A large body of clinical evidence exists to suggest that tumor hypoxia negatively impacts radiotherapy. As a result, there has been longstanding active research into novel methods of improving tumor oxygenation, targeting hypoxic tumor cells, and otherwise modulating the effect hypoxia has on how tumors respond to radiation. Over time, as more has been learned about the many ways hypoxia affects tumors, our understanding of the mechanisms connecting hypoxia to radiosensitivity has become increasingly broad and complicated. This has opened up new potential avenues for interrupting hypoxia's negative effects on tumor radiosensitivity. Here, we will review what is currently known about the spectrum of influence hypoxia has over the way tumors respond to radiation. Particular focus will be placed on recent discoveries suggesting that hypoxia-inducible factor-1 (HIF-1), a transcription factor that upregulates its target genes under hypoxic conditions, plays a major role in determining tumor radiosensitivity. HIF-1 and/or its target genes may represent therapeutic targets which could be manipulated to influence hypoxia's impact on tumor radiosensitivity.

Lactate accumulation in tumors has been associated with metastases and poor overall survival in cancer patients. Lactate promotes angiogenesis and metastasis, providing rationale for understanding how it is processed by cells. The concentration of lactate in tumors is a balance between the amount produced, amount carried away by vasculature and if/how it is catabolized by aerobic tumor or stromal cells. We examined lactate metabolism in human normal and breast tumor cell lines and rat breast cancer: 1. at relevant concentrations, 2. under aerobic vs. hypoxic conditions, 3. under conditions of normo vs. hypoglucosis. We also compared the avidity of tumors for lactate vs. glucose and identified key lactate catabolites to reveal how breast cancer cells process it. Lactate was non-toxic at clinically relevant concentrations. It was taken up and catabolized to alanine and glutamate by all cell lines. Kinetic uptake rates of lactate in vivo surpassed that of glucose in R3230Ac mammary carcinomas. The uptake appeared specific to aerobic tumor regions, consistent with the proposed “metabolic symbiont” model; here lactate produced by hypoxic cells is used by aerobic cells. We investigated whether treatment with alpha-cyano-4-hydroxycinnamate (CHC), a MCT1 inhibitor, would kill cells in the presence of high lactate. Both 0.1 mM and 5 mM CHC prevented lactate uptake in R3230Ac cells at lactate concentrations at ≤20 mM but not at 40 mM. 0.1 mM CHC was well-tolerated by R3230Ac and MCF7 cells, but 5 mM CHC killed both cell lines ± lactate, indicating off-target effects. This study showed that breast cancer cells tolerate and use lactate at clinically relevant concentrations in vitro (± glucose) and in vivo. We provided additional support for the metabolic symbiont model and discovered that breast cells prevailingly take up and catabolize lactate, providing rationale for future studies on manipulation of lactate catabolism pathways for therapy.

Tumor hypoxia is a persistent obstacle for traditional therapies in solid tumors. Strategies for mitigating the effects of hypoxic tumor cells have been developed under the assumption that chronically hypoxic tumor cells were the central cause of treatment resistance. In this study, we show that instabilities in tumor oxygenation are a prevalent characteristic of three tumor lines and previous characterization of tumor hypoxia as being primarily diffusion-limited does not accurately portray the tumor microenvironment. Phosphorescence lifetime imaging was used to measure fluctuations in vascular pO 2 in rat fibrosarcomas, 9L gliomas, and R3230 mammary adenocarcinomas grown in dorsal skin-fold window chambers (n = 6 for each tumor type) and imaged every 2.5 minutes for a duration of 60 to 90 minutes. O 2 delivery to tumors is constantly changing in all tumors, resulting in continuous reoxygenation events throughout the tumor. Vascular pO 2 maps show significant spatial heterogeneity at each time point, as well as between time points. The fluctuations in oxygenation occur with a common periodicity within and between tumors, suggesting a common mechanism, but have tumor type-dependent spatial patterns. The widespread presence of fluctuations in tumor oxygenation has broad ranging implications for tumor progression, stress response, and signal transduction, which are altered by oxygenation/reoxygenation events. [Cancer Res 2008;68(14):5812-9]

Abstract-In erythrocytes, S-nitrosohemoglobin (SNO-Hb) arises from S-nitrosylation of oxygenated hemoglobin (Hb). Ithas been shown that SNO-Hb behaves as a nitric oxide (NO) donor at low oxygen tensions. This property, in combination with oxygen transport capacity, suggests that SNO-Hb may have unique potential to reoxygenate hypoxic tissues. The present study was designed to test the idea that the allosteric properties of SNO-Hb could be manipulated to enhance oxygen delivery in a hypoxic tumor. Using Laser Doppler flowmetry, we showed that SNO-Hb infusion to animals breathing 21% O 2 reduced tumor perfusion without affecting blood pressure and heart rate. Raising the pO 2 (100% O 2 ) slowed the release of NO bioactivity from SNO-Hb (ie, prolonged the plasma half-life of the SNO in Hb), preserved tumor perfusion, and raised the blood pressure. In contrast, native Hb reduced both tumor perfusion and heart rate independently of the oxygen concentration of the inhaled gas, and did not elicit hypertensive effects. Window chamber (to image tumor arteriolar reactivity in vivo) and hemodynamic measurements indicated that the preservation of tissue perfusion by micromolar concentrations of SNO-Hb is a composite effect created by reduced peripheral vascular resistance and direct inhibition of the baroreceptor reflex, leading to increased blood pressure. Overall, these results indicate that the properties of SNO-Hb are attributable to allosteric control of NO release by oxygen in central as well as peripheral issues. Key Words: nitric oxide Ⅲ hemoglobin Ⅲ oxygen Ⅲ hemodynamics Ⅲ blood flow H emoglobin (Hb) of red blood cells (RBC) is a tetrameric protein composed of 2 ␣ and 2 ␤ chains, each containing a heme prosthetic group. One ␣ and 1 ␤ chain is combined in stable ␣␤ dimers, and 2 dimers are more loosely associated to form tetramers. The physiological role of Hb depends on its ability to reversibly bind O 2 at its heme iron centers. This transport capacity is governed by a cycle of allosteric transitions in which Hb assumes the R (relaxed, high O 2 affinity) conformation to bind O 2 in the lungs and, on partial deoxygenation, the T (tense, low O 2 affinity) conformation to efficiently deliver O 2 to peripheral tissues. This transition also controls the reactivity of 2 conserved cysteines on the ␤ chains (Cys␤93). Thiols of the cysteines can react with the potent vasodilator nitric oxide (NO) to form S-nitrosohemoglobin (SNO-Hb) in the R conformation, and preferentially unload SNO in the T conformation. [1][2][3] It has therefore been proposed that Hb would carry NO equivalents from the lungs to the periphery, thereby bringing tissue blood flow in line with oxygen demand. Although mechanisms are debated, 4 -7 some consensus has been reached, namely that Hb can act as an oxygen sensor and oxygen-dependent transducer of NO bioactivity, 7-9 and the direct coupling between SNO and O 2 content of Hb has been recently affirmed in intact human erythrocytes (Doctor et al, unpublished observations).Outside the red blood cell (R...