Vascular Endothelium - Tissue Functioning and Remodeling in the Circulatory and Ventilatory Systems - page 522

Small Rho GTPase and its effector RoCK kinase are implicated in the formation
of stress fibers from bundles of actin microfilaments and myosin-2. Kinase RoCK
is activated by binding to RhoA GTP . Small RhoA GTPase also regulates actin
polymerization by its effectors diaphanous protein (Dia) and RoCK kinase [ 854 ].
Effectors Dia and RoCK activate LIM kinase that phosphorylates (inactivates) actin-
severing cofilin. In addition, RoCK also activates crosslinkers ezrin-radixin-moesin
between cortical actin and plasma membrane. On the other hand, P21-activated
kinase that is an effector of both small GTPases CDC42 and Rac, not only inhibits
MLCK, but also phosphorylates MLC independently of MLCK kinase.
Actin crosslinkers of the family of actin-binding proteins, such as filamin and
spectrin, stabilize cortical actin and control organization and distribution of actin
filaments at intercellular junctions [ 854 ]. They then regulate endothelial barrier. In
endothelial cells, ligation of spectrin with band-4.1 is required for TRPC4-induced
Ca 2 + entry. Filamin also links to E-selectin upon leukocyte binding to the luminal
surface of endothelial cells.
Other actin-binding proteins, such as cortactin and WASP, bind to the heptameric
ARP2-ARP3 complex. They not only foster actin polymerization, but also partic-
ipate in the assembly of adherens junctions and focal adhesions, as ARP2-ARP3
interacts with E-cadherin and vinculin. Phosphorylation of WASP by focal adhesion
kinase prevents its nuclear translocation and thus prolongs WASP-induced actin
polymerization caused by CDC42 to restore barrier function.
Vasodilator-stimulated phosphoprotein (VASP) cooperates with Rac1, as it facil-
itates Rac1 activation downstream from PKA and PKG, for the maintenance of the
endothelial barrier. Furthermore, it modulates the activity of capping (e.g., gelsolin)
and actin-binding proteins (e.g., profilin).
In endothelial cells, VASP is phosphorylated by cAMP- and cGMP-dependent
protein kinases PKA (Ser157) and PKG (Ser239), as well as by AMP-activated
protein kinase (AMPK; Thr278) [ 958 ]. Phosphorylation of VASP (Ser157) causes
VASP localization to the cell cortex, but has a minor impact on F actin assembly.
Upon phosphorylation by PKA, VASP lodges in adhesion complexes with occludin,
zonula occludens protein ZO1, and junctional adhesion molecule JAM1 to stabilize
cell junctions. Phosphorylation of VASP (Ser239 or Thr278) impedes actin filament
formation.
Synthesis of VASP is regulated by several transcription factors. During hypoxia,
HIF1 binds to the VASP promoter [ 958 ]. Inflammatory cytokines, such as tumor-
necrosis factor- α and interleukin-1 and -6 provoke NF κ B binding to the VASP

promoter and downregulate Vasp gene expression.

α 2B β 3 -integrin is impeded by tyrosine phosphorylation of the β 3 -integrin tail. On the other hand,

γ

Src kinase activates PLC 2 and provokes MLC phosphorylation.