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From the Departments of Pathology*
andUrology,
and the Division ofBiostatistics,
Indiana University School ofMedicine, Indianapolis; and the Lilly ResearchLaboratories,
Eli Lilly and Company, LillyCorporate Center, Indianapolis, Indiana
| Abstract |
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Several types of PLA2s have been identified.7,8 Seven low molecular weight (14 kd) sPLA2s (including group IB, IIA, IID, IIE, III, V, and X) are known to be present in both the intracellular compartment and extracellular milieu. The 40-kd form is a calcium-independent enzyme, which is referred to as iPLA2. Cytosolic PLA2 (cPLA2) is a high molecular weight (85 kd) form found predominantly within the cytosol of cells. cPLA2 activity is regulated by intracellular Ca2+ concentrations8,9 and shows characteristic preference for hydrolyzing arachidonic acid at the sn-2 position. In contrast, sPLA2s have a broad substrate preference.8
Reports have been published linking arachidonic acid and its metabolites with prostatic malignancy.10-15 Because phospholipase activity is required for phospholipid metabolism and subsequent generation of arachidonic acid, aberrant expression and function of sPLA2 may play a role in prostatic carcinogenesis. However, little is known about sPLA2 expression in human prostatic tissues. In this study, we determined the level of group IIA sPLA2 expression in specimens of human prostatic adenocarcinoma, its precursor lesion [(high-grade prostatic intraepithelial neoplasia PIN)], low-grade PIN, and benign prostatic tissue.
| Materials and Methods |
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Seventy-eight cases of radical retropubic prostatectomy and bilateral lymphadenectomy between 1990 and 1994 were obtained from the surgical pathology files of Indiana University Medical Center. Patients ranged in age from 51 to 78 years (mean, 63 years). Grading of the primary tumor from radical prostatectomy specimens was performed according to the Gleason system.16 The Gleason grade ranged from 4 to 10. Pathological stage was performed according to the 1997 TNM (tumor, lymph nodes, and metastasis) system. Pathological stages were T2a (n = 11 patients), T2b (n = 35), T3a (n = 26), and T3b (n = 6). Six (8%) patients had lymph node metastasis at the time of surgery.
Generation of Rabbit Polyclonal Antibody to sPLA-IIA
Rabbit polyclonal antibody specific to sPLA2-IIA was generated by immunizing rabbits with purified, recombinant human sPLA2-IIA protein. The antisera were affinity-purified. The specificity of the purified IgG antibody was confirmed by staining Chinese hamster ovary (CHO) cells that stably expressed sPLA2-IIA. CHO cells expressing sPLA2-X or sPLA-V did not stain with this antibody (Eli Lilly and Company, Indianapolis, IN).
Immunohistochemical Studies
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 grade were selected. One representative slide from each case was analyzed and we recognized the limitation of sample variation. Slides were deparaffinized in xylene twice for 5 minutes and rehydrated through graded ethanols to distilled water. Endogenous peroxidase activity was inactivated by incubation in 3% H2O2 for 15 minutes. The nonspecific binding sites were blocked by incubating with 10% normal horse serum in phosphate-buffered saline (PBS) (0.01 mol/L phosphate, ph7.4, 0.137 mol/L NaCl). Tissue sections were then incubated with the polyclonal rabbit antibody against human sPLA2 (1:76,000 dilution) for 60 minutes at room temperature. After washing with PBS, biotinylated goat anti-rabbit IgG was applied for 30 minutes. Additional washing was followed by incubation with peroxidase-labeled streptavidin for 30 minutes. Immunoreactivity was visualized by incubation of sections with diaminobenzidine in the presence of hydrogen peroxide. Sections were counterstained with light hematoxylin and mounted with a coverslip. All of the procedures were performed at room temperature. No enzymatic pretreatment was required for antigen retrieval. Positive and negative controls were run in parallel with each series and appropriate results were obtained.
The extent and intensity of staining were evaluated in benign epithelium, low-grade PIN, 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 between 0 and 3 was assigned to each case on a scale from 0 to 3 (0, no staining; 1, weak staining; 2, moderate staining; and 3, strong staining).
Statistical Analysis
The mean percentage of immunoreactive cells in benign epithelium, low-grade PIN, high-grade PIN, and adenocarcinoma were compared using one-way analysis of variance with a random subject effect to correlate the within-subject measurements. The intensity of staining in benign epithelium, low-grade PIN, high-grade PIN, and adenocarcinoma were compared using Cochran-Mantel-Haenszel tests for correlated ordered categorical outcomes. Pairwise comparisons between the tissue types were made if the analysis of variance revealed significant treatment effects. 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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Increased immunoreactivity for group IIA sPLA2 in prostatic neoplasia seen in the present study agrees with previously published observations by Faas and colleagues.15 Using enzymatic analysis, these investigators demonstrated a two-fold enhancement of sPLA2 activity in human prostatic adenocarcinoma compared to benign prostatic tissue. Evidence indicates that malignant prostatic tissue contains significantly lower levels of arachidonic acid in phospholipids.17,18 Altered phospholipid metabolism in malignant prostatic tissues may result from increased utilization of arachidonic acid for the formation of prostaglandins and eicosanoids. These arachidonic acid metabolites may be crucial for the growth and progression of malignant lesions. Chaudry and colleagues19 demonstrated a 10-fold increase in prostaglandin E2 synthesis from labeled arachidonic acid in malignant human prostatic tissues. Shaw and colleagues20 also showed increased prostaglandin E2 levels in the effusions of a fast-growing, metastasizing subline of the Dunning R-3327 rat prostatic adenocarcinoma when compared to a slow-growing derivative. Taken together, these findings are consistent with our observation of enhanced expression of sPLA2 immunoreactivity in high-grade PIN and prostatic adenocarcinoma.
sPLA2 has previously been localized using immunohistochemistry using human Paneth cells, chondrocytes, amniotic epithelial cells, and lacrimal gland cells.21-23 Expression of different sPLA2 mRNA isoforms have been evaluated in various tissues.24 Comparative expression of sPLA2 protein expression in other organs and their malignant counterparts has been published.25-29 The distinctive staining pattern of group IIA sPLA2 between the benign and malignant prostatic glands suggests a utility of this antigen as a potentially useful diagnostic maker. Given the significant differences evident in the staining patterns, sPLA2 immunostaining might have an advantage over prostate-specific antigen and prostate-specific membrane antigen to differentiate between benign epithelium, high-grade PIN, and malignant prostatic cells. If present at all, benign epithelia show mainly focal sPLA2 staining. Even when positive, the staining intensity in benign epithelium is usually much weaker (one or two grades) than in adjacent malignant tissues. Because a minority (8.7% for high-grade PIN and 9.0% for adenocarcinoma) of the neoplastic cases were not immunoreactive, sPLA2 staining is not able to detect all of the prostatic cancers or high-grade PIN lesions. Further, a number of cases showed weak or moderate sPLA2 immunoreactivity in benign epithelium. Therefore, the utility of sPLA2 as a diagnostic marker to distinguish benign from neoplastic glands is still limited.
In summary, group IIA sPLA2 expression is elevated in the neoplastic human prostatic tissue raising the possibility that dysregulation of this enzyme may play a role in prostatic carcinogenesis. Our findings may have implications for target validation and development of therapeutic strategies modulating phospholipid metabolic pathways in prostatic neoplasia.
| Footnotes |
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Supported in part by Clarion Health Value Fund grant and Indiana Biomedical Research Fund grant (to L.C.).
Accepted for publication November 8, 2001.
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