Abstract 341: Both Beta-1 and Beta-2 Adrenergic Receptors (ARs) are Required for Pressure Overload Cardiac Hypertrophy
In isolated myocytes, hypertrophy induced by norepinephrine is mediated via β-ARs, however, in vivo, mice with deletions of both major cardiac β-ARs still develop hypertrophy with pressure overload. The mechanism by which the heart adapts to pressure overload, producing either adaptive or maladaptive remodeling is still not completely understood. To study the role of β-ARs in pressure overload hypertrophy, we performed transverse aortic constriction (TAC) in congenic mice with targeted deletions of β1, β2 and both β1 and β2-ARs and in sham controls. After 3 wks, β1−/− mice showed a 21% increase in heart weight to body weight ratio (HW/BW) vs. sham, similar to WT (HW/BW 5.02 ± 0.72 for β1−/− vs. 5.20 ± 0.92 for WT). β2−/− mice showed an exaggerated (49%) hypertrophic response (HW/BW 5.81 ± 0.53, p < 0.001 vs. WT). Only when both β-ARs were ablated was hypertrophy fully attenuated: in β1β2−/− mice HW/BW increased only 8% (HW/BW 4.30 ± 0.31, p < 0.01 vs. WT). Echocardiography showed that peak band gradient was not different between groups (WT 45.3 ± 4.1, β1−/− 47.2 ± 10.2, β2−/− 49.0 ± 9.7, β1β2−/− 53.2 ± 11.3 mmHg) and all groups maintained normal LV function. Morphometric analysis confirmed the absence of hypertrophy in the β1β2−/−: mean cross-sectional area for WT was 254.7 ± 34.9 vs. β1β2−/− 115.8 ± 16.7μm2, which was not different from sham. Gene microarray analysis detected a set of genes which were differentially expressed in β1β2−/− vs. WT, β1−/−, or β2−/−:
S100 calcium binding protein A9/calgranulin B (S100a9, 4.5-fold up);
Cyclin-dependent kinase inhibitor 1A/P21 (Cdkn1a, 3.8-fold up);
Metallothioneins Mt1 (3-fold up) and Mt2 (2.7-fold up);
FK506 binding protein 5, a glucocorticoid receptor-regulating co-chaperone and calcineurin inhibitor (3.2-fold up).
In contrast, TGFβ2 was upregulated in WT, β1−/− and β2−/− but not in β1β2−/−. Differentially regulated genes were validated by SYBR QRT-PCR on the same RNA samples. Thus, β2-AR signaling may serve to limit the hypertrophic response to pressure afterload. However, both β-ARs are required for the development of a normal hypertrophic response. Ablation of both β-AR subtypes alters expression of several genes, some of which may be critical to the hypertrophic program.