title: Regulation of inositol 1,4,5-trisphosphate receptors by cAMP independent of cAMP-dependent protein kinase.
abstract: In HEK cells stably expressing type 1 receptors for parathyroid hormone (PTH), PTH causes a sensitization of inositol 1,4,5-trisphosphate receptors (IP(3)R) to IP(3) that is entirely mediated by cAMP and requires cAMP to pass directly from type 6 adenylyl cyclase (AC6) to IP(3)R2. Using DT40 cells expressing single subtypes of mammalian IP(3)R, we demonstrate that high concentrations of cAMP similarly sensitize all IP(3)R isoforms to IP(3) by a mechanism that does not require cAMP-dependent protein kinase (PKA). IP(3) binding to IP(3)R2 is unaffected by cAMP, and sensitization is not mediated by the site through which ATP potentiates responses to IP(3). In single channel recordings from excised nuclear patches of cells expressing IP(3)R2, cAMP alone had no effect, but it increased the open probability of IP(3)R2 activated by a submaximal concentration of IP(3) alone or in combination with a maximally effective concentration of ATP. These results establish that cAMP itself increases the sensitivity of all IP(3)R subtypes to IP(3). For IP(3)R2, this sensitization results from cAMP binding to a novel site that increases the efficacy of IP(3). Using stably expressed short hairpin RNA to reduce expression of the G-protein, G alpha(s), we demonstrate that attenuation of AC activity by loss of G alpha(s) more substantially reduces sensitization of IP(3)R by PTH than does comparable direct inhibition of AC. This suggests that G alpha(s) may also specifically associate with each AC x IP(3)R complex. We conclude that all three subtypes of IP(3)R are regulated by cAMP independent of PKA. In HEK cells, where IP(3)R2 selectively associates with AC6, G alpha(s) also associates with the AC x IP(3)R signaling junction.
authors.affiliation: Department of Pharmacology, University of Cambridge, Tennis Court Road, Cambridge CB2 1PD, United Kingdom.
authors.forename: Stephen C
authors.initials: SC
authors.lastname: Tovey
authors.forename: Skarlatos G
authors.initials: SG
authors.lastname: Dedos
authors.forename: Taufiq
authors.initials: T
authors.lastname: Rahman
authors.forename: Emily J A
authors.initials: EJ
authors.lastname: Taylor
authors.forename: Evangelia
authors.initials: E
authors.lastname: Pantazaka
authors.forename: Colin W
authors.initials: CW
authors.lastname: Taylor
mesh_terms: D000262:Adenylyl Cyclases / Q000235:genetics / Q000378:metabolism*; D000818:Animals; D002460:Cell Line; D000242:Cyclic AMP / Q000235:genetics / Q000378:metabolism*; D017868:Cyclic AMP-Dependent Protein Kinases / Q000235:genetics / Q000378:metabolism; D044385:GTP-Binding Protein alpha Subunits / Q000235:genetics / Q000378:metabolism*; D006801:Humans; D015544:Inositol 1,4,5-Trisphosphate / Q000235:genetics / Q000378:metabolism*; D053496:Inositol 1,4,5-Trisphosphate Receptors / Q000235:genetics / Q000378:metabolism*; D020033:Protein Isoforms / Q000235:genetics / Q000378:metabolism; D051381:Rats; D044168:Receptor, Parathyroid Hormone, Type 1 / Q000235:genetics / Q000378:metabolism; D015398:Signal Transduction / Q000502:physiology*
chemical_list: D044385:GTP-Binding Protein alpha Subunits; D053496:Inositol 1,4,5-Trisphosphate Receptors; D020033:Protein Isoforms; D044168:Receptor, Parathyroid Hormone, Type 1; D015544:Inositol 1,4,5-Trisphosphate; D000242:Cyclic AMP; D017868:Cyclic AMP-Dependent Protein Kinases; D000262:Adenylyl Cyclases; C121100:adenylyl cyclase 6