Transcriptional Activation and Cell Cycle Block Are the Keys for 5-Fluorouracil Induced Up-Regulation of Human Thymidylate Synthase Expression

Background: 5-fluorouracil, a commonly used chemotherapeutic agent, up-regulates expression of human thymidylate synthase (hTS). Several different regulatory mechanisms have been proposed to mediate this up-regulation in distinct cell lines, but their specific contributions in a single cell line have not been investigated to date. We have established the relative contributions of these previously proposed regulatory mechanisms in the ovarian cancer cell line 2008 and the corresponding cisplatin-resistant and 5-FU cross-resistant-subline C13*. Methodology/Principal Findings: Using RNA polymerase II inhibitor DRB treated cell cultures, we showed that 70-80% of up-regulation of hTS results from transcriptional activation of TYMS mRNA. Moreover, we report that 5-FU compromises the cell cycle by blocking the 2008 and C13* cell lines in the S phase. As previous work has established that TYMS mRNA is synthesized in the S and G₁ phase and hTS is localized in the nuclei during S and G₂-M phase, the observed cell cycle changes are also expected to affect the intracellular regulation of hTS. Our data also suggest that the inhibition of the catalytic activity of hTS and the up-regulation of the hTS protein level are not causally linked, as the inactivated ternary complex, formed by hTS, deoxyuridine monophosphate and methylenetetrahydrofolate, was detected already 3 hours after 5-FU exposure, whereas substantial increase in global TS levels was detected only after 24 hours. Conclusions/Significance: Altogether, our data indicate that constitutive TYMS mRNA transcription, cell cycle-induced hTS regulation and hTS enzyme stability are the three key mechanisms responsible for 5-fluorouracil induced up-regulation of human thymidylate synthase expression in the two ovarian cancer cell lines studied. As these three independent regulatory phenomena occur in a precise order, our work provides a feasible rationale for earlier observed synergistic combinations of 5-FU with other drugs and may suggest novel therapeutic strategies.