Inhibition of the mammalian transcription factor LSF induces S-phase-dependent apoptosis by downregulating thymidylate synthase expression

The thymidylate synthase (TS) gene, which is induced at the G(1)-S transiti on in growth-stimulated cells, encodes an enzyme that is essential for DNA replication and cell survival. Here we demonstrate that LSF (LBP-1c, CP2) b inds to sites within the TS promoter and intronic regions that are required for this induction. Mutation of the LSF binding sites inhibits G(1)-S indu ction of mRNA derived from a TS minigene, Furthermore, expression of domina nt-negative LSF (LSFdn) prevents the increase in TS enzyme levels during G( 1)-S, and induces apoptosis in growth-stimulated mouse and human cell lines . Such apoptosis can be prevented either by circumventing the TS requiremen t through addition of low concentrations of thymidine, or by coexpression o f the TS gene driven by a heterologous promoter. Induction of apoptosis by LSFdn parallels the process known as thymineless death, which is induced by the TS inhibitor and chemotherapeutic drug 5-fluorodeoxyuridine. Thus, LSF is a novel regulatory factor that supports progression through S-phase by targeting a single gene that is critical for cell survival.