Stimulation by tumor necrosis factor of HL‐60 thymidine salvage pathway metabolism dissociated from proliferation

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Abstract
The effect of human tumor necrosis factor (TNF) on early-passage HL-60 cells was studied. A transient phase of increased [3H]thymidine (TdR) incorporation was noted at 20-24 hr of exposure to TNF. This increase was disproportionate to the much slighter stimulation of the percentage of S-phase cells, which was measured by flow cytometry. Evidence for increased metabolic trapping of [3H] TdR following TNF treatment was apparent from whole cell uptake experiments. The salvage pathway enzyme TdR kinase was therefore measured and was found to be elevated comparably to [3H]TdR uptake. The mechanism of TNF regulation of TdR kinase was further investigated by a series of combination treatment experiments using other biologic factors and pharmacologic inhibitors of various intracellular steps. The response to TNF was not potentiated or reproduced by IL-1, IL-2, IL-3, IL-4, G-CSF, M-CSF, GM-CSF or α- or γ-interferon. Blockers of early signal transduction steps, including H7, W7, sphingosine, and pertussis toxin, failed to inhibit TNF stimulation of [3H]TdR incorporation. mRNA synthesis inhibition with α-amanitin blocked this TNF effect, as did cAMP but not cGMP analogues. A sensitizing effect was noted with amiloride or cytochalasin B, characterized by greater relative increases of [3H]TdR incorporation and TdR kinase activity in response to TNF. In the presence of cytochalasin B, TNF treatment resulted in no change or slight decreases in the percentage of S-phase cells. Regulation of TdR kinase could thereby be dissociated from the usual cell cycle control. This study thus documents a unique example of stimulation of thymidine salvage pathway metabolism by a biologic factor, dissociable from overall cell cycle regulation.