Imaging molecular pathways: reporter genes

Abstract

Molecular imaging is a rapidly advancing field that allows cancer biologists to look deeper into the complex inner workings of tumor cells, or whole tumors, in a non-invasive manner. In this review, we will summarize some recent advances that enable investigators to study various important biological processes in tumors in vivo. We will discuss novel imaging approaches that allow investigators to visualize and quantify molecular pathways, such as receptor tyrosine kinase activation, hypoxia signal transduction, apoptosis, and DNA double-strand breaks. Select examples of these applications will be discussed. Because of the limited scope of this review, we will only focus on natural reporters, such as bioluminescence and fluorescent proteins.

FIG. 1

Imaging EGFR activation through the use of the split-luciferase system. Panel A: Graphic illustration of the principle of a split-luciferase EGFR-Shc activity reporter. Panel B: The dose-response curve for the EGFR/Shc-luc reporter and the Grb2/Shc-luc reporter. The top panel shows representative images of reporter-transduced cells (in 48-well plates) treated with different concentrations of EGF at 37°C 15 min and then imaged in the IVIS200 instrument, while the lower panel shows the quantitative dose-response of the reporter activation after EGF addition. The error bars represent standard deviations derived from 3 to 5 data points. Panel C: Radiotherapy-induced activation of EGFR/Shc-luc (broken lines) and Grb2/Shc-luc (solid lines) reporters in H322 xenograft tumors. After reporter-transduced H322 lung tumor cell (5 × 10⁶) implantation (subcutaneous) and tumor formation (with diameters around 5 to 7 mm), the tumors were irradiated with X rays (6 Gy). The activities of the EGFR were then imaged.

FIG. 2

Imaging caspase activation during radiotherapy of cancer. Caspase 3 activation in 4T1 tumors as detected by a caspase 3 reporter. Left panels depict the structure of a proteasome-based caspase 3 reporter (top left) and its principle of action (lower left). Right panels showed caspase 3 activities in 4T1 tumors transduced with the control, as well as caspase reporter genes. The difference between the control and caspase 3 reporter groups are significant at days 3, 5, and 7 (P < 0.01, n = 5, t test). Error bars, SEM.