Expression of platelet-derived growth factor (PDGF) and PDGF receptors in human malignant mesothelioma in vitro and in vivo

The expression of platelet-derived growth factor (PDGF) and PDGF receptors was studied in human normal and malignant mesothelial cells in vitro and in vivo. Staining with anti-cytokeratin and ME1 antibodies and ultrastructural analysis confirmed the mesothelial nature of the cell lines used to study PDGP and PDGF receptor expression in vitro. Using antibodies, mesothelioma cell lines were found to express PDGP and both the PDGF α- and the PDGF β-receptor, whereas cultured normal mesothelial cells expressed PDGF and PDGP a-receptor. This PDGF and PDGF receptor staining pattern largely reflects the earlier described mRNA expression in these cell lines. The only exception was the immunocytochemical detection of PDGF a-receptors in the mesothelioma cell lines, which is different from the inability to detect a-receptor transcripts on Northern blots. Expression was also investigated in mesothelial cells in vivo. Expression of PDGF was observed in malignant mesothelioma cells on frozen tissue sections. In pleural effusions, a double immunofluorescence staining procedure for PDGF and epithelial membrane antigen (EMA) revealed PDGF expression by EMA-\positive malignant mesothelioma cells. PDGF β-receptors and occasionally PDGF α-receptors were detected in frozen tissue sections of malignant mesotheliomas, whereas mesothelioma cells in effusions showed faint expression of only the PDGF P-receptor. In contrast, in effusions containing non-malignant mesothelial cells, only a very low level of PDGP a-receptor could be detected. Taken together, these results indicate that the pattern of PDGF and PDGF receptor expression in mesothelial cells in vivo largely corresponds to expression of PDGF and its receptors in vitro. Malignant mesothelioma sell lines thus constitute a good model system for studies on the role of PDGF in this malignancy. Furthermore, the data reported in this paper are consistent with the idea that an autocrine growth stimulatory effect of PDGP via PDGP receptors may play a role in the pathogenesis of malignant mesothelioma.