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Cancer Res. Author manuscript; available in PMC Apr 28, 2011.
Published in final edited form as:
PMCID: PMC3083842

High level of SOX9 in the prostate contributes to increased proliferation and can cooperate with PTEN loss to accelerate neoplasia formation


Developmental pathways have been shown to be important in the initiation and progression of cancer in various tissues. We showed that the transcription factor SOX9 is expressed in the epithelia of the mouse embryonic prostate and is required for proper prostate development. We have performed an in vivo investigation into the role of SOX9 in prostate cancer in mouse and human. Studies on Pten and Nkx3.1 mutant mice show that cells with an increased level of SOX9 appear within the epithelia at the early stages of prostate neoplasia and this high expression correlates with all stages of neoplastic progression. Using genetically modified mice we show that overexpression of SOX9 in prostate epithelia leads to an increase in cell proliferation without inducing hyperplasia. In mice that were heterozygous for the conditional mutant allele of Pten, overexpression of SOX9 gave rise to an earlier induction of high-grade prostate intraepithelial neoplasia. Consistent with this role, loss of Sox9 in prostate epithelia led to a decrease in proliferating cells in normal and in homozygous Pten mutant mice with prostate neoplasia. Analysis of a cohort of 880 human prostate cancer samples showed that SOX9 expression is associated with increasing Gleason grades and higher Ki67 staining. These studies identify SOX9 as part of a developmental pathway that is reactivated in prostate neoplasia where it is involved in regulating proliferation and suggests it can contribute to carcinogenesis in specific genetic contexts.


Prostate cancer is a leading cause of mortality in older men of the western world (1). Understanding the processes that lead to prostate neoplasia is essential for the assessment of disease progression and choice of treatment. It is frequently the case that pathways involved in embryonic development are reactivated in cancer. Gene expression studies have indicated that programs that regulate prostate development and growth are reactivated in prostate cancer (2, 3).

The mouse has been used as a model for the study of prostate development and cancer. Although spontaneous prostate tumours do not arise in mice, several genetically modified mice have been shown to develop prostate intraepithelial neoplasia (PIN) and invasive carcinoma that is similar to the human neoplasia. An example of this is mice lacking the tumour suppressor gene Pten (phophatase and tensin homolog deleted on chromosome 10) in the prostate that develop PIN at 6 weeks and invasive carcinoma after 9 weeks (4). Mutations in PTEN have also been associated with prostate cancer in humans (5).

The transcription factor SOX9 has been shown to be essential for many processes during embryogenesis including early prostate development (6). Studies on mice lacking Sox9 in the prostate showed a requirement in ventral prostate development and proper anterior prostate differentiation (7). High levels of SOX9 were associated with the prostate epithelia from the first stages of bud development from the urogenital sinus. In the ventral prostate, the absence of Sox9 gave rise to a decrease of proliferation within the epithelia and a lack of expression of genes specific to prostate bud development that led to a complete loss of the structure.

Studies on prostate cancer cell lines have implicated SOX9 in prostate cancer, although its role has not been clearly determined. One study demonstrated that it can suppress growth and tumourigenesis (8). In contrast, other studies have shown that it can enhance proliferation and invasion of prostate cell lines in a xenograft model (9, 10). Analysis of human tissue samples showed an association between SOX9 expression and prostate cancer, with a higher protein level in samples from patients with hormone refractory cancer (2, 9). SOX9 expression has been associated with other cancers such as colorectal and melanoma (11, 12). However, in some cases expression is associated with tumour formation but in others it is downregulated in neoplasia. This suggests that the role of SOX9 is distinct in different cancers.

In this study we have performed an extensive analysis of the role of SOX9 in prostate cancer in mouse and human. We show that high levels of SOX9 associate with PIN lesions of all grades in mice and correlate with high Gleason score in prostate cancer in humans. Overexpression of SOX9 in adult mouse prostate epithelia gave rise to an increase in proliferation and induced early high-grade PIN lesions when mice were also heterozygous for Pten. Our studies show that high levels of SOX9 contribute to regulate proliferation within the prostate epithelia and can cooperate with PTEN loss to accelerate prostate neoplasia.

Materials and Methods

Mouse strains

The mouse strain containing the Sox9 conditional allele was kindly provided by Andreas Schedl (INSERM U470, Nice, France) (7). The mice with the mutant Pten allele and the PbCre4 transgene were obtained from the MMHCC repository unit (13, 14) and mice with the conditional allele of Pten was obtained from The Jackson Laboratories (15). Mice deficient for Nkx3.1 were obtained from Michael Shen (Columbia University, New York, US) (7). The animals were bred on a mixed genetic background.

Human and mouse samples

Details of the human prostate samples containing the three different zones are in the supplemental Fig S1. Tissue arrays were constructed from unselected transurethral resections as described previously (16). Assignment of areas of cancer and non-cancer in each core was carried out after antibody staining on the basis of histopathological examination. The primary endpoints were time to death from prostate cancer and time to death from any cause. Univariate and multivariate analysis were performed by proportional hazard (Cox) regression analysis (17). All follow-up times commenced at the point of 6 months following diagnosis as in the previous report (18). The association between SOX9 expression and Gleason score and Ki67 staining was examined using the χ2 test. The following variables were included in the multivariate analyses: centrally reviewed Gleason score, baseline PSA (last PSA value within 6 months of diagnosis), extent of disease and age at diagnosis.

For the mouse studies, the histological phenotype of all samples used for immunohistochemistry were initially assessed on haematoxylin and eosin stained sections. The assessment was based on published guidelines and assisted by a pathologist (19, 20). PIN lesions noted as low grade were equivalent to PIN I-II and those noted as high grade were equivalent to PIN III-IV in Park et al (20).


Antibody stains were done on paraffin sections as described previously (7). The following antibodies were used: for SOX9, two antibodies were used with amplification protocols, one was a gift from Francis Poulat (CNRS, Montpellier, France)(1/2000 dilution) and was used for the human tissue array samples and the other was a gift from Robin Lovell-Badge (NIMR, London, UK) (1/16,000); Ki67 (TEC-3, Dako) was used at a 1/25 and 1/200 dilution (with amplification protocols); pAKT (736E11, Cell Signaling Technologies) was used at a 1/25 dilution and 1/200 dilution (with amplification protocols); p63 (4A4, Santa Cruz Biotechnology) was used at a 1/50 dilution; GFP (ab290-50, Abcam) was used at a 1/3000 dilution. Secondary fluorescent antibodies were obtained from Molecular Probes and were used a 1/1000 dilution. The ABC vector kit (Vector Laboratories) was used for amplification and stained with DAB chromogen or the TSA kit for fluorescence (NEL741, Perkin Elmer).

Quantification of proliferation

Ki67 staining was performed on a number of sections per animal and positive cells were counted from 4-6 different fields from different sections. The number of total cells per animal ranged from 600 to 2500.

Quantitative RTPCR

Tissues from the distal regions of the different prostatic lobes were dissected and RNA was isolated using the Rneasy Micro Kit (Qiagen). cDNA synthesis was primed with oligo dT and performed with Superscript II reverse transcriptase (Invitrogen). Real time PCR was performed using the Taqman system (ABI7900) with the following primers/probe sets: Mm00448840_m1 for Sox9 and Mm00486906_m1 for e-cadherin. Relative mRNA accumulation was determined by the ΔΔCt method and Sox9 levels were normalized to e-cadherin levels.


SOX9 expression in the adult mouse and human prostate

Using an antibody to SOX9 that showed high levels of protein in the prostate epithelia of mouse embryos (7), we investigated the expression of this factor in the adult mouse prostate. This analysis determined that SOX9 was expressed in the epithelia of the adult mouse prostate but at different levels between lobes. Highest levels were found in the lateral and ventral prostate (Fig 1A). This difference was confirmed by real time RTPCR (Fig 1B). SOX9 was expressed in both luminal and basal cells of the lateral and ventral prostate (Supplemental Fig S1A). In contrast, all zones in the human expressed similar levels of SOX9. Basal cell staining was predominantly found, with all zones showing some luminal cell staining (Fig 1A and Supplemental Fig S1B).

Figure 1
SOX9 expression in the mouse and human adult prostate.

The mouse prostatic lobes can be divided into three regions relative to the urethra, distal, intermediate and proximal. Recent studies have shown that the proximal region harbours cells that have properties of adult stem cells (21, 22). Staining with an antibody to SOX9 revealed high levels of this protein within the proximal region relative to the intermediate and distal regions (Fig 1C). Higher levels of SOX9 were also found in the epithelia of all lobes of prostates from mice that had been castrated when compared to intact animals (Fig 1D).

SOX9 expression in mouse models of prostate neoplasia

SOX9 has been implicated in human prostate cancer and therefore we investigated the levels of SOX9 in various mouse models of prostate neoplasia. Nkx3.1 deficient mice have been shown to develop hyperplasia and dysplasia at 12 months that will progress to PIN in mice over 18 months old (23, 24). Sections of 12-month-old anterior prostates from these mice showed a general hyperplastic lobe with cells filling the lumen. SOX9 antibody staining of these samples revealed discrete areas of high SOX9 expression within the epithelia (Fig 2A).

Figure 2
Upregulation of SOX9 is associated with early and late stages of prostate neoplasia.

Mice heterozygous for a deletion in Pten develop spontaneous tumours in various organs (13, 25-27). Within the prostate, they show hyperplasia that progresses to PIN after 10 months. Staining with an antibody to SOX9 of 6-month-old anterior prostates from these mice showed that high levels of this factor were associated with areas of hyperplasia (Fig 2A).

Mice with a conditional allele of Pten (Ptenfl) and the prostate specific Cre expressing construct, PbCre4, develop PIN at 6 weeks of age that progresses to invasive carcinoma after 9 weeks (4). Antibody staining showed high levels of SOX9 expression associated with low and high grade PIN (28)(Fig 2B). Therefore these studies show that an increase in SOX9 levels was observed at early stages of prostate hyperplasia and was associated with progression to high grade PIN lesions.

Overexpression of SOX9 in prostate epithelia

The association between increased levels of SOX9 and prostate neoplasia indicated that this factor may contribute to prostate cancer initiation and/or progression. To investigate this possibility, we created mice that overexpressed high levels of SOX9 in the adult prostate. For this we used transgenic mice that were hemizygous for a construct where Sox9 and GFP, through the use of an internal ribosomal entry site, were driven by the CAG (CMV early enhancer/chicken β actin) promoter in tissues where Cre recombinase is expressed (Z/Sox9 mice; see supplemental Fig S2 for details of the Z/Sox9 construct). These mice were mated to the PbCre4 mice, creating mice that expressed high levels of SOX9 within the adult prostate epithelia. A variable number of GFP positive cells could be found in all lobes in a mosaic pattern, with highest levels in the lateral and dorsal prostate. As expected, cells that expressed GFP showed high levels of SOX9 (Fig 3A). These cells were mainly luminal, although some positive basal cells could be found (Fig 3A and supplemental Fig S2A). In the dorsal prostate, these mice expressed up to 3.75 fold more Sox9 than control animals without the PbCre4 construct (Supplemental Fig S2B). Analysis of the prostate of these mice of various ages (6 to 12 months) showed no major histological abnormalities, when compared to wild type animals (five animals analysed). However, staining with an antibody to Ki67 revealed a 2.8 fold increase in the number of proliferating prostate epithelial cells in the dorsal lobe of overexpressing animals when compared to animals without PbCre4 (3 mice per genotype analysed, p<0.05, data not shown). Co-staining with Ki67 and GFP antibodies showed that, within the dorsal lobe, the few Ki67 positive cells were 4.7-fold more likely to express transgenic SOX9 (Fig 3A). These studies indicate that high levels of SOX9 lead to an increase in cell proliferation. However, this increase is not sufficient to induce hyperplasia within the prostate epithelia in animals up to 12 months of age.

Figure 3
Overexpression of SOX9 leads to an increase in proliferation in normal mice and PIN in heterozygous Pten deficient mice.

To establish if high levels of SOX9 can contribute to the prostate neoplasia observed in Pten mutant animals, we created mice that overexpress SOX9 and are heterozygous for the conditional Ptenfl allele. Analysis of prostates from 6-7 month old heterozygous Ptenfl mutant animals without the Z/Sox9 construct showed few areas of mild hyperplasia in the anterior prostate (6 animals analysed). In contrast, two out of four mice containing the Z/Sox9 construct revealed areas of high grade PIN in the anterior prostate (Fig 3B). These areas showed high levels of GFP and Sox9 (Fig 3C and supplemental Fig S2C). Staining with an antibody to Ki67 showed a clustering of positive cells associated with these areas (Fig 3C). Homozygous loss of Pten is associated with high levels of phosphorylated AKT (pAKT) in prostate neoplasia (4). Therefore to investigate if increased levels of SOX9 could contribute to a Pten loss of heterozygosity phenotype, we stained sections from these animals with a pAKT antibody. We found that high levels of pAKT were specifically associated with the PIN lesions (Fig 3D). These studies show that increased levels of SOX9 lead to an increase in proliferation and can contribute to the process of neoplastic transformation in cells that have lost one allele of Pten.

Effect of Sox9 deletion on normal and neoplastic prostate epithelia

Our studies on mice that overexpress SOX9 indicate a role for this factor in promoting proliferation. To investigate if absence of SOX9 in the adult prostate epithelia could lead to changes in cell proliferation, we created mice that contained the conditional loss-of-function allele of Sox9 (Sox9fl) and the PbCre4 construct. These mice showed no obvious phenotype in the lateral and ventral prostates, where SOX9 levels were highest in normal prostate. Analysis of Ki67 expression showed a significant 3.1-fold decrease in positive cells in the prostate epithelia of the mutant mice (Fig 4A). Therefore these studies show a strong correlation between SOX9 levels and the number of Ki67 positive cells within the prostate epithelia.

Figure 4
Loss of Sox9 leads to a decrease in proliferation in normal and neoplastic prostate epithelia.

To establish if SOX9 can contribute to the phenotype observed in homozygous Pten mutant animals, the conditional allele of Sox9 (Sox9fl) was included in our breeding programme. Two types of mice were generated that contained PbCre4 and were homozygous for Ptenfl (Ptenfl/fl): those that were heterozygous for Sox9fl (Sox9fl/+) or were homozygous for Sox9fl (Sox9fl/fl). Histological analysis of mice at 3 and 6 months showed no major difference in the overall prostate neoplasia phenotype between these two groups. Prostates from mutant Pten animals have been shown to have an increase in proliferation as measured by Ki67 antibody staining (4). Interestingly, we observed a difference in the number of Ki67 positive cells in the prostate between the two groups. The Sox9fl/fl containing animals showed a reduction in the number of Ki67 positive cells (Fig 4B). Consistent with previous studies, most of the Ki67 positive cells were found near the stromal compartment of the gland (29).

To confirm that Sox9 has been deleted in the Sox9fl/fl containing animals, we stained prostate sections with an antibody to SOX9. As expected, most cells of the prostate of these animals lacked SOX9, however, cells with high levels of SOX9 were found in the region near the stromal compartment of the prostate of these mice (Fig 4B). Staining with an antibody to phosphorylated AKT (pAKT) showed that the SOX9 positive cells in the Sox9fl/fl containing animals lacked an upregulation of this protein in the plasma membrane (Fig 4B). Upregulation of pAKT levels has been shown to be associated with loss of Pten in the prostate (4), therefore, these studies indicate that these cells did not express Cre recombinase and no recombination between flox sites at the Pten and Sox9 loci had occurred. Double staining with Ki67 and either SOX9 or pAKT antibodies showed that most of the Ki67 positive cells in the Sox9fl/fl animals were also positive for SOX9 and negative for pAKT (Fig 4B). Consistent with this, quantification of cells that were positive for Ki67 and negative for pAKT between the two groups showed a significant two-fold decrease in proliferation in Sox9fl/fl animals compared to the Sox9fl/+ animals (Fig 4B). These data show that loss of Sox9 does not inhibit neoplastic progression in the prostate of homozygous mutant Pten mice, but does lead to a reduction in the number of proliferating prostate epithelial cells that have retained PI3 kinase signalling.

SOX9 expression in human prostate cancer

Our studies in mice suggested that high levels of SOX9 in the prostate might be associated with prostate cancer. Therefore to establish the relationship between SOX9 levels and prostate cancer in humans we stained 880 prostate cancer cores with an antibody to SOX9. These cores were derived from 387 patients with localised disease and Gleason scores ranging from 4 to 10. In contrast to our mouse studies, not all cancer samples showed SOX9 expression, with 54% percent of cores (N=478) showing no staining while the remaining showed either low or high levels of nuclear staining (Fig 5A and B). The level of SOX9, either 0 or some positive staining (> 0), was determined for each patient and used in statistical analysis. There was a significant positive correlation between SOX9 expression and Gleason score (46% of SOX9 positive for Gleason <7, 67% for Gleason 7 and 61% for Gleason > 7, Pearson’s χ2 = 13.6, p=0.001) (Table 1). In univariate analysis, SOX9 expression correlated with poorer prognosis and was significant for overall survival (Fig 5C) but not for cause-specific survival (Fig 5D). However, SOX9 expression did not remain significant independently of age, Gleason score, PSA and extent of disease in a multivariate model (Δχ2 (1df) = 0.17, p = 0.19). Higher levels of Ki67 have been shown to correlate with lower chance of survival in this group of samples (30). Consistent with our mouse studies, expression of SOX9 was significantly positively associated with Ki67 in the prostate cancer samples (χ2 for trend =7.25, p=0.007) (Table 1). These studies indicate that expression of SOX9 correlates with increased proliferation and tumour progression and decreased survival in humans.

Figure 5
SOX9 expression in tissue arrays from human prostate cancer samples
Table 1
Distribution of Gleason score categories and the percentage of Ki67 positive cells between SOX9 negative (0) and positive (>0) cancer cores as indicated


Various studies have implicated SOX9 in prostate cancer. However, the data has been conflicting with studies indicating that it can promote, but also repress, cell proliferation and that it is involved in metastasis and may be required in the early stages of prostate carcinogenesis (2, 8, 10, 31). Our results in mice show that SOX9 expression is upregulated within the epithelia in areas where neoplastic abnormalities are first observed and high levels of SOX9 are associated with all stages of PIN progression. This is consistent with our data on human prostate cancer where SOX9 expression was more frequent in samples with higher Gleason score. Our functional data shows that high levels of SOX9 in the prostate contribute to an increase in proliferation that can lead to PIN in certain mutant genetic backgrounds. Therefore our results are consistent with a cooperative role for SOX9 in the progression of the early stages of prostate cancer formation.

Our expression data had suggested that high levels of SOX9 could promote early stages of prostate neoplasia. Our overexpression studies demonstrated that a high level of SOX9 on its own is not sufficient to induce hyperplasia and neoplasia in vivo, although increased proliferation was observed. However, in the context where one allele of Pten was lost, focal areas of high grade PIN were detected, where all cells showed high GFP and SOX9 expression. This indicates that cells with increased levels of SOX9 and decreased levels of Pten are more likely to become neoplastic. The focal nature of this event could be due to a stochastic process or the nature of the cell of origin, which, if relatively rare in Cre expressing cells, would be made even more infrequent if both transgenic SOX9 expression and loss of one Ptenfl allele needed to occur in the same cell. The areas of high grade PIN in these mice showed increased pAKT in most cells. This phenotype was not observed in GFP positive cells that were not part of the PIN lesion. This suggests that a high level of SOX9 does not directly regulate pAKT levels, rather it increases the likelihood of an event that leads to an increase in cells with high PI3 kinase signalling. A similar situation occurs in mice that are heterozygous for Pten and Nkx3.1 mutations (32). Our data suggest that an increase in proliferation is a potential mechanism in which SOX9 can contribute to the acceleration of neoplasia progression. Overall, our studies indicate that SOX9 does not act like a classic oncogene but rather it cooperates with other events such as the loss of PTEN to induce carcinogenesis.

Our genetic data show that Sox9 deletion leads to a decrease in proliferation in wild type and homozygous Ptenfl mutant mice. However, this decrease did not inhibit prostate neoplasia progression in the mutant animals. Presumably other pathways regulating proliferation are also active in these mice and are sufficient to ensure neoplastic progression. In the Ptenfl mutant mice the proliferating cells were more likely to harbour cells where Cre recombination had not occurred (see figure 4B) and therefore contained wild type Pten and Sox9. This suggests that the neoplastic epithelia associated with Pten loss produces non-cell autonomous factors that regulate proliferation in neighbouring cells. Paracrine effects from the epithelia have been observed to control processes within the stroma in models of prostate cancer (33). Our results are consistent with a contribution of SOX9 to the regulation of the factors controlling proliferation. This effect was also observed in the early neoplastic lesions where Ki67 positive cells tended to surround the areas of high SOX9 in the prostate of Nkx3.1 mutant mice (Supplemental Fig. S3). This suggests that the production of paracrine factors regulating proliferation might be a general mechanism associated with the initial stages of carcinogenesis.

A striking difference between mouse and human in our studies is that we observed high expression of SOX9 in prostate neoplasia in mouse models but only 46% of human cancer cores were positive for this factor. One technical explanation is that the SOX9 antibody we used did not react with all the samples in the same way due to the tissue array process. We do not think this is the case as our studies with a different SOX9 antibody and two other studies using different antibodies and samples found a similar percentage of cancer samples that were negative for SOX9 (2, 9). Cancer samples that were positive for SOX9 tended to have a higher Gleason score and increased Ki67 positive cells. A multivariate analysis of our human data show that expression of SOX9 is not an independent prognostic factor of survival. Therefore our analysis suggests that SOX9 expression correlates with disease progression in a subset of prostate cancers and that other SOX9 independent pathways can also lead to a low survival prognosis. Classification of prostate cancers into different subtypes may be required to uncover whether expression of SOX9 has any predictive value. Screening of other mouse models of prostate cancer for SOX9 levels may also uncover differences in the dependence of SOX9 in the different pathways that lead to neoplasia.

SOX9 has been implicated in other cancers such as melanoma in humans (12). In this example, higher levels of SOX9 were associated with normal melanocytes and were downregulated in premalignant and malignant cells. In addition, overexpression of SOX9 inhibited proliferation and led to cell cycle arrest. This shows that the function of this factor in cancer is context specific. This is consistent with its many roles during embryonic development where it can cooperate with tissue specific factors to initiate different cell fates such as Sertoli cell differentiation, chondrogenesis and neural crest development (34-36). This also highlights the value of in vivo studies to delineate the role of SOX9 in specific tissues.

In summary, this study identifies SOX9 as a factor involved in the regulation of proliferation of prostate epithelial cells, which can contribute to neoplastic transformation in specific contexts. Future identification of different prostate cancer subtypes may uncover the contributions of SOX9 to specific pathways that lead to neoplasia.

Supplementary Material


We thank Andreas Schedl for providing the Sox9fl/fl mice and Michael Shen for the Nkx3.1 mutant mice. We thank Francis Poulat and Robin Lovell-Badge for SOX9 antibodies. We thank Afshan McCarthy for help with the Pten mutant animals. We thank Andrew Dodson for help with the scoring of the human tissue array. We would like to thank Jeff Francis and Bill Buaas for helpful comments on the manuscript. D. M. B. is supported by The Orchid Appeal. KSEC and SW were supported by the Research Grants Council of Hong Kong (HKU7337/01M, HKU2/02C, AoE/M-04/04). This work was funded by the National Cancer Research Institute (NCRI).


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