The inositol trisphosphate pathway mediates platelet-activating-factor-induced pulmonary oedema

Abstract

Platelet-activating factor (PAF) is a pro-inflammatory lipid mediator that increases vascular permeability by simultaneous activation of two pathways, one dependent on the cyclooxygenase metabolite prostaglandin E 2 and the other on the sphingomyelinase metabolite ceramide. The hypothesis that part of the PAF-induced oedema is mediated via the inositol 1,4,5-trisphosphate (IP 3 ) pathway or Rho kinase pathway was investigated.Oedema formation was induced in isolated perfused rat lungs by injection of 5 nmol PAF into the pulmonary artery. Lungs were pre-treated with specific inhibitors: edelfosine (L108) to block phosphatidyl-inositol-specific phospholipase C, xestospongin to block the IP 3 receptor, 5-iodonaphthalene-1-sulphonyl-homopiperazine (ML-7) to block myosin light chain kinase, and (+)-R-trans-4-(aminoethyl)-N-(4-pyridyl)cyclohexanecarboxamide (Y27632) to block Rho-associated protein kinase.Pre-treatment with L108 or xestospongin reduced PAF-induced oedema formation by 58 and 56%, respectively. The effect of L108 was additive to that of the cyclooxygenase inhibitor acetyl salicylic acid (88% oedema reduction). PAF-induced oedema formation was also reduced if extracellular calcium concentrations were lowered. Furthermore, treatment with ML-7 reduced oedema formation by 54%, whereas Y27632 was without effect.It is concluded that platelet-activating-factor-triggered oedema is mediated by activation of the inositol 1,4,5-trisphosphate pathway, influx of extracellular calcium and subsequent activation of a myosin light chain kinase-dependent and Rho-associated-protein-kinase-independent mechanism.

Vascular Barrier Regulation by PAF, Ceramide, Caveolae, and NO - an Intricate Signaling Network with Discrepant Effects in the Pulmonary and Systemic Vasculature

“…Consistently, PAF did not cause MLCK phosphorylation in human umbilical vein endothelial cells (HUVEC) [23]. In contrast, in the lungs ML-7 (35 µM) attenuated PAFinduced edema formation [14]. These conflicting findings allude to the possibility that the mechanisms underlying PAF-induced edema formation may differ between microvascular beds, namely that PAF increases lung microvascular permeability at least partly via an actinmyosin contractile mechanism, while paracellular gap formation in mesenteric venules is caused mainly by the untightening of endothelial-endothelial cell adhesion structures.…”

“…Sphingosine-1-phosphate (S1P) enhances vascular barrier function by activation of its G-protein coupled receptor S1P 1 and subsequent downstream activation of small Rho GTPases, cytoskeletal reorganization, adherens junction and tight junction assembly, and focal adhesion formation [12]. Ceramide, on the other hand, increases vascular permeability by a mechanism that is not yet fully understood, but involves the regulation of both endothelial Ca 2+ signaling and NO formation [13][14][15][16]. Here, we will focus on the role of caveolae -membrane rafts rich in sphingolipids [17] which play important roles in the regulation of endothelial Ca 2+ signaling [18,19] and NO synthesis [20] -as integrating signaling platforms in the pathogenesis of permeability-type edema.…”

Differential Regulation of Lung Endothelial Permeability <b><i>in Vitro</i></b> and <b><i>in Situ</i></b>

“…This is an important difference, because the induction of high NO levels by PAF is a response typical for non-pulmonary vascular beds [11]. Another notable discrepancy relates to the relevance of the myosin light chain kinase that seems to play no critical role in endothelial cell monolayers [29], whereas ML-7, an inhibitor of myosin light chain kinase, attenuated the PAF-induced increase in vascular permeability in intact lungs [38]. Again, this behavior of the cultured cells is reminiscent of extra-pulmonary endothelial cells in which ML-7 did also have no effect on the PAF-induced increase in vascular permeability [39].…”

“…Further, intratracheal LPS challenge does not result in activation of RhoA [125] and RhoA-mediated contractile mechanisms are not involved in bradykinin-, PAF- or leukotriene C4-induced hyperpermeability in various vascular beds in situ (see Fig. 5 and refs [38,126]). When overexpressed, RhoA may possibly play a role in vascular permeability as was suggested by studies in RhoGDI-1-deficient mice [127].…”

“…Analogously, inhibition of Ca 2+ entry or release has proven effective to attenuate vascular permeability increases in response to inflammatory mediators such as PAF [32,38] or mechanical stimulation such as endothelial cell stretch [3,4,113] in isolated lungs. Yet, the finding that pro-inflammatory stimuli such as TNF-α or thrombin are able to elicit distinct endothelial [Ca 2+ ] i responses, yet fail to induce marked increases in endothelial permeability in isolated perfused lung preparations (see above) suggests that an increase in endothelial [Ca 2+ ] i in response to a physiological stimulus rather than a pharmacological agonist may by itself not be sufficient to trigger an endothelial barrier leak.…”