Caveolin-1 is essential for activation of Rac1 and NAD(P)H oxidase after angiotensin II type 1 receptor stimulation in vascular smooth muscle cells: Role in redox signaling and vascular hypertrophy

Objective - Angiotensin II (Ang II) is a potent mediator of vascular hypertrophy in vascular smooth muscle cells (VSMCs). These effects are mediated through the Ang II type 1 receptor (AT₁R) and require its trafficking through caveolin-1 (Cav1)-enriched lipid rafts and reactive oxygen species (ROS) derived from Rac1-dependent NAD(P)H oxidase. The specific role(s) of Cav1 in AT₁R signaling is incompletely understood. Methods and Results - Knockdown of Cav1 protein by small interfering RNA (siRNA) inhibits Ang II-stimulated Rac1 activation and membrane translocation, H₂O ₂ production, ROS-dependent epidermal growth factor receptor (EGF-R) transactivation, and subsequent phosphorylation of Akt without affecting ROS-independent extracellular signal-regulated kinase 1/2 phosphorylation. Ang II stimulates tyrosine phosphorylation of Sos-1, a Rac-guanine nucleotide exchange factor, which is inhibited by Cav1 siRNA, demonstrating involvement of Cav1 in Rac1 activation. Detergent-free fractionation showed that EGF-Rs are found basally in Cav1-enriched lipid raft membranes and associate with Cav1. Ang II stimulates AT₁R movement into these microdomains contemporaneously with the egress of EGF-R. Both aspects of this bidirectional receptor trafficking are inhibited by Cav1 siRNA. Moreover, Cav1 siRNA inhibits Ang II-induced vascular hypertrophy. Conclusions - Cav1 plays an essential role in AT₁R targeting into Cav1-enriched lipid rafts and Rac1 activation, which are required for proper organization of ROS-dependent Ang II signaling linked to VSMC hypertrophy.