Time-dependent changes in protein kinase C distribution and disappearance in phorbol ester-treated human osteosarcoma...

To test directly whether protein kinase C activation is one of the required events leading to stimulation of prostaglandin production by bone cells, protein kinase C activity and prostaglandin E2 release were measured in monolayer cultures of the clonal human osteosarcoma cell lines G-292 and SaOS-2 after exposure to phorbol myristate acetate (PMA). Both cell lines have specific receptors for PMA but only G-292 cells respond with increased prostaglandin E2 production (M. A. Shupnik and A. H. Tashjian, Jr., J. Biol. Chem., 257: 12161-12164, 1982). The subcellular distribution of protein kinase C in both unstimulated osteosarcoma cell lines was similar; in an EDTA- and leupeptin-containing homogenization buffer, between 70 and 80% of the total enzyme activity was cytosolic. Short (less than 60 min) incubations with PMA induced marked decreases in cytosolic enzyme activity and parallel increases in particulate protein kinase C; thereafter, total measured cellular protein kinase C activity declined, mediated by decreases in both cytosolic and particulate protein kinase C specific activities. By 24 h cytosolic, particulate, and total protein kinase C activities were less than 10% of basal. Because the protein kinase C responses in both cell types were essentially the same, but only G-292 cells give a prostaglandin response to PMA, we conclude that protein kinase C activation by PMA is itself insufficient to stimulate prostaglandin E2 production and that the lack of a prostaglandin response in SaOS-2 cells cannot be explained by lack of protein kinase C activation.