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Examination of effects of GSK3beta phosphorylation, beta-catenin phosphorylation, and beta-catenin degradation on kinetics of Wnt signaling pathway using computational method.

Sun YC - Theor Biol Med Model (2009)

Bottom Line: It has also been found that the kinase PKA attenuates beta-catenin degradation.However, the effects of these kinases on the level and degradation of beta-catenin and the resulting downstream transcription activity remain to be clarified.The rate laws of reactions in the modified model were solved numerically to examine these effects on beta-catenin level.

View Article: PubMed Central - HTML - PubMed

Affiliation: Department of Chemistry, National Taiwan Normal University, Taipei, Taiwan. sun@ntnu.edu.tw

ABSTRACT

Background: Recent experiments have explored effects of activities of kinases other than the well-studied GSK3beta, in wnt pathway signaling, particularly at the level of beta-catenin. It has also been found that the kinase PKA attenuates beta-catenin degradation. However, the effects of these kinases on the level and degradation of beta-catenin and the resulting downstream transcription activity remain to be clarified. Furthermore, the effect of GSK3beta phosphorylation on the beta-catenin level has not been examined computationally. In the present study, the effects of phosphorylation of GSK3beta and of phosphorylations and degradation of beta-catenin on the kinetics of the wnt signaling pathway were examined computationally.

Methods: The well-known computational Lee-Heinrich kinetic model of the wnt pathway was modified to include these effects. The rate laws of reactions in the modified model were solved numerically to examine these effects on beta-catenin level.

Results: The computations showed that the beta-catenin level is almost linearly proportional to the phosphorylation activity of GSK3beta. The dependence of beta-catenin level on the phosphorylation and degradation of free beta-catenin and downstream TCF activity can be analyzed with an approximate, simple function of kinetic parameters for added reaction steps associated with effects examined, rationalizing the experimental results.

Conclusion: The phosphorylations of beta-catenin by kinases other than GSK3beta involve free unphorphorylated beta-catenin rather than GSK3beta-phosphorylated beta-catenin*. In order to account for the observed enhancement of TCF activity, the beta-catenin dephosphorylation step is essential, and the kinetic parameters of beta-catenin phosphorylation and degradation need to meet a condition described in the main text. These findings should be useful for future experiments.

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Dependence of level of β-catenin on GSK3β phosphorylation. Circles are computed results. The line shows that the dependence is almost linear. The other kinetic parameters of added reaction steps, kG' and kβ/kβ', were set equal to 1 and 0.1/1 min-1, respectively.
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Figure 2: Dependence of level of β-catenin on GSK3β phosphorylation. Circles are computed results. The line shows that the dependence is almost linear. The other kinetic parameters of added reaction steps, kG' and kβ/kβ', were set equal to 1 and 0.1/1 min-1, respectively.

Mentions: To examine how the kinetic parameters of the added reaction steps affect the kinetics of the wnt pathway, selected parameters were varied and the rate laws of the modified model were solved. The SS solutions were examined first. The effect of varying kG on the total β-catenin level and free unphosphorylated β-catenin are listed in Table 2. The higher the kG value, the higher the β-catenin level. This dependence qualitatively demonstrates the negative role of GSK3β in wnt/β-catenin signaling. This is because a decrease in unphosphorylated GSK3β level results in a decrease in the APC/axin/GSK3β complex. This complex is a central component of the β-catenin destruction cycle. Therefore, the level of β-catenin accumulates and increases. Experimentally, it was found that phosphorylation of GSK3β led to enhancement of the β-catenin level in HEK293-TPα cells [18] but not HEK293 cells [19]. The discrepancy is due to differences in cell type, experimental conditions, etc. The present result is consistent with the former cell type. The GSK3β/GSK3β+ ratio is determined by the kG/kG' ratio. The dependence of the kG/kG' ratio on β-catenin level is shown in Figure 2, a plot of ratio of β-catenin level versus kG that represents the strength of phosphorylation of GSK3β by other kinases. This plot shows an almost linear dependence. Because of the significant effect when kG/kG' = 1 (Table 2), this parameter set was used as a reference parameter set to analyze the effects of β-catenin non-GSK3β phosphorylation on the wnt pathway in the discussion below.


Examination of effects of GSK3beta phosphorylation, beta-catenin phosphorylation, and beta-catenin degradation on kinetics of Wnt signaling pathway using computational method.

Sun YC - Theor Biol Med Model (2009)

Dependence of level of β-catenin on GSK3β phosphorylation. Circles are computed results. The line shows that the dependence is almost linear. The other kinetic parameters of added reaction steps, kG' and kβ/kβ', were set equal to 1 and 0.1/1 min-1, respectively.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC2724396&req=5

Figure 2: Dependence of level of β-catenin on GSK3β phosphorylation. Circles are computed results. The line shows that the dependence is almost linear. The other kinetic parameters of added reaction steps, kG' and kβ/kβ', were set equal to 1 and 0.1/1 min-1, respectively.
Mentions: To examine how the kinetic parameters of the added reaction steps affect the kinetics of the wnt pathway, selected parameters were varied and the rate laws of the modified model were solved. The SS solutions were examined first. The effect of varying kG on the total β-catenin level and free unphosphorylated β-catenin are listed in Table 2. The higher the kG value, the higher the β-catenin level. This dependence qualitatively demonstrates the negative role of GSK3β in wnt/β-catenin signaling. This is because a decrease in unphosphorylated GSK3β level results in a decrease in the APC/axin/GSK3β complex. This complex is a central component of the β-catenin destruction cycle. Therefore, the level of β-catenin accumulates and increases. Experimentally, it was found that phosphorylation of GSK3β led to enhancement of the β-catenin level in HEK293-TPα cells [18] but not HEK293 cells [19]. The discrepancy is due to differences in cell type, experimental conditions, etc. The present result is consistent with the former cell type. The GSK3β/GSK3β+ ratio is determined by the kG/kG' ratio. The dependence of the kG/kG' ratio on β-catenin level is shown in Figure 2, a plot of ratio of β-catenin level versus kG that represents the strength of phosphorylation of GSK3β by other kinases. This plot shows an almost linear dependence. Because of the significant effect when kG/kG' = 1 (Table 2), this parameter set was used as a reference parameter set to analyze the effects of β-catenin non-GSK3β phosphorylation on the wnt pathway in the discussion below.

Bottom Line: It has also been found that the kinase PKA attenuates beta-catenin degradation.However, the effects of these kinases on the level and degradation of beta-catenin and the resulting downstream transcription activity remain to be clarified.The rate laws of reactions in the modified model were solved numerically to examine these effects on beta-catenin level.

View Article: PubMed Central - HTML - PubMed

Affiliation: Department of Chemistry, National Taiwan Normal University, Taipei, Taiwan. sun@ntnu.edu.tw

ABSTRACT

Background: Recent experiments have explored effects of activities of kinases other than the well-studied GSK3beta, in wnt pathway signaling, particularly at the level of beta-catenin. It has also been found that the kinase PKA attenuates beta-catenin degradation. However, the effects of these kinases on the level and degradation of beta-catenin and the resulting downstream transcription activity remain to be clarified. Furthermore, the effect of GSK3beta phosphorylation on the beta-catenin level has not been examined computationally. In the present study, the effects of phosphorylation of GSK3beta and of phosphorylations and degradation of beta-catenin on the kinetics of the wnt signaling pathway were examined computationally.

Methods: The well-known computational Lee-Heinrich kinetic model of the wnt pathway was modified to include these effects. The rate laws of reactions in the modified model were solved numerically to examine these effects on beta-catenin level.

Results: The computations showed that the beta-catenin level is almost linearly proportional to the phosphorylation activity of GSK3beta. The dependence of beta-catenin level on the phosphorylation and degradation of free beta-catenin and downstream TCF activity can be analyzed with an approximate, simple function of kinetic parameters for added reaction steps associated with effects examined, rationalizing the experimental results.

Conclusion: The phosphorylations of beta-catenin by kinases other than GSK3beta involve free unphorphorylated beta-catenin rather than GSK3beta-phosphorylated beta-catenin*. In order to account for the observed enhancement of TCF activity, the beta-catenin dephosphorylation step is essential, and the kinetic parameters of beta-catenin phosphorylation and degradation need to meet a condition described in the main text. These findings should be useful for future experiments.

Show MeSH
Related in: MedlinePlus