Beta-adrenergic receptor (beta-AR) stimulation serves as the most powerful means to regulate energy metabolism and cardiac performance. Both beta1-AR and beta2-AR, the closely related beta-AR subtypes, are expressed in mammalian hearts, but they display subtype-specific G protein coupling and functions. While beta1-AR couples to Gs, beta2-AR couples dually to the Gs and pertussis toxin (PTX)-sensitive Gi proteins with the Gi coupling negating the Gs-mediated contractile response and promoting cell survival in the heart. It has been shown that phosphorylation of beta2-AR by PKA promotes the receptor-Gi coupling. However, the mechanism inhibiting the beta2-AR-Gi coupling is poorly understood. Here, we define regulator of G protein signaling 2 (RGS2) as a novel negative regulator of the beta2-AR-Gi coupling. First, while beta1-AR induced contractile response remained intact, beta2-AR mediated contractile response was profoundly sensitized and resistant to PTX in adult mouse cardiomyocytes cultured for 24 h, in the absence of alterations in the expression of G proteins or the density of either beta-AR subtype, indicating the beta2-AR-Gi coupling is severely impaired in cultured cells. Second, RGS2 protein was selectively elevated in cultured cells without changes in other major cardiac RGS proteins, including RGS3, RGS4, RGS5, RGS10 and GAIP. Third, both upregulation of RGS2 and beta2-AR-Gi dysfunction were fully prevented by beta-AR stimulation with isoproterenol (1.0 nM) during cell culture, suggesting that the beta2-AR-Gi signaling is mediated by the receptor activation-induced downregulation of RGS2. This conclusion is corroborated by the fact that gene-targeted knockout of RGS2 sustained the beta2-AR-Gi signaling in cultured cardiomyocytes even in the absence of beta-AR agonist stimulation. In sharp contrast, RGS2 adenoviral overexpression abrogated the agonist-induced beta2-AR-Gi signaling in cultured cardiomyocytes. Thus, RGS2 functions as a powerful negative regulator of the beta2-AR-coupled Gi signaling, sensitizing beta2-AR-mediated contractile response in mammalian hearts.
