Loss of Smooth Muscle α-Actin Leads to NF-κB-Dependent Increased Sensitivity to Angiotensin II in Smooth Muscle Cells and Aortic Enlargement

Rationale: Mutations in ACTA2, encoding the smooth muscle isoform of alpha-actin, cause thoracic aortic aneurysms, acute aortic dissections, and occlusive vascular diseases. Objective: We sought to identify the mechanism by which loss of smooth muscle alpha-actin causes aortic disease. Methods and Results: Acta2(-/-) mice have an increased number of elastic lamellae in the ascending aorta and progressive aortic root dilation as assessed by echocardiography that can be attenuated by treatment with losartan, an angiotensin II (AngII) type 1 receptor blocker. AngII levels are not increased in Acta2(-/-) aortas or kidneys. Aortic tissue and explanted smooth muscle cells from Acta2(-/-) aortas show increased production of reactive oxygen species and increased basal nuclear factor kappa B signaling, leading to an increase in the expression of the AngII receptor type I a and activation of signaling at 100-fold lower levels of AngII in the mutant compared with wildtype cells. Furthermore, disruption of smooth muscle alpha-actin filaments in wild-type smooth muscle cells by various mechanisms activates nuclear factor kappa B signaling and increases expression of AngII receptor type I a. Conclusions: These findings reveal that disruption of smooth muscle alpha-actin filaments in smooth muscle cells increases reactive oxygen species levels, activates nuclear factor kappa B signaling, and increases AngII receptor type I a expression, thus potentiating AngII signaling in vascular smooth muscle cells without an increase in the exogenous levels of AngII.