Hyperlipidemia is a recognized risk factor for atherosclerotic vascular disease. The underlying mechanisms that link lipoproteins and vascular disease are undefined. Connective tissue growth factor (CTGF) is emerging as a key determinant of progressive fibrotic diseases and its expression is upregulated by diabetes. To define the mechanisms through which low density lipoproteins (LDL) promotes vascular injury, we evaluated whether LDL can modulate the expression of CTGF and collagen IV in human aortic endothelial cells (HAEC). Treatment of HAEC with LDL (50µg/ml) for 24 h produced a significant increase in the mRNA and the protein levels of CTGF and collagen IV compared to unstimulated controls. To explore the mechanisms by which LDL regulates CTGF and collagen IV expression in HAEC, we determined first if CTGF and collagen IV are downstream targets for regulation by transforming growth factor- (TGF-). The results demonstrated that TGF-beta produced a concentration-dependent increase in the protein levels of CTGF. To assess whether the induction of CTGF in response to LDL is mediated via autocrine activation of TGF-, HAECs were treated with LDL for 24h in the presence and absence of neutralizing anti-TGF-beta (TGF- NA) antibodies. The results demonstrated that the increase in CTGF induced by LDL was significantly inhibited by the anti-TGF- NA. To investigate the upstream mediators of TGF- on activity of CTGF in response to LDL, HAECs were treated with LDL for 24h in the presence and absence of cell permeable MAPK inhibitors. Inhibition of p38^mapk activities did not affect LDL induced TGF- , CTGF and collagen IV expression. On the other hand, SP600125 a specific inhibitor of c-Jun NH2-terminal kinase (JNK) suppressed LDL induced TGF- , CTGF and collagen IV expression and PD98059, a selective inhibitor of p44/42^mapk suppressed LDL induced TGF- and CTGF expression. These findings are the first to implicate MAPK pathway and TGF- as key players in LDL signaling leading to CTGF and collagen IV expression in HAEC. The data also points to a potential mechanistic pathway through which lipoproteins may promote vascular injury.