A preclinical model for noninvasive imaging of hypoxia-induced gene expression; comparison with an exogenous marker of tumor hypoxia.
OBJECTIVE
Hypoxia is associated with tumor aggressiveness and is an important cause of resistance to radiation therapy and chemotherapy. Assays of tumor hypoxia could provide selection tools for hypoxia-modifying treatments. The purpose of this study was to develop and characterize a rodent tumor model with a reporter gene construct that would be transactivated by the hypoxia-inducible molecular switch, i.e., the upregulation of HIF-1.


METHODS
The reporter gene construct is the herpes simplex virus 1-thymidine kinase (HSV1-tk) fused with the enhanced green fluorescent protein (eGFP) under the regulation of an artificial hypoxia-responsive enhancer/promoter. In this model, tumor hypoxia would up-regulate HIF-1, and through the hypoxia-responsive promoter transactivate the HSV1-tkeGFP fusion gene. The expression of this reporter gene can be assessed with the 124I-labeled reporter substrate 2'-fluoro-2'-deoxy-1-beta-D-arabinofuranosyl-5-iodouracil (124I-FIAU), which is phosphorylated by the HSV1-tk enzyme and trapped in the hypoxic cells. Animal positron emission tomography (microPET) and phosphor plate imaging (PPI) were used in this study to visualize the trapped 124I-FIAU, providing a distribution of the hypoxia-induced molecular events. The distribution of 124I-FIAU was also compared with that of an exogenous hypoxic cell marker, 18F-fluoromisonidazole (FMISO).


RESULTS
Our results showed that 124I-FIAU microPET imaging of the hypoxia-induced reporter gene expression is feasible, and that the intratumoral distributions of 124I-FIAU and 18F-FMISO are similar. In tumor sections, detailed radioactivity distributions were obtained with PPI which also showed similarity between 124I-FIAU and 18F-FMISO.


CONCLUSIONS
This reporter system is sufficiently sensitive to detect hypoxia-induced transcriptional activation by noninvasive imaging and might provide a valuable tool in studying tumor hypoxia and in validating existing and future exogenous markers for tumor hypoxia.
Wen
Bixiu
B
Department of Medical Physics, Memorial Sloan-Kettering Cancer Center, 1275 York Avenue, NY, New York 10021, USA.
Burgman
Paul
P
Zanzonico
Pat
P
O'donoghue
Joseph
J
Cai
Shangde
S
Finn
Ron
R
Serganova
Inna
I
Blasberg
Ronald
R
Gelovani
Juri
J
Li
Gloria C
GC
Ling
C Clifton
CC
D000230:Adenocarcinoma / Q000000981:diagnostic imaging* / Q000235:genetics / Q000378:metabolism*; D000818:Animals; D001086:Arabinofuranosyluracil / Q000031:analogs & derivatives* / Q000493:pharmacokinetics*; D014408:Biomarkers, Tumor / Q000378:metabolism*; D015687:Cell Hypoxia; D045744:Cell Line, Tumor; D004268:DNA-Binding Proteins / Q000378:metabolism*; D004195:Disease Models, Animal; D004353:Drug Evaluation, Preclinical; D005240:Feasibility Studies; D005462:Fluorine Radioisotopes / Q000493:pharmacokinetics; D020869:Gene Expression Profiling / Q000379:methods*; D051793:Hypoxia-Inducible Factor 1; D051795:Hypoxia-Inducible Factor 1, alpha Subunit; D007457:Iodine Radioisotopes / Q000493:pharmacokinetics; D008920:Misonidazole / Q000031:analogs & derivatives* / Q000493:pharmacokinetics; D009687:Nuclear Proteins / Q000378:metabolism*; D011877:Radionuclide Imaging; D019275:Radiopharmaceuticals / Q000493:pharmacokinetics; D051381:Rats; D013937:Thymidine Kinase / Q000235:genetics / Q000378:metabolism; D014157:Transcription Factors / Q000378:metabolism*; D014764:Viral Proteins / Q000235:genetics / Q000378:metabolism
D014408:Biomarkers, Tumor; D004268:DNA-Binding Proteins; D005462:Fluorine Radioisotopes; C497444:Hif1a protein, rat; D051793:Hypoxia-Inducible Factor 1; D051795:Hypoxia-Inducible Factor 1, alpha Subunit; D007457:Iodine Radioisotopes; D009687:Nuclear Proteins; D019275:Radiopharmaceuticals; D014157:Transcription Factors; D014764:Viral Proteins; C031843:fluoromisonidazole; D001086:Arabinofuranosyluracil; C043457:fialuridine; D008920:Misonidazole; C099241:thymidine kinase, Canid herpesvirus 1; D013937:Thymidine Kinase