Phospholipase C activity in NCB-20 cells is inhibited by protein kinase A-mediated phosphorylation of low molecular mass GTP-binding proteins.
We have previously shown that bradykinin-induced production of second messengers such as inositol trisphosphate and diacylglycerol in neurotumor cells is inhibited by raising cellular cyclic AMP levels, which in turn inhibit phospholipase C. A monoclonal antibody to phospholipase C-II immunoprecipitated the 140-kDa form of phospholipase C-II from [35S]methionine/[3H]eucine-labeled cells, but not [32P]orthophosphate-labeled phospholipase C-II, following treatment with either forskolin or dibutyryl cyclic AMP. This suggested that phospholipase C is not the target for cyclic AMP-dependent protein kinase-mediated phosphorylation. In vitro studies confirmed that phospholipase C activity was inhibited by raising cellular cAMP levels, and partial sensitivity to Bordetella pertussis toxin suggested the involvement of a GTP-binding protein which could be the target for protein kinase A. The involvement of a GTP-binding protein in coupling the bradykinin receptor to phospholipase C was further suggested by the ability of both guanosine 5'-O-(thio-triphosphate) and fluoride (NaF) to release inositol phosphates from NCB-20 cell membranes previously labeled with [3H]inositol. Both effects were blocked by pretreatment of the cells with protein kinase A activators, further suggesting a GTP-binding protein as the target for protein kinase A-mediated phosphorylation. When whole NCB-20 cell extracts were blotted onto nitrocellulose and incubated with [alpha- 32P]GTP, a major 24-kDa band plus minor bands at 22 and 20 kDa were revealed by autoradiography. A pH 3.0/6.0 soluble (basic protein) NCB-20 cell extract revealed the major 24-kDa band plus the 20-kDa band, and similar basic proteins were shown to be heavily phosphorylated following [32P]orthophosphate labeling and pretreatment with forskolin. The size and ability to bind GTP on Western blots are characteristic of the ras, rho, smg, etc. family of GTP-binding proteins recently suggested to be the much sought after GPLC (Lapetina, E.G., Lacal, J. C., Reep, B. R., and Molina y Vedia, L. (1989) Proc. Natl. Acad. Sci. U.S.A. 86, 3131-3134; Wang, P., Nishihata, J., Takabori, E., Yamamoto, K., Toyoshima, S., and Osawa, T. (1989) J. Biochem. (Tokyo) 105, 461-466; Nagata, K.-I., Nagao, S., and Nozawa, Y. (1989) Biochem. Biophys. Res. Commun. 160, 235-242). We propose that GPLC is uniquely sensitive to protein kinase A-mediated phosphorylation and that phosphorylation inhibits stimulus-secretion coupling in these cells.
McAtee
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Department of Biochemistry, University of Chicago, Illinois 60637.
Dawson
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D015056:1-Methyl-3-isobutylxanthine / Q000494:pharmacology; D000818:Animals; D016718:Arachidonic Acid; D001095:Arachidonic Acids / Q000378:metabolism; D001920:Bradykinin / Q000494:pharmacology; D001921:Brain / Q000166:cytology / Q000378:metabolism; D002460:Cell Line; D002772:Cholera Toxin / Q000494:pharmacology; D005576:Colforsin / Q000494:pharmacology; D006224:Cricetinae; D004622:Embryo, Mammalian; D004789:Enzyme Activation; D019204:GTP-Binding Proteins / Q000378:metabolism*; D016244:Guanosine 5'-O-(3-Thiotriphosphate); D006160:Guanosine Triphosphate / Q000031:analogs & derivatives / Q000494:pharmacology; D006822:Hybrid Cells / Q000187:drug effects / Q000201:enzymology; D007295:Inositol Phosphates / Q000378:metabolism; D007700:Kinetics; D008970:Molecular Weight; D009447:Neuroblastoma; D010766:Phosphorylation; D011494:Protein Kinases / Q000378:metabolism*; D013873:Thionucleotides / Q000494:pharmacology; D010738:Type C Phospholipases / Q000037:antagonists & inhibitors*
D001095:Arachidonic Acids; D007295:Inositol Phosphates; D013873:Thionucleotides; D005576:Colforsin; D016718:Arachidonic Acid; D016244:Guanosine 5'-O-(3-Thiotriphosphate); D006160:Guanosine Triphosphate; D002772:Cholera Toxin; D011494:Protein Kinases; D010738:Type C Phospholipases; D019204:GTP-Binding Proteins; D001920:Bradykinin; D015056:1-Methyl-3-isobutylxanthine