Linkage of β1-adrenergic stimulation to apoptotic heart cell death through protein kinase A–independent activation of Ca2+/calmodulin kinase II

Abstract

Stimulation of β-adrenergic receptor (βAR), a prototypical G protein-coupled receptor, is broadly involved in metabolic regulation, growth control, muscle contraction, cell survival, and cell death.In the heart, βAR stimulation by catecholamines serves as the most powerful regulatory mechanism to enhance myocardial performance in response to stress or exercise by activating the classic stimulatory pathway comprising the G protein G s , adenylyl cyclase, cAMP, and protein kinase A (PKA) (1, 2).However, sustained activation of β 1 AR, the predominant βAR subtype expressed in the heart, also induces cardiac myocyte apoptosis (3-6).Apoptotic heart cell death has been implicated in the overall process of myocardial remodeling and the transition from cardiac hypertrophy to chronic heart failure (7-10), an illness afflicting more than five million Americans, with a 5-year mortality greater than 80% (11).However, the mechanism underlying the β 1 AR apoptotic effect remains poorly understood.Previous studies suggested that β 1 AR-induced cardiac myocyte apoptosis was mediated by the cAMP/PKA pathway (12, 13), the only known intracellular mechanism underlying β 1 AR-elicited cellular responses (1, 2).However, transgenic overexpression of type V (14) or type VI (15) adenylyl cyclase in mouse hearts does not cause cell death, although it markedly augments basal PKA activity (14) and cardiac contractility (14, 15).More ironically, in cultured cardiac myocytes or in vivo, selective β 2 AR subtype stimulation elicits a profound cardiac protective effect (4, 5, 16), in spite of overtly enhanced cAMP formation (16-18).The goal of the present study is to determine the mechanism of β 1 AR-induced apoptosis.In addition to pharmacological approaches used in WT mouse cardiac myocytes, we created genetically "pure" β 1 AR experimental settings using adult cardiac myocytes from β 2 AR KO mice (19) or by adenovirus-mediated gene transfer (20) of the mouse β 1 AR in myocytes from β 1 β 2 double knockout (DKO) mice (21).These approaches enabled us to avoid the complicated interactions between the