Untersuchung der Expression von Cyclooxygenase-2, VEGF und der Gefäßdichte im Nierenzellkarzinom

Abstract

Cyclooxygenase-2 (COX-2) influences the cell transformation to malignant cells, their proliferation, migration and tissue invasion, apoptosis and tumorangiogenesis. The latter is a fundamental process during tumor development, and vascular endothelial growth factor (VEGF) has a central position among the growth factors involved in angiogenesis. The aim of this study was to investigate the expression of COX-2 in renal cell carcinoma. In particular, it should be clarified whether COX-2 contributes to tumor angiogenesis in renal cell carcinoma together with VEGF, and to what extent this process is affected by the lack of oxygen.We examined tissue samples of 24 tumors and adjacent normal tissue. The protein expression of COX-2, VEGF and the microvessel density (MVD) were assessed by immunohistochemistry. The expression of COX-2 mRNA and VEGF mRNA were analysed by real-time PCR. In cell culture experiments, tumor cells and macrophages were examined with regard to the expression of COX-2 and VEGF under hypoxic conditions and under stimulation with necrotic tumor material.COX-2 protein could be detected in tumor epithelia, vascular endothelia and in macrophages. In tumor-free tissue, COX-2 was detected in the cells of collecting ducts and in tubular epithelia. There was no significant difference in COX-2 expression between tumor tissue and normal tissue. The expression of COX-2 mRNA was decreased in tumor tissue compared to normal tissue. There was no correlation between COX-2 expression and VEGF expression or MVD of the tumors. VEGF protein was detected in tumor epithelia and in macrophages. In tumor-free tissue, VEGF was detected in the cells of collecting ducts and in tubular epithelia. The expression of VEGF protein and VEGF mRNA were increased in tumor tissue. Tumors with high MVD showed a low expression of VEGF protein and a high expression of VEGF mRNA. In cell culture experiments, tumor cells under hypoxia showed only moderate changes in the COX-2 and VEGF expression depending on the cell line. In contrast, in macrophages, the expression of COX-2 and VEGF under hypoxic conditions and under stimulation with necrotic tumor material was significantly upregulated.Taken together, an increased expression of COX-2 in tumor tissue could not be confirmed. The fact, that COX-2 could be detected in normal kidney tissue, may be attributed to the physiological role of COX-2 in the kidney regulating water and electrolyte balance. The decreased expression of COX-2 mRNA in tumors may be explained by epigenetic changes like DNA methylation or by posttranscriptional modifications of the RNA. While the results question the importance of COX-2 in the control of VEGF expression and angiogenesis in renal cell carcinoma, they emphasize the central role of VEGF in tumor angiogenesis. It is likely that, in renal cell carcinoma, VEGF is subject to other regulatory mechanisms, and mutations in the von Hippel-Lindau gene are likely to play a role. The results of cell culture experiments suggest, that macrophages contribute to the progress of renal cell carcinomas because of their tumormodulating properties.