Open Access

Suppression of HIF-1α expression and radiation resistance in acute hypoxic conditions

  • Authors:
    • Takahiro Oike
    • Yoshiyuki Suzuki
    • Wael Al-Jahdari
    • Abdulelah Mobaraki
    • Jun‑Ichi Saitoh
    • Kohta Torikai
    • Katsuyuki Shirai
    • Takashi Nakano
  • View Affiliations

  • Published online on: October 21, 2011     https://doi.org/10.3892/etm.2011.373
  • Pages: 141-145
Metrics: Total Views: 0 (Spandidos Publications: | PMC Statistics: )
Total PDF Downloads: 0 (Spandidos Publications: | PMC Statistics: )


Abstract

Recently, it has become clear that acute hypoxia affecting radioresistance exists widely in tumor tissues. Concurrently, hypoxia-inducible factor-1α (HIF-1α) is recognized as an essential transcriptional factor, enabling cells to survive through hypoxia. However, it is unclear as to whether HIF-1α plays a direct role in the radioresistance caused by acute hypoxia. Therefore, in this study, we investigated the in vitro response of the human lung adenocarcinoma cell line, A549, to ionizing radiation in an experimental model that imitates acute hypoxia in the presence and absence of HIF-1α expression, using the HIF-1α inhibitor 5-[1‑(phenylmethyl)‑1H‑indazol‑3-yl]‑2-furanmethanol (YC-1). Cells were treated with or without 10 µM YC-1 for 2 h. Cells were exposed to either 95% N2 and 5% CO2 (hypoxic condition of <0.1 mmHg) or atmospheric air (normoxic condition) for 1 h, and irradiated with 2, 5 and 10 Gy. Western blot analysis revealed that, without YC-1, cells exposed to hypoxic conditions expressed increased levels of HIF-1α compared with those exposed to normoxic conditions. Under hypoxic conditions, HIF-1α expression was suppressed by YC-1 to the same extent as that observed in cells exposed to normoxic conditions without YC-1. Clonogenic survival assay revealed that under hypoxic conditions there was no significant difference between the surviving fraction of cells treated with YC-1 and without YC-1 at any dose point examined. The oxygen enhancement ratio at 10% surviving fraction was calculated as 2.7 and 2.6 in the presence and the absence of YC-1, respectively. These results indicate that HIF-1α itself is not an immediate cause of acute hypoxia-induced radioresistance in A549 cells.

Introduction

Hypoxia in the microenvironment of tumors is well recognized as one of the major factors affecting resistance against radiation therapy (14). In the clinical setting, a number of investigators have proven the existence of hypoxic cells in human cancers by directly measuring intratumoral oxygen partial pressure (pO2) using special electrodes (57). We have also previously reported that low pretreatment intratumoral pO2 correlates with poor clinical outcome in cervical cancer treated with photon therapy (7).

It is recognized that hypoxia in tumors may result from two quite different mechanisms, namely ‘acute hypoxia’ and ‘chronic hypoxia’ (8). Acute hypoxia is the result of the abrupt temporary closing of a tumor blood vessel owing to the malformed vasculature of the tumor, which lacks smooth muscle functionality and often has an incomplete endothelial lining and basement membrane (9). Meanwhile, chronic hypoxia arises in regions distant from the blood vessels owing to the limited diffusion distance of oxygen through solid and massive tumor tissues. Although the compositional contribution of acute hypoxia to total hypoxia in tumor tissues has been considered to be low (10,11), recently, Cárdenas-Navia et al proved the pervasive presence of fluctuating pO2 in rat tumors (12), indicating that acute hypoxia is a more common phenomenon in tumor tissues than previously presumed. In clinical practice, fractionated radiation therapy, which leads to tumor reoxygenation is considered to be an effective method for killing cancer cells under conditions of chronic hypoxia (8). However, therapeutic significance of acute hypoxia in radiation therapy remains unclear.

Hypoxia-inducible factor-1α (HIF-1α) is considered to be an essential transcriptional factor, which mediates a number of physiological responses in tumor cells under hypoxic conditions (13,14). HIF-1α is synthesized under normoxia but undergoes rapid ubiquitination and proteosomal degradation (15). Under hypoxia, HIF-1α stabilizes, translocates to the nucleus and dimerizes with HIF-1β forming the heterodimer, HIF-1 (16,17). HIF-1 then binds to hypoxia-responsive elements of genes mediating physiological responses to hypoxia. As a result, HIF-1α promotes cell survival by increasing glucose transport, converting cellular metabolism to the obligatory glycolytic system and also inducing vascular endothelial growth factor (VEGF) expression, which leads to angiogenesis (18). In clinical specimens, high HIF-1α expression levels correlate with poor responses to radiation therapy and chemotherapy in several studies (1923). However, the exact role of HIF-1α in tumor resistance to radiation therapy under hypoxia has not yet been fully elucidated. Moreover, to the best of our knowledge, previous studies investigating the relationship between HIF-1α expression and hypoxia-induced radioresistance do not take into account the different circumstances between acute and chronic hypoxia.

Taking all these things into consideration, the present study was conducted to investigate whether HIF-1α has a direct fundamental role in the radioresistance induced by acute hypoxia. The in vitro response of the human lung adenocarcinoma cell line, A549, to ionizing radiation was examined in an experimental model that imitates acute hypoxia with or without HIF-1α expression using the well known HIF-1α inhibitor, 5-[1-(phenylmethyl)-1H-indazol-3-yl]-2-furanmethanol (YC-1).

Materials and methods

Cell culture and drugs

The human lung adenocarcinoma cell line, A549, was generously provided from the Cell Resources Center for Biomedical Research, Institute of Development, Aging and Cancer, Tohoku University (Sendai, Japan). A549 was cultured in RPMI-1640 (Invitrogen, Carlsbad, CA, USA) supplemented with 10% fetal bovine serum (Biowest, Miami, FL, USA) and 5% penicillin-streptomycin (Gibco, Grand Island, NY, USA) in a humidified incubator at 37°C and 5% CO2. YC-1 was purchased from Cayman Chemicals (Gruenberg, Germany), resuspended in dimethyl sulfoxide (DMSO) at a concentration of 10 μg/ml and stored at −20°C.

Western blot analysis for measuring expression of HIF-1α

Western blot analysis was performed to assess the effect of YC-1 on HIF-1α suppression. Cultured A549 cells were trypsinized, counted, seeded to glass dishes at a concentration of 1×104 cells in 2 ml of medium (RPMI-1640 with 10% fetal bovine serum and 5% penicillin-streptomycin) and allowed to attach overnight at 37°C. The medium was replaced the following day by fresh medium with 10 μM YC-1 and incubated for 2 h at 37°C. The cells were exposed to hypoxic conditions of <0.1 mmHg for 1 and 24 h, as an experimental model of acute and chronic hypoxia, respectively. For exposure of the cells to radiation under hypoxic conditions, a hypoxic chamber with an atmospheric gas-concentration control apparatus designed by Furusawa et al was used (24). Briefly, for exposure under hypoxic conditions, the glass dishes containing the cells were placed in the steel chamber. The chamber was flushed with 95% N2 gas and 5% CO2 gas. A pO2 monitoring machine with electrodes (UOE-04T, Unique Medical, Tokyo) placed into the glass dishes was used for real-time monitoring of the pO2 level.

The cells were washed with phosphate-buffered saline, scraped from the culture plates, centrifuged briefly and then solubilized into the mixture of RIPA lysis buffer (Millipore, Billerica, MA, USA), 1% phosphatase inhibitor cocktail 1 (Sigma-Aldrich, St. Louis, MO, USA), phosphatase inhibitor cocktail 2 (Sigma-Aldrich) and protease inhibitor cocktail set 3 (Merck, Darmstadt, Germany). Equal amounts of protein from the lysates were fractionated on 12.5% polyacrylamide gels (Bio-Rad Japan, Tokyo, Japan) and then transferred to Pure Nitrocellulose Membranes (Bio-Rad Japan). The Western blots were probed with an antibody directed against HIF-1α (BD Transduction Laboratories, Lexington, KY, USA) and actin (Chemicon International Inc., Temecula, CA, USA) and incubated with secondary antibodies conjugated to horse-radish peroxidase (Dako, Glostrup, Denmark). The enhanced chemiluminescence (ECL) Western Blot Detection System (GE Healthcare Japan, Tokyo, Japan) was used for protein detection.

Clonogenic survival assay of irradiated cells

A clonogenic survival assay was performed to assess the effect of HIF-1α suppression on the radiosensitivity of A549 cells under acute hypoxia. A schematic representation of this assay is described in Fig. 1. Cultured A549 cells were trypsinized, counted, seeded to glass dishes at a concentration of 1×104 cells in 2 ml of medium and allowed to attach overnight at 37°C. The medium was replaced the following day by fresh medium with or without 10 μM YC-1 and incubated for 2 h at 37°C. pO2 levels reached <0.1 mmHg approximately 30 min following induction of hypoxia. The cells were maintained under the hypoxic condition of <0.1 mmHg for 1 h, irradiated with 2, 5 and 10 Gy of 4 MV X-ray (Elekta Synergy, Elekta AB, Stockholm, Sweden), and maintained under the same hypoxic condition for a further hour. As a control, cells in the chamber were irradiated 2, 5 and 10 Gy under atmospheric air and exposed under hypoxic conditions for 2 h in order to match the condition of the case and the control with regard to the effect of hypoxia on cell survival.

Cultured dishes were then removed from the chamber. The cells were trypsinized, counted, re-seeded into 6-well plates at a specified number at in least triplicate with fresh medium, and incubated at 37°C for 7 days under normoxic conditions to enable colony formation. Cells were stained with crystal violet (Sigma-Aldrich) solution (2%) in 100% ethanol and colonies of at least 50 cells were counted. Plating efficiency was calculated as the ratio of the number of colonies formed to the number of cells seeded in the absence of radiation.

The D10 value, which is a survival parameter representing a radiation dose required to reduce the surviving fraction to 10%, was calculated from each survival data set by a curve fitting method using the linear-quadratic (LQ) model: SF = exp(−αD−βD2); where SF is the surviving fraction and D is the dose. The oxygen enhancement ratio (OER) was calculated as the ratio of the D10 value obtained under hypoxic conditions to that obtained in the control condition.

Statistical analysis

Statistical analysis was performed by StatMateIII Version 3.17 (ATMS, Tokyo, Japan). An F-test was used to test whether the standard deviations (SDs) of the two populations were equal or not. To compare the surviving fractions obtained by different experimental conditions, the unpaired Student’s t-test and Welch's test were used in the cases where the SDs of two populations were equal or not equal, respectively. Differences were considered significant at p<0.05.

Results

HIF-1α expression in A549 cells according to YC-1 modulation

Fig. 2 displays HIF-1α expression in A549 cells. Cells exposed to hypoxia for both 1 and 24 h expressed higher levels of HIF-1α compared with normoxic cells. These expression levels appeared to be similar. In cells exposed to hypoxia for 1 h, HIF-1α expression was suppressed by YC-1 to a similar extent as that observed in YC-1-free normoxic cells.

Role of HIF-1α in acute hypoxia-induced radioresistance

Fig. 3 shows the results of a clonogenic survival assay of A549 cells irradiated under normoxic conditions without YC-1 and under hypoxic conditions with or without YC-1. The plating efficiency of normoxic and hypoxic cells was 72 and 82%, respectively. There was no significant difference between the plating efficiency of cells exposed to normoxic and hypoxic conditions (p=0.094), demonstrating that exposure to the hypoxic condition of <0.1 mmHg for 1 h was not cytotoxic. As shown in Table I, the surviving fractions of A549 cells exposed to hypoxic conditions with or without YC-1 were significantly higher than those exposed to normoxic conditions at any dose point examined (p<0.01, <0.05 at 2 Gy; p<0.001, <0.001 at 5 Gy; p<0.001, <0.001 at 10 Gy, respectively). Under hypoxic conditions, there was no significant difference between the surviving fraction of cells treated with YC-1 and those without at any dose point examined (p=0.17, 0.10, 0.43 at 2, 5 and 10 Gy, respectively). The D10 values of normoxic cells without YC-1, hypoxic cells with YC-1 and those without YC-1 were 5.3, 14 and 13 Gy, respectively. Therefore, in the hypoxic groups, the addition of YC-1 did not significantly affect the OER value at 10% surviving fraction in comparison to the YC-1-free medium group (Table I).

Table I.

Surviving fraction of A549 cells obtained from clonogenic survival assay.

Table I.

Surviving fraction of A549 cells obtained from clonogenic survival assay.

2 Gy5 Gy10 GyD10OER
Normoxic conditions without YC-10.55±0.140.17±0.10 0.00012±0.000175.3
Hypoxic conditions with YC-10.71±0.24a0.63±0.31c0.24±0.09c142.7
Hypoxic conditions without YC-10.79±0.20b0.74±0.11c0.26±0.02c132.6

a, b, c Significantly different from the corresponding values obtained under normoxic conditions without YC-1 (ap<0.05, bp<0.01, cp<0.001).

{ label (or @symbol) needed for fn[@id='tfn2-etm-03-01-0141'] } D10, dose required to obtain 10% surviving fraction; OER, oxygen enhancement ratio. Each value represents an average surviving fraction with the standard deviation. YC-1, 5-[1-(phenylmethyl)-1H-indazol-3-yl]-2-furanmethanol.

Discussion

In the present study, the radiosensitivity of A549 cells under acute hypoxia was examined in the presence and absence of HIF-1α expression by suppression of HIF-1α with YC-1. The results showed that the OER in the 10% surviving fraction was consistent irrespective of HIF-1α expression.

It has been recognized that, in response to hypoxia, HIF-1α promotes cell survival by increasing glucose transport, converting cellular metabolism to the obligatory glycolytic system, and also inducing VEGF expression, which leads to angiogenesis (18). However, the exact role of HIF-1α in hypoxia-induced radioresistance remains unclear. Moreover, to the best of our knowledge, previous studies investigating the relationship between HIF-1α expression and hypoxia-induced radioresistance do not take into account the different circumstances between acute and chronic hypoxia. In this regard, since the mechanism and clinical significance of radioresistance induced by acute hypoxia have not been fully elucidated, the present study focused on acute hypoxia and investivated whether or not HIF-1α has a fundamental role in acute hypoxia-induced radioresistance.

In the present study, the OER value of 2.6 obtained in the absence of YC-1 following exposure to hypoxia for 1 h, which is the model for acute hypoxia, was considered to be a value of the typical response of tumor cells to severe hypoxia (8). Meanwhile, HIF-1α was induced to a similar extent as that following the exposure to hypoxia for 24 h, representing the model for chronic hypoxia. These observations are in accordance with a previous study by Vordermark et al (25). In addition, under acute hypoxia, the expression of HIF-1α was effectively suppressed by YC-1 (Fig. 2). The exposure to hypoxia for 1 h was not itself cytotoxic. Taken together, the OER value of 2.7 obtained in the presence of YC-1 clearly indicates that HIF-1α alone is not an immediate or direct cause of acute hypoxia-induced radioresistance in A549 cells in our experimental system.

There are a number of in vitro studies investigating the effect of HIF-1α on hypoxia-induced radioresistance. In these reports, cancer cells were subjected to hypoxia for various time periods (2527). Vordermark et al investigated the correlation between intracellular HIF-1α levels and radiosensitivity in FaDu and HT1080 cell lines following exposure to hypoxia for 1 h, and found the correlation was only in FaDu but not in HT1080 cells, suggesting that the correlation between HIF-1α levels and hypoxia-induced radioresistance is cell-type specific (25). Arvold et al demonstrated that the radiosensitivity of a mouse embryonic fibroblast (MEF) cell line following exposure to hypoxia for 4 h was independent of HIF-1α (26), which is in accordance with our results. They compared the radiosensitivity of a HIF-1α-wild-type MEF cell line to that of a HIF-1α-null MEF cell line under hypoxic conditions and showed that the OER of both cell lines in the 10% surviving fraction was identical. By contrast, Staab et al exposed HT1080 cells to hypoxia for 12 h with the HIF-1 inhibitor, chetomin, and found that the treatment by chetomin reduced the OER in these cells (27). These results suggest that it is controversial as to whether HIF-1α plays a fundamental role in hypoxia-induced radioresistance, since a distinction is not made between acute and chronic hypoxia in these studies. Combined with our results, it could be said that the contribution of HIF-1α to hypoxia-induced radioresistance is low for cells under acute hypoxia or hypoxic conditions for a relatively short time period.

Since the results of the present study reveal that HIF-1α does not play a direct role in acute hypoxia-induced radioresistance, there is a possibility that other proteins that interact with HIF-1α at higher regulation levels may be involved. In fact, there are some proteins that are considered to be candidates for the cause of radioresistance in response to hypoxia. We also examined the effect of nitric oxide as a factor that regulates hypoxic cells. Nitric oxide showed its ability to regulate HIF-1α and the epidermal growth factor receptor (EGFR). By adding the nitric oxide synthase (NOS) inhibitor, radiosensitivity was enhanced by the suppression of these two proteins (data not shown). In the present study, exposure to hypoxic conditions for 1 h increased HIF-1α expression, as well as exposure to hypoxia for 24 h.

In the microenvironment of solid tumors, hypoxia occurs as a result of the complex interaction of numerous factors, including tumor cells, blood vessels, interstitial tissues and others. Therefore, in vivo investigations would be of great importance to fully understand the exact role of HIF-1α in acute hypoxia-induced radioresistance. In the literature, there are some in vivo models that would be applicable to the real-time evaluation of intratumoral pO2 and HIF-1α activity. Cárdenas-Navia et al proved the pervasive presence of acute hypoxia in rat tumor models (12). In their study, phosphorescence lifetime imaging was used to measure fluctuations in vascular pO2 in rat tumors grown in dorsal skin-fold window chambers. Meanwhile, Harada et al demonstrated real-time imaging of intratumoral HIF-1α activity in tumor xenografts using a cell line expressing luciferase bioluminescence, in conjunction with intracellular accumulation of HIF-1α protein (28). The combination of these techniques will be beneficial for the simultaneous evaluation of intratumoral pO2 and HIF-1α activity.

In conclusion, subjecting A549 cells to hypoxia for 1 h revealed a comparable OER value regardless of HIF-1α expression. The results indicate that HIF-1α expression does not contribute primarily to the radiosensitivity of A549 cells under acute hypoxia. Further investigations should be conducted to elucidate the responsible proteins involved in acute hypoxia-induced radioresistance in tumor cells.

References

1. 

JM BrownWR WilsonExploiting tumour hypoxia in cancer treatmentNat Rev Cancer4437447200410.1038/nrc1367

2. 

RA GatenbyHB KesslerJS RosenblumLR CoiaPJ MoldofskyWH HartzOxygen distribution in squamous cell carcinoma metastases and its relationship to outcome of radiation therapyInt J Radiat Oncol Biol Phys14831838198810.1016/0360-3016(88)90002-83360652

3. 

RF KallmanThe phenomenon of reoxygenation and its implications for fractionated radiotherapyRadiology105135412197210.1148/105.1.1354506641

4. 

M NordsmarkM OvergaardJ OvergaardPretreatment oxygenation predicts radiation response in advanced squamous cell carcinoma of the head and neckRadiother Oncol413139199610.1016/S0167-8140(96)91811-38961365

5. 

P VaupelK SchlengerC KnoopM HöckelOxygenation of human tumors: evaluation of tissue oxygen distribution in breast cancers by computerized O2 tension measurementsCancer Res5133162219912040005

6. 

KA YehS BiadeRM LancianoDQ BrownMC FenningJS BabbPolarographic needle electrode measurements of oxygen in rat prostate carcinomasInt J Radiat Oncol Biol Phys33111118199510.1016/0360-3016(95)00036-X7642408

7. 

Y SuzukiT NakanoT OhnoS KatoY NiibeS MoritaOxygenated and reoxygenated tumors show better local control in radiation therapy for cervical cancerInt J Gynecol Cancer16306311200610.1111/j.1525-1438.2006.00341.x16445650

8. 

EJ HallAJ GiacciaOxygen effect and reoxygenationRadiobiology for the RadiologistL McAllister6th editionLippincott Williams & WilkinsPhiladelphia, PA851052006

9. 

RK JainNormalization of tumor vasculature: an emerging concept in antiangiogenic therapyScience3075862200510.1126/science.110481915637262

10. 

MJ TrotterDJ ChaplinRE DurandPL OliveThe use of fluorescent probes to identify regions of transient perfusion in murine tumorsInt J Radiat Oncol Biol Phys16931934198910.1016/0360-3016(89)90889-42703399

11. 

I TuftoEK RofstadTransient perfusion in human melanoma xenograftsBr J Cancer71789793199510.1038/bjc.1995.1537536022

12. 

LI Cárdenas-NaviaD MaceRA RichardsonDF WilsonS ShanMW DewhirstThe pervasive presence of fluctuating oxygenation in tumorsCancer Res6858125819200818632635

13. 

MC Brahimi-HornJ PouysségurHIF at a glanceJ Cell Sci12210551057200910.1242/jcs.03502219339544

14. 

EB RankinAJ GiacciaThe role of hypoxia-inducible factors in tumorigenesisCell Death Differ15678685200810.1038/cdd.2008.2118259193

15. 

LE HuangZ AranyDM LivingstonHF BunnActivation of hypoxia-inducible transcription factor depends primarily upon redox-sensitive stabilization of its alpha subunitJ Biol Chem2713225332259199610.1074/jbc.271.50.32253

16. 

T HoferI DesbailletsG HöpflM GassmannRH WengerDissecting hypoxia-dependent and hypoxia-independent steps in the HIF-1 alpha activation cascade: implications for HIF-1 alpha gene therapyFASEB J1527152717200111606485

17. 

D ChilovG CamenischI KvietikovaU ZieglerM GassmannRH WengerInduction and nuclear translocation of hypoxia- inducible factor-1 (HIF-1): heterodimerization with ARNT is not necessary for nuclear accumulation of HIF-1alphaJ Cell Sci11212031212199910085255

18. 

GL SemenzaHypoxia-inducible factor 1 and the molecular physiology of oxygen homeostasisJ Lab Clin Med131207214199810.1016/S0022-2143(98)90091-99523843

19. 

L XueguanW XiaoshenZ YongshengH ChaosuS ChunyingF YanHypoxia inducible factor-1alpha and vascular endothelial growth factor expression are associated with a poor prognosis in patients with nasopharyngeal carcinoma receiving radiotherapy with carbogen and nicotinamideClin Oncol20606612200810.1016/j.clon.2008.07.001

20. 

DM AebersoldP BurriKT BeerJ LaissueV DjonovRH GreinerExpression of hypoxia-inducible factor-1 alpha: a novel predictive and prognostic parameter in the radiotherapy of oropharyngeal cancerCancer Res6129112916200111306467

21. 

R BosH ZhongCF HanrahanEC MommersGL SemenzaHM PinedoLevels of hypoxia-inducible factor-1 alpha during breast carcinogenesisJ Natl Cancer Inst93309314200110.1093/jnci/93.4.30911181778

22. 

P BirnerM SchindlA ObermairC PlankG BreiteneckerG OberhuberOverexpression of hypoxia-inducible factor 1alpha is a marker for an unfavorable prognosis in early-stage invasive cervical cancerCancer Res6046934696200010987269

23. 

P BirnerB GatterbauerG OberhuberM SchindlK RösslerA ProdingerExpression of hypoxia-inducible factor-1 alpha in oligodendrogliomas: Its impact on prognosis and on neoangiogenesisCancer92165171200110.1002/1097-0142(20010701)92:1%3C165::AID-CNCR1305%3E3.0.CO;2-F11443623

24. 

Y FurusawaK FukutsuM AokiH ItsukaichiK Eguchi-KasaiH OharaInactivation of aerobic and hypoxic cells from three different cell lines by accelerated (3)He-, (12)C- and (20)Ne-ion beamsRadiat Res154485496200010.1667/0033-7587(2000)154%5B0485:IOAAHC%5D2.0.CO;211025645

25. 

D VordermarkA KatzerK BaierP KraftM FlentjeCell type-specific association of hypoxia-inducible factor-1 alpha (HIF-1 alpha) protein accumulation and radiobiologic tumor hypoxiaInt J Radiat Oncol Biol Phys5812421250200410.1016/j.ijrobp.2003.11.03015001269

26. 

ND ArvoldN GuhaD WangM MatliDF DeenRS WarrenHypoxia-induced radioresistance is independent of hypoxia-inducible factor-1 in vitroInt J Radiat Oncol Biol Phys62207212200510.1016/j.ijrobp.2005.01.01915850923

27. 

A StaabJ LoefflerHM SaidD DiehlmannA KatzerM BeyerEffects of HIF-1 inhibition by chetomin on hypoxia-related transcription and radiosensitivity in HT 1080 human fibrosarcoma cellsBMC Cancer7213220200710.1186/1471-2407-7-21317999771

28. 

H HaradaS Kizaka-KondohS ItasakaK ShibuyaA MorinibuK ShinomiyaThe combination of hypoxia-response enhancers and an oxygen-dependent proteolytic motif enables real-time imaging of absolute HIF-1 activity in tumor xenograftsBiochem Biophys Res Commun360791796200710.1016/j.bbrc.2007.06.14917624305

Related Articles

Journal Cover

January 2012
Volume 3 Issue 1

Print ISSN: 1792-0981
Online ISSN:1792-1015

Sign up for eToc alerts

Recommend to Library

Copy and paste a formatted citation
x
Spandidos Publications style
Oike T, Suzuki Y, Al-Jahdari W, Mobaraki A, Saitoh JI, Torikai K, Shirai K and Nakano T: Suppression of HIF-1α expression and radiation resistance in acute hypoxic conditions. Exp Ther Med 3: 141-145, 2012
APA
Oike, T., Suzuki, Y., Al-Jahdari, W., Mobaraki, A., Saitoh, J., Torikai, K. ... Nakano, T. (2012). Suppression of HIF-1α expression and radiation resistance in acute hypoxic conditions. Experimental and Therapeutic Medicine, 3, 141-145. https://doi.org/10.3892/etm.2011.373
MLA
Oike, T., Suzuki, Y., Al-Jahdari, W., Mobaraki, A., Saitoh, J., Torikai, K., Shirai, K., Nakano, T."Suppression of HIF-1α expression and radiation resistance in acute hypoxic conditions". Experimental and Therapeutic Medicine 3.1 (2012): 141-145.
Chicago
Oike, T., Suzuki, Y., Al-Jahdari, W., Mobaraki, A., Saitoh, J., Torikai, K., Shirai, K., Nakano, T."Suppression of HIF-1α expression and radiation resistance in acute hypoxic conditions". Experimental and Therapeutic Medicine 3, no. 1 (2012): 141-145. https://doi.org/10.3892/etm.2011.373