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Carcinogenesis Advance Access originally published online on February 21, 2007
Carcinogenesis 2007 28(8):1814-1823; doi:10.1093/carcin/bgm041
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© The Author 2007. Published by Oxford University Press. All rights reserved. For Permissions, please email: journals.permissions@oxfordjournals.org

Delayed expression of apoptotic and cell-cycle control genes in carcinogen-exposed bladders of mice lacking p53.S389 phosphorylation

Wendy Bruins, Martijs J. Jonker1, Oskar Bruning1, Jeroen L.A. Pennings, Mirjam M. Schaap, Esther M. Hoogervorst, Harry van Steeg, Timo M. Breit1 and Annemieke de Vries*

Laboratory of Toxicology, Pathology and Genetics (TOX), National Institute of Public Health and the Environment (RIVM), Bilthoven, The Netherlands
1 MicroArray Department, Swammerdam Institute for Life Sciences, Faculty of Science, University of Amsterdam, Amsterdam, The Netherlands

* To whom correspondence should be addressed. Tel: +31 30 274 3483; Fax: +31 30 274 4446; Email: annemieke.de.vries{at}rivm.nl


   Abstract
Top
 Abstract
Introduction
 Materials and methods
 Results
 Discussion
 Supplementary material
 References
 
Mice with non-phosphorylated serine 389 in p53 are susceptible for bladder tumors induced by 2-acetylaminofluorene (2-AAF). Since p53 is a transcription factor, this might well be preceded by differences in the regulation of gene expression. Microarray analysis was used to determine early transcriptional changes that might underlie this cancer-prone phenotype. Interestingly, lack of Ser389 phosphorylation led to endogenously different gene expression levels. The number of genes affected was, however, rather small. Conversely, after short-term exposure to 2-AAF, wild-type and p53.S389A bladders demonstrated a significant number of differentially expressed genes. Differences between wild-type and p53.S389A could mainly be attributed to a delayed, rather than complete absence of, transcriptional response of a group of genes, including well-known p53 target genes involved in apoptosis and cell-cycle control like Bax, Perp and P21. An analysis of differentially expressed genes in non-tumorigenic tissue and bladder tumors of p53.S389A after long-term exposure to 2-AAF revealed 319 genes. Comparison of these with those found after short-term exposure resulted in 23 transcripts. These possible marker genes might be useful for the early prediction of bladder tumor development. In conclusion, our data indicate that lack of Ser389 phosphorylation results in aberrant expression of genes needed to execute vital responses to DNA damage. Post-translational modifications, like Ser389 phosphorylation, seem crucial for fine-tuning the transcription of a specific set of genes and do not appear to give rise to major changes in transcription patterns. As such, Ser389 phosphorylation is needed for some, but certainly not all, p53 functions.

Abbreviations: 2-AAF, 2-acetylaminofluorene; GO, Gene Ontology


   Introduction
Top
 Abstract
 Introduction
Materials and methods
 Results
 Discussion
 Supplementary material
 References
 
When chemical or physical compounds damage DNA, several defense mechanisms are activated to prevent the DNA from harmful structural alterations that may ultimately lead to cancer. All these defense mechanisms are aimed at eliminating DNA damage. Examples are cell-cycle arrest, DNA repair and apoptotic processes. The latently present p53 protein becomes stabilized and activated in response to DNA damage. Once activated, p53 acts as a tumor suppressor through transcriptional activation of a large variety of target genes (1–3). Activation of p53 occurs exclusively at the post-translational level through a broad range of modifications, namely (de)phosphorylation, acetylation, ubiquitination, sumoylation, glycosylation, methylation and neddylation. Of these processes, phosphorylation appears to be the most frequent modification of the protein (4–6).

Phosphorylation of the p53 N-terminal region, in particular Ser15 and Ser20, disrupts the interaction with the MDM2 protein, a ubiquitin ligase that inactivates p53 by targeting the protein for proteasomal degradation (7,8). Ser315 and Ser392 were the first phosphorylation sites identified at the C-terminal domain (9). Ser315 is phosphorylated in response to both ionizing and UV radiation, whereas Ser392 is only phosphorylated specifically after UV radiation (4,10–12). Kinases targeting this Ser392 site in vitro are casein kinase II (13,14), the double-stranded, RNA-activated protein kinase (15), p38 MAP kinase (16,17) and the recently identified cyclin-dependent kinase 9 through direct physical interaction (18,19). However, it is still not clear how certain types of DNA damage result in phosphorylation of Ser392, and what the ultimate effect on specific cellular defense systems is in vivo.

To investigate the function of the Ser392 phosphorylation site (equivalent of mouse Ser389), we generated mice with a single-point mutation in the p53 gene that resulted in a substitution of a serine to an alanine, the p53.S389A mouse model (20). Phosphorylation of Ser389 is not required for p53 functioning in either spontaneous or ionizing radiation-induced lymphoma tumor development, since tumor responses were highly comparable in p53.S389A and wild-type mice (20,21). However, p53.S389A mice appeared to be more susceptible for 2-acetylaminofluorene (2-AAF)-induced urinary bladder tumors and UV-B-induced skin tumors (20,21). This is in line with our in vitro findings where cells of these mice demonstrated an impaired DNA-binding capacity and a reduced apoptotic response after UV radiation (20). We therefore hypothesized that phosphorylation of p53 at codon 389 is substrate dependent, and that lack of Ser389 phosphorylation only has an adverse effect on the functioning of p53 as a tumor suppressor in the case of specific substrates.

In the current study, we further explored which in vivo molecular responses are affected when phosphorylation of p53 at codon 389 is impossible. For this, mice were exposed to 2-AAF and gene expression profiles were analyzed in urinary bladder tissue (further referred to as bladder) of male mice. This was done because the increased incidence of tumors in p53.S389A mice was observed in this specific tissue, and was furthermore restricted to male mice only (21). Given the well-known function of p53 as a transcription factor, we hypothesized that this increased incidence of bladder tumors is probably preceded early in the exposure period by a different gene expression profile in bladders of p53.S389A mice compared with wild-type mice not susceptible to exposure to 2-AAF. To investigate this, we performed a microarray analysis on wild-type and p53.S389A male bladders exposed for 1 or 2 weeks (short term) to 2-AAF. Interestingly, a significant number of genes are differentially expressed in bladders of p53.S389A compared with wild-type mice, including known p53 target genes involved in apoptotic and cell-cycle processes. Finally, we also analyzed gene expression patterns in tumorigenic bladders of p53.S389A mice after a long-term exposure to 2-AAF, and compared these with the differentially expressed genes found after a short-term exposure to 2-AAF. Several overlapping genes could be identified. Possibly, these genes are useful as early markers for bladder cancer development.


   Materials and methods
Top
 Abstract
 Introduction
 Materials and methods
Results
 Discussion
 Supplementary material
 References
 
Wild-type and p53.S389A mice exposed to 2-AAF
Mice were exposed to 300 p.p.m. 2-AAF mixed in feed as used in previous studies (21) for 1 or 2 weeks (short term) or 39 weeks (long term) (n = 4). The 2-AAF-containing feed (Altromin, Lage, Germany), normal feed and water were available ad libitum throughout the course of the studies. All mice used were males of 6–9 weeks of age and at least 10 times backcrossed in C57BL/6. Furthermore, control mice received normal diet.

RNA isolation
Total RNA was isolated from mouse bladder using the Rneasy Mini kit (Qiagen, Valencia, CA), followed by a DNase treatment with RNase-Free DNase Set (Qiagen). RNA quality was assessed with the Bioanalyzer 2100 (Agilent Technologies, Palo Alto, CA). All assays were performed according to the manufacturer's protocols.

Microarray analysis
The mouse oligonucleotide libraries (cat. no. MOULIBST and cat. no. MOULIB384B) were obtained from Sigma–Compugen. Technical support was supplied by LabOnWeb (http://www.labonweb.com/cgi-bin/chips/full_loader.cgi). The libraries represent in total 21 766 LEADSTM clusters plus 231 controls. The oligonucleotide library was printed with a Lucidea spotter (Amersham Pharmacia Biosciences, Piscataway, NJ) on commercial UltraGAPS slides (amino-silane-coated slides, Corning 40017) and processed according to the manufacturer's instructions. The slides contained 55mer oligonucleotides and the batch was checked for the quality of spotting by hybridizing with SpotCheck Cy3-labeled nonamers (Genetix, New Milton, Hampshire, UK).

Total RNA samples (1.5 µg) were used in randomized batches, according to a common reference design, with a mix of all samples as common reference. The RNA was amplified using the Amino Allyl MessageAmp aRNA kit (Ambion, Austin, TX) and labeled with Cy3 (experimental samples) and Cy5 (common reference) reactive dye according to the manufacturer's instructions. The microarrays were hybridized overnight with 200 µl hybridization mixture, consisting of 50 µl Cy3- and Cy5-labeled aRNA (in a 1 to 3 molar ratio), 100 µl formamide and 50 µl 4x RPK0325 microarray hybridization buffer (Amersham Pharmacia Biosciences) at 37°C, washed in an automated slide processor (Amersham Pharmacia Biosciences) and subsequently scanned (Agilent DNA MicroArray Scanner, Agilent Technologies).

Data analysis and statistics
Microarray spot intensities were quantified as artifact-removed densities, using Array Vision software (version 6.0). Further processing of the data was performed using R (version 2.2.1) and the Bioconductor MAANOVA package (version 0.98.8). All slides were subjected to a set of quality control checks, i.e. visual inspection of the scans, examining the consistency among the replicated samples by principal component analysis, testing against criteria for signal to noise ratios, testing for consistent performance of the labeling dyes, pen grid plots to check consistent pen performance and visual inspection of pre- and post-normalized data with box plots and Ratio-Intensity (RI) plots.

After log2 transformation, the data were normalized by a spatial lowess smoothing procedure and analyzed using a two-stage mixed model analysis of variance (22,23). First, array, dye and array-by-dye effects were modeled globally. Subsequently, the residuals from this first model are fed into the gene-specific model to fit treatment, and spot effects on a gene-by-gene basis using a mixed model analysis of variance. These residuals are normalized expression values and throughout used in the graphs to depict (differential) gene expression. For hypothesis testing, a permutation-based F1 test was used which allows relaxing the assumption that the data are normally distributed (24). To account for multiple testing, P values from the permutation procedure were adjusted to represent a false discovery rate of 10% (25). Four tests were performed to identify differential gene expression. (i) A test to find differences between control, 1 or 2 weeks 2-AAF-exposed wild-type bladder. (ii) A test to find differences between control, 1 or 2 weeks 2-AAF-exposed p53.S389A bladder. (iii) A test to find differences between genotypes across control, 1 or 2 weeks 2-AAF-exposed bladder. (iv) A test to find differences between tumorigenic and non-tumorigenic p53.S389A bladder.

The differences in gene expression between the wild-type and p53.S389A mice in their response to 2-AAF was qualitatively determined as follows. First, the normalized expression values for all genes found to be involved in 2-AAF exposure were calculated [the union of list (i) and (ii)]. Second, for each time point, the difference in gene expression between 2-AAF-exposed and control bladder was calculated; this difference represents a rate of change in expression. Third, for each time point, this difference in gene expression between 2-AAF-exposed and control bladder of wild-type and p53.S389A were compared in a scatter plot. A deviation from a one-to-one relationship in these plots indicated a difference in gene expression rate between wild-type and p53.S389A in their response to 2-AAF exposure. This enabled the identification of delayed up-regulated and delayed down-regulated genes in p53.S389A bladder.

To relate the differences in gene expression to changes in functional biological processes, lists of differentially expressed genes were analyzed for over-representation of specific Gene Ontology (GO) terms using Onto-Express (http://vortex.cs.wayne.edu/projects.htm). The list of all gene products on the microarray was then used as the reference set. Relevant GO terms were selected according to the false discovery rate-corrected P values and the number of genes involved. The F1 statistics from list (i), (ii) and (iii) were used for gene set enrichment analysis (26). All pathways and gene expression signatures contained in the c2 database of the Molecular Signature Database (MsigDb 1.0; http://www.broad.mit.edu/gsea/msigdb/msigdb_index.html), and an additional p53 pathway reported by Harris et al. (27) were tested for significance using the Gene-Set-Test facility provided by the Limma package (version 2.7.3) in Bioconductor. Pathways and gene expression signatures with P values < 0.05 and with at least one gene from list (i), (ii) or (iii) were reported.

Real-time PCR
To verify the microarray results, cDNA was generated from RNA by using the high-capacity cDNA archive kit containing random hexamer primers (Applied Biosystems, Foster City, CA). mRNA presence was measured with Taqman gene expression assays (Applied Biosystems) on a 7500 Fast Real-Time PCR System, with a two-step PCR procedure according to the manufacturer's protocol. The assays we used were Ccnb2, Mm00432351_m1; Ccng1, Mm00438084_m1; Pttg1, Mm00479224_m1 and Pmaip1, Mm00451763_m1.


   Results
Top
 Abstract
 Introduction
 Materials and methods
 Results
Discussion
 Supplementary material
 References
 
Differentially expressed genes in unexposed wild-type and p53.S389A bladders
Absence of biologically active p53 protein has a dramatic influence on expression profiles, even in unexposed cells (28,29). To test comparable effects in cells carrying modified p53 protein, we compared gene expression profiles between unexposed wild-type and p53.S389A bladders. A total of 44 genes with a clear statistically significant difference between the genotypes were identified and these are presented in a bar plot in Figure 1. Twenty-three genes had a lower expression level {log2 [expression ratio (p53.S389A:wild-type)] <0} and 21 genes had a higher {log2 [expression ratio (p53.S389A:wild-type)] >0} expression level in p53.S389A bladders compared with wild-type bladders. Interestingly, the two genes with the highest induced ratios were the bladder tumor-related gene Sfrp1 and the cofactor of transcription Pparbp (30). The two genes with the most reduced ratio were Igh-6 and the presumed proto-oncogene Pttg1 (31). The biological categories, based on GO terms [analyzed using Onto-Express (32)], containing the most affected genes were development, regulation of transcription and transport. The complete lists of differentially regulated genes found on the basis of genotype differences between wild-type and p53.S389A bladders together with their log2 (expression ratios) are provided in supplementary Table I (available at Carcinogenesis Online). Lack of Ser389 phosphorylation therefore results in clear differences in gene expression levels in unexposed bladders.


Figure 1
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  		Fig. 1. Bar plot of differentially expressed genes in wild-type and p53.S389A bladders. Differences in expression of genes across genotypes were tested as described in Materials and methods. A bar plot representing ratios of the log2 (expression values) of p53.S389A divided by wild-type is shown. Each bar represents an individual gene; corresponding values are depicted in supplementary Table I available at Carcinogenesis Online. The genes with the highest ratios are magnified and represented as a bar plot with the average of log2 (expression values) of both WT (wild-type, black) and SA (p53.S389A, white) control samples.

 
Differentially expressed genes in wild-type and p53.S389A bladders after exposure to 2-AAF
To analyze whether differences in gene expression shortly after exposure to 2-AAF can explain differences in tumor outcome later in life, we exposed wild-type and p53.S389A mice for 1 or 2 weeks to 2-AAF and analyzed gene expression by microarrays in RNA isolated from bladder tissues.

A principal component analysis with a two-dimensional representation of the relationship between the wild-type and p53.S389A samples demonstrated the segregation between the control and samples exposed to 2-AAF for 1 or 2 weeks (Figure 2A). Importantly, a clear segregation between the genotypes was observed as well. Apparently, lack of Ser389 phosphorylation of p53 resulted in different gene expression responses in vivo after exposure to 2-AAF.


Figure 2
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  		Fig. 2. Presentation of differentially expressed genes in wild-type and p53.S389A bladders after short-term exposure to 2-AAF. (A) Principal component analysis (PCA) of differentially expressed genes in wild-type (WT) and p53.S389A (SA) bladders, providing a two-dimensional representation of the relationships between wild-type and p53.S389A gene expression of control bladders and bladders after short-term exposure to 2-AAF. Each group represents four individual replicates. The dashed line shows segregation between wild-type and p53.S389A and the dotted line between the control group and groups exposed short term (1 or 2 weeks) to 2-AAF. (B) Self-organizing map of differentially expressed genes in bladders wild-type and p53.S389A mice after short-term exposure to 2-AAF. All differentially expressed genes were grouped into four clusters per genotype (panels A–D for wild-type and panels E–H for p53.S389A). Each self-organizing map cluster represents the gene expression pattern for genes within the specific cluster, with the number of genes in each cluster indicated. Log2 (expression values) scaled relative to the mean is shown on the y-axis, with the corresponding time points on the x-axis. 1 = 1 week exposure to 2-AAF and 2 = two weeks exposure to 2-AAF.

 
Differential expression of the genes found to have significantly changed over time was analyzed with the help of a one-way analysis of variance on the different groups of mice (control, 1 or 2 weeks 2-AAF) and the construction of self-organizing maps. Figure 2B shows the expression profiles of both genotypes as well as the number of genes per self-organizing map. The complete lists of differentially expressed genes found in wild-type and p53.S389A bladders together with fold-change levels based on their corresponding controls are provided in supplementary Tables II and III (available at Carcinogenesis Online), respectively. Expression levels measured by microarray analysis were highly similar to results obtained with real-time PCR (supplementary Figure 1 available at Carcinogenesis Online).

As a consequence of 2-AAF exposure, there were groups of genes in both genotypes that exhibited continuous changes in gene expression. However, induction and repression levels were rather small (panels A, B, E and F). Furthermore, there were groups with more pronounced expression profiles (panels C, D, G and H). With respect to differences between wild-type and p53.S389A, a group of 82 genes (panels C and D) in the wild-type demonstrated a clear increase in gene expression after 1 week and a marked decrease after 2 weeks, which was not observed in p53.S389A (panels G and H). A careful evaluation revealed that those genes peak at 1 week in wild-type, whereas in p53.S389A, they only reached this peak after 2 weeks. Interestingly, this group included apoptosis-related genes such as Pmaip1 (Noxa) (33), Bax (34) and Birc5 (Survivin) (35), genes involved in damage recovery and growth promotion like Ccng1 (Cyclin G1) and Ccnb2 (Cyclin B2) (36) and the DNA repair gene Mgmt.

Exploring the differentially expressed genes in wild-type and p53.S389A bladders after exposure to 2-AAF
To gain a better understanding of the processes involved in mouse bladders after short-term exposure to 2-AAF, we analyzed all genes that were differentially expressed in wild-type and/or p53.S389A (i.e. union, see Materials and methods) using Onto-Express (32). Supplementary Table IV (available at Carcinogenesis Online) shows the GO categories with a corrected P value ≤ 0.05 and at least three genes present. Within the GO categories found to be significant, the most interesting were cell cycle (arrest), DNA repair and induction of apoptosis. These are processes known to be affected after exposure to a genotoxic agent (37). As expected after exposure to 2-AAF, the category ‘response to DNA damage stimulus’ was also found. Furthermore, mitosis and mitotic-related processes such as DNA replication, cytokinesis and DNA metabolism are significantly present. In addition to the GO analysis, we used gene set enrichment analysis (recently described as a powerful analytical method for interpreting microarray data) to obtain more insight into the biological pathways involved (26). Table I provides a complete list of pathways that were significantly affected (P ≤ 0.005). Four processes with known p53 involvement were ranked in the top 10 when selecting for highest P values in wild-type bladders. For example, the p53-signaling pathway was highly significantly altered, as well as a general p53 pathway and the p53 hypoxia pathway. Furthermore, a variety of cell cycle and apoptotic-related processes were found. In conclusion, short-term in vivo exposure to 2-AAF of wild-type mice resulted in altered expression of genes in bladders mostly involved in p53-related pathways, such as cell-cycle regulation and apoptosis. Given this observation, it might be expected that these p53-related pathways, in particular, are aberrantly regulated by exposure to 2-AAF when p53 functioning is inhibited through mutational inactivation of the Ser389 phosphorylation site. Indeed, gene set enrichment analysis of the differentially regulated genes in bladders from p53.S389A mice exposed to 2-AAF resulted in an almost entirely different order of this list (Table I). Only two of the p53-related processes, p53-UP and p53 signaling, were also found to be significantly present. However, for the corresponding processes, the associated P values were considerably higher in the p53.S389A bladders than those found in wild-type mice, indicating that these p53-related pathways are activated/deactivated to a lesser extent in p53.S389A bladders exposed to 2-AAF.


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  		Table I. Biological pathways in mouse bladders after exposure to 2-AAF that were found to be differential using gene set enrichment analysis

 
Delayed gene expression in p53.S389A bladders exposed to 2-AAF
To unravel the exact nature of the observed differences in gene expression between wild-type and p53.S389A mice, the union of the 2-AAF-induced differential genes was tested for its distribution of genes along the log2 (expression values) axis. As depicted in Figure 3A, left panel, the line chart with the log2 (expression values) of genes in wild-type and p53.S389A control bladders had a slope of 1.04 and a correlation coefficient of 0.98, indicating equal log2 (expression values) of the genes in unexposed bladders of mice of both genotypes (i.e. lack of significant differences). However, after 1 week of exposure to 2-AAF, the distribution along the 1:1 line of these genes was divergent since the slope was only 0.71 and the correlation coefficient 0.64. This means the log2 values (fold-change levels) compared with their corresponding control values of genes in p53.S389A bladders were either higher or lower than those of the same genes in wild-type bladders. Intriguingly, after 2 weeks of exposure to 2-AAF, the distribution of log2 (fold-change levels) of the selected genes was becoming more comparable again with a slope of 1.16 and a correlation coefficient of 0.83. These results indicated that although expression levels in unexposed samples were highly comparable, a mutation at Ser389 resulted in different kinetics of gene expression after short-term exposure to 2-AAF compared with wild-type mice. Since this difference in transcriptional response was most evident after 1 week, and seemed to decrease again after 2 weeks of exposure toward endogenous levels, responses in p53.S389A bladders appeared to be delayed compared with their wild-type counterparts.


Figure 3
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  		Fig. 3. Graphical configuration of differentially expressed genes in wild-type and p53.S389A bladders after short-term exposure to 2-AAF. (A) All differentially expressed genes are displayed in a scatter plot to compare the expression values of the differentially expressed genes found in bladders of wild-type (WT) and p53.S389A (SA) mice after short-term exposure to 2-AAF. Each spot represents an individual gene, where the axis shows the log2 (expression values) for the first plot and log2 (fold-change levels) based on their corresponding controls for the second and third plot of wild-type (x-axis) and p53.S389A (y-axis). An orthogonal regression line is added to show the slope of the data points. Equal expression values of a gene in both wild-type and p53.S389A give a slope of 1. The slope (sl) of this line is displayed in the chart, as well as the corresponding correlation coefficient (cc). (B) A selection of genes (n = 247) with a delayed response in p53.S389A (SA) compared with wild-type (WT) mice after 1 week of exposure [represented in (A)] is shown. Six pairs of line graphs have been made for these genes. In each pair, the left plot is wild-type and the right plot is p53.S389A values of the selected genes. The number of genes in each plot is indicated above the corresponding pair of graphs. (C) Log2 (expression values) of six individual genes corresponding with the line graphs depicted under (B). Black line: wild type, dashed line: p53.S389A, C = control, 1 = 1 week exposure to 2-AAF and 2 = 2 weeks exposure to 2-AAF.

 
K-means clustering was subsequently performed on a selection of these genes (n = 247), in which induction or repression were delayed in p53.S389A compared with wild-type bladders after 1 week of exposure to 2-AAF (Figure 3B). Although the resulting groups of genes did not have similar response patterns, responses in p53.S389A bladders were consistently delayed compared with wild-type responses, both in down-regulation as well as in up-regulation of genes. In some groups (I, IV and V), the expression value in p53.S389A bladders after 2 weeks of exposure to 2-AAF reached more or less the same value as observed in wild-type bladders after 1 week of exposure to 2-AAF. In the remaining groups, the minimum expression values in p53.S389A bladders were either higher (II) or reached a lower (III) or higher (VI) maximum than those in the wild-type. This indicated that although all 247 genes demonstrated delayed responses, the exact nature of this delay was highly dependent on the individual gene concerned. A representative gene was selected for each group (I–VI) and the expression levels of these are depicted in Figure 3C. Use of the GO categories revealed that these delayed genes were involved in transport and mitotic-related processes (cytokinesis, DNA replication, electron transport, metabolism and mitosis). In addition, categories involved in the p53-dependent processes like apoptosis and cell-cycle regulation were also present (see supplementary Table V, available at Carcinogenesis Online).

Based on the above-mentioned results, we investigated the behavior of the known p53-related pathways of cell-cycle arrest and apoptosis, and the p53 target genes involved in these processes. Harris et al. (27) recently reviewed the downstream events of the p53 pathway and this led to a model of important p53 target genes and their function. Figure 4 shows an adapted version of this model, provides an overview of the genes present on our microarray and lists the genes showing a delayed response in our mutant mouse model. Several genes involved in these p53-dependent pathways were severely altered by the mutation at Ser389. In the cell-cycle arrest pathway, three of the 10 p53 target genes, Cdkn1a (P21), Reprimo and Ccnb2 (Cyclin B), demonstrated a delayed response. Also, five of the nine analyzed genes involved in the intrinsic apoptosis pathway were affected by the p53.S389A mutation, Wig1 (PAG608), Perp, Bax, Pmaip1 (Noxa) and Ei24 (Pig8). Interestingly, the extrinsic apoptotic pathway seemed not to be affected, at least not those genes described in the Harris et al. (27) model. Even in the pathway involving angiogenesis and metastasis expression, one of the four genes analyzed was affected, Maspin. In summary, differences in gene expression between p53.S389A and wild-type bladders after exposure to 2-AAF, as detected by microarray analysis, could be mapped to specific p53-dependent pathways.


Figure 4
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  		Fig. 4. Adversely affected p53 target genes in bladders of p53.S389A mice after short-term exposure to 2-AAF. Known p53 target genes involved in the apoptotic, cell-cycle arrest, inhibition of angiogenesis and metastasis and DNA repair processes are presented [model adapted from (27)]. Genes represented in yellow show a delayed response in bladders of p53.S389A mice compared with wild-type after exposure to 2-AAF. Genes represented in blue are present on our microarray but not differential between p53.S389A and wild-type; genes in white are absent on our microarray.

 
Analysis of gene expression in p53.S389A bladder tumors
The detected differences in gene expression profiles of bladders of wild-type and p53.S389A mice after short-term exposure to 2-AAF might explain the increased susceptibility of our mutant model for development of bladder tumors. To analyze whether these differentially expressed genes could be directly linked to bladder tumor development, we compared gene expression profiles in both non-tumorigenic bladders and bladder tumor tissues. Since wild-type mice did not develop bladder tumors using the current exposure protocol, these analyses were restricted to p53.S389A mice only. A total of 319 significant genes were found between non-tumorigenic and bladder tumors of p53.S389A mice (supplementary Table VI available at Carcinogenesis Online). Hierarchical clustering of the individual samples of the control, tumorigenic and non-tumorigenic bladders separated the groups, as a strong correlation across samples was found between the 319 differentially expressed genes (Figure 5). Non-tumorigenic tissue exposed long term to 2-AAF clustered more closely to the controls than tumor-bearing bladders. In addition, absolute expression levels of the majority of genes appeared to be higher in the tumorigenic bladders compared with the non-tumorigenic bladders or controls. This seemed plausible given the high levels of cell proliferation or suppression of survival genes known to be present in tumors in general. Examples of significant biological processes found after GO analysis in this group of differential genes were metabolism, cell adhesion, muscle development, apoptosis and cytoskeleton organization. All these are processes known to be involved in or affected by tumor formation (supplementary Table VII available at Carcinogenesis Online). Some processes demonstrated a reduced expression level of genes in tumorigenic compared with non-tumorigenic bladder (e.g. cytoskeleton organization and muscle development), whereas others show an increased expression of most genes (e.g. cell adhesion).


Figure 5
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  		Fig. 5. Hierarchical clustering of genes found to be differentially expressed in tumorigenic versus non-tumorigenic p53.S389A bladders after long-term exposure to 2-AAF. A total of 319 differentially expressed transcripts were found by analysis of tumorigenic versus non-tumorigenic bladders of p53.S389A mice after long-term exposure to 2-AAF. Clusters were created using hierarchical clustering of the gene expression profiles, where each row represents an individual gene. The degree of redness and greenness represent induction and repression, respectively. Non-tumor = bladders after 39 weeks exposure to 2-AAF without tumorigenic bladder and tumor = bladders after 39 weeks of exposure to 2-AAF with tumorigenic bladder.

 
To find out whether altered gene expression after short-term exposure coincided with genes affected in tumors, a comparison was made of genes found to be differentially expressed in tumorigenic bladders (long-term exposure to 2-AAF) and genes found in p53.S389A bladders exposed short term to 2-AAF. Interestingly, this comparison revealed 23 genes found both in short-term exposed bladders as well as in bladder tumors. Ratios of short-term exposed (i.e. 1 week) p53.S389A:wild-type and long-term exposed tumorigenic:non-tumorigenic were subsequently calculated, and are presented in Table II. Of the genes over-expressed in tumorigenic bladders after long-term exposure to 2-AAF, only two genes (Sfrp1 and Tuba6) were also over-expressed in p53.S389A bladders exposed short term compared with wild-type. However, most of the genes repressed in the tumors were also decreased in p53.S389A bladders exposed short term compared with wild-type.


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  		Table II. Differentially expressed genes detected in p53.S389A bladders after both long-term and short-term exposure to 2-AAF

 

   Discussion
Top
 Abstract
 Introduction
 Materials and methods
 Results
 Discussion
Supplementary material
 References
 
Previously, we demonstrated that 39 weeks of in vivo 2-AAF exposure results in an increased incidence of bladder tumors in p53.S389A compared with wild-type mice (21). Although the underlying molecular and cellular mechanisms remained unclear, it was thought that changes in gene expression might, at least in part, provide some insight in these processes.

Microarray experiments were performed with bladders from wild-type and p53.S389A mice that were exposed to the genotoxic carcinogen 2-AAF for either 1 or 2 weeks. These time points were chosen because a clear and significant induction of cell proliferation and apoptotic cells were previously observed after exposure to 2-AAF in wild-type and p53 knockout mice (38). Short-term 2-AAF exposure resulted in a large group of differentially expressed genes, confirming the involvement of gene expression in response to a genotoxic carcinogen at early time points.

Differentially expressed genes between unexposed wild-type and p53.S389A bladders
Merely the lack of Ser389 phosphorylation resulted in a different expression pattern without the introduction of damage by 2-AAF, even though only 44 genes were found. An explanation for the difference in genotypes under unexposed conditions could lie in different responses to spontaneously formed DNA damage, like reactive oxygen species or depurination [reviewed in (39)]. Apparently, the basal p53 tumor suppressor system is readjusted to cope with spontaneous DNA damage, since p53.S389A mice are not tumor prone under unexposed conditions (20). However, these readjusted responses together with an altered response to genotoxic compounds might contribute to an increased susceptibility for bladder tumor development. A good example is the gene with the highest fold endogenous change level, Sfrp1. This gene is frequently affected in bladder tumors, and is used as a biomarker for bladder cancer detection in humans (40). Another interesting example with one of the lowest fold endogenous change levels is Pttg1 (Securin), which plays a role in p53-mediated cellular response to DNA damage. Rustgi (41) have already demonstrated that the C-terminus of p53 can interact with the N-terminus of Pttg1, regulating apoptosis and transcription activity. The S389A alteration at the C-terminus of p53 might therefore prohibit binding of Pttg1. In our experiment, Pttg1 was not specifically induced after exposure to 2-AAF but the expression level was already found to be reduced in p53.S389A unexposed bladders compared with wild-type. A constitutive expression level of Pttg1 probably gives an overall steady level of some target genes involved in DNA damage response, so that wild-type cells can immediately react to 2-AAF exposure. Alternatively, Pttg1 might be necessary to respond to endogenous DNA damage, and this response is constitutively activated through Ser389 phosphorylation. Pttg1 is also known to be involved in the induction of apoptosis through the p53-dependent, pro-apoptotic gene Bax (42), and mice lacking Pttg1 have aberrant cell-cycle progression, premature centromere division and problems with chromosomal stability (43). In our experiment, levels of Bax induction are indeed reduced in p53.S389A compared with wild-type bladders after exposure to 2-AAF, and this might finally lead to a less effective defense against carcinogens. Altogether, it seems probable that readjustments of basal gene expression level in response to the p53.S389A mutation plays a role in the bladder tumorigenesis upon exposure to 2-AAF.

Processes involved in the responses to 2-AAF
Short-term 2-AAF exposure induced rather large effects on gene expression levels in bladders of mice, with most of the differentially expressed genes functioning in apoptotic and cell-cycle control processes. These findings are in line with a previous study using 2-AAF as the damaging agent, where it was found that apoptotic- and cell proliferation-related genes were up-regulated, whereas immune system related genes were down-regulated [note that these assays were examined in rat livers and B cells, respectively (44)]. In our present study, however, we did not find differences in expression of immune response-related genes in the bladders of mice after short-term exposure to 2-AAF.

Delayed transcriptional response to 2-AAF in p53.S389A bladders
For the first time, we demonstrated in vivo that the mutation at Ser389, a site involved in the post-translational modification of p53, caused altered gene expression patterns in bladders after exposure to a genotoxic carcinogen (Figure 4). In particular, we identified a set of genes whose change of expression after 1 week of exposure was clearly delayed in the p53.S389A mutant bladders. This group of ‘delayed genes’ included several p53 target genes, such as Mgmt (O-6-methylguanine–DNA methyltransferase), which is involved in DNA repair (45). Recently, a microarray study demonstrated the largest induction for Mgmt after 28 days of 2-AAF administration (46). Even though this was specifically described for rat livers, our experiment also clearly revealed an induction of Mgmt in wild-type bladders after short-term exposure to 2-AAF, with a reduced activation of Mgmt after 1 week in p53.S389A mutant bladders. Apparently, phosphorylation of Ser389 is, at least in part, necessary for a rapid induction of Mgmt by p53.

The group of delayed genes also included the well-known p53 target genes involved in the p53-dependent apoptotic or cell-cycle arrest responses to DNA damage [see also model by Harris et al. (27), Figure 4] like Pmaip1 (Noxa), (47), Bax (48), Wig1 (49), Perp (50), Ei24 (Pig8) (51), P21 (52), Reprimo (53), Ccnb2 (Cyclin B2) (54) and Ccng1 (Cyclin G1) (55). Inactivation of these targets commonly results in an oncogenic phenotype. Therefore, the reduced expression levels in p53.S389A mutant bladders after exposure to 2-AAF, as observed in this study, can be clearly linked to increased tumor susceptibility. Preliminary results using immunohistochemical analysis of apoptosis in epithelial tissue of bladders after short-term exposure to 2-AAF indicate a slight reduction of apoptotic levels in vivo in the p53.S389A mice compared with wild-type.

Genes found differentially expressed in tumors and after short-term exposure to 2-AAF
To identify possible tumor markers, i.e. genes that are associated with tumor formation, differentially expressed genes found with the analysis of tumorigenic versus non-tumorigenic bladders after long-term exposure to 2-AAF were compared with genes found differentially expressed in bladders after short-term exposure. A fraction of these genes, 23, indeed overlapped, indicating that these tumor-related genes are already affected by 2-AAF exposure at early time points. The list of genes as presented in Table II contains some genes related to drug metabolism and tumor development, like Fbln5 and Sfrp1. Expression of Fbln5 has previously been associated with the suppression of tumor formation through its control of cell proliferation (56), and reduced expression of this gene has also been associated with the progression of malignant lymphoma (57). In line with this, expression levels in tumorigenic tissue were decreased compared with non-tumorigenic p53.S389A bladders in our study, indicating phosphorylation of Ser389 is needed for optimal functioning of Fbln5 in tumor suppression in the case of bladders after long-term exposure to 2-AAF. Other examples of overlapping genes are the cytochrome P450 family member Cyp1b1 and the key enzyme involved in defense against reactive forms of oxygen Nqo1, which was also found to be required for the stabilization of p53 protein in response to DNA-damaging stimuli. Both are repressed after short-term 2-AAF exposure in p53.S389A bladders and in the bladder tumor. A final example is the Wnt antagonist Sfrp1 with induced expression levels both in unexposed p53.S389A bladders and in the bladder tumor. However, for the above-mentioned genes in our experiment, the ratio of expression levels of genes found in tumorigenic compared with non-tumorigenic tissue was the opposite of that in studies performed by others (40,58–62). This might be because our experiment was performed in bladders of mutant mice, whereas the other studies mainly used bladder cancer samples or related cell lines from human patients that probably contain other genetic alterations that affect gene expression. In summary, our work has identified some genes differentially expressed after short-term exposure to 2-AAF, which are associated with tumor formation in bladders of mice lacking the Ser389 phosphorylation event.

Concluding remarks
As tumor formation is a process of several sequential stages like initiation, promotion and progression, the differences in response to bladder tumors between wild-type and p53.S389A mice are expected to be present at all these stages. This was particularly the case in our experimental set up, since it was previously reported that arylamines (such as 2-AAF) are involved in both initiating as well as promoting activities in carcinogenesis (63). In this study, we were interested in the gene expression profiles after 1 or 2 weeks of exposure to 2-AAF in relation to a p53.S389A mutation, as this can give an impression of the initiation step of tumor development. In general, we measured many subtle differences in gene expression. Mapping these findings to a recently published model of the p53 activation pathway demonstrated that a considerable number of these model genes were differentially expressed in bladders of p53.S389A compared with wild-type mice after exposure to 2-AAF. These subtle changes in gene expression can be important for tumor development, as bladder cancer develops due to the accumulation of various molecular changes: (i) chromosomal alterations; (ii) loss of cell-cycle regulation; (iii) growth control events such as angiogenesis, resulting in metastasis and (iv) decrease in cellular apoptosis (64). Our results demonstrated a possible effect on chromosomal stability, as Pttg1 expression is differential between the genotypes. Further, a clear effect on cell-cycle regulation and apoptotic responses could be detected. Although angiogenesis-related genes and pathways were not found, the regulation of these processes might arise at a later stage in tumor development, i.e. far beyond the 2 weeks time point. In conclusion, a mutation at the phosphorylation site Ser389 results, in vivo, in specific altered gene expression profiles in bladders exposed short term to carcinogens, which finally leads to an increased susceptibility for tumor development in the bladder. Since responses of several (p53) target genes are not completely absent but only partially disturbed, and since only 14% of the p53.S389A mice developed tumors after 39 weeks, p53 still seems to be partially active in response to DNA damage induced by 2-AAF. Activation of p53 in the p53.S389A mice might presumably go through other post-translational modifications (21). Mouse models with mutations at other phosphorylation sites also displayed intermediate cellular responses, underlining the idea of p53 functioning being fine-tuned through post-translational modifications (65–68). These results do, however, indicate the importance of post-translational modifications for proper p53 functioning. Phosphorylation of Ser389 appears to play a key role in fine-tuning the expression of a variety of important genes, as opposed to being the result of major changes in a few genes, because a reduction or delay in expression is measurable as opposed to a complete elimination. Finally, we demonstrated that phosphorylation of Ser389 is involved in some, but not all, functions of p53.


   Supplementary material
Top
 Abstract
 Introduction
 Materials and methods
 Results
 Discussion
 Supplementary material
References
 
Supplementary Tables I–VII and Figure 1 can be found at http://carcin.oxfordjournals.org/


   Acknowledgments
 
We thank the Central Animal Facility Laboratory (NVI-CDF) for their skilful (bio)technical support and S. Banus for help with preparation of the figures. The work presented here was supported by grants from the Dutch Cancer Society (KWF 2000-2352) and the National Institutes of Health/National Institute of Environmental Health Sciences (Comparative Mouse Genomics Centers Consortium) grant 1UO1 ES11044.

Conflict of Interest Statement: None declared.


   References
Top
 Abstract
 Introduction
 Materials and methods
 Results
 Discussion
 Supplementary material
 References
 

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Received October 1, 2006; revised December 22, 2006; accepted February 12, 2007.


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