Abstract 289: Retinoblastoma Gene Deletion Promotes Cardiac Dysfunction, Fibrosis and Apoptosis in Response to Pressure Overload

Abstract

The retinoblastoma (Rb) protein is a universal cell cycle regulator in mammals. When the Rb protein is phosphorylated by Cyclins/Cdks, it dissociates from E2F, and Rb-dependent E2F repression is subsequently inactivated. Furthermore, the Rb protein has also been implicated in the regulation of cardiac hypertrophy and apoptosis in cardiomyocytes (CMs). To elucidate the role of Rb in response to mechanical stress, we conducted transverse aortic constriction (TAC) in cardiac-specific Rb knockout mice (cRb-KO) in vivo (C57BL/6J). Cardiac-specific deletion of Rb was achieved by crossing Rb flox/flox mice with αMHC-Cre mice. Under basal conditions, 3- to 5-month-old cRb-KO mice showed increased heart weight (HW) (left ventricular weight/ tibial length (TL): 5.93 ± 029 vs. 4.76 ± 0.14, p< 0.01), increased apoptosis as determined by TUNEL staining (0.12% vs. 0.02%, p< 0.05) and a trend towards cardiac dysfunction (-dP/dt: 4320 ± 388 vs. 5933 ± 489 mmHg/sec, p < 0.05) compared to control mice (Rb flox/flox) Following 2 weeks of TAC, cRb-KO mice showed increased heart weight (HW/TL: 8.58 ± 0.35 vs. 7.50 ± 0.24, p < 0.05), cardiac dysfunction (ejection fraction (EF): 51.1% ± 4.0 vs. 74.3% ± 0.9, p < 0.01) , increased apoptosis as determined by TUNEL staining (0.48% vs. 0.05%, p < 0.01) and increased fibrosis as determined by Masson’s Trichrome staining (1.84% vs. 1.03%, p < 0.05) compared to Rb flox/flox mice after TAC. In response to 4 weeks of TAC, cRb-KO mice showed increased heart weight (HW/TL: 12.93 ± 085 vs. 9.32 ± 0.34, p < 0.01), lung weight (LW) (LW/TL: 18.35 ± 2.66 vs. 10.21 ± 1.93, p < 0.01), cardiac dysfunction (EF: 34.5% ± 8.3 vs. 64.3% ± 8.9, p < 0.01), increased apoptosis as determined by TUNEL staining (0,42% vs. 0,18%, p < 0.05) and increased fibrosis as determined by Masson’s Trichrome staining (4.2 % vs. 1.1 %, p < 0.05) compared to Rb flox/flox mice after TAC. Pressure gradients were similar between the cRb-KO mice submitted to 2 and 4 weeks of TAC and their respective controls. In conclusion, our results suggest that endogenous Rb plays an important role in mediating cell survival in CMs and negatively regulates cardiac hypertrophy at baseline. Furthermore, we showed that the Rb protein is important for the maintenance of cardiac function in response to pressure overload.