Interactions between diphtheria toxin entry and anion transport in vero cells. III. Effect on toxin binding and anion transport of tumor-promoting phorbol esters, vanadate, fluoride, and salicylate.

When Vero cells were incubated with TPA (12-O-tetradecanoylphorbol 13-acetate) and related tumor promoters, their ability to bind diphtheria toxin in a functional way was rapidly reduced to less than 1% of the normal value. Upon further incubation with TPA, the cells recovered their ability to bind the toxin, apparently because they became resistant to TPA. Treatment with Na3VO4 reduced the ability of the cells to bind diphtheria toxin to approximately the same extent as treatment with TPA, but the reduction required longer time to develop and it persisted upon prolonged incubation with Na3VO4. ATP depletion of the cells prevented the reduction in binding capability. Such treatment also prevented the reduction in toxin binding induced by treatment with salicylate or fluoride. Treatment with TPA, fluoride, vanadate, and salicylate altered the ability of the cells to carry out anion transport and interfered with their ability to regulate the transport. The results indicate that the binding sites for diphtheria toxin on Vero cells are modulated by TPA, Na3VO4, salicylate, and fluoride by a process which requires ATP. The possibility is discussed that the modulation consists in phosphorylation of the toxin binding sites, which may be identical with, or closely linked to, the anion antiporter in the cells.