Imaging transcriptional regulation of p53-dependent genes with positron emission tomography in vivo

A noninvasive method for molecular imaging of the activity of different sig nal transduction pathways and the expression of different genes in vivo wou ld be of considerable value. It would aid in understanding the role specifi c genes and signal transduction pathways have in various diseases, and coul d elucidate temporal dynamics and regulation at different stages of disease and during various therapeutic interventions. We developed and assessed a method for monitoring the transcriptional activation of endogenous genes by positron-emission tomography (PET) imaging. The HSV1-tk/GFP (TKGFP) dual r eporter gene was used to monitor transcriptional activation of p53-dependen t genes. A retrovirus bearing the Cis-p53/TKGFP reporter system was constru cted in which the TKGFP reporter gene was placed under control of an artifi cial cis-acting p53-specific enhancer. U87 glioma and SaOS-2 osteosarcoma c ells were transduced with this retrovirus and used to establish xenografts in rats. We demonstrated that DNA damage-induced up-regulation of p53 trans criptional activity correlated with the expression of p53-dependent downstr eam genes, such as p21, in U87 (wild-type p53), but not in SaOS-2 osteosarc oma (p53 -/-) cells. We showed that PET, with [I-124]FIAU (2 ' -fluoro-2 ' -deoxy-1-beta -D-arabinofuranosyl-5-[I-124]iodouracil) and the Cis-p53TKGFP reporter system, is sufficiently sensitive to image the transcriptional re gulation of genes in the p53 signal transduction pathway. These imaging res ults were confirmed by independent measurements of p53 activity and the exp ression levels of downstream genes (e.g., p21} by using conventional molecu lar biological assays. PET imaging of p53 transcriptional activity in tumor xenografts by using the Cis-p53TKGFP reporter system may be useful in asse ssing novel therapeutic approaches.