Hyperexpression and activation of extracellular signal-regulated kinases (ERK1/2) in atherosclerotic lesions of cholesterol-fed rabbits

A hallmark of hyperlipidemia-induced atherosclerosis is altered gene expres sion that initiates cell proliferation and (de)differentiation in the intim a of the arterial wall. The molecular signaling that mediates this process in vivo has yet to be identified. Extracellular signal-regulated kinases (E RKs) are thought to play a pivotal role in transmitting transmembrane signa ls required for cell proliferation in vitro. The present studies were desig ned to investigate the activity, abundance, and localization of ERK1/2 in a therosclerotic lesions of cholesterol-fed rabbits. Immunofluorescence analy sis revealed abundant and heterogeneous distribution of ERK1/2, mainly loca lized in the cap and basal regions of atheromas. A population of ERK-enrich ed cells was identified as alpha-actin-positive smooth muscle cells (SMCs). ERK1 and 2 were heavily phosphorylated on tyrosyl residues and coexpressed with proliferating cell nuclear antigen in atherosclerotic lesions. ERK1/2 protein levels in protein extracts from atherosclerotic lesions were 2- to 3-fold higher than the vessels of chow-fed rabbits, and their activities w ere elevated 3- to 5-fold over those of the normal vessel. SMCs derived fro m atherosclerotic lesions had increased migratory/proliferative ability and higher ERK activity in response to LDL stimulation compared with cells fro m the normal vessel. Inhibition of ERK activation by PD98059,a specific inh ibitor of mitogen-activated protein kinase kinases (MEK1/2), abrogated LDL- induced SMC proliferation in vitro. Taken together, our findings support th e proposition that persistent activation and hyperexpression of ERK1/2 may be a critical element to initiate and perpetuate cell proliferation during the development of atherosclerosis.