Kinetic analysis of molecular mechanisms of HIF-1 signalling during acute hypoxia and reoxygenation

Motivation Although the basic layout of the HIF-1 pathway is known, many molecular details and especially the kinetics remain to be determined. Here, the aim was to explore differences in the pathway response as function of individual protein kinetics, hereby analyzing different possible scenarios and identifying persistent behavior present irrespective of kinetic variations. Methods A mathematical model of the HIF-1 pathway, including HIF-1 hydroxylation and degradation, and translocation of activated HIF-1 complex to the nucleus, has been developed. Different datasets from HeLaS3 cells were used as input (kinetic data of acute hypoxia, O2 from 20% to 0.5%) and for model testing (reoxygenation). Results Network analyses showed consistent activation kinetics in response to hypoxia. Analysis of the predicted reoxygenation responses revealed two classes: a rapid inactivation (time-to-half-decay <2 min.) and a slower response (time-to-half-decay > 10 min.), the latter in agreement with experimental data. The importance of this difference became apparent when the responses to varying shifts in oxygen were investigated (dose-response). The fast reoxygenation appeared to correlate with a switch-like dose-response, whereas the response corresponding to slower reoxygenation was exponential. Conclusion When oxygen decreases HIF-1 activity increases sharply. The dose-response has been hypothesized to be switch-like, but experimental data is inconclusive and an exponential response cannot be excluded. We have been able to quantitatively integrate data of different experiments into a consistent model. The model predicts that the difference in dose-response does not affect the dynamic response to acute hypoxia. However, the difference is closely related to the rate by which the pathway is inactivated when oxygen becomes available again. This suggests that experimental efforts focused on reoxygenation are important to gain further understanding of the HIF-1 pathway.