Calcineurin-NFAT signaling pathway mediates NHE1 induced cardiac hypertrophy

Calcineurin-NFAT Signaling Pathway Mediates NHE1 Induced Cardiac Hypertrophy Mohamed Mlih, Fatima Mraiche College of Pharmacy, Qatar University, Doha, Qatar In the myocardium, the Na+/H+ exchanger isoform 1 (NHE1), a plasma membrane protein that regulates intracellular pH has been shown to be critical in the development of cardiac hypertrophy. Inhibition of NHE1 activity/expression has previously been demonstrated to produce cardioprotective effects. Recent reports have reported that transgenic mice overexpressing active NHE1 developed spontaneous cardiac hypertrophy in association with elevated levels of osteopontin (OPN). Osteopontin is a secreted protein involved in many physiological pathways including bone formation, immune response, vascularization and cardiac hypertrophy. The physiological pathway in which active NHE1 induces OPN expression in the heart remains unknown. Recently, NHE1 was described as a key regulator of the calcineurin-NFAT pathway. The calcineurin-NFAT pathway is a hypertrophic pathway induced by intracellular increase in calcium concentration that induces NFAT translocation to the nucleus. In the nucleus, NFAT interacts with the transcription factor GATA-4 and together activate hypertrophic gene. Our hypothesis is that NHE1 through the calcineurin/Nfat pathway induces OPN expression leading to cardiac hypertrophy. To verify this hypothesis , rat cardiomyoblasts (H9c2 cells) were infected with activate NHE1 adenovirus in the presence and absence of a calcineurin inhibitor, FK506 . H9c2 cells infected with active NHE1 showed an increase of NHE1 activity (NHE1: 376.5%) and exhibit a 1.6 fold increase in cell area compared to the control, which was attenuated in the presence of FK506. Our preliminary results show that NHE1 activation increases osteopontin protein expression. In addition, H9c2 cells infected with active NHE1 also demonstrate a significant activation of the transcription factor GATA-4 (NHE1: 149% ±28% , p<0.05). The activation of GATA-4 induced by active NHE1 is inhibited by FK506 treatment. In conclusion, NHE1 through the calcineurin/NFAT pathway induces cardiac hypertrophy in H9c2 cells. This new finding will give us a better understanding of signaling pathway mediating NHE1 induced cardiac hypertrophy and allow us to identify alternative therapeutic targets for the treatment heart failure.