Oxygen sensitivity of reporter genes: Implications for preclinical imaging of tumor hypoxia

Reporter gene techniques have been applied toward studying the physiologic phenomena associated with tumor hypoxia, a negative prognostic indicator. The purpose of this study was to assess the potential adverse effects of hypoxic conditions on the effectiveness of four commonly used reporter genes: Renilla luciferase, monomeric red fluorescent protein, thymidine kinase, and lacZ. Tumorforming A375 cells expressing a trifusion reporter consisting of Renilla luciferase, monomeric red fluorescent protein, and thymidine kinase were subjected to decreasing oxygen tensions and assayed for reporter expression and activity. A375 cells expressing ßgalactosidase were similarly exposed to hypoxia, with activity of the reporter monitored by cleavage of the fluorescent substrate 7hydroxy-9H-(1,3-dichloro-9,9-dimethylacridin-2-one)-β- galactoside (DDAOG). Generation of signal in in vivo tumor models expressing bioluminescent or β-galactosidase reporters were also examined over the course of hypoxic stresses, either by tumor clamping or the antivascular agent 5,6-dimethylxanthenone-4-acetic acid (DMXAA). Our findings indicate that bioluminescent and fluorescent reporter activity are decreased under hypoxia despite minimal variations in protein production, whereas β-galactosidase reporter activity per unit protein was unchanged. These results demonstrate that combining β-galactosidase with the DDAOG optical probe may be a robust reporter system for the in vivo study of tumor hypoxia.