Synthesis of adenophostin A analogues conjugating an aromatic group at the 5'-position as potent IP3 receptor ligands

Previous structure-activity relationship studies of adenophostin A, a potent IP(3) receptor agonist, led us to design the novel adenophostin A analogues 5a-c, conjugating an aromatic group at the 5'-position to develop useful IP(3) receptor ligands. The common key intermediate, a D-ribosyl alpha-D-glucoside 10alpha, was stereoselectively synthesized by a glycosidation with the 1-sulfinylglucoside donor 11, which was conformationally restricted by a 3,4-O-cyclic diketal protecting group. After introduction of an aromatic group at the 5-position of the ribose moiety, an adenine base was stereoselectively introduced at the anomeric beta-position to form 7a-c, where the tetra-O-i-butyryl donors 9a-c were significantly more effective than the corresponding O-acetyl donor. Thus, the target compounds 5a-c were synthesized via phosphorylation of the 2', 3' ', and 4' '-hydroxyls. The potencies of compounds 5a-c for Ca(2+) release were shown to be indistinguishable from that of adenophostin A, indicating that bulky substitutions at the 5'-position of adenophostin A are well-tolerated in the receptor binding. This biological activity of 5a-c can be rationalized by molecular modeling using the ligand binding domain of the IP(3) receptor.

Signalling from parathyroid hormone

PTH (parathyroid hormone), acting via type 1 PTH receptors, is a major regulator of plasma [Ca(2+)]. The G-protein, G(s), is an essential component of the sequence linking PTH to plasma Ca(2+) regulation, but the relative importance of intracellular signals, including Ca(2+) and cAMP, that lie downstream of G(s) is not resolved.

Ca2+ entry through plasma membrane IP3 receptors

Inositol 1,4,5-trisphosphate receptors (IP3Rs) release calcium ions, Ca2+, from intracellular stores, but their roles in mediating Ca2+ entry are unclear. IP3 stimulated opening of very few (1.9 +/- 0.2 per cell) Ca2+-permeable channels in whole-cell patch-clamp recording of DT40 chicken or mouse B cells. Activation of the B cell receptor (BCR) in perforated-patch recordings evoked the same response. IP3 failed to stimulate intracellular or plasma membrane (PM) channels in cells lacking IP3R. Expression of IP3R restored both responses. Mutations within the pore affected the conductances of IP3-activated PM and intracellular channels similarly. An impermeant pore mutant abolished BCR-evoked Ca2+ signals, and PM IP3Rs were undetectable. After introduction of an alpha-bungarotoxin binding site near the pore, PM IP3Rs were modulated by extracellular alpha-bungarotoxin. IP(3)Rs are unusual among endoplasmic reticulum proteins in being also functionally expressed at the PM, where very few IP3Rs contribute substantially to the Ca2+ entry evoked by the BCR.

Stimulation of arachidonic acid release by vasopressin in A7r5 vascular smooth muscle cells mediated by Ca2+-stimulated phospholipase A2

Arachidonic acid (AA) regulates many aspects of vascular smooth muscle behaviour, but the mechanisms linking receptors to AA release are unclear. In A7r5 vascular smooth muscle cells pre-labelled with (3)H-AA, vasopressin caused a concentration-dependent stimulation of 3H-AA release that required phospholipase C and an increase in cytosolic [Ca2+]. Ca2+ release from intracellular stores and Ca2+ entry via L-type channels or the capacitative Ca2+ entry pathway were each effective to varying degrees. Selective inhibitors of PLA2 inhibited the 3H-AA release evoked by vasopressin, though not the underlying Ca2+ signals, and established that cPLA2 mediates the release of AA. We conclude that in A7r5 cells vasopressin stimulates AA release via a Ca2+-dependent activation of cPLA2.

Cardiac Risks of Breast-cancer Radiotherapy: A Contemporary View

For some time, there has been compelling evidence both from randomised-controlled trials and from observational studies, that some of the breast-cancer radiotherapy regimens used in the past have led to increased risk of mortality from heart disease. There is also some evidence that the more recent regimens used in the USA are associated with lower risks than previous ones, but it is not clear whether current regimens are free from cardiac risk, especially in the light of recent evidence from the survivors of the bombings of Hiroshima and Nagasaki, in whom a clear relationship was observed between the risk of mortality from heart disease and radiation dose for doses in the range 0-4 Gy. Mortality from radiation-induced heart disease usually occurs at least a decade after irradiation. Symptomatic heart disease might have a much shorter induction period, but little information about it is available at present. Subclinical vascular abnormalities have been observed within months of irradiation, via myocardial perfusion imaging studies, but little is known about the relationship between these and later overt heart disease. At present, few data relate heart dose and other specific characteristics of breast radiotherapy to cardiac outcome. Further information on these topics is needed to enable estimation of the cardiac risk, that is likely to arise from radiotherapy regimens in current use and from those being considered for future use. Such knowledge would facilitate radiotherapy treatment planning and enable a reduction in cardiac risk while maintaining the known benefit in terms of breast cancer mortality.

Design and Synthesis of 5‘-Deoxy-5‘-Phenyladenophostin A, a Highly Potent IP3 Receptor Ligand1

[structure: see text] 5'-Deoxy-5'-phenyladenophostin A (5), designed as a useful IP(3) receptor ligand based on the previous structure-activity relationship studies, was successfully synthesized via two key stereoselective glycosidation steps. This compound proved to be a highly potent IP(3) receptor agonist.

A Systematic Study of C-Glucoside Trisphosphates as myo-Inositol Trisphosphate Receptor Ligands. Synthesis of β-C-Glucoside Trisphosphates Based on the Conformational Restriction Strategy

Beta-C-glucoside trisphosphates having a C2 side chain (3,7-anhydro-2-deoxy-D-glycero-D-gulo-octitol 1,5,6-trisphosphate, 11) and a C3 side chain (4,8-anhydro-2,3-dideoxy-D-glycero-D-gulo-nonanitol 1,6,7-trisphosphate, 12) were designed as structurally simplified analogues of a potent D-myo-inositol 1,4,5-trisphosphate (IP3) receptor ligand, adenophostin A. Construction of the beta-C-glucosidic structure, which was the key to their synthesis, was achieved by two different methods based on the conformational restriction strategy: (1) radical cyclization with a temporary connecting silicon tether and (2) silane reduction of glyconolactols having an anomeric allyl substituent. Using these methods, the target beta-C-glycoside trisphosphates 11 and 12 were successfully synthesized. A structure-activity relationship was established on a series of C-glucoside trisphosphates, including the previously synthesized related compounds, which were a C-glycosidic analogue 3 of adenophostin A, its uracil congener 5, alpha-C-glucoside trisphosphates 7-9 having a C1, C2, or C3 side chain, and the beta-C-glucoside trisphosphates 10-12 having a C1, C2, or C3 side chain. The O-glycosidic linkage of adenophostin A and its analogues proved to be replaced by the chemically and biologically more stable C-glycosidic linkage. The alpha-C2-glucoside trisphosphate 8 stimulates Ca2+ release with a potency similar to that of IP3 in spite of its simplified structure, indicating a better fit to the receptor than the beta-C-glucoside trisphosphates and also the alpha-congeners having a shorter or longer C1 side chain, which was supported by molecular modeling using the ligand binding domain of the IP3 receptor.

Guanophostin A: Synthesis and evaluation of a high affinity agonist of the d-myo-inositol 1,4,5-trisphosphate receptor

Guanophostin A, the guanosine counterpart of the inositol 1,4,5-trisphosphate receptor agonist adenophostin A, has been synthesized and is the first synthetic adenophostin A-like analogue to be equipotent to its parent in stimulating intracellular Ca2+ release; its nucleotide moiety is proposed to interact with the receptor binding core by guanine base cation-pi stacking with Arg504 and hydrogen bonding with Glu505 and interaction of the ribosyl 2'-phosphate group with the helix-dipole of alpha6.

Regulation of capacitative and non-capacitative Ca2+ entry in A7r5 vascular smooth muscle cells

A capacitative Ca2+ entry (CCE) pathway, activated by depletion of intracellular Ca2+ stores, is thought to mediate much of the Ca2+ entry evoked by receptors that stimulate phospholipase C (PLC). However, in A7r5 vascular smooth muscle cells, vasopressin, which stimulates PLC, empties intracellular Ca2+ stores but simultaneously inhibits their ability to activate CCE. The diacylglycerol produced with the IP3 that empties the stores is metabolized to arachidonic and this leads to activation of nitric oxide (NO) synthase, production of NO and cyclic GMP, and consequent activation of protein kinase G. The latter inhibits CCE. In parallel, NO directly activates a non-capacitative Ca2+ entry (NCCE) pathway, which is entirely responsible for the Ca2+ entry that occurs in the presence of vasopressin. This reciprocal regulation of two Ca2+ entry pathways ensures that there is sequential activation of first NCCE in the presence of vasopressin, and then a transient activation of CCE when vasopressin is removed. We suggest that the two routes for Ca2+ entry may selectively direct Ca2+ to processes that mediate activation and then recovery of the cell.

Rapid functional assays of recombinant IP3 receptors

Functional assays of inositol 1,4,5-trisphosphate receptors (IP3R) currently use 45Ca2+ release methods, fluorescent Ca2+ indicators within either the ER or cytosol, or electrophysiological analyses of IP3R in the nuclear envelope or artificial bilayers. None of the methods is presently amenable to the rapid, high-throughput quantitative analyses of IP3R function needed to address the structural determinants of IP3R behavior. We use a low-affinity Ca2+ indicator (Mag-fluo-4) to measure free [Ca2+] within the ER of permeabilized DT40 cells expressing only rat type 1 IP(3)R, and establish that the indicator is capable of reliably reporting the Ca(2+) release evoked by IP3. A 96-well fluorescence plate reader equipped for automated fluid additions (FlexStation, Molecular Devices) is used to monitor IP3-evoked Ca2+ release. The method allows quick and economical functional assays of recombinant IP3R in small volumes (< or = 100 microl).

Leukemic infiltration of the orbit: report of three cases and literature review

Orbital infiltration by acute lymphoblastic leukemia is rare. The authors present 3 patients, 2 with optic nerve involvement and 1 with anterior chamber infiltration, treated by chemotherapy and radiotherapy. Two are in continuous remission at 64 and 59 months and 1 relapsed in the central nervous system 35 months after ocular relapse. Visual deterioration was prevented in two. Early diagnosis and treatment are important for preservation of vision.

Oncological aspects of breast reconstruction

Breast reconstruction has become increasingly popular over the past 20 years. There is concern that it may mask locoregional recurrence or that immediate reconstruction may compromise adjuvant treatments. We review available evidence regarding its oncological safety. The literature consists almost entirely of single institution, small retrospective reviews with variable follow-up and varying conclusions. Most reviews suggest that breast reconstruction does not adversely affect disease-free or overall survival and that there is no significant delay in presentation with recurrent disease. Three retrospective series compared chemotherapy delivery after immediate breast reconstruction with controls having mastectomy alone. No delay in chemotherapy delivery or effect on dose intensity was demonstrated. Irradiation of a prosthetic implant has been shown to increase the rate of capsular contracture; irradiation of autogenous tissue reconstruction is usually well tolerated.

The effect of immediate breast reconstruction on adjuvant chemotherapy

Immediate breast reconstruction is being increasingly offered to patients requiring mastectomy for breast cancer. An audit was carried out to determine whether it affected time to initiation of chemotherapy, delays during chemotherapy, percentage intended dose and need for support with antibiotics or granulocyte-colony stimulating factor. A total of 44 patients undergoing a variety of reconstructive procedures followed by chemotherapy were identified and patient records were reviewed. These were compared with a control group of 49 patients undergoing mastectomy alone and chemotherapy in the same 4-year period and institution. Patients undergoing transverse rectus abdominis myocutaneous flap reconstruction experienced an average of 5 more days delay to chemotherapy initiation than controls with the commonest reason being poor wound healing. Percentage intended dose, delays during chemotherapy and need for support with granulocyte-colony stimulating factor or antibiotics were comparable in all groups.

Long lasting inhibition of adenylyl cyclase selectively mediated by inositol 1,4,5-trisphosphate-evoked calcium release

In A7r5 smooth muscle cells, vasopressin stimulates release of Ca2+ from intracellular stores and Ca2+ entry, and it inhibits adenylyl cyclase (AC) activity. Inhibition of AC is prevented by inhibition of phospholipase C or when the increase in cytosolic [Ca2+] is prevented by the Ca2+ buffer, BAPTA. It is unaffected by pertussis toxin, inhibition of protein kinase C, or L-type Ca2+ channels or by removal of extracellular Ca2+. The independence of extracellular Ca2+ occurs despite inhibition of AC by vasopressin persisting for at least 15 min, whereas the cytosolic [Ca2+] returns to its basal level within 1-2 min in Ca2+-free medium. Although capacitative Ca2+ entry (CCE), activated by emptying stores with thapsigargin, inhibits AC, Ca2+ entry via CCE or L-type Ca2+ channels activated by vasopressin is ineffective. Temporally separating vasopressin-evoked Ca2+ release from the assessment of AC activity revealed that the transient Ca2+ signal resulting from Ca2+ mobilization causes a long lasting inhibition of AC. By contrast, inhibition of AC by thapsigargin-evoked CCE reverses rapidly after removal of extracellular Ca2+. Inhibition of AC by vasopressin is prevented by inhibition of Ca2+-calmodulin-dependent protein kinase II. We conclude that persistent inhibition of AC (probably AC-3) by vasopressin is mediated by inositol trisphosphate-evoked Ca2+ release causing activation of Ca2+-calmodulin-dependent protein kinase II. Our results establish that an important interaction between two ubiquitous signaling pathways is tuned selectively to Ca2+ release via inositol trisphosphate receptors and that the interaction transduces a transient Ca2+ signal into a long lasting inhibition of AC.

Adenophostin A and analogues modified at the adenine moiety: synthesis, conformational analysis and biological activity

The synthesis of adenophostin A (2) and two analogues [etheno adenophostin (4) and 8-bromo adenophostin (5)] modified at the adenine moiety, is reported. A combination of NMR analysis and molecular modelling was used to compare their structures in solution and determined that they all adopt very similar conformations. The analogues were tested for their ability to mobilise Ca(2+) from DT40 cells expressing recombinant Type 1 rat Ins(1,4,5)P(3)R which reveals etheno adenophostin as a high affinity fluorescent probe of the Ins(1,4,5)P(3)R. 8-Bromo adenophostin was only slightly less potent. The biological results support our current hypothesis regarding the binding mode of adenophostin A at the Ins(1,4,5)P(3)R, i. e. that a cation-pi interaction between the base moiety and Arg 504 of the receptor in combination with H-bonding may be responsible for the high potency of adenophostin A relative to Ins(1,4,5)P(3).

Ca2+Regulation of Inositol 1,4,5-trisphosphate Receptors: Can Ca2+Function without Calmodulin?

All Ca(2+) channels are regulated by Ca(2+), a feature that allows them to respond to their own activity and to the activities of neighboring Ca(2+) channels. Inhibition by Ca(2+) protects cells from potentially hazardous increases in cytosolic [Ca(2+)], and stimulation can mediate facilitation and regenerative propagation of Ca(2+) signals. Calmodulin is emerging as a key player in regulation of Ca(2+) channels by Ca(2+), but its role is more complex and more beautiful than might have been imagined.

IP(3) receptors: the search for structure

Inositol (1,4,5)-trisphosphate receptors (IP(3)R) are intracellular Ca(2+) channels that are regulated by Ca(2+) and IP(3), and are modulated by many additional signals. They thereby allow both receptors that stimulate IP(3) formation and Ca(2+) to control release of Ca(2+) from intracellular stores. IP(3)Rs share many features with their close relatives, ryanodine receptors; each provides insight into the structure and function of the other. The structural basis of IP(3)R behaviour is beginning to emerge from intermediate-resolution structures of the complete IP(3)R, a 2.2-A structure of the IP(3)-binding core and comparisons with the pore structures of other tetrameric cation channels. The binding of IP(3) to a site towards the N-terminal of each IP(3)R subunit promotes binding of Ca(2+). This destabilizes an inhibitory interaction between N-terminal residues and a C-terminal 'gatekeeper' sequence, enabling the pore to open.

Targeting of Inositol 1,4,5-Trisphosphate Receptors to the Endoplasmic Reticulum by Multiple Signals within Their Transmembrane Domains

Most inositol 1,4,5-trisphosphate receptors (IP3R) are expressed in the endoplasmic reticulum (ER), where their precise distribution underlies the spatially complex Ca2+ signals evoked by extracellular stimuli. The signals that target IP3R to the ER or, less commonly, to other membranes are unknown. We expressed yellow fluorescent protein-tagged fragments of type 1 IP3R alone or fused with a plasma membrane protein to establish the determinants of ER targeting in COS-7 cells. By using a combination of confocal imaging and glycoprotein analyses, we demonstrated that any pair of the six transmembrane domains (TMD) linked by a luminal loop retains the protein within the ER, and when attached to a plasma membrane protein (ICAM-1), prevents it from reaching the medial Golgi. TMD1 or TMD2 alone were accumulated in mitochondria, whereas TMD5 and TMD6 were retained in ER, but were unable to prevent ICAM from reaching the plasma membrane. We conclude that IP3R are targeted to the ER membrane only after synthesis of TMDs 1 and 2, and that after co-translational insertion of the remaining TMDs, redundant retention signals present in any pair of TMD retain IP3R in the ER.

Dimers of d-myo-Inositol 1,4,5-Trisphosphate: Design, Synthesis, and Interaction with Ins(1,4,5)P3 Receptors

The design and synthesis of dimeric versions of the intracellular signaling molecule d-myo-inositol 1,4,5-trisphosphate [Ins(1,4,5)P(3)] are reported. Ins(1,4,5)P(3) dimers in a range of sizes were constructed by conjugation of a partially protected 2-O-(2-aminoethyl)-Ins(1,4,5)P(3) intermediate with activated oligo- and poly(ethylene glycol) (PEG) tethers, to give benzyl-protected dimers with amide or carbamate linkages. After deprotection, the resulting water-soluble Ins(1,4,5)P(3) dimers were purified by ion-exchange chromatography. The interaction of the Ins(1,4,5)P(3) dimers with tetrameric Ins(1,4,5)P(3) receptors was explored, using equilibrium [(3)H]Ins(1,4,5)P(3)-binding to membranes from cerebellum, and (45)Ca(2+)-release from permeabilized hepatocytes. The results showed that dimers, even when they incorporate large PEG tethers, interact potently with Ins(1,4,5)P(3) receptors, and that the shorter dimers are more potent than Ins(1,4,5)P(3) itself. A very small dimer, consisting of two Ins(1,4,5)P(3) motifs joined by a short N,N'-diethylurea spacer, was synthesized. Preliminary studies of (45)Ca(2+) release from the intracellular stores of permeabilized hepatocytes showed this shortest dimer to be almost as potent as adenophostin A, the most potent Ins(1,4,5)P(3) receptor ligand known. Possible interpretations of this result are considered in relation to the recently disclosed X-ray crystal structure of the type 1 Ins(1,4,5)P(3) receptor core binding domain.

Effect of an oxytocin receptor antagonist and rho kinase inhibitor on the [Ca++]i sensitivity of human myometrium

OBJECTIVE

The purpose of this study was to assess the possible mechanism and effect of oxytocin receptor on the sensitization of the contractile proteins to [Ca(++)](i).

STUDY DESIGN

Myometrial strips were prepared from biopsy specimens that had been taken at term (37-40 weeks of gestation) cesarean delivery from 22 women, before the onset of labor. Simultaneous measurements of [Ca(++)](i) and tension were performed on spontaneously contracting strips. The effects of a specific oxytocin receptor antagonist, L371,257, and a rho kinase inhibitor, Y-27632, on the [Ca(++)](i) and tension transients were determined.

RESULTS

Application of L371,257 (90 nmol/L) to spontaneously contracting muscle strips reduced peak tension to 43%+/-12% of its original value without affecting peak [Ca(++)](i) (105%+/-15%). More tension is developed at each [Ca(++)](i) during the falling phase, relative to the rising phase of each spontaneous contraction; this asymmetric [Ca(++)](i)-tension relationship was abolished by L371,257. The subsequent application of oxytocin (3 micromol/L) reversed the effects of the antagonist on tension. An inhibitor of rho kinase (Y-27632) caused a concentration-dependent inhibition of spontaneous contractions without affecting the underlying Ca(++) signals. The addition of oxytocin (1 nmol/L) to spontaneously active myometrium increased both the duration of the Ca(++) signal and the sensitivity of the contractile machinery to [Ca(++)](i); only the latter effect was prevented by Y-27632.

CONCLUSION

Oxytocin independently regulates the duration of the periodic Ca(++) signals and the sensitivity of the contractile machinery to Ca(++). The latter is likely to be mediated by rho kinase, which is essential for the effective coupling of increases in [Ca(++)](i) to tension.

Synthesis and Ca2+-Mobilizing Activity of Purine-Modified Mimics of Adenophostin A: A Model for the Adenophostin−Ins(1,4,5)P3Receptor Interaction

The synthesis of a series of adenophostin A analogues modified at C-6 and C-2 of adenine is described. The target compounds were synthesized by a convergent route involving a modified Vorbrüggen condensation of either 6-chloropurine or 2,6-dichloropurine with a protected disaccharide, yielding two versatile intermediates capable of undergoing substitution with a range of nucleophiles. The new analogues showed a range of abilities to mobilize Ca(2+) from the intracellular stores of permeabilized hepatocytes and are among the first totally synthetic compounds to approach the activity of adenophostin A. In agreement with the biological results, docking studies of adenophostin A using the recently reported X-ray crystal structure of the type 1 Ins(1,4,5)P(3) receptor binding core suggested that, in likely binding modes of adenophostin A, the area around N(6) may be relatively open, identifying this region of the adenophostin A molecule as a promising target for further elaboration. The docking results also point to specific interactions involving residues within the binding domain of the Ins(1,4,5)P(3) receptor that may be involved in the molecular recognition of the adenophostins.

Analogues of cyclic adenosine 5'-diphosphate ribose and adenophostin A, nucleotides in cellular signal transduction

Nicotinamide 8-Br-hypoxanthine dinucleotide (8-Br-NHD+) was cyclised at the N-1 position by ADP-ribosyl cyclase from Aplysia californica to give cyclic 8-Br-inosine diphosphoribose, a novel membrane-permeant analogue of cyclic-ADP ribose and agonist in human T cells. Adenine-substituted analogues of adenophostin A with potent Ca2+ releasing activity were synthesized; docking studies using the binding core of the Ins(1,4,5)P3 receptor identified specific residues that could be of importance in determining the hyperagonist nature of adenophostin activity.

Domain organization of the type 1 inositol 1,4,5-trisphosphate receptor as revealed by single-particle analysis

The inositol 1,4,5-trisphosphate receptor (IP(3)R) is a tetrameric intracellular Ca(2+) channel, which mediates the release of Ca(2+) from the endoplasmic reticulum in response to many different extracellular stimuli. We present a 3D structure of the type 1 IP(3)R obtained by electron microscopy and single-particle analysis that reveals its domain organization. The IP(3)R has a flower-like appearance with fourfold symmetry and is made up of three distinct domains connected by slender links. By relating the organization of the structural domains to secondary-structure predictions and biochemical data we develop a model in which structural domains are mapped onto the amino acid sequence to deduce the location of the channel region and the cytoplasmic inositol 1,4,5-trisphosphate-binding and modulatory subdomains. The structure of the IP(3)R is compared with that of other tetrameric cation channels. The channel domain is similar in size and shape to its counterparts in the ryanodine receptor and the Shaker voltage-gated K(+) channel.