The phorbol ester tumor promoter, 12-O-tetradecanoylphorbol-13-acetate ((TPA) or phorbol 12-myristate 13-acetate), directly activates the calcium- and phospholipid-dependent protein kinase C (protein kinase C), which, in turn, generates a number of cellular responses. The bryostatins, a family of macrocyclic lactones isolated from marine bryozoans, also bind to and activate protein kinase C. However, they differ from TPA in the selectivity of their responses in that they behave either as agonists or antagonists of protein kinase C actions. We used several bryostatins and TPA to examine the role of protein kinase C in the regulation of GH4C1 rat pituitary tumor cell proliferation. TPA inhibited [3H]thymidine incorporation in GH4 cells in a stereoselective concentration-dependent manner. Examination of cell cycle distribution by flow cytometry revealed that TPA decreased the percentage of cells in S-phase and proportionally increased the percentage of G1-phase cells. Bryostatin 1 alone did not affect cell proliferation, but prevented the TPA inhibition of cell proliferation. Bryostatin 1 treatment from 30 min to 6 h after TPA treatment also prevented the growth-inhibitory action of TPA, suggesting that prolonged stimulation of protein kinase C is necessary for growth inhibition. Both bryostatin 1 and TPA down-regulated protein kinase C, indicating that down regulation of the enzyme cannot account for the growth inhibitory action of TPA. Bryostatin 2, which differs from bryostatin 1 by a hydroxyl substitution for the acetyl group at the C-7 carbon of the macrocyclic lactone ring (R1), inhibited cell proliferation and did not reduce the growth-inhibitory action of TPA. Bryostatins 3 and 8 (each of which has an ester group in the R1 position, yet contains other structural modifications) are antagonists for TPA inhibition of GH4 cell proliferation like bryostatin 1. We next examined the effect of bryostatins 3 and 8 on cell-substratum adhesion, a cellular response observed after GH4 cells are treated with growth-inhibitory agents. Bryostatin 8 (like bryostatin 1) did not enhance cell-substratum adhesion and blocked the action of TPA. In contrast, bryostatin 3 enhanced cell-substratum adhesion. Because bryostatin 3 blocked TPA inhibition of cell proliferation, yet did not block TPA-enhanced cell-substratum adhesion, these responses are not interdependent. We next examined the effect of bryostatin on other growth-inhibitory agents for GH4 cells. Bryostatin 8 blocks the effect of TPA on [3H]thymidine incorporation and the entry of G1 cells into S-phase, but does not block the growth-inhibitory action of thyrotropin-releasing hormone or epidermal growth factor. This suggests that thyrotropin-releasing hormone and epidermal growth factor inhibit GH4 cell proliferation by a mechanism different from that of TPA. Taken together, these results suggest that sustained activation of protein kinase C inhibits GH4 cell proliferation by blocking G1 cells from entering S-phase and also demonstrate that bryostatins show a high degree of selectivity which appears to be due to the acetyl ester linkage in the R1 position.
|Original language||English (US)|
|Number of pages||8|
|Journal||Journal of Biological Chemistry|
|State||Published - 1991|
ASJC Scopus subject areas
- Molecular Biology
- Cell Biology