epidermal-growth-factor and palytoxin

Palytoxin, a nonphorbol ester-type tumor promoter, stimulated the production of prostaglandin E2 (PGE2) and bone resorption in neonatal mouse calvariae in organ culture. The action of palytoxin on bone resorption occurred at extraordinarily low concentrations; enhanced resorption was regularly observed at 0.5 pg/ml, and the ED50 was 1-2 pg/ml (approximately 3 X 10(-13) M). Palytoxin-induced formation of PGE2 and bone resorption were inhibited completely by indomethacin (200 ng/ml). Concentrations of palytoxin above 10 pg/ml led to progressively decreasing enhancement of bone resorption; by 100-250 pg/ml no stimulation of resorption was observed despite continued high production of PGE2. Treatment with high concentrations of palytoxin (100 or 250 pg/ml) for 24-72 h inhibited cAMP accumulation stimulated by exogenous PGE2 or PTH and inhibited bone resorption induced by PGE2, PTH, or an analog of cAMP. Thus, palytoxin exhibited a biphasic dose-response curve for enhanced bone resorption, with stimulation at low concentrations (0.5-10 pg/ml) and toxic inhibition at high concentrations (greater than 50 pg/ml). Palytoxin is one of the most potent stimulators of bone resorption yet identified.

Topics: 1-Methyl-3-isobutylxanthine; 8-Bromo Cyclic Adenosine Monophosphate; Acrylamides; Animals; Bone and Bones; Bone Resorption; Cnidarian Venoms; Cyclic AMP; Dinoprostone; Epidermal Growth Factor; Growth Substances; Indomethacin; Mice; Organ Culture Techniques; Parathyroid Hormone; Peptides; Prostaglandins E; Tetradecanoylphorbol Acetate; Transforming Growth Factors

Previous results have established that 12-O-tetradecanoylphorbol-13-acetate (TPA)-type tumor promoters can alter the properties of the epidermal growth factor (EGF) receptor through activation of protein kinase C. In order to determine whether other, non-TPA-type tumor promoters might similarly influence growth-mediating receptors, we investigated the effect of palytoxin on EGF binding in Swiss 3T3 fibroblasts and human epidermal carcinoma (A431) cells. In both cell types, pretreatment with a low dose of palytoxin (1-11 pM) at 37 degrees C causes a decrease in EGF binding. In Swiss 3T3 cells the inhibitory effect is temperature dependent and does not occur at 4 degrees C, indicating that palytoxin is not directly competing with EGF for binding. As assessed by effects on DNA synthesis, palytoxin is not toxic at these concentrations and does not appear to be mitogenic for these cells. Although palytoxin, like phorbol esters, alters EGF binding, its action in Swiss 3T3 cells differs from that of TPA-type tumor promoters in at least 4 respects: (a) the kinetics and dose dependence differ significantly from that of phorbol dibutyrate; (b) the effect is not readily reversible; (c) there is loss of low-affinity as well as high-affinity binding sites; (d) the effect is independent of cellular protein kinase C levels. These results indicate that palytoxin is capable of heterologous regulation of the EGF receptor through a novel mechanism and suggest that certain non-TPA-type tumor promoters as well as TPA-type tumor promoters may act in part through modulation of growth regulatory pathways.

Topics: Acrylamides; Cell Line; Cnidarian Venoms; DNA; Dose-Response Relationship, Drug; Electrolytes; Epidermal Growth Factor; ErbB Receptors; Phorbol 12,13-Dibutyrate; Phorbol Esters; Protein Kinase C

Skin tumor-promoting agents, including the 12-O-tetradecanoylphorbol-13-acetate (TPA)-type tumor promoters, such as diterpine phorbol esters, teleocidin and aplysiatoxin, and a non-TPA-type tumor promoter (the newly described palytoxin, present in the coelenterate of the genus Palythoa), stimulated arachidonic acid metabolism by rat liver cells in culture. Palytoxin was 1000-3000 times more effective than TPA-type tumor promoters. The stimulations of arachidonic acid metabolism by palytoxin and the TPA-type tumor promoters were synergistic, whereas the stimulations among the TPA-type tumor promoters were not. The stimulation of arachidonic acid metabolism by palytoxin was synergistic with that of epidermal growth factor (EGF), transforming growth factor-alpha (TGF-alpha) and transforming growth factor-beta (TGF-beta). An antiserum to the EGF-receptor that blocks EGF binding partially inhibited the synergistic responses to palytoxin and EGF and palytoxin and TGF-alpha, whereas an antiserum to the EGF-receptor that does not block EGF binding or a non-immune rabbit serum did not.

Topics: Acrylamides; Animals; Arachidonic Acid; Arachidonic Acids; Carcinogens; Cell Line; Cnidarian Venoms; Drug Synergism; Epidermal Growth Factor; Liver; Peptides; Rats; Stimulation, Chemical; Tetradecanoylphorbol Acetate; Transforming Growth Factors