Hypoxic stress proteins: Survival of the fittest

The importance of hypoxia in radiotherapy for the immune response, metastatic potential and FLASH-RT

PURPOSE

Hypoxia (low oxygen) is a common feature of solid tumors that has been intensely studied for more than six decades. Here we review the importance of hypoxia to radiotherapy with a particular focus on the contribution of hypoxia to immune responses, metastatic potential and FLASH radiotherapy, active areas of research by leading women in the field.

CONCLUSION

Although hypoxia-driven metastasis and immunosuppression can negatively impact clinical outcome, understanding these processes can also provide tumor-specific vulnerabilities that may be therapeutically exploited. The different oxygen tensions present in tumors and normal tissues may underpin the beneficial FLASH sparing effect seen in normal tissue and represents a perfect example of advances in the field that can leverage tumor hypoxia to improve future radiotherapy treatments.

Hypoxia signaling pathway: A central mediator in endocrine tumors

Adequate oxygen levels are essential for the functioning and maintenance of biological processes in virtually every cell, albeit based on specific need. Thus, any change in oxygen pressure leads to modulated activation of the hypoxia pathway, which affects numerous physiological and pathological processes, including hematopoiesis, inflammation, and tumor development. The Hypoxia Inducible Factors (HIFs) are essential transcription factors and the driving force of the hypoxia pathway; whereas, their inhibitors, HIF prolyl hydroxylase domain (PHDs) proteins are the true oxygen sensors that critically regulate this response. Recently, we and others have described the central role of the PHD/HIF axis in various compartments of the adrenal gland and its potential influence in associated tumors, including pheochromocytomas and paragangliomas. Here, we provide an overview of the most recent findings on the hypoxia signaling pathway in vivo, including its role in the endocrine system, especially in adrenal tumors.

HIF-Prolyl Hydroxylase Domain Proteins (PHDs) in Cancer-Potential Targets for Anti-Tumor Therapy?

Simple Summary

In solid tumors, proliferation of cancer cells typically outpaces the growth of functional vessels. The net result is often an obstructed blood circulation and areas of deprived oxygen (hypoxia). To overcome this acute stress, hypoxia inducible factors (HIFs) stimulate the expression of numerous proteins that will support adaptation to this situation and stimulate further growth, differentiation, and even dissemination. The HIF-response is closely controlled by a class of enzymes known as the HIF prolyl hydroxylases (PHDs). They are true oxygen sensors and directly regulate the activity of HIFs. Although many studies are currently focusing on inhibiting the activity of HIFs in tumors, the role of hypoxia signaling is complex and regulating PHDs in a number of tumor settings might be beneficial. This review gives an overview of the literature on the nature of PHDs in tumor-associated cells and discusses available PHD inhibitors and their potential use as an anti-tumor therapy.

Abstract

Solid tumors are typically associated with unbridled proliferation of malignant cells, accompanied by an immature and dysfunctional tumor-associated vascular network. Consequent impairment in transport of nutrients and oxygen eventually leads to a hypoxic environment wherein cells must adapt to survive and overcome these stresses. Hypoxia inducible factors (HIFs) are central transcription factors in the hypoxia response and drive the expression of a vast number of survival genes in cancer cells and in cells in the tumor microenvironment. HIFs are tightly controlled by a class of oxygen sensors, the HIF-prolyl hydroxylase domain proteins (PHDs), which hydroxylate HIFs, thereby marking them for proteasomal degradation. Remarkable and intense research during the past decade has revealed that, contrary to expectations, PHDs are often overexpressed in many tumor types, and that inhibition of PHDs can lead to decreased tumor growth, impaired metastasis, and diminished tumor-associated immune-tolerance. Therefore, PHDs represent an attractive therapeutic target in cancer research. Multiple PHD inhibitors have been developed that were either recently accepted in China as erythropoiesis stimulating agents (ESA) or are currently in phase III trials. We review here the function of HIFs and PHDs in cancer and related therapeutic opportunities.

Modulating Hypoxia via Nanomaterials Chemistry for Efficient Treatment of Solid Tumors

The common existence of hypoxia in solid tumors has been heavily researched because it renders tumors more resistant to most standard therapeutic methods, such as radiotherapy (RT), chemotherapy, and photodynamic therapy (PDT), and is associated with a more malignant phenotype and poor survival in patients with tumors. The development of hypoxia modulation methods for advanced therapeutic activity is therefore of great interest but remains a considerable challenge. Since the significant development of nanotechnology and nanomedicine, functionalized nanomaterials can be exploited as adjuvant "drugs" for these oxygen-dependent standard therapies or as hypoxia initiators for advanced new therapies to solid tumors. In this Account, we summarize our recent studies on the design and synthesis of nanomaterials with a set of desired chemistry benefits achievable by modulating hypoxia, suggesting a valid therapeutic option for tumors. The investigated strategies can be categorized into three groups: The first strategy is based on countering hypoxia. Considering that O₂ deficiency is the major obstacle for the oxygen-dependent therapies, we initially developed methods to supply O₂ by taking advantage of the hypoxia-responsive properties of nano-MnO₂ or nanomaterials' photothermal effects for increased intratumoral blood flow. The second approach is to disregard hypoxia. Possible benefits of nanoagents include reducing/eliminating reliance on O₂ or making O₂ replacements as adjuvants to standard therapies. To this end, we investigated a nano-upconversion/scintillator with the capacity toup-/down-convert near-infrared light (NIR)/X-ray to luminescence in the ultraviolet/visible region fortype-I PDT with minimized oxygen-tension dependency or developed Fe-based nanomaterials for chemodynamic therapy (CDT) without external energy and oxygen participation for efficient free radical killing of deep tumors. The third strategy involves exploiting hypoxia. The unique biological characteristics of hypoxia are exploited to activate nanoagents for new therapies. To address the discrepancy between the nanoagents' demand and supply within the hypoxia region, a smart "molecule-nano" medicine that stays small-molecule-like in the bloodstream and turns into self-assembled nanovesicles after entry into the hypoxia region was constructed for hypoxia-adaptive photothermal therapy (PTT). In addition to traditional anti-angiogenesis therapy, we prepared Mg₂Si nanoparticles by a special self-propagating high-temperature synthesis approach. These nanoparticles can directly remove the intratumoral oxygen via the oxidation reactions of Mg₂Si and later efficiently block the rapid reoxygenation via tumor blood vessels by the resultant SiO₂ microsheets for cancer starvation therapy. Taken together, these findings indicate that nanomaterials will assume a valuable role for anticancer exploration based on either their properties to make up oxygen deficiency or the use of hypoxia for therapeutic applications.

In Vivo Application of Proton-Electron Double-Resonance Imaging

SIGNIFICANCE

Proton-electron double-resonance imaging (PEDRI) employs electron paramagnetic resonance irradiation with low-field magnetic resonance imaging so that the electron spin polarization is transferred to nearby protons, resulting in higher signals. PEDRI provides information about free radical distribution and, indirectly, about the local microenvironment such as partial pressure of oxygen (pO2), tissue permeability, redox status, and acid-base balance. Recent Advances: Local acid-base balance can be imaged by exploiting the different resonance frequency of radical probes between R and RH+ forms. Redox status can also be imaged by using the loss of radical-related signal after reduction. These methods require optimized radical probes and pulse sequences.

CRITICAL ISSUES

High-power radio frequency irradiation is needed for optimum signal enhancement, which may be harmful to living tissue by unwanted heat deposition. Free radical probes differ depending on the purpose of PEDRI. Some probes are less effective for enhancing signal than others, which can reduce image quality. It is so far not possible to image endogenous radicals by PEDRI because low concentrations and broad line widths of the radicals lead to negligible signal enhancement.

FUTURE DIRECTIONS

PEDRI has similarities with electron paramagnetic resonance imaging (EPRI) because both techniques observe the EPR signal, directly in the case of EPRI and indirectly with PEDRI. PEDRI provides information that is vital to research on homeostasis, development of diseases, or treatment responses in vivo. It is expected that the development of new EPR techniques will give insights into novel PEDRI applications and vice versa. Antioxid. Redox Signal. 28, 1345-1364.

Co-imaging of the tumor oxygenation and metabolism using electron paramagnetic resonance imaging and 13-C hyperpolarized magnetic resonance imaging before and after irradiation

To examine the relationship between local oxygen partial pressure and energy metabolism in the tumor, electron paramagnetic resonance imaging (EPRI) and magnetic resonance imaging (MRI) with hyperpolarized [1-¹³C] pyruvate were performed.

SCCVII and HT29 solid tumors implanted in the mouse leg were imaged by EPRI using OX063, a paramagnetic probe and ¹³C-MRI using hyperpolarized [1-¹³C] pyruvate. Local partial oxygen pressure and pyruvate metabolism in the two tumor implants were examined. The effect of a single dose of 5-Gy irradiation on the pO₂ and metabolism was also investigated by sequential imaging of EPRI and ¹³C-MRI in HT29 tumors.

A phantom study using tubes filled with different concentration of [1-¹³C] pyruvate, [1-¹³C] lactate, and OX063 at different levels of oxygen confirmed the validity of this sequential imaging of EPRI and hyperpolarized ¹³C-MRI. In vivo studies revealed SCCVII tumor had a significantly larger hypoxic fraction (pO₂ < 8 mmHg) compared to HT29 tumor. The flux of pyruvate-to-lactate conversion was also higher in SCCVII than HT29. The lactate-to-pyruvate ratio in hypoxic regions (pO₂ < 8 mmHg) 24 hours after 5-Gy irradiation was significantly higher than those without irradiation (0.76 vs. 0.36) in HT29 tumor. The in vitro study showed an increase in extracellular acidification rate after irradiation.

In conclusion, co-imaging of pO₂ and pyruvate-to-lactate conversion kinetics successfully showed the local metabolic changes especially in hypoxic area induced by radiation therapy.

FBP1 expression is associated with basal-like breast carcinoma

The present study aimed to investigate the value of liver fructose 1,6-bisphophatase (FBP1) and hypoxia-inducible factor-1α (HIF-1α) in the molecular subtyping of breast carcinoma. Tissue obtained from 60 surgical specimens from patients with breast carcinoma underwent immunohistochemical staining for cytokeratin 5/6, HIF-1α and FBP1. The variation in the expression levels of these markers and clinicopathological factors were compared between molecular subtypes. In addition, disease-free survival was compared between basal-like and luminal breast carcinoma, according to differing expression levels of HIF-1α and FBP1. The results revealed that HIF-1α expression was detectable in 20/60 (33.3%) of the breast carcinoma cases, and was positively associated with lymph node metastasis (P=0.007). HIF-1α-positive patients exhibited a shorter disease-free survival, compared with HIF-1α-negative patients with invasive breast cancer. The expression levels of FBP1 were positive in 33/60 tumor tissues (55%; P<0.001), and FBP1 expression was associated with nuclear grade (P=0.017) and tumor stage (P=0.012). In breast carcinoma, HIF-1α expression levels were significantly negatively correlated with FBP1 levels (r=-0.711; P<0.001). Cox regression analysis identified FBP1 and tumor size as independent prognostic factors. Therefore, the present study demonstrated that patients with basal-like breast carcinoma exhibited lower levels of FBP1 expression in tumor tissues, compared with patients with luminal type breast cancer, and that low or absent expression levels of FBP1 may be associated with reduced disease-free survival.

Correlation Between Hypoxia Proteins and EPR-Detected Hypoxia in Tumors

Rapid expansion of tumor cells that outpace existing vasculature essential for nutrient and oxygen support as well as waste removal, correlates with profound changes in the microenvironment including angiogenesis, vasodilation, glucose metabolism, and cell cycle perturbations. Since hypoxic cells are up to three times more radioresistant than normoxic cells, identification of hypoxic populations to predict radiotherapeutic outcome is important. The consequences of hypoxia and activated proteins contribute to radioresistant tumors and radiotherapeutic failure. Stereotactic MCa4 tumor tissue biopsies from mouse tumors that were guided by electron paramagnetic resonance (EPR) O₂ imaging were examined for hypoxia-induced proteins. The oxygen broadening of narrow EPR spectral lines or, equivalently, the increase in relaxation rates of electron magnetization, report pO₂ with 1-2 torr resolution in image voxels less than 1 mm³. The pO₂ reporter molecule OX063d64 (trityl) was used to acquire the data described here. Trityl appears to be selectively retained in tumors with a half-life of ~30 min. We used an inversion recovery electron spin echo (IRESE) to measure the T1 rate of the trityl inside the tumor bearing leg. We estimate our uncertainty in pO₂ measurement to be 1-3 torr per voxel. Three hypoxic cell biomarkers, hypoxic-induced factor 1-alpha (HIF-1α), vascular endothelial growth factor (VEGF), and carbonic anhydrase IX (CA9), were examined using the ELISA assay. Quantification of these proteins based on results from the ELISA immunoassay kits indicate a strong correlation between EPR pO₂-identified hypoxic fractions (<10 torr) and HIF-1α, VEGF, and CA9. We clearly demonstrate that hypoxic regions in tumors generate substantial amounts of HIF- 1α, VEGF, and CA9 protein.

The tumor as an organ: comprehensive spatial and temporal modeling of the tumor and its microenvironment

BACKGROUND

Research related to cancer is vast, and continues in earnest in many directions. Due to the complexity of cancer, a better understanding of tumor growth dynamics can be gleaned from a dynamic computational model. We present a comprehensive, fully executable, spatial and temporal 3D computational model of the development of a cancerous tumor together with its environment.

RESULTS

The model was created using Statecharts, which were then connected to an interactive animation front-end that we developed especially for this work, making it possible to visualize on the fly the on-going events of the system's execution, as well as the effect of various input parameters. We were thus able to gain a better understanding of, e.g., how different amounts or thresholds of oxygen and VEGF (vascular endothelial growth factor) affect the progression of the tumor. We found that the tumor has a critical turning point, where it either dies or recovers. If minimum conditions are met at that time, it eventually develops into a full, active, growing tumor, regardless of the actual amount; otherwise it dies.

CONCLUSIONS

This brings us to the conclusion that the tumor is in fact a very robust system: changing initial values of VEGF and oxygen can increase the time it takes to become fully developed, but will not necessarily completely eliminate it.

Hypoxia Strongly Affects Mitochondrial Ribosomal Proteins and Translocases, as Shown by Quantitative Proteomics of HeLa Cells

Hypoxia is an important and common characteristic of many human tumors. It is a challenge clinically due to the correlation with poor prognosis and resistance to radiation and chemotherapy. Understanding the biochemical response to hypoxia would facilitate the development of novel therapeutics for cancer treatment. Here, we investigate alterations in gene expression in response to hypoxia by quantitative proteome analysis using stable isotope labeling with amino acids in cell culture (SILAC) in conjunction with LCMS/MS. Human HeLa cells were kept either in a hypoxic environment or under normoxic conditions. 125 proteins were found to be regulated, with maximum alteration of 18-fold. In particular, three clusters of differentially regulated proteins were identified, showing significant upregulation of glycolysis and downregulation of mitochondrial ribosomal proteins and translocases. This interaction is likely orchestrated by HIF-1. We also investigated the effect of hypoxia on the cell cycle, which shows accumulation in G1 and a prolonged S phase under these conditions. Implications. This work not only improves our understanding of the response to hypoxia, but also reveals proteins important for malignant progression, which may be targeted in future therapies.

Modulation of the tumor vasculature and oxygenation to improve therapy

The tumor microenvironment is increasingly recognized as a major factor influencing the success of therapeutic treatments and has become a key focus for cancer research. The progressive growth of a tumor results in an inability of normal tissue blood vessels to oxygenate and provide sufficient nutritional support to tumor cells. As a consequence the expanding neoplastic cell population initiates its own vascular network which is both structurally and functionally abnormal. This aberrant vasculature impacts all aspects of the tumor microenvironment including the cells, extracellular matrix, and extracellular molecules which together are essential for the initiation, progression and spread of tumor cells. The physical conditions that arise are imposing and manifold, and include elevated interstitial pressure, localized extracellular acidity, and regions of oxygen and nutrient deprivation. No less important are the functional consequences experienced by the tumor cells residing in such environments: adaptation to hypoxia, cell quiescence, modulation of transporters and critical signaling molecules, immune escape, and enhanced metastatic potential. Together these factors lead to therapeutic barriers that create a significant hindrance to the control of cancers by conventional anticancer therapies. However, the aberrant nature of the tumor microenvironments also offers unique therapeutic opportunities. Particularly interventions that seek to improve tumor physiology and alleviate tumor hypoxia will selectively impair the neoplastic cell populations residing in these environments. Ultimately, by combining such therapeutic strategies with conventional anticancer treatments it may be possible to bring cancer growth, invasion, and metastasis to a halt.

The clinical importance of assessing tumor hypoxia: relationship of tumor hypoxia to prognosis and therapeutic opportunities

Tumor hypoxia is a well-established biological phenomenon that affects the curability of solid tumors, regardless of treatment modality. Especially for head and neck cancer patients, tumor hypoxia is linked to poor patient outcomes. Given the biological problems associated with tumor hypoxia, the goal for clinicians has been to identify moderately to severely hypoxic tumors for differential treatment strategies. The "gold standard" for detecting and characterizing of tumor hypoxia are the invasive polarographic electrodes. Several less invasive hypoxia assessment techniques have also shown promise for hypoxia assessment. The widespread incorporation of hypoxia information in clinical tumor assessment is severely impeded by several factors, including regulatory hurdles and unclear correlation with potential treatment decisions. There is now an acute need for approved diagnostic technologies for determining the hypoxia status of cancer lesions, as it would enable clinical development of personalized, hypoxia-based therapies, which will ultimately improve outcomes. A number of different techniques for assessing tumor hypoxia have evolved to replace polarographic pO2 measurements for assessing tumor hypoxia. Several of these modalities, either individually or in combination with other imaging techniques, provide functional and physiological information of tumor hypoxia that can significantly improve the course of treatment. The assessment of tumor hypoxia will be valuable to radiation oncologists, surgeons, and biotechnology and pharmaceutical companies who are engaged in developing hypoxia-based therapies or treatment strategies.

Targeting the tumor vasculature: a strategy to improve radiation therapy

A continuously expanding vasculature is an essential requirement of a growing neoplastic mass. The blood-vessel network provides not only nutritional support and waste product management, but also offers opportunities for the secondary spread of tumor cells. Given its crucial role in tumor development, growth and spread, considerable efforts have been spent on developing therapeutic strategies that compromise the growth and/or function of the tumor neovasculature. Two primary approaches are being pursued. Angiogenic inhibitors seek to interrupt the process of angiogenesis to prevent new tumor blood-vessel formation. Vascular disrupting agents aim to cause direct damage to the existing tumor endothelium. Lead agents in both categories have now advanced into clinical trials. Still, their greatest utility may ultimately lie in combinations with conventional anticancer therapies. Indeed, the application of such strategies as adjuvants to conventional radiation treatments offers unique opportunities to develop more effective cancer therapies.

Imaging hypoxia in gliomas

Hypoxia plays a central role in tumour development, angiogenesis, growth and resistance to treatment. Owing to constant developments in medical imaging technology, significant advances have been made towards in vitro and in vivo imaging of hypoxia in a variety of tumours, including gliomas of the central nervous system. The aim of this article is to review the literature on imaging approaches currently available for measuring hypoxia in human gliomas and provide an insight into recent advances and future directions in this field. After a brief overview of hypoxia and its importance in gliomas, several methods of measuring hypoxia will be presented. These range from invasive monitoring by Eppendorf polarographic O(2) microelectrodes, positron electron tomography (PET) tracers based on 2-nitroimidazole compounds [(18)F-labelled fluoro-misonidazole ((18)F-MISO) or 1-(2-[((18))F]fluoro-1-[hydroxymethyl]ethoxy)methyl-2-nitroimidazole (FRP-170)], (64)Cu-ATSM Cu-diacetyl-bis(N4-methylthiosemicarbazone) (Cu-ATSM) or (99m)Tc- and (68)Ga-labelled metronidazole (MN) agents to advanced MRI methods, such as blood oxygenation level dependent (BOLD) MRI, oxygen-enhanced MRI, diffusion-weighted MRI (DWI-MRI), dynamic contrast-enhanced MRI (DCE-MRI) and (1)H-magnetic resonance spectroscopy.

Development of (99m)Tc-N4-NIM for molecular imaging of tumor hypoxia

The nitro group of 2-nitroimidazole (NIM) enters the tumor cells and is bioreductively activated and fixed in the hypoxia cells. 1,4,8,11-tetraazacyclotetradecane (N4) has shown to be a stable chelator for ^99mTc. The present study was aimed to develop ^99mTc-cyclam-2-nitroimidazole (^99mTc-N4-NIM) for tumor hypoxia imaging. N4-NIM precursor was synthesized by reacting N4-oxalate and 1,3-dibromopropane-NIM, yielded 14% (total synthesis). Cell uptake of ^99mTc-N4-NIM and ^99mTc-N4 was obtained in 13762 rat mammary tumor cells and mesothelioma cells in 6-well plates. Tissue distribution of ^99mTc-N4-NIM was evaluated in breast-tumor-bearing rats at 0.5–4 hrs. Tumor oxygen tension was measured using an oxygen probe. Planar imaging was performed in the tumor-bearing rat and rabbit models. Radiochemical purity of ^99mTc-N4-NIM was >96% by HPLC. Cell uptake of ^99mTc-N4-NIM was higher than ^99mTc-N4 in both cell lines. Biodistribution of ^99mTc-N4-NIM showed increased tumor-to-blood and tumor-to-muscle count density ratios as a function of time. Oxygen tension in tumor tissue was 6–10 mmHg compared to 40–50 mmHg in normal muscle tissue. Planar imaging studies confirmed that the tumors could be visualized clearly with ^99mTc-N4-NIM in animal models. Efficient synthesis of N4-NIM was achieved. ^99mTc-N4-NIM is a novel hypoxic probe and may be useful in evaluating cancer therapy.

Quantitative assessment of hypoxia subtypes in microcirculatory supply units of malignant tumors using (immuno-)fluorescence techniques

BACKGROUND AND PURPOSE

Hypoxia is a characteristic of tumors, is known to increase aggressiveness, and causes treatment resistance. Traditional classification suggests two types of hypoxia: chronic and acute. Acute hypoxia is mostly caused by transient disruptions in perfusion, while chronic hypoxia is caused by diffusion limitations. This classification may be insufficient in terms of pathogenetic and pathophysiological mechanisms. Therefore, we quantified hypoxia subtypes in tumors based on (immuno-)fluorescent marker distribution patterns in microcirculatory supply units (MCSUs).

MATERIAL AND METHODS

Cryosections from hSCC lines (SAS, FaDu, UT-SCC-5, UT-SCC-14, UT-SCC-15) were analyzed. Hypoxia was identified by pimonidazole, perfusion by Hoechst 33342, and endothelial cells by CD31. The following patterns were identified in vital tumor tissue: (1) normoxia: Hoechst 33342 fluorescence around microvessels, no pimonidazole, (2) chronic hypoxia: Hoechst 33342 fluorescence around microvessels, pimonidazole distant from microvessels, (3) acute hypoxia: no Hoechst 33342 fluorescence around microvessels, pimonidazole in immediate vicinity of microvessels, and (4) hypoxemic hypoxia: Hoechst 33342 fluorescence and pimonidazole directly around microvessels.

RESULTS

Quantitative assessment of MCSUs show predominance for normoxia in 4 out of 5 tumor lines (50.1-72.8%). Total hypoxia slightly prevails in UT-SCC-15 (56.9%). Chronic hypoxia is the dominant subtype (65.4-85.9% of total hypoxia). Acute hypoxia only accounts for 12.9-29.8% and hypoxemic hypoxia for 1.2-6.4% of total hypoxia. The fraction of perfused microvessels ranged from 82.5-96.6%.

CONCLUSION

Chronic hypoxia is the prevailing subtype in MCSUs. Acute hypoxia and hypoxemic hypoxia account for only a small fraction. This approach enables assessment and recognition of different hypoxia subtypes including hypoxemic hypoxia and may facilitate methods to (clinically) identify and eliminate hypoxia.

Oxygenation status of urogenital tumors

In malignant urogenital tumors, tissue oxygenation is compromised and very heterogeneous,with steep and fluctuating spatio-temporal oxygen gradients signaling a complex instability in tumor oxygenation (complex "4D-heterogeneity"). Tumor hypoxia is highly dynamic, and rapidly changing pO(2) gradients may be key factors driving hypoxia-dependent adaptive processes leading to malignant progression. The grand median oxygen tension in malignant urogenital tumors is 7-11 mmHg. In contrast, benign leiomyomas of the uterus are severely, but uniformly, hypoxic with only shallow oxygen gradients ("static hypoxia"). In these benign tumors, the median pO(2) is 1 mmHg and signs of hypoxia-driven processes are missing.

Pathophysiological and vascular characteristics of tumours and their importance for hyperthermia: heterogeneity is the key issue

Tumour blood flow before and during clinically relevant mild hyperthermia exhibits pronounced heterogeneity. Flow changes upon heating are not predictable and are both spatially and temporally highly variable. Flow increases may result in improved heat dissipation to the extent that therapeutically relevant tissue temperatures may not be achieved. This holds especially true for tumours or tumour regions in which flow rates are substantially higher than in the surrounding normal tissues. Changes in tumour oxygenation tend to reflect alterations in blood flow upon hyperthermia. An initial improvement in the oxygenation status, followed by a return to baseline levels (or even a drop to below baseline at high thermal doses) has been reported for some tumours, whereas a predictable and universal occurrence of sustained increases in O(2) tensions upon mild hyperthermia is questionable and still needs to be verified in the clinical setting. Clarification of the pathogenetic mechanisms behind possible sustained increases is mandatory. High-dose hyperthermia leads to a decrease in the extracellular and intracellular pH and a deterioration of the energy status, both of which are known to be parameters capable of acting as direct sensitisers and thus pivotal factors in hyperthermia treatment. The role of the tumour microcirculatory function, hypoxia, acidosis and energy status is complex and is further complicated by a pronounced heterogeneity. These latter aspects require additional critical evaluation in clinically relevant tumour models in order for their impact on the response to heat to be clarified.

Radiotherapy in combination with vascular-targeted therapies

BACKGROUND

Given the critical role of tumor vasculature in tumor development, considerable efforts have been spent on developing therapeutic strategies targeting the tumor vascular network. A variety of agents have been developed, with two general approaches being pursued. Antiangiogenic agents (AAs) aim to interfere with the process of angiogenesis, preventing new tumor blood vessel formation. Vascular-disrupting agents (VDAs) target existing tumor vessels causing tumor ischemia and necrosis. Despite their great therapeutic potential, it has become clear that their greatest clinical utility may lie in combination with conventional anticancer therapies. Radiotherapy is a widely used treatment modality for cancer with its distinct therapeutic challenges. Thus, combining the two approaches seems reasonable.

CONCLUSIONS

Strong biological rationale exist for combining vascular-targeted therapies with radiation. AAs and VDAs were shown to alter the tumor microenvironment in such a way as to enhance responses to radiation. The results of preclinical and early clinical studies have confirmed the therapeutic potential of this new treatment strategy in the clinical setting. However, concerns about increased normal tissue toxicity, have been raised.

Effects of irradiation on tumor cell survival, invasion and angiogenesis

Ionizing irradiation is a widely applied therapeutic method for the majority of solid malignant neoplasms, including brain tumors where, depending on localization, this might often be the only feasible primary intervention.Without doubt, it has been proved to be a fundamental tool available in the battlefield against cancer, offering a clear survival benefit in most cases. However, numerous studies have associated tumor irradiation with enhanced aggressive phenotype of the remaining cancer cells. A cell population manages to survive after the exposure, either because it receives sublethal doses and/or because it successfully utilizes the repair mechanisms. The biology of irradiated cells is altered leading to up-regulation of genes that favor cell survival, invasion and angiogenesis. In addition, hypoxia within the tumor mass limits the cytotoxicity of irradiation, whereas irradiation itself may worsen hypoxic conditions, which also contribute to the generation of resistant cells. Activation of cell surface receptors, such as the epidermal growth factor receptor, utilization of signaling pathways, and over-expression of cytokines, proteases and growth factors, for example the matrix metalloproteinases and vascular endothelial growth factor, protect tumor and non-tumor cells from apoptosis, increase their ability to invade to adjacent or distant areas, and trigger angiogenesis. This review will try to unfold the various molecular events and interactions that control tumor cell survival, invasion and angiogenesis and which are elicited or influenced by irradiation of the tumor mass, and to emphasize the importance of combining irradiation therapy with molecular targeting.

Regulation of HIF-1alpha and VEGF by miR-20b tunes tumor cells to adapt to the alteration of oxygen concentration

The regulation of HIF-1α is considered to be realized by pVHL-mediated ubiquitin-26S proteasome pathway at a post-transcriptional level. The discovery of a class of small noncoding RNAs, called microRNAs, implies alternative mechanism of regulation of HIF-1α. Here, we show that miR-20b plays an important role in fine-tuning the adaptation of tumor cells to oxygen concentration. The inhibition of miR-20b increased the protein levels of HIF-1α and VEGF in normoxic tumor cells; the increase of miR-20b in hypoxic tumor cells, nevertheless, decreased the protein levels of HIF-1α and VEGF. By using luciferase reporter vector system, we confirmed that miR-20b directly targeted the 3′UTR of Hif1a and Vegfa. On the other hand, the forced overexpression of HIF-1α in normoxic tumor cells downregulated miR-20b expression. However, HIF-1α knockdown in hypoxic tumor cells caused the increase of miR-20b. The differential expression of miR-20b has important biological significance in tumor cells, either enhancing the growth or favoring the survival of tumor cells upon the oxygen supply. Thus, we identify a novel molecular regulation mechanism through which miR-20b regulates HIF-1α and VEGF and is regulated by HIF-1α so to keep tumor cells adapting to different oxygen concentrations.

Taking advantage of tumor cell adaptations to hypoxia for developing new tumor markers and treatment strategies

Cancer cells in hypoxic areas of solid tumors are to a large extent protected against the action of radiation as well as many chemotherapeutic drugs. There are, however, two different aspects of the problem caused by tumor hypoxia when cancer therapy is concerned: One is due to the chemical reactions that molecular oxygen enters into therapeutically targeted cells. This results in a direct chemical protection against therapy by the hypoxic microenvironment, which has little to do with cellular biological regulatory processes. This part of the protective effect of hypoxia has been known for more than half a century and has been studied extensively. However, in recent years there has been more focus on the other aspect of hypoxia, namely the effect of this microenvironmental condition on selecting cells with certain genetic prerequisites that are negative with respect to patient prognosis. There are adaptive mechanisms, where hypoxia induces regulatory cascades in cells resulting in a changed metabolism or changes in extracellular signaling. These processes may lead to changes in cellular intrinsic sensitivity to treatment irrespective of oxygenation and, furthermore, may also have consequences for tissue organization. Thus, the adaptive mechanisms induced by hypoxia itself may have a selective effect on cells, with a fine-tuned protection against damage and stress of many kinds. It therefore could be that the adaptive mechanisms may take advantage of for new tumor labeling/imaging and treatment strategies. One of the Achilles' heels of hypoxia research has always been the exact measurements of tissue oxygenation as well as the control of oxygenation in biological tumor models. Thus, development of technology that can ease this control is vital in order to study mechanisms and perform drug development under relevant conditions. An integrated EU Framework project 2004-2009, termed EUROXY, demonstrates several pathways involved in transcription and translation control of the hypoxic cell phenotype and evidence of cross-talk with responses to pH and redox changes. The carbonic anhydrase isoenzyme CA IX was selected for further studies due to its expression on the surface of many types of hypoxic tumors. The effort has led to marketable culture flasks with sensors and incubation equipment, and the synthesis of new drug candidates against new molecular targets. New labeling/imaging methods for cancer diagnosing and imaging of hypoxic cancer tissue are now being tested in xenograft models and are also in early clinical testing, while new potential anti-cancer drugs are undergoing tests using xenografted tumor cancers. The present article describes the above results in individual consortium partner presentations.

Brain tumor hypoxia: tumorigenesis, angiogenesis, imaging, pseudoprogression, and as a therapeutic target

Hypoxia is implicated in many aspects of tumor development, angiogenesis, and growth in many different tumors. Brain tumors, particularly the highly aggressive glioblastoma multiforme (GBM) with its necrotic tissues, are likely affected similarly by hypoxia, although this involvement has not been closely studied. Invasion, apoptosis, chemoresistance, resistance to antiangiogenic therapy, and radiation resistance may all have hypoxic mechanisms. The extent of the influence of hypoxia in these processes makes it an attractive therapeutic target for GBM. Because of their relationship to glioma and meningioma growth and angiogenesis, hypoxia-regulated molecules, including hypoxia inducible factor-1, carbonic anhydrase IX, glucose transporter 1, and vascular endothelial growth factor, may be suitable subjects for therapies. Furthermore, other novel hypoxia-regulated molecules that may play a role in GBM may provide further options. Emerging imaging techniques may allow for improved determination of hypoxia in human brain tumors to better focus therapeutic treatments; however, tumor pseudoprogression, which may be prompted by hypoxia, poses further challenges. An understanding of the role of hypoxia in tumor development and growth is important for physicians involved in the care of patients with brain tumors.

Hypoxia and aggressive tumor phenotype: implications for therapy and prognosis

Tumor hypoxia, mostly resulting from poor perfusion and anemia, is one of the key factors in inducing the development of cell clones with an aggressive and treatment-resistant phenotype that leads to rapid progression and poor prognosis. Studies in patients with solid tumors suggest that there is a range of hemoglobin (Hb) concentrations that is optimum for tumor oxygenation. When used to achieve an Hb level within this range, erythropoiesis-stimulating agents (ESAs) can be expected to increase tumor oxygenation, and this may favorably influence sensitivity to treatment as well as quality of life. There is no robust evidence that ESAs, when used as indicated, have a negative effect on survival in patients with solid tumors. When used outside the indications recommended, the rise in Hb level that results may reduce tumor blood flow and tissue oxygenation because of a raised viscosity within the abnormal tumor microvasculature. In the current situation, it remains important to use ESAs within the approved indications and according to treatment guidelines such as those developed by the European Organization for Research and Treatment of Cancer.

Her2/neu signaling blockade improves tumor oxygenation in a multifactorial fashion in Her2/neu+ tumors

PURPOSE

Tumor hypoxia reduces the efficacy of radiation and chemotherapy as well as altering gene expression that promotes cell survival and metastasis. The growth factor receptor, Her2/neu, is overexpressed in 25-30% of breast tumors. Tumors that are Her2(+) may have an altered state of oxygenation, relative to Her2(-) tumors, due to differences in tumor growth rate and angiogenesis.

METHODS

Her2 blockade was accomplished using an antibody to the receptor (trastuzumab; Herceptin). This study examined the effects of Her2 blockade on tumor angiogenesis, vascular architecture, and hypoxia in Her2(+) and Her2(-) MCF7 xenograft tumors.

RESULTS

Treatment with trastuzumab in Her2(+) tumors significantly improved tumor oxygenation, increased microvessel density, and improved vascular architecture compared with the control-treated Her2(+) tumors. The Her2(+) xenografts treated with trastuzumab also demonstrated decreased proliferation indices when compared with control-treated xenografts. These results indicate that Her2 blockade can improve tumor oxygenation by decreasing oxygen consumption (reducing tumor cell proliferation and inducing necrosis) and increasing oxygen delivery (vascular density and architecture).

CONCLUSIONS

These results support the use of trastuzumab as an adjunct in the treatment of breast tumors with chemotherapy or radiotherapy, as improvements in tumor oxygenation should translate into improved treatment response.

Colonisation of Clostridium in the body is restricted to hypoxic and necrotic areas of tumours

The use of gene therapy is one of the most recent molecular strategies for the treatment of cancer. It is essential, however, to have an efficient transfer system by which the desired gene can be delivered to the correct environment. The experiments described in this report investigate apathogenic Clostridium as a possible vector to transfer a specific gene product into the extracellular microenvironment of the tumour which is hypoxic/necrotic in parts, using WAG/Rij rats with transplantable rhabdomyosarcomas as a model. Our data show that Clostridium, after systemic administration of at least 10(7) spores, specifically colonises the hypoxic/necrotic areas of our tumour model, the most efficient species being C. acetobutylicum (NI-4082) and C. oncolyticum. Although spores were also detected in normal tissues for up to 4 weeks, they did not germinate in these tissues. We conclude that it seems likely that these bacteria can be used as a selective transfer system into the extracellular environment of tumours which have hypoxic regions. This strategy would be more tumour-specific than various other strategies that are currently being investigated in anti-cancer gene therapy.

Relation of a hypoxia metagene derived from head and neck cancer to prognosis of multiple cancers

Affymetrix U133plus2 GeneChips were used to profile 59 head and neck squamous cell cancers. A hypoxia metagene was obtained by analysis of genes whose in vivo expression clustered with the expression of 10 well-known hypoxia-regulated genes (e.g., CA9, GLUT1, and VEGF). To minimize random aggregation, strongly correlated up-regulated genes appearing in >50% of clusters defined a signature comprising 99 genes, of which 27% were previously known to be hypoxia associated. The median RNA expression of the 99 genes in the signature was an independent prognostic factor for recurrence-free survival in a publicly available head and neck cancer data set, outdoing the original intrinsic classifier. In a published breast cancer series, the hypoxia signature was a significant prognostic factor for overall survival independent of clinicopathologic risk factors and a trained profile. The work highlights the validity and potential of using data from analysis of in vitro stress pathways for deriving a biological metagene/gene signature in vivo.

Chemoradiotherapy for localized esophageal cancer: regimen selection and molecular mechanisms of radiosensitization

Concurrent chemoradiotherapy administered either before surgery or as definitive treatment has a central role in the multimodality treatment of locally advanced esophageal cancer. Initial studies of this combined-modality regimen were based on models of squamous-cell cancers from other primary sites; this approach progressed from use of bleomycin or fluorouracil plus cisplatin concurrent with radiation in early trials, to the integration of taxanes, camptothecins and platinum analogs in recent trials. These trials demonstrated the tumoricidal effect of concurrent chemotherapy and radiotherapy and showed the survival advantages of this approach. Preoperative concurrent chemoradiation is used to downstage the tumor, ideally to a pathological complete response status in which there is no residual tumor in the resected primary and nodal tissues. A pathological complete response is associated with long-term survival but occurs in a minority (30%) of patients. While clinical trials have demonstrated an improvement in survival with concurrent chemoradiotherapy this effect is limited, as indicated by the plateau in survival beyond 5 years of approximately 30% or less. The recent clinical development of biologic, targeted therapies provides a new avenue for the study of chemoradiotherapy and an opportunity to increase long-term survival.

Vascular damaging agents

To provide a comprehensive overview on vascular targeting agents and the application of radiobiological principles in pre-clinical and clinical studies, we completed a comprehensive review of published medical studies on vascular targeting agents using Pub Med. Vascular targeting agents are now divided into vascular disrupting agents (VDAs), which target the pre-existing tumour vasculature, and angiogenesis inhibitors (AIs), which prevent the formation of new blood vessels. Modest success has been seen when VDAs and AIs are used as single agents and therefore combination therapies that can work in a complimentary and synergistic manner, targeting both the tumour cells and endothelial cells, are needed. Radiobiological principles have been used to increase our understanding of these agents, and can explain the increased efficacy of combination treatments. In particular, the alteration of the tumour microenvironment by AIs and VDAs can lead to enhanced efficacy when combined with chemotherapy or radiotherapy, with phase II/III trials showing encouraging results. The optimal use and scheduling of AIs and VDAs remains to be determined. Further understanding of the mechanisms of action of these potentially very exciting anti-neoplastic agents is urgently required.

Salmonella typhimurium lacking ribose chemoreceptors localize in tumor quiescence and induce apoptosis

The effectiveness of most chemotherapeutics is limited by their inability to penetrate deep into tumor tissue and their ineffectiveness against quiescent cells. Motile Salmonella typhimurium, which are specifically attracted to compounds produced by quiescent cancer cells, could overcome this therapeutic barrier. We hypothesized that individual chemoreceptors target S. typhimurium to specific tumor microenvironments. To test this hypothesis, we used time-lapse fluorescent microscopy and tumor cylindroids to quantify the accumulation of chemotaxis machinery knockouts, including strains lacking individual cell surface chemoreceptors, chemotaxis signal transduction pathway enzymes, and the flagella and motor assemblies. To measure the extent of apoptosis induced by individual bacterial strains, caspase-3 activity was measured as a function of time. Our results showed how chemoreceptors directed bacterial chemotaxis within cylindroids: the aspartate receptor initiated chemotaxis toward cylindroids, the serine receptor initiated penetration, and the ribose/galactose receptor directed S. typhimurium toward necrosis. In addition, strains lacking proper flagella constructs, signal transduction proteins, or active motor function did not chemotax toward tumor cylindroids, indicating that directed chemotaxis is necessary to promote accumulation in tumors. By deleting the ribose/galactose receptor, bacterial accumulation localized to tumor quiescence and had a greater individual effect on inducing apoptosis than wild-type S. typhimurium. This new understanding of the mechanisms of Salmonella migration in tumors will allow for the development of bacterial therapies with improved targeting to therapeutically inaccessible regions of tumors.

Influence of hemoglobin levels on survival after radical treatment of esophageal carcinoma with radiotherapy

INTRODUCTION

The objective was to investigate the possible prognostic value of blood hemoglobin concentration in the outcome of radical treatment for locally advanced esophageal carcinoma.

MATERIALS AND METHOD

This was a retrospective analysis of data for 85 patients treated for locally advanced esophageal carcinoma between January 1991 and January 1997 with chemoradiotherapy alone or as neoadjuvant therapy. All patients received chemotherapy (4 cycles of cisplatin 100 mg/m2 on day 1, and continuous infusion 5-fluorouracil 1 g/m2 per day on days 1-5) with concomitant radiotherapy (40 Gy at 2 Gy/session to the esophageal tumor and mediastinum). The response was evaluated after 4 weeks. 69 patients continued to receive chemoradiotherapy only to a total dose of 60-64 Gy to the esophageal tumor with a 2-cm margin. Sixteen patients underwent radical surgery. Hemoglobin levels were measured before combined treatment in all patients. The prognostic value of hemoglobin concentration was analyzed statistically, along with other patient-, tumor- and treatment-related factors.

RESULTS

Mean follow-up time: 82 months (range 60- 99 months). Chemoradiotherapy was followed by an overall clinical response of 69.4%, with complete clinical response in 24.7% of the patients. Mean survival time was 12 months, and overall likelihood of survival after 3 years was 13%. Mean time to progression: 5 months. Median survival time was 12 months in the 69 patients who underwent chemoradiotherapy alone, and 26 months in patients who underwent radical surgery. Univariate analysis showed a hemoglobin value of > 13 g/dl to be a prognostic factor for better survival, along with performance status according to the ECOG classification, weight loss < 10%, tumor stage, tumor length, and complete response to chemoradiotherapy. Multivariate analysis showed that only hemoglobin concentration was an independent prognostic factor: for each unit increase in hemoglobin level, the risk of death from esophageal carcinoma decreased by 5%. In the subgroup of patients who did not undergo surgery, hemoglobin concentration was also an independent prognostic factor along with complete clinical response.

CONCLUSIONS

As found for other solid tumors, hemoglobin level was a determining factor in the prognosis for treatment outcome in patients with esophageal carcinoma. Our findings require confirmation in randomized studies and further documentation of the probable benefits of correcting hemoglobin levels.

PET and planar imaging of tumor hypoxia with labeled metronidazole

RATIONALE AND OBJECTIVES

This study was aimed to develop 99mTc- and 68Ga-labeled metronidazole (MN) using ethylenedicysteine (EC) as a chelator and evaluate their potential use to assess tumor hypoxia.

MATERIALS AND METHODS

EC-MN was labeled with 99mTc in the presence of tin (II) chloride. Labeling EC-MN with 68Ga was achieved by adding 68GaCl3 (2 mCi with 3.4 microg cold GaCl3). In vitro cellular uptakes of 99mTc- and 68Ga-EC-MN were obtained in various types of tumor cells at 0.5-4 hours. Tissue distribution and PET imaging of 99mTc and 68Ga-EC-MN were evaluated in breast tumor-bearing rats at 0.5-4 hours. Tumor oxygen tension was measured using an oxygen probe.

RESULTS

There were similar cellular uptakes (2-10%) between 99mTc- and 68Ga-EC-MN at 0.5-4 hours. In vivo biodistribution of 99mTc- and 68Ga-EC-MN in breast tumor-bearing rats showed increased tumor-to-blood and tumor-to-muscle count density ratios as a function of time. Positron emission tomography images confirmed that the tumors could be visualized clearly with 68Ga-EC-MN. Oxygen tension in tumor tissue was determined to be 6-10 mm Hg compared with 40-50 mm Hg in normal muscle tissue.

CONCLUSIONS

The results indicated that it is feasible to use 99mTc- and 68Ga-EC-MN for assessment of tumor hypoxia. These agents may be useful in selecting and evaluating cancer therapy.

Novel vascular targeting/disrupting agents: combretastatin A4 phosphate and related compounds

Novel anticancer compounds are being developed that attempt to exploit the unique properties of the vascular endothelium, which supplies rapidly dividing neoplasms. The goal of these vascular targeting agents (VTAs) or endothelial disrupting agents is to cause rapid shutdown of tumor blood supply with subsequent tumor death from hypoxia and nutrient deprivation. VTAs are classified into two broad categories: biologic therapies or small molecule compounds. A variety of VTAs are in early clinical development. These agents have demonstrated clinical activity in phase I trials and are being evaluated with cytotoxic chemotherapy and radiotherapy.

Hypoxia in the tumorigenesis of gliomas and as a potential target for therapeutic measures

✓ In this article, the author provides a brief description of the role of hypoxia in the tumorigenesis of gliomas and suggests potential ways of exploiting this role to design treatment modalities. Tumor hypoxia predicts the likelihood of metastases, tumor recurrence, resistance to chemotherapy and radiation therapy, invasive potential, and decreased patient survival for many human malignancies. Various methods of measurement of tumor hypoxia are discussed, including direct measurement and imaging methods.

The role of hypoxia-responsive molecules, especially hypoxia-inducible factor-1 (HIF-1), in glioma tumorigenesis is explored. Treatment modalities regulated by hypoxia are proposed and some potential strategies reviewed. The progression of a low-grade astrocytoma to a glioblastoma multiforme may be mediated by hypoxia-induced phenotypic changes and subsequent clonal selection of cells that overexpress hypoxia-responsive molecules, such as HIF-1. In this model, intratumoral hypoxia causes genetic changes that produce a microenvironment that selects for cells of a more aggressive phenotype.

Angiogenic and cell survival functions of vascular endothelial growth factor (VEGF)

Vascular endothelial growth factor (VEGF) was originally identified as an endothelial cell specific growth factor stimulating angiogenesis and vascular permeability. Some family members, VEGF C and D, are specifically involved in lymphangiogenesis. It now appears that VEGF also has autocrine functions acting as a survival factor for tumour cells protecting them from stresses such as hypoxia, chemotherapy and radiotherapy. The mechanisms of action of VEGF are still being investigated with emerging insights into overlapping pathways and cross-talk between other receptors such as the neuropilins which were not previously associated with angiogenesis. VEGF plays an important role in embryonic development and angiogenesis during wound healing and menstrual cycle in the healthy adult. VEGF is also important in a number of both malignant and non-malignant pathologies. As it plays a limited role in normal human physiology, VEGF is an attractive therapeutic target in diseases where VEGF plays a key role. It was originally thought that in pathological conditions such as cancer, VEGF functioned solely as an angiogenic factor, stimulating new vessel formation and increasing vascular permeability. It has since emerged it plays a multifunctional role where it can also have autocrine pro-survival effects and contribute to tumour cell chemoresistance. In this review we discuss the established role of VEGF in angiogenesis and the underlying mechanisms. We discuss its role as a survival factor and mechanisms whereby angiogenesis inhibition improves efficacy of chemotherapy regimes. Finally, we discuss the therapeutic implications of targeting angiogenesis and VEGF receptors, particularly in cancer therapy.

Gene expression profiling of the long-term adaptive response to hypoxia in the gills of adult zebrafish

Low oxygen levels (hypoxia) play a role in clinical conditions such as stroke, chronic ischemia, and cancer. To better understand these diseases, it is crucial to study the responses of vertebrates to hypoxia. Among vertebrates, some teleosts have developed the ability to adapt to extremely low oxygen levels. We have studied long-term adaptive responses to hypoxia in adult zebrafish. We used zebrafish that survived severe hypoxic conditions for 3 wk and showed adaptive behavioral and phenotypic changes. We used cDNA microarrays to investigate hypoxia-induced changes in expression of 15,532 genes in the respiratory organs (the gills). We have identified 367 differentially expressed genes of which 117 showed hypoxia-induced and 250 hypoxia-reduced expressions. Metabolic depression was indicated by repression of genes in the TCA cycle in the electron transport chain and of genes involved in protein biosynthesis. We observed enhanced expression of the monocarboxylate transporter and of the oxygen transporter myoglobin. The hypoxia-induced group further included the genes for Niemann-Pick C disease and for Wolman disease [lysosomal acid lipase (LAL)]. Both diseases lead to a similar intra- and extracellular accumulation of cholesterol and glycolipids. The Niemann-Pick C protein binds to cholesterol from internal lysosomal membranes and is involved in cholesterol trafficking. LAL is responsible for lysosomal cholesterol degradation. Our data suggest a novel adaptive mechanism to hypoxia, the induction of genes for lysosomal lipid trafficking and degradation. Studying physiological responses to hypoxia in species tolerant for extremely low oxygen levels can help identify novel regulatory genes, which may have important clinical implications.

Optical Imaging of Tumor Hypoxia and Evaluation of Efficacy of a Hypoxia-Targeting Drug in Living Animals

Solid tumors containing more hypoxic regions show a more malignant phenotype by increasing the expression of genes encoding angiogenic and metastatic factors. Hypoxia-inducible factor-1 (HIF-1) is a master transcriptional activator of such genes, and thus, imaging and targeting hypoxic tumor cells where HIF-1 is active are important in cancer therapy. In the present study, HIF-1 activity was monitored via an optical in vivo imaging system by using a luciferase reporter gene under the regulation of an artificial HIF-1-dependent promoter, 5HRE. To monitor tumor hypoxia, we isolated a stable reporter-transfectant, HeLa/5HRE-Luc, which expressed more than 100-fold luciferase in response to hypoxic stress, and observed bioluminescence from its xenografts. Immunohistochemical analysis of the xenografts with a hypoxia marker, pimonidazole, confirmed that the luciferase-expressing cells were hypoxic. Evaluation of the efficacy of a hypoxia-targeting prodrug, TOP3, using this optical imaging system revealed that hypoxic cells were significantly diminished by TOP3 treatment. Immunohistochemical analysis of the TOP3-treated xenografts confirmed that hypoxic cells underwent apoptosis and were removed after TOP3 treatment. These results demonstrate that this model system using the 5HRE-luciferase reporter construct provides qualitative information (hypoxic status) of solid tumors and enables one to conveniently evaluate the efficacy of cancer therapy on hypoxia in malignant solid tumors.

Hyperspectral imaging of hemoglobin saturation in tumor microvasculature and tumor hypoxia development

Tumor hypoxia has been shown to have prognostic value in clinical trials involving radiation, chemotherapy, and surgery. Tumor oxygenation studies at microvascular levels can provide understanding of oxygen transport on scales at which oxygen transfer to tissue occurs. To fully grasp the significance of blood oxygen delivery and hypoxia at microvascular levels during tumor growth and angiogenesis, the spatial and temporal relationship of the data must be preserved and mapped. Using tumors grown in window chamber models, hyperspectral imaging can provide serial spatial maps of blood oxygenation in terms of hemoglobin saturation at the microvascular level. We describe our application of hyperspectral imaging for in vivo microvascular tumor oxygen transport studies using red fluorescent protein (RFP) to identify all tumor cells, and hypoxia-driven green fluorescent protein (GFP) to identify the hypoxic fraction. 4T1 mouse mammary carcinoma cells, stably transfected with both reporter genes, are grown in dorsal skin-fold window chambers. Hyperspectral imaging is used to create image maps of hemoglobin saturation, and classify image pixels where RFP alone is present (tumor cells), or both RFP and GFP are present (hypoxic tumor cells). In this work, in vivo calibration of the imaging system is described and in vivo results are shown.

Hypoxia and anemia: effects on tumor biology and treatment resistance

In locally advanced solid tumors, oxygen (O2) delivery is frequently reduced or even abolished. This is due to abnormalities of the tumor microvasculature, adverse diffusion geometries, and tumor-associated and/or therapy-induced anemia. Up to 50-60% of locally advanced solid tumors may exhibit hypoxic and/or anoxic tissue areas that are heterogeneously distributed within the tumor mass. In approximately 30% of pretreatment patients, a decreased O2 transport capacity of the blood as a result of tumor-associated anemia can greatly contribute to the development of tumor hypoxia. While normal tissues can compensate for this O2 deficiency status by a rise in blood flow rate, locally advanced tumors (or at least larger tumor areas) cannot adequately counteract the restriction in O2 supply and thus the development of hypoxia. Hypoxia-induced alteration in gene expression and thus in the proteome (< 1% O2, or < 7 mmHg), and/or genome changes (< 0.1% O2, or < 0.7 mmHg) may promote tumor progression via mechanisms enabling cells to overcome nutritive deprivation, to escape from the hostile metabolic microenvironment and to favor unrestricted growth. Sustained hypoxia may thus lead to cellular changes resulting in a more clinically aggressive phenotype. In addition, hypoxia is known to directly or indirectly confer resistance to X- and gamma-radiation, and some chemotherapies leading to treatment failures. Whereas strong evidence has accumulated that hypoxia plays a pivotal role in tumor progression and acquired treatment resistance, the mechanism(s) by which treatment efficacy and survival may be compromised by anemia (independent of hypoxia) are not fully understood.

The role of hypoxia-induced factors in tumor progression

Hypoxia is a common characteristic of locally advanced solid tumors that has been associated with diminished therapeutic response and, more recently, with malignant progression, that is, an increasing probability of recurrence, locoregional spread, and distant metastasis. Emerging evidence indicates that the effect of hypoxia on malignant progression is mediated by a series of hypoxia-induced proteomic and genomic changes activating angiogenesis, anaerobic metabolism, and other processes that enable tumor cells to survive or escape their oxygen-deficient environment. The transcription factor hypoxia-inducible factor 1 (HIF-1) is a major regulator of tumor cell adaptation to hypoxic stress. Tumor cells with proteomic and genomic changes favoring survival under hypoxic conditions will proliferate, thereby further aggravating the hypoxia. The selection and expansion of new (and more aggressive) clones, which eventually become the dominant tumor cell type, lead to the establishment of a vicious circle of hypoxia and malignant progression.