Transforming growth factor  1 (TGF 1 ) activation leads to tissue fibrosis. Here, we report on the role of LOX-1, a lectin-like 52-kDa receptor for oxidized low density lipoprotein, in TGF 1 -mediated collagen expression and underlying signaling in mouse cardiac fibroblasts. TGF 1 was overexpressed in wildtype (WT) and LOX-1 knock-out mouse cardiac fibroblasts by transfection with adeno-associated virus type 2 vector carrying the active TGF 1 moiety (AAV/TGF 1 ACT ). Transfection of WT mouse cardiac fibroblasts with AAV/TGF 1 ACT markedly enhanced the expression of NADPH oxidases (p22 phox , p47 phox , and gp91 phox subunits) and LOX-1, formation of reactive oxygen species, and collagen synthesis, concomitant with an increase in the activation of p38 and p44/42 mitogen-activated protein kinases (MAPK). The TGF 1 -mediated increase in collagen synthesis was markedly attenuated in the LOX-1 knock-out mouse cardiac fibroblasts as well as in WT mouse cardiac fibroblasts treated with a specific anti-LOX-1 antibody. Treatment with anti-LOX-1 antibody also reduced NADPH oxidase expression and MAPK activation. The NADPH oxidase inhibitors and gp91phox small interfering RNA reduced LOX-1 expression, MAPK activation, and collagen formation. The p38 MAPK inhibitors as well as the p44/42 MAPK inhibitors reduced collagen formation without affecting LOX-1 expression in cardiac fibroblasts. These observations suggest that collagen synthesis in cardiac fibroblasts involves a facilitative interaction between TGF 1 -NADPH oxidase and LOX-1. Further, the activation of MAPK pathway appears to be downstream of TGF 1 -reactive oxygen species-LOX-1 cascade.Cardiac fibrosis, characterized by accumulation of matrix proteins in the extracellular space in the perivascular region, is closely associated with the development of heart failure. Among the extracellular matrix proteins, up to 85% are collagens, of which type 1 (collagen I) and type 3 (collagen III) constitute two-thirds of total collagens in most tissues (1). Transforming growth factor (TGF) 3  1 is one of the most pleiotropic and multifunctional peptides known (2). It exerts potent effects on many different cell types and is involved in a wide variety of biological processes (2). The cellular actions of TGF 1 are dependent not only on the cell type but also on its state of differentiation and the cytokine milieu (3). Although there is evidence that TGF 1 stimulates fibroblast growth, enhances collagen synthesis, and suppresses collagen degradation (2) ACT that appears to be functionally relevant in the process of ischemia-reperfusion (5).LOX-1 is a lectin-like receptor for oxidized low density lipoprotein (6). It is also up-regulated in response to oxidative stress (7). Activation of LOX-1 enhances the growth of cardiac fibroblasts and promotes collagen synthesis (8, 9). It has been reported that TGF 1 can regulate LOX-1 expression in vascular endothelial cells, smooth muscle cells, and monocytes/macrophages (10, 11). Another study showed that LOX-1 blockade reduc...