Abstract
OBJECTIVE: Unloading of the rodent heart activates the fetal gene program, decreases peroxisome proliferator-activated receptor alpha (PPARalpha) and PPARalpha-regulated gene expression (MCAD), and induces cardiomyocyte atrophy. NF-kappaB regulates the fetal gene program and PPARalpha-regulated gene expression during cardiac hypertrophy and induces atrophy in skeletal muscle. Our objective was to test the hypothesis that NF-kappaB is the regulator for activation of the fetal gene program, for downregulation of PPARalpha and PPARalpha-regulated gene expression, and for cardiomyocyte atrophy in the heart subjected to mechanical unloading. METHODS: Activation of the inhibitory kappa B kinase beta (IKKbeta)/NF-kappaB pathways were measured in the heterotopically transplanted rat heart using Western blotting of total and phospho-IKKbeta and using transcription factor ELISA's for the five members of the NF-kappaB family (p65 (Rel A), p105/p50, c-Rel, RelB, and p100/p52). In loss of function experiments, we transplanted hearts of p105/p50 knockout mice into wildtype mice and compared changes in gene expression and cardiomyocyte size with wildtype hearts transplanted into wildtype mice. RESULTS: Total and phospho-IKKbeta levels significantly increased in the transplanted heart seven days after surgery. The activation of IKKbeta was paralleled by increased DNA binding activity of p65 and p105/p50. Mechanical unloading induced myosin heavy chain beta expression and decreased cardiomyocyte size in hearts of both wildtype and p105/p050 knockout animals. In contrast, the downregulation of PPARalpha and MCAD was significantly attenuated or prevented in the hearts of p105/p50 knockout mice. CONCLUSIONS: The IKKbeta/p65/p50 pathway is activated in the unloaded rodent heart and a likely regulator for the downregulation of PPARalpha and PPARalpha-regulated gene expression.