Role of lipid peroxidation and DNA damage in paraquat toxicity and the interaction of paraquat with ionizing radiation.
Since the introduction of paraquat (PQ) as a herbicide in 1963, there have been many speculations concerning the critical lesion in PQ toxicity. Damage to membrane lipids might be an initial event leading to PQ-induced cell killing. The ability of PQ to induce lipid peroxidation was tested in liver homogenates of the mouse. Lipid peroxidation was indeed induced by PQ and shown to be dose dependent, starting to be significant at 2.5 mM. Subsequently, a possible correlation between lipid peroxidation and PQ-induced cell death was investigated in mouse fibroblasts (LM) and Ehrlich ascites tumour (EAT) cells using a clonogenic assay. It was found that in order to be cytotoxic PQ needs enzymatic activation (incubation at 37 degrees). In both cell lines, PQ-induced cell killing appeared to be dose dependent, starting at a dose of 0.5 mM. Supplementation of LM cells with the antioxidant vitamin E had no effect on PQ-induced cell killing and modification of the membranes of LM cells by incorporation of the polyunsaturated fatty acid 20:4 (arachidonic acid) did not sensitize the cells to PQ toxicity. PQ had no effect on the glutathione (GSH) level in EAT cells and complete GSH depletion by DL-buthionine-(SR)-sulfoximine could not sensitize the cells to PQ toxicity. In LM cells PQ-induced cell killing was enhanced after complete GSH depletion by DEM. This sensitization might, however, be attributed to the binding of DEM to proteins. From these results it seems unlikely that lipid peroxidation is the primary cause for PQ-induced cell killing. Another critical target in PQ toxicity is DNA. This possibility was investigated in EAT cells. PQ was found to induce DNA damage (detected by the alkaline unwinding assay) in the same dose range that caused cell death. A good correlation was obtained for cell killing after PQ treatment and DNA damage measured 2 hr after 37 degrees post-incubation. A proposed possible interaction between PQ and X-rays was also investigated. In EAT cells, X-ray-induced cell death was significantly enhanced by pre-incubation with PQ at doses of 0.5 mM and above. At the level of 10% survival an enhancement factor of 1.6 could be observed by treatment with 1 mM PQ when cell killing by PQ is not taken into account. Induction as well as processing of radiation-induced DNA damage seems to be unaffected by pre-incubation with PQ. The mechanism of radiosensitization by PQ is yet unclear.
Peter
B
B
Department of Radiobiology, University of Groningen, The Netherlands.
Wartena
M
M
Kampinga
H H
HH
Konings
A W
AW
D000818:Animals; D019328:Buthionine Sulfoximine; D002460:Cell Line / Q000187:drug effects / Q000528:radiation effects; D002462:Cell Membrane / Q000187:drug effects; D002470:Cell Survival / Q000187:drug effects / Q000528:radiation effects; D004249:DNA Damage; D004305:Dose-Response Relationship, Drug; D005227:Fatty Acids / Q000494:pharmacology; D015227:Lipid Peroxidation; D008099:Liver / Q000187:drug effects / Q000378:metabolism; D008297:Male; D008298:Maleates / Q000494:pharmacology; D008717:Methionine Sulfoximine / Q000031:analogs & derivatives / Q000494:pharmacology; D051379:Mice; D008810:Mice, Inbred C57BL; D010269:Paraquat / Q000633:toxicity*; D011829:Radiation Dosage; D011838:Radiation-Sensitizing Agents; D013696:Temperature; D014407:Tumor Cells, Cultured / Q000187:drug effects / Q000528:radiation effects; D014810:Vitamin E / Q000494:pharmacology
D005227:Fatty Acids; D008298:Maleates; D011838:Radiation-Sensitizing Agents; D014810:Vitamin E; D008717:Methionine Sulfoximine; D019328:Buthionine Sulfoximine; C014476:diethyl maleate; D010269:Paraquat