PDGF-induced signaling in proliferating and differentiated vascular smooth muscle: effects of altered intracellular Ca2+ 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)g expression and activation of extracellular signal-regulated kinase (ERK) 1/2 was determined by immunoblotting or confocal immunolabelling. Activation of the Ca2+-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 PLCg 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 Ca2+-dependent and protein kinase C-dependent. However in 2–4-day-old PV, PDGF-induced ERK1/2 activation was via a Ca2+-independent, atypical protein kinase C. PLCg 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 PLCg 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.