Modification of β-adrenoceptor signal transduction pathway by genetic manipulation and heart failure

The beta -adrenoceptor (beta -AR) mediated signal transduction pathway in cardiomyocytes is known to involve beta (1)- and beta (2)-ARs, stimulatory (G(s)) and inhibitory (G(i)) guanine nucleotide binding proteins, adenylyl cyclase (AC) and cAMP-dependent protein kinase (PKA). The activation of beta (1)- and beta (2)-ARs has been shown to increase heart function by increasing Ca2+-movements across the sarcolemmal membrane and sarcoplasmic reticulum through the stimulation of G(s)-proteins, activation of AC and PKA enzymes and phosphorylation of the target sites. The activation of PKA has also been reported to increase phosphorylation of some myofibrillar proteins (for promoting cardiac relaxation) and nuclear proteins (for cardiac hypertrophy). The activation of beta (2)-AR has also been shown to affect G(i)-proteins, stimulate mitogen activated protein kinase and increase protein synthesis by enhancing gene expression. beta (1)- and beta (2)-ARs as well as AC are considered to be regulated by PKA- and protein kinase C (PKC)-mediated phosphorylations directly; both PKA and PKC also regulate beta -AR indirectly through the involvement of beta -AR kinase (beta ARK), beta -arrestins and G(beta gamma)-protein subunits. Genetic manipulation of different components and regulators of beta -AR signal transduction pathway by employing transgenic and knockout mouse models has provided insight into their functional and regulatory characteristics in cardiomyocytes. The genetic studies have also helped in understanding the pathophysiological role of beta ARK in heart dysfunction and therapeutic role of beta ARK inhibitors in the treatment of heart failure. Varying degrees of defects in the beta -AR signal transduction system have been identified in different types of heart failure to explain the attenuated response of the failing heart to sympathetic stimulation or catecholamine infusion. A decrease in beta (1)-AR density, an increase in the level of G(i)-proteins and overexpression of beta ARK are usually associated with heart failure; however, these attenuations have been shown to be dependent upon the type and stage of heart failure as well as region of the heart. Both local and circulating renin-angiotensin systems, sympathetic nervous system and endothelial cell function appears to regulate the status of beta -AR signal transduction pathway in the failing heart. Thus different components and regulators of the beta -AR signal transduction pathway appears to represent important targets for the development of therapeutic interventions for the treatment of heart failure.