Description: Aims Expression and activity of the transcription factor AP-1 are enhanced during cardiac remodelling and heart failure progression. In order to test if AP-1 inhibition may limit processes contributing to cardiac remodelling, ventricular cardiomyocytes of mice with cardiac overexpression of the AP-1 inhibitor JDP2 were analysed under stimulation of hypertrophy, apoptosis, or contractile function. Methods and results Three models of JDP2 overexpressing mice were analysed: JDP2 was overexpressed either life-long, for 7 weeks, or 1 week. Then cardiomyocytes were isolated and stimulated with β-adrenoceptor agonist isoprenaline (ISO, 50 nM). This enhanced cross-sectional area and the rate of protein synthesis in WT but not in JDP2 overexpressing cardiomyocytes. To induce apoptosis, cardiomyocytes were stimulated with 3 ng/mL TGFβ 1 . Again, JDP2 overexpression prevented apoptosis induction compared with WT cells. Determination of contractile function under electrical stimulation at 2 Hz revealed enhancement of cell shortening, and contraction and relaxation velocities under increasing ISO concentrations (0.3–30 nM) in WT cells. This inotropic effect was abrogated in JDP2 overexpression cells. Responsiveness to increased extracellular calcium concentrations was also impaired in JDP2 overexpressing cardiomyocytes. Simultaneously, a reduction of SERCA expression was found in JDP2 mice. Conclusion A central role of AP-1 in the induction of hypertrophy and apoptosis in cardiomyocytes is demonstrated. Besides these protective effects of AP-1 inhibition on factors of cardiac remodelling, AP-1-inhibition impairs contractile function. Therefore, AP-1 acts as a double-edged sword that mediates mal-adaptive cardiac remodelling, but is required for maintaining a proper contractile function of cardiomyocytes.