J Nucl Med 2002 Sep;43(9):1210-7
Validation of FLT uptake as a measure of thymidine kinase-1 activity in A549
carcinoma cells.
Rasey JS, Grierson JR, Wiens LW, Kolb PD, Schwartz JL.
The thymidine analog (18)F-3'-deoxy-3' -fluorothymidine (FLT) is being used
clinically for PET imaging of tumor proliferation. Appropriate use of this
tracer requires validating the mechanisms by which it accumulates in dividing
cells. We tested the accuracy with which FLT uptake predicted the activity of
cytosolic thymidine kinase-1 (TK(1)), an enzyme that is upregulated before and
during DNA synthesis. METHODS: Cultured A549 human lung carcinoma cells were
manipulated to a range of proliferation rates from actively dividing to growth
arrested. Uptake of radiolabeled FLT was compared with cell cycle activity,
which was expressed as the percentage of cells in S phase, and with activity of
cytosolic TK(1). We also compared uptake of FLT and deoxyglucose. We genetically
manipulated A549 cells by transfecting them with human papillomavirus type 16 E6
(designated A549-E6) to abrogate function of the tumor suppressor gene, p53.
Although radiation typically inhibits progression of mammalian cells through the
cell cycle, abrogation of p53 function eliminates this inhibition. We then
compared FLT uptake with the percentage of cells in S phase and TK(1) activity
in irradiated A549-E6 cells and in irradiated control cells having normal p53
function and the expected radiation-induced growth delay. RESULTS: A549 cells
with only 3%-5% cells in S phase took up little FLT and had low levels of TK(1)
activity. When cells were stimulated to grow by being placed into fresh medium,
we observed a strong correlation between increased FLT uptake and increased
TK(1) activity. As expected, FLT uptake varied much more as a function of growth
than did uptake of deoxyglucose. Nonproliferating A549 cells did not enter the
cell cycle if they were irradiated before being placed into fresh medium, and
they did not accumulate FLT or show elevated TK(1) activity. In contrast,
radiation did not inhibit the cell cycle progression of A549-E6 cells. When
subcultured, they began to grow and showed increased uptake of FLT commensurate
with greater TK(1) activity. CONCLUSION: In cultured A549 cells FLT uptake is
positively correlated with cell growth and TK(1) activity. Inhibition of cell
cycle progression prevents FLT uptake and increased TK(1) activity. These
results suggest that FLT images reflect TK(1) activity and the percentage of
cells in S phase.
