Background: Pharmacological studies have shown that increasing the activity of Na+/H+ exchanger isoform 1 (NHE1) plays a critical role in the development of cardiac hypertrophy. Despite the importance of NHE1, direct inhibition of NHE1 has demonstrated several adverse side effects. It is has been demonstrated that p90 ribosomal S6 kinase (RSK) regulates the activity of NHE1. RSK, a downstream regulator of the mitogen activated pathway, has also been implicated in cardiac hypertrophy both in in vitro and in vivo models. The interplay between NHE1 and RSK in NHE1 induced cardiomyocyte hypertrophy remains unknown.
Methods and results: H9c2 cardiomyoblasts were infected with active NHE1 adenovirus in the absence and presence of dominant negative (DN) (N-terminal kinase dead protein) RSK adenovirus. H9c2 cardiomyoblasts expressing active NHE1 demonstrated a significant increase in cardiomyocyte hypertrophic parameters including cell area (NHE1 = 170.8±7.38%; P=0.05), protein content (NHE1 = 210±34.47%; P=0.05), NHE1 activity (NHE1=885.5±156.96%; P=0.05) and ANP mRNA expression (NHE1= 176.6 ± 19.39%; P=0.05). These parameters were significantly reduced upon concomitant administration of the DN RSK adenovirus. The NHE1 induced cardiomyocyte hypertrophy was associated with a significant increase in ROS production(NHE1: 127.16±1.2%; P=0.05), phosphorylation and activation of GATA4 (NHE1: 266.56±26.69%; P=0.05), which was significantly reduced upon adding DN RSK to NHE1 infected cells.
Stimulation with phenylephrine (PE) induced a marked increase in phosphorylation and activation of RSK and GATA4, which were reduced upon treatment with NHE1 inhibitor (EMD). Treatment with a RSK inhibitor (BID 1870) caused a marked decrease of GATA4 phosphorylation without affecting ERK phosphorylation following stimulation with PE.
Conclusion: Taken together, our study demonstrates that active NHE1 induces cardiomyocyte hypertrophy.Indirect inhibition of NHE1 by targeting RSK may regress the hypertrophic effect, thus making RSK a potential therapeutic target for alleviating cardiomyocyte hypertrophy.