Sphingosine 1-phosphate (S1P) in blood is phosphorylated, stored, and transported by red blood cells (RBC). Release of S1P from RBC into plasma is a regulated process that does not occur in plasma- or serum-free media. Plasma fractionation and incubations with isolated and recombinant proteins identified high density lipoprotein (HDL) and serum albumin (SA) as non-redundant endogenous triggers for S1P release from RBC. S1P bound to SA and HDL was able to stimulate the S1P(1) receptor in calcium flux experiments. The binding capability of acceptor molecules triggers S1P release, as demonstrated with the anti-S1P antibody Sphingomab. More S1P was extracted from RBC membranes by HDL than by SA. Blood samples from anemic patients confirmed a reduced capacity for S1P release in plasma. In co-cultures of RBC and endothelial cells (EC), we observed transcellular transportation of S1P as a second function of RBC-associated S1P in the absence of SA and HDL and during tight RBC-EC contact, mimicking conditions in tissue interstitium and capillaries. In contrast to S1P bound to SA and HDL, RBC-associated S1P was significantly incorporated by EC after S1P lyase (SGPL1) inhibition. RBC-associated S1P, therefore, has two functions: (1) It contributes to the cellular pool of SGPL1-sensitive S1P in tissues after transcellular transportation and (2) it helps maintain extracellular S1P levels via SA and HDL independently from SGPL1 activity.

These results suggest that SAA enrichment of HDL during disease conditions contributes to the decreased protective function. It is a novel finding that SAA acts as a pro-inflammatory molecule to reduce the anti-inflammatory properties of HDL.

BACKGROUND AND PURPOSEPurinergic signalling plays an important role in vascular tone regulation in humans. We have identified uridine adenosine tetraphosphate (Up4A) as a novel and highly potent endothelial-derived contracting factor. Up4A induces strong vasoconstrictive effects in the renal vascular system mainly by P2X1 receptor activation. However, other purinoceptors are also involved and were analysed here. EXPERIMENTAL APPROACHThe rat isolated perfused kidney was used to characterize vasoactive actions of Up4A. KEY RESULTSAfter desensitization of the P2X1 receptor by a,b-methylene ATP (a,b-meATP), Up4A showed dose-dependent P2Y2-mediated vasoconstriction. Continuous perfusion with Up4A evoked a biphasic vasoconstrictor effect: there was a strong and rapidly desensitizing vasoconstriction, inhibited by P2X1 receptor desensitization. In addition, there is a long-lasting P2Y2-mediated vasoconstriction. This vasoconstriction could be blocked by suramin, but not by PPADS or reactive blue 2. In preparations of the rat isolated perfused kidney model with an elevated vascular tone, bolus application of Up4A showed a dose-dependent vasoconstriction that was followed by a dose-dependent vasodilation. The vasoconstriction was in part sensitive to P2X1 receptor desensitization by a,b-meATP, and the remaining P2Y2-mediated vasoconstriction was only inhibited by suramin. The Up4A-induced vasodilation depended on activation of nitric oxide synthases, and was mediated by P2Y1 and P2Y2 receptor activation. CONCLUSIONS AND IMPLICATIONSUp4A activated P2X1 and P2Y2 receptors to act as a vasoconstrictor, whereas endothelium-dependent vasodilation was induced by P2Y1/2 receptor activation. Up4A might be of relevance in the physiology and pathophysiology of vascular tone regulation.Abbreviations a,b-meATP, a,b-methylene ATP; AngII, angiotensin II; ApnA, diadenosine-n-phophate (n: number of phosphates); ApnG, adenosine-guanosine-n-phosphate (n: number of phosphates); ApoE, apolipoprotein E; CI, confidence interval; DMSO, dimethyl sulphoxide; eNOS, endothelial NOS; GpnG, diguanosine-n-phosphate (n: number of phosphates); L-NAME, N G -nitro-L-arginine methyl ester; MAP, mean arterial blood pressure; MCP-1, monocyte chemoattractant protein-1; MRS2179, 2′-deoxy-N6-methyladenosine 3′,5′-bisphosphate; PPADS, pyridoxal-phosphate-6-azophenyl-2;4-disulphonic acid; RB2, reactive blue 2; Up4A, uridine adenosine tetraphosphate BJP British Journal of Pharmacology

Nimesulide is a nonsteroidal anti-inflammatory drug (NSAID) marketed in more than 50 countries. This drug has caused rare and idiosyncratic but severe hepatotoxicity. The mechanisms associated with and factors responsible for this toxicity remain unknown. One of the nimesulide metabolites identified in human urine is 4-amino-2-phenoxy-methanesulfonanilide (M1). In the current study, we demonstrate that M1 is a stable metabolite that is highly susceptible to facile oxidation by cytochrome P450 enzymes (P450s) to form a reactive diiminoquinone intermediate (M2). Direct detection of M2 was difficult by LC-MS. However, its formation was confirmed indirectly by identification of N-acetyl-cysteine (NAC) adducts of M2. The formation of diiminoquinone M2 was P450 mediated with 2C19 and 1A2 as the two principal P450 enzymes catalyzing M1 oxidation. M1 metabolism irreversibly inhibited 2C19 but activated 1A2 in a time-dependent manner. P450 2C19 exclusively mediated further metabolism of M1 to the amino hydroxynimesulide M3 and its diiminoquinone M4. Similar to M2, M4 is also reactive and can be observed indirectly as its NAC adduct. Nucleophilic addition to diiminoquinone M2 occurs with low regioselectivity, yielding three adducts (the peak area ratio 1:0.08:12). The three regioisomers have the same m/z for [M + H](+), presumably due to nucleophilic addition at the three possible electrophilic sites (C-3, -5, and -6 positions of the sulfonaniline ring). The primary adduct, R, was derived from the attack of the nucleophile at the C-5 position of the sulfonaniline ring and was determined by MS/MS and (1)H and (13)C NMR analyses. The structural assignments were confirmed by chemical synthesis of the adduct R. M2 demonstrated its electrophilic reactivity by selectively alkylating human serum albumin (HSA) at the only free thiol, Cys-34. This suggests the possibility that other proteins may undergo a similar conjugation to form irreversible adducts. Under oxidizing conditions in the presence of cumene hydroperoxide (CHP), the formation of M2 was enhanced, indicating that oxidative stress may accelerate the production of reactive diiminoquinone species (M2 and M4).

Purinergic signaling has a crucial role in different vascular processes. The endothelial-derived vasoconstrictor uridine adenosine tetraphosphate (Up(4)A) is a potent activator of the purinoceptor P2Y and is released under pathological conditions. Here we sought to measure purinergic effects on vascular calcification and initially found that Up(4)A plasma concentrations are increased in patients with chronic kidney disease. Exploring this further we found that exogenous Up(4)A enhanced mineral deposition under calcifying conditions ex vivo in rat and mouse aortic rings and in vitro in rat vascular smooth muscle cells. The addition of Up(4)A increased the expression of different genes specific for osteochondrogenic vascular smooth muscle cells such as Cbfa1, while decreasing the expression of SM22α, a marker specific for vascular smooth muscle cells. The influence of different P2Y antagonists on Up(4)A-mediated process indicated that P2Y(2/6) receptors may be involved. Mechanisms downstream of P2Y signaling involved phosphorylation of the mitogen-activated kinases MEK and ERK1/2. Thus, Up(4)A activation of P2Y influences phenotypic transdifferentiation of vascular smooth muscle cells to osteochondrogenic cells, suggesting that purinergic signaling may be involved in vascular calcification.

It is very well established that purinergic signaling plays a relevant role in vascular physiology and pathophysiology. Recently, a new purinoceptor agonist uridine adenosine tetraphosphate (Up(4)A) has been identified as a highly potent endothelial-derived contracting factor (EDCF). The purpose of the study was to investigate Up(4)A's influence on pro-inflammatory mechanisms. An early component of the inflammatory response in atherogenesis is the oxidative stress-induced formation of monocyte chemoattractant protein-1 (MCP-1). Here, we investigated the influence of Up(4)A on MCP-1 formation and characterized the underlying signaling transduction mechanisms in rat vascular smooth muscle cells (VSMCs). Up(4)A induced MCP-1 expression and secretion in VSMCs via the activation of P2Y(2) in a concentration-dependent manner. MCP-1 formation depends on generation of reactive oxygen species (ROS). To determine whether the predominant source of ROS in the vasculature, the NAD(P)H oxidase (Nox), is involved, we used different approaches. The ROS scavenger, tiron, the Nox inhibitor, apocynin and diphenyl-iodonium, as well as Nox1 knockdown, diminished the Up(4)A-induced MCP-1 formation. Rac1 activation and p47(phox) translocation from cytosol to the plasma membrane-both required for assembling and activation of Nox, were stimulated by Up(4)A. ERK1/2 and p38 activation is essential for the intracellular signal transduction. In summary, Up(4)A induced Nox1-dependent ROS generation, which further stimulated MCP-1 formation via MAPK phosphorylation in VSMCs. This process requires the activation of the G-protein coupled receptor P2Y(2). Therefore, Up(4)A is not only a potent EDCF but also a potent inductor of pro-inflammatory response in the vascular wall.

The interplay of sphingosine 1-phosphate (S1P) synthetic and degradative enzymes as well as S1P exporters creates concentration gradients that are a fundamental to S1P biology. Extracellular S1P levels, such as in blood and lymph, are high relative to cellular S1P. The blood-tissue S1P gradient maintains endothelial integrity while local S1P gradients influence immune cell positioning. Indeed, the importance of S1P gradients was recognized initially when the mechanism of action of an S1P receptor agonist used as a medicine for multiple sclerosis was revealed to be inhibition of T-lymphocytes’ recognition of the high S1P in efferent lymph. Furthermore, the increase in erythrocyte S1P in response to hypoxia influences oxygen delivery during high altitude acclimatization. However, understanding of how S1P gradients are maintained is incomplete. For example, S1P is synthesized but is only slowly metabolized by blood yet circulating S1P turns over quickly by an unknown mechanism. Prompted by the counterintuitive observation that blood S1P increases markedly in response to inhibition S1P synthesis (by sphingosine kinase 2 (SphK2)), we studied mice wherein several tissues were made deficient in either SphK2 or S1P degrading enzymes. Our data reveal a mechanism whereby S1P is de-phosphorylated at the hepatocyte surface and the resulting sphingosine is sequestered by SphK phosphorylation and in turn degraded by intracellular S1P lyase. Thus, we identify the liver as the primary site of blood S1P clearance and provide an explanation for the role of SphK2 in this process. Our discovery suggests a general mechanism whereby S1P gradients are shaped.

Sulfonyl-triazole probes modified with a kinase recognition element are developed for live cell activity-based profiling to identify tyrosine sites located in catalytic and regulatory domains that are important for kinase function.