Arginine-178 is an essential residue for ITPA function

The inosine triphosphate pyrophosphatase (ITPA) enzyme plays a critical cellular role by removing noncanonical nucleoside triphosphates from nucleotide pools. One of the first pathological ITPA mutants identified is R178C (rs746930990), which causes a fatal infantile encephalopathy, termed developmental and epileptic encephalopathy 35 (DEE 35). The accumulation of noncanonical nucleotides such as inosine triphosphate (ITP), is suspected to affect RNA and/or interfere with normal nucleotide function, leading to development of DEE 35. Molecular dynamics simulations have shown that the very rare R178C mutation does not significantly perturb the overall structure of the protein, but results in a high level of structural flexibility and disrupts active-site hydrogen bond networks, while preliminary biochemical data indicate that ITP hydrolyzing activity is significantly reduced for the R178C mutant. Here we report Michaelis-Menten enzyme kinetics data for the R178C ITPA mutant and three other position 178 ITPA mutants. These data confirm that position 178 is essential for ITPA activity and even conservative mutation at this site (R178K) results in significantly reduced enzyme activity. Our data support that disruption of the active-site hydrogen bond network is a major cause of diminished ITP hydrolyzing activity for the R178C mutation. These results suggest an avenue for developing therapies to address DEE 35.

The GTP responsiveness of PI5P4Kβ evolved from a compromised trade-off between activity and specificity

Unlike most kinases, phosphatidylinositol 5-phosphate 4-kinase β (PI5P4Kβ) utilizes GTP as a physiological phosphate donor and regulates cell growth under stress (i.e., GTP-dependent stress resilience). However, the genesis and evolution of its GTP responsiveness remain unknown. Here, we reveal that PI5P4Kβ has acquired GTP preference by generating a short dual-nucleotide-recognizing motif called the guanine efficient association (GEA) motif. Comparison of nucleobase recognition with 660 kinases and 128 G proteins has uncovered that most kinases and PI5P4Kβ use their main-chain atoms for adenine recognition, while the side-chain atoms are required for guanine recognition. Mutational analysis of the GEA motif revealed that the acquisition of GTP reactivity is accompanied by an extended activity toward inosine triphosphate (ITP) and xanthosine triphosphate (XTP). Along with the evolutionary analysis data that point to strong negative selection of the GEA motif, these results suggest that the GTP responsiveness of PI5P4Kβ has evolved from a compromised trade-off between activity and specificity, underpinning the development of the GTP-dependent stress resilience.

Inosine Triphosphate Pyrophosphatase (ITPase): Functions, Mutations, Polymorphisms and Its Impact on Cancer Therapies

Inosine triphosphate pyrophosphatase (ITPase) is an enzyme encoded by the ITPA gene and functions to prevent the incorporation of noncanonical purine nucleotides into DNA and RNA. Specifically, the ITPase catalyzed the hydrolysis of (deoxy) nucleoside triphosphates ((d) NTPs) into the corresponding nucleoside monophosphate with the concomitant release of pyrophosphate. Recently, thiopurine drug metabolites such as azathioprine have been included in the lists of ITPase substrates. Interestingly, inosine or xanthosine triphosphate (ITP/XTP) and their deoxy analogs, deoxy inosine or xanthosine triphosphate (dITP/dXTP), are products of important biological reactions such as deamination that take place within the cellular compartments. However, the incorporation of ITP/XTP, dITP/dXTP, or the genetic deficiency or polymorphism of the ITPA gene have been implicated in many human diseases, including infantile epileptic encephalopathy, early onset of tuberculosis, and the responsiveness of patients to cancer therapy. This review provides an up-to-date report on the ITPase enzyme, including information regarding its discovery, analysis, and cellular localization, its implication in human diseases including cancer, and its therapeutic potential, amongst others.

Biochemical characteristics of the Ca2+ pumping ATPase in the peribacteroid membrane from broad bean root nodules

Ca(2+)-ATPase in the peribacteroid membrane (PBM) of symbiosomes isolated from Vicia faba root nodules was characterized in terms of its hydrolytic and transport activities. Both activities were found to be pH-dependent and exhibit pH optimum at pH 7.0. Translocation of Ca(2+) through the PBM by the Ca(2+)-ATPase was shown to be fueled by ATP and other nucleotide triphosphates in the following order: ATP > ITP ≅ GTP ≅ UTP ≅ CTP, the K m of the enzyme for MgATP being about 100 μM. Ca-dependent ITP-hydrolytic activity of symbiosomes was investigated in the presence of the Ca-EGTA buffer system and showed the affinity of PBM Ca(2+)-ATPase for Ca(2+) of about 0.1 μM. The transport activity of Ca(2+)-ATPase was inhibited by erythrosin B as well as orthovanadate, but markedly stimulated by calmodulin from bovine brain. These results allowed us to conclude that this enzyme belongs to IIB-type Ca(2+)-ATPases which are present in other plant membranes.

The effect of ITPA polymorphisms on the enzyme kinetic properties of human erythrocyte inosine triphosphatase toward its substrates ITP and 6-Thio-ITP

The role of inosine triphosphatase (ITPase) in adverse drug reactions associated with thiopurine therapy is still under heavy debate. Surprisingly, little is known about the way thiopurines are handled by ITPase. We studied the effect of ITPA polymorphisms on the handling of inosine triphosphate (ITP) and thioinosine triphosphate (TITP) to gain more insight into this phenomenon. Human erythrocyte ITPase activity was measured by incubation with ITP using established protocols, and the generated inosine monophosphate (IMP) was measured using ion-pair RP-HPLC. Molecular analysis of the ITPA gene was performed to establish the genotype. Kinetic parameters were established for the two common polymorphisms for both ITP and TITP as substrates using the above mentioned protocol. Both ITP and TITP are substrates for ITPase and their enzyme activities are comparable. Substrate binding is not altered in the different ITPA polymorphisms. It is shown that the velocity of pyrophosphohydrolysis is compromised when the c.94C > A polymorphism is present, both in the heterozygous and in the homozygous state. TITP is handled by ITPase in a similar way as for ITP, which implies that TITP will accumulate in the erythrocytes of patients with an ITPase deficiency, resulting in adverse drug reactions (ADRs) on thiopurine therapy. In carriers of ITPA polymorphisms, the matter is more complex and the development of ADR may depend on additional epigenetic factors rather than on the accumulation of thiopurinenucleotides.

Melatonin sensitizes human myometrial cells to oxytocin in a protein kinase C alpha/extracellular-signal regulated kinase-dependent manner

CONTEXT

Studies have shown that labor occurs primarily in the night/morning hours. Recently, we identified the human myometrium as a target for melatonin (MEL), the neuroendocrine output signal coding for circadian night.

OBJECTIVE

The purpose of this study was to determine the signaling pathway underlying the effects of MEL on contractility and the contractile machinery in immortalized human myometrial cells.

DESIGN

To ascertain the signaling pathway of MEL leading to its effects on myometrial contractility in vitro, we performed gel retraction assays with cells exposed to iodo-MEL (I-MEL) with or without oxytocin and the Rho kinase inhibitor Y27632. I-MEL effects on inositol trisphosphate (IP(3))/diacylglycerol (DAG)/protein kinase C (PKC) signaling were also investigated. Additionally, we assayed for caldesmon phosphorylation and ERK1/2 activation.

RESULTS

I-MEL was found to activate PKC alpha via the phospholipase C/IP(3)/DAG signaling pathway, which was confirmed by PKC enzyme assay. I-MEL did not affect myosin light chain phosphatase activity, and its effects on contractility were insensitive to Rho kinase inhibition. I-MEL did increase phosphorylation of ERK1/2 and caldesmon, which was inhibited by the MAPK kinase inhibitor PD98059 or the PKC inhibitor C1.

CONCLUSIONS

MEL sensitizes myometrial cells to subsequent procontractile signals in vitro through activation of the phospholipase C/IP(3)/DAG signaling pathway, resulting in specific activation of PKC alpha and ERK1/2, thereby phosphorylating caldesmon, which increases actin availability for myosin binding and cross-bridging. In vivo, this sensitization would provide a mechanism for the increased nocturnal uterine contractility and labor that has been observed in late-term human pregnancy.

NUDT16 and ITPA play a dual protective role in maintaining chromosome stability and cell growth by eliminating dIDP/IDP and dITP/ITP from nucleotide pools in mammals

Mammalian inosine triphosphatase encoded by ITPA gene hydrolyzes ITP and dITP to monophosphates, avoiding their deleterious effects. Itpa(-) mice exhibited perinatal lethality, and significantly higher levels of inosine in cellular RNA and deoxyinosine in nuclear DNA were detected in Itpa(-) embryos than in wild-type embryos. Therefore, we examined the effects of ITPA deficiency on mouse embryonic fibroblasts (MEFs). Itpa(-) primary MEFs lacking ITP-hydrolyzing activity exhibited a prolonged doubling time, increased chromosome abnormalities and accumulation of single-strand breaks in nuclear DNA, compared with primary MEFs prepared from wild-type embryos. However, immortalized Itpa(-) MEFs had neither of these phenotypes and had a significantly higher ITP/IDP-hydrolyzing activity than Itpa(-) embryos or primary MEFs. Mammalian NUDT16 proteins exhibit strong dIDP/IDP-hydrolyzing activity and similarly low levels of Nudt16 mRNA and protein were detected in primary MEFs derived from both wild-type and Itpa(-) embryos. However, immortalized Itpa(-) MEFs expressed significantly higher levels of Nudt16 than the wild type. Moreover, introduction of silencing RNAs against Nudt16 into immortalized Itpa(-) MEFs reproduced ITPA-deficient phenotypes. We thus conclude that NUDT16 and ITPA play a dual protective role for eliminating dIDP/IDP and dITP/ITP from nucleotide pools in mammals.

[Vasodilating effect and its mechanism of ethanol on isolated rat thoracic aorta at different resting tension]

AIM

To investigate the vasodilating effect and its mechanism of ethanol on isolated rat thoracic aorta at different resting tension.

METHODS

The tension of the isolated Sprague-Dawley rat thoracic aorta rings perfused with different concentrations of ethanol was measured using organ bath technique.

RESULTS

At different resting tension (1.0, 1.5, 2.0, 2.5, 3.0, 3.5 and 4.0 g), ethanol (0.1-7.0 per thousand) caused a concentration-dependent relaxation on endothelium-denuded aortic rings precontracted with KCl (6 x 10(-2)mol/L) or phenylephrine (PE, 10(-6) mol/L), and the vasodilating effect was the most potent when the aortic rings were at the resting tension of 3 g. Ethanol had much less vasodilating effect on endothelium-intact aortic rings. Ethanol at 3 per thousand (the maximum-effect concentration) inhibited the CaCl2 induced contraction and downward shifted concentration-response curve of endothelium-denuded aortic rings pre-contracted with KCI or PE at the resting tension of 3 g. Incubation of aorta with ruthenium red (10(-5) mol/L) or heparin (50 mg/L) decreased the vasodilating effect of ethanol (3.0 per thousand) on endothelium-denuded aorta precontracted with PE at the resting tension of 3 g.

CONCLUSION

Ethanol induces endothelium-independent relaxation on rat thoracic aorta, which is concerned with the resting tension. This effect of ethanol may be mediated by the inhibition of voltage-dependent and receptor-operated Ca2+ channels in the vascular smooth muscle cells. The inhibition of the ryanodine receptor and trisphosphate inositol (IP3) pathway may also contribute to this effect.

[Substrate specificity and kinetic properties of a soluble nucleoside triphosphatase from bovine kidneys]

Soluble nucleoside triphosphatase differing in its properties from all known proteins with NTPase activity was partially purified from bovine kidneys. The enzyme has pH optimum of 7.5, molecular mass of 60 kDa, as estimated by gel chromatography, and shows an absolute dependence on divalent metal ions. NTPase obeyed Michaelis-Menten kinetics in the range of substrate concentration tested from 45 to 440 microM; the apparent Km for inosine-5'-triphosphate was calculated to be 23.3 microM. The enzyme was found to possess a broad substrate specificity, being capable of hydrolyzing various nucleoside-5'-tri- as well as diphosphates.

Enhanced ryanodine-mediated calcium release in mutant PS1-expressing Alzheimer's mouse models

Intracellular Ca(2+) signaling involves Ca(2+) liberation through both inositol triphosphate and ryanodine receptors (IP(3)R and RyR). However, little is known of the functional interactions between these Ca(2+) sources in either neuronal physiology, or during Ca(2+) disruptions associated with Alzheimer's disease (AD). By the use of whole-cell recordings and 2-photon Ca(2+) imaging in cortical slices we distinguished between IP(3)R- and RyR-mediated Ca(2+) components in nontransgenic (non-Tg) and AD mouse models and demonstrate powerful signaling interactions between these channels. Ca(2+)-induced Ca(2+) release (CICR) through RyR contributed modestly to Ca(2+) signals evoked by photoreleased IP(3) in cortical neurons from non-Tg mice. In contrast, the exaggerated signals in 3xTg-AD and PS1(KI) mice resulted primarily from enhanced CICR through RyR, rather than through IP(3)R, and were associated with increased RyR expression levels. Moreover, membrane hyperpolarizations evoked by IP(3) in neurons from AD mouse models were even greater than expected simply from the exaggerated Ca(2+) signals, pointing to an increased coupling efficiency between cytosolic [Ca(2+)] and K(+) channel regulation. Our results highlight the critical roles of RyR-mediated Ca(2+) signaling in both neuronal physiology and pathophysiology, and point to presenilin-linked disruptions in RyR signaling as an important genetic factor in AD.

[Nucleoside-5'-triphosphate hydrolysis in the liver and kidney of rats with chronic alloxan diabetes]

Activity and some properties of a soluble enzyme hydrolyzing nucleoside-5'-triphosphates were studied in the liver and kidney of normal and diabetic rats. The enzyme activity was shown to be reduced by 34% (p < 0.01) in the liver extracts of diabetic animals, while no difference was observed in the kidney. When ITP was used as substrate, the apparent Michaelis constant of the enzyme was significantly lower in the liver of controls as compared to experimental rats (32.3 +/- 1.3 microM and 54.3 +/- 1.0 microM, respectively, p < 0.01). The KM values of the enzyme in the kidney were not distinguishable in both groups. NTPase exhibits maximal activity at pH 7.0 and has a broad substrate specificity with respect to different nucleoside-5'-tri- and diphosphates. Molecular mass of the enzyme was estimated by gel filtration to be 63.7 +/- 0.9 kD.

3-O-acetyl-11-keto-boswellic acid decreases basal intracellular Ca2+ levels and inhibits agonist-induced Ca2+ mobilization and mitogen-activated protein kinase activation in human monocytic cells

Previously, we showed that 11-keto-boswellic acid and 3-O-acetyl-11-keto-BA (AKBA) stimulate Ca(2+) mobilization and activate mitogen-activated protein kinases (MAPKs) in human polymorphonuclear leukocytes (PMNLs). Here, we addressed the effects of boswellic acids on the intracellular Ca(2+) concentration ([Ca(2+)](i)) and on the activation of p38(MAPK) and extracellular signal-regulated kinase (ERK) in the human monocytic cell line Mono Mac (MM) 6. In contrast to PMNLs, AKBA concentration dependently (1-30 microM) decreased the basal [Ca(2+)](i) in resting MM6 cells but also in cells where [Ca(2+)](i) had been elevated by stimulation with platelet-activating factor (PAF). AKBA also strongly suppressed the subsequent elevation of [Ca(2+)](i) induced by N-formyl-methionyl-leucyl-phenylalanine (fMLP), PAF, or by the direct phospholipase C activator 2,4, 6-trimethyl-N-(meta-3-trifluoromethyl-phenyl)-benzenesulfonamide, but AKBA failed to prevent Ca(2+) signals induced by thapsigargin or ionomycin. Suppression of Ca(2+) homeostasis by AKBA was also observed in primary monocytes, isolated from human blood. Moreover, AKBA inhibited the activation of p38(MAPK) and ERKs in fMLP-stimulated MM6 cells. Although the effects of AKBA could be mimicked by the putative phospholipase C (PLC) inhibitor U-73122 (1-[6-[[17beta-methoxyestra-1,3,5(10)-trien-17-yl]amino]hexyl]-1H-pyrrole-2,5-dione), AKBA appears to operate independent of PLC activity since the release of intracellular inositol-1,4,5-trisphosphate evoked by 2,4,6-trimethyl-N-(meta-3-trifluoromethyl-phenyl)-benzenesulfonamide was hardly diminished by AKBA. Inhibitor studies indicate that AKBA may decrease [Ca(2+)](i) by blocking store-operated Ca(2+) and/or nonselective cation channels. Together, AKBA interferes with pivotal signaling events in monocytic cells that are usually required for monocyte activation by proinflammatory stimuli. Interruption of these events may represent a possible mechanism underlying the reported anti-inflammatory properties of AKBA.

Characterization of an ecto-ATPase activity in Cryptococcus neoformans

Cryptococcus neoformans is the causative agent of pulmonary cryptococcosis and cryptococcal meningoencephalitis, which are major clinical manifestations in immunosuppressed patients. In the present study, a surface ATPase (ecto-ATPase) was identified in C. neoformans yeast cells. Intact yeasts hydrolyzed adenosine-5'-triphosphate (ATP) at a rate of 29.36+/-3.36nmol Pi/hx10(8) cells. In the presence of 5 mM MgCl(2), this activity was enhanced around 70 times, and an apparent K(m) for Mg-ATP corresponding to 0.61mM was determined. Inhibitors of phosphatases, mitochondrial Mg(2+)-ATPases, V-ATPases, Na(+)-ATPases or P-ATPases had no effect on the cryptococcal ATPase, but extracellular impermeant compounds reduced enzyme activity in living cells. ATP was the best substrate for the cryptococcal ecto-enzyme, but it also efficiently hydrolyzed inosine 5'-triphosphate (ITP), cytidine 5'-triphosphate (CTP), guanosine 5'-triphosphate (GTP) and uridine-5'-triphosphate (UTP). In the presence of ATP, C. neoformans became less susceptible to the antifungal action of fluconazole. Our results are indicative of the occurrence of a C. neoformans ecto-ATPase that may have a role in fungal physiology.

Repriming the actomyosin crossbridge cycle

The central features of the mechanical cycle that drives the contraction of muscle are two translational steps: the working stroke, whereby an attached myosin crossbridge moves relative to the actin filament, and the repriming step, in which the crossbridge returns to its original orientation. Although the mechanism of the first of these is understood in some detail, that of the second has received less attention. Here, we show that repriming occurs after detachment of the crossbridge from the actin, rather than intervening between two actomyosin states with ATP bound [Eisenberg, E. & Greene, L. E. (1980) Annu. Rev. Physiol. 42, 293-309]. To discriminate between these two models we investigated the single-molecule mechanics of the myosin-actin interaction in the presence of ATP analogues such as GTP, for which the hydrolytic step itself limits the actomyosin GTPase rate to a much lower rate than for ATP. The lifetimes of bound states was proportional to 1/[GTP], indicating that during the bound period myosin was in the actomyosin rigor configuration. Moreover, despite the very low actomyosin GTPase, the rate of actin binding and formation of the rigor state was higher than with ATP; it follows that most interactions with actin result in the release of GTP and not of the products, GDP and phosphate. There was no significant movement of the actin during this interaction, so repriming must occur while myosin is dissociated, as in the original Lymn-Taylor scheme [Lymn, R. W. & Taylor, E. W. (1971) Biochemistry 10, 4617-4624].

Adverse drug reactions to azathioprine therapy are associated with polymorphism in the gene encoding inosine triphosphate pyrophosphatase (ITPase)

Adverse drug reactions to azathioprine (AZA), the pro-drug of 6-mercaptopurine (6-MP), occur in 15% to 28% of patients and the majority are not explained by thiopurine methyltransferase (TPMT) deficiency. Inosine triphosphate pyrophosphatase (ITPase) deficiency results in the benign accumulation of the inosine nucleotide ITP. 6-MP is activated through a 6-thio-IMP intermediate and, in ITPase deficient patients, potentially toxic 6-thio-ITP is predicted to accumulate. The association between polymorphism in the ITPA gene and adverse drug reactions to AZA therapy was studied in patients treated for inflammatory bowel disease. Sixty-two patients with inflammatory bowel disease suffering adverse drug reactions to AZA therapy were genotyped for ITPA 94C>A and IVS2 + 21A>C polymorphisms, and TPMT*3A, *3C, *2 polymorphisms. Genotype frequencies were compared to a consecutive series of 68 controls treated with AZA for a minimum of 3 months without adverse effect. The ITPA 94C>A deficiency-associated allele was significantly associated with adverse drug reactions [odds ratio (OR) 4.2, 95% confidence interval (CI) 1.6-11.5, P = 0.0034]. Significant associations were found for flu-like symptoms (OR 4.7, 95% CI 1.2-18.1, P = 0.0308), rash (OR 10.3, 95% CI 4.7-62.9, P = 0.0213) and pancreatitis (OR 6.2,CI 1.1-32.6, P = 0.0485). Overall, heterozygous TPMT genotypes did not predict adverse drug reactions but were significantly associated with a subgroup of patients experiencing nausea and vomiting as the predominant adverse reaction to AZA therapy (OR 5.5, 95% CI 1.4-21.3, P = 0.0206). Polymorphism in the ITPA gene predicts AZA intolerance. Alternative immunosuppressive drugs, particularly 6-thioguanine, should be considered for AZA-intolerant patients with ITPase deficiency.

A Japanese case with inosine triphosphate pyrophosphohydrolase deficiency attributable to an enzymatic defect in white blood cells

Inosine triphosphate pyrophosphohydrolase (ITPase) deficiency is characterized by abnormal accumulation of inosine triphosphate. We describe the first Japanese case with ITPase deficiency and demonstrate that the deficiency of ITPase activity is not only found in erythrocytes but also in white blood cells.

Three-strand exchange by the Escherichia coli RecA protein using ITP as a nucleotide cofactor: mechanistic parallels with the ATP-dependent reaction of the RecA protein from Streptococcus pneumoniae

The RecA protein from Escherichia coli promotes an ATP-dependent three-strand exchange reaction between a circular single-stranded DNA (ssDNA) and a homologous linear double-stranded (dsDNA). We have now found that under certain conditions, the RecA protein is also able to promote the three-strand exchange reaction using the structurally related nucleoside triphosphate, ITP, as the nucleotide cofactor. However, although both reactions are stimulated by single-stranded DNA-binding (SSB) protein, the ITP-dependent reaction differs from the ATP-dependent reaction in that it is observed only at low SSB protein concentrations, whereas the ATP-dependent reaction proceeds efficiently even at high SSB protein concentrations. Moreover, the circular ssDNA-dependent ITP hydrolysis activity of the RecA protein is strongly inhibited by SSB protein (suggesting that SSB protein displaces RecA protein from ssDNA when ITP is present), whereas the ATP hydrolysis activity is uninhibited even at high SSB protein concentrations (because RecA protein is resistant to displacement by SSB protein when ATP is present). These results suggest that SSB protein does not stimulate the ITP-dependent strand exchange reaction presynaptically (by facilitating the binding of RecA protein to the circular ssDNA substrate) but may act postsynaptically (by binding to the displaced strand that is generated when the circular ssDNA invades the linear dsDNA substrate). Interestingly, the mechanistic characteristics of the ITP-dependent strand exchange reaction of the E. coli RecA protein are similar to those of the ATP-dependent strand exchange reaction of the RecA protein from Streptococcus pneumoniae. These findings are discussed in terms of the relationship between the dynamic state of the RecA-ssDNA filament and the mechanism of the SSB protein-stimulated three-strand exchange reaction.

The role of the amino-terminal domain of tachykinins in neurokinin-1 receptor signaling and desensitization

Neurokinin A (NKA) has previously been shown to be a full agonist of the neurokinin-1 receptor (NK-1R) but is only able to cause partial homologous desensitization of the receptor compared to substance P (SP). NKA and SP share the same amino acid sequence at their C-terminal active site domains but differ in structure at their N-terminal domains. These observations have led to the proposal that the N-terminal domains of tachykinin peptides affect the desensitization but not the agonist activities of the peptides. Some of the preprotachykinin proteins contain SP and the NKA-like tachykinins neuropeptide K (NPK) and neuropeptide gamma (NPgamma), which contain NKA at their C-terminals and are N-terminally extended. In this study, the abilities of NKA, NPK, and NPgamma to stimulate NK-1R second messenger (IP(3)) signaling and rapid homologous desensitization of the NK-1R were examined. In addition, a similar analysis was performed using several nonmammalian tachykinin peptides in order to obtain additional insight into the role of the tachykinin N-terminal domain in these NK-1R functions. NPK and NPgamma were found, like NKA, to be full agonists of rat NK-1R IP(3) signaling but, unlike NKA, were also able to cause full rapid homologous desensitization of the receptor. The extended N-terminal domains of NPK and NPgamma thus increase the desensitization activities of these NKA-like peptides. Of the nonmammalian tachykinins tested, all were full agonists but kassinin and eledoisin had only partial homologous desensitization activity, suggesting that the N-terminal structures of these peptides also differentially affect agonist versus desensitization activities of the NK-1R.

Endogenous adenosine regulates neutrophil pro-inflammatory activities by cyclic AMP-dependent accelerated clearance of cytosolic calcium

OBJECTIVE AND DESIGN

To identify the involvement of adenosine in restoration of Ca2+ homeostasis to activated human neutrophils.

MATERIALS

Neutrophils were isolated from venous blood taken from healthy, adult, human volunteers.

TREATMENT

The cells were exposed to adenosine deaminase (ADA, 0.1-2 units/ml) for 10 min at 37 degrees C prior to activation with N-formyl-L-methionyl-L-leucyl-L-phenylala-nine (FMLP, 1 microM).

METHODS

Cytosolic Ca2+ concentrations and transmembrane fluxes of the cation in FMLP-activated neutrophils +/- ADA were measured using spectrofluorimetric and radiometric procedures respectively, while intracellular cAMP and inositol triphosphate were measured by radioassay, and superoxide production and elastase release by, chemiluminescence and colourimetric methods respectively. Levels of statistical significance were calculated using the Mann-Whitney U-test and ANOVA.

RESULTS

Although FMLP-activated generation of inositol triphosphate and mobilisation of Ca2+ from neutrophil internal stores, as well as the magnitude of the subsequent efflux and store-operated influx of the cation were unaffected by ADA, there was a prolonged elevation in cytosolic Ca2+ in the presence of the enzyme, which was associated with failure to activate adenylate cyclase and with increased production of superoxide and release of elastase. These effects of ADA were attenuated by dibutyryl cAMP (4 mM), CGS 21680 (1 microM) and rolipram (0.5 microM), as well as by EGTA (10 mM).

CONCLUSIONS

These results are compatible with a physiological role for adenosine in promoting deactivation of neutrophils, possibly by promoting cAMP-dependent clearance of Ca2+ from the cytosol of the cells by the endo-membrane Ca2+-ATPase.

Identification of the dITP- and XTP-hydrolyzing protein from Escherichia coli

A hypothetical 21.0 kDa protein (ORF O197) from Escherichia coli K-12 was cloned, purified, and characterized. The protein sequence of ORF O197 (termed EcO197) shares a 33.5% identity with that of a novel NTPase from Methanococcus jannaschii. The EcO197 protein was purified using Ni-NTA affinity chromatography, protease digestion, and gel filtration column. It hydrolyzed nucleoside triphosphates with an O6 atom-containing purine base to nucleoside monophosphate and pyrophosphate. The EcO197 protein had a strong preference for deoxyinosine triphosphate (dITP) and xanthosine triphosphate (XTP), while it had little activity in the standard nucleoside triphosphates (dATP, dCTP, dGTP, and dTTP). These aberrant nucleotides can be produced by oxidative deamination from purine nucleotides in cells; they are potentially mutagenic. The mutation protection mechanisms are caused by the incorporation into DNA of unwelcome nucleotides that are formed spontaneously. The EcO197 protein may function to eliminate specifically damaged purine nucleotide that contains the 6-keto group. This protein appears to be the first eubacterial dITP- and XTPhydrolyzing enzyme that has been identified.

Distinct interactions of G(salpha-long), G(salpha-short), and G(alphaolf) with GTP, ITP, and XTP

The G(s)-proteins G(salpha-short) (G(salphaS)) and G(salpha-long) (G(salphaL)), and the olfactory G(s) protein (G(alphaolf)) mediate activation of adenylyl cyclase by the beta(2)-adrenoceptor (beta(2)AR). Early studies showed that the purine nucleotides GTP, ITP, and XTP differentially support receptor-mediated adenylyl cyclase activation in various native membrane systems, but those findings have remained unexplained thus far. We systematically analyzed the effects of GTP, ITP, and XTP on the coupling of the beta(2)AR to G(salphaS), G(salphaL), and G(alphaolf), respectively, using fusion proteins expressed in Sf9 insect cells. Fusion proteins ensure defined receptor/G-protein stoichiometry and efficient coupling. At all three fusion proteins, GTP, ITP, and XTP exhibited unique profiles with respect to their potency and efficacy at disrupting high-affinity agonist binding and supporting adenylyl cyclase activation by partial and full agonists. Our data can be interpreted in two ways: (i) GTP, ITP, and XTP may stabilize different active conformations in various G(s)-proteins, or (ii) GTP, ITP, and XTP may differ from one another in the kinetics of interaction with various G(s)-proteins. Regardless of which of the two explanations is correct, our present data demonstrate that GTP, ITP, and XTP are highly efficient regulators of signal transduction mediated through a specific G-protein. Also discussed is the possibility that G-protein activation by ITP and XTP may be of relevance in Lesch-Nyhan syndrome, a defect of the purine salvage pathway associated with abnormalities in various neurotransmitter systems.

Acceleration by fructose of the ATP-sensitive K(+) channel-independent pathway of glucose-induced insulin secretion

The mechanism with which fructose augments glucose-induced insulin secretion is still unclear. The present study was aimed at examining whether the ketohexose potentiates the ATP-sensitive K(+) channel-independent pathway of glucose-induced insulin secretion and, if so, how this happens. When isolated rat islets were depolarized by incubating them with 50 mM KCl in the presence of 150 microM diazoxide (an opener of ATP-sensitive K(+) channels), 10 mM glucose plus 20 mM fructose elicited significantly higher insulin secretion than 10 mM glucose alone, whereas 20 mM fructose alone did not stimulate insulin secretion. The fructose 1,6-bisphosphate and inositol trisphosphate contents were markedly higher in islets incubated with glucose plus fructose than in islets incubated with glucose alone. The results demonstrate that fructose has the ability to potentiate the ATP-sensitive K(+) channel-independent pathway of glucose-induced insulin secretion. The increase in fructose 1,6-bisphosphate content induced by the co-presence of fructose with glucose, resulting in the rise in inositol trisphosphate content, is likely to be one of the signals involved in the fructose potentiation of glucose-induced insulin secretion.

Bi-site catalysis in F1-ATPase: does it exist?

The mechanism of action of F(1)F(0)-ATP synthase is controversial. Some favor a tri-site mechanism, where substrate must fill all three catalytic sites for activity, others a bi-site mechanism, where one of the three sites is always unoccupied. New approaches were applied to examine this question. First, ITP was used as hydrolysis substrate; lower binding affinities of ITP versus ATP enable more accurate assessment of sites occupancy. Second, distributions of all eight possible enzyme species (with zero, one, two or three sites filled) as fraction of total enzyme population at each ITP concentration were calculated, and compared with measured ITPase activity. Confirming data were obtained with ATP as substrate. Third, we performed a theoretical analysis of possible bi-site mechanisms. The results argue convincingly that bi-site hydrolysis activity is negligible, and may not even exist. Effectively, tri-site hydrolysis is the only mechanism. We argue that only tri-site hydrolysis drives subunit rotation. Theoretical analyses of possible bi-site mechanisms reveal serious flaws, not previously recognized. One is that, in bi-site catalysis, the predicted direction of subunit rotation is the same for both ATP synthesis and hydrolysis; a second is that infrequently occurring enzyme species are required.

Stabilities and isomeric equilibria in solutions of monomeric metal-ion complexes of guanosine 5'-triphosphate (GTP4-) and inosine 5'-triphosphate (ITP4-) in comparison with those of adenosine 5'-triphosphate (ATP4-)

Under experimental conditions in which the self-association of the purine-nucleoside 5'-triphosphates (PuNTPs) GTP and ITP is negligible, potentiometric pH titrations were carried out to determine the stabilities of the M(H;PuNTP) and M(PuNTP)2-complexes where M2+ = Mg2+, Ca2+, Sr2+. Ba2+, Mn2+, Co2+, Ni2+, Cu2+, Zn2+, or Cd2+ (I = 0.1 M, 25 degrees C). The stabilities of all M(GTP)2- and M(ITP)2- complexes are significantly larger than those of the corresponding complexes formed with pyrimidine-nucleoside 5'-triphosphates (PyNTPs), which had been determined previously under the same conditions. This increased complex stability is attributed, in agreement with previous 1H MNR shift studies, to the formation of macrochelates of the phosphate-coordinated metal ions with N7 of the purine residues. A similar enhanced stability (despite relatively large error limits) was observed for the M(H;PuNTP) complexes, in which H+ is bound to the terminal y-phosphate group, relative to the stability of the M(H;PyNTP)- species. The percentage of the macrochelated isomers in the M(GTP)2- and M(ITP)2- systems was quantified by employing the difference log KMM(PuNTP)-log KMM(PyNTP); the lowest and highest formation degrees of the macrochelates were observed for Mg(ITP)2- and Cu(GTP)2- with 17 +/- 11% and 97 +/- 1%, respectively. From previous studies of M(ATP)2- complexes, it is known that innersphere and outersphere macrochelates may form; that is, in the latter case a water molecule is between N7 and the phosphate-coordinated M2+. Similar conclusions are reached now by comparisons with earlier 1H MNR shift measurements, that is, that Mg(GTP)2- (21 +/- 11%), for example, exists largely in the form of an outersphere macrochelate and Zn(GTP)2- (68 +/- 4%) as an innersphere one. Generally, the overall percentage of macrochelate falls off for a given metal ion in the order M(GTP)2- > M(ITP)2- > M(ATP)2-; this is in accord with the decreasing basicity of N7 and the steric inhibition of the (C6)NH2 group in the adenine residue. Furthermore, although the absolute stability constants of the previously studied M(GMP), M(IMP), and M(AMP) complexes differ by about two to three log units from the present M(PuNTP)2- results, the formation degrees of the macrochelates are astonishingly similar for the two series of nucleotides for a given metal ion and purine-nucleobase residue. The conclusion that N7 of the guanine residue is an especially favored binding site for metal ions is also in accord with observations made for nucleic acids.

Identification of angiotensin II type 2 (AT2) receptor domains mediating high-affinity CGP 42112A binding and receptor activation

Chimeric angiotensin II (AngII) receptors constructed of portions of the AT2 receptor substituted into the AT1 receptor revealed the AT2 third extracellular loop and seventh transmembrane-spanning domain as major determinants for the ability to bind and activate in response to the AT2 receptor-selective agonist CGP 42112A. Radioligand binding experiments showed that chimeric AngII receptors possessing the AT2 third extracellular loop and seventh transmembrane-spanning domain bound CGP 42112A with high affinity approaching that of the wild-type AT2 receptor. The presence of the AT2 third extracellular loop appeared sufficient for high-affinity CGP 42112A binding, which was further enhanced by the additional presence of the AT2 seventh transmembrane-spanning domain. Experiments with PD 123319, losartan, and [Sar1,Ile8]-AngII showed that increases in binding affinity associated with these domains were specific for CGP 42112A. Use of phosphoinositide hydrolysis as a functional index to measure activation of these chimeric AngII receptors further demonstrated that the AT2 seventh transmembrane-spanning domain was especially critical for CGP 42112A to act as an agonist. The absence of the AT2 seventh transmembrane-spanning domain prohibited CGP 42112A-induced activation of these receptors, even in the presence of high concentrations of CGP 42112A sufficient to saturate the binding sites. This study is the first to identify binding determinants of the AT2 receptor that are selective for CGP 42112A, and indicates that these determinants are at least partially distinct from those for the AT2-selective antagonist PD 123319. These differences may be a factor in the pharmacodynamic difference between these two ligands.

Biochemical characterization of a novel hypoxanthine/xanthine dNTP pyrophosphatase from Methanococcus jannaschii

A novel dNTP pyrophosphatase, Mj0226 from Methanococcus jannaschii, which catalyzes the hydrolysis of nucleoside triphosphates to the monophosphate and PPi, has been characterized. Mj0226 protein catalyzes hydrolysis of two major substrates, dITP and XTP, suggesting that the 6-keto group of hypoxanthine and xanthine is critical for interaction with the protein. Under optimal reaction conditions the k(ca)(t) /K(m) value for these substrates was approximately 10 000 times that with dATP. Neither endonuclease nor 3'-exonuclease activities were detected in this protein. Interestingly, dITP was efficiently inserted opposite a dC residue in a DNA template and four dNTPs were also incorporated opposite a hypoxanthine residue in template DNA by DNA polymerase I. Two protein homologs of Mj0226 from Escherichia coli and Archaeoglobus fulgidus were also cloned and purified. These have catalytic activities similar to Mj0226 protein under optimal conditions. The implications of these results have significance in understanding how homologous proteins, including Mj0226, act biologically in many organisms. It seems likely that Mj0226 and its homologs have a major role in preventing mutations caused by incorporation of dITP and XTP formed spontaneously in the nucleotide pool into DNA. This report is the first identification and functional characterization of an enzyme hydrolyzing non-canonical nucleotides, dITP and XTP.

Purine but not pyrimidine nucleotides support rotation of F(1)-ATPase

The binding change model for the F(1)-ATPase predicts that its rotation is intimately correlated with the changes in the affinities of the three catalytic sites for nucleotides. If so, subtle differences in the nucleotide structure may have pronounced effects on rotation. Here we show by single-molecule imaging that purine nucleotides ATP, GTP, and ITP support rotation but pyrimidine nucleotides UTP and CTP do not, suggesting that the extra ring in purine is indispensable for proper operation of this molecular motor. Although the three purine nucleotides were bound to the enzyme at different rates, all showed similar rotational characteristics: counterclockwise rotation, 120 degrees steps each driven by hydrolysis of one nucleotide molecule, occasional back steps, rotary torque of approximately 40 piconewtons (pN).nm, and mechanical work done in a step of approximately 80 pN.nm. These latter characteristics are likely to be determined by the rotational mechanism built in the protein structure, which purine nucleotides can energize. With ATP and GTP, rotation was observed even when the free energy of hydrolysis was -80 pN.nm/molecule, indicating approximately 100% efficiency. Reconstituted F(o)F(1)-ATPase actively translocated protons by hydrolyzing ATP, GTP, and ITP, but CTP and UTP were not even hydrolyzed. Isolated F(1) very slowly hydrolyzed UTP (but not CTP), suggesting possible uncoupling from rotation.

Cloning, expression, and characterization of a human inosine triphosphate pyrophosphatase encoded by the itpa gene

ITP and dITP exist in all cells. dITP is potentially mutagenic, and the levels of these nucleotides are controlled by inosine triphosphate pyrophosphatase (EC ). Here we report the cloning, expression, and characterization of a 21.5-kDa human inosine triphosphate pyrophosphatase (hITPase), an enzyme whose activity has been reported in many animal tissues and studied in populations but whose protein sequence has not been determined before. At the optimal pH of 10.0, recombinant hITPase hydrolyzed ITP, dITP, and xanthosine 5'-triphosphate to their respective monophosphates whereas activity with other nucleoside triphosphates was low. K(m) values for ITP, dITP, and xanthosine 5'-triphosphate were 0.51, 0.31, and 0.57 mm, respectively, and k(cat) values were 580, 360, and 640 s(-1), respectively. A divalent cation was absolutely required for activity. The gene encoding the hITPase cDNA sequence was localized by radiation hybrid mapping to chromosome 20p in the interval D20S113-D20S97, the same interval in which the ITPA inosine triphosphatase gene was previously localized. A BLAST search revealed the existence of many similar sequences in organisms ranging from bacteria to mammals. The function of this ubiquitous protein family is proposed to be the elimination of minor potentially mutagenic or clastogenic purine nucleoside triphosphates from the cell.

Nucleoside triphosphate specificity of tubulin

We have determined the binding affinity for binding of the four purine nucleoside triphosphates GTP, ITP, XTP, and ATP to E-site nucleotide- and nucleoside diphosphate kinase-depleted tubulin. The relative binding affinities are 3000 for GTP, 10 for ITP, 2 for XTP, and 1 for ATP. Thus, the 2-exocyclic amino group in GTP is important in determining the nucleotide specificity of tubulin and may interact with a hydrogen bond acceptor group in the protein. The 6-oxo group also makes a contribution to the high affinity for GTP. NMR ROESY experiments indicate that the four nucleotides have different average conformations in solution. ATP and XTP are characterized by a high anti conformation, ITP by a medium anti conformation, and GTP by a low anti conformation. Possibly, the preferred solution conformation contributes to the differences in affinities. When the tubulin E-site is saturated with nucleotide, there appears to be little difference in the ability of the four nucleotides to stimulate assembly. The critical protein concentration is essentially identical in reactions using the four nucleotides. All four of the nucleotides were hydrolyzed during the assembly reaction, and the NDPs were incorporated into the microtubule. We also examined the binding of two gamma-phosphoryl-modified GTP photoaffinity analogues, p(3)-1, 4-azidoanilido-GTP and p(3)-1,3-acetylanilido-GTP. These analogues are inhibitors of the assembly reaction and bind to tubulin with affinities that are 15- and 50-fold lower, respectively, than the affinty for GTP. The affinity of GTP is less sensitive to substitutions at the gamma-phosphoryl position that to changes in the purine ring.

Improved cycle sequencing of GC-rich templates by a combination of nucleotide analogs

A common problem in automated DNA sequencing when applying the Sanger chain termination method is ambiguous base calling caused by band compressions. Band compressions are caused by anomalies in the migration behavior of certain DNA fragments in the polyacrylamide gel because of intramolecular base pairing between guanine and cytosine residues. To reduce such undesired secondary structures, several modifications of the sequencing reaction parameters have been performed previously. Here, we have applied mixtures of the nucleotide analogs 7-deaza-dGTP and dITP instead of dGTP in the cycle sequencing reaction and in combination with varying buffer conditions. Band compressions were particularly well resolved, and reading length was optimal when a ratio of 7-deaza-dGTP:dITP of 4:1 was used in the in vitro DNA synthesis with AmpliTaq FS DNA polymerase. We conclude that the incorporation of both nucleotide analogs at these particular ratios leads to heterogeneous DNA chains that result in a reduction or elimination of intramolecular base pairing and thus a higher accuracy in the base assignment.

Hydrolysis of nucleoside triphosphates other than ATP by nitrogenase

The hydrolysis of ATP to ADP and P(i) is an integral part of all substrate reduction reactions catalyzed by nitrogenase. In this work, evidence is presented that nitrogenases isolated from Azotobacter vinelandii and Clostridium pasteurianum can hydrolyze MgGTP, MgITP, and MgUTP to their respective nucleoside diphosphates at rates comparable to those measured for MgATP hydrolysis. The reactions were dependent on the presence of both the iron (Fe) protein and the molybdenum-iron (MoFe) protein. The oxidation state of nitrogenase was found to greatly influence the nucleotide hydrolysis rates. MgATP hydrolysis rates were 20 times higher under dithionite reducing conditions (approximately 4,000 nmol of MgADP formed per min/mg of Fe protein) as compared with indigo disulfonate oxidizing conditions (200 nmol of MgADP formed per min/mg of Fe protein). In contrast, MgGTP, MgITP, and MgUTP hydrolysis rates were significantly higher under oxidizing conditions (1,400-2,000 nmol of MgNDP formed per min/mg of Fe protein) as compared with reducing conditions (80-230 nmol of MgNDP formed per min/mg of Fe protein). The K(m) values for MgATP, MgGTP, MgUTP, and MgITP hydrolysis were found to be similar (330-540 microM) for both the reduced and oxidized states of nitrogenase. Incubation of Fe and MoFe proteins with each of the MgNTP molecules and AlF(4)(-) resulted in the formation of non-dissociating protein-protein complexes, presumably with trapped AlF(4)(-) x MgNDP. The implications of these results in understanding how nucleotide hydrolysis is coupled to substrate reduction in nitrogenase are discussed.

Identification of residues of Escherichia coli phosphofructokinase that contribute to nucleotide binding and specificity

The apparent affinity of phosphofructo-1-kinase (PFK) of Escherichia coli for ATP is at least 10 times higher than for other nucleotides. Mutagenesis was directed toward five residues that may interact with ATP: Y41, F76, R77, R82, and R111. Alanine at position 41 or 76 increased the apparent Km by 49- and 62-fold, respectively. Position 41 requires the presence of a large hydrophobic residue and is not restricted to aromatic rings. Tryptophan and, to a lesser extent, phenylalanine could substitute at position 76. None of the mutants at 41 or 76 showed a change in the preference for alternative purines, although F76W used CTP 3 times better than the wild type enzyme. Mutations of R77 suggested that the interaction was hydrophobic with no influence on nucleotide preference. Mutation of R82 to alanine or glutamic acid increased the apparent Km for ATP by more than 20-fold and lowered the kcat/Km with ATP more than 30-fold. However, these mutants had a higher kcat/Km than wild type for both GTP and CTP, reflecting a loss of substrate preference. A loss in preference is seen as well with R111A where the kcat/Km for ATP decreases by only 68%, but the kcat/Km with GTP increases more than 10-fold. Activities with ITP, CTP, and UTP are also higher than with the wild type enzyme. Arginine residues at positions 82 and 111 are important dictators of nucleoside triphosphate preference.

Purification and properties of mycobacterial GDP-mannose pyrophosphorylase

The enzyme that catalyzes the formation of GDP-d-mannose from GTP and alpha-d-mannose-1-P was purified about 2300-fold to near homogeneity from the soluble fraction of Mycobacterium smegmatis. At the final stage of purification, a major protein band of 37 kDa was observed and this band was specifically labeled, and in a concentration-dependent manner, by the photoaffinity probe 8-N3-GDP[32P]-d-mannose. The purified enzyme was stable for several months when kept in the frozen state. The 37-kDa band was subjected to protein sequencing and one peptide sequence of 25 amino acids showed over 80% identity to GDP-mannose pyrophosphorylases of pig liver and Saccharomyces cerevesiae. In contrast to some other bacterial GDP-mannose pyrophosphorylases, the mycobacterial enzyme was not multifunctional and did not have phosphomannose isomerase or phosphoglucose isomerase activity. Also, in contrast to the pig liver enzyme which uses mannose-1-P or glucose-1-P plus GTP to synthesize either GDP-mannose or GDP-glucose, the mycobacterial enzyme was specific for mannose-1-P as the sugar phosphate substrate. The enzyme was also relatively specific for GTP as the nucleoside triphosphate substrate. ITP was about 18% as effective as GTP, but ATP, CTP, and UTP were inactive. The activity of the enzyme was inhibited by GDP-glucose and glucose-1-P, although neither was a substrate for this enzyme. The pH optimum for the enzyme was 8.0, and Mg2+ was the best cation with optimum activity at about 5 mM. This enzyme is important for producing the activated form of mannose for formation of cell wall lipoarabinomannan and various mannose-containing glycolipids and polysaccharides.

Identification of stable RNA hairpins causing band compression in transcriptional sequencing and their elimination by use of inosine triphosphate

To identify stable RNA secondary structure causing band compression, 30 lambda DNA clones and four cDNA clones (about 10 kb in total length) were sequenced using Transcriptional Sequencing, which is based on the phage RNA polymerase chain termination reaction with fluorescent 3' deoxynucleoside triphosphate, using the canonical set of rNTPs for the substrate. Electrophoresis was performed on acrylamide gel containing 7 M urea at 50 degrees C using ABI 377 DNA sequencer. A total of 159 band compressions were identified, and most compression sites seem to be due to hairpin structures. We also found that the presence of rITP in place of rGTP in the sequencing reaction can entirely eliminate all band compressions. The use of rITP gave a better peak uniformity and resolution in the sequencing gel in the case of lambda DNA than with c7rGTP, leading to improved accuracy in the sequence determination. Substitution of the base analog rITP for rGTP should be useful for accurate sequencing determination.

Thiamine triphosphatase activity in bovine kidney

Properties of soluble thiamine triphosphatase (ThTPase), adenosine triphosphatase, nucleoside triphosphatase and alkaline phosphatase activities in bovine kidney were compared. ThTPase and the other phosphatases differed clearly in their pH-dependences, K(m) and molecular masses. Apparent K(m) and pH optimum for ThTPase were determined to be 45.5 microM and 8.9, respectively. Molecular mass of the enzyme was 29.1 kDa as estimated by Sephadex G-100 gel filtration. The results obtained show bovine kidney to contain a specific soluble ThTPase, this enzyme being the only one hydrolyzing low concentrations of ThTP.

Transfer of second messengers through gap junction connexin 43 channels reconstituted in liposomes

Gap junction channels reconstituted in liposomes provide a pathway for the transfer of second messengers. Gap junction channels were formed in the artificial unilamellar liposomes using immunoaffinity-purified connexin 43 gap junction protein from rat brain. Sucrose-permeable and -impermeable liposomes were separated on the basis of sucrose permeability in the iso-osmolar sucrose density gradient. The liposomes permeable to sucrose were also permeable to a communicating dye molecule, Lucifer yellow. In the present study, we examined the transfer of second messengers through the connexin 43 channels reconstituted in liposomes and first report the direct evidence that the gap junction channels are permeable to second messengers including adenosine 3',5'-cyclic phosphate and inositol 1,4,5-trisphosphate.

Single-channel properties of inositol (1,4,5)-trisphosphate receptor heterologously expressed in HEK-293 cells

The inositol (1,4,5)-trisphosphate receptor (InsP3R) mediates Ca2+ release from intracellular stores in response to generation of second messenger InsP3. InsP3R was biochemically purified and cloned, and functional properties of native InsP3-gated Ca2+ channels were extensively studied. However, further studies of InsP3R are obstructed by the lack of a convenient functional assay of expressed InsP3R activity. To establish a functional assay of recombinant InsP3R activity, transient heterologous expression of neuronal rat InsP3R cDNA (InsP3R-I, SI- SII+ splice variant) in HEK-293 cells was combined with the planar lipid bilayer reconstitution experiments. Recombinant InsP3R retained specific InsP3 binding properties (Kd = 60 nM InsP3) and were specifically recognized by anti-InsP3R-I rabbit polyclonal antibody. Density of expressed InsP3R-I was at least 20-fold above endogenous InsP3R background and only 2-3-fold lower than InsP3R density in rat cerebellar microsomes. When incorporated into planar lipid bilayers, the recombinant InsP3R formed a functional InsP3-gated Ca2+ channel with 80 pS conductance using 50 mM Ba2+ as a current carrier. Mean open time of recombinant InsP3-gated channels was 3.0 ms; closed dwell time distribution was double exponential and characterized by short (18 ms) and long (130 ms) time constants. Overall, gating and conductance properties of recombinant neuronal rat InsP3R-I were very similar to properties of native rat cerebellar InsP3R recorded in identical experimental conditions. Recombinant InsP3R also retained bell-shaped dependence on cytosolic Ca2+ concentration and allosteric modulation by ATP, similar to native cerebellar InsP3R. The following conclusions are drawn from these results. (a) Rat neuronal InsP3R-I cDNA encodes a protein that is either sufficient to produce InsP3-gated channel with functional properties identical to the properties of native rat cerebellar InsP3R, or it is able to form a functional InsP3-gated channel by forming a complex with proteins endogenously expressed in HEK-293 cells. (b) Successful functional expression of InsP3R in a heterologous expression system provides an opportunity for future detailed structure-function characterization of this vital protein.

Inosine 5'-triphosphate can dramatically increase the yield of NASBA products targeting GC-rich and intramolecular base-paired viroid RNA

Nucleic acid sequence-based amplification (NASBA) according to the standard protocol failed to amplify cRNA of viroids, probably because of their GC-rich and intramolecular base-paired structure. However, NASBA in the presence of inosine 5'-triphosphate successfully amplified the cRNAs to viroids in total nucleic acid extracts from citrus plants. As sequence specificity of the cRNA to viroids was confirmed by northern analysis, the amplification and fidelity of cRNAs are sufficient for the sensitive and specific detection of viroids.

Functionally nonequivalent interactions of guanosine 5'-triphosphate, inosine 5'-triphosphate, and xanthosine 5'-triphosphate with the retinal G-protein, transducin, and with Gi-proteins in HL-60 leukemia cell membranes

G-proteins mediate signal transfer from receptors to effector systems. In their guanosine 5'-triphosphate (GTP)-bound form, G-protein alpha-subunits activate effector systems. Termination of G-protein activation is achieved by the high-affinity GTPase [E.C. 3.6.1.-] of their alpha-subunits. Like GTP, inosine 5'-triphosphate (ITP) and xanthosine 5'-triphosphate (XTP) can support effector system activation. We studied the interactions of GTP, ITP, and XTP with the retinal G-protein, transducin (TD), and with G-proteins in HL-60 leukemia cell membranes. TD hydrolyzed nucleoside 5'-triphosphates (NTPs) in the order of efficacy GTP > ITP > XTP. NTPs eluted TD from rod outer segment disk membranes in the same order of efficacy. ITP and XTP competitively inhibited TD-catalyzed GTP hydrolysis. In HL-60 membranes, the chemoattractants N-formyl-L-methionyl-L-leucyl-L-phenylalanine (fMLP) and leukotriene B4 (LTB4) effectively activated GTP and ITP hydrolysis by Gi-proteins. fMLP and LTB4 were at least 10-fold more potent activators of ITPase than of GTPase. Complement C5a effectively activated the GTPase of Gi-proteins but was only a weak stimulator of ITPase. The potency of C5a to activate GTP and ITP hydrolysis was similar. The fMLP-stimulated GTPase had a lower Km value than the fMLP-stimulated ITPase, whereas the opposite was true for the Vmax values. fMLP, C5a, and LTB4 did not stimulate XTP hydrolysis. Collectively, our data show that GTP, ITP, and XTP bind to G-proteins with different affinities, that G-proteins hydrolyze NTPs with different efficacies, and that chemoattractants stimulate GTP and ITP hydrolysis by Gi-proteins in a receptor-specific manner. On the basis of our results and the data in the literature, we put forward the hypothesis that GTP, ITP, and XTP act as differential signal amplifiers and signal sorters at the G-protein level.

Properties of a new radioiodinated antagonist for human vasopressin V2 and V1a receptors

A vasopressin receptor antagonist, [1-(beta-mercapto-beta,beta-pentamethylenepropionic acid), 2-o-ethyl-D-tyrosine, 4-valine, 9-tyrosylamide] arginine vasopressin (d(CH2)5[o-ethyl-D-Tyr2,Val4,Tyr-NH9(2)]AVP), has been prepared. This antagonist is a potent antiantidiuretic, antivasopressor and antioxytocic peptide with pA2 values of 7.69-7.94 and affinities of 1.12-11.0 nM. When radioiodinated at the phenyl moiety of the tyrosylamide residue at position 9, this peptide was demonstrated to bind to vasopressin V2 and V1a receptors with a dissociation constant of 0.22-0.75 nM. This ligand is a good tool for further studies on human vasopressin V2 receptor localization and characterization, when used in combination with a selective vasopressin V1a ligand.

Corticotropin-releasing factor-like peptides increase cytosolic [Ca2+] in human epidermoid A-431 cells

This study investigated whether sauvagine and urotensin I change [Ca2+]i in human epidermoid A-431 cells and whether these changes are correlated with their anti-edema properties in vivo. A-431 cells were used because they possess the corticotropin-releasing factor (CRF) receptor 2. Treatment with either sauvagine or urotensin I led to an immediate increase in [Ca2+]i, the magnitude of which depended on the concentration of the drug. Sauvagine was more effective than urotensin I, with a median effective concentration (EC50) of 1.4 +/- 0.2 fM, compared to an EC50 of 66 +/- 7 fM for urotensin I. Both were more effective at stimulating increases in [Ca2+]i than CRF (EC50 of 6.8 +/- 0.1 pM). There was a correlation between the EC50 for increasing [Ca2+]i and the median effective dose (ED50) for inhibiting edema induced by heating rat paw (r = 0.99). Removal of extracellular Ca2+ or incubation with La3+ eliminated the increase in [Ca2+]i stimulated by either peptide. Pretreatment with a CRF receptor antagonist reduced the increase in [Ca2+]i by these peptides. This occurred in an antagonist concentration-dependent manner, with median inhibitory concentrations (IC50) of 1.99 +/- 0.04 nM and 0.85 +/- 0.04 nM, respectively. Both pertussis toxin (an inhibitor of G proteins) and U-73122 (an inhibitor for inositol trisphosphate (InsP3) production) partially inhibited the increases. InsP3 was measured to determine whether these peptides mobilized Ca2+ from an InsP3-sensitive store. Both sauvagine and urotensin I increased InsP3. The InsP3 increases were inhibited by U-73 122 and CRF antagonist, but not by removal of external Ca2+. Both peptides elevated protein tyrosine phosphorylation. In summary, these peptides increase [Ca2+]i as a result of Ca2+ influx via CRF receptor-operated Ca2+ channels coupled to pertussis toxin-sensitive G proteins and a Ca2+ mobilization from InsP3-sensitive Ca2+ pools. Their in vivo effectiveness at inhibiting edema is related to their respective capacities to stimulate elevations of [Ca2+]i, supporting a role for intracellular Ca2+ in this process.

Effect of an active metabolite of the antiallergic agent tazanolast on histamine release from rat mast cells

WP-871 (3'-(1H-tetrazol-5-yl)oxanilic acid monohydrate, CAS 114607-46-4) is a monohydrate of a main active metabolite of tazanolast (butyl 3'-(1H-tetrazol-5-yl) oxanilate, CAS 82989-25-1), an orally active antiallergic drug. WP-871 inhibited dose-dependently compound 48/80-induced histamine release from rat peritoneal mast cells. In a similar dose range, WP-871 was effective in inhibiting compound 48/80-induced 45Ca uptake into mast cells from extracellular medium and compound 48/80-induced translocation of protein kinase C from the cytosol to the membrane fraction of mast cells. WP-871 also inhibited inositol trisphosphate production but did not exhibit a direct inhibitory effect on phospholipase C in mast cells. WP-871 caused no increase in cAMP content in mast cells. These results suggest that WP-871 may inhibit histamine release mainly by preventing the increase in intracellular Ca2+ concentration, which is a critical event in signal transduction leading to histamine release in mast cells.

Function of the novel subdomain in the RNA binding domain of transcription termination factor Rho from Micrococcus luteus

Transcription termination factor Rho from Micrococcus luteus, a high G + C Gram-positive bacterium, contains an unusual extra sequence within its RNA binding domain that is rich in Arg, Glu, and Asp residues and deficient in hydrophobic residues. To determine the role of this extra sequence, we compared the biochemical properties of a variant lacking nearly all the extra sequence, des(60-300) Rho, to that of wild-type M. luteus Rho. The two forms had very similar properties except that the des(60-300) Rho was unable to terminate transcription with Escherichia coli RNA polymerase at the promoter proximal sites used by the wild-type Rho on a lambda cro DNA template but could cause termination at more distal sites and did cause termination at proximal sites when ITP replaced GTP in the reaction mixture. The RNA binding properties of the two forms of this Rho with normal and inosine-substituted RNAs were found to correlate fully with their termination properties. These results indicate that the arginine-rich extra sequence is directly involved in the selection of the termination site and support the hypothesis that the sequence is present in M. luteus Rho to facilitate its binding to M. luteus transcripts, which are likely to have a high degree of base-paired secondary structure because of their high proportion of G residues.

Neuropeptide Y Y2 receptor and somatostatin sst2 receptor coupling to mobilization of intracellular calcium in SH-SY5Y human neuroblastoma cells

1. In this study we have investigated neuropeptide Y (NPY) and somatostatin (SRIF) receptor-mediated elevation of intracellular Ca2+ concentration ([Ca2+]i) in the human neuroblastoma cell line SH-SY5Y. 2. The Ca(2+)-sensitive dye fura 2 was used to measure [Ca2+]i in confluent monolayers of SH-SY5Y cells. Neither NPY (30-100 nM) nor SRIF (100 nM) elevated [Ca2+]i when applied alone. However, when either NPY (300 pM-1 microM) or SRIF (300 pM-1 microM) was applied in the presence of the cholinoceptor agonist carbachol (1 microM or 100 microM) they evoked an elevation of [Ca2+]i above that caused by carbachol alone. 3. The elevation of [Ca2+]i by NPY was independent of the concentration of carbachol. In the presence of 1 microM or 100 microM carbachol NPY elevated [Ca2+]i with a pEC50 of 7.80 and 7.86 respectively. 4. In the presence of 1 microM carbachol the NPY Y2 selective agonist peptide YY(3-36) (PYY(3-36)) elevated [Ca2+]i with a pEC50 of 7.94, the NPY Y1 selective agonist [Leu31, Pro34]-NPY also elevated [Ca2+]i when applied in the presence of carbachol, but only at concentrations > 300 nM. The rank order of potency, PYY(3-36) > or = NPY > > [Leu31, Pro34]-NPY indicates that an NPY Y2-like receptor is involved in the elevation of [Ca2+]i. 5. In the presence of 1 microM carbachol, SRIF elevated [Ca2+]i with a pEC50 of 8.24. The sst2 receptor-preferring analogue BIM-23027 (c[N-Me-Ala-Tyr-D-Trp-Lys-Abu-Phe]) elevated [Ca2+]i with a pEC50 of 8.63, and the sst5-receptor preferring analogue L-362855 (c[Aha-Phe-Trp-D-Trp-Lys-Thr-Phe]) elevated [Ca2+]i with a pEC50 of approximately 6.1. Application of the sst3 receptor-preferring analogue BIM-23056 (D-Phe-Phe-Tyr-D-Trp-Lys-Val-Phe-D-Nal-NH2, 1 microM) to SH-SY5Y cells in the presence of carbachol neither elevated [Ca2+]i nor affected the elevations of [Ca2+]i caused by a subsequent coapplication of SRIF. The rank order of potency, BIM-23026 > or = SRIF > > L-362855 > > > BIM-23026 suggests that an sst2-like receptor is involved in the elevation of [Ca2+]i. 6. Block of carbachol activation of muscarinic receptors with atropine (1 microM) abolished the elevation of [Ca2+]i by the SRIF and NPY. 7. Muscarinic receptor activation, not a rise in [Ca2+]i, was required to reveal the NPY or SRIF response. The Ca2+ channel activator maitotoxin (2 ng ml-1) also elevated [Ca2+]i but subsequent application of either NPY or SRIF in the presence of maitotoxin caused no further changes in [Ca2+]i. 8. The elevations of [Ca2+]i by NPY and SRIF were abolished by pretreatment of the cells with pertussis toxin (200 ng-ml-1, 16 h). This treatment did not significantly affect the response of the cells to carbachol. 9. NPY and SRIF appeared to elevate [Ca2+]i by mobilizing Ca2+ from intracellular stores. Both NPY and SRIF continued to elevate [Ca2+]i when applied in nominally Ca(2+)-free external buffer. Thapsigargin (100 nM), an agent which discharges intracellular Ca2+ stores, also blocked the NPY and SRIF elevations of [Ca2+]i. 10. Delta-Opioid receptor agonists applied in the presence of carbachol also elevate [Ca2+]i in SH-SY5Y cells. When NPY (30 nM) or SRIF (100 nM) was applied together with a maximally effective concentration of the delta-opioid receptor agonist DPDPE ([D-Pen2,5]-enkephalin) (1 microM), the resulting elevations of [Ca2+]i were not greater than those caused by application of DPDPE alone. 11. Thus, in SH-SY5Y cells, NPY and SRIF can mobilize Ca2+ from intracellular stores via activation of NPY Y2 and sst2-like receptors, respectively. Neither NPY nor SRIF elevated [Ca2+]i when applied alone. The requirements for the elevations of [Ca2+]i by NPY and SRIF are the same as those for delta- and mu-opioid receptor and nociceptin receptor mobilization of [Ca2+]i in SH-SY5Y cells.

Modification of membrane-bound F1 by p-fluorosulfonylbenzoyl-5'-adenosine: sites of binding and effect on activity

Bovine heart submitochondrial particles (smp) were incubated with p-fluorosulfonylbenzoyl-5'-adenosine (FSBA) in order to study the binding of this ligand and its effect on ATP synthesis and ATP hydrolysis in smp and to compare the results with those obtained with isolated F1. The binding was measured with the 14C-labeled compound. ATP hydrolysis was in all cases as much inhibited as succinate-driven ATP synthesis and ITP hydrolysis was more inhibited than ATP hydrolysis. The binding experiments show that modification of three nucleotide binding sites results in nearly complete inhibition of ATPase activity. In the presence of pyrophosphate up to 6 mol [14C]SBA/mol F1 can be bound. FSBA preferentially modifies amino acids of the alpha-subunits but also beta-subunits are modified. It is concluded that modification of both subunits results in inhibition of activity. The results are very well comparable with the results obtained with isolated F1, which indicates that our preparation of F1 is a good model for F1 in the intact system. Furthermore it is concluded that each alpha-subunit of F1 in smp, just like in the isolated form, contains two pockets where adenosine moieties can bind, one located above the P-loop, modifying alpha-Tyr-244 and alpha-Tyr-300 and the other one located below the P-loop where also the adenosine moiety of AD(T)P binds, modifying beta-Tyr-368.

Selective amplification of RNA utilizing the nucleotide analog dITP and Thermus thermophilus DNA polymerase

The ability to selectively amplify RNA in the presence of genomic DNA of analogous sequence is cumbersome and requires implementation of critical controls for genes lacking introns. The convenient approaches of either designing oligonucleotide primers at the splice junction or differentiating the target sequence based on the size difference obtained by the presence of the intron are not possible. Our strategy for the selective amplification of RNA targets is based on the enzymology of a single thermostable DNA polymerase and the ability to modulate the strand separation temperature requirements for PCR amplification. Following reverse transcription of the RNA by recombinant Thermus thermophilus DNA polymerase (rTth pol), the resulting RNAxDNA hybrid is digested by the RNase H activity of rTth pol, allowing the PCR primer to hybridize and initiate second-strand cDNA synthesis. Substitution of one or more conventional nucleotides with nucleotide analogs that decrease base stacking interactions and/or hydrogen bonding (e.g. hydroxymethyldUTP or dITP) during the first- and second-strand cDNA synthesis step reduces the strand separation temperature of the resultant DNAxDNA duplex. Alteration of the thermal cycling parameters of the subsequent PCR amplification, such that the strand separation temperature is below that required for denaturation of genomic duplex DNA composed of standard nucleotides, prevents the genomic DNA from being denatured and therefore amplified.

Inhibition by HAJ11 of respiratory burst in neutrophils and the involvement of protein tyrosine phosphorylation and phospholipase D activation

1. The possible mechanisms of the inhibitory effect of ethyl 2-(3-hydroxyanilino)-4-oxo-4,5-dihydrofuran-3-carboxylate (HAJ11) on the respiratory burst of rat neutrophils in vitro was investigated. 2. HAJ11 caused a reversible and a concentration-dependent inhibition of formyl-Met-Leu-Phe (fMLP)-induced superoxide anion (O2.-) generation (IC50 4.9 +/- 0.7 microM) and O2 consumption (IC50 4.9 +/- 1.5 microM). Concanavalin A (Con A)- and NaF-induced O2.- generation were also suppressed by HAJ11. However, HAL11 was a weak inhibitor of the phorbol 12-myristate 13-acetate (PMA)-induced responses. 3. HAJ11 did not scavenge the /2.- generation in the xanthine-xanthine oxidase system and dihydroxyfumaric acid (DHF) autoxidation. 4. HAJ11 showed no activity on fMLP-induced inositol phosphates formation and [Ca2+]i elevation in intact neutrophils. In addition, HAJ11 had no effect on neutrophil cytosolic phospholipase C (PLC) activity. 5. HAJ11 reduced fMLP-induced phosphatidic acid (PA) (IC50 29.1 +/- 6.5 microM) and phosphatidylethanol (PE+) (IC50 22.6 +/- 1.9 microM) formation in a concentration-dependent manner. HAJ11 also reduced protein tyrosine phosphorylation in neutrophils stimulated by fMLP. 6. HAJ11 was a weak inhibitor of neutrophil cytosolic protein kinase C (PKC) activity, and had a negligible effect on brain PKC. Cellular cyclic nucleotides levels were not altered by HAJ11. In addition, HAJ11 did not affect protein kinase A (PKA) activity. 7. HAJ11 had not effect on the O2.- generation of PMA-activated and arachidonic acid (AA)-activated NADPH oxidase preparations. 8. Taken together these results indicate that the inhibition of respiratory burst by HAJ11 probably mainly occurs through inhibition of protein tyrosine phosphorylation and phospholipase D (PLD) activity.

Cellular signaling and proliferative action of AVP in mesangium of SHR: effect of low density lipoprotein

The present study was undertaken to determine whether low density lipoprotein (LDL) modulates the cellular action of arginine vasopressin (AVP) in cultured glomerular mesangial cells of spontaneously hypertensive rats (SHR) and normotensive Wistar-Kyoto rats (WKY). The AVP-induced cellular signal transduction, including inositol 1,4,5-trisphosphate (IP3) production, fura-2 intracellular calcium measurements and cellular alkalinization, was significantly greater in cells of SHR than those of WKY. This is based on an increase in AVP V1 receptor number in cells of the SHR. Also, the AVP activation of mitogen-activated protein (MAP) kinase and [3H]thymidine incorporation was significantly exaggerated in cells of SHR compared with those of WKY. LDL at a concentration of 10 micrograms/ml augmented the cellular signaling and proliferative action of AVP in cells of WKY, but not in those of SHR. Since [3H]AVP receptor binding was not affected by the LDL pretreatment, LDL modulates the signal transduction between a location distal to the AVP receptors and proximal from the production of IP3 and diacylglycerol. These results indicate that an increase in AVP receptor capacity has a profound effect on the AVP-induced cellular signaling and proliferation, and that LDL has a slight alteration on the action of AVP in glomerular mesangial cells of SHR.

Reengineering the nucleotide cofactor specificity of the RecA protein by mutation of aspartic acid 100

We have recently obtained evidence for a direct linkage between the S0.5 (S0.5 is the substrate concentration required for half-maximal velocity) value of a nucleoside triphosphate and the conformational state of the RecA-ssDNA complex, with an S0.5 value of 125 microM or less required for stabilization of the strand exchange-active conformation. For example, although ATP and ITP are hydrolyzed by the RecA protein with the same turnover number (18 min-1), ATP (S0.5 = 45 microM) functions as a cofactor for the strand exchange reaction, whereas ITP (S0.5 = 500 microM) is inactive as a strand exchange cofactor. The RecA protein crystal structure suggests that cofactor specificity is determined by Asp100, which likely forms a hydrogen bond with the exocyclic 6-amino group of ATP; the higher S0. 5 value for ITP is presumably due to unfavorable interactions between Asp100 and the 6-carbonyl group of the inosine ring. To test this hypothesis, we prepared a mutant RecA protein in which Asp100 was replaced by an asparagine residue. The S0.5(ITP) for the [D100N]RecA protein is 125 microM, indicating favorable interactions between the Asn100 side chain and the 6-carbonyl group of ITP. Correspondingly, ITP functions as a cofactor for the strand exchange activity of the [D100N]RecA protein. This result demonstrates the importance of the residue at position 100 in determining nucleotide cofactor specificity and underscores the importance of the S0.5 value in the RecA protein-promoted strand exchange reaction.

Characterization of the PGE2 receptor subtype in bovine chondrocytes in culture

1. Prostaglandin E2 (PGE2) is an autacoid that decreases proteoglycan synthesis, increases metalloprotease production by cultured chondrocytes, and can modulate some of the actions of interleukin-1 on cartilage. The objective of the present study was to characterize the subtype of prostaglandin E2 receptor present in bovine chondrocytes in culture. 2. Primary cultures of articular chondrocytes were prepared from slices of bovine carpal cartilage by sequential digestion with type III hyaluronidase, trypsin, type II collagenase, followed by overnight incubation in Dulbecco's Modified Eagle's Medium (DMEM) with type II collagenase, washing, and seeding at a density of 2 x 10(5) cells cm-2 in DMEM with 10% foetal bovine serum. 3. PGE2 and carbaprostacyclin induced dose-dependent increases in intracellular cyclic AMP in bovine chondrocytes in culture. The potencies of these compounds were different, and maximal doses of PGE2 and carbaprostacyclin had an additive effect. PGD2 induced a small increase in intracellular cyclic AMP only at a high concentration (10(-5) M). 4. PGE2 was more potent that the EP2 agonist 11-deoxy-PGE1 at inducing increases in intracellular cyclic AMP. The EP2 agonist butaprost, however, induced only a small increase at a concentration of 10(-5)M. 17-Phenyl-PGE2 (EP1 agonist), sulprostone and MB 28767 (15S-hydroxy-9-oxo-16-phenoxy-omega-tetranorprost-13E-enoic acid) (EP3 agonists) did not induce an increase in intracellular cyclic AMP at concentrations up to 10(-5)M. 5. The EP4 antagonist AH 23848B ([1 alpha(Z),2 beta, 5 alpha]-(+/-) -7-[5-[[(1,1'-biphenyl)-4-yl]methoxyl-2-(4-morpholinyl) -3-oxocyclopentyl]-5-heptenoic acid) antagonized PGE2 but not carbaprostacyclin effects on intracellular cyclic AMP. The Schild plot slope was different from 1 but this could be due to an interaction of PGE2 with IP receptors in high doses. The exact nature of the antagonism by compound AH 23848B could not be definitely established in these experimental conditions. 6. Neither PGE2 nor any of its analogues inhibited the increase in intracellular cyclic AMP induced by forskolin, and pertussis toxin did not alter the response to PGE2, suggesting that no Gi-coupled PGE2 receptors are present in these cells. Stimulation with PGE2 did not induce significant increases in intracellular inositol-trisphosphate levels nor increases in intracellular free calcium as determined by confocal microscopy, suggesting the absence of phospholipase-C-coupled or of calcium channel-coupled PGE2 receptors in bovine chondrocytes in these experimental conditions. 7. These results show for the first time that bovine chondrocytes in culture present a functional PGE2 receptor that has some pharmacological characteristics of an EP4 subtype, as well as an IP receptor.