Chemotherapeutic agents up-regulate the cytomegalovirus promoter: implications for bioluminescence imaging of tumor response to therapy.

Bioluminescence imaging is widely used to evaluate tumor growth and response to therapy in living animals. In cells expressing luciferase under the control of a constitutive promoter, light output in part depends on viable cell number, so that changes in bioluminescence intensity may be correlated with changes in viable tumor mass over time. We have found that treatment of cancer cell lines expressing luciferase under control of the cytomegalovirus (CMV) promoter with staurosporine, doxorubicin, and paclitaxel results in a transient increase in bioluminescence, which is positively correlated with apoptosis and inversely correlated with cell viability. In contrast, similar treatment of cell lines expressing luciferase under control of the SV40 promoter did not exhibit this result. We found that low doses of staurosporine induced bioluminescence in CMV- but not SV40-driven luciferase cell lines, whereas high doses elicited induction in both, indicating promoter-dependent and promoter-independent mechanisms of bioluminescence induction. The promoter-dependent increase in bioluminescence intensity from CMV-driven luciferase is a result of induction of luciferase mRNA and protein expression. We extended these findings in vivo; doxorubicin treatment resulted in a transient induction in bioluminescence when normalized to tumor volume in CMV- but not SV40-driven luciferase-expressing xenografts. We found that inhibition of the p38 mitogen-activated protein kinase pathway blocked bioluminescence induction by doxorubicin, paclitaxel, and staurosporine in CMV-driven luciferase-expressing cells. These findings have important implications when using bioluminescence to monitor the efficacy of anticancer therapy and underscore the complex regulation of the CMV promoter, which is widely used for high-level protein expression in mammalian cells.