Platelet lipid(s) bound to albumin increases endothelial electrical resistance: mimicked by LPA

Abstract

The objectives were to determine whether the permeability-decreasing activity of platelet-conditioned medium (PCM) is associated with a lipid bound to albumin and whether lysophosphatidic acid (LPA) is present in the PCM. A decrease in permeability was assessed by an increase in electrical resistance across endothelial cell monolayers derived from bovine pulmonary arteries and microvessels. The Sephacryl S-200 fraction of PCM that contained albumin, the albumin immunoprecipitate from the PCM, and the methanol extract from the albumin immunoprecipitate all increased endothelial electrical resistance. Increased electrical resistance induced by PCM was not abolished by boiling and was mimicked by 1-oleoyl-LPA and 1-palmitoyl-LPA. Analysis of a methanol-chloroform extract of one sample of PCM by electrospray mass spectrometry revealed many fatty acids, ceramide, diacylglycerol, phosphatidic acid, and palmitoyl-LPA, but analysis of a second sample of PCM and the methanol extract of its albumin immunoprecipitate revealed no LPA, only lipids. These findings indicate that a bioactive lipid(s), possibly LPA, released from platelets and subsequently bound to albumin forms an active complex that decreases endothelial permeability.

Role of sphingosine-1 phosphate in the enhancement of endothelial barrier integrity by platelet-released products

“…Our data agree with published reports that plateletreleased products decrease endothelial permeability (18) and increase TER (32). Although our data strongly support the concept that S1P is a platelet-derived factor that significantly underlies this effect, our data do not exclude the possibility that other platelet-derived products may also participate in the barrier-protective effect of platelets.…”

“…Although our data strongly support the concept that S1P is a platelet-derived factor that significantly underlies this effect, our data do not exclude the possibility that other platelet-derived products may also participate in the barrier-protective effect of platelets. We and others have demonstrated that LPA, released from thrombin-stimulated platelets, possesses endothelial barrier-stabilizing activity (1,8,32). Although in our experiments LPA-induced endothelial barrier enhancement was weak in vitro (8), LPA released from platelets may form an active complex with serum albumin that further enhances its barrier-enhancing activity (32), which may in part explain the previously observed trypsin sensitivity of the barrier-enhancing platelet-derived factor (18).…”

“…We and others have reported on vasoactive plateletderived lipids that activate endothelium, including our first report that S1P activated EC phospholipase D (7,10,23,33,48). Subsequent studies have since demonstrated that platelet-derived phospholipids modulate the barrier function of the vascular endothelium via both endothelial barrier-perturbing lipid products such as phosphatidate (6) and barrier-stabilizing/enhancing lipid products such as lysophosphatidic acid (LPA) (8,32) and S1P (11). Although these platelet-derived factors are implicated in the vascular endothelial barrierenhancing effect of platelets, the specific platelet-derived factors remain incompletely characterized.…”

“…Although the mechanism underlying these protective effects remains elusive, it has been demonstrated that platelets as well as platelet supernatants provide similar barrier-enhancing results (18,39,44), which suggests a released barrier-enhancing factor. Preliminary characterization indicates that the active barrier-promoting factor is heat stable and resistant to inactivation by exposure to freeze-thaw cycles; therefore, it is likely not a peptide (1,18,32). Additionally, the barrier-enhancing effect of platelet-conditioned media was shown to involve signaling via the small GTPase G i (10).…”

“…A variety of platelet-released products including adenosine, adenine nucleotides, serotonin, norepinephrine, and arachidonic acid metabolites have been excluded as possible factors involved in the plateletinduced decrease in endothelial permeability (1,18,37,45). Although the specific identity of the platelet-de-rived endothelial permeability-decreasing factor remains unknown, subsequent work has provided evidence that this factor is a phospholipid with albumincoupled activity (32).…”