Aims: Growth factor-induced repression of smooth muscle (SM) cell marker genes is an integral part of vascular SM (VSM) cell proliferation. This is partly regulated via translocation of extracellular signal-regulated kinase 1/2 (ERK1/2) to the nucleus which activates the transcription factor Elk-1. The mediators involved in ERK1/2 nuclear translocation in VSM cells are unknown. The aim of this study is to examine the mechanisms which regulate growth factor-induced nuclear translocation of ERK1/2 and gene expression in VSM cells.
Methods and results: In cultured human VSM cells, phospholipase C (PLC)γ1 expression was required for platelet-derived growth factor (PDGF)-induced ERK1/2 nuclear translocation, Elk-1 phosphorylation, and subsequent repression of SM α-actin gene expression. The mechanisms of a role for PLCγ1 in ERK1/2 nuclear localization were further examined by investigating interacting proteins. The ERK1/2-binding phosphoprotein, protein enriched in astrocytes-15 (PEA-15), was phosphorylated by PDGF and this phosphorylation required activation of PLCγ1. In cells pre-treated with PEA-15 siRNA, ERK1/2 distribution significantly increased in the nucleus and resulted in decreased SM α-actin expression and increased VSM cell proliferation. Overexpression of PEA-15 increased ERK1/2 localization in the cytoplasm. The regulatory role of PEA-15 phosphorylation was assessed. In VSM cells overexpressing a non-phosphorylatable form of PEA-15, PDGF-induced ERK1/2 nuclear localization was inhibited.
Conclusion: These results suggest that PEA-15 phosphorylation by PLCγ1 is required for PDGF-induced ERK1/2 nuclear translocation. This represents an important level of phenotypic control by directly affecting Elk-1-dependent transcription and ultimately SM cell marker protein expression in VSM cells.