Etoposide increases equilibrative nucleoside transporter 1 activity and fluorothymidine uptake: screening of 60 cytotoxic agents.

Equilibrative nucleoside transporter 1 (ENT1) is a major regulator for the uptake of [(18)F]fluorothymidine ([(18)F]FLT), a promising positron emission tomography tracer for treatment monitoring. Various antimetabolites such as 5-fluorouracil often increase ENT1 activity and [(18)F]FLT uptake (flare) in spite of cell death. However, it has not yet been defined which agents induce [(18)F]FLT flare and what is the role of [(18)F]FLT flare in cell viability. Sixty cytotoxic agents from the LOPAC1280 library were screened for ENT1 activity in HeLa cells which predominantly express ENT1, and topoisomerase inhibitors (i.e., aurintricarboxylic acid, idarubicin, camptothecin and etoposide) were identified as potent inducers of ENT1 activity. The changes in ENT1 activity were closely correlated with [(3)H]FLT uptake (Spearman's correlation coefficient, r=0.66; P<0.01). Etoposide significantly increased ENT1 activity and [(3)H]FLT uptake accompanying cell cycle arrest at S/G2/M phase and the increase in TK1 expression and activity in both ENT1-low expressing HT29 and ENT1-high expressing MDA-MB-231 cells. The inhibition of ENT1 activity by dipyridamol or S-(p-nitrobenzyl)-6-thioinosine repressed the etoposide-induced cell death in HeLa cells, whereas it induced no changes in the other cell lines. In conclusion, etoposide is identified as a potent inducer for ENT1 activity and [(3)H]FLT uptake. The role of ENT1 activity by etoposide was cell-type dependent, which requests caution for the application of ENT1-mediated [(18)F]FLT flare for treatment monitoring.