title: Thyrotropin-releasing hormone activates a Ca2+-dependent polyphosphoinositide phosphodiesterase in permeable GH3 cells. GTP gamma S potentiation by a cholera and pertussis toxin-insensitive mechanism.
abstract: Numerous hormones are known to rapidly activate polyphosphoinositide turnover in target cells by promoting phosphodiesteratic cleavage of the phospholipids; however, little is known about the enzymology of receptor-mediated phosphoinositide breakdown. In the present study, thyrotropin-releasing hormone (TRH) stimulation of polyphosphoinositide turnover has been characterized in electrically permeabilized, [3H]myoinositol-labeled GH3 cells. The permeable cells allow the influence of small molecular weight (Mr less than or equal to 1000) cofactors to be determined. We present evidence for the following: 1) TRH stimulates inositol phosphate generation in permeable cells; 2) optimal hormone-stimulated inositol phosphate generation requires Mg2+, ATP, and Ca2+; 3) Mg2+ and ATP requirements reflect polyphosphoinositide kinase reactions; 4) in the absence of MgATP, TRH stimulates the phosphodiesteratic breakdown of pre-existing polyphosphoinositides in a reaction which requires only low Ca2+ (10(-7) M); 5) hormone activation is potentiated in the presence of the stable guanine nucleotide, GTP gamma S; neither TRH-stimulated nor GTP gamma S-potentiated hydrolysis is inhibited by cholera or pertussis toxin treatment. These results demonstrate that hormone-induced phospholipid hydrolysis involves activation of a phosphoinositide phosphodiesterase; activation results in lowering the Ca2+ requirement of the phosphodiesterase such that maximal activity is observed at Ca2+ levels characteristic of a resting cell (10(-7) M). Furthermore, TRH regulation of polyphosphoinositide hydrolysis is modulated by guanine nucleotides; however, nucleotide regulation appears to involve a GTP-binding factor (Np) other than Ns or Ni.
mesh_terms: D000227:Adenine Nucleotides / Q000378:metabolism; D000255:Adenosine Triphosphate / Q000378:metabolism; D000818:Animals; D002118:Calcium / Q000378:metabolism*; D002460:Cell Line; D002463:Cell Membrane Permeability; D002772:Cholera Toxin / Q000494:pharmacology; D004305:Dose-Response Relationship, Drug; D004558:Electric Stimulation; D004789:Enzyme Activation; D016244:Guanosine 5'-O-(3-Thiotriphosphate); D006160:Guanosine Triphosphate / Q000031:analogs & derivatives* / Q000494:pharmacology; D006868:Hydrolysis; D007295:Inositol Phosphates / Q000378:metabolism; D008094:Lithium / Q000494:pharmacology; D037342:Pertussis Toxin; D054801:Phosphoinositide Phospholipase C; D010727:Phosphoric Diester Hydrolases / Q000378:metabolism*; D010911:Pituitary Neoplasms / Q000201:enzymology*; D011494:Protein Kinases / Q000378:metabolism; D051381:Rats; D013873:Thionucleotides / Q000494:pharmacology*; D013973:Thyrotropin-Releasing Hormone / Q000494:pharmacology*; D010566:Virulence Factors, Bordetella / Q000494:pharmacology
chemical_list: D000227:Adenine Nucleotides; D007295:Inositol Phosphates; D013873:Thionucleotides; D010566:Virulence Factors, Bordetella; D016244:Guanosine 5'-O-(3-Thiotriphosphate); D013973:Thyrotropin-Releasing Hormone; D006160:Guanosine Triphosphate; D000255:Adenosine Triphosphate; D002772:Cholera Toxin; D008094:Lithium; D037342:Pertussis Toxin; D011494:Protein Kinases; D010727:Phosphoric Diester Hydrolases; D054801:Phosphoinositide Phospholipase C; D002118:Calcium