Synergism between AT1 receptor and hyperhomocysteinemia during vascular remodeling. 
Hyperhomocysteinemia (HHcy) is an independent risk factor of cardiovascular diseases.  Extracellular signal-regulated kinase-1/2 (ERK-1/2) and the JAK/STAT pathway kinase, signal transducer and activator of transcription 3 (STAT3), are involved in matrix metalloproteinase-9 (MMP-9) induction and matrix remodeling.  However, their role in homocysteine (Hcy)-mediated MMP-9 induction and matrix remodeling is unclear.  Clinical and experimental evidence indicates that HHcy and activation of the renin-angiotensin system, mediated by angiotensin II type 1 (AT1) receptor, are involved in a variety of vascular pathologies.  Despite this fact, the relationship between HHcy and activation of the renin-angiotensin system has not been comprehensively characterized.  Therefore, we hypothesized that Hcy activates AT1 receptor that potentiates STAT3 via ERK-1/2 phosphorylation.  STAT3 modulates target MMP-9 and collagen, resulting in vascular remodeling.  Mouse aortic endothelial cells (MAEC) were treated with various doses of Hcy for different time periods.  The levels of AT1 receptor, ERK-1/2, STAT3, MMP-9 and collagen type-1 were measured by immunoblot analyses.  The activation of ERK-1/2 and STAT3 were determined by measuring ERK-1/2 phosphorylation and phosphoserine (727) STAT3.  Although Hcy dose-dependently induced AT1 receptor expression in the endothelial cells, a significant induction was observed at 100 microM at 48 h. We investigated Hcy-induced ERK-1/2 and STAT3 phosphorylation through AT1 receptor induction, and our results suggest that Hcy activated AT1 receptor which led to ERK-1/2 and STAT3 phosphorylation.  In addition, findings of this study suggest that Hcy-mediated STAT3 activation regulated MMP-9 and collagen type-1.  However, AT1 receptor blocker, valsartan, and the specific STAT3 inhibitor peptide attenuated MMP-9 and collagen type-1 induction.  These findings demonstrate for the first time the contribution of AT1 receptor in HHcy-induced atherosclerotic diseases; Hcy-induced activation of AT1 receptor involves MMP-9 and collagen type-1 modulation using ERK-1/2 and STAT3 signaling cascades.