Mechanical stress-evoked but angiotensin II-independent activation of angiotensin II type 1 receptor induces cardiac hypertrophy through calcineurin pathway

Mechanical stress can induce cardiac hypertrophy through angiotensin II (AngII) type 1 (AT1) receptor independently of AngII, however, the intracellular mechanisms remain largely indeterminate. Since calcineurin, a Ca2+-dependent phosphatase, plays a critical role in pressure overload-induced cardiac hypertrophy, we therefore, asked whether calcineurin is involved in the AT1 receptor-mediated but AngII-independent cardiac hypertrophy. Mechanical stretch failed to elicit hypertrophic responses in COS7 cells co-transfected with plasmid of AT1 receptor and siRNA of calcineurin. Mechanical stresses for 2 weeks in vivo and for 24 h in vitro significantly induced upregulation of calcineurin expression and hypertrophic responses, such as the increases in cardiomyocytes size and specific gene expressions, in cardiomyocytes of angiotensinogen gene knockout (ATG−/−) mice, both of which were significantly suppressed by a specific calcineurin inhibitor FK506, suggesting a critical role of calcineurin in mechanical stress-induced cardiac hypertrophy in the ATG−/− mice. Furthermore, an AT1 receptor blocker Losartan not only attenuated cardiac hypertrophy but also abrogated upregulation of cardiac calcineurin expression induced by mechanical stresses in the AngII-lacking mice, indicating that calcineurin expression is regulated by AT1 receptor without the involvement of AngII after mechanical stress. These findings collectively suggest that mechanical stress-evoked but AngII-independent activation of AT1 receptor induces cardiac hypertrophy through calcineurin pathway.