Cell Surface ATP Synthase: A Potential Target for Anti-Angiogenic Therapy - Extracellular ATP and Adenosine as Regulators of Endothelial Cell Function - page 148

9.4 Coupling Factor 6

ATP synthase on the surface of endothelial cells has recently been identified as
the receptor for another molecule—coupling factor 6 (CF6). Interestingly, CF6
itself is actually an integral subunit in the stalk of ATP synthase, required for
oxidative phosphorylation [17]. In 1998 this peptide was isolated from cardiac
tissue of spontaneously hypertensive rats and identified as an inhibitor of prosta-
cyclin synthesis. CF6 is thought to act by blocking the Ca 2+ -dependent activity
of phospholipase A 2 , which normally catalyzes the release of the precursor of the
vasodilator prostacyclin—arachidonic acid (AA)—from the plasma membrane [31].
The concentration of this peptide is significantly greater in the plasma of sponta-
neously hypertensive rats than in normotensive control rats (294 ± 126 pg/ml versus

2310 813 pg/ml). Furthermore, intravenous injection of recombinant CF6 results

in decreased plasma prostacyclin levels and increased blood pressure, indicating a
potential role in systemic vasoregulation and hypertension [31]. CF6 is present on
the surface of endothelial cells, from which it is released into systemic circulation.
Expression and release of this peptide are increased in response to shear stress, thus
opposing the synthesis of prostacyclin that is also induced by shear stress [33].
Cell surface ATP synthase serves as at least one receptor for CF6 on endothe-
lial cells. CF6 binds to the

±

subunit—at a distinct location from angiostatin—and
causes increased ATP hydrolysis by ATP synthase. The intracellular pH of HUVEC
also decreases upon treatment with CF6, as ATP hydrolysis is coupled with the
transport of protons from the extracellular milieu into the cytoplasm (Fig. 9.1a).
Pre-treatment of rats with anti-

β

β subunit antibody reduces the CF6-induced rise in

blood pressure by 50%. Evidence supports the involvement of ATP synthase in

the pathway through which CF6 inhibits prostacyclin synthesis. CF6 suppresses the
release of AA in HUVEC, but this effect is reduced by pre-treating with anti-

∼

β
subunit antibody. ATP also stimulates ATP hydrolysis and suppresses AA release,
though to a lesser degree than CF6. This effect is not additive with the suppression
by CF6, indicating that a common signaling pathway is likely. The mechanism by
which CF6 binding to the

subunit of ATP synthase causes inhibition of AA release
remains unclear, but Osanai et al. propose that the decrease in pH induced by CF6
might inhibit the activity of cytosolic phospholipase A 2 , which has been shown to be
pH-sensitive. The P2X and P2Y receptors do not appear to be involved, as treatment
with pyridoxal-phosphate-6-azophenyl-2 ,4 -disulphonic acid (PPAD), an inhibitor
of these receptors, does not prevent the suppression of AA release induced by ATP
[32]. Regardless of the precise post-receptor signaling pathway, CF6 and ATP syn-
thase could play a role in the development or maintenance of hypertension through
effects on prostacyclin levels, which are decreased in many models of hypertension
[47].

β

9.5 Endothelial Monocyte-Activating Polypeptide II

Endothelial monocyte-activating polypeptide II (EMAP II) is a tumor-derived pro-
inflammatory cytokine produced by cleavage of the C-terminus from p43, a protein