PDGF-induced signaling in proliferating and differentiated vascular smooth muscle: Effects of altered intracellular Ca regulation

OBJECTIVE: Platelet-derived growth factor-BB (PDGF)-induced intracellular signaling is involved in phenotypic modulation of vascular smooth muscle (VSM). This study has examined the PDGF-induced Ca2+ increase and the resultant effect on signaling pathways in proliferative compared with fully differentiated VSM. METHODS: PDGF-induced changes in Ca2+ were measured in portal vein (PV) myocytes from 2-4-day-old (proliferating), compared to 6-week-old (differentiated), Sprague Dawley rats. Phospholipase C (PLC)gamma expression and activation of extracellular signal-regulated kinase (ERK) 1/2 was determined by immunoblotting or confocal immunolabelling. Activation of the Ca(2+)-dependent transcription factor, nuclear factor of activated T-cells (NFATc), was assessed by electromobility shift assay. RESULTS: PDGF increased the intracellular Ca2+ concentration in differentiated, but not in proliferating, PV myocytes. This is probably due to very low expression of PLC(gamma) in proliferating PV. In 6-week-old PV, PDGF stimulation induced nuclear translocation and activation of NFATc. PDGF did not induce NFATc activation in neonatal PV. PDGF-induced ERK1/2 activation was observed in both 2-4-day-old and 6-week-old PV. In 6-week-old PV, ERK1/2 activation was Ca(2+)-dependent and protein kinase C-dependent. However in 2-4-day-old PV, PDGF-induced ERK1/2 activation was via a Ca(2+)-independent, atypical protein kinase C. PLC(gamma) expression was also decreased in the neointima, compared to media, of balloon-injured rabbit subclavian arteries. CONCLUSIONS: The regulation of PDGF-induced Ca2+ increases by PLC(gamma) expression in VSM may provide a mechanism for coordinating different signaling pathways leading to activation of specific transcription factors. This may play an important role in the phenotypic modulation of VSM.