Metoprolol increases the expression of β3-adrenoceptors in the diabetic heart: Effects on nitric oxide signaling and forkhead transcription factor-3

We have previously shown that the β-blocking drug metoprolol improves cardiac function in the streptozotocin-diabetic rat partly by inducing parallel improvements in cardiac metabolism and gene expression. β-blockers have been previously reported to increase the expression of β1 and β2-adrenoceptors, but their effects on the expression of β3-adrenoceptors are unknown. The aim of the present study was to investigate whether metoprolol increases β3-adrenoceptor expression and downstream Akt-mediated signaling. Left ventricular function was measured in paced isolated working hearts. β1, β2 and β3 adrenoceptor-expression levels were measured using Western blotting. Protein kinase A (PKA) and calcium/calmodulin dependent protein kinase II (CAMK-II) activities, as well as Akt phosphorylation, were measured as indices of downstream target activation. Chronic metoprolol treatment improved cardiac function and produced a marked increase in the expression of all three β-adrenoceptor subtypes which was associated with a decrease in PKA activity and an increase in Akt phosphorylation. Akt-mediated phosphorylation of endothelial nitric oxide synthase (eNOS) was not altered, but phosphorylation of the transcription factor FOXO-3 was increased. Metoprolol increased the expression of β1, β2 and β3 adrenoceptors, associated with repression of FOXO-3 expression. β-adrenoceptor signaling shifted from PKA to Akt-mediated signaling, associated with phosphorylation of FOXO-3 but not eNOS.