Department of Immunology , Erasmus University , Rotterdam , The Netherlands .
Eur J Biochem 208 : 589-96 ( 1992)
Abstract
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 .