Figure 2.

Mechanism for shear stress–induced activation of eNOS- and NO-mediated relaxation of vascular smooth muscle cells (VSMCs). A complex consisting of PECAM-1, VE-cadherin, and VEGFR-2 localized at cell-cell junctions serves as a mechanotransducer for shear stress. The tension exerted by shear stress on PECAM-1 activates a Src family kinase, resulting in ligand-independent transactivation of VEGFR-2, followed by activation of Akt (also known as protein kinase B), phosphorylation of serine 1177 (S1177) of eNOS, and production of NO. Activation of VEGFR-2 can also cause S1177 phosphorylation via PKA. Heat shock protein 90 (Hsp90), when associated with eNOS, can recruit Akt into the eNOS complex and maintains Akt activity. In contrast to S1177, the phosphorylated threonine 495 (T495) of eNOS exerts inhibitory effect on the enzyme. T495 can also be phosphorylated by PKC. Shear stress may enhance eNOS activity by inhibiting PKC activity. NO released from the endothelial cells (ECs) can readily diffuse into the underlying VSMCs, where it activates sGC, resulting in the conversion of guanosine triphosphate (GTP) to cGMP (cyclic guanosine monophosphate). The elevated cGMP causes vasodilatation, primarily via PKG, by reducing the intracellular Ca²⁺ concentration and decreasing the sensitivity of myofilaments to Ca²⁺. PKG stimulates Ca²⁺-activated K⁺ (BK_Ca) channels, resulting in membrane hyperpolarization and suppression of Ca²⁺ entry. PKG also reduces the release of Ca²⁺ from sarcoplasmic reticulum (SR) into cytosol by inhibiting IP₃ receptor (IP₃R) activity. The decreased cytosolic Ca²⁺ concentration leads to diminished activity of myosin light-chain kinase (MLCK), decreased phosphorylation of myosin light chain (MLC), and reduced contractility. PKG causes reduced Ca²⁺ sensitivity of myofilaments by directly stimulating the activity of myosin light-chain phosphatase (MLCP) as well as interfering with the inhibitory action of RhoA/Rho kinase (ROK) signaling on MLCP. These actions leads to increased dephosphorylation of MLC and thus relaxation of VSMCs. Solid arrows indicate activation and dashed arrows inhibition. eNOS: endothelial nitric oxide synthase; IP₃R: inositol 1,4,5-trisphosphate receptor; L-Arg: l-arginine; NO: nitric oxide; PECAM-1: platelet endothelial cell adhesion molecule; PKA: protein kinase A; PKC: protein kinase C; PKG: cGMP-dependent protein kinase; sGC: soluble guanylyl cyclase; Src: tyrosine-protein kinase CSK; VGCC: voltage-gated Ca²⁺ channel; VE-cad: vascular endothelial cadherin; VEGFR-2: vascular endothelial growth factor receptor 2.