Activators of
protein kinase C (PKC), such as 12-O-tetradecanoylphorbol-13-acetate (TPA) and
bryostatins 1 and 2, inhibit the growth of A549 cells. At high concentrations the
bryostatins do not affect cell growth. Here the hypothesis has been tested that modulation of A549 cell growth is the consequence of agent-induced changes in location or extent of cellular PKC activity. PKC activity was measured after semi-purification with nondenaturing
polyacrylamide gel electrophoresis in the cytosol and the particulate fraction of A549 cells. When cells were exposed to TPA or
mezerein, PKC activity underwent rapid and concentration-dependent translocation from the cytosol to the membrane. TPA at 0.1 microM or
mezerein at 1 microM caused almost complete translocation within 30 min. Incubation with
bryostatins 1 or 2 also led to
enzyme translocation, which was, however, much weaker than that observed with the
tumor promoters. Neither 4 alpha-phorboldidecanoate nor the synthetic
diacylglycerols 1,2-sn-dioctanoylglycerol or
1-oleoyl-2-acetyl-sn-glycerol mimicked TPA in this way. Exposure of cells to TPA or the
bryostatins for longer than 30 min caused the gradual disappearance of total cellular PKC activity. PKC downregulation was concentration dependent and complete after 24 h. A549 cells which had acquired temporary resistance toward the growth-arresting potential of TPA were completely devoid of any measurable PKC activity. The
bryostatins were potent inhibitors of the binding of [3H]
phorbol-12,13-dibutyrate to its receptors in intact cells, and the inhibition was dependent on
bryostatin concentration. The results support the contention that PKC is involved in the mediation of growth inhibition caused by TPA or the
bryostatins. However, the relationship between growth arrest and PKC translocation or downregulation seems to be a complex one.