Expression of minichromosome maintenance proteins in vascular smooth muscle cells is ERK/MAPK dependent

Abstract

Proliferation of vascular smooth muscle cells (VSMC) represents a key event for the pathogenesis of postangioplasty restenosis. Minichromosome maintenance proteins (MCM) form essential components of the prereplicative complex at DNA replication origins and are regulated by E2F. The present studies were designed to investigate the signal transduction pathways controlling the expression of MCM6 and MCM7 in VSMC in response to mitogenic stimuli. MCM6 and MCM7 expression was substantially increased after stimulation with platelet-derived growth factor-BB and insulin. Pretreatment with PD98059, a specific inhibitor of the extracellular signal-regulated kinases (ERK)–mitogen-activated protein kinase (MAPK), competely inhibited the mitogen-induced MCM6 and MCM7 mRNA and protein expression, demonstrating a critical role for this pathway in transmitting transmembrane signals required for the initiation of DNA replication. The p38MAPK inhibitor SB203580, the phosphatidylinositol 3 kinase (PI3-kinase) pathway inhibitor wortmannin, and the protein kinase C pathway (PKC) inhibitor Gö 6976 did not significantly affect mitogen-induced MCM6 and MCM7 expression. Transient transfection experiments revealed that PD98059 inhibited mitogen-induced MCM6 and MCM7 transcriptional activation. In addition, blockade of ERK/MAPK signaling with PD98059 strongly inhibited phosphorylation of the retinoblastoma protein (Rb) and activity of a luciferase reporter plasmid driven by multiple E2F elements. Inhibition of mitogen-induced MCM6 and MCM7 expression by PD98059 was reversed by ectopic overexpression of E2F, indicating that ERK/MAPK signaling is required for events that occur upstream of E2F release from phosphorylated Rb. In combination, these data demonstrate that the ERK/MAPK signal transduction pathway plays a central role in regulating E2F-dependent MCM expression and DNA replication in VSMC.