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Cell Cycle Control Processes Determine Cytostasis or Cytotoxicity in Thymineless Death of Colon Cancer Cells¹

Department of Molecular Pharmacology, St. Jude Children's Research Hospital, Memphis, Tennessee 38101

Thymidylate synthase (TS) is a target of critical importance to the survival of human colon cancer cells since, upon inhibition, cells subsequently undergo thymineless death induced by dTTP deficiency. Using genetically marked mutants deficient in TS (TS^-) and a derived population (Thy4) that is resistant to commitment to thymineless death, resistance was conferred by the ability of cells to arrest at a point either in late G₁ or at the onset of S induced by dThd deprivation. Thus, Thy4 cells initially synchronized in G₀ by leucine deprivation and released in the absence of dThd remained viable at 5 days, demonstrated delayed onset of nucleosomal ladder formation, and retained clonogenic potential (cytostatic response). In contrast, TS^- and asynchronous Thy4 cells lost 50% clonogenic potential in 65 h and >90% in 5 days (cytotoxic response). [³H]DNA precursor studies indicated failure of synchronized Thy4 but not TS^- cells to progress through S, with arrest of Thy4 close to the G₁/S boundary. Cell cycle control processes including: (a) the locus of dThd deprivation in G₁; and (b) a potential checkpoint close to the G₁/S border, may dictate whether consequences of dThd or dTTP restriction become cytostatic or cytotoxic.

¹ Supported by National Cancer Institute Grants CA32613 and CA21765 (Cancer Center Support Grant) and by the American Lebanese Syrian Associated Charities.

² To whom requests for reprints should be addressed.

Received 5/10/94. Accepted 7/12/94.

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