Rapamycin Attenuates Cardiac Fibrosis in Experimental Uremic Cardiomyopathy by Reducing Marinobufagenin Levels and Inhibiting Downstream Pro‐Fibrotic Signaling

BACKGROUND: Experimental uremic cardiomyopathy causes cardiac fibrosis and is causally related to the increased circulating levels of the cardiotonic steroid, marinobufagenin (MBG), which signals through Na/K‐ATPase. Rapamycin is an inhibitor of the serine/threonine kinase mammalian target of rapamycin (mTOR) implicated in the progression of many different forms of renal disease. Given that Na/K‐ATPase signaling is known to stimulate the mTOR system, we speculated that the ameliorative effects of rapamycin might influence this pathway. METHODS AND RESULTS: Biosynthesis of MBG by cultured human JEG‐3 cells is initiated by CYP27A1, which is also a target for rapamycin. It was demonstrated that 1 μmol/L of rapamycin inhibited production of MBG in human JEG‐2 cells. Male Sprague‐Dawley rats were subjected to either partial nephrectomy (PNx), infusion of MBG, and/or infusion of rapamycin through osmotic minipumps. PNx animals showed marked increase in plasma MBG levels (1025±60 vs 377±53 pmol/L; P<0.01), systolic blood pressure (169±1 vs 111±1 mm Hg; P<0.01), and cardiac fibrosis compared to controls. Plasma MBG levels were significantly decreased in PNx‐rapamycin animals compared to PNx (373±46 vs 1025±60 pmol/L; P<0.01), and cardiac fibrosis was substantially attenuated by rapamycin treatment. CONCLUSIONS: Rapamycin treatment in combination with MBG infusion significantly attenuated cardiac fibrosis. Our results suggest that rapamycin may have a dual effect on cardiac fibrosis through (1) mTOR inhibition and (2) inhibiting MBG‐mediated profibrotic signaling and provide support for beneficial effect of a novel therapy for uremic cardiomyopathy.