The cellular and molecular mechanisms of oxidant-induced cardiomyocyte hypertrophy

Abstract

Cardiac hypertrophy is a common consequence of many cardiovascular diseases. It is often a transition to heart failure. Although oxidants have been indicated to mediate heart failure, whether they induce hypertrophy and how they are able to induce hypertrophy are still unknown. The objective of this dissertation is to investigate the underlying molecular and cellular mechanisms of oxidant-induced heart hypertrophy. My hypothesis is that H₂O₂ induces cardiomyocyte hypertrophy through activating specific signaling pathways. First, signaling pathways that contribute to H₂O₂-induced cell size enlargement were investigated. In vitro kinase assay and Western Blot analysis were conducted to examine activation of PI3K and p70S6K1. We found that H₂O₂ is able to activate PI3K and p70S6K1 in a time- and dose-dependent manner. Inhibitor studies indicate that the activation of PI3K is upstream of p70S6K1. Both PI3K and p70S6K1 inhibitors are able to abolish H₂O₂ induced cell size enlargement and protein content increase, suggesting that this pathway is necessary for H₂O₂ induced hypertrophy. We also tested the activation of three branches of MAPKs: ERK, p38 and JNK. Experiments examining the possible relationship between MAPKs and p70S6K1 revealed that p38 MAPK is able to regulate p70S6K1 in part. Second, signaling pathways related to gene expression alteration associated with hypertrophy induced by H₂O₂ were investigated. This study focused on two important hypertrophy transcription factors, AP-1 and NFAT3. Using both gel shift assay and promoter reporter activation analysis, we found that H₂O₂ is a potent inducer of AP-1. This activation is downstream of ERK. NFAT3 was recently implicated to play a role in cardiac hypertrophy. Whether and how it is activated by H2O 2 is unknown. When cardiomyocytes transfected with pNFAT-luc, a NFAT luciferase plasmid with a luciferase gene under control of repetitive NFAT/AP-1 composite sites derived from the IL-2 promoter, were treated by H₂O₂, luciferase activity increased in a time and dose dependent manner. Experiments using pharmacological inhibitors demonstrated that this NFAT3 activation detected by luciferase assay is ERK and AP-1 dependent but calcineurin independent. This discovery reveals a novel mechanism of NFAT3 regulation in cardiomyocytes that can play an important role in hypertrophic gene regulation.