Comparison of PDGF-AA- and PDGF-BB-induced phosphoinositide formation in human and mouse fibroblasts

In certain cells, such as human fibroblasts (AG 1523), there is a clear difference in the cell motility response induced by the different isoforms of platelet-derived growth factor (PDGF). PDGF-BB induces extensive actin reorganization and is a potent chemotactic agent, whereas PDGF-AA has a limited effect on actin reorganization and is not chemotactic. In the present study, we wanted to compare these effects on cell motility with the effects of the PDGF isoforms on phosphoinositide (PtdIns) turnover. We find that stimulation of serum-starved AG 1523 cells with PDGF-AA or PDGF-BB caused an initial increase of the phosphatidylinositol phosphate and bisphosphate (PtdInsP and PtdInsP2) pools, suggesting that activation of the phosphoinositide kinases is an initial response to PDGF stimulation. Despite a lower number of PDGF alpha-receptors than beta-receptors on these cells, the initial formation of PtdInsP and PtdInsP2 appears to be stimulated to a similar degree by the two PDGF isoforms. In contrast, PtdInsP2 hydrolysis, indirectly measured as formation of phosphatidic acid, was correlated to the number of receptors. During prolonged exposure to PDGF-BB the stimulated PtdIns turnover remained at a high level, whereas the effect of PDGF-AA appeared more transient. A marked increase in the synthesis of a component migrating as phosphatidylinositol trisphosphate (PtdInsP3) was also detected after stimulation with PDGF-BB for 5 min. With PDGF-AA minor amounts were found, indicating that activation of the PtdIns 3'-kinase occurs also via the PDGF alpha-receptor. Stimulation with PDGF-BB, but not -AA, also induced a 50% decrease in lyso-PtdIns. In murine fibroblasts (Swiss 3T3), where the two PDGF isoforms have a similar effect on cell motility, the two PDGF isoforms also similarly induced PtdIns turnover, PtdInsP3 formation, and a decrease in lyso-PtdIns. Thus, there seems to be a correlation between PDGF-induced PtdIns turnover and PDGF-induced actin reorganization. This is compatible with previous evidence suggesting the microfilament formation is directly linked to an increased turnover of polyphosphoinositides in stimulated cells.