An α1A-adrenergic -: Extracellular signal-regulated kinase survival signaling pathway in cardiac myocytes

Background - In alpha 1-AR knockout (alpha 1ABKO) mice that lacked cardiac myocyte alpha 1-adrenergic receptor (alpha 1-AR) binding, aortic constriction induced apoptosis, dilated cardiomyopathy, and death. However, it was unclear whether these effects were attributable to a lack of cardiac myocyte alpha 1-ARs and whether the alpha 1A, alpha 1B, or both subtypes mediated protection. Therefore, we investigated alpha 1A and alpha 1B subtype-specific survival signaling in cultured cardiac myocytes to test for a direct protective effect of alpha 1-ARs in cardiac myocytes. Methods and Results - We cultured alpha 1ABKO myocytes and reconstituted alpha 1-AR signaling with adenoviruses expressing alpha 1-GFP fusion proteins. Myocyte death was induced by norepinephrine, doxorubicin, or H2O2 and was measured by annexin V/ propidium iodide staining. In alpha 1ABKO myocytes, all 3 stimuli significantly increased apoptosis and necrosis. Reconstitution of the alpha 1A subtype, but not the alpha 1B, rescued alpha 1ABKO myocytes from cell death induced by each stimulus. To address the mechanism, we examined alpha 1-AR activation of extracellular signal-regulated kinase ( ERK). In alpha 1ABKO hearts, aortic constriction failed to activate ERK, and in alpha 1ABKO myocytes, expression of a constitutively active MEK1 rescued alpha 1ABKO myocytes from norepinephrine-induced death. In addition, only the alpha 1A-AR activated ERK in alpha 1ABKO myocytes, and expression of a dominant-negative MEK1 completely blocked alpha 1A survival signaling in alpha 1ABKO myocytes. Conclusions - Our results demonstrate a direct protective effect of the alpha 1A subtype in cardiac myocytes and define an alpha 1A-ERK signaling pathway that is required for myocyte survival. Absence of the alpha 1A-ERK pathway can explain the failure to activate ERK after aortic constriction in alpha 1ABKO mice and can contribute to the development of apoptosis, dilated cardiomyopathy, and death.