Biology Reference
In-Depth Information
contraction (Jho et al.
2005
). However, the detailed mechanisms of angiopoietin-1
action on barrier function remain to be elucidated.
4 The Role of cAMP in the Regulation of Endothelial
Barrier Function
While the study of S1P and angiopoietin-1 as agonists of endothelial barrier
enhancement is just beginning, the role of cAMP and agonists that increase this
second messenger, such as
b
-adrenergic agonists and serotonin, has been studied
extensively (Mehta and Malik
2006
; van Hinsbergh
1997
). In general, cAMP has
been shown to improve barrier function and decrease permeability under both basal
and stimulated conditions in vitro and in vivo. Until recently, it had been accepted
that cAMP improves endothelial barrier function only via activation of PKA
(Mehta and Malik
2006
). PKA has been shown to inhibit thrombin-induced RhoA
activation (Qiao et al.
2003
). RhoA is a member of the superfamily of Rho GTPases
whose activity is determined by the binding of GTP. PKA has been shown to
directly phosphorylate and inhibit RhoA (Lang et al.
1996
). Another possible
mechanism by which PKA inhibits RhoA is via the phosphorylation of GTP
dissociation inhibitor (GDI) (Qiao et al.
2008
). Since RhoA has been implicated
in inhibiting MLC phosphatase, which can result in increased MLC phosphoryla-
tion and contraction, PKA activation would lead to increased phosphatase activity,
decreased MLC phosphorylation, and relaxation. This should improve barrier
function. PKA has also been shown to phosphorylate and inhibit MLCK (Verin
et al.
1998
). By preventing MLC phosphorylation by MLCK, PKA can reduce
endothelial cell contraction. These decreases in contraction caused by PKA pre-
sumably lead to improved tight and adherent junction stabilization, decreased gap
formation, and improved barrier function.
More recently, another cAMP effector molecule, Epac, was also shown to
mediate at least part of the barrier-enhancing actions of cAMP (Fukuhara et al.
2005
; Kooistra et al.
2005
; Wittchen et al.
2005
). Epac1 and Epac2 are guanine
nucleotide exchange factors for the small G protein Rap (Gloerich and Bos
2010
).
The discovery of Epac as a target of cAMP signaling explained the various effects of
cAMP that could not be attributed to PKA and cyclic nucleotide-gated ion channels.
It was demonstrated that Epac can also inhibit thrombin-induced RhoA activation
(Cullere et al.
2005
). Similar to PKA, Epac-mediated inhibition of RhoA activity
presumably leads to decreased MLC phosphorylation, endothelial relaxation, and
increased barrier function. The relative contributions of PKA and Epac in mediating
the barrier-enhancing effects of cAMP are just now beginning to be determined. Of
the early studies describing the role of Epac in mediating the barrier-enhancing
effects of cAMP, only one attempted to rule out the involvement of PKA via the use
of H89, a PKA inhibitor (Fukuhara et al.
2005
). However, H89 has also been shown
to inhibit other kinases including Rho kinase (Leemhuis et al.
2002
; Murray
2008
).
