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Glutathione-dependent enzymes alone can produce paraquat resistance
1University of California, San Diego Medical Center 92103.
Abstract:
HL60 cells exposed to increasing paraquat concentrations were screened for clones without increased superoxide dismutase activities in an effort to examine cytotoxic events occurring after superoxide production. The resulting resistance to paraquat was not associated with alterations in paraquat uptake, catalase, or NADPH-P450 reductase activity, but to alterations in glutathione-dependent enzyme activities. While increases in glutathione-dependent enzymes upon exposure to paraquat have been reported, the increases were considered a secondary response to increases in superoxide dismutase activities. Our results demonstrate that glutathione-dependent enzymes alone provide protection against paraquat toxicity, and their increase upon exposure to paraquat can be independent of the response of superoxide dismutases. This supports a previous finding that cells resistant to Adriamycin, based on elevated glutathione peroxidase and transferase activities are also cross-resistant to paraquat. Unlike this previous report, the increase in glutathione peroxidase was not a persistent genetic event, as activities returned to normal upon removal of paraquat. An isolated increase in glutathione peroxidase without accompanying increases in superoxide dismutases was a rare event, as nearly all clones examined after exposure to paraquat had increased superoxide dismutase.
Insights
Glutathione-dependent enzymes protect cells from paraquat toxicity independently of superoxide dismutase. This finding reveals a direct protective mechanism against oxidative stress, crucial for understanding cellular defense strategies.
Area of Science:
- Biochemistry
- Cell Biology
- Toxicology
Background:
- Paraquat induces oxidative stress via superoxide production.
- Elevated superoxide dismutase (SOD) activity is often linked to paraquat resistance.
- The role of glutathione-dependent enzymes in paraquat resistance is not fully elucidated.
Purpose of the Study:
- To investigate paraquat cytotoxicity mechanisms.
- To determine if glutathione-dependent enzymes can confer paraquat resistance independently of SOD.
- To examine the regulation of glutathione-dependent enzymes in response to paraquat.
Main Methods:
- Screening of HL60 cell clones for resistance to paraquat without increased SOD activity.
- Assessing paraquat uptake, catalase, NADPH-P450 reductase, and glutathione-dependent enzyme activities.
- Evaluating the persistence of enzyme activity changes upon paraquat removal.
Main Results:
- Paraquat resistance was associated with altered glutathione-dependent enzyme activities, not paraquat uptake or other measured enzymes.
- Increased glutathione-dependent enzyme activity provided protection against paraquat toxicity, independent of SOD activity.
- Glutathione peroxidase activity increases were not persistent and returned to normal after paraquat removal.
Conclusions:
- Glutathione-dependent enzymes play a direct role in protecting cells against paraquat-induced oxidative stress.
- The upregulation of these enzymes can occur independently of superoxide dismutase activity.
- This suggests potential therapeutic targets for mitigating paraquat toxicity.