Cell Surface ATP Synthase: A Potential Target for Anti-Angiogenic Therapy - Extracellular ATP and Adenosine as Regulators of Endothelial Cell Function - page 150

elucidated. Other components of the electron transport chain, normally present in
the inner mitochondrial membrane, have also been demonstrated to exist on the cell
surface of eukaryotic cells. Some of the subunits for these complexes are trans-
lated on cytoplasmic ribosomes and targeted to mitochondria, whereas others are
encoded by mitochondrial DNA and synthesized in mitochondria. This suggests
that the complexes are most likely assembled in mitochondria and then transported
to the cell surface.
Yonally and Capaldi propose two potential mechanisms for this trafficking to
the plasma membrane: fusion with the mitochondrial membrane or transport by a
shuttling mechanism, e.g. lipid rafts [53]. This latter hypothesis is supported by the
observation that ATP synthase and other mitochondrial proteins are distributed in
caveolae—specialized lipid rafts [40, 46, 49, 53]. However, Yamamoto et al. demon-
strated that ATP synthase levels on the cell surface are not diminished when caveolae
formation is disrupted by caveolin-1 siRNA or methyl- CD).

Although these findings indicate that transport to the cell surface does not require
lipid rafts, ATP synthase function in shear stress-mediated ATP release is suppressed
when the complex is not localized to caveolae. The authors suggest that the role
of ATP synthase in this pathway might be signal transduction through P2X and/or
P2Y receptors, concentrated in proximity to ATP synthase in caveolae [49]. A recent
publication purports that ATP synthase is in fact not localized in lipid rafts/caveolae,
given that these authors did not observe sensitivity of surface ATP synthase expres-
sion to cholesterol disruption via M

β -cyclodextrin (M β

CD [55]. However, both Yamamoto et al. and
Wang et al. demonstrated that increasing and depleting plasma membrane choles-
terol levels in endothelial cells affects cell surface ATP synthase expression [46, 49].
These different findings have not yet been resolved, but could be due to the different
cell types examined.
Collaborators of our laboratory studying ATP synthase in neural cells propose
that at least the

β

subunit is transported to the cell surface through the secretory path-
way, i.e. moving from the rough endoplasmic reticulum (ER) to the Golgi apparatus
to the plasma membrane via secretory vesicles. Utilizing a neuroblastoma cell line
(B103), they show that inhibiting trafficking from the ER to the Golgi with brefeldin
A prevented the

α

-subunit from reaching the cell surface. Further supporting this
hypothesis, they provide evidence that surface

α

α

-ATP synthase is N-glycosylated
during the secretory process, unlike the mitochondrial form of this protein [37].
Although this is possible for the subunit, which is encoded in nucleic DNA, this

transport process would not account for trafficking of ATP synthase subunits 6 and
8, which are synthesized in mitochondria, to the cell surface. Given the conflicting
evidence provided by various research groups, the question of how the various com-
ponents of ATP synthase, as well as other members of the electron transport chain
traditionally considered mitochondrial proteins, remains unanswered.
Aside from this trafficking mystery, the precise mechanisms by which ATP syn-
thase carries out its other functions in endothelial cells continue to be unresolved
(Fig. 9.8). With regards to its role in angiogenesis, it is unclear whether it promotes
endothelial cell survival and proliferation through pH i regulation, ATP signaling,
and/or another mechanism. If ATP signaling is involved, the purinergic receptors

α