Abstract 18528: Knocking out of E2F4 Aggravates Cardiac Function During Pressure Overload
Background: The E2F family constitutes a family of transcription factors involved in the cell cycle. E2F4 is considered to be a repressive E2F due to its lacking the N-terminal domain characteristic of the activating E2Fs, including E2F1, E2F2, and E2F3a. The activating E2Fs are known not only as cell cycle regulators but also as pro-apoptotic factors. Cardiac specific ablation of Rb, a negative regulator of the activating E2Fs, enhances cardiac hypertrophy and failure in response to transverse aortic constriction (TAC). Currently, however, the function of the repressive E2Fs in cardiac hypertrophy and failure is unknown.
Methods: To evaluate the role of E2F4 in regulating cardiac hypertrophy and function during pressure overload (PO), E2F4 knockout (E2F4-/-) and wild type (WT) mice were subjected to TAC for 4 weeks.
Results: In the sham operation group, there was no significant difference in left ventricular (LV) weight/ tibial length (TL) or ejection fraction (EF) between E2F4-/- and WT (LV weight/ TL: E2F4-/-= 5.5±0.2 mg, WT= 5.7±0.3 mg/mm, n.s.; EF: E2F4-/-= 70.3±3.3%, WT= 71.6±2.9%, n.s.; n=5-8). In the TAC group (4 weeks), there was no significant difference in LV weight/ TL between E2F4-/- and WT (E2F4-/-= 8.6±1.1, WT= 8.9±0.6 mg/mm, n.s.; n=4-7), whereas the EF was significantly lower in E2F4-/- than in WT (E2F4-/-= 48.0±2.9%, WT= 64.6±3.4%, p<0.01; n=4-7). In histological analyses, although the cell size of cardiomyocytes in E2F4-/- was not different from that in WT (E2F4-/-= 690±13 μ m2, WT= 637±64 μ m2, n.s.; n-3-4), fibrosis and the number of TdT-mediated dUTP nick-end labeling (TUNEL) positive cells were significantly greater in E2F4-/- than in WT (fibrosis: E2F4-/-= 5.2±1.3%, WT= 0.8±0.6%, p<0.05; TUNEL: E2F4-/-= 0.50±0.06%, WT= 0.07±0.03%, p<0.01; n=3-4). RT-PCR showed that mRNA expression of E2F1 and its target genes that are involved in apoptosis were increased in E2F4-/- compared to in WT (E2F1: 2.5-fold, p<0.05; caspase-8: 2.6-fold, p<0.05), indicating upregulation of the pro-apoptotic mechanism in E2F4-/-.
Conclusions: These results suggest that E2F4 plays an essential role in maintaining cardiac function during pressure overload by negatively regulating the pro-apoptotic mechanisms and suppressing cardiac fibrosis.
- © 2012 by American Heart Association, Inc.