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From the Departments of Pathology and Laboratory Medicine,* Medicine,
Urology,
and Division of Biostatistics, ¶ Indiana University School of Medicine, Indianapolis, Indiana; and MedImmune Inc.,
Gaithersburg, Maryland
| Abstract |
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EphA2 has been implicated in carcinogenesis of melanoma, breast, colon, lung, and esophageal cancers.6,7,9-12 These studies have identified high levels of EphA2 in both tissue and cell models of these diseases. Moreover, ectopic overexpression of EphA2 is sufficient to confer malignant transformation and tumorgenic potential on non-transformed mammary epithelial cells.12 The potential importance of EphA2 in cancer is also indicated by recent studies, which indicate that specific targeting of EphA2 with monoclonal antibodies is sufficient to inhibit the growth, migration, and invasiveness of aggressive breast and prostate cancer cells.12,13
While much investigation has focused on EphA2 levels in aggressive cancers, most of these studies have involved a relatively small sample size. We therefore studied EphA2 expression in a relatively large collection of prostate carcinomas. Moreover, we sought to determine whether the expression of EphA2 is increased in pre-malignant conditions such as high-grade prostatic intraepithelial neoplasia (PIN), a putative precursor of prostatic adenocarcinoma.
| Materials and Methods |
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Ninety-three cases of radical retropubic prostatectomy were obtained from the surgical pathology files of Indiana University Medical Center (Table 1)
. These cases were selected as representative of the full spectrum of Gleason grades and pathological stage. None had received hormonal or radiation therapy before surgery. Patients ranged in age from 44 to 77 years (mean, 63 years).
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Murine Monoclonal Antibody to Human EphA2 Protein
Murine monoclonal antibody specific to EphA2 was generated by immunizing mouse with purified, recombinant human EphA2 protein. A plasmid encoding a fusion protein of the extracellular domain of EphA2 linked to immunoglobulin chain was used as the immunogen. The antisera was affinity purified. The specificity of the purified IgG antibody was confirmed by immunoprecipitation, ELISA, BiaCore, and functional activation assays (ie, increased EphA2 phosphotyrosine content) (MedImmune Inc, Gaithersburg, MD). In addition, fluorescence microscopy study revealed the overexpression of EphA2 protein in a right pattern and in the right cellular location in the EphA2-overexpressing cells but not EphA2-deficient cells.17-19
Immunohistochemistry
Serial 5-µm-thick sections of formalin-fixed slices of radical prostatectomy specimens were used for the studies. Tissue blocks that contained the maximum amount of tumor and highest Gleason score were selected. One representative slide from each case was analyzed. We recognized the limitation of sample variation. Slides were deparaffinized in xylene twice for 5 minutes and rehydrated through graded ethanols to distilled water. Antigen retrieval was carried out by heating sections in 1 mmol/L ethylene diaminetetraacetic acid (EDTA) (pH 8.0) for 30 minutes. Endogenous peroxidase activity was inactivated by incubation in 3% H2O2 for 15 minutes. Non-specific binding sites were blocked using Protein Block (DAKO, Carpinteria, CA) for 20 minutes. Tissue sections were then incubated with the mouse monoclonal antibody against human EphA2 (IgG1, 1:100 dilution; MedImmune, Inc.) overnight at room temperature, followed by biotinylated secondary antibody (DAKO) and peroxidase-labeled streptavidin. 3,3-diaminobenzidine was used as the chromogen in the presence of hydrogen peroxide. Positive controls consisted of human prostate cancer samples that had been shown to express EphA2 and PC-3 cells.20 LnCaP cells were used as negative control.20 As an additional parallel negative control, negative controls were performed using blocking serum in place of primary antibody.
Evaluation of EphA2 Expression
The extent and intensity of staining were evaluated in benign epithelium, high-grade PIN and adenocarcinoma from the same slide for each case. Microscopic fields with the highest degree of immunoreactivity were chosen for analysis. At least 1000 cells were analyzed in each case. The percentage of cells exhibiting staining in each case was evaluated semiquantitatively on a 5% incremental scale ranging from 0 to 95%. A numeric intensity score is set from 0 to 3 (0, no staining; 1, weak staining; 2, moderate staining; and 3, strong staining). These methods follow our previous work.21,22
Statistical Analysis
The mean percentage of immunoreactive cells in benign epithelium, high-grade PIN and adenocarcinoma were compared using the Wilcoxon paired signed rank test. The intensity of staining for EphA2 in benign epithelium, high-grade PIN, and adenocarcinoma were compared using Cochran-Mantel-Haenszel tests for correlated ordered categorical data. A P value <0.05 was considered significant, and all P values were two-sided.
| Results |
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| Discussion |
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High levels of EphA2 have been reported in other cancers including melanoma, breast, lung, colon, and esophageal cancers.6,7,9-12 Our data are unique in part because they provide the first indication that high levels of EphA2 are found early in disease progression. For example, pre-malignant conditions of the prostate such as high-grade PIN overexpress EphA2. This suggests that EphA2 may play an important role in the early stage of prostatic carcinogenesis. This hypothesis is supported by evidence that overexpression of EphA2 is sufficient to induce malignant transformation of non-transformed mammary epithelial cells.12,13 It will be interesting to determine whether high levels of EphA2 are also found in pre-malignant conditions of other cancers as well (eg, ductal carcinoma in situ). Taken together, EphA2 may be an important component during the early carcinogenesis of many human cancers.
Overexpression of several tyrosine kinases including fibroblast growth factor, epidermal growth factor, and focal adhesion kinase has been reported in prostate cancer.1,23-25 However, these PTKs are generally found to be sporadically overexpressed in prostate cancer. Multiple and different lines of evidence suggest Eph family members contribute to the high tyrosine phosphorylation levels in prostate cancer.20 We found that EphA2 is overexpressed in both high-grade PIN and prostatic adenocarcinoma. Although information about the expression and regulation of other Eph family members in prostate cancer is rather limited, Robinson et al26 showed that EphA1 and htk are expressed in some prostatic cancer cells. Members of the Eph family have been shown to have contrasting effects on cell proliferation and tumorigenic potential in cell lines of different origins. For example, EHK-1 and CEK8 have been linked to tumorigenesis of glioma. However, EphA5 has no implication in promotion of cell proliferation in human glioblastoma cell line. Other data showed that EphA3 and EphB1 have no oncogenic effect.
In addition to its overexpression, the EphA2 in malignant cells generally functions differently than the EphA2 in non-transformed cells. For example, defective cell-cell contacts decrease the ability of EphA2 to interact with its ligands, which are located on apposing cells.27 This observation is interesting in light of a recent report, which showed that overexpression of EphA2 in non-transformed mammary epithelial cells (MCF-10A) is sufficient to destabilize cell-cell contact. Thus, the high levels of EphA2 in high-grade PIN and prostate cancer cells likely do not interact efficiently with its ligand.
These changes in EphA2-ligand binding are important because ligand binding negatively regulates tumor cell growth and invasiveness and can reverse the malignant phenotypes of highly aggressive tumor cells.12,13,27,28 Moreover, much recent investigation has sought to exploit these changes in EphA2 function by using monoclonal antibodies to mimic the actions of the endogenous ligands of EphA2. In this light, our present findings are notable, in part, because they suggest that antibody-based targeting of EphA2 could also have utility for blocking the progression of PIN toward malignant disease.
The biological role and regulation of EphA2 expression in prostate cancer is unclear. Overexpression of EphA2 in high-grade PIN and prostatic adenocarcinoma are consistent with emerging evidence that EphA2 may be involved in the process of prostatic carcinogenesis. Further studies on the regulation of the EphA2 signaling transduction pathway may provide insights leading to approaches to prostate cancer prevention and treatment.
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| Footnotes |
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Guangyuan Zeng and Zhiqiang Hu contributed equally to this study.
Accepted for publication August 12, 2003.
| References |
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