Regulation of Weibel–Palade Body Exocytosis

doi:10.1016/j.tcm.2005.09.005

Trends in Cardiovascular Medicine

Volume 15, Issue 8, November 2005, Pages 302-308

https://doi.org/10.1016/j.tcm.2005.09.005 Get rights and content

Weibel–Palade bodies (WPBs) are endothelial granules that store von Willebrand factor (VWF), P-selectin, and other vascular modulators. Endothelial cells secrete WPBs in response to vascular injury, releasing VWF, which triggers platelet rolling, and externalizing P-selectin, which activates leukocyte trafficking. Endothelial exocytosis is one of the earliest responses to vascular damage and plays a pivotal role in thrombosis and inflammation. This review examines the regulation of WPB exocytosis—the exocytic machinery, activators, and inhibitors of exocytosis—and speculates about the development of novel anti-exocytic drugs.

Section snippets

History

Edward Weibel and George Palade discovered endothelial granules in 1964 while examining human lungs with an electron microscope. They described a “hitherto unknown rod-shaped cytoplasmic component…in endothelial cells of small arteries” (Weibel and Palade 1964). The rods measured 0.1 μm wide and 3 μm long and contained fibers running lengthwise. Weibel and Palade concluded that the “nature and significance of these cytoplasmic components are yet unknown.” In 1982, Wagner et al. discovered that

WPB Contents

WPBs store proteins that regulate thrombosis and inflammation (Table 1). VWF is the major component inside WPBs (Wagner et al. 1982). Vesicular transport carries VWF from the endoplasmic reticulum, where it is glycosylated and dimerized, to the Golgi apparatus, where it is further glycosylated and sulfated and linked into multimers, and thence to WPBs (Ruggeri 2003, Wagner and Marder 1983). Endothelial exocytosis releases multimeric VWF into the blood (Figure 1), where the metalloproteinase

Exocytic Machinery Inside Endothelial Cells

The proteins that drive fusion of the WPB with the endothelial plasma membrane are vesicular trafficking proteins conserved in yeast, Drosophila, and mammalian cells. Exocytosis of WPBs involves a series of discrete stages: loading of cargo into the nascent granule, granule budding, targeting of the granule to the endothelial membrane, priming of the granule, fusion of the granule and plasma membranes, and, finally, granule recycling (Figure 2) (Jahn 2004, Jahn et al. 2003, Jahn and Sudhof 1999

Activators of Endothelial Exocytosis

Endothelial exocytosis occurs within minutes of stimulation. WPB release is, thus, an immediate early response of endothelial cells, a rapid response to injury independent of gene transcription (Pober and Cotran 1990). (In contrast, vascular injury also activates a delayed transcriptional response evident hours after stimulation.) For example, thrombin triggers an immediate endothelial response: within minutes of exposure to thrombin, endothelial cells release VWF, which mediates platelet

Nitric Oxide Inhibits Vascular Inflammation in Animals and Humans

Nitric oxide (NO) inhibits vascular inflammation: vascular injury and atherosclerosis are more severe in knockout mice lacking endothelial NO synthase (eNOS) or inducible NOS; conversely, gene therapy with NOS ameliorates arteriosclerosis (Kuhlencordt et al. 2001a, Kuhlencordt et al. 2001b, Rudic et al. 1998, Shears et al. 1997). An inability to synthesize NO, the hallmark of endothelial dysfunction, is a risk factor for the development of atherosclerosis (Davignon and Ganz 2004, Hansson 2005).

Unanswered Questions

Several critical questions regarding NO inhibition of exocytosis remain to be answered. If NO targets NSF, does NO also inhibit general vesicle trafficking? If so, why does NO not kill the cell by interrupting vesicle trafficking between the endoplasmic reticulum and the Golgi apparatus? Finally, how does S-nitrosylation of cysteine 91 interfere with NSF disassembly?

Another Inhibitor of Endothelial Exocytosis: Hydrogen Peroxide

Hydrogen peroxide (H₂O₂) can also inhibit exocytosis: exogenous and endogenous H₂O₂ decrease thrombin-stimulated exocytosis in cultured endothelial cells (Matsushita et al. 2005b). Conversely, ectopic expression of catalase diminishes endogenous H₂O₂ levels, increasing exocytosis ex vivo. Finally, chemical inhibition of catalase increases H₂O₂ production and decreases exocytosis in mice.

Exocytosis: A Novel Therapeutic Target

Excessive endothelial exocytosis may play a role in inflammatory and thrombotic disorders such as atherosclerosis, acute coronary syndromes, myocardial infarction, and deep vein thrombosis. Patients with defective NO synthesis—the hallmark of endothelial dysfunction—may be at increased risk for acquiring atherosclerosis and coronary events partly because less NO synthesis enables more exocytosis in response to vascular injury. Drugs directed at the exocytic machinery of endothelial cells may be

Conclusion

Upon discovering the rod-shaped organelles that now bear their name, Weibel and Palade noted that these granules “must be a structure of some functional significance which for the moment remains obscure…It is hoped that further…studies may shed some light on the nature of the rods and their components” (Weibel and Palade 1964). Researchers have defined the major components of WPBs, and we are now beginning to glimpse the molecular machinery that drives endothelial exocytosis. These discoveries

Acknowledgments

This study was supported by grants from the National Institutes of Health (R01 HL63706-04, R01 HL074061, P01 HL65608, and P01 HL56091), the Ciccarone Center, and the John and Cora H. Davis Foundation to C.J.L. and by those from the National Institutes of Health (RR07002 and HL074945) to C.N.M.

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Estrogen therapy is used to treat patients with post-menopausal symptoms, such as hot flashes and dyspareunia. Estrogen therapy also decreases the risk of fractures from osteoporosis in post-menopausal women. However, estrogen increases the risk of venous thromboembolic events, such as pulmonary embolism, but the pathways through which estrogen increase the risk of thromboembolism is unknown. Here, we show that estrogen elicits endothelial exocytosis, the key step in vascular thrombosis and inflammation. Exogenous 17β-estradiol (E2) stimulated endothelial exocytosis of Weibel-Palade bodies (WPBs), releasing von Willebrand factor (vWF) and interleukin-8 (IL-8). Conversely, the estrogen antagonist ICI-182,780 interfered with E2-induced endothelial exocytosis. The ERα agonist propyl pyrazole triol (PPT) but not the ERβ agonist diarylpropionitrile (DPN) induced vWF release, while ERα silencing counteracted vWF release by E2, suggesting that ERα mediates this effect. Exocytosis triggered by E2 occurred rapidly within 15 min and was not inhibited by either actinomycin D or cycloheximide. On the contrary, it was inhibited by the pre-treatment of U0126 or SB203580, an ERK or a p38 inhibitor, respectively, suggesting that E2-induced endothelial exocytosis is non-genomically mediated by the MAP kinase pathway. Finally, E2 treatment enhanced platelet adhesion to endothelial cells ex vivo, which was interfered with the pre-treatment of ICI-182,780 or U0126. Taken together, our data show that estrogen activates endothelial exocytosis non-genomically through the ERα-MAP kinase pathway. Our data suggest that adverse cardiovascular effects such as vascular inflammation and thrombosis should be considered in patients before menopausal hormone treatment.
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Secretory phospholipase A₂ (PLA₂) hydrolyzes LDL phospholipids generating modified LDL particles (PLA₂-LDL) with increased atherogenic properties. Exocytosis of Weibel–Palade bodies (WPB) releases angiopoietin 2 (Ang2) and externalizes P-selectin, which both play important roles in vascular inflammation.
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Review articleRegulation of Weibel–Palade Body Exocytosis

Section snippets

History

WPB Contents

Exocytic Machinery Inside Endothelial Cells

Activators of Endothelial Exocytosis

Nitric Oxide Inhibits Vascular Inflammation in Animals and Humans

Unanswered Questions

Another Inhibitor of Endothelial Exocytosis: Hydrogen Peroxide

Exocytosis: A Novel Therapeutic Target

Conclusion

Acknowledgments

Blood

Trends Pharmacol Sci

Trends Mol Med

J Biol Chem

J Biol Chem

Blood

J Biol Chem

Neuron

Cell

Blood

Biochim Biophys Acta

Cell

Cell

Blood

Cell

Cell

J Thromb Haemost

Biochem Pharmacol

Cell

Blood

J Biol Chem

Cell

Int Rev Cytol

Blood

J Pediatr Surg

Ceramide triggers Weibel–Palade body exocytosis

Circ Res

Purification of an N-ethylmaleimide-sensitive protein catalyzing vesicular transport

Proc Natl Acad Sci U S A

Regulated secretion in endothelial cells: biology and clinical implications

Thromb Haemost

Role of endothelial dysfunction in atherosclerosis

Circulation

Small GTP-binding protein RalA associates with Weibel–Palade bodies in endothelial cells

Thromb Haemost

Small GTP-binding protein Ral modulates regulated exocytosis of von Willebrand factor by endothelial cells

Arterioscler Thromb Vasc Biol

A mouse model of severe von Willebrand disease: defects in hemostasis and thrombosis

Proc Natl Acad Sci U S A

Composition of the von Willebrand factor storage organelle (Weibel–Palade body) isolated from cultured human umbilical vein endothelial cells

J Cell Biol

Regulated exocytosis in vascular endothelial cells can be triggered by intracellular guanine nucleotides and requires a hydrophobic, thiol-sensitive component. Studies of regulated von Willebrand factor secretion from digitonin permeabilized endothelial cells

Endothelium

C5a-induced expression of P-selectin in endothelial cells

J Clin Invest

P-Selectin glycoprotein ligand 1 (PSGL-1) is expressed on platelets and can mediate platelet–endothelial interactions in vivo

J Exp Med

Ionizing radiation mediates expression of cell adhesion molecules in distinct histological patterns within the lung

Cancer Res

Accumulation of P-selectin in the lumen of irradiated blood vessels

Radiat Res

X-Ray-induced P-selectin localization to the lumen of tumor blood vessels

Cancer Res

Weibel–Palade bodies recruit Rab27 by a content-driven, maturation-dependent mechanism that is independent of cell type

J Cell Sci

Review article
Regulation of Weibel–Palade Body Exocytosis