Studies on new proteins involved in vascular endothelial injury

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An elevated level of homocysteine is associated with arteriosclerosis and thrombosis. The mechanisms by which homocysteine may promote vascular diseases have not been elucidated yet. We identified a novel homocysteine responsive protein, RTP, in human umbilical vein endothelial cells (HUVEC). Its mRNA was increased by a variety of endoplasmic reticulum-stress inducing agents such as tunicamycin, A23187, and thapsigargin. Cytokines had little or no effect on RTP mRNA expression. Western blot analysis using recombinant RTP antiserum showed that protein level of RTP (47 kDa) was detectable in untreated HUVEC and increased in homocysteine- or tunicamycin-treated cells. It was also demonstrated that RTP was partially phosphorylated and contained seven or more phosphorylation sites. Phosphorylation of RTP was enhanced by the forskolin treatment and inhibited by a protein kinase A inhibitor. Furthermore, protein kinase A directly phosphorylated recombinant RTP in vitro. The phosphorylated forms were abundant in the proliferating cells and decreased in relation to increased cell density. Immunocytochemical analysis indicated that RTP was mainly located in the cytoplasm of cultured cells. Immunohistochemistry of mouse tissues demonstrated that RTP was present abundantly in proximal tubule of kidney and in villi of intestine. The present study raises the possibility that cellular functions in patients with hyperhomocysteinemia might be altered through changes in the RTP expression level and its phosphorylation state.