decrease in type I collagen, osterix, osteopontin, and osteocalcin mRNA expressions in Aldh2(-/-)/Al compared with Aldh2(-/-)/Wa. In bone marrow cell cultures, mineralized nodule formation in Aldh2(-/-)/Al was significantly decreased compared with that in Aldh2(+/+)/Wa and Aldh2(-/-)/Wa, while PAK18, a p21-activated kinase inhibitor, recovered the decreased mineralized nodule formation in Aldh2(-/-)/Al.

CONCLUSION

Alcohol consumption suppressed the differentiation and mineralization of osteoblasts and then reduced trabecular bone formation and bone volume in association with the elevated p21 expression in bone marrow cells, especially in aldehyde dehydrogenase 2-disrupted mice.

p53/p63/p73 isoforms: an orchestra of isoforms to harmonise cell differentiation and response to stress

p63, p73 and p53 compose a family of transcription factors involved in cell response to stress and development. p53 is the most frequently mutated gene in cancer (50%) and loss of p53 activity is considered to be ubiquitous to all cancers. Recent publications may have a profound impact on our understanding of p53 tumour suppressor activity. p63, p73 and p53 genes have a dual gene structure conserved in drosophila, zebrafish and man. They encode for multiple p63, p73 or p53 proteins containing different protein domains (isoforms) due to multiple splicing, alternative promoter and alternative initiation of translation. In this review, we describe the different isoforms of p63, p73, p53 and their roles in development and cancer. The changes in the interactions between p53, p63 and p73 isoforms are likely to be fundamental to our understanding in the transition between normal cell cycling and the onset of tumour formation.

A Novel Human p53 Isoform Is an Essential Element of the ATR-Intra-S Phase Checkpoint

The archetypal human tumor suppressor p53 is considered to have unique transactivation properties. The assumption is based on the fact that additionally identified human p53 isoforms lack transcriptional activity. However, we provide evidence for the existence of an alternatively spliced p53 isoform (Deltap53) that exerts its transcriptional activity independent from p53. In contrast to p53, Deltap53 transactivates the endogenous p21 and 14-3-3sigma but not the mdm2, bax, and PIG3 promoter. Cell cycle studies showed that Deltap53 displays its differential transcriptional activity only in damaged S phase cells. Upon activation of the ATR-intra-S phase checkpoint, Deltap53, but not p53, transactivates the Cdk inhibitor p21. Induction of p21 results in downregulation of cyclin A-Cdk activity and accordingly attenuation of S phase progression. Data demonstrate that the Deltap53-p21-cyclin A-Cdk pathway is crucial to facilitate uncoupling of repair and replication events, indicating that Deltap53 is an essential element of the ATR-intra-S phase checkpoint.

p53 protein variants: structural and functional similarities with p63 and p73 isoforms

Since its discovery in 1979, many studies have reported that the p53 tumour suppressor protein could be expressed in the form of products smaller than those predicted by the full-length amino-acid sequence. These products differ from full-length p53 in their N- or C-terminal regions, but generally conserve the central, DNA-binding domain. They appear to be expressed at rather low levels and to be restricted to particular cell types and/or physiological circumstances, suggesting that they play very narrow and specific roles. Several mechanisms have been proposed to explain their timely occurrence, including alternative splicing, internal initiation of translation or proteolytic cleavage. A precise assessment of the various 'p53 isoforms' reveals striking similarities with several isoforms of the p53 homologous proteins p63 or p73, suggesting that regulated production of specific, N- or C-terminal variants may be a 'trademark' of all family members. In this review, we summarize the published evidence on the structure, mode of production, expression and function of the p53 isoforms, and discuss their properties in the light of recent data on the structure and function of p63/p73 isoforms.

Disruption of the p53 gene results in preserved trabecular bone mass and bone formation after mechanical unloading

We tested the hypothesis that mechanical unloading facilitates signaling of p53, an important modulator of cell cycling and apoptosis, in bone marrow cells and thereby reduces trabecular bone volume (BV). We performed histomorphometric analyses and bone marrow cell cultures in tail-suspended (TS) p53 null (p53-/-) and wild-type (p53+/+) mice. Eight-week-old male mice were assigned to four groups after 1-week acclimatization: p53+/+ + ground control (GC), p53+/+ + TS, p53-/- + GC, and p53-/- + TS. Bilateral tibial samples were used for analysis. The histomorphometric parameters of trabecular structure, formation and resorption did not differ between the p53-/- + GC and p53+/+ + GC groups. Trabecular BV in p53+/+ + TS mice was significantly reduced to 45% of that in the p53+/+ + GC group after one week of TS. In contrast, BV in p53-/- + TS mice was preserved at the same level as that in the p53-/- + GC group. The bone formation rate (BFR) was significantly reduced in p53+/+ + TS but not in p53-/- + TS mice. Unloading significantly increased trabecular osteoclast number (Oc.N) and surface in p53+/+ + TS mice compared with the p53+/+ + GC group, but the difference was not significant between p53-/- + TS and p53-/- + GC mice. In bone marrow cell culture, the numbers of alkaline phosphatase-positive (ALP+) colony-forming units fibroblastic (CFU-f) and mineralized nodules were significantly reduced in p53+/+ + TS, but not p53-/- + TS mice. [3H]thymidine incorporation into bone marrow cells was higher in p53-/- mice than in p53+/+ mice, independent of mechanical loading or unloading. Flow cytometric cell cycle analysis revealed that unloading significantly increased the percentage of hypoploid bone marrow cells in p53+/+ mice relative to that in p53+/+ + GC mice, but there was no significant difference in ploidy between p53-/- + TS and p53-/- + GC mice. Expression levels of p53 and p21 mRNAs were enhanced after TS in bone marrow cells from p53+/+ mice. Our data show that trabecular bone mass and bone formation were preserved after tail-suspension in p53-/- mice, closely associated with ALP+ CFU-f and mineralized nodule formation in marrow cultures obtained from tibias of p53-/- mice. We speculate that bone loss due to mechanical unloading may be related to facilitation of intracellular p53-p21 signaling.

Identification of a novel p53 mutation in JCV-induced mouse medulloblastoma

Medulloblastoma, a malignant invasive tumor of the cerebellum, is one of the most common neoplasms of the nervous system in children. Utilization of the human neurotropic virus JC virus (JCV) early gene T-antigen allowed the development of a transgenic animal that models human medulloblastoma. Here we describe the characterization of two distinct populations of cells derived from the JCV-induced mouse medulloblastoma. Results from immunohistochemical and biochemical studies revealed the expression of T-antigen in some but not all tumor cells. In T-antigen-producing cells, T-antigen was found in association with wild-type p53 and pRb, two tumor suppressors that control cell growth and differentiation. In cells that lack expression of T-antigen, a novel mutant p53 with a deletion between residues 35 and 123 was detected. Morphological differences were observed between the two populations of cells, though there was no significant difference in their growth rates. However, subcutaneous transplantation of the T-antigen-positive, but not T-antigen-negative, cells resulted in the development of massive tumors in experimental animals. In light of earlier reports on the association of JCV with human medulloblastoma, the mouse cell lines described in this study may provide a valuable tool for deciphering the pathways involved in the formation and progression of medulloblastoma.

p53-dependent apoptosis is regulated by a C-terminally alternatively spliced form of murine p53

It is now well accepted that the p53 C-terminus plays a central role in controlling the activity of the wild-type molecule. In our previous studies, we observed that a C-terminally altered p53 protein (p53AS), generated by an alternative spliced p53 mRNA, induces an attenuated p53-dependent apoptosis, compared to that induced by the regularly spliced form (p53RS). In the present study we analysed the interrelationships between these two physiological variants of wild-type p53, and found that in cells co-expressing both forms, in contrast to the expected additive effect on the induction of apoptosis, p53AS inhibits apoptosis induced by p53RS. This inhibitory effect is specific for p53-dependent apoptosis and was not evident in a p53-independent apoptotic pathway induced by growth factor deprivation. Furthermore, the expression of p53AS in transiently transfected cells caused both inhibition of apoptosis and inhibition of the p53RS-dependent transactivation of a number of p53 target genes. These results suggest that expression of an alternatively spliced p53 form may serve as an additional level in controlling the complexity of p53 function by the C-terminal domain.

Species-specific regulation of alternative splicing in the C-terminal region of the p53 tumor suppressor gene

Alternative splicing occurs in the C-terminal region of the p53 tumor suppressor gene between two alternative 3' splice sites in intron 10. This alternative splicing event has been detected in murine cells, but not in rat or human tissues. In this paper, we have characterized the pattern of p53 alternative splicing in cell lines from five different species. Our results confirm that p53 alternative splicing is species-specific, being detected only in cell lines of rodent origin. Using transient transfection assays, we have established that the rat p53 gene undergoes efficient alternative splicing in both mouse and rat cell lines, thus demonstrating that it has all the necessary cis -acting sequences to be alternatively spliced. In contrast, we were unable to detect any usage of the human alternative 3' splice site under the same experimental conditions. Thus, the low levels or absence of alternatively spliced p53 mRNA in rat and human cell lines seems to be the result of different mechanisms. Our results support the hypothesis that there are species-specific mechanisms implicated in the regulation of p53 activity.

Cadmium-induced apoptosis and changes in expression of p53, c-jun and MT-I genes in testes and ventral prostate of rats

Apoptosis and a change in the expression of p53, c-jun and MT-I genes occurred in rats exposed to cadmium in a way known to cause carcinogenesis in testes and ventral prostate. In situ end labelling (ISEL), DNA electrophoresis, and RT-PCR methods were used in present study. Adult male Wistar rats were given a single (s.c.) injection of 0, 5, 10, or 20 micromol/kg CdCl2. Then 12, 48 or 96 h after administration of cadmium, animals were sacrificed. It was observed that cadmium markedly induced apoptosis in the testes at the dose of 5 micromol/kg while 10 and 20 micromol/kg cadmium caused more necrosis than apoptosis. Apoptosis in the ventral prostate was markedly induced by all the doses of cadmium and there was an obvious time- and dose-dependent relationship between apoptotic index (AI) and cadmium treatment. Far fewer apoptotic cells appeared in liver, compared to the testes and ventral prostate. p53 mRNA expression was clearly enhanced in the ventral prostate but clearly suppressed in the testes by cadmium exposure, and the time- and dose-effect was very clear. The expression level of p53 in the liver was not affected by cadmium treatment. Cadmium-induced overexpression of c-jun gene appeared at 12 h in the liver, but not until 96 h in the testes and ventral prostate. Although the MT-I gene was found to be expressed in all tissues, marked induction by cadmium of the expression of MT-I gene was only observed in the liver. These results indicate: (1) that apoptosis is an early mechanism of acute tissue damage by cadmium in the testes and ventral prostate; (2) that p53 and c-jun genes may be involved in cadmium-induced cytotoxicity (apoptosis) and related carcinogenicity in male reproductive tissues; and (3) that the enhanced expression of MT-I in the liver could protect this organ from cadmium-induced cytotoxicity (apoptosis) and carcinogenicity.

Estrogen receptor-alpha mRNA variants in murine and human tissues

A side-by-side comparison of several normal mouse and human tissues was undertaken in order to determine if exon-deleted variant ER-alpha mRNAs are expressed in the mouse. The data showed that the complex pattern of ER-alpha alternative splicing that is detected in multiple human tissues was not apparent in murine tissues. Only low levels of an exon-4 deleted ER-alpha transcript were detected in murine tissues, although multiple relatively abundant exon-deleted ER-alpha transcripts were detected in human tissues. The data support a species-specific difference in the expression of ER-alpha variant mRNAs between mouse and human.

p53-mediated regulation of proliferating cell nuclear antigen expression in cells exposed to ionizing radiation

The proliferating cell nuclear antigen (PCNA) is a highly conserved cellular protein that functions both in DNA replication and in DNA repair. Exposure of a rat embryo fibroblast cell line (CREF cells) to gamma radiation induced simultaneous expression of PCNA with the p53 tumor suppressor protein and the cyclin-dependent kinase inhibitor p21(WAF1/Cip1). PCNA mRNA levels transiently increased in serum-starved cells exposed to ionizing radiation, an observation suggesting that the radiation-associated increase in PCNA expression could be dissociated from cell cycle progression. Irradiation of CREF cells activated a transiently expressed PCNA promoter chloramphenicol acetyltransferase construct through p53 binding sequences via a mechanism blocked by a dominant negative mutant p53. Electrophoretic mobility shift assays with nuclear extracts prepared from irradiated CREF cells produced four p53-specific DNA-protein complexes with the PCNA p53 binding site. Addition of monoclonal antibody PAb421 (p53-specific) or AC238 (specific to the transcriptional coactivator p300/CREB binding protein) to the mobility shift assay distinguished different forms of p53 that changed in relative abundance with time after irradiation. These findings suggest a complex cellular response to DNA damage in which p53 transiently activates expression of PCNA for the purpose of limited DNA repair. In a population of nongrowing cells with diminished PCNA levels, this pathway may be crucial to survival following DNA damage.

The murine C'-terminally alternatively spliced form of p53 induces attenuated apoptosis in myeloid cells

The onset of p53-dependent apoptosis results from the accumulation of damaged DNA. Recently, it was shown that the C' terminus of the p53 protein plays a central role in sensing damaged DNA. In our present study, we examined the role of the C' terminus in the induction of apoptosis. A temperature-sensitive (ts) mutant of the alternatively spliced form of p53 (p53AS-ts) and the ts mutant of the regularly spliced form (p53RS-ts) were used to generate series of stable clones with increasing amounts of p53 protein. Apoptotic patterns induced by either the regularly spliced p53 product (p53RS) or a C'-terminally alternatively spliced p53 product (p53AS) were compared. We found that although both forms of p53 induced apoptosis following expression of the wild-type protein conformation, the kinetics were different. Apoptosis induced by the p53AS protein was attenuated compared to that induced by p53RS. The delay in the manifestation of the apoptotic features following p53AS expression was in agreement with a delay in the regulation of the expression of apoptosis-related genes. The observation that p53 with an altered C' terminus is still capable of inducing apoptosis suggests that the actual onset of the apoptotic process most probably involves structural domains other than the C' terminus of the p53 molecule. However, the fact that the apoptotic activity mediated by the p53AS product was slower than that mediated by the p53RS product suggests that the C' terminus indeed exerts a certain control on the apoptotic activity of the p53 molecule.

Studies of oncogene activation and tumor suppressor gene inactivation in normal and neoplastic rodent tissue

Emerging short-term bioassays for chemically-induced carcinogenesis are dependent for their relevance to human risk assessment on the degree of coincidence of human and rodent tumor pathways. Since these pathways do not always converge, these new tests may have a number of unanticipated pitfalls. Models of liver and renal tumors are described. The results from Rb and p53 tumor suppressor gene transgenic animals are compared to human tumor syndromes. The question of mutagenic and epigenetic fingerprints of chemicals versus the cell-specific selection of spontaneous mutations is debated. Examples of specific pitfalls, such as the recently discovered Helicobacter hepaticus promoted liver tumors in mice are presented. The rat pseudogenes for p53 and the rare role of p53 in most important rodent tumor models other than epithelial tumors present experimental quandaries. The differential effects of carcinogens during various stages of rodent perinatal and adult development are also discussed. It is concluded that the pathways of both animal models and their human counterparts should be better identified so that realistic endpoint markers can be chosen for human carcinogenic risk assessment.