Arterial hyperplasia occurs as a result of increased vascular smooth muscle (VSM) cell proliferation. It is now understood that this is initiated by modulation of VSM cells towards a more proliferative and diseased phenotype. An important event in this phenotypic change is the active repression of smooth muscle (SM) marker proteins associated with contractility, such as SM22α, calponin and smooth muscle α-actin. This decrease in SM marker proteins is controlled by growth factor-induced gene expression via activation of the mitogen-activated protein kinases, extracellular signal-regulated kinase (ERK)1/2. When an appropriate growth signal is received by the VSM cell, this leads to phosphorylation of ERK1/2 which in turn regulates specific transcription factors including Elk-1. Elk-1, localised in the nucleus, has a critical role in gene expression and in VSM cells is responsible for repressing SM marker genes. We have recently revealed a novel mechanism regulating ERK1/2 and subsequently Elk-1 in VSM cells which is likely to be important in regulating VSM cell phenotype. An essential step in ERK1/2-induced changes in gene expression is the translocation of ERK1/2 from the cytoplasm to the nucleus (Hunter et al, 2011). As we have demonstrated, if this translocation is prevented in VSM cells, the ability of growth factors to repress SM marker genes is inhibited. Our experimental evidence now indicates that a protein previously uncharacterized in VSM cells, phosphoprotein enriched in astrocytes-15 (PEA-15), has an essential role in regulating ERK1/2 nuclear localization. PEA-15 has a cytoplasmic distribution and one of its primary roles is to function as an ERK1/2-binding protein (Ramos, 2008). Studies have now revealed that PEA-15 binds and sequesters ERK1/2 in the cytoplasm and we have confirmed this in VSM cells. In cultured human coronary artery smooth muscle cells, knockdown of PEA-15 results in ERK1/2 nuclear localization, which can be reversed by PEA-15 overexpression. Importantly, PEA-15 binding to ERK1/2 is regulated by phosphorylation of PEA-15 at sites phosphorylated by Ca2+/calmodulin-dependent protein kinase II (CaMKII). In an unstimulated cell PEA-15 is unphosphorylated and bound to ERK1/2. Our results demonstrate that an increase in intracellular Ca2+ and activation of CaMKII is essential for PEA-15 phosphorylation and subsequently ERK1/2 localization to the nucleus. This phosphorylation event is the “release” mechanism which uncouples ERK1/2 from PEA-15 thereby allowing ERK1/2 translocation. In a typical growth factor-induced response in VSM cells, phosphorylation of tyrosine kinase receptors activates ERK1/2 and simultaneously releases intracellular Ca2+ via phospholipase Cγ1 (PLCγ1). Knockdown of PLCγ1 inhibits PEA-15 phosphorylation although does not affect growth factor-induced ERK1/2 activation. PLCγ1 knockdown however prevents ERK1/2 nuclear localization, Elk-1 activation and repression of smooth muscle marker genes such as smooth muscle α-actin. A rise in intracellular Ca2+ is therefore required in growth factor-induced proliferation as this directly leads to PEA-15 phosphorylation and permits normal ERK1/2 signalling to occur. As PEA-15 has an important role in regulating VSM cell proliferation, changes in PEA-15 expression could have profound effects on blood vessel structure and function. Our results reveal that in mouse injured arteries PEA-15 expression is reduced in the neointima compared to the medial layer. We predict that a decreased PEA-15 expression allows increased ERK1/2 translocation to the nucleus thereby inducing VSM cell proliferation and contributing to hyperplasia in injured arteries. Physiological regulation of PEA-15 expression in the context of vascular disease will be further discussed.