TNF-alpha is a proinflammatory cytokine that is upregulated in many cardiac diseases. The increase of TNF-alpha expression affects both heart function and the structure of the extracellular matrix. Lysyl oxidase (LOX) is a key enzyme responsible for the maturation of extracellular matrix proteins, including collagens type I and III. In this study, we investigated the regulation of LOX expression and activity by TNF-alpha using adult rat cardiac fibroblasts. Our results indicate that TNF-alpha has a dichotomous effect on LOX expression by cardiac fibroblasts. Low dose TNF-alpha (1-5 ng/ml) decreased LOX expression, whereas higher doses (10-30 ng/ml) increased expression. The higher dose TNF-alpha effect on LOX expression was attenuated by the inhibition of PI3Kinase/Akt pathway. TGF-beta 1 signaling played a significant role in mediating the TNF-alpha, effect. TNF-alpha increased the expression of TGF-beta, and TGF-beta receptors type I and II, and also stimulated Smad3 phosphorylation. Inhibition of TGF-beta receptor I or Smad3 prevented increased LOX expression by TNF-alpha. These findings indicate that TNF-alpha stimulated LOX expression may play an important role in progressive cardiac fibrosis. (C) 2011 Elsevier Inc. All rights reserved.