Cardiac b2-AR activates both Gs and Gi proteins whereas b1-AR couples only to Gs. The goal of this study is to determine whether 1-AR and 2-AR differ in regulating cardiomyocyte survival and apoptosis, if so, to explore underlying mechanisms. To avoid complicated crosstalks between -AR subtypes, we express b1-AR or b2-AR individually in the null background of b1b2double knockout mouse cardiomyocytes using adult mouse myocyte culture and adenoviral gene transfer techniques. Stimulation of b1-AR, but not b2-AR, markedly induces myocyte apoptosis, as indicated by increased TUNEL or Hoechst staining positive cells and DNA fragmentation. Inhibition Gi signaling with pertussis toxin converts b2-AR to b1-AR in terms of its apoptotic effect, suggesting that Gi is essential for b2-AR-mediated survival effect. To explore the downstream signaling events of b2-AR-coupled Gi, we first examined the possible involvement of p38 MAPK, since recent studies propose that p38 MAPK underlies Gi-dependent anti-apoptotic effects. We found that although stimulation of either b-AR subtype increases p38 MAPK activity, this effect is insensitive to PTX, excluding a role of p38 MAPK in b2-AR-mediated cell survival. In contrast, b2-AR (but not b1-AR) elevates the activity of Akt, a powerful survival signal; this effect is fully abolished by inhibiting Gi with pertussis toxin, scavenging Gbg with ARK-ct, or blocking PI3K with LY294002, indicating that b2-AR activates Akt via a Gi -PI3K pathway. Most importantly, inhibition of the Gi-Gbg-PI3K-Akt pathway converts b2-AR signaling from survival to apoptotic. Thus, b2-AR, unlike b1-AR, activates concurrent apoptotic and survival signals in cardiomyocytes, and the survival effect is mediated by the Gibg-PI3K-Akt pathway. The strikingly different effects of b-AR subtypes on cardiac cell survival and apoptosis may have important pathophysiological and therapeutic implications in chronic heart failure.
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