Akt induces enhanced myocardial contractility and cell size in vivo in transgenic mice

The serine-threonine kinase Akt seems to be central in mediating stimuli from different classes of receptors. In fact, both IGF-1 and IL6-like cytokines induce hypertrophic and antiapoptotic signals in cardiomyocytes through PI3K-dependent Akt activation. More recently, it was shown that Akt is involved also in the hypertrophic and antiapoptotic effects of β-adrenergic stimulation. Thus, to determine the effects of Akt on cardiac function in vivo, we generated a model of cardiac-specific Akt overexpression in mice. Transgenic mice were generated by using the E40K, constitutively active mutant of Akt linked to the rat α-myosin heavy chain promoter. The effects of cardiac-selective Akt overexpression were studied by echocardiography, cardiac catheterization, histological and biochemical techniques. We found that Akt overexpression produced cardiac hypertrophy at the molecular and histological levels, with a significant increase in cardiomyocyte cell size and concentric LV hypertrophy. Akt-transgenic mice also showed a remarkable increase in cardiac contractility compared with wild-type controls as demonstrated by the analysis of left ventricular (dP/dt_max) in an invasive hemodynamic study, although with graded dobutamine infusion, the maximum response was not different from that in controls. Diastolic function, evaluated by left ventricular dP/dt_min, was not affected at rest but was impaired during graded dobutamine infusion. Isoproterenol-induced cAMP levels, β-adrenergic receptor (β-AR) density, and β-AR affinity were not altered compared with control mice. Moreover, studies on signaling pathway activation from myocardial extracts demonstrated that glycogen synthase kinase3-β is phosphorylated, whereas p42/44 mitogen-activated protein kinases is not, indicating that Akt induces hypertrophy in vivo by activating the glycogen synthase kinase3-β/GATA 4 pathway. In summary, our results not only demonstrate that Akt regulates cardiomyocyte cell size in vivo, but, importantly, show that Akt modulates cardiac contractility in vivo without directly affecting β-AR signaling capacity.