Stokes Research Institute, Children's Hospital of Philadelphia, Department of Biochemistry and Biophysics, The University of Pennsylvania, Philadelphia, Pennsylvania 19104
IN GREEK MYTHOLOGY, the Chimera was a
fearsome monster with the head of a lion, the body of a goat, and the
tail of a serpent, which was slain by Bellerophon. In their paper (the
current article in focus, Ref. 1, see p. L917
in this issue) on the construction of a chimeric superoxide
dismutase (SOD), it appears that Gao et al. may have slain the
monstrous task of constructing a therapeutically beneficial SOD. Since
its discovery, the family of SODs has offered the potential for an
effective antioxidant therapy that would reduce undesired consequences
of inflammatory diseases as well as a number of conditions associated
with uncontrolled overproduction of superoxide. However, for reasons
that are not entirely clear, this goal has become somewhat of a monster
for researchers. By constructing a chimera of two of the isotypes of
SOD, Gao et al. may have achieved the construction of a therapeutically
viable form of the enzyme.
The three SOD isomers, cytosolic Cu,Zn SOD (SOD1),
mitochondrial MnSOD (SOD2), and extracellular Cu,Zn SOD (SOD3), have
been shown to have some therapeutic utility in protecting organ systems from oxidative stress, particularly in animal model systems of disease
(5, 6). However, the success of these therapies has been
limited due to a variety of reasons such as the short half-life of the
protein in circulation, inability to associate with the cellular
surface, and slow rates of equilibration between the vascular and
interstitial spaces. Primarily due to the small molecular radius of
SOD1 injected into circulation, it is rapidly (half-life of 10 min)
cleared by the kidneys. Furthermore, its negative charge does not allow
SOD1 to interact with cell surfaces and reduces its ability to enter
the interstitium. Moreover, the therapeutic efficacy of SOD1 exhibits a
bell-shaped curve after systemic administration, which, although not
well understood, further limits the concentration of this protein that
can be administered pharmacologically (5, 6). These
limitations are partially alleviated by the use of SOD2, which is the
least negatively charged SOD, and in the tetrameric form has a
molecular radius of 40 Å, which retards its clearance by the
kidneys (plasma half-life of 4 h). Despite its larger size, SOD2
equilibrates nearly four times faster that SOD1 within interstitial
spaces (5).
SOD3 is normally tagged to the cellular surface via its hydrophilic
positively charged "tail", which gives the protein its heparin-binding ability (4, 8). Previously, it has been shown that cleavage of this tail results in the release of SOD3 from
the cellular surface and that this loss may contribute to the
sensitivity of the endothelium to oxidative insults. Furthermore, a
major contributor of reactive intermediates near or at the endothelial plasma membrane is the NADPH oxidase. It is now recognized that a
family of membrane-associated proteins (NOX) are responsible for
generating superoxide and hydrogen peroxide in vascular endothelium and
smooth muscle cells potentially for defense purposes and for cell
signaling (9). The NOX enzymes appear to be composed of the typical low-potential membrane gp91phox flavoprotein
that reduces oxygen to superoxide, as well as of cytosolic proteins, which in response to stimuli assemble into a functional oxidase (9). The generation of superoxide in the vascular
compartment not only from activated inflammatory cells but also from
vascular cells contributes to adverse effects of tissue injury during
inflammation and other vascular disorders.
Therefore, adherence to the endothelium appears to be critical for the
protective and anti-inflammatory function of SODs. The pharmacological
efficacy of SOD2 may be limited by the inability of the protein to
adhere to endothelial cell surface. For reasons not completely
understood, SOD3 cannot be expressed and purified in large quantities,
prompting investigators to utilize SOD1 and SOD2 primarily. However, on
the basis of limitations of SOD1 and SOD2 discussed above,
investigators have employed chemical and molecular approaches to
generate SODs that combine some of the most beneficial features of SODs
(2, 3). Examples of chemical modifications that extend the
half-life of SOD1 and improve its pharmacological profile include
coupling of polyethylene glycol, lecithin, putrescine, and sugars (for
a review, see Ref. 6). Molecular approaches include the
generation of chimeric proteins such as an SOD1/3 chimera protein that
contains the positively charged tail of SOD3 and has been shown to be
more effective in protecting tissues than SOD1 (3),
although this chimera was still retained in the kidney and was not
optimal for therapeutic utility.
To overcome these limitations, Gao et al. (1) have
generated a new chimera utilizing the SOD3 COOH terminus linked to
SOD2. This new protein combines the endothelial localization of SOD3 with the extended half-life and pharmacological profile of SOD2 to
produce a novel bioactive agent against the negative effects of
oxidative stress. Like the mythological Chimera, this new protein displays three different characteristics: it has the head of all SODs,
namely the ability to remove superoxide; the body of a tetramer, such
that its large molecular size reduces its clearance from the plasma;
and the tail of positively charged residues, which allows it to be
preferentially tagged to the endothelial surface. This novel chimeric
protein shows considerable promise within two animal models of
inflammation, IL-1-induced lung injury and carrageenan-injected paw
inflammation. In the first model, administration of SOD2/3 chimera
prevented the vascular leak and edema and reduced the number of
infiltrating neutrophils in the lung after a IL-1 challenge. Similarly,
SOD2/3 administration reduced the foot edema induced by injection of
carrageenan. Thus through this novel strategy, Gao et al.
(1) may have provided us with the appropriate strategy with which to utilize the power of SOD to combat oxidative stress induced by inflammatory disease. This major advancement coupled with
the development of chemical and molecular approaches to target the
chimeric protein to the lung (7) or other tissues could provide a powerful approach to generate a clinically therapeutic agent
to fulfill the promise for SOD-based therapies.
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FOOTNOTES |
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Address for reprint requests and other correspondence: H. Ischiropoulos, Stokes Research Institute, Children's Hospital of Philadelphia, 416D Abramson Center, 34th St. and Civic Center Blvd., Philadelphia, PA 19104-4318 (E-mail: ischirop{at}mail.med.upenn.edu).
10.1152/ajplung.00014.2003
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