Abstract P221: Retinoblastoma Protein--Associated Proteins 48 and 46 Are Novel p300/GATA4-Binding Partners Involved in Hypertrophic Responses in Cardiomyocytes

Abstract

Background: A zinc finger protein GATA4 is one of hypertrophy-responsive transcription factors, and increases its DNA-binding and transcriptional activities in response to hypertrophic stimuli in cardiomyocytes. Activation of GATA4 during this process is mediated, in part, through acetylation by intrinsic histone acetyltransferases such as a transcriptional coactivator p300. Here, we show that retinoblastoma protein (Rb)-associated protein 48 and 46 (RbAp48, RbAp46), components of NuRD (nucleosome remodeling and deacetylase) complex that has been implicated in chromatin remodeling and transcriptional repression associated with histone deacetylation, are novel components of p300/GATA4 complex. However, the precise functional relationships among p300, GATA4, RbAp48, and RbAp46 remain unknown.

Methods and Results: A series of GST pull-down assays revealed that the C-terminal domain of RbAp48/46 bound to the N-terminal transcriptional activation domain of GATA4 and C/H-3 domain of p300, respectively. Immunoprecipitation followed by western blotting demonstrated that RbAp48/46 repressed p300-induced acetylation of GATA4 and histones. While overexpressions of RbAp48/46 inhibited p300/GATA4-induced atrial natriuretic factors (ANF) and endotheline-1 (ET-1) promoter activities, knockdown of neither RbAp48 nor RbAp46 by RNAi enhanced these promoter activities in HEK293 cells. Stimulation of cardiomyocytes with phenylephrine (PE) decreased the binding of GATA4/p300 with RbAp48/46. RbAp48/46 repressed PE-induced hypertrophic responses such as myofibrillar organization, increase in cell size and promoter activation of the ANF and ET-1 in cardiomyocytes.

Conclusion: These findings demonstrate that RbAp48 and RbAp46 form a functional protein complex with GATA4/p300 and regulated hypertrophic responses in cardiomyocytes.