Activation of PLC and PI 3 kinase by PDGF receptor alpha is not sufficientfor mitogenesis and migration in mesangial cells

Background. Platelet-derived growth factor (PDGF) isoforms act through two distinct cell surface alpha and beta receptors. Glomerular mesangial cells express both receptors. PDGF BE and AB are potent mitogens for glomerular m esangial cells, and PDGF BE stimulates cell migration in a phosphatidylinos itol 3 (PI 3) kinase-dependent manner. In this study, we investigated the e ffect of PDGF AA on cell migration, PI 3 kinase and phospholipase C (PLC) a ctivation. and the role of these two enzymes in mediating biological respon ses in these cells in response to all three isoforms. Methods. H-3-thymidine incorporation and modified Boyden chamber assay were used to determine DNA synthesis and directed migration, respectively.. in response to all three PDGF isoforms. Differential activation of alpha and b eta receptors was studied by immunecomplex tyrosine kinase assay of corresp onding receptor immunoprecipitates. PLC gamma 1 activity was determined by measuring total inositol phosphates in response to different PDGF isoforms. PI 3 kinase activity was determined in anti-phosphotyrosine or PDGF recept or immunoprecipitates. Results. Both PDGF BE and AB resulted in stimulation of DNA synthesis and d irected migration of mesangial cells. AA a was neither chemotactic nor mito genic, However, all three isoforms increased tyrosine phosphorylation of a 180 kD protein in antiphosphotyrosine immunoprecipitates. suggesting activa tion of respective receptors. Direct immunecomplex tyrosine kinase assay of alpha and beta receptors demonstrated significant activation of both of th ese receptors when cells are treated with PDGF BE or AB. PDGF AA increased tyrosine kinase activity of the alpha receptor but not the beta receptor. A ll three isoforms significantly stimulated the production of inositol phosp hates with order of potency being BE > AB > AA. PDGF AA also dose dependent ly stimulated PI 3 kinase activity measured in antiphosphotyrosine immunopr ecipitates of treated cells. A comparison of PI 3 kinase activity in antiph osphotyrosine immunoprecipitates from mesangial cells stimulated with three different PDGF isoforms showed significant activation of this enzyme with a decreasing order of activity: BE > AB > AA. Conclusion. Taken together, these data demonstrate that all three isoforms of PDGF significantly stimulate: PLC gamma 1 and PI 3 kinase, two enzymes n ecessary for both DNA synthesis and directed migration. However, activation of alpha receptor bg PDGF AA with a subsequent increase in PLC and PI 3 ki nase activities is not sufficient to induce these biological responses in m esangial cells. These data indicate that the extent of activation of signal transduction pathways may be a major determinant of the biological activit y of different PDGF isoforms in mesangial cells.