G-proteins in growth and apoptosis: lessons from the heart.

The acute contractile function of the heart is controlled by the effects of released nonepinephrine (NE) on cardiac adrenergic receptors. NE can also act in a more chronic fashion to induce cardiomyocyte growth, characterized by cell enlargement (hypertrophy), increased protein synthesis, alterations in gene expression and addition of sarcomeres. These responses enhance cardiomyocyte contractile function and thus allow the heart to compensate for increased stress. The hypertrophic effects of NE are mediated through Gq-coupled alpha(1)-adrenergic receptors and are mimicked by the actions of other neurohormones (endothelin, prostaglandin F(2alpha) angiotensin II) that also act on Gq-coupled receptors. Activation of phospholipase C by Gq is necessary for these responses, and protein kinase C and MAP kinases have also been implicated. Gq stimulated cardiac hypertrophy is also evident in transgenic mouse models. In contrast, stimulation of G(s)-coupled beta-adrenergic receptors or G(i)-coupled receptors do not directly effect cardiomyocyte hypertrophy. Apoptosis is also induced by G-protein-coupled receptor stimulation in cardiomyocytes. Sustained or excessive activation of either Gq- or Gs-signaling pathways results in apoptotic loss of cardiomyocytes both in vitro and in vivo. Apoptosis is associated with decreased ventricular function in the failing heart. Cardiomyocytes provide an ideal model system for understanding the basis for G-protein mediated hypertrophy and apoptosis, and the mechanisms responsible for the transition from compensatory to deleterious levels of signaling. This information may prove critical for designing interventions that prevent the pathophysiological consequences of heart failure.