Biology Reference
In-Depth Information
6 Endothelial Cell Cyclic Nucleotide Phosphodiesterases
While PKA, Epac, PKG, and cyclic nucleotide-gated channels (CNG) play critical
roles in mediating the effects of cAMP and cGMP in endothelial cells, the role of
cyclic nucleotide phosphodiesterases (PDEs) is emerging as an equally important
player in endothelial function. In addition to regulating the amplitude, duration, and
compartmentalization of cyclic nucleotides, PDEs are also effector molecules of
cyclic nucleotides and calcium, making them critical mediators of crosstalk
between various second messengers. Endothelial cells have been shown to primar-
ily express cGMP-stimulated PDE2, cGMP-inhibited PDE3, cAMP-specific PDE4,
and cGMP-specific PDE5 (Netherton and Maurice
2005
). The expression of Ca
2+
/
CaM-regulated PDE1 and the cAMP-specific PDE7A has also been described
(Keravis et al.
2000
; Miro et al.
2000
). While these PDEs have been shown to be
expressed in the endothelium, significant differences in their relative expression
levels have been found in endothelial cells of different origin. The relative expres-
sion and PDE activity levels in bovine aortic endothelial cells (BAEC), human
aortic endothelial cells (HAEC), human umbilical vein endothelial cells (HUVEC),
and human microvascular endothelial cells (HMVEC) were determined, and large
differences in the expression levels of PDE2, PDE3, PDE4, and PDE5 were found
among these cells (Netherton and Maurice
2005
). For example, measuring PDE
activity at a substrate concentration of 1
m
M cAMP, it was shown that PDE3
accounted for 15 and 36% of the cAMP activity in BAEC and HAEC, respectively,
while only 7 and 6% of the cAMP activity in HUVEC and HMVEC, respectively.
Differences were also seen at the protein level by immunoblot analysis. In addition
to differences in PDE expression between endothelial cells of different origins,
there have been differences reported in endothelial cells over time in culture. For
example, early passage BAEC (4-6) expressed PDE2 and PDE5 cGMP hydrolytic
activity, which were lost with passage (
10) (Ashikaga et al.
1997
). As for cAMP,
PDE2 and PDE4 cAMP hydrolytic activity was found in early passage BAEC that
later become predominantly PDE4 at later passages. Furthermore, there are
reported differences in PDE expression in endothelial cells of different phenotypes.
Confluent and resting endothelial cells take on a cobblestone-shaped morphology,
while proliferating, noncontact-inhibited endothelial cells appear elongated and
spindle-like. The PDE profile of BAEC has been characterized in these two
different states (Keravis et al.
2000
). While resting cobblestone BAEC expressed
PDE2 and PDE4 cAMP hydrolytic activity, measured at a substrate concentration
of 1
m
M cAMP, spindle-shaped proliferating BAEC expressed increased cAMP
hydrolytic activity due to increased PDE2 and PDE4 activity as well as PDE1
and PDE3 activity. cGMP-PDE activity was primarily due to PDE2 in resting
cobblestone BAEC, while spindle-shaped proliferating BAEC expressed both
PDE2 and PDE5. Finally, various stimuli can alter PDE expression in endothelial
cells. The inflammatory cytokine, TNF-
a
, shown to increase endothelial expression
of various adhesion molecules and decrease barrier function has also been shown
to upregulate the expression of PDE2, PDE4, and PDE7 in endothelial cells
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