miR-17-3p Is Required for Exercise-induced Cardiac Growth. (A) miR-17-3p antagomiR significantly decreased miR-17-3p in hearts from both sedentary control and swum mice. n=7 per group. (B) Heart weight/body weight (HW/BW) and heart weight/tibia length (HW/TL) ratios were shown from sedentary control and swum mice. n=6 per group. (C) Wheat germ agglutinin (WGA) staining for determination of cardiomyocyte area showed that miR-17-3p antagomiR partly blocked exercise-induced cardiomyocyte hypertrophy. n=4 per group. (D) Immunohistochemical staining for EdU and phospho-histoneH3 (pHH3) indicated that miR-17-3p antagomiR partly reduced markers of cardiomyocyte proliferation. n=4 for control and n=8 for swim group. (E) Western blot showed that exercise-induced downregulation of TIMP3 and PTEN, and activation of AKT were partly altered by miR-17-3p antagomiR in vivo. n=3 per group. At least 4000 cells were quantified in each group. Data are shown as mean±SEM. Scale bar: 100 μm. *, P<0.05, **, P<0.01, ***, P<0.001 versus respective control.