Stunning and its effect on 3H-FDG uptake and key gene expression in breast cancer cells undergoing chemotherapy

Shortly after chemotherapy, relatively little is known about the expression of key genes and proteins involved in glycolysis. Doxorubicin (DOX) and 5-fluorouracil (5FU) are two commonly used chemotherapy agents that work through differing pathways. Glucose transporter-1 (Glut-1) and hexokinase II (HKII) proteins are highly expressed in many breast carcinomas, but their status while undergoing DOX or 5FU chemotherapy has not been systematically evaluated.

We evaluated, in vitro, the messenger RNA (mRNA) and protein levels of Glut-1 and HKII in MCF-7, a breast adenocarcinoma cell line, and (3)H-FDG uptake, both in untreated conditions and during treatment with either DOX or 5FU for 24 h. Six time points were evaluated: untreated at time 0; treated for 1 h; treated for 24 h; and 1, 2, and 3 d after chemotherapy. We analyzed tumor cell Glut-1 and HKII mRNA expression with real-time polymerase chain reaction and Western blotting, (3)H-FDG uptake per cell, and cell viability with a 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazoliumbromide assay.

DOX was more effective than 5FU in killing the cancer cells under our study conditions. In untreated MCF-7 cells, the mRNA levels of HKII were typically higher than those of Glut-1, and (3)H-FDG uptake was strongly related to Glut-1 mRNA expression (R(2) = 0.85). Once treated with either drug, (3)H-FDG uptake declined initially, the mRNA ratio was reversed, and Glut-1 mRNA levels were higher than HKII levels. This was verified in the protein assay. With DOX treatment, the cells showed increased Glut-1 mRNA and decreased HKII mRNA for the duration of active treatment; these levels returned to those seen in the untreated cells once the treatment was stopped for 24 h. However, HKII protein levels remained somewhat low. No correlation was seen between (3)H-FDG uptake and HKII mRNA in DOX- and 5FU-treated cells (R(2) = 0.14 and 0.0038, respectively).

After DOX or 5FU therapy, the relationship between (3)H-FDG uptake and viable cell number can become disjointed, with transient declines in (3)H-FDG uptake in excess of the decline in cell number despite increased Glut-1 mRNA levels. This transient "stunning" has potential implications for (3)H-FDG PET, especially soon after treatment is initiated. However, (3)H-FDG remains a generally valid marker of viable cell number after cancer chemotherapy.