Editorial Commentary
Vasoconstriction Caused by the ATP Synthase
Subunit–Coupling Factor 6
A New Function for a Historical Enzyme
Stephanie W. Watts
E
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nhancement of arterial vasoconstriction and depression of endothelial cell– dependent relaxation have
been reported by many investigators as characteristics of vascular dysfunction in hypertension.1 These
dysfunctions support the increase in total peripheral resistance observed in experimental and human forms of
hypertension. Because arterial smooth-muscle cell hyperreactivity and reduced vasoactive function of the endothelial cell are observed in response to multiple stimuli (eg,
serotonin, norepinephrine, KCl in smooth muscle; acetylcholine, bradykinin, A23187 in endothelial cell) and are
thus not necessarily agonist-specific, researchers have
investigated the idea that more general mechanisms of
signal transduction are altered and support global changes
in vascular function. These include alterations in composition and function of elements as diverse as potassium
channels, calcium channels, sodium-potassium ATPase,
G-proteins, membrane lipid composition, etc. In this issue
of Hypertension, the study by Osanai et al2 reveals a new
function for a protein subunit of an enzyme that is
ubiquitously expressed in all tissues, the ATP synthase.
oligomycin sensitivity-conferral protein, subunit b, subunit
d, and coupling factor 6 (CF6).4
Functions of CF6: Old and New
CF6 is synthesized in an immature form in the cell cytosol as
a 108-amino acid peptide and is led to the mitochondria
where a 32-amino acid signal peptide is cleaved, forming the
mature 78-amino acid peptide (⬇9 kDa in Western analyses)
that is essential for energy transduction through the ATP
synthase.5 Half a decade of studies have suggested that this
protein has a function in the cardiovascular system previously
unappreciated. Researchers have demonstrated that the ATP
synthase, of which CF6 is a part, is a receptor for angiostatin
and HDL (6,7; Figure 2).
The models used in the preceding and present studies that
focus on CF6 include human umbilical vein endothelial cells
(HUVECs), rat vascular endothelial cells, and the Wistar
Kyoto (WKY) and spontaneously hypertensive rat (SHR).
The first line of evidence that CF6 was potentially important
to the cardiovascular system was discovery of the localization
of CF6 to the plasma membrane of endothelial cells using a
rabbit-raised antibody8; this and other important pieces of
data suggested the presence of the CF6/ATP synthase in a site
other than the mitochondria. CF6 is capable of being shedded
or released. Several lines of evidence support this, including
measurement of CF6 in the media of HUVEC and rat
vascular endothelial cells8,9 and measurement of circulating
CF6 in rats and humans.9,10 Circulating levels of CF6 in the
normal rat is ⬇0.1 nM. Figure 2 depicts a current understanding of potential physiological regulators of endothelial cell
CF6.
The story of the cardiovascular function of CF6 begins
with the observation that a peptide extracted from the
hearts of SHR reduced prostacyclin synthesis.11 This
protein was isolated and identified as CF6. Recombinant
CF6 was made, and it reduced baseline and bradykininstimulated prostacyclin synthesis as well as arachidonate
release when used in HUVECs (1 to 100 nM CF6). Further
work has suggested that CF6 does not inhibit prostacyclin
synthase directly but may inhibit activation of calciumdependent phospholipase A2.11 The mechanism by which
this occurs may be through a reduction in intracellular pH
of endothelial cells; this activity is one focus of the study
by Osanai et al.2
Two lines of evidence suggest that CF6 has physiological
function. First, administration of recombinant CF6 to WKY
and SHR increased blood pressure of ketamine/xylazineanesthetized rats. Administered in the left femoral vein,
Historic Function of ATP Synthase
Figure 1 depicts our classic understanding of the ATP
synthase. This ancient enzyme is present in the inner
membrane of mitochondria and is responsible for generation of ATP. Synthesis of ATP is generated by the
movement of hydrogen ions from the intermembrane space
into the matrix of the mitochondria through the transmembrane H⫹ carrier (Fo) with subsequent movement of the Fi
portion of the enzyme that coordinates the interaction of
the subunits and ATP synthesis.3 This powerhouse of an
enzyme can synthesize more than 100 molecules of ATP
per second; this enzyme can also function in the reverse
direction, hydrolyzing ATP. The H⫹ channel and the
enzymatic portion of the ATP synthesis are connected by a
protein stalk. This stalk is composed of 4 subunits: the
The opinions expressed in this editorial are not necessarily those of the
editors or of the American Heart Association.
From the Department of Pharmacology and Toxicology, Michigan
State University, East Lansing.
Correspondence to Stephanie W. Watts, B445 Life Sciences Building,
Department of Pharmacology and Toxicology, Michigan State University, East Lansing, 48824-1317. E-mail [email protected]
(Hypertension. 2005;46:1100-1102.)
© 2005 American Heart Association, Inc.
Hypertension is available at http://www.hypertensionaha.org
DOI: 10.1161/01.HYP.0000186476.62399.6c
1100
Watts
New Function for Old Protein: CF6
1101
ical activity because administration of the ␤-subunit antibody
dampened the function of CF6. Importantly, activity was not
obliterated by the antibody, suggesting there may be other
potential sites of action for CF6.
Unanswered Questions
Figure 1. Depiction of the classic function of ATP synthase in
the mitochondria, an ATP generating machine.
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recombinant CF6 (0.1 to 1 ␮g/kg) increased blood pressure a
maximum of 5 mm Hg in WKY and 11 to 12 mm Hg in SHR.
This blood pressure reduction was virtually abolished by
indomethacin. Second, administration of an antibody to CF6
modestly reduced blood pressure of WKY (7 mm Hg),
whereas it reduced blood pressure of SHR over 30 mm Hg.9
The CF6 antibody potentiated the depressor effects of bradykinin infusion in SHR but not in WKY. These studies suggest
an increased sensitivity to CF6 in hypertension and potentially a greater function of CF6 in hypertension. This idea is
supported by findings that circulating levels of CF6 in SHR
were greater than in WKY. Importantly, CF6 plasma levels
were greater in essential hypertensive humans compared with
age-matched normotensive individuals.10 An interesting note
is that vitamin C normalized levels of circulating CF6 in
hypertensive individuals, building a potential connection
between reactive oxygen species and regulation of CF6
levels.
How does CF6 elicit a biological function? Is there a
receptor for CF6? Current evidence suggests that CF6 stimulates ATPase activity in HUVECs, whereas biological functions of CF6 can be blocked with efrapeptin, an ATP synthase
inhibitor. Radioligand-binding studies presented in the present article suggest there is a saturable binding site in plasma
membranes of HUVECs,2 though the pharmacological tools
to perform these studies were limited and may be biased (eg,
[125I-CF6] competed with cold CF6). The interaction of CF6
with the ATP synthase may be directly important for biolog-
Figure 2. Current understanding of the potential role played by
CF6 in the endothelial cell and blood.
This work leads to a number of interesting questions. The first
set of questions pertains to the general function of CF6.
Although CF6 is present in the plasma membrane of endothelial cells, the mechanism by which CF6 is transported to
the membrane is unclear. Another question is whether endothelial cells are the primary source for circulating CF6, and
the identification of physiological stimuli for the release of
CF6, considered an endogenous inhibitor of arachidonate
release (Figure 2).8,11,12 Given the ubiquitous expression of
ATP synthase, it would be surprising that endothelial cells
would be the only source. The receptor for CF6 also remains
an issue: is ATP synthase the only receptor for CF6? If so,
how does stimulation of ATP synthase by CF6 occur through
the ␤ subunit? There is something of a conundrum with the
CF6-induced reduction in pH being association with vasoconstriction. As the authors point out, vasoconstriction is
typically associated with an alkalinization of cells. It would
be interesting to examine the effects of CF6 on the function
of isolated blood vessels with respect to arachidonate-induced
relaxation and general vessel contractility.
The second set of questions is in regard to the relevance of
CF6 to hypertension. Reductions in prostacyclin synthesis
have been observed in a number of different hypertension
models, suggesting a common underlying dysfunction that
lowers the levels of this important vasodilator. The authors
have begun to build a case that CF6 may play a role in
determination of prostacyclin levels in vivo. There are questions as to the application of the finding of higher plasma
levels of CF6 to models other than SHR, as to how antioxidants affect the plasma levels of CF6, and as to whether CF6
could play an initiating or maintenance role in hypertension.
In summary, this interesting work by Osanai et al2 points to
a new function—that of an endogenous prostacyclin inhibitor
with potentially direct and/or indirect vascular effects—for
CF6 and widens our view of the functions of the ancient
enzyme ATP synthase.
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Vasoconstriction Caused by the ATP Synthase Subunit−Coupling Factor 6: A New
Function for a Historical Enzyme
Stephanie W. Watts
Downloaded from http://hyper.ahajournals.org/ by guest on June 17, 2017
Hypertension. 2005;46:1100-1102; originally published online October 17, 2005;
doi: 10.1161/01.HYP.0000186476.62399.6c
Hypertension is published by the American Heart Association, 7272 Greenville Avenue, Dallas, TX 75231
Copyright © 2005 American Heart Association, Inc. All rights reserved.
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