Abstract 3472: Endogenous Muscle Atrophy F-box Mediates Pressure Overload Induced Cardiac Hypertrophy via Activation of NF-κB
Muscle atrophy F-box (MAFbx, Atrogin-1) is an E3 ubiquitin ligase, which plays a critical role in mediating skeletal muscle atrophy. Gene deletion of MAFbx causes enhancement of cardiac hypertrophy in response to exercise, suggesting that MAFbx negatively regulates physiological hypertrophy in the heart. To examine the role of endogenous MAFbx in regulating cardiac hypertrophy in response to pathological stimuli, transverse aortic constriction (TAC) was applied for 2 weeks to MAFbx−/− and wild type (WT) mice. TAC-induced increases in left ventricular weight/body weight (LVW/BW) were significantly smaller in MAFbx−/− than in WT mice (4.49, 5.68, p<0.05). Interstitial fibrosis was significantly less in MAFbx−/− than in WT mice (7.7, 13.7 p<0.05). Thus, the absence of endogenous MAFbx attenuated pathological hypertrophy by pressure overload. To elucidate the mechanism by which MAFbx regulates pathological hypertrophy, DNA microarray analysis was conducted using heart samples obtained from MAFbx−/− and WT mice with or without TAC. Comparisons of gene expression patterns between WT-Sham vs WT-TAC and WT-TAC vs MAFbx KO-TAC revealed that genes upregulated by TAC in WT are inversely regulated by MAFbx KO. Using transcription factor binding site analysis, the consensus binding sequence for c-Rel, a member of the NF-κB family, was frequently found in the promoter region of the genes whose upregulation was suppressed in MAFbx KO after TAC, suggesting that NF-κB may be inhibited by the lack of MAFbx. Although the level of IκB was significantly decreased after TAC in WT hearts, it was increased 1.49 fold in MAFbx−/−hearts after TAC. To determine whether MAFbx could regulate the activity of NFκ-B, neonatal myocytes were transfected with NFκ-B binding sites-luciferase (Luc) reporter gene and siRNA targeting MAFbx, and stimulated with phenylephrine (PE) for 24 hours. PE-induced activation of NFκ-B mediated transcription was significantly suppressed by the MAFbx knock-down, consistent with the notion that MAFbx positively regulates NF-κB. In conclusion, endogenous MAFbx plays an essential role in mediating cardiac hypertrophy in response to pressure overload through downregulation of IκB and subsequent activation of NFκ-B.