Structurally related nucleotides as selective agonists and antagonists at P2Y1 receptors

Fluorescent N2,N3-epsilon-adenine nucleoside and nucleotide probes: synthesis, spectroscopic properties, and biochemical evaluation

N1,N(6)-ethenoadenine, epsilon-A, nucleos(t)ides have been previously applied as fluorescent probes in numerous biochemical systems. However, these epsilon-A analogues lack the H-bonding capability of adenine. To improve the fluorescence characteristics while preserving the H-bonding pattern required for molecular recognition, we designed a novel probe: N(2),N3-etheno-adenosine, (N(2),N3-epsilon-A). Here, we describe four novel syntheses of the target epsilon-nucleoside and related analogues. These methods are short, facile, and provide the product regiospecifically. In addition, we report the absorption and emission spectra of N(2),N3-epsilon-A and the dependence of the spectral features on the pH and polarity of the medium. Specifically, maximum emission of N(2),N3-epsilon-A in water is observed at 420 nm (phi=0.03, excitation at 290 nm). The biochemical relevance of the new probe was evaluated with respect to the P2Y(1) receptor and NTPDases 1 and 2. N(2),N3-epsilon-ATP was found to be almost equipotent with ATP at the P2Y(1) receptor and was hydrolyzed by NTPDases 1 and 2 at about 80 % of the rate of ATP. Furthermore, protein binding does not seem to shift the fluorescence of N(2),N3-epsilon-ATP. Based on the fluorescence and full recognition by ATP-binding proteins, we propose N(2),N3-epsilon-ATP and related nucleo(s)tides as unique probes for the investigation of adenine nucleo(s)tide-binding proteins as well as for other biochemical applications.

(N)-methanocarba-2MeSADP (MRS2365) is a subtype-specific agonist that induces rapid desensitization of the P2Y1 receptor of human platelets

Adenosine diphosphate (ADP) initiates and maintains sustained aggregation of platelets through simultaneous activation of both the Gq-coupled P2Y1 receptor and the Gi-coupled P2Y12 receptor. We recently described the synthesis and P2Y1 receptor-specific agonist activity of (N)-methanocarba-2MeSADP (MRS2365). Consequences of selective activation of the P2Y1 receptor by MRS2365 have been further examined in human platelets. Whereas MRS2365 alone only induced shape change, addition of MRS2365 following epinephrine treatment, which activates the Gi/z-linked, alpha2A-adrenergic receptor, resulted in sustained aggregation that was indistinguishable from that observed with ADP. Conversely, the platelet shape change promoted by ADP in the presence of the GPIIb/IIIa antagonist eptifibatide was similar to that promoted by MRS2365. Preaddition of the high affinity P2Y1 receptor antagonist MRS2500 inhibited the effect of MRS2365, whereas addition of MRS2500 subsequent to MRS2365 reversed the MRS2365-induced shape change. Preactivation of the P2Y1 receptor with MRS2365 for 2 min resulted in marked loss of capacity of ADP to induce aggregation as evidenced by a greater than 20-fold rightward shift in the concentration effect curve of ADP. This inhibitory effect of P2Y1 receptor activation was dependent on the concentration of MRS2365 (EC50 = 34 nm). The inhibitory effect of preincubation with MRS2365 was circumvented by activation of the Gq-coupled 5-HT2A receptor suggesting that MRS2365 induces loss of the ADP response as a consequence of desensitization of the Gq-coupled P2Y1 receptor. The time course of MRS2365-induced loss of aggregation response to epinephrine was similar to that observed with ADP. These results further demonstrate the P2Y1 receptor selectivity of MRS2365 and illustrate the occurrence of agonist-induced desensitization of the P2Y1 receptor of human platelets studied in the absence of P2Y12 receptor activation .

Differential effects of P2Y1 and P2Y12 nucleotide receptors on ERK1/ERK2 and phosphatidylinositol 3-kinase signalling and cell proliferation in serum-deprived and nonstarved glioma C6 cells

We have previously shown that, in glioma C6 cells, two nucleotide ADP-sensitive receptors coexist: P2Y1, coupled to PLC and responsible for Ca2+ release, and P2Y12, negatively coupled to adenylate cyclase. In the present study, we examined the effects of the stimulation of these two receptors on ERK1/2 and PI3-K activation, and cell proliferation in either serum-deprived or nonstarved C6 cells. In response to ADP and its analogues, in serum-starved cells, both p44 ERK1 and p42 ERK2 were activated in a time-dependent manner, as monitored by Western blot analysis using an antiphospho-p42/p44 MAPK antibody. The phosphorylation was reduced both by removal of the extracellular Ca2+ and partially or almost completely by MRS2179 or AR-C69931MX, specific antagonists of the P2Y1 and P2Y12 receptors, respectively. The inhibitory effect of antagonists was additive. These data indicate the involvement of both receptors, P2Y1 and P2Y12, in the ERK1/2 activation, but the P2Y12 receptor contribution predominates. ERK1/2 activity was positively correlated with cell proliferation of cultured glioma C6 cells. In nonstarved cells, ADP markedly decreased the PI3-K activity. In contrast, in serum-starved cells, ADP evoked an increase in the PI3-K activity. Blocking of the P2Y1 receptor by MRS2179 additionally increased this ADP response. These results suggest that the P2Y1 receptor has an inhibitory and the P2Y12 receptor a stimulatory effect on PI3-K signalling pathway. RT-PCR analysis revealed different mRNA expression of both receptors in starved and nonstarved cells. In nonstarved cells, the P2Y1 receptor mRNA predominates, whereas in serum-deprived cells the expression of P2Y12 mRNA becomes more pronounced. British Journal of Pharmacology (2004) 141, 497-507. doi:10.1038/sj.bjp.0705639

ADP-evoked phospholipase C stimulation and adenylyl cyclase inhibition in glioma C6 cells occur through two distinct nucleotide receptors, P2Y(1) and P2Y(12)

In this study we characterized the subtypes of nucleotide P2Y receptors that respond to ADP in glioma C6 cells. Direct visualization of phosphatidylinositol 4,5-bisphosphate at the cell surface revealed that extracellular ADP activates phospholipase C (PLC). Knock-down of P2Y(1) receptor with antisense oligonucleotide, as well as treatment with MRS2179 and pyridoxal-phosphate-6-azophenyl-2',4'-disulfonic acid (P2Y(1) antagonists), attenuates receptor-mediated PLC activity. Adenylyl cyclase inhibition by ADP remains unchanged under these conditions. Reverse transcription-PCR analysis showed that P2Y(12) receptor is expressed in C6 cells. We therefore conclude that, in glioma C6 cells, two P2Y receptor subtypes are present: P2Y(1), coupled to PLC, and P2Y(12), negatively coupled to adenylyl cyclase.