Chimeric molecules map and dissociate the potent transforming and secretory properties of PDGF A and PDGF B

The effects of N-ras oncogene expression on PDGF-BB stimulated responses in cultured mouse myoblasts

The role of the ras oncogene in the signalling pathway triggered by platelet-derived growth factor BB (PDGF-BB) has been investigated in a cell line which normally differentiates into myotubes. Following the activation of the N-ras oncogene, however, the cells proliferate and form foci. PDGF-BB stimulated the phosphorylation of tyrosine in several cellular proteins of molecular weight 185, 160, 94, 54, 44, 42 kDa and furthermore Ca2+ was released from internal stores. Activation of the N-ras gene by treatment of cells with dexamethasone (DEX) inhibited these responses to PDGF-BB. On the other hand, both ras-induced and -non induced cells responded to bradykinin (BK), foetal calf serum (FCS) and ionomycin (ION) by releasing Ca2+ from intracellular stores. The inhibition of the response to PDGF-BB in ras-activated cells has been further investigated. The binding of [125I]-PDGF-BB to its receptors was low and western blotting showed a low level of PDGF-BB receptor protein. This was in marked contrast to the receptor number seen in cells grown in growth medium or fusion promoting medium. These results indicate that cells transformed with the N-ras oncogene fail to respond to platelet-derived growth factor and exhibit a very low level of PDGF receptors. This suggests a role for the ras oncogene in the earliest steps of the signalling pathway.

Accumulation of PDGF B and cell-binding forms of PDGF A in the extracellular matrix

PDGF is a powerful mitogen initially identified within platelets, but also shown to be produced by a wide variety of cell types. PDGF is encoded on two separate genes. These give rise to three polypeptides, PDGF B and two forms of PDGF A (SA and LA), resulting from alternative splicing of the PDGF A gene primary transcript. We report that in CHO cells transfected with PDGF gene constructs and producing moderate levels of PDGF homodimers, much of the PDGF LA and B produced, but little if any SA, is found in the matrix laid down beneath the cells. Immunoreactive PDGF in cells, and in matrix below expressing cells, was visualized by laser confocal microscopy. Western blotting of protein in matrix extracts, cell extracts, and secreted into the growth medium was used to demonstrate that the range of PDGF A polypeptides seen in the matrix was overlapping with those reported previously to be cell associated in cell types such as NIH3T3 and COS 7. This suggests that attachment to matrix or cell surface may be alternative fates for these polypeptides, with fate dependent on the characteristics of the producing cells. Immunoreactive PDGF A and B could be partially released by incubation of matrix material with heparin but not with other glycosaminoglycans. Digestion of matrix with chondroitin ABC lyase but not heparitinase or collagenase displaced some PDGF from its attachment sites. The results indicate attachment of PDGF to matrix proteoglycans, at least partly through the glycosaminoglycan moieties, and perhaps to additional components. The significance of matrix deposition for PDGF action is discussed.