Objective-Although erythrocytes have been suggested to play a role in blood clotting, mediated through phosphatidylserine (PS) exposure and/or PS-bearing microvesicle generation, an endogenous substance that triggers the membrane alterations leading to a procoagulant activity in erythrocytes has not been reported. We now demonstrated that lysophosphatidic acid (LPA), an important lipid mediator in various pathophysiological processes, induces PS exposure and procoagulant microvesicle generation in erythrocytes, which represent a biological significance resulting in induction of thrombogenic activity. Methods and Results-In human erythrocytes, LPA treatment resulted in PS exposure on remnant cells and PS-bearing microvesicle generation in a concentration-dependent manner. Consistent with the microvesicle generation, scanning electron microscopic study revealed that LPA treatment induced surface changes, alteration of normal discocytic shape into echinocytes followed by spherocytes. Surprisingly, chelation of intracellular calcium did not affect LPA-induced PS exposure and microvesicle generation. On the other hand, protein kinase C (PKC) inhibitors significantly reduced PS exposure and microvesicle generation induced by LPA, reflecting the role of calcium-independent PKC. Activation of PKC was confirmed by Western blot analysis showing translocation of calcium-independent isoform, PKC, to erythrocyte membrane. The activity of flippase, which is important in the maintenance of membrane asymmetry, was also inhibited by LPA. Furthermore, LPA-exposed erythrocytes actually potentiated the thrombin generation as determined by prothrombinase assay and accelerated the coagulation process initiated by recombinant human tissue factor in plasma. The adherence of erythrocytes to endothelial cells, another important feature of thrombogenic process, was also stimulated by LPA treatment. Conclusion-These results suggested that LPA-exposed erythrocytes could make an important contribution to thrombosis mediated through PS exposure and procoagulant microvesicle generation.

Agonist-dependent activation of G protein-coupled receptors induces diversified receptor cellular and signaling properties. Norepinephrine (NE) and epinephrine (Epi) are two endogenous ligands that activate adrenoceptor (AR) signals in a variety of physiological stress responses in animals. Here we use cardiomyocyte contraction rate response to analyze the endogenous ␤ 2 AR signaling induced by Epi or NE in cardiac tissue. The Epi-activated ␤ 2 AR induced a rapid contraction rate increase that peaked at 4 min after stimulation. In contrast, the NE-activated ␤ 2 AR induced a much slower contraction rate increase that peaked at 10 min after stimulation. Whereas both drugs activated ␤ 2 AR coupling to G s proteins, only Epi-activated receptors were capable of coupling to G i proteins. Subsequent studies showed that the Epi-activated ␤ 2 AR underwent a rapid phosphorylation by G protein-coupled receptor kinase 2 (GRK2) and subsequent dephosphorylation on serine residues 355 and 356, which was critical for sufficient receptor recycling and G i coupling. In contrast, the NE-activated ␤ 2 ARs underwent slow GRK2 phosphorylation, receptor internalization and recycling, and failed to couple to G i . Moreover, inhibiting ␤ 2 AR phosphorylation by ␤ARK C terminus or dephosphorylation by okadaic acid prevented sufficient recycling and G i coupling. Together, our data revealed that distinct temporal phosphorylation of ␤ 2 AR on serine 355 and 356 by GRK2 plays a critical role for dictating receptor cellular events and signaling properties induced by Epi or NE in cardiomyocytes. This study not only helps us understand the endogenous agonist-dependent ␤ 2 AR signaling in animal heart but also offers an example of how G protein-coupled receptor signaling may be finely regulated by GRK in physiological settings.GPCRs 3 comprise the largest known family of cell-surface receptors and are fundamentally involved in mammalian physiology (1, 2). This receptor superfamily represents the largest single target for modern drug therapy. A growing body of evidence indicates that divergent efficacies of an activated GPCR are both agonist-and tissue-dependent (3-5), which presents a great challenge on clinical application when a specific receptor is targeted with drugs. Many of the drug-dependent effects have been attributed to the distinct receptor conformational changes induced by different ligands, leading to different subsequent cellular events and signaling properties (5, 6). Thus, there is great interest in elucidating the mechanisms underlying the drug-dependent cellular events in physiologically relevant contexts.Interestingly, ␤ARs, a family of prototypical GPCRs, can be activated by two endogenous ligands NE and Epi. The receptors play critical roles in the regulation of cardiovascular (7) and pulmonary function (8), as well as other physiological processes. NE is primarily released from sympathetic nerve terminal on the innervated tissues, whereas Epi is primarily released from adrenal gland to the circulating plasma. The distinct ...

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