Mark W. Dewhirst

Glioma stem cells promote radioresistance by preferential activation of the DNA damage response

Ionizing radiation represents the most effective therapy for glioblastoma (World Health Organization grade IV glioma), one of the most lethal human malignancies, but radiotherapy remains only palliative because of radioresistance. The mechanisms underlying tumour radioresistance have remained elusive. Here we show that cancer stem cells contribute to glioma radioresistance through preferential activation of the DNA damage checkpoint response and an increase in DNA repair capacity. The fraction of tumour cells expressing CD133 (Prominin-1), a marker for both neural stem cells and brain cancer stem cells, is enriched after radiation in gliomas. In both cell culture and the brains of immunocompromised mice, CD133-expressing glioma cells survive ionizing radiation in increased proportions relative to most tumour cells, which lack CD133. CD133-expressing tumour cells isolated from both human glioma xenografts and primary patient glioblastoma specimens preferentially activate the DNA damage checkpoint in response to radiation, and repair radiation-induced DNA damage more effectively than CD133-negative tumour cells. In addition, the radioresistance of CD133-positive glioma stem cells can be reversed with a specific inhibitor of the Chk1 and Chk2 checkpoint kinases. Our results suggest that CD133-positive tumour cells represent the cellular population that confers glioma radioresistance and could be the source of tumour recurrence after radiation. Targeting DNA damage checkpoint response in cancer stem cells may overcome this radioresistance and provide a therapeutic model for malignant brain cancers.

Tumor hypoxia adversely affects the prognosis of carcinoma of the head and neck

PURPOSE Tumor hypoxia adversely affects short term clinical radiation response of head and neck cancer lymph node metastases and long term disease-free survival (DFS) in cervix carcinoma. This study was performed to evaluate the relationship between tumor hypoxia and DFS in patients with squamous carcinoma of the head and neck (SCCHN). METHODS AND MATERIALS Pretreatment tumor pO2 was assessed polarographically in SCCHN patients. All patients were AJCC Stage IV and had pretreatment oxygen measurements taken from locally advanced primaries (T3 or T4) or neck nodes > or = 1.5 cm diameter. Treatment consisted of once daily (2 Gy/day to 66-70 Gy) or twice daily irradiation (1.25 Gy B.I.D. to 70-75 Gy) +/- planned neck dissection (for > or = N2A disease) according to institutional treatment protocols. RESULTS Twenty-eight patients underwent tumor pO2 measurement. The average pre-treatment median pO2 was 11.2 mm Hg (range 0.4-60 mm Hg). The DFS at 12 months was 42%. The DFS was 78% for patients with median tumor pO2 > 10 mm Hg but only 22% for median pO2 < 10 mm Hg (p = 0.009). The average tumor median pO2 for relapsing patients was 4.1 mm Hg and 17.1 mm Hg in non-relapsing (NED) patients (p = 0.007). CONCLUSION Tumor hypoxia adversely affected the prognosis of patients in this study. Understanding of the mechanistic relationship between hypoxia and treatment outcome will allow for the development of new and rational treatment programs in the future.

Targeting lactate-fueled respiration selectively kills hypoxic tumor cells in mice

Tumors contain oxygenated and hypoxic regions, so the tumor cell population is heterogeneous. Hypoxic tumor cells primarily use glucose for glycolytic energy production and release lactic acid, creating a lactate gradient that mirrors the oxygen gradient in the tumor. By contrast, oxygenated tumor cells have been thought to primarily use glucose for oxidative energy production. Although lactate is generally considered a waste product, we now show that it is a prominent substrate that fuels the oxidative metabolism of oxygenated tumor cells. There is therefore a symbiosis in which glycolytic and oxidative tumor cells mutually regulate their access to energy metabolites. We identified monocarboxylate transporter 1 (MCT1) as the prominent path for lactate uptake by a human cervix squamous carcinoma cell line that preferentially utilized lactate for oxidative metabolism. Inhibiting MCT1 with alpha-cyano-4-hydroxycinnamate (CHC) or siRNA in these cells induced a switch from lactate-fueled respiration to glycolysis. A similar switch from lactate-fueled respiration to glycolysis by oxygenated tumor cells in both a mouse model of lung carcinoma and xenotransplanted human colorectal adenocarcinoma cells was observed after administration of CHC. This retarded tumor growth, as the hypoxic/glycolytic tumor cells died from glucose starvation, and rendered the remaining cells sensitive to irradiation. As MCT1 was found to be expressed by an array of primary human tumors, we suggest that MCT1 inhibition has clinical antitumor potential.

Radiation activates HIF-1 to regulate vascular radiosensitivity in tumors: Role of reoxygenation, free radicals, and stress granules

Through a poorly understood mechanism, tumors respond to radiation by secreting cytokines capable of inhibiting apoptosis in endothelial cells, thereby diminishing treatment response by minimizing vascular damage. We reveal here that this pathway is governed by a major angiogenesis regulator, HIF-1. Following radiotherapy, tumor reoxygenation leads to: (1) nuclear accumulation of HIF-1 in response to reactive oxygen, and (2) enhanced translation of HIF-1-regulated transcripts secondary to stress granule depolymerization. The resulting increase in HIF-1-regulated cytokines enhances endothelial cell radioresistance. Inhibiting postradiation HIF-1 activation significantly increases tumor radiosensitivity as a result of enhanced vascular destruction. These data describe novel pathways contributing significantly to our understanding of HIF-1 regulation which may be major determinants of tumor radiosensitivity, potentially having high clinical relevance.

Cycling hypoxia and free radicals regulate angiogenesis and radiotherapy response

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.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.

Basic principles of thermal dosimetry and thermal thresholds for tissue damage from hyperthermia

This paper is one of several in this Special Issue of the International Journal of Hyperthermia that discusses the current state of knowledge about the human health risks of hyperthermia. This special issue emanated from a workshop sponsored by the World Health Organization in the Spring of 2002 on this topic. It is anticipated that these papers will help to establish guidelines for human exposure to conditions leading to hyperthermia. This comprehensive review of the literature makes it clear that much more work needs to be done to clarify what the thresholds for thermal damage are in humans. This review summarizes the basic principles that govern the relationships between thermal exposure (temperature and time of exposure) and thermal damage, with an emphasis on normal tissue effects. Methods for converting one time-temperature combination to a time at a standardized temperature are provided as well as a detailed discussion about the underlying assumptions that go into these calculations. There are few in vivo papers examining the type and extent of damage that occurs in the lower temperature range for hyperthermic exposures (e.g. 39-42°C). Therefore, it is clear that estimation of thermal dose to effect at these thermal exposures is less precise in that temperature range. In addition, there are virtually no data that directly relate to the thermal sensitivity of human tissues. Thus, establishment of guidelines for human exposure based on the data provided must be done with significant caution. There is detailed review and presentation of thermal thresholds for tissue damage (based on what is detectable in vivo ). The data are normalized using thermal dosimetric concepts. Tables are included in an Appendix Database which compile published data for thresholds of thermal damage in a variety of tissues and species. This database is available by request (contact MWD or PJH), but not included in this manuscript for brevity. All of the studies reported are for single acute thermal exposures. Except for brain function and physiology (as detailed in this issue by Sharma et al ) one notes the critical lack of publications examining effects of chronic thermal exposures as might be encountered in occupational hazards. This review also does not include information on the embryo, which is covered in detail elsewhere in this volume (see article by Edwards et al. ) as well as in a recent review on this subject, which focuses on thermal dose.

Oxygenation of head and neck cancer: changes during radiotherapy and impact on treatment outcome.

BACKGROUND AND PURPOSE To evaluate the long term clinical significance of tumor oxygenation in a population of head and neck cancer patients receiving radiotherapy and to assess changes in tumor oxygenation during the course of treatment. METHODS AND MATERIALS Patients with head and neck cancer receiving primary RT underwent pretreatment polarographic tumor oxygen measurement of the primary site or a metastatic neck lymph node. Treatment consisted of once daily (2 Gy/fraction to a total dose of 66-70 Gy) or twice daily irradiation (1.25 Gy/fraction to 70-75 Gy) to the primary site. Twenty-seven patients underwent a second series of measurements early in the course of irradiation. RESULTS Sixty-three patients underwent pretreatment tumor oxygen assessment (primary site, n = 24; nodes, n = 39). The median pO2 for primary lesions was 4.8 mmHg, and it was 4.3 mmHg for cervical nodes. There was a weak association between anemia and more poorly oxygened tumors, but many non-anemic patients still had poorly oxygenated tumors. Repeat assessments of tumor oxygenation after 10-15 Gy were unchanged compared to pretreatment baselines. Poorly oxygenated nodes pretreatment were more likely to contain viable residual disease at post-radiation neck dissection. Median follow-up time for surviving patients was 20 months (range 3-50 months). Hypoxia (tumor median pO2 <10 mmHg) adversely affected 2 year local-regional control (30 vs. 73%, P = 0.01), disease-free survival (26 vs. 73%, P = 0.005), and survival (35 vs. 83%, P = 0.02). CONCLUSION Tumor oxygenation affects the prognosis of head and neck cancer independently of other known prognostic variables. This parameter may be a useful tool for the selection of patients for investigational treatment strategies.

Drug targeting using thermally responsive polymers and local hyperthermia.

We report a new thermal targeting method in which a thermally responsive drug carrier selectively accumulates in a solid tumor that is maintained above physiological temperature by externally applied, focused hyperthermia. We synthesized two thermally responsive polymers that were designed to exhibit a lower critical solution temperature (LCST) transition slightly above physiological temperature: (1) a genetically engineered elastin-like polypeptide (ELP) and (2) a copolymer of N-isopropylacrylamide (NIPAAm) and acrylamide (AAm). The delivery of systemically injected polymer-rhodamine conjugates to solid tumors was investigated by in vivo fluorescence video microscopy of ovarian tumors implanted in dorsal skin fold window chambers in nude mice, with and without local hyperthermia. When tumors were heated to 42 degrees C, the accumulation of a thermally responsive ELP with a LCST of 40 degrees C was approximately twofold greater than the concentration of the same polymer in tumors that were not heated. Similar results were also obtained for a thermally responsive poly(NIPAAM-co-AAm), though the enhanced accumulation of this carrier in heated tumors was lower than that observed for the thermally responsive ELP. These results suggest that enhanced delivery of drugs to solid tumors can be achieved by conjugation to thermally responsive polymers combined with local heating of tumors.

Randomized trial of hyperthermia and radiation for superficial tumors.

PURPOSE Randomized clinical trials have demonstrated hyperthermia (HT) enhances radiation response. These trials, however, generally lacked rigorous thermal dose prescription and administration. We report the final results of a prospective randomized trial of superficial tumors (</= 3 cm depth) comparing radiotherapy versus HT combined with radiotherapy, using the parameter describing the number of cumulative equivalent minutes at 43 degrees C exceeded by 90% of monitored points within the tumor (CEM 43 degrees C T(90)) as a measure of thermal dose. METHODS This trial was designed to test whether a thermal dose of more than 10 CEM 43 degrees C T(90) results in improved complete response and duration of local control compared with a thermal dose of </= 1 CEM 43 degrees C T(90). Patients received a test dose of HT </= 1 CEM 43 degrees C T(90) and tumors deemed heatable were randomly assigned to additional HT versus no additional HT. HT was given using microwave spiral strip applicators operating at 433 MHz. RESULTS One hundred twenty-two patients were enrolled; 109 (89%) were deemed heatable and were randomly assigned. The complete response rate was 66.1% in the HT arm and 42.3% in the no-HT arm. The odds ratio for complete response was 2.7 (95% CI, 1.2 to 5.8; P = .02). Previously irradiated patients had the greatest incremental gain in complete response: 23.5% in the no-HT arm versus 68.2% in the HT arm. No overall survival benefit was seen. CONCLUSION Adjuvant hyperthermia with a thermal dose more than 10 CEM 43 degrees C T(90) confers a significant local control benefit in patients with superficial tumors receiving radiation therapy.

Tie2 Expression and Phosphorylation in Angiogenic and Quiescent Adult Tissues

Angiogenesis, the process of new vessels sprouting from the existing vasculature, is a critical process during early development. However, angiogenesis rarely occurs in the adult, except in response to cyclic hormonal stimulation in the ovary and uterus, in response to injury, and in response to pathological conditions such as tumorigenesis and diabetes mellitus. Tie2 (also known as Tek) is a novel endothelium-specific receptor tyrosine kinase, which has been demonstrated to be essential for the development of the embryonic vasculature; Tie2 knockout mice die by embryonic day 10.5 with specific defects in the formation of microvessels. Tie2 is downregulated later in embryogenesis, and its function in the adult has been relatively unexplored. To gain insight into the potential functions of Tie2 in the adult vasculature, Tie2 expression was examined in adult tissues undergoing angiogenesis and in quiescent tissues. Tie2 expression was localized by immunohistochemistry to the endothelium of neovessels in rat tissues undergoing angiogenesis during hormonally stimulated follicular maturation and uterine development and in healing skin wounds. Immunoprecipitation and RNase protection assay demonstrated upregulation of Tie2 protein and mRNA in rat and mouse skin wounds, respectively. Moreover, Tie2 immunoprecipitated from skin wounds was tyrosine-phosphorylated, indicating active downstream signaling. Surprisingly, Tie2 was also expressed in the entire spectrum of the quiescent vasculature (arteries, veins, and capillaries) in a wide range of adult tissues, and Tie2 immunoprecipitated from quiescent adult tissues was also tyrosine-phosphorylated. Together, these results suggest a dual function for Tie2 in adult tissues involving both angiogenesis and vascular maintenance.