Peroxisome proliferator-activated receptor-γ ligands inhibit nuclear but not cytosolic extracellular signal-regulated kinase/mitogen-activated protein kinase-regulated steps in vascular smooth muscle cell migration

Vascular smooth muscle cell (VSMC) migration involves adhesion, locomotion, and invasion regulated by various signaling molecules, among which the extracellular signal-regulated kinase (ERK)/mitogen-activated protein kinases (MAPK) play a critical role. We have shown that the peroxisome proliferator-activated receptor-gamma (PPAR-gamma) ligands troglitazone and rosiglitazone inhibit VSMC migration downstream of ERK MAPK. The purpose of the current study was to more specifically determine which step(s) in VSMC migration are targeted by inhibition of the ERK MAPK pathway or activation of PPAR-gamma. VSMC adhesion was not affected by the ERK MAPK pathway inhibitor PD98059 or PPAR-gamma ligands. Phosphorylation and activation of myosin light chain kinase (MLCK) play important roles in cell locomotion. Platelet-derived growth factor (PDGF)-induced MLCK phosphorylation (1.7-fold) was completely blocked by PD98059 at 30 muM (p < 0.05), but not by troglitazone or rosiglitazone. PDGF-directed migration (5.8-fold) was inhibited by PD98059 -88% at 30 <mu>M and the MLCK inhibitor ML9 (0.1-1 muM, -84% at 1 muM) (all p < 0.05). The transcription factor Ets-1 mediates matrix metalloproteinase induction required for tissue invasion by VSMC. PDGF (20 ng/ml) stimulated an Ets-1 protein expression (14-fold at 60 min) in VSMC, which was inhibited by PD98059 (-72% at 30 <mu>M), troglitazone (-69% at 20 muM), and rosiglitazone (-54% at 10 muM) (all p < 0.05). Immunohistochemistry of rat aortae 2 h after balloon injury showed a dramatic upregulation of Ets-1, which was markedly inhibited in animals that had received troglitazone treatment. In contrast, phosphorylated ERK MAPK was not affected by troglitazone. These data are consistent with PPAR-<gamma> ligands exerting their anti-migratory effects downstream of ERK MAPK activation by blocking nuclear events, such as Ets-1 expression. required for cell invasion in response to arterial injury.