Differential expression of platelet derived growth factor-beta in malignant mesothelioma: a clue to future therapies?

Malignant mesothelioma (MM) is resistant to most standard forms of treatment. Accordingly, novel therapies based on the genetic and autocrine growth characteristics are being investigated. Platelet-derived growth factor (PDGF), a potent mitogen for mesenchymal cells, is produced by several human malignant cell lines including MM and therefore may promote tumourigenesis by an autocrine mechanism. We investigated the expression of PDGF-beta mRNA in tumour specimens excised from 18 patients with MM. Total cellular RNA was successfully extracted from 16/18 frozen tumour specimens with guanidine isothiocyanate and purified by centrifugation through a caesium chloride gradient. Northern blots were prepared and probed sequentially with 32P-labelled PDGF-beta and beta-actin cDNA. Gene expression was quantitated by optical densitometry. Freshly elutriated human peripheral blood monocytes were stimulated to induce PDGF-mRNA expression with transforming growth factor-beta-1. This positive control was assigned an expression index (EI) of 1, with the EI for the tumour sample calculated as: PDGFpatient/Actinpatient/EI positive control. In the 16 tumour specimens with useable RNA, transcripts for PDGF-beta MRNA were detected. Northern blot analyses revealed elevation of PDGF-beta expression above control in 10/16 (63%) of MM patients. A 230% increase in PDGF-beta expression (EI = 1.62 vs. EI = 0.49) was found between the lowest and highest expression samples. The specimens from which the PDGF transcripts were derived were found histologically to contain 87% tumour and 13% contaminating normal cells, predominantly lymphocytes. The elucidation of the transcriptional regulation of growth factors which are implicated in the pathogenesis of MM may guide the development of more effective biologic therapies.