Abstract
Sphingosine 1‐phosphate (S1P) has been shown to regulate expression of several genes in vascular smooth muscle cells (VSMCs) and contributes to arteriosclerosis. However, the mechanisms regulating epidermal growth factor receptor (EGFR) expression by S1P in aortic VSMCs remain unclear. Western blotting and RT‐PCR analyses showed that S1P induced EGFR mRNA and protein expression in a time‐ and concentration‐dependent manner, which was attenuated by inhibitors of MEK1/2 (U0126) and phosphatidylinositide 3‐kinase (PI3K; wortmannin), and transfection with dominant negative mutants of ERK and Akt, respectively. These results suggested that S1P‐induced EGFR expression was mediated through p42/p44 MAPK and PI3K/Akt pathways in VSMCs. In accordance with these findings, S1P stimulated phosphorylation of p42/p44 MAPK and Akt which was attenuated by U0126 and wortmannin, respectively. Furthermore, S1P‐induced EGFR upregulation was blocked by a selective NF‐κB inhibitor helenalin. Immunofluorescent staining and reporter gene assay revealed that S1P‐induced activation of NF‐κB was blocked by wortmannin, but not by U0126, suggesting that activation of NF‐κB was mediated through PI3K/Akt. Moreover, S1P‐induced EGFR expression was inhibited by an AP‐1 inhibitor curcumin and tanshinone IIA. S1P‐stimulated AP‐1 subunits (c‐Jun and c‐Fos mRNA) expression was attenuated by U0126 and wortmannin, suggesting that MEK and PI3K/ERK cascade linking to AP‐1 was involved in EGFR expression. Upregulation of EGFR by S1P may exert a phenotype modulation of VSMCs. This hypothesis was supported by pretreatment with AG1478 or transfection with shRNA of EGFR that attenuated EGF‐stimulated proliferation of VSMCs pretreated with S1P, determined by XTT assay. These results demonstrated that in VSMCs, activation of Akt/NF‐κB and ERK/AP‐1 pathways independently regulated S1P‐induced EGFR expression in VSMCs. Understanding the mechanisms involved in S1P‐induced EGFR expression on VSMCs may provide potential therapeutic targets in the treatment of arteriosclerosis. J. Cell. Biochem. 103: 1732–1746, 2008. © 2007 Wiley‐Liss, Inc.