phorbol and Glioma

Cross-regulation from the stimulatory phospholipase C to the adenylyl cyclase pathways was explored in neuroblastoma-glioma NG-108-15 cells in culture. Activation of protein kinase C by phorbol myristic acid resulted in a markedly attenuated activation of the inhibitory adenylyl cyclase response to delta-opiate agonists and epinephrine but not to the muscarinic agonist carbachol. The ability of okadaic acid to mimic the effects of phorbol myristic acid on the inhibitory response suggested a role for protein phosphorylation. Adenylyl cyclase activity from cells in which protein kinase C had been activated demonstrated a loss in the inhibitory adenylyl cyclase response at the level of the G-protein. Activation of protein kinase C prompted a 2-4-fold increase in phosphorylation of G1 alpha 2 in cells metabolically labeled with [32P]orthophosphate. The phosphate content of Gi alpha 2 was determined to be approximately 0.5 mol/mol subunit in the unstimulated cells and approximately 1.5 mol/mol subunit for cells in which protein kinase C was activated. The effects of okadaic acid, 4-alpha-phorbol, and calphostin C on inhibition of adenylyl cyclase in cells treated with phorbol myristic acid correlate with the effects of these agents on phosphorylation of Gi alpha 2. The time courses for attenuation of inhibitory adenylyl cyclase and that for phosphorylation of Gi alpha 2 were similar in cells challenged with phorbol myristic acid. These data argue for cross-regulation from the stimulatory protein kinase C to inhibitory adenylyl cyclase pathways at the level of Gi alpha 2 via protein phosphorylation.

Topics: Adenylyl Cyclases; Enkephalin, Leucine-2-Alanine; Enzyme Activation; Ethers, Cyclic; Glioma; GTP-Binding Proteins; Hybrid Cells; Naphthalenes; Neuroblastoma; Okadaic Acid; Phorbols; Phosphorylation; Polycyclic Compounds; Protein Kinase C; Tetradecanoylphorbol Acetate; Tumor Cells, Cultured

The mechanisms by which lymphokine-activated killer (LAK) cells exert their cytotoxic effects are not well understood. This study demonstrates that phorbol ester pretreatment of a LAK cell-sensitive glioma cell line (SNB-19) induced a significant decrease in the susceptibility of cells to LAK cell-mediated lysis. This effect was produced by low concentrations of the tumor-promoting phorbol ester, phorbol-12,13-myristate acetate (PMA), and was reversible. Protein kinase C (PKC) inhibitors failed to block this phenomenon. No apparent alteration in the ability of LAK cells to bind to their targets was observed. Thus, PMA may have exerted its effects by a mechanism that does not require PKC, or these glioma cells may possess an isozyme of PKC which is insensitive to the inhibitors used in these studies.

Topics: Cell Division; Glioma; Humans; Killer Cells, Lymphokine-Activated; Phorbol 12,13-Dibutyrate; Phorbol Esters; Phorbols; Protein Kinase C; Tetradecanoylphorbol Acetate; Thymidine; Tumor Cells, Cultured