Abstract 2428: Role of Rac1, Connective Tissue Growth Factor and Lysyl Oxidase for Atrial Fibrosis in Atrial Fibrillation
Background: Despite the importance of interstitial fibrosis for atrial remodeling during atrial fibrillation (AF) is the underlying signal transduction poorly understood.
Methods and Results: Expression profiling by Affymetrix array analysis of the left atrial appendage from patients with AF revealed that these atria are characterised by markedly increased Lysyl Oxidase (LOX; key enzyme for the crosslink of elastin and collagen) and Connective Tissue Growth factor (CTGF) gene expression compared to samples from sinus rhythm (SR) patients with same atrial diameter. RT-PCR and Western Blot analysis confirmed a 2-fold up-regulation of LOX as well as CTGF on mRNA- and protein-level. This was associated with a significantly higher collagen content (14.9±2.1% vs. 8.5±1.3%, n=5), a 4-fold higher total protein expression of Rac1-GTPase and an increased Rac1 activity (362±98%, PAK pull-down assays) compared to SR patients. In order to test whether these pro-fibrotic effects were mediated by Rac1, neonatal cardiomyocytes und fibrocytes were pre-incubated with angiotensin II and the specific small molecule inhibitor of Rac1, NSC23766. NSC23766 completely prevented the angiotensin II induced upregulation of CTGF (194±34%) expression in fibrocytes as well as in cardiomyocytes. Treatment of rat neonatal cardiomyocytes und fibrocytes with recombinant CTGF (1ng/ml) increased LOX protein expression markedly whereas Rac1 expression was unaffected. To test the importance of Rac1 in the pathogenesis of fibrosis in vivo, transgenic mice with cardiac overexpression of Rac1 (RacET) developing atrial fibrillation at high age, were compared to wildtype (WT). Left atria of RacET mice showed a significant elevation of CTGF as well as LOX protein expression. Interstitial collagen content quantified by sirius red staining was markedly increased in the atria of RacET (44±1%) compared to WT (19±5%). All effects are significant with p<0.05.
Conclusion: Left atria of patients with AF exhibit upregulation of Rac1 activity and increased CTGF and LOX expression. In cell culture, Rac1 mediates upregulation of LOX via CTGF. Inhibition of this signaling pathway may therefore represent a target for the prevention of atrial fibrillation.