PDGF-INDUCED GLYCOSAMINOGLYCAN SYNTHESIS IS MEDIATED VIA PHOSPHATIDYLINOSITOL 3-KINASE

Platelet-derived growth factor (PDGF)-BB has been shown previously to increase glycosaminoglycan (GAG) synthesis but not DNA synthesis in fr eshly isolated fetal lung fibroblasts. In the present study, we found that PDGF-BB also enhanced (SO4)-S-35 incorporation into the small, so luble proteoglycan biglycan without affecting biglycan's core protein mRNA expression, suggesting that PDGF-BB mainly affects GAG chain elon gation and/or sulfation. PDGF-BB-stimulated GAG synthesis was abrogate d by tyrphostin 9, a PDGF receptor-associated tyrosine kinase inhibito r, implying that the stimulatory effect is mediated via the PDGF beta- receptor (PDGFR). The intracellular signal transduction pathways that mediate PDGF-BB-stimulated GAG synthesis in fetal lung fibroblasts wer e investigated. On Ligand-induced tyrosine phosphorylation, PDGFR asso ciated with phospholipase C (PLC)-gamma 1, Ras GTPase activating prote in (RasGAP), and phosphatidylinositol 3-kinase (PI3K) but not with the Syp-growth factor receptor-bound protein 2-Son of Sevenless complex. Association of PDGFR with PLC-gamma l and RasGAP followed by their tyr osine phosphorylation failed, however, to activate PLC-gamma l, protei n kinase C (PKC), and nas. Neither a PLC-gamma inhibitor, U-73122; a P KC inhibitor, calphostin C; nor a mitogen-activated protein kinase kin ase inhibitor, PD-98059, inhibited PDGF-BB-induced GAG synthesis. In c ontrast, PDGF-BB stimulation triggered PDGFR-associated PI3K activity Both PDGF-BB-induced PI3K activation and GAG synthesis were abolished by the PI3K inhibitors wortmannin and LY-294002. The results suggest t hat PI3K is a downstream mediator of PDGF-BB-stimulated GAG synthesis in fetal rat lung fibroblasts.