Arjunolic acid, a peroxisome proliferator-activated receptor α agonist, regresses cardiac fibrosis by inhibiting non-canonical TGF-β signaling

Cardiac hypertrophy and associated heart fibrosis remain a major cause of death worldwide. Phytochemicals have gained attention as alternative therapeutics for managing cardiovascular diseases. These include the extract from the plant Terminalia arjuna, which is a popular cardioprotectant and may prevent or slow progression of pathological hypertrophy to heart failure. Here, we investigated the mode of action of a principal bioactive T. arjuna compound, arjunolic acid (AA), in ameliorating hemodynamic load-induced cardiac fibrosis and identified its intracellular target. Our data revealed that AA significantly represses collagen expression and improves cardiac function during hypertrophy. We found that AA binds to and stabilizes the ligand-binding domain of peroxisome proliferator-activated receptor alpha (PPAR alpha) and increases its expression during cardiac hypertrophy. PPAR alpha knockdown during AA treatment in hypertrophy samples, including angiotensin II-treated adult cardiac fibroblasts and renal artery-ligated rat heart, suggests that AA-driven cardioprotection primarily arises from PPAR alpha agonism. Moreover, AA-induced PPAR alpha up-regulation leads to repression of TGF-beta signaling, specifically by inhibiting TGF-beta-activated kinase1 (TAK1) phosphorylation. We observed that PPAR alpha directly interacts with TAK1, predominantly via PPAR alpha N-terminal transactivation domain (AF-1) thereby masking the TAK1 kinase domain. The AA-induced PPAR alpha bound TAK1 level thereby shows inverse correlation with the phosphorylation level of TAK1 and subsequent reduction in p38 MAPK and NF-kappa Bp65 activation, ultimately culminating in amelioration of excess collagen synthesis in cardiac hypertrophy. In conclusion, our findings unravel the mechanism of AA action in regressing hypertrophy-associated cardiac fibrosis by assigning a role of AA as a PPAR alpha agonist that inactivates non-canonical TGF-beta signaling.