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  • Review Article
  • Published:

Cycling hypoxia and free radicals regulate angiogenesis and radiotherapy response

A Corrigendum to this article was published on 03 July 2008

Key Points

  • This Review discusses four important and interrelated features of tumour hypoxia: the hypoxic response, the factors that influence tumour hypoxia, the role of hypoxia in the initiation of angiogenesis (angiogenic switch) and how hypoxia influences treatment responses.

  • The hypoxia response, driven primarily by the heterodimeric transcription factor hypoxia-inducible factor 1 (HIF1) influences cell survival, behaviour and angiogenesis.

  • Several pathophysiological factors contribute to the development of tumour hypoxia, which is typified by heterogeneity in oxygenation in space and in time.

  • Conflicting theories exist with respect to whether hypoxic stabilization of HIF1 is the primary feature of the angiogenic switch. There is clear evidence that HIF1 upregulation is associated with angiogenesis acceleration as opposed to angiogenesis initiation.

  • The appearance of perivascular (oxygenated regions) HIF1 expression during angiogenesis acceleration might be the result of increased levels of reactive oxygen species, associated with proliferation and/or instability in flow and hypoxia–reoxygenation injury.

  • Cytotoxic therapies, such as radiation therapy, improve tumour oxygenation but also increase HIF1 levels and transactivation of target genes through mechanisms associated with stress granule depolymerization and the production of free radicals. The upregulation of HIF1 in these circumstances protects tumour and endothelial cells from damage by the cytotoxic therapy.

Abstract

Hypoxia and free radicals, such as reactive oxygen and nitrogen species, can alter the function and/or activity of the transcription factor hypoxia-inducible factor 1 (HIF1). Interplay between free radicals, hypoxia and HIF1 activity is complex and can influence the earliest stages of tumour development. The hypoxic environment of tumours is heterogeneous, both spatially and temporally, and can change in response to cytotoxic therapy. Free radicals created by hypoxia, hypoxia–reoxygenation cycling and immune cell infiltration after cytotoxic therapy strongly influence HIF1 activity. HIF1 can then promote endothelial and tumour cell survival. As discussed here, a constant theme emerges: inhibition of HIF1 activity will have therapeutic benefit.

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Figure 1: Features of hypoxia-inducible factor (HIF1) regulation.
Figure 2: Seven points of regulation of tumour oxygenation.
Figure 3: Composite model for the effect of cycling tumour hypoxia on radial diffusion of oxygen.
Figure 4: Models for role of hypoxia in initiation and acceleration of angiogenesis.
Figure 5: Mechanisms for hypoxia-inducible factor 1 (HIF1) upregulation and consequences after radiation therapy.

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Acknowledgements

The authors acknowledge the helpful discussions with I. Cardenas-Navia, T. Schroeder, E. Moon and A. Manzoor in the preparation of this manuscript. B. Sorg's contributions to the laboratory form important bases for the concepts presented. Two decades of collaboration with T. Secomb led to many of the insights provided in this Review. The authors also acknowledge the support of I. Fridovich, B. Haberle and Z. Vujaskovic for introducing them to SOD mimetics that were used to test important hypotheses regarding the role of reactive oxygen species in HIF1α regulation and angiogenesis. Supported by grants from the NIH CA40355 and the Duke SPORE for Breast Cancer.

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Supplementary information

Supplementary information S1 (movie)

Phosphorescence lifetime imaging was performed to image vascular oxygen concentration in 2x3 mm regions of skin fold window chamber tumors of Fischer rats. The windows contained either the 9L glioma (movie 1) or a fibrosarcoma (movie 2). Oxygen measurements were made every 2.5 min for 60-90 min. The movies depict changes in pO2 that are >5 mmHg from the immediately preceding image. Red indicates an increase, Blue indicates a decrease and yellow indicates no changes >5mmHg. Note that in both examples there are a significant number of pixels showing this behavior, with the fibrosarcoma showing more of a tendency to exhibit contiguous regions of similar behaviour. (AVI 8525 kb)

Supplementary information S2 (movie)

Phosphorescence lifetime imaging was performed to image vascular oxygen concentration in 2x3 mm regions of skin fold window chamber tumors of Fischer rats. The windows contained either the 9L glioma (movie 1) or a fibrosarcoma (movie 2). Oxygen measurements were made every 2.5 min for 60-90 min. The movies depict changes in pO2 that are >5 mmHg from the immediately preceding image. Red indicates an increase, Blue indicates a decrease and yellow indicates no changes >5mmHg. Note that in both examples there are a significant number of pixels showing this behavior, with the fibrosarcoma showing more of a tendency to exhibit contiguous regions of similar behaviour. (AVI 12787 kb)

Supplementary information S3 (figure)

Expression of HIF-1 reporter protein (Green fluorescence protein; GFP) during angiogenesis initiation and acceleration. (PDF 793 kb)

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DATABASES

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breast cancer

cervical cancer

colorectal cancer

glioma

head and neck cancer

National Cancer Institute Drug Dictionary

tirapazamine

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Glossary

Microangiography

This method permits visualization of microvessels in a tissue following injection of an X-ray contrast agent and exposure to X-rays; X-ray film or digital imaging are used to visualize the vessels.

Vascular casting

With this method, a cast of microvessels as they actually exist in tissues is preserved to permit visualization. Vasculature is filled with a polymer that sets inside the vessels. The tissue is then digested away to leave just the cast of the microvessels.

Shunt vessels

These are large-diameter vessels that directly connect between feeding and draining vessels at the periphery of a tumour. These can shunt flow around the main body of the tumour, thereby starving the tumour of nutrients.

Redox ratio

This ratio, which is derived from the relative abundance of two naturally fluorescent coenzymes, FAD and NADH, is related to the metabolic activity of a tissue.

Hypoxic cytotoxin

Hypoxic cytotoxins are drugs that are selectively toxic to hypoxic cells.

Laser Doppler flowmetry

This method measures velocity of red blood cells in tissue. When a laser illuminates tissue the light strikes red blood cells that are moving. The reflected light undergoes a detectable change in shift in frequency (Doppler shift) that is related to red blood cell velocity.

Blood oxygen level detection magnetic resonance imaging

(BOLD MRI.) This is an MRI method that is sensitive to the difference in magnetic properties of deoxyhaemoglobin versus oxyhaemoglobin.

Dynamic contrast enhanced MRI

(DCE MRI.) This is a method to measure perfusion/permeability of an MR contrast agent as it enters and leaves a tissue following bolus intravenous injection. Kinetic analysis permits derivation of parameters related to vascular permeability and perfusion.

Vasomotion

Arterioles in the peripheral circulation exhibit fluctuations in diameter that control perfusion of dependent tissues.

Bifurcation point

This term refers to a branch point in the microcirculation, in which flow splits from one vessel to two or more daughter vessels.

Carotid body

The carotid body is a collection of pH-, partial pressure of O2 (pO2)- and pCO2-sensitive chemoreceptor cells located on the carotid artery wall. When stimulated they send signals to the central nervous system to regulate respiratory and heart rates.

Dormancy

This is a state of tumour growth at a time when cell loss is equal to cell proliferation and before the onset of angiogenesis.

Stress granules

Cell stress initiators, such as nutritional deprivation or hypoxia, cause a general downregulation of protein translation, involving prevention of mRNA entry into ribosomes, forming stress granules (in complex with RNA-binding proteins) in the cytoplasm. Stress granules rapidly disaggregate upon removal of stress, permitting subsequent protein translation.

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Dewhirst, M., Cao, Y. & Moeller, B. Cycling hypoxia and free radicals regulate angiogenesis and radiotherapy response. Nat Rev Cancer 8, 425–437 (2008). https://doi.org/10.1038/nrc2397

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