GATA4 is a survival factor in adult cardiac myocytes but is not required for α1A-adrenergic receptor survival signaling

Recently, we defined an alpha 1A-adrenergic receptor-ERK (alpha 1A-AR-ERK) survival signaling pathway in adult cardiac myocytes. Previous studies in neonatal cardiac myocytes indicated that the cardiac-specific transcription factor GATA4 is a downstream mediator of alpha 1-ERK signaling and that phosphorylation of GATA4 by ERK increases DNA binding and transcriptional activity. Therefore, we examined GATA4 as a potential downstream effector of alpha 1A-ERK survival signaling in adult cardiac myocytes. We measured norepinephrine (NE)-induced cell death in cultured cardiac myocytes lacking alpha 1-ARs (cultured from alpha 1A/B-AR double-knockout mice, alpha 1ABKO mice) that are susceptible to cell death induced by several proapoptotic stimuli, including NE. Our results show that overexpression of GATA4 is sufficient to protect alpha 1ABKO cardiac myocytes from NE-induced cell death. However, we found that the alpha 1A-subtype did not induce phosphorylation or increase the activity of GATA4 in adult mouse cardiac myocytes in culture or in vivo. Furthermore, we examined the effect of siRNA-mediated knockdown of GATA4 on alpha 1A-survival signaling. In alpha 1B-knockout cardiac myocytes, which express only the alpha 1A-subtype and are protected from NE-induced cell death, GATA4 knockdown did not reverse alpha 1A-survival signaling in response to NE. In summary, we found that GATA4 acted as a survival factor by preventing cell death in alpha 1ABKO cardiac myocytes, but GATA4 was not activated by alpha 1-AR stimulation and was not required for alpha 1A-survival signaling in adult cardiac myocytes. This also identifies an important mechanistic difference in alpha 1-signaling between adult and neonatal cardiac myocytes.