Tumor promoter-dependent phosphorylation of a Triton X-100 extractable form of lipocortin I in T51B rat liver cells.
The phosphorylation of lipocortin (a substrate of EGF-receptor kinase, and a putative phospholipase A2 inhibitor) was examined in T51B cells. By using Western blot procedures and antisera specific to lipocortin I, we found that most immunoreactive lipocortin I was located in the cytosol (lipocortin(cvt] of cells extracted in Ca2+-free buffers These cells however had another pool of immunoreactive lipocortin I located in the particulate fraction that was Triton X-100 extractable (lipocortin(mem]. Increasing Ca2+ concentrations in the extraction buffer resulted in more lipocortin(mem) recovered. In vitro phosphorylation of endogenous proteins demonstrated that lipocortin I became phosphorylated in a Ca2+ and phosphatidylserine-dependent manner, suggesting an involvement of protein kinase C. Treatment of cells with 100 ng/ml 12-0-tetradecanoylphorbol-13-acetate (TPA) but not with 4 alpha-phorbol 12,13-didecanoate (4 alpha-PDD) resulted in the in vitro phosphorylation of lipocortin(mem) by protein kinase C. TPA also increased the phosphorylation of lipocortin(mem) in [32P]phosphate-labeled cells.
Campos-Gonzalez
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R
Cancer Research Center of Hawaii, Honolulu 96813.
Kanemitsu
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M
Boynton
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D000818:Animals; D017302:Annexins; D002135:Calcium-Binding Proteins / Q000378:metabolism*; D002273:Carcinogens / Q000494:pharmacology*; D002478:Cells, Cultured; D003600:Cytosol / Q000378:metabolism; D004847:Epithelial Cells; D004848:Epithelium / Q000187:drug effects / Q000378:metabolism; D008099:Liver / Q000166:cytology* / Q000187:drug effects / Q000378:metabolism; D017830:Octoxynol; D010740:Phospholipases / Q000037:antagonists & inhibitors*; D010766:Phosphorylation; D011092:Polyethylene Glycols; D011493:Protein Kinase C / Q000032:analysis / Q000378:metabolism / Q000494:pharmacology; D051381:Rats
D017302:Annexins; D002135:Calcium-Binding Proteins; D002273:Carcinogens; D011092:Polyethylene Glycols; D017830:Octoxynol; D011493:Protein Kinase C; D010740:Phospholipases