Cell Surface ATP Synthase: A Potential Target for Anti-Angiogenic Therapy - Extracellular ATP and Adenosine as Regulators of Endothelial Cell Function - page 143

and nuclear protein kinases-I and -II [23, 30, 34, 36]. Given this lack of specificity
and its potential effects on oxidative phosphorylation, sangivamycin is not an ideal
candidate for use as an anti-angiogenic therapeutic.
Two additional research groups reported developing monoclonal antibodies
against the

subunit of ATP synthase in 2008. Zhang et al. reported inhibition of
cell surface ATP synthesis and endothelial cell tube formation with the monoclonal
antibody McAb178-5G10. In addition to blocking endothelial cell proliferation and
migration in vitro, this antibody decreased proliferation of MDA-MB-231 breast
cancer cells, which also express cell surface ATP synthase [54]. Most recently, the
Fan laboratory produced the murine monoclonal antibody mAb6F2C4 and demon-
strated its potential as a cancer therapeutic in vivo. Like McAb178-5G10, this
antibody decreases endothelial cell growth and tube formation, as well as prolifera-
tion of an ATP synthase-expressing hepatoma cell line (SMMC-7721). Furthermore,
intra-tumoral injection of this antibody every three days inhibits in vivo growth
of SMMC-7721 flank tumors in nude mice. Although the authors show in vitro
binding of the antibody to endothelial cells, immunohistochemical staining of the
tumors demonstrates mAb6F2C4 binding to the tumor cells only, with no apparent
vascular staining. Despite this puzzling lack of staining, the tumor growth delay
is likely attributable to a combination of the direct anti-tumorigenic effects of the
antibody and its anti-angiogenic effects [45]. The direct inhibitory effect of these
two monoclonal antibodies on tumor growth also mimics angiostatin, since angio-
statin is cytotoxic under acidic conditions to tumor cells expressing cell surface
ATP synthase [13]. Monoclonal antibodies targeting ATP synthase provide several
advantages over the use of angiostatin as a cancer therapeutic. They have increased
specificity and binding affinity for the target, limited side effect profiles, and a longer
serum half-life.
The potential for stimulation of angiogenesis by extracellular production of
ATP points to one potential mechanism by which targeting the enzymatic activ-
ity of ATP synthase can inhibit angiogenesis. A recent report demonstrates that
extracellular ATP induces a pro-angiogenic effect on vasa vasorum endothelial
cells (VVEC) isolated from pulmonary artery adventitia [18]. These cells demon-
strated a dose-dependent increase in proliferation in response to exogenous addition
of ATP. Additionally, they were stimulated to migrate and to form endothelial
cell tube networks in the presence of 100

β

μ

M ATP. Thus, inhibition of extra-
cellular ATP production by ATP synthase may reduce pro-angiogenic purinergic
signaling.

9.2.3 pH Regulation: Potential Mechanism for the
Anti-Angiogenic Effect of ATP Synthase Inhibition

Inhibition of ATP synthase disrupts translocation of protons out of the cell and
decreases extracellular ATP production. Therefore, the anti-angiogenic effect of
blocking this enzyme is likely due to changes in intracellular pH regulation and/or