LOX and LOX-Like Proteins as Potential Therapeutic Target for Atrial Fibrillation

Lysyl oxidase (LOX) and LOX-like (LOXL-1, 2, 3 and 4) proteins have a crucial role in extracellular matrix (ECM) remodeling in several types of heart disease, such as heart failure (HF) and fibrosis. The main objective of this thesis was to address the matrix-dependent and matrix-independent functions of LOX and LOXL proteins in signal transduction, leading to atrial fibrosis and atrial fibrillation (AF). We noted upregulation of LOX and LOXL-1 in tissues, LOX and LOXL-2 in fibroblasts and LOX, LOXL-1, LOXL-3 and LOXL-4 in myocytes of the canine left atrium (LA) in congestive heart failure (CHF). Based on these findings, we studied the roles of LOX isoforms as upstream targets in the signaling pathways of LA fibrosis and AF in a rat model following myocardial infarction (MI). Additionally, we explored the physiological roles of LOX and LOXL proteins in matrix-dependent and matrix-independent functions of cardiac fibroblasts and myocytes through angiotensin II (Ang II) treatment and siRNA-mediated knockdown of LOX isoforms in canine and neonatal rat cells. Upregulation of the mRNA expression of LOX and LOXL was accompanied by an increase in mRNA expression of COL 1A1, FN 1, TGF-β1, CTGF, periostin, α-SMA and MMP-2 in the infarcted area of the left ventricle (LV). mRNA expression of LOXL-1, LOXL-3, COL 1A1, TGF-β1 and periostin were significantly increased in the LA post-MI. Administration of β-aminopropionitrile (BAPN) post-MI significantly reduced the mRNA expression of LOXL-1, LOXL-2 and LOXL-3 along with a decrease in the mRNA expression of several profibrotic markers in the LA without significant changes to those in the LV. Moreover, the administration of BAPN post-MI reduced LA fibrosis and the collagen cross-linking ratio without significantly changing those in the LV. BAPN administration post-MI reduced the adverse structural and functional remodeling of LA without significantly changing those in the LV. Furthermore, MI caused an increase in the P-wave duration, AF duration and AF inducibility, while the values of those parameters were significantly reduced upon BAPN administration post-MI. The protein expression of secreted LOX and LOXL-2 were increased in cultured neonatal rat ventricular fibroblasts and myocytes along with an increase in the collagen cross-linking ratio in fibroblasts upon treatment with Ang II. The secretion of LOX and LOXL-2 from cardiac fibroblasts and myocytes may contribute to ECM remodeling. Moreover, the contractility of canine LA myocytes was enhanced upon knockdown of LOX or LOXL-1 along with slight changes in Ca2+ transients. Upon knockdown of LOXL-3 in fibroblasts, LOXL-2, LOXL-3 and LOXL-4 mRNA expression levels were reduced along with reduced proliferation and mRNA expression of COL 1A1, COL 3A1 and CCNE 2. The results showed that LOXL-2, LOXL-3 and LOXL-4 may promote fibroblast proliferation and collagen synthesis. LOXL-4 knockdown increased the mRNA expression of the BAX/BCL-2 ratio, suggesting that LOXL-4 may protect against apoptosis in cardiac fibroblasts and myocytes. In conclusion, LOX and LOXL proteins are prominent mediators of LA fibrosis and AF post-MI. Additionally, these findings suggested that LOX and LOXL proteins are implicated in the regulation of various aspects of cardiac fibroblast and myocyte functions.

Keyphrases: Heart Failure, Atrial Fibrillation, Fibrosis, Lysyl Oxidase (LOX), LOX-Like (LOXL) Proteins, Collagen Cross-linking, Fibroblast, Myocyte, Lysyl-Oxydase (LOX), LOX-like (LOXL)