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Kinetic Modeling Reveals the Roles of Reactive Oxygen Species Scavenging and DNA Repair Processes in Shaping the Dose-Response Curve of KBrO₃-Induced DNA Damage.

Spassova MA, Miller DJ, Nikolov AS - Oxid Med Cell Longev (2015)

Bottom Line: We used as an example chemical KBrO3 which is activated by glutathione and forms reactive intermediates that directly interact with DNA to form 8-hydroxy-2-deoxyguanosine DNA adducts (8-OH-dG).Our modeling revealed that sustained exposure to KBrO3 can lead to fast scavenger exhaustion, in which case the dose-response shapes for both endpoints are not substantially affected.The results are important to consider when forming conclusions on a chemical's toxicity dose dependence based on the dose-response of early genotoxic events.

View Article: PubMed Central - PubMed

Affiliation: National Center for Environmental Assessment, Office of Research and Development, U.S. Environmental Protection Agency, Washington, DC 20460, USA.

ABSTRACT
We have developed a kinetic model to investigate how DNA repair processes and scavengers of reactive oxygen species (ROS) can affect the dose-response shape of prooxidant induced DNA damage. We used as an example chemical KBrO3 which is activated by glutathione and forms reactive intermediates that directly interact with DNA to form 8-hydroxy-2-deoxyguanosine DNA adducts (8-OH-dG). The single strand breaks (SSB) that can result from failed base excision repair of these adducts were considered as an effect downstream from 8-OH-dG. We previously demonstrated that, in the presence of effective base excision repair, 8-OH-dG can exhibit threshold-like dose-response dependence, while the downstream SSB can still exhibit a linear dose-response. Here we demonstrate that this result holds for a variety of conditions, including low levels of GSH, the presence of additional SSB repair mechanisms, or a scavenger. It has been shown that melatonin, a terminal scavenger, inhibits KBrO3-caused oxidative damage. Our modeling revealed that sustained exposure to KBrO3 can lead to fast scavenger exhaustion, in which case the dose-response shapes for both endpoints are not substantially affected. The results are important to consider when forming conclusions on a chemical's toxicity dose dependence based on the dose-response of early genotoxic events.

No MeSH data available.


Related in: MedlinePlus

Base model simulation results. (a) Model used for simulation of 8-OH-dG formation in absence of any DNA repair mechanisms. (b) Base Model used for simulations of DNA adducts and SSB formation in the presence of a BER repair mechanism. (c) Time course of KBrO3 exposure, 8-OH-dG, and SSB levels determined by simulations using the model in (b). A brief period of KBrO3 exposure (blue) in the presence of GSH, followed by quick KBrO3 removal. A base excision repair (BER) mechanism is active (b). The time course of 8-OH-dG adducts (green) shows a fast initial increase of adducts and consequent decrease as the BER repairs the adducts. However, infrequent repair failure results in persistent single strand breaks (SSB) that gradually accumulate over the course of the simulation (red). (d) A dose-response plot sectioned from a time point marked in (c) (dashed line) shows that the SSB response can be linear despite the nonlinear, threshold-like appearance of the 8-OH-dG adducts.
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fig2: Base model simulation results. (a) Model used for simulation of 8-OH-dG formation in absence of any DNA repair mechanisms. (b) Base Model used for simulations of DNA adducts and SSB formation in the presence of a BER repair mechanism. (c) Time course of KBrO3 exposure, 8-OH-dG, and SSB levels determined by simulations using the model in (b). A brief period of KBrO3 exposure (blue) in the presence of GSH, followed by quick KBrO3 removal. A base excision repair (BER) mechanism is active (b). The time course of 8-OH-dG adducts (green) shows a fast initial increase of adducts and consequent decrease as the BER repairs the adducts. However, infrequent repair failure results in persistent single strand breaks (SSB) that gradually accumulate over the course of the simulation (red). (d) A dose-response plot sectioned from a time point marked in (c) (dashed line) shows that the SSB response can be linear despite the nonlinear, threshold-like appearance of the 8-OH-dG adducts.

Mentions: We initially modeled the dose-response of 8-OH-dG in the absence of BER and SSB repair mechanisms (Figure 2(a)). The simulation revealed a linear increase of 8-OH-dG with exposure to increasing KBrO3 levels, as expected (data not shown). In the next step, a BER mechanism was added to the model and SSB were generated when repair was not completed (Figure 2(b)). For this model, a brief exposure to different levels of KBrO3 was applied. The time course of KBrO3 exposure, 8-OH-dG formation, and SSB formation is plotted on Figure 2(c). Five levels of KBrO3 concentration within the range we used in the simulation are plotted (blue) with the corresponding time course of 8-OH-dG levels (green) and SSB levels (red). A similar format is used in the consecutive figures showing time course of KBrO3 exposure, 8-OH-dG levels, and SSB levels (Figures 3(a), 4(b), and 4(d)). We used only relative units of time as we did not attempt to predict the actual time course but we were rather interested in the resulting shape of the dose-responses of 8-OH-dG and SSB for different scenarios. Initially, the 8-OH-dG lesions increased steeply as the repair lags behind. However, shortly after the end of the exposure, the 8-OH-dG lesion levels steeply decreased due to successful and failed repair. The SSB continuously accumulated with time as the rates of failed repair were kept constant (Figure 2(c)). It is important to note that if experimental measurements are made at a time shortly after the exposure (red arrow), the SSB would not be detected for the entire KBrO3 concentration range. On the other hand, measurements made a long time after the exposure would not be able to detect 8-OH-dG lesions (green arrow). These results can explain to some extent discrepancies between different experimental studies. The dose-response dependence of 8-OH-dG and SSB was plotted at Figure 2(d) at an intermediate time after exposure (t = 500; the dotted line in Figure 2(c)). The simulation of the 8-OH-dG dose-response revealed highly sublinear, threshold-like behavior. At low KBrO3 doses repair of 8-OH-dG lesions completes, creating a dose threshold for KBrO3 effects. It is also important to mention that in this scenario 8-OH-dG would not be a suitable measure of exposure and should not be used as an exposure biomarker, as it may not be detected if measured a long time after exposure, even after exposure to high concentrations of KBrO3. However, SSB generated downstream from the 8-OH-dG, accumulated over time, and the dose-response dependence for SSB is strictly linear (Figure 2(d), red) suggesting that consecutive downstream points may have preserved linear dependence.


Kinetic Modeling Reveals the Roles of Reactive Oxygen Species Scavenging and DNA Repair Processes in Shaping the Dose-Response Curve of KBrO₃-Induced DNA Damage.

Spassova MA, Miller DJ, Nikolov AS - Oxid Med Cell Longev (2015)

Base model simulation results. (a) Model used for simulation of 8-OH-dG formation in absence of any DNA repair mechanisms. (b) Base Model used for simulations of DNA adducts and SSB formation in the presence of a BER repair mechanism. (c) Time course of KBrO3 exposure, 8-OH-dG, and SSB levels determined by simulations using the model in (b). A brief period of KBrO3 exposure (blue) in the presence of GSH, followed by quick KBrO3 removal. A base excision repair (BER) mechanism is active (b). The time course of 8-OH-dG adducts (green) shows a fast initial increase of adducts and consequent decrease as the BER repairs the adducts. However, infrequent repair failure results in persistent single strand breaks (SSB) that gradually accumulate over the course of the simulation (red). (d) A dose-response plot sectioned from a time point marked in (c) (dashed line) shows that the SSB response can be linear despite the nonlinear, threshold-like appearance of the 8-OH-dG adducts.
© Copyright Policy - open-access
Related In: Results  -  Collection

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fig2: Base model simulation results. (a) Model used for simulation of 8-OH-dG formation in absence of any DNA repair mechanisms. (b) Base Model used for simulations of DNA adducts and SSB formation in the presence of a BER repair mechanism. (c) Time course of KBrO3 exposure, 8-OH-dG, and SSB levels determined by simulations using the model in (b). A brief period of KBrO3 exposure (blue) in the presence of GSH, followed by quick KBrO3 removal. A base excision repair (BER) mechanism is active (b). The time course of 8-OH-dG adducts (green) shows a fast initial increase of adducts and consequent decrease as the BER repairs the adducts. However, infrequent repair failure results in persistent single strand breaks (SSB) that gradually accumulate over the course of the simulation (red). (d) A dose-response plot sectioned from a time point marked in (c) (dashed line) shows that the SSB response can be linear despite the nonlinear, threshold-like appearance of the 8-OH-dG adducts.
Mentions: We initially modeled the dose-response of 8-OH-dG in the absence of BER and SSB repair mechanisms (Figure 2(a)). The simulation revealed a linear increase of 8-OH-dG with exposure to increasing KBrO3 levels, as expected (data not shown). In the next step, a BER mechanism was added to the model and SSB were generated when repair was not completed (Figure 2(b)). For this model, a brief exposure to different levels of KBrO3 was applied. The time course of KBrO3 exposure, 8-OH-dG formation, and SSB formation is plotted on Figure 2(c). Five levels of KBrO3 concentration within the range we used in the simulation are plotted (blue) with the corresponding time course of 8-OH-dG levels (green) and SSB levels (red). A similar format is used in the consecutive figures showing time course of KBrO3 exposure, 8-OH-dG levels, and SSB levels (Figures 3(a), 4(b), and 4(d)). We used only relative units of time as we did not attempt to predict the actual time course but we were rather interested in the resulting shape of the dose-responses of 8-OH-dG and SSB for different scenarios. Initially, the 8-OH-dG lesions increased steeply as the repair lags behind. However, shortly after the end of the exposure, the 8-OH-dG lesion levels steeply decreased due to successful and failed repair. The SSB continuously accumulated with time as the rates of failed repair were kept constant (Figure 2(c)). It is important to note that if experimental measurements are made at a time shortly after the exposure (red arrow), the SSB would not be detected for the entire KBrO3 concentration range. On the other hand, measurements made a long time after the exposure would not be able to detect 8-OH-dG lesions (green arrow). These results can explain to some extent discrepancies between different experimental studies. The dose-response dependence of 8-OH-dG and SSB was plotted at Figure 2(d) at an intermediate time after exposure (t = 500; the dotted line in Figure 2(c)). The simulation of the 8-OH-dG dose-response revealed highly sublinear, threshold-like behavior. At low KBrO3 doses repair of 8-OH-dG lesions completes, creating a dose threshold for KBrO3 effects. It is also important to mention that in this scenario 8-OH-dG would not be a suitable measure of exposure and should not be used as an exposure biomarker, as it may not be detected if measured a long time after exposure, even after exposure to high concentrations of KBrO3. However, SSB generated downstream from the 8-OH-dG, accumulated over time, and the dose-response dependence for SSB is strictly linear (Figure 2(d), red) suggesting that consecutive downstream points may have preserved linear dependence.

Bottom Line: We used as an example chemical KBrO3 which is activated by glutathione and forms reactive intermediates that directly interact with DNA to form 8-hydroxy-2-deoxyguanosine DNA adducts (8-OH-dG).Our modeling revealed that sustained exposure to KBrO3 can lead to fast scavenger exhaustion, in which case the dose-response shapes for both endpoints are not substantially affected.The results are important to consider when forming conclusions on a chemical's toxicity dose dependence based on the dose-response of early genotoxic events.

View Article: PubMed Central - PubMed

Affiliation: National Center for Environmental Assessment, Office of Research and Development, U.S. Environmental Protection Agency, Washington, DC 20460, USA.

ABSTRACT
We have developed a kinetic model to investigate how DNA repair processes and scavengers of reactive oxygen species (ROS) can affect the dose-response shape of prooxidant induced DNA damage. We used as an example chemical KBrO3 which is activated by glutathione and forms reactive intermediates that directly interact with DNA to form 8-hydroxy-2-deoxyguanosine DNA adducts (8-OH-dG). The single strand breaks (SSB) that can result from failed base excision repair of these adducts were considered as an effect downstream from 8-OH-dG. We previously demonstrated that, in the presence of effective base excision repair, 8-OH-dG can exhibit threshold-like dose-response dependence, while the downstream SSB can still exhibit a linear dose-response. Here we demonstrate that this result holds for a variety of conditions, including low levels of GSH, the presence of additional SSB repair mechanisms, or a scavenger. It has been shown that melatonin, a terminal scavenger, inhibits KBrO3-caused oxidative damage. Our modeling revealed that sustained exposure to KBrO3 can lead to fast scavenger exhaustion, in which case the dose-response shapes for both endpoints are not substantially affected. The results are important to consider when forming conclusions on a chemical's toxicity dose dependence based on the dose-response of early genotoxic events.

No MeSH data available.


Related in: MedlinePlus