Abstract 19232: The Retinoblastoma Protein is Inactivated in Response to Cardiac Pressure Overload and Regulates Apoptosis and Autophagy in Cardiomyocytes
The retinoblastoma protein (pRb) is a well-known regulator of cell cycle, apoptosis and differentiation. The best studied binding partner of pRb is the E2f transcription factor family. Bound to E2fs, pRb inhibits the ability of E2fs to promote transcription. Here we investigated the role of pRb in mediating growth and death of cardiomyocytes (CMs) in vivo during pressure overload (PO). We first investigated the phosphorylation and activity status of pRb in response to PO. Transverse aortic constriction (TAC) was performed in wild type mice. Since phosphorylation of pRb at Ser780 is known to de-repress E2fs, we conducted Western blot analyses using a specific antibody against pRb phosphorylated at Ser780. TAC for 4 weeks induced a significant increase in Ser780 phosphorylation of pRb compared to sham operation. In order to elucidate the function of endogenous pRb during PO, cardiac specific pRb knockout (cRb-KO) mice were also subjected to TAC. Cardiac hypertrophy (heart weight/tibial length: 12.93 ± 0.85 vs. 9.32 ± 0.34, p < 0.01) and echocardiographically evaluated LV dysfunction (LV ejection fraction: 34.5% ± 8.3 vs. 64.3% ± 8.9, p 8 times) and Bcl-2 (>2.5 times) mRNA, well-known targets of E2f mediated transcription, at baseline and after TAC, suggesting that E2f transcription is stimulated by downregulation of endogenous pRb. cRb-KO mice exhibited an increase in TUNEL-positive nuclei (0,42% vs. 0,18%, p < 0.05) and an increase in cleaved caspase 3 (2.3 fold in response to TAC and 5.9 fold at baseline ) in response to TAC compared to control mice. Furthermore, cRb-KO mice showed decreased levels of LC3 II (1.75 fold) and increased levels of p62 (3.4 fold), indicators of autophagy, suggesting that autophagy is inhibited in cRb-KO mice. In conclusion, these results suggest that pRb is phosphorylated at Ser780 and that E2f-mediated gene expression is enhanced in the heart in response to TAC. pRb plays an important role in suppressing cardiac hypertrophy and LV dysfunction in response to PO, possibly through inhibition of apoptosis and stimulation of autophagy in the heart.
Author Disclosures: P.B. Ammann: None. T. Yamamoto: None. P. Zhai: None. J. Sadoshima: None.
This research has received full or partial funding support from the American Heart Association.
- © 2014 by American Heart Association, Inc.