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Serine/threonine phosphatase 2B regulates protein kinase C-alpha 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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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-alpha and -beta 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-alpha 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-alpha but not of PKC-beta in bovine pulmonary microvascular endothelial cells. FK506 also prolongs the thrombin-induced barrier dysfunction that was rescued to normal when PKC-alpha 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-alpha activity, indicating the specificity of FK506. Additionally, their findings demonstrated a time-dependent correlation between thrombin-mediated increase in endothelial cell permeability and PKC-alpha phosphorylation, thus supporting their conclusion that PKC-alpha plays a critical role in protecting barrier dysfunction to normal after thrombin challenge. Thus their data identify the PP2B-PKC-alpha 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-alpha 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.


    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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Am J Physiol Lung Cell Mol Physiol 281(3):L544-L545
1040-0605/01 $5.00 Copyright © 2001 the American Physiological Society




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