EDITORIAL
The conundrum of oxidant-induced barrier dysfunction
Arnold
Johnson
The Lung Biology Laboratory, Upstate New York Veterans Affairs Healthcare Stratton Medical Center, Albany, New York 12208, American Journal of Physiology-, Lung Cellular and Molecular Physiology, September 2000, Volume 279 (23)
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ARTICLE |
IN THIS ISSUE
of the American Journal of Physiology-Lung Cellular and Molecular
Physiology, Shi et al. (7) have shown that activation
of p60c-Src mediates the diperoxovanadate
(DPV)-induced increase in endothelial permeability of bovine pulmonary
arterial endothelial cells. DPV, which is produced by the reaction of
equimolar amounts of orthovanadate and hydrogen peroxide
(H2O2), has the unique ability to both activate tyrosine kinases and inhibit protein tyrosine phosphatases. DPV is used
as a model for H2O2-induced activation of
tyrosine kinase activity and barrier dysfunction (10). The
conclusions of Shi et al. (7) are based on their data
indicating that the c-Src inhibitors genistein and PP-2 and
overexpression of a dominant negative plasmid for p60c-Src
attenuate the DPV-induced increases in albumin clearance and/or decrease in electrical resistance. Importantly, tyrosine
phosphorylation, autophosphorylation, and peptide phosphorylation
assays were used to substantiate the noted changes in tyrosine kinase
activity. A most novel finding of this paper is an increase in
p60c-Src-associated cortactin and myosin light chain kinase
(MLCK) immunoactivity in response to DPV. These findings substantiate
their previous findings (5, 10) and those of others
(2, 3, 9) noting that in response to reactive oxygen
species (ROS), the endothelial cell is not simply the victim of
nonspecific mediated reactions such as an oxidation attack on membranes
followed by the loss of membrane integrity and necrosis. Indeed, as
exemplified in this paper (7), the endothelium responds in
a orchestrated series of intracellular reactions characterized by
activation of c-Src with phosphorylation of cortactin and MLCK,
resulting in a decrease in barrier selectivity to albumin.
A caveat for the previous work in this field (2, 6, 8) is
the selectivity of the pharmacological probes that are used to alter
the activity of kinases such as genistein for c-Src and, in the case of
our own work, calphostin C for protein kinase C-
. Thus an important
aspect of the study by Shi et al. (7) is the verification
of the role of p60c-Src in barrier function. They showed
that expression of constitutively active and wild-type
p60c-Src increased the response to DPV (i.e., a further
decrease in the electrical resistance), whereas expression of a
dominant negative mutant p60c-Src decreased the response to
DPV (i.e., increased the electrical resistance). Notwithstanding the
intricate "cross-talk network" in signal transduction, this paper
notably acclaims a specific role for p60c-Src in regulating
barrier function.
In the paper by Shi et al. (7), the increased
association of phosphorylated cortactin and MLCK with
p60c-Src is noted in pulmonary arterial endothelial cells.
Yet, it is worth documenting these events in endothelial cells derived
from the microcirculation because there may be DPV- and ROS-induced responses that are different between endothelial cells derived from
large and small vessels (4). These needed studies are underscored by the large surface area available for fluid exchange that
occurs in the lung microcirculation. In addition, what is the role of
tyrosine phosphorylated cortactin and MLCK in barrier dysfunction in
response to tyrosine kinase activation and ROS? With today's
technology, expression of mutant inactive MLCK and cortactin that
contain critical sites unavailable for phosphorylation or, conversely,
expression of a constitutively active endothelial MLCK can yield much
needed information. Interestingly, the data by Shi et al.
(7) indicate that p60c-Src only partially
mediates the effect of DPV. Obviously, all the members of the signal
transduction orchestra have not been identified, and we need more
investigation of the response to tyrosine kinase activation and ROS.
Apparently, other possible signals would be those hosted by other
kinases as Ferro et al. (1) demonstrated for
protein kinase C-
or as Shi et al. (7) mentioned for
the activity of mitogen-activated protein kinase and the phospholipase D-phosphatidic acid pathway. Thus as we are all aware and as
exemplified in the important work of Shi et al., "Science is the
knowledge of consequences, and dependence of one fact upon another"
(Thomas Hobbes, English philosopher and author, 1588-1679).
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