Diketopiperazines (DKPs) are a common motif in various biologically active natural products, and hence they may be useful scaffolds for the rational design of receptor probes and therapeutic agents. We constructed a new bicyclic scaffold that combines a DKP bridged with a 10-membered ring. In this way we obtained a three-dimensional molecular skeleton, with several amendable sites that provide a starting point to design a new combinatorial library having diverse substituent groups. Structural variation is based upon the flexibility of alkylation of the nitrogen atoms of the DKP and on the side-chain olefin. We obtained a 10-membered secondary ring through a ring-closure metathesis reaction using the second generation Grubbs catalyst. Rings containing both O-ethers and S-ethers were compared. N-Alkyl or arylalkyl groups were introduced optionally at the two Nα-atoms. This is a general scheme that will allow us to test rings of varying sizes, linkages, and stereochemical parameters. The DKP derivatives were tested for activity in astrocytoma cells expressing receptors coupled to phospholipase C. Inhibitory effects were observed for signaling elicited by activation of human nucleotide P2Y receptors but not m3 muscarinic receptors. Compound 20 selectively inhibited calcium mobilization (IC50 value of 486 ± 16 nM) and phosphoinositide turnover elicited by a selective P2Y1 receptor agonist, but this compound did not compete for binding of a radiolabeled nucleotide-competitive receptor antagonist. Therefore, the new class of DKP derivatives shows utility as pharmacological tools for P2Y receptors.
An alternative approach to overcome the inherent lack of specificity of conventional agonist therapy can be the reengineering of the GPCRs and their agonists. A reengineered receptor (neoceptor) could be selectively activated by a modified agonist, but not by the endogenous agonist. Assisted by rhodopsin-based molecular modeling, we pinpointed mutations of the A3 adenosine receptor (AR) for selective affinity enhancement following complementary modifications of adenosine. Ribose modifications examined included, at 3′: amino, aminomethyl, azido, guanidino, ureido; and at 5′: uronamido, azidodeoxy. N6-variations included: 3-iodobenzyl, 5-chloro-2-methyloxybenzyl, and methyl. An N6-3-iodobenzyl-3′-ureido adenosine derivative 10 activated phospholipase C in COS-7 cells (EC50=0.18 μM) or phospholipase D in chick primary cardiomyocytes mediated by a mutant (H272E), but not the wild-type, A3AR. The affinity enhancements for 10 and the corresponding 3′-acetamidomethyl analogue 6 were >100-fold and >20-fold, respectively. 10 concentration-dependently protected cardiomyocytes transfected with the neoceptor against hypoxia. Unlike 10, adenosine activated the wild-type A3AR (EC50 of 1.0 μM), but had no effect on the H272E mutant A3AR (100 μM). Compound 10 was inactive at human A1, A2A, and A2BARs. The orthogonal pair comprising an engineered receptor and a modified agonist should be useful for elucidating signaling pathways and could be therapeutically applied to diseases following organ-targeted delivery of the neoceptor gene.
Activation of either the A1 or the A3 adenosine receptor (A1R or A3R, respectively) elicits delayed cardioprotection against infarction, ischemia, and hypoxia. Mitochondrial contribution to the progression of cardiomyocyte injury is well known; however, the protective effects of adenosine receptor activation in cardiac cells with a respiratory chain deficiency are poorly elucidated. The aim of our study was to further define the role of A1R and A3R activation on functional tolerance after inhibition of the terminal link of the mitochondrial respiratory chain with sodium azide, in a state of normoxia or hypoxia, compared with the effects of the mitochondrial ATP-sensitive K+ channel opener diazoxide. Treatment with 10 mM sodium azide for 2 h in normoxia caused a considerable decrease in the total ATP level; however, activation of adenosine receptors significantly attenuated this decrease. Diazoxide (100 µM) was less effective in protection. During treatment of cultured cardiomyocytes with hypoxia in the presence of 1 mM sodium azide, the A1R agonist 2-chloro-N6-cyclopentyladenosine was ineffective, whereas the A3R agonist 2-chloro-N6-iodobenzyl-5′-N-methylcarboxamidoadenosine (Cl-IB-MECA) attenuated the decrease in ATP level and prevented cell injury. Cl-IB-MECA delayed the dissipation in the mitochondrial membrane potential during hypoxia in cells impaired in the mitochondrial respiratory chain. In cells with elevated intracellular Ca2+ concentration after hypoxia and treatment with NaN3 or after application of high doses of NaN3, Cl-IB-MECA immediately decreased the elevated intracellular Ca2+ concentration toward the diastolic control level. The A1R agonist was ineffective. This may be especially important for the development of effective pharmacological agents, because mitochondrial dysfunction is a leading factor in the pathophysiological cascade of heart disease.
Ca2+ transience; hypoxia; ATP-sensitive K+ channel; sodium azide; heart disease; ischemia
Human P2Y receptors encompass at least eight subtypes of Class A G protein-coupled receptors (GPCRs), responding to adenine and/or uracil nucleotides. Using a BLAST search against the Homo sapiens subset of the SWISS–PROT and TrEMBL databases, we identified 68 proteins showing high similarity to P2Y receptors. To address the problem of low sequence identity between rhodopsin and the P2Y receptors, we performed a multiple-sequence alignment of the retrieved proteins and the template bovine rhodopsin, combining manual identification of the transmembrane domains (TMs) with automatic techniques. The resulting phylogenetic tree delineated two distinct subgroups of P2Y receptors: Gq-coupled subtypes (e.g., P2Y1) and those coupled to Gi (e.g., P2Y12). On the basis of sequence comparison we mutated three Tyr residues of the putative P2Y1 binding pocket to Ala and Phe and characterized pharmacologically the mutant receptors expressed in COS-7 cells. The mutation of Y306 (7.35, site of a cationic residue in P2Y12) or Y203 in the second extracellular loop selectively decreased the affinity of the agonist 2-MeSADP, and the Y306F mutation also reduced antagonist (MRS2179) affinity by 5-fold. The Y273A (6.48) mutation precluded the receptor activation without a major effect on the ligand-binding affinities, but the Y273F mutant receptor still activated G proteins with full agonist affinity. Thus, we have identified new recognition elements to further define the P2Y1 binding site and related these to other P2Y receptor subtypes. Following sequence-based secondary-structure prediction, we constructed complete models of all the human P2Y receptors by homology to rhodopsin. Ligand docking on P2Y1 and P2Y12 receptor models was guided by mutagenesis results, to identify the residues implicated in the binding process. Different sets of cationic residues in the two subgroups appeared to coordinate phosphate-bearing ligands. Within the P2Y1 subgroup these residues are R3.29, K/R6.55, and R7.39. Within the P2Y12 subgroup, the only residue in common with P2Y1 is R6.55, and the role of R3.29 in TM3 seems to be fulfilled by a Lys residue in EL2, whereas the R7.39 in TM7 seems to be substituted by K7.35. Thus, we have identified common and distinguishing features of P2Y receptor structure and have proposed modes of ligand binding for the two representative subtypes that already have well-developed ligands.
The physiological role of the P2Y6 nucleotide receptor may involve cardiovascular, immune and digestive functions based on the receptor tissue distribution, and selective antagonists for this receptor are lacking. We have synthesized a series of symmetric aryl diisothiocyanate derivatives and examined their ability to inhibit phospholipase C (PLC) activity induced by activation of five subtypes of recombinant P2Y receptors. Several derivatives were more potent at inhibiting action of UDP at both human and rat P2Y6 receptors expressed in 1321N1 human astrocytes than activation of human P2Y1, P2Y2, P2Y4 and P2Y11 receptors. The inhibition by diisothiocyanate derivatives of 1,2-diphenylethane (MRS2567) and 1,4-di-(phenylthioureido) butane (MRS2578) was concentration-dependent and insurmountable, with IC50 values of 126 ± 15 nM and 37 ± 16 nM (human) and 101 ± 27 nM (rat), respectively. A derivative of 1,4-phenylendiisothiocyanate (MRS2575) inhibited only human but not rat P2Y6 receptor activity. MRS2567 and MRS2578 at 10 μM did not affect the UTP (100 nM)-induced responses of cells expressing P2Y2 and P2Y4 receptors, nor did they affect the 2-methylthio-ADP (30 nM)-induced responses at the P2Y1 receptor or the ATP (10 μM)-induced responses at the P2Y11 receptor. Other antagonists displayed mixed selectivities. The selective antagonists MRS2567, MRS2575 and MRS2578 (1 μM) completely blocked the protection by UDP of cells undergoing TNFα-induced apoptosis. Thus, we have identified potent, insurmountable antagonists of P2Y6 receptors that are selective within the family of PLC-coupled P2Y receptors.
P2Y6 nucleotide receptor; GPCR; Pyrimidines; Purines; Isothiocyanate; Apoptosis
The affinity and efficacy at four subtypes (A1, A2A, A2B and A3) of human adenosine receptors (ARs) of a wide range of 2-substituted adenosine derivatives were evaluated using radioligand binding assays and a cyclic AMP functional assay in intact CHO cells stably expressing these receptors. Similar to previous studies of the N6-position, several 2-substituents were found to be critical structural determinants for the A3AR activation. The following adenosine 2-ethers were moderately potent partial agonists (Ki, nM): benzyl (117), 3-chlorobenzyl (72), 2-(3-chlorophenyl)ethyl (41), and 2-(2-naphthyl)ethyl (130). The following adenosine 2-ethers were A3AR antagonists: 2,2-diphenylethyl, 2-(2-norbornan)ethyl, R- and S-2-phenylbutyl, and 2-(2-chlorophenyl)ethyl. 2-(S-2-Phenylbutyloxy)a-denosine as an A3AR antagonist right-shifted the concentration–response curve for the inhibition by NECA of cyclic AMP accumulation with a KB value of 212 nM, which is similar to its binding affinity (Ki = 175 nM). These 2-substituted adenosine derivatives were generally less potent at the A1AR in comparison to the A3AR, but fully efficacious, with binding Ki values over 100 nM. The 2-phenylethyl moiety resulted in higher A3AR affinity (Ki in nM) when linked to the 2-position of adenosine through an ether group (54), than when linked through an amine (310) or thioether (1960). 2-[2-(l-Naphthyl)ethyloxy]adenosine (Ki = 3.8 nM) was found to be the most potent and selective (>50-fold) A2A agonist in this series. Mixed A2A/A3AR agonists have been identified. Interestingly, although most of these compounds were extremely weak at the A2BAR, 2-[2-(2-naphthyl)ethyloxy]adenosine (EC50 = 1.4 µM) and 2-[2-(2-thienyl)-ethyloxy]adenosine (EC50 = 1.8 (M) were found to be relatively potent A2B agonists, although less potent than NECA (EC50 = 140 nM).
Adenosine receptors; Purines; Nucleosides; GPCR; Efficacy; Structure–activity relationships
We have synthesized a series of derivatives of the known P2 receptor antagonist PPADS (pyridoxal-5′-phosphate-6-azo-phenyl-2,4-disulfonate) and examined their ability to inhibit functional activity of the recombinant human P2Y13 nucleotide receptor expressed in 1321N1 human astrocytoma cells co-expressing Gα16 protein (AG32). Analogues of PPADS modified through substitution of the phenylazo ring, including halo and nitro substitution, and 5′-alkyl phosphonate analogues were synthesized and tested. A 6-benzyl-5′-methyl phosphonate analogue was prepared to examine the effect of stable replacement of the azo linkage. The highest antagonistic potency was observed for 6-(3-nitrophenylazo) derivatives of pyridoxal-5′-phosphate. The 2-chloro-5-nitro analogue (MRS 2211) and 4-chloro-3-nitro analogue (MRS 2603) inhibited ADP (100 nM)-induced inositol trisphosphate (IP3) formation with pIC50 values of 5.97 and 6.18, respectively, being 45- and 74-fold more potent than PPADS. The antagonism of MRS 2211 was competitive with a pA2 value of 6.3. MRS2211 and MRS2603 inhibited phospholipase C (PLC) responses to 30 nM 2-methylthio-ADP in human P2Y1 receptor-mediated 1321N1 astrocytoma cells with IC50 values of >10 and 0.245 μM, respectively. Both analogues were inactive (IC50 > 10 μM) as antagonists of human P2Y12 receptor-mediated PLC responses in 1321N1 astrocytoma cells. Thus, MRS2211 displayed >20-fold selectivity as antagonist of the P2Y13 receptor in comparison to P2Y1 and P2Y12 receptors, while MRS2603 antagonized both P2Y1 and P2Y13 receptors.
PPADS (pyridoxal-5′-phosphate-6-azo-phenyl-2,4-disulfonate); Pyridoxal phosphate derivatives; Adenine nucleotides; P2Y13 receptor; Inositol trisphosphate; Purines
We have found previously that structural features of adenosine derivatives, particularly at the N6- and 2-positions of adenine, determine the intrinsic efficacy as A3 adenosine receptor (AR) agonists. Here, we have probed this phenomenon with respect to the ribose moiety using a series of ribose-modified adenosine derivatives, examining binding affinity and activation of the human A3 AR expressed in CHO cells. Both 2′- and 3′-hydroxyl groups in the ribose moiety contribute to A3 AR binding and activation, with 2′-OH being more essential. Thus, the 2′-fluoro substitution eliminated both binding and activation, while a 3′-fluoro substitution led to only a partial reduction of potency and efficacy at the A3 AR. A 5′-uronamide group, known to restore full efficacy in other derivatives, failed to fully overcome the diminished efficacy of 3′-fluoro derivatives. The 4′-thio substitution, which generally enhanced A3 AR potency and selectivity, resulted in 5′-CH2OH analogues (10 and 12) which were partial agonists of the A3 AR. Interestingly, the shifting of the N6-(3-iodobenzyl)adenine moiety from the 1′- to 4′-position had a minor influence on A3 AR selectivity, but transformed 15 into a potent antagonist (16) (Ki = 4.3 nM). Compound 16 antagonized human A3 AR agonist-induced inhibition of cyclic AMP with a KB value of 3.0 nM. A novel apio analogue (20) of neplanocin A, was a full A3 AR agonist. The affinities of selected, novel analogues at rat ARs were examined, revealing species differences. In summary, critical structural determinants for human A3 AR activation have been identified, which should prove useful for further understanding the mechanism of receptor activation and development of more potent and selective full agonists, partial agonists and antagonists for A3 ARs.
Nucleosides; A3 adenosine receptor agonist; A3 adenosine receptor antagonist; Adenylyl cyclase; Phospholipase C; Partial agonist
ADP is the endogenous agonist for both P2Y1 and P2Y12 receptors, which are important therapeutic targets. It was previously demonstrated that ADP and a synthetic agonist, 2-methylthioadenosine 5′-diphosphate (2MeSADP), can induce apoptosis by activating the human P2Y1 receptor heterologously expressed in astrocytoma cells. However, it was not known whether the P2Y12 receptor behaved similarly. We demonstrated here that, unlike with the Gq-coupled P2Y1 receptor, activation of the Gi-coupled P2Y12 receptor does not induce apoptosis. Furthermore, activation of the P2Y12 receptor by either ADP or 2MeSADP significantly attenuates the tumor necrosis factor α (TNFα)-induced apoptosis in 1321N1 human astrocytoma cells. This protective effect was blocked by the P2Y12 receptor antagonist 2-methylthioAMP and by inhibitors of phospholipase C (U73122) and protein kinase C (chelerythrin), but not by the P2Y1 receptor antagonist MRS2179. Toward a greater mechanistic understanding, we showed that hP2Y12 receptor activation by 10 nM 2MeSADP, activates Erk1/2, Akt, and JNK by phosphorylation. However, at a lower protective concentration of 100 pM 2MeSADP, activation of the hP2Y12 receptor involves only phosphorylated Erk1/2, but not Akt or JNK. This activation is hypothesized as the major mechanism for the protective effect induced by P2Y12 receptor activation. Apyrase did not affect the ability of TNFα to induce apoptosis in hP2Y12-1321N1 cells, suggesting that the endogenous nucleotides are not involved. These results may have important implications for understanding the signaling cascades that follow activation of P2Y1 and P2Y12 receptors and their opposing effects on cell death pathways.
Apoptosis; Nucleotides; G protein-coupled receptors; Tumor necrosis factor; Phospholipase C; Protein kinase C
Activation of the Gq-coupled P2Y6 receptor heterologously expressed in astrocytes significantly attenuates apoptosis induced by tumor necrosis factor α (TNFα). We have extended the analysis of P2Y6 receptor-induced cytoprotection to mouse skeletal muscle cells endogenously expressing this receptor. The endogenous P2Y6 receptor agonist UDP and synthetic agonist MRS2693 protected C2C12 skeletal muscle cells against apoptosis in a concentration-dependent manner (0.1−10 nM) as determined by propidium iodide staining, histochemical analysis using hematoxylin and Hoechst 33258, and DNA fragmentation. The insurmountable P2Y6 receptor antagonist MRS2578 blocked the protection. TNFα-induced apoptosis in C2C12 cells correlated with activation of the transcription factor NF-κB. The NF-κB activation was attenuated by 10 nM MRS2693, which activated the antiapoptic ERK1/2 pathway. In an in vivo mouse hindlimb model, MRS2693 protected against skeletal muscle ischemia/reperfusion injury. The P2Y6 receptor is a novel cytoprotective receptor that deserves further exploration in ameliorating skeletal muscle injury.
ischemia; uracil nucleotide; G protein-coupled receptor; tumor necrosis factor; skeletal muscle; UDP
Combining molecular dynamics (MD) in a hydrated phospholipids (DOPC) bilayer, Monte Carlo search, and synthesis of locked nucleotide analogues we discovered that the Southern conformation of the ribose is preferred for ligand recognition by the P2Y6 receptor. 2′-Deoxy-(S)-methanocarbaUDP was found to be a full agonist of the receptor and displayed a 10-fold higher potency than the corresponding flexible 2′-deoxyUDP. MD results also suggested a conformational change of the second extracellular loop consequent to agonist binding.
Cardioprotection (delaying of irreversible damage in hypoxia or prevention of doxorubicin [DOX] toxicity) is achieved by increasing the energy supply, or decreasing the energy demand in the cell and may be regulated through adenosine (ADO) receptor (AR) signalling. The aim of this study was to define of the protective role of ADO A1R and A3R against these two different kinds of stress conditions via direct action on isolated cardiomyocytes. Effects of A1 and A3 adenosine receptors were assessed by comparing morphological-functional tolerance, cellular energy state and contribution of the mitochondrial KATP channels during development of hypoxia and DOX cytotoxicity.
The primary cardiac myocyte cultures were treated in a hypoxic chamber of N2 (100%) in glucose-free media. A second group of cells were treated on day 4 in culture with 0.5 to 5 μM DOX for 18 h and then incubated in drug-free growth medium for an additional 24 h or 72 h. The hypoxic and cytotoxic damage was characterized by morphological and biochemical evaluations.
The A1R and A3R selective agonists (CCPA and Cl-IB-MECA, respectively) significantly decreased damage to cardiac myocytes under hypoxic conditions. Activation of both A1R and A3R together (100 nM) was more efficient in protection against hypoxia than by each one alone. The A3R agonist Cl-IB-MECA (100 nM) shows cardioprotective activity to the DOX-treated cells; however, the A1R agonist CCPA (10 nM to 10 μM) was not effective in protection against DOX toxicity.
Activation of both the ADO receptors (A1R and A3R) leads to positive beneficial effects in cultured cardiomyocytes in 90 min hypoxia, but only A3R activation renders positive response against slowly developed DOX toxicity. Hence, the cascade of events involved in cardioprotection appears to be distinct for A1 and A3 receptor signalling.
Adenosine receptors; Cardiomyocytes; Cardioprotection; Doxorubicin; Hypoxia