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Visualization of Telomerase Reverse Transcriptase (hTERT) Promoter Activity Using a Trimodality Fusion Reporter Construct

¹ Department of Radiology and Molecular Imaging Program at Stanford (MIPS), Stanford University, Stanford, California; ² Department of Medicine, Veterans Affairs Palo Alto Health Care System, Palo Alto, California; and ³ Department of Radiology, University of Southern California, Los Angeles, California

Correspondence: For correspondence or reprints contact either of the following: Gary A. Ulaner, MD, PhD, Division of Nuclear Medicine, Department of Radiology, University of Southern California, 1200 N. State St., Room 5250, Los Angeles, CA 90033. E-mail: ulaner{at}usc.edu Sandip Biswal, MD, Department of Radiology, Stanford University School of Medicine, 300 Pasteur Dr., Room S-062B, Stanford, CA 94305. E-mail: biswals{at}stanford.edu

Our goal was to noninvasively measure chemotherapy-induced changes in the expression of critical tumor growth genes. To achieve this goal, we used radionuclide and optical methods to measure changes in human telomerase reverse transcriptase (hTERT) gene expression in tumor cells before and after 5-fluorouracil treatment. Methods: A fusion reporter construct, containing humanized Renilla luciferase (hrl, for bioluminescent imaging), monomeric red fluorescence protein 1 (mrfp1, for fluorescent imaging), and a truncated thymidine kinase (ttk, for imaging of radiolabeled acycloguanosines), was placed under the control of hTERT promoter fragments. These constructs were introduced into tumor cell lines with and without hTERT expression. Transfected cells were treated with 5-fluorouracil, a chemotherapeutic that decreases hTERT gene expression, and treatment-induced changes in hTERT promoter activity were imaged. Results: When the fusion construct is introduced into cell lines that express hTERT, all 3 reporter systems are highly expressed and hTERT promoter activity can be visualized. Cell lines lacking hTERT transcription show no significant reporter expression. Decreases in hTERT gene expression caused by 5-fluorouracil treatment could be visualized in living 293T cells by both fluorescent microscopy and bioluminescent imaging. Conclusion: hTERT promoter activity can be monitored by 1 radionuclide and 2 optical reporter systems using a single reporter construct. This in vitro study provides evidence that our multimodality reporter construct can be used to study the expression of a critical tumor growth gene in living subjects.

Key Words: molecular imaging • reporter genes • telomerase • telomerase reverse transcriptase

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