A computational model of intracellular oxygen sensing by hypoxia-inducible factor HIF1α

Abstract

Hypoxia-inducible factor-1, HIF1, transcriptionally activates over 200 genes vital for cell homeostasis and angiogenesis. We developed a computational model to gain a detailed quantitative understanding of how HIF1 acts to sense oxygen and respond to hypoxia. The model consists of kinetic equations describing the intracellular variation of 17 compounds, including HIF1, iron, prolyl hydroxylase, oxygen, ascorbate, 2-oxoglutarate, von Hippel Lindau protein and associated complexes. We tested an existing hypothesis of a switch-like change in HIF1 expression in response to a gradual decrease in O2 concentration. Our model predicts that depending on the molecular environment, such as intracellular iron levels, the hypoxic response varies considerably. We show HIF1-activated cellular responses can be divided into two categories: a steep, switch-like response to O2 and a gradual one. Discovery of this dual response prompted comparison of two therapeutic strategies, ascorbate and iron supplementation, and prolyl hydroxylase targeting, to predict under what microenvironments either effectively increases HIF1α hydroxylation. Results provide crucial insight into the effects of iron and prolyl hydroxylase on oxygen sensing. The model advances quantitative molecular level understanding of HIF1 pathways - an endeavor that will help elucidate the diverse responses to hypoxia found in cancer, ischemia and exercise.

Age Decreases Endothelial Progenitor Cell Recruitment Through Decreases in Hypoxia-Inducible Factor 1α Stabilization During Ischemia

“…This selective effect has been confirmed by Duffy et al, 34 who demonstrated no significant effect of DFO on human endothelial cells in the absence of ischemic coronary artery disease. The reasons for this are unclear, but recent work has examined the complex interplay between intracellular iron, PHD activity, and HIF stabilization 35 and suggests that iron becomes limiting only in conditions of low oxygen availability. 27 From this, one would predict that in vivo administration of DFO would not completely inhibit PHD function in normoxia but would linearly decrease PHD activity as oxygen availability decreases in the ischemic tissue, as we observed in the present study.…”

Reactive Oxygen Species Regulate Hypoxia-Inducible Factor 1α Differentially in Cancer and Ischemia

“…A model of oxygen sensing by HIF1␣ was introduced and validated elsewhere (77). Here, we built an HIF1 computational model to incorporate potential mechanisms of ROS and antioxidants reacting within the HIF1␣ pathway.…”

“…A combination of enzymesubstrate saturation assumptions was used for the binding of iron, ascorbate, 2-oxoglutarate, and oxygen to PHD2, PHD2 hydroxylation of HIF1␣, and VHLmediated ubiquitination. In the hydroxylation reaction of PHDs with HIF1␣, we represented the binding of PHD2 with the substrates iron, 2-oxoglutarate, and oxygen sequentially; the redox reactions for ascorbate and iron are included as separate equations (77). These hydroxylation steps and their output were validated against experiments previously (77,78).…”

Multi-scale Computational Models of Pro-angiogenic Treatments in Peripheral Arterial Disease

“…A more detailed model of HIF1a behavior in response to changing oxygen 48 may be incorporated to better simulate the oxygen-VEGF link. In vivo experimentation-measurement of microenvironmental oxygen, HIF activation, and VEGF secretion-would also be extremely useful for clarification of this important pathway.…”

“…The oxygen-VEGF secretion relationship 24,32 is based on experimental data on the regulation by oxygen of the expression of VEGF via its transcription factor, hypoxia-inducible factor 1a (HIF1a). 26,29,48,59 The complexity of this system-including the interactions of VEGF ligands with their receptors and co-receptors, as well as the development of spatial gradients in an anatomically representative geometry-makes it impossible to achieve a quantitative understanding using experimental methods alone. We build here on our previously constructed models: studying VEGF interactions with VEGF receptors in healthy and ligated rat extensor digitorum longus (EDL) 24,31,32 and in human vastus lateralis muscle, 36 and interactions with Neuropilin-1 (NRP1) and placental growth factor (PlGF) in vitro.…”