EDITORIAL FOCUS
Serine/threonine phosphatase 2B regulates protein kinase C-
activity and endothelial barrier function
Dolly
Mehta
Department of Pharmacology, College of Medicine, The University
of Illinois at Chicago, Chicago, Illinois 60612
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ARTICLE |
IMPAIRMENT OF ENDOTHELIAL BARRIER
FUNCTION and the development of tissue edema are serious
conditions underlying acute inflammatory diseases of the lung. The loss
of barrier integrity in response to edemagenic agents such as thrombin
results from a repertoire of signaling events that lead to cell shape
change and interendothelial gap formation (6, 10). How
individual signaling events contribute to endothelial barrier
dysfunction and how these pathways can be "switched off"
to protect barrier function has been an intense area of research in
endothelial cell biology.
Protein phosphorylation is known to regulate a vast number of cellular
signaling pathways. One of the signaling pathways that has received
much attention, independent of the well-established myosin light chain
phosphorylation pathway, is the protein kinase C (PKC)-dependent
pathway (10). PKC is a family of serine/threonine (Ser/Thr) kinases that regulate endothelial cell cytoskeleton and shape
via regulating not only cell-cell (i.e., catenin-cadherin complex) and
cell-matrix linking proteins (i.e., vinculin, paxillin, and focal
adhesion kinase) but also the proteins that control actin
polymerization and microtubule assembly (1-3, 8, 14, 16). Furthermore, PKC activation may have broad effects on the signaling events mediated by heterotrimeric G proteins as well as by
Rho GTPases. For example, PKC can phosphorylate
G12/G13 proteins (7, 13) and the
modulators of Rho GTPases such as guanine nucleotide exchange factors,
GTPase-activating proteins, and guanine nucleotide dissociation
inhibitors (11a, 15).
Of the 12 different PKC isozymes discovered so far, PKC-
and -
have been shown to play an important role in the mechanism of
agonist-induced endothelial barrier dysfunction (5, 12, 17). However, despite such an important role for PKC activation in the endothelium, the mechanism regulating its activity has not been
elucidated. Recent studies (4, 9) indicate that two
sequential, and critical, mechanisms regulate PKC activity: 1) phosphorylation triggered by the
3-phosphoinositide-dependent kinase (PDK)-1 and 2) binding
to the lipid second messenger diacylglycerol. The activation of PKC by
phosphorylation may require a balance of activities between PDK-1 and
Ser/Thr phosphatases; however, the possible contribution of Ser/Thr
phosphatases in regulating PKC activity remains unclear.
In this issue of the American Journal of Physiology-Lung Cellular
and Molecular Biology, Lum et al. (11) provide
compelling evidence that the activation of Ser/Thr protein phosphatase
type 2B (PP2B) is a significant mechanism for regulating PKC-
activity as well as endothelial barrier function in response to
thrombin. Their results demonstrated that the PP2B inhibitor FK506
potentiated a thrombin-induced increase in PKC phosphotransferase
activity and phosphorylation of PKC-
but not of PKC-
in bovine
pulmonary microvascular endothelial cells. FK506 also prolongs the
thrombin-induced barrier dysfunction that was rescued to normal when
PKC-
expression was downregulated by phorbol ester treatment. In
contrast, commonly used inhibitors such as okadaic acid and calyculin
A, which inhibit PP1 and PP2A, respectively, had no effect on
thrombin-induced PKC-
activity, indicating the specificity of FK506.
Additionally, their findings demonstrated a time-dependent correlation
between thrombin-mediated increase in endothelial cell permeability and PKC-
phosphorylation, thus supporting their conclusion that PKC-
plays a critical role in protecting barrier dysfunction to normal after
thrombin challenge. Thus their data identify the PP2B-PKC-
pathway
as a novel component of signaling events that regulate endothelial
barrier function, an area of scientific endeavor that holds great
promise in understanding the mechanism underlying acute inflammatory
diseases of the lung, especially 1) how PP2B is inactivated
after thrombin challenge of endothelial cells; 2) how PP2B
regulates PKC-
activity whether it occurs at the level of PDK-1, at
the level of substrate supply, or through its direct action on PKC; and
3) identification of other targets of PP2B that trigger
endothelial dysfunction.
Inhibition of protein phosphatases has been implemented as a
therapeutic target in the control of diverse pathological conditions such as asthma, hypertension, restenosis, and metastasis and no doubt
in controlling endothelial barrier function and thus pulmonary edema.
It is expected that in the near future, studies on PP2B will elucidate
the potential role of a critical switch-off signal by which
endothelial barrier function may be rescued to normal.
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FOOTNOTES |
Address for reprint requests and other correspondence: D. Mehta, Dept. of Pharmacology, The Univ. of Illinois at Chicago College of Medicine, 835 S. Wolcott Ave., Chicago, IL 60612 (E-mail:
dmehta{at}uic.edu).
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