Regulation of differential expression of platelet-derived growth factor alpha- and beta-receptor mRNA in normal and malignant human mesothelial cell lines.

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