GPR4 plays a critical role in endothelial cell function and mediates the effects of sphingosylphosphorylcholine

Signal transduction underlying the vascular effects of sphingosine 1-phosphate and sphingosylphosphorylcholine

Two related lysosphingolipids, sphingosine 1-phosphate (S1P) and sphingosylphosphorylcholine (SPC) mediate diverse cellular responses through signals transduced by either activation of G-protein coupled receptors or possibly by acting intracellularly. Vascular responses to S1P and SPC measured both in vivo and in dissected vessels show predominantly vasoconstriction with some evidence for vasodilation. Although stimulation with S1P or SPC generally leads to similar vascular responses, the signalling pathways stimulated to produce these responses are often distinct. Nevertheless, mobilization of Ca2+ from intracellular stores and influx of extracellular Ca2+, which both increase [Ca2+]i, occur in response to S1P and SPC. Both mobilization of Ca2+ from intracellular stores and influx of extracellular Ca2+ occur in response to S1P and SPC. As well, both S1P and SPC induce Ca2+-sensitization in vascular smooth muscle which is mediated through Rho kinase activation. In the endothelium, S1P and SPC stimulate the production of the vasodilator, nitric oxide through activation of endothelial nitric oxide synthase. This activation occurs through phosphorylation by Akt and through binding of Ca2+-calmodulin upon increased [Ca2+]i. These lysosphingolipids also activate cyclooxygenase-2 which produces prostaglandins with both vasoconstrictor and vasodilator properties. A balance between the signals inducing vasodilation versus the signals inducing vasoconstriction will determine the vascular outcome. Thus, perturbations in S1P and SPC concentrations, relative expression of receptors or downstream signalling pathways may provide a mechanism for pathophysiological conditions such as hypertension. Given this background, recent studies examining a potential role for S1P and SPC in hypertension and vascular dysfunction in aging are discussed.

Two ligands for a GPCR, proton vs lysolipid

Recently, two different chemicals have been matched as ligands with the same G-protein-coupled receptor (GPCR). Double-pairing of OGR1 family GPCRs with proton and lysolipid raises several questions. First, whether both are the real ligands for the GPCRs. Second, whether modulation of a GPCR by two chemicals could be possible. Third, one of the chemicals is proton. Proton-sensing not only is a new action mode of GPCR activation, but also it could be generalized in other GPCRs. In this review, I would like to summarize the issue and discuss questions with pharmacological criteria.

Lysophosphatidylcholine increases endothelial permeability: role of PKCalpha and RhoA cross talk

Lysophosphatidylcholine (LPC) is a bioactive proinflammatory lipid that can be generated by pathological activities. We investigated the hypothesis that LPC signals increase in endothelial permeability. Stimulation of human dermal microvascular endothelial cells and bovine pulmonary microvascular endothelial cells with LPC (10-50 microM) induced decreases (within minutes) in transendothelial electrical resistance and increase of endothelial permeability. LPC activated (within 5 min) membrane-associated PKC phosphotransferase activity in the absence of translocation. Affinity-binding analysis indicated that LPC induced increases (also by 5 min) of GTP-bound RhoA, but not Rac1 or Cdc42. By 60 min, both signaling pathways decreased toward baseline. Inhibition of RhoA with C3 transferase inhibited approximately 50% of LPC-induced resistance decrease. Pretreatment with PKC inhibitor Gö-6983 (concentrations selective for classic PKC), PMA-induced depletion of PKCalpha, and transfection of antisense PKCalpha oligonucleotide each prevented 40-50% of the LPC-induced resistance decrease. Furthermore, these three PKC inhibition strategies inhibited 60-80% of the LPC-induced GTP-bound RhoA. These results show that LPC directly impairs the endothelial barrier function that was dependent, at least in part, on cross talk of PKCalpha and RhoA signals. The evidence indicates that elevated LPC levels can contribute to the activation of a proinflammatory endothelial phenotype.