title: Defining the minimal structural requirements for partial agonism at the type I myo-inositol 1,4,5-trisphosphate receptor.
abstract: The novel synthetic analogues D-3-fluoro-myo-inositol 1,5-bisphosphate-4-phosphorothioate, [3F-Ins(1,5)P2-4PS], D-3-fluoro-myo-inositol 1,4-bisphosphate-5-phosphorothioate [3F-Ins(1,4)P2-5PS], and D-3-fluoro-myo-inositol 1-phosphate-4,5-bisphosphorothioate [3F-Ins(1)P-(4,5)PS2] were utilised to define the structure-activity relationships which could produce partial agonism at the Ca2+ mobilising myo-inositol 1,4,5-trisphosphate [Ins(1,4,5)P3] receptor. Based on prior structure-activity data we hypothesised that the minimal structural requirements for lns(1,4,5)P3 receptor partial agonism, were phosphorothioate substitution of the crucial vicinal 4,5-bisphosphate pair accompanied by another structural perturbation, such fluorination of 3-position of the myo-inositol ring. All the analogues fully displaced [3H]Ins(1,4,5)P3 from a single Ins(1,4,5)P3 binding site in pig cerebellar membranes [3F-Ins(1,5)P2-4PS (1C50 = 26 nM), 3F-Ins(1,4)P2-5PS (IC50 = 80 nM) and 3F-Ins(1)P-(4,5)PS2 (IC50 = 109 nM) cf. Ins(1,4,5)P3 (IC50 = 11 nM)]. In contrast, 3F-Ins(1,5)P2-4PS (IC50 = 424 nM) and 3F-Ins(1,4)P2-5PS (IC50 = 3579 nM) were weak full agonists at the Ca2+ mobilising Ins(1,4,5)P3 receptor of permeabilised SH-SY5Y neuroblastoma cells, being respectively 4- and 36-fold less potent than Ins(1,4,5)P3 (EC50 = 99 nM). While 3F-Ins(1)P-(4,5)PS2 (EC50 = 11345 nM) was a partial agonist releasing only 64.3 +/- 1.9% of the Ins(1,4,5)P3-sensitive intracellular Ca2+ pools. 3F-Ins(1)P-(4,5)PS2 was unique among the Ins(1,4,5)P3 receptor partial agonists so far identified in having a relatively high affinity for the Ins(1,4,5)P3 binding site, accompanied by a significant loss of intrinsic activity for Ca2+ mobilisation. This improved affinity was probably due to the retention of the 1-position phosphate, which enhances interaction with the Ins-(1,4,5)P3 receptor. 3F-Ins(1)P-(4,5)PS2 may be an important lead compound for the development of efficient Ins(1,4,5)P3 receptor antagonists.
authors.affiliation: Department of Cell Physiology and Pharmacology, University of Leicester, UK. RAW6@leicester.ac.uk
authors.forename: R A
authors.initials: RA
authors.lastname: Wilcox
authors.forename: A
authors.initials: A
authors.lastname: Fauq
authors.forename: A P
authors.initials: AP
authors.lastname: Kozikowski
authors.forename: S R
authors.initials: SR
authors.lastname: Nahorski
mesh_terms: D000818:Animals; D001667:Binding, Competitive; D002118:Calcium / Q000378:metabolism; D015220:Calcium Channels / Q000737:chemistry* / Q000187:drug effects; D002462:Cell Membrane / Q000378:metabolism; D002531:Cerebellum / Q000378:metabolism*; D006801:Humans; D015544:Inositol 1,4,5-Trisphosphate / Q000031:analogs & derivatives* / Q000138:chemical synthesis / Q000378:metabolism / Q000494:pharmacology*; D053496:Inositol 1,4,5-Trisphosphate Receptors; D007700:Kinetics; D009447:Neuroblastoma; D018160:Receptors, Cytoplasmic and Nuclear / Q000737:chemistry* / Q000187:drug effects; D013329:Structure-Activity Relationship; D013552:Swine; D014407:Tumor Cells, Cultured
chemical_list: D015220:Calcium Channels; C506926:ITPR1 protein, human; D053496:Inositol 1,4,5-Trisphosphate Receptors; D018160:Receptors, Cytoplasmic and Nuclear; D015544:Inositol 1,4,5-Trisphosphate; D002118:Calcium