Identification of a homocysteine receptor in the peripheral endothelium and its role in proliferation

Abstract

Homocysteine, a risk factor for atherosclerosis, increases intimal hyperplasia after carotid endarterectomy with associated smooth muscle cell proliferation and modulation of cytokines. The N-methyl-D-aspartate receptor (NMDAr), a glutamate-gated ion channel receptor, is associated with homocysteine-induced cerebrovascular injury; however, the receptor has not been identified in peripheral vascular cells, nor has any interaction with homocysteine been clarified. Our objectives were first, to identify NMDAr in rat carotid artery and rat aorta endothelial cells (RAEC); and second, to determine whether homocysteine activates NMDAr in the endothelium. NR1 and NR2A, two NMDAr subunits, were probed in rat carotid arteries by immunohistochemistry. RNA was isolated from RAECs, and expression of all NMDAr subunits (NR1, 2A, 2B, 2C, and 2D) were examined by RT-PCR and sequencing. For receptor protein expression, RAEC were incubated with different homocysteine concentrations and incubation times and also were treated with 50 microM homocysteine and/or preincubated with 50 microM dizocilpine MK-801, an NMDAr inhibitor. Both NR1 and NR2A were expressed in rat carotid arteries. All NMDAr subunits were expressed in the RAECs, and there was 92% to 100% similarity compared with rat NMDAr from the National Center for Biotechnology Information (NCBI) GenBank. Homocysteine upregulated NR1 expression and increased cell proliferation. RAEC pretreatment with MK-801 reduced homocysteine-mediated cell proliferation. This study is the first to show that NMDAr exists in the peripheral vasculature, and that homocysteine may act via NMDAr to increase intimal hyperplasia. Our objectives included the identification of a homocysteine receptor in the peripheral vasculature. The possible inhibition of a homocysteine receptor to prevent intimal hyperplasia rather than treat established stenosis would make a significant clinical impact. This will open further avenues of study in determining the role of homocysteine in the pathogenesis of intimal hyperplasia.