Abstract-Relaxin is a naturally occurring peptide hormone that mediates systemic hemodynamic and renal adaptive changes during pregnancy and abrogates aberrant scar tissue formation (fibrosis) in diverse pathogeneses. However, its efficacy relative to renin-angiotensin system blockade, the most effective antifibrotic strategy currently available, is not known. We compared the individual versus combined antifibrotic effects of serelaxin (a recombinant form of human gene-2 relaxin) and the angiotensin-converting enzyme inhibitor enalapril, in preventative (started before injury) and therapeutic (treatment of established fibrosis) strategies, in a mouse model of isoprenaline-induced cardiac injury (at 17 days). Changes in systolic blood pressure, organ hypertrophy, and tissue remodeling/fibrosis were assessed. Pretreatment with serelaxin (0.5 mg/kg per day via subcutaneous administration) alone reduced cardiac fibrosis to a greater extent than enalapril (200 mg/L via drinking water; equivalent to 48 mg/kg per day) alone (P<0.05 versus enalapril alone). Additionally, the combined effects of serelaxin and enalapril reduced cardiac fibrosis by at least 2-fold compared with enalapril alone, when administered preventatively or therapeutically; by suppressing transforming growth factor-β1 expression and phosphorylation of Smad2 (an intracellular regulator of transforming growth factor-β1 activity; both P<0.05 versus enalapril alone) to a greater extent. The effects of serelaxin were independent of blood pressure, while enalapril lowered systolic blood pressure in the model studied. These findings suggest that serelaxin alone and in combination with an angiotensin-converting enzyme inhibitor more effectively ameliorates fibrosis than angiotensin-converting enzyme inhibition alone in the diseased heart, in a clinically relevant experimental scenario. (Hypertension. 2014;64:315-322.)

Background Recombinant human H2 relaxin (serelaxin) has emerged as a potential agent to treat fibrosis, the pathological hallmark of chronic disease. As we now know that serelaxin requires the angiotensin II (Ang II) type 2 receptor (AT2R) to ameliorate renal fibrogenesis in vitro and in vivo, we sought to determine if its anti-fibrotic actions were affected by Ang II type 1 receptor (AT1R) modulation. Methods We examined the signal transduction mechanisms of serelaxin when applied to primary rat renal and human cardiac myofibroblasts in vitro, and in three models of renal-or cardiomyopathy-induced fibrosis in vivo. Results The anti-fibrotic signal transduction of serelaxin via its cognate receptor, relaxin family peptide receptor 1 (RXFP1), was abrogated by the AT1R blockers, irbesartan or candesartan in vitro and in vivo. Candesartan also ameliorated serelaxin's anti-fibrotic actions in the left ventricle of mice with cardiomyopathy, indicating that the inhibitory effects of candesartan were not confined to the kidney. In a transfected cell system, we demonstrated that serelaxin did not directly bind to AT1Rs but that constitutive AT1R-RXFP1 interactions could form. To potentially explain these findings, we also demonstrated that all three receptors were expressed by renal and cardiac (myo)fibroblasts and that antagonists acting at each receptor directly/allosterically blocked the anti-fibrotic effects of either serelaxin or the AT2R agonist, Compound 21. Conclusions These findings have significant implications for the concomitant use of RXFP1 or AT2R agonists with AT1R blockers and suggest that functional AT1R-AT2R-RXFP1 interactions on myofibroblasts may represent new targets for controlling fibrosis progression.