Abstract P194: CCAAT/Enhancer-Binding Protein Beta Negatively Regulates Hemodynamic Stress-Induced Cardiac Hypertrophy

Abstract

The CCAAT/enhancer-binding proteins (C/EBP) beta is a member of family of leucine-zipper transcription factors that regulate gene expression to control cellular proliferation, differentiation, inflammation and metabolism. Previous reports showed that the protein and mRNA levels of C/EBP beta were increased in rat ischemia-reperfused or human failing hearts. Recent study indicates an important role of C/EBP beta in physiological cardiac hypertrophic responses. In the present study, we generated cardiac-specific C/EBP beta-deficient mice (CKO) to elucidate its in vivo function in pathological cardiac hypertrophy. We crossed floxed C/EBP beta mice with mice expressing Cre recombinase in cardiac-specific manner. Echocardiographic and physiological analyses revealed that CKO showed no cardiac phenotypes under basal conditions at 10 weeks old, indicating that the C/EBP beta is not essential for mouse heart development. Then, we subjected CKO and control mice (CTL) to pressure overload by means of transverse aortic constriction (TAC). In wild-type mouse hearts, the expression level of C/EBP beta was increased after TAC. One week after TAC, CKO showed left ventricle (LV) hypertrophy (LV/body weight, CKO 4.64 ± 0.08 mg/g versus CTL 4.14 ± 0.06 mg/g, p<0.01, LV mass index, CKO 106.1 ± 1.7 versus CTL 98.5 ± 1.8, p<0.05), without showing contractile dysfunction. Cross-sectional area of cardiomyocytes was increased in CKO after TAC (CKO 373.6 ± 2.1 μm 2 versus CTL 300.5 ± 3.6 μm 2 , p<0.01). The increase in the atrial natriuretic factor (ANF) or alpha skeletal actin mRNA expression, molecular markers for cardiac remodeling, was observed in CKO hearts. Furthermore, overexpression of C/EBP beta in isolated neonatal rat cardiomyocytes inhibited an increase in 3 H-leucine uptake induced by phenylephrine stimulation. C/EBP beta is expressed as three distinct protein isoforms, namely FL, LAP, and LIP, which are encoded by a single gene, but transcribed from different initiation sites. Luciferase reporter gene assay showed that overexpression of LAP isoform attenuated the transcriptional activity of ANF induced by phenylephrine, but not LIP. Thus, we conclude that C/EBP beta attenuates the pathological cardiac hypertrophy induced by hemodynamic stresses.