Splicing of the platelet-derived-growth-factor A-chain mRNA in human malignant mesothelioma cell lines and regulation of its expression.

Platelet-derived-growth-factor (PDGF) A-chain transcripts differing in the presence or absence of an alternative exon-derived sequence have been described. In some publications, the presence of PDGF A-chain transcripts with this exon-6-derived sequence was suggested to be tumour specific. However, in this paper it was shown by reverse-transcription polymerase-chain-reaction (PCR) analysis that both normal mesothelial cells and malignant mesothelioma cell lines predominantly express the PDGF A-chain transcript without the exon-6-derived sequence. This sequence encodes a cell-retention signal, which means that the PDGF A-chain protein is most likely to be secreted by both cell types. In cultured normal mesothelial cells, the secreted PDGF A-chain protein might be involved in autocrine growth stimulation via PDGF alpha receptors. However, human malignant mesothelioma cell lines only possess PDGF beta receptors. If this also holds true in vivo, the PDGF A-chain protein produced and secreted by malignant mesothelial cells might have a paracrine function. In a previous paper, we described elevated expression of the PDGF A-chain transcript in human malignant mesothelioma cell lines, compared to normal mesothelial cells. In this paper, the possible reason for this elevation was studied. First, alterations at the genomic level were considered, but cytogenetic and Southern-blot analysis revealed neither consistent chromosomal aberrations, amplification nor structural rearrangement of the PDGF A-chain gene in the malignant cells. Possible differences in transcription rate of the PDGF A-chain gene, and stability of the transcript between normal and malignant cells, were therefore studied. The presence of a protein-synthesis inhibitor, cycloheximide, in the culture medium did not significantly influence the PDGF A-chain mRNA level in normal mesothelial and malignant mesothelioma cell lines. Furthermore, nuclear run-off analysis showed that nuclear PDGF A-chain mRNA levels varied in both cell types to the same extent as the levels observed in Northern blots. Taken together, this suggests that increased transcription is the most probable mechanism for the elevated mRNA level of the PDGF A-chain gene in human malignant mesothelioma cell lines.