The p53 Isoform Deltap53 lacks intrinsic transcriptional activity and reveals the critical role of nuclear import in dominant-negative activity

Role of Δ133p53 isoform in NF-κB inhibitor PDTC-mediated growth inhibition of MKN45 gastric cancer cells

AIM

To investigate the role of Δ133p53 isoform in nuclear factor-κB (NF-κB) inhibitor pyrrolidine dithiocarbamate (PDTC)-mediated growth inhibition of MKN45 gastric cancer cells.

METHODS

The growth rate of MKN45 cells after treatment with different concentrations of only PDTC or PTDC in combination with cisplatin was detected by the CCK-8 assay. mRNA expression levels of Δ133p53, p53β, and the NF-κB p65 subunit and p65 protein levels were detected by reverse transcription-polymerase chain reaction (RT-PCR) and immunofluorescence, respectively. Growth of MKN45 cells was significantly inhibited by PDTC alone in a dose-dependent manner (P < 0.01). Moreover, the inhibitory effect of cisplatin was remarkably enhanced in a dose-dependent manner by co-treatment with PDTC (P < 0.01).

RESULTS

RT-PCR analysis revealed that mRNA expression of p65 was curbed significantly in a dose-dependent manner by treatment with only PDTC (P < 0.01), and this suppressive effect was further enhanced when co-treated with cisplatin (P < 0.01). With respect to the other p53 isoforms, mRNA level of Δ133p53 was significantly reduced in a dose-dependent manner by treatment with only PDTC or PTDC in combination with cisplatin (P < 0.01), whereas p53β mRNA expression was not altered by PDTC treatment (P > 0.05). A similar tendency of change in p65 protein expression, as observed for the corresponding mRNA, was detected by immunofluorescence analysis (P < 0.01). Pearson correlation analysis demonstrated that Δ133p53 and p65 mRNA expression levels were positively related, while no significant relationship was observed between those of p65 and p53β (r = 0.076, P > 0.01).

CONCLUSION

Δ133p53 isoform (not p53β) is required in PDTC-induced inhibition of MKN45 gastric cancer cells, indicating that disturbance in the cross-talk between p53 and NF-κB pathways is a promising target in pharmaceutical research for the development of treatment strategies for gastric cancer.

Effect of p53β on human gastric cancer cells treated with recombinant mutated human TNF and cisplatin

The present study aimed to investigate the role of tumour protein 53 isoform b (p53β) on human gastric cancer (GC) cell lines treated with recombinant mutated human tumour necrosis factor (rmhTNF) and cisplatin. The Cell Counting Kit‑8 assay was used to assess growth in the GC cell lines MKN45 and SGC7901, following treatment with rmhTNF in the presence or absence of cisplatin. Levels of p53β and bcl‑2 apoptosis regulator (bcl‑2) mRNA were assessed using reverse transcription‑polymerase chain reaction. The results demonstrated that growth was significantly inhibited by either cisplatin or rmhTNF treatments alone in MKN45 cells, and combination treatment with cisplatin and rmhTNF had a synergistic effect on growth inhibition of MKN45 cells. Notably, these observations were not evident in SGC7901 cells, where a mutant form of p53 is present. Treatment of MKN45 cells with rmhTNF did not affect bcl‑2 or p53β mRNA expression levels. However, treatment of MKN45 cells with cisplatin induced upregulation of p53β and downregulation of bcl-2 mRNA expression levels, and these effects were enhanced by combination treatment with rmhTNF. Pearson correlation analysis revealed a negative correlation between the expression of p53β and bcl‑2 mRNA, and a negative correlation between bcl-2 mRNA expression and the inhibition of cell growth. In conclusion, the inhibitory effect of cisplatin on the growth of MKN45 GC cells was enhanced by rmhTNF via unknown mechanisms that involved p53β, indicating that p53β may be an appropriate therapeutic target for the treatment of GC.

Role of p53β in the inhibition of proliferation of gastric cancer cells expressing wild-type or mutated p53

p53 is a tumor suppressor gene whose mutation is highly associated with tumorigenesis. The present study investigated the role of p53β in the inhibition of proliferation of gastric cancer cell lines expressing wild-type or mutated p53. Wild-type p53 is expressed in MKN45 cells, but deleted in KATOIII cells, whereas mutated p53 is expressed in SGC7901 cells. The mRNA expression levels of p53β and Δ133p53 were detected in MKN45, SGC-7901 and KATOIII gastric cancer cell lines using nested polymerase chain reaction (PCR). The mRNA expression levels of p53, p53β and B-cell lymphoma 2-associated X protein (Bax) were detected in the MKN45 and SGC-7901 cells following treatment with cisplatin by reverse transcription-PCR. The inhibition of cellular proliferation following treatment with cisplatin was measured by MTT assay. The results of the present study demonstrated that both p53β and Δ133p53 mRNA were expressed in the MKN45 cells, whereas only p53β mRNA was expressed in the SGC7901 cells. No expression of p53β or Δ133p53 mRNA was detected in the KATOIII cells. Following treatment with cisplatin, the number of both MKN45 and SGC-7901 cells was significantly reduced (P<0.001). In the MKN45 cells, p53β, p53 and Bax mRNA expression levels gradually increased with the dose of cisplatin, and the expression of p53β was positively correlated with the expression of p53 (tr=6.358, P<0.05) and Bax (tr=8.023, P<0.05). In the SGC-7901 cells, the expression levels of p53β, p53 and Bax mRNA did not alter with the dose of cisplatin, and the expression of p53β was positively correlated to the expression of p53 (tr=26.41, P<0.01) but not that of Bax. The present study identified the different roles of the p53β isoform in gastric cancer cells with different p53 backgrounds. Enhanced knowledge regarding the p53 status is required for the development of specific biological therapies against gastric cancer.

Detection of p53 protein aggregation in cancer cell lines and tumor samples

The p53 protein plays a central role in regulating apoptosis. The loss of functional p53 is common in many cancers. In cancer cells, the dysfunctional p53 protein often maintains a misfolded, inactive conformation due to genetic mutations or posttranslational deregulation. The misfolded p53 protein can aggregate and form amyloid-like oligomers and fibrils, which abrogate the pro-apoptotic functions of p53. Therefore, the aggregation of p53 may be a crucial factor in carcinogenesis, tumor progression, and the response of cancer cells to apoptotic signals. In this chapter, we provide details on various methods for detecting p53 aggregation in cancer cell lines and tumor samples.

Identification of p53 and its isoforms in human breast carcinoma cells

In breast carcinoma, disruption of the p53 pathway is one of the most common genetic alterations. The observation that the p53 can express multiple protein isoforms adds a novel level of complexity to the outcome of p53 mutations. p53 expression was analysed by Western immunoblotting and immunohistochemistry using monoclonal antibodies DO-7, Pab240, and polyclonal antiserum CM-1. The more frequently p53-positive nuclear staining has been found in the invasive breast tumors. One of the most intriguing findings is that mutant p53 appears as discrete dot-shaped regions within the nucleus of breast cancer cells. In many malignant cells, the nucleolar sequestration of p53 is evident. These observations support the view that the nucleolus is involved directly in the mediation of p53 function or indirectly by the control of the localization of p53 interplayers. p53 expressed in the nuclear fraction of breast cancer cells revealed a wide spectrum of isoforms. p53 isoforms ΔNp53 (47 kDa) and Δ133p53 β (35 kDa), known as dominant-negative repressors of p53 function, were detected as the most predominant variants in nuclei of invasive breast carcinoma cells. The isoforms expressed also varied between individual tumors, indicating potential roles of these p53 variants in human breast cancer.

Specific inhibition of NF-Y subunits triggers different cell proliferation defects

Regulated gene expression is essential for a proper progression through the cell cycle. The transcription factor NF-Y has a fundamental function in transcriptional regulation of cell cycle genes, particularly of G2/M genes. In order to investigate common and distinct functions of NF-Y subunits in cell cycle regulation, NF-YA, NF-YB and NF-YC have been silenced by shRNAs in HCT116 cells. NF-YA loss led to a delay in S-phase progression, DNA damage and apoptosis: we showed the activation of the replication checkpoint, through the recruitment of Δp53 and of the replication proteins PCNA and Mcm7 to chromatin. Differently, NF-YB depletion impaired cells from exiting G2/M, but did not interfere with S-phase progression. Gene expression analysis of NF-YA and NF-YB inactivated cells highlighted a common set of hit genes, as well as a plethora of uncommon genes, unveiling a different effect of NF-Y subunits loss on NF-Y binding to its target genes. Chromatin extracts and ChIP analysis showed that NF-YA depletion was more effective than NF-YB in hitting NF-Y recruitment to CCAAT-promoters. Our data suggest a critical role of NF-Y expression, highlighting that the lack of the single subunits are differently perceived by the cells, which activate diverse cell cycle blocks and signaling pathways.

DeltaN133p53 expression levels in relation to haplotypes of the TP53 internal promoter region

The transcription of the DeltaN133p53 isoform of the TP53 gene is controlled by an internal promoter region (IPR) containing eight polymorphisms in 11 common haplotypes, following a resequencing of 47 Caucasians. We assayed the functional effects of the commonest six haplotypes on the promoter activity with a luciferase reporter system, in HeLa and 293T cells. These studies showed that different IPR haplotypes are associated with differences in the promoter activity resulting in marked variation in the baseline expression of DeltaN133p53. In vivo quantitative-polymerase chain reaction (PCR) on human tissues confirmed that the baseline levels of DeltaN133p53 showed haplotype specific differences that paralleled those seen in vitro. When cell lines were treated with camptothecin, the fold-increase in DeltaN133p53 levels was dose-dependent but haplotype-independent (i.e., similar for all the haplotypes). Finally, we used an electrophoretic mobility shift assay to analyze the rs1794287 polymorphism and found changes in the pattern of protein binding. This partially confirmed our in silico analysis showing that the polymorphism rs1794287 can affect the function of the internal promoter by changing its affinity for several transcription factors. Thus, we showed that the expression of DeltaN133p53 is under genetic control, and suggested the presence of interindividual differences underlying this mechanism.

Nuclear accumulation and activation of p53 in embryonic stem cells after DNA damage

BACKGROUND

P53 is a key tumor suppressor protein. In response to DNA damage, p53 accumulates to high levels in differentiated cells and activates target genes that initiate cell cycle arrest and apoptosis. Since stem cells provide the proliferative cell pool within organisms, an efficient DNA damage response is crucial.

RESULTS

In proliferating embryonic stem cells, p53 is localized predominantly in the cytoplasm. DNA damage-induced nuclear accumulation of p53 in embryonic stem cells activates transcription of the target genes mdm2, p21, puma and noxa. We observed bi-phasic kinetics for nuclear accumulation of p53 after ionizing radiation. During the first wave of nuclear accumulation, p53 levels were increased and the p53 target genes mdm2, p21 and puma were transcribed. Transcription of noxa correlated with the second wave of nuclear accumulation. Transcriptional activation of p53 target genes resulted in an increased amount of proteins with the exception of p21. While p21 transcripts were efficiently translated in 3T3 cells, we failed to see an increase in p21 protein levels after IR in embryonal stem cells.

CONCLUSION

In embryonic stem cells where (anti-proliferative) p53 activity is not necessary, or even unfavorable, p53 is retained in the cytoplasm and prevented from activating its target genes. However, if its activity is beneficial or required, p53 is allowed to accumulate in the nucleus and activates its target genes, even in embryonic stem cells.

The novel p53 isoform "delta p53" is a misfolded protein and does not bind the p21 promoter site

The tumor suppressor p53 can be expressed as different isoforms because of promoter selection and mRNA editing. One isoform, "delta p53" (Delta p53), results from what would be an unusual alternative splicing of exons 7/8 of the p53 gene, conserving the reading frame and generating a novel protein with proposed transcriptional activity essential for the intra S-phase checkpoint. Here, we show that the deletion of the 66 residues that correspond to strand beta10 and the C-terminal helix of the core domain and the interconnecting linker to the tetramerization domain occurring in the Delta p53 isoform leads to a misfolded and unstable protein, prone to form soluble aggregates, which does not bind the p21 promoter site. The complex of coexpressed Delta p53 and flp53 is soluble in vitro and binds poorly to DNA. Our results provide a structural explanation for the dominant-negative effect of Delta p53 and its lack of transcriptional activity.

p53 and its isoforms in cancer

p53, p63 and p73 are members of the p53 gene family involved in development, differentiation and response to cellular stress. p53 gene is a transcription factor essential for the prevention of cancer formation. The p53 pathway is ubiquitously lost in human cancer either by p53 gene mutation (60% of cancers) or by lost of cell signalling upstream and downstream of p53 in the remaining cancers expressing WTp53 gene. As p53 pathway inactivation is a common denominator to all cancers, the understanding of p53 tumour suppressor activity is likely to bring us closer to cancer therapy. However, despite all the experimental evidences showing the importance of p53 in preventing carcinogenesis, it is difficult in clinical studies to link p53 status to cancer treatment and clinical outcome. The recent discovery that p53 gene encodes for nine different p53 proteins (isoforms) may have a profound impact on our understanding of p53 tumour suppressor activity. Studies in several tumour types have shown that the nine different p53 isoforms are abnormally expressed in tumour tissues compared to normal cells. p53 protein isoforms modulate p53 transcriptional activity and cell fate outcome in response to stress. Regulation of p53 function in normal and tumour tissues in man is likely to be more complex than has been hitherto appreciated. Therefore, the tumour p53 status needs to be determined more accurately by integrating p53 isoform expression, functional p53 mutation analysis and a panel of antibodies specific of p53 and of its target genes.