Regulation of differential expression of platelet-derived growth factor α- and β-receptor mRNA in normal and malignant human mesothelial cell lines

Abstract

In earlier studies we showed that the expression patterns of platelet-derived growth factor (PDGF) α- and β-receptors differ between normal and malignant mesothelial cell lines. Normal mesothelial cells predominantly express PDGF α-receptor mRNA and protein, whereas most malignant mesothelioma cell lines produce PDGF β-receptor mRNA and protein. In this paper we studied regulation of this differential PDGF receptor mRNA expression. Such an analysis is of importance in view of the suggested PDGF autocrine activity involving the PDGF β-receptor in mesothelioma cells. The results obtained in this study demonstrate that malignant mesothelioma cell lines are not only capable of PDGF β-receptor transcription but of α-receptor transcription as well, as evidenced from run off analysis and RT-PCR using α-receptor specific primers. However, the fact that PDGF α-receptor mRNA could not be detected by Northern blot analysis, even after cycloheximide treatment, suggests a difference in steady-state PDGF α-receptor mRNA expression levels between normal and malignant mesothelial cell lines, which is likely to be caused by a post-transcriptional mechanism. In normal mesothelial cells a half-life of more than 6 h was observed for PDGF α-receptor mRNA. In the majority of malignant mesothelioma cell lines clear PDGF β-receptor mRNA expression was seen. The half-life of the PDGF β-receptor transcript was at least 6 h in these cells. In contrast, hardly any PDGF β-receptor transcription was observed in run off assays in normal mesothelial cells, suggesting that differences in β-receptor transcriptional initiation most probably account for the inability to clearly detect PDGF β-receptor transcripts in these cells. Transforming growth factor β-1 (TGF-β1), which is being produced in active form by mesothelial cells was evaluated for its potential role in regulation of the differential PDGF receptor expression in these cells. Stimulation with TGF-β1 revealed decreased PDGF α-receptor mRNA expression in normal mesothelial cells. The effect on PDGF β-receptor mRNA in the malignant mesothelioma cell lines was variable. Although the TGF-β1 effect cannot entirely explain the differential PDGF receptor expression pattern, TGF-β1 may nevertheless play a role in downregulation of an (already) low PDGF α-receptor mRNA level in malignant mesothelioma cell lines.