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Monensin blocks the transport of diphtheria toxin to the cell cytoplasm
Abstract:
Lysosomotropic amines are believed to inhibit the transport of diphtheria toxin to the cell cytoplasm by raising the pH within intracellular vesicles. If so, then other drugs that dissipate intracellular proton gradients should have a similar effect on toxin transport. We found that monensin, a proton ionophore unrelated to lysosomotropic amines, is a potent inhibitor of the cytotoxic effect of diphtheria toxin. Monensin appears to block the escape of endocytosed toxin from a vesicle to the cytoplasm. Monensin fails to protect cells from the effects of diphtheria toxin that is bound to the cell surface and exposed to acidic medium, suggesting that the step normally blocked by the drug is circumvented under these conditions. The inhibition of toxin transport caused by monensin could not be relieved when monensin was replaced by ammonium chloride, nor when ammonium chloride was again replaced by monensin. This suggests that both drugs block the same step of toxin transport. The effect of monensin on the transport of diphtheria toxin to the cytoplasm is consistent with the proposal (Draper and Simon. 1980. J. Cell Biol. 87:849-854; Sandvig and Olsnes. 1980. J. Cell Biol. 87:828-832) that the toxin is endocytosed and then, in response to an acidic environment, penetrates through the membrane of an intracellular vesicle to reach the cytoplasm.
Insights
Lysosomotropic amines and monensin inhibit diphtheria toxin entry into cells by affecting intracellular vesicle pH. Both drugs block the toxin
Area of Science:
- Cell Biology
- Toxicology
- Molecular Biology
Background:
- Lysosomotropic amines are known to inhibit diphtheria toxin transport.
- This inhibition is thought to occur by increasing the pH within intracellular vesicles.
- The study investigates if other drugs affecting proton gradients share this inhibitory effect.
Purpose of the Study:
- To determine if monensin, a proton ionophore, inhibits diphtheria toxin's cytotoxic effects.
- To elucidate the mechanism by which monensin affects toxin transport.
- To compare the inhibitory action of monensin with lysosomotropic amines.
Main Methods:
- Treating cells with diphtheria toxin and monensin.
- Assessing the cytotoxic effects of diphtheria toxin.
- Investigating toxin transport by exposing cells to different conditions (e.g., surface-bound toxin, acidic medium).
- Comparing the effects of monensin and ammonium chloride.
Main Results:
- Monensin potently inhibits the cytotoxic effects of diphtheria toxin.
- Monensin blocks the escape of endocytosed diphtheria toxin from vesicles to the cytoplasm.
- Monensin does not protect cells when toxin is cell-surface bound and exposed to acid, indicating a specific step is blocked.
- Monensin and ammonium chloride inhibit the same step in toxin transport.
Conclusions:
- Monensin, a proton ionophore, effectively inhibits diphtheria toxin cytotoxicity.
- The findings support the model where diphtheria toxin is endocytosed and requires an acidic environment to penetrate the vesicle membrane.
- Monensin's action is consistent with blocking the toxin's escape from intracellular vesicles.