Expression of PDGF, VEGF and their receptors in non-small cell lung tumor cell lines

Cell lines derived from non-small cell carcinomas of the lung (NSCLC) often express a variety of polypeptide growth factors able to activate receptors that encode tyrosine kinases. These receptors initiate numerous biological responses, including cell proliferation, and constitutive activation can result in cellular transformation. Consequently, it is possible that the aberrant growth properties of NSCLC cell lines is due at least in part to the action of mutated or perpetually activated receptor tyrosine kinases (RTK)s. As a first step in the process of testing this hypothesis we set out to examine the different RTKs expressed in NSCLC cell lines. A small group of NSCLC cell lines was screened for the expression of RTKs using a reverse transcriptase PCR approach with nested degenerate primers. We found four different RTKs expressed: the fibroblast growth factor receptor (FGFR) type III, the FGFR type IV, the platelet-derived growth factor beta receptor (beta PDGFR) and Flk-1/KDR, one of the receptors that binds vascular endothelial growth factor (VEGF). Since Flk-1/KDR and the beta PDGFR are not usually expressed in epithelial cells, we verified that they are expressed in NSCLC by Northern and Western blot analysis. To assess the incidence of expression of these RTKs in NSCLC cell lines a larger panel of cell lines was examined by Western blot analysis. The beta PDGFR was expressed in 30% (3/10) of the cell lines examined, while Flk-1/KDR was expressed in 10% (1/10). We also examined the expression of ligands for these RTKs. PDGF-A, PDGF-B and VEGF were expressed in 89%, 0% and 78%, respectively. While the high incidence of ligand expression made it likely that receptor and growth factor would be coexpressed, we found that in most instances this was not the case. Furthermore, in the cell lines in which the RTK and its ligand were coexpressed, we were unable to detect a functional autocrine loop. These studies indicate that while NSCLC cell lines aberrantly express various RTKs and growth factors, this does not always result in the establishment of an autocrine loop. These findings suggest that growth factors such as PDGF and VEGF act in a paracrine manner to contribute to the growth, survival and angiogenic program of a lung tumor.