Enhanced cytotoxicity with interleukin-1α and 5-fluorouracil in HCT116 colon cancer cells

Recombinant human interleukin-1α (rIL-1α), at concentrations that were not growth-inhibitory when given alone (100-10,000 U/ml), enhanced the growth inhibition resulting from a 72-h fluorouracil (FUra) exposure in HCT116 colon cancer cells. Median-effect analysis of clonogenic assays indicated that rIL- 1α, given 24 h prior to and following a 24-h exposure to FUra, increased lethality in a more than additive fashion. rIL-1α did not appear to significantly affect [³H]-FUra metabolism, total [³H]-FUra-RNA incorporation or RNA retention after drug removal, inhibition of thymidylate synthase, or thymidine triphosphate pool depletion. During continuous exposure to rIL-1α, transient stimulation of RNA and DNA synthesis was observed at 72 h, with a return to normal by 96 h. A 24-h exposure to 10 μM FUra altered the elution profile of newly synthesized DNA as monitored by pH step alkaline elution. An accumulation of lower-MW single-stranded DNA species was noted with FUra compared to control, accompanied by a significantly decreased proportion of DNA retained on the polycarbonate filter: 10% retained vs. 32% for control (P = 0.01). A 48-h exposure to rIL- 1α alone did not affect the elution profile of nascent DNA species, nor did it enhance the effects of FUra. Although FUra did not appreciably affect pulse [³H]-uridine incorporation into RNA for the initial 8-24 h of FUra exposure, progressive inhibition of net RNA synthesis was observed thereafter. FUra prevented the stimulatory effect of rIL-1α on RNA synthesis, and net RNA synthesis was significantly inhibited (by 64-79% after 72 and 96 h) with the combination compared to rIL-1α alone. Continuous exposure to 10 μM thymidine did not rescue cells from the lethality of FUra alone or the combination of FUra plus rIL-1α suggesting that depletion of deoxythymidine triphosphate as a consequence of thymidylate synthase inhibition was not the most important component of FUra toxicity. In contrast, 1 mM uridine provided partial protection against the toxicity of FUra alone or with rIL-1α. Although uridine did not affect FUra metabolism, it decreased FUra-RNA incorporation by 42-60%, presumably as a consequence of the 2-fold expansion of UTP pools. [¹²⁵I]-rIL-1α binding was nonspecific; with a 24-h exposure, however, internalized [¹²⁵I]-rIL-1α exceeded cell surface-bound material by 2-fold. Concurrent exposure to 10 μM FUra increased the binding and internalization of [¹²⁵I]-rIL-1α at 37°C by 2.7-fold. While the precise mechanism of interaction between FUra and IL-1α is unclear, the data suggest that RNA-directed cytotoxicity by FUra is contributory.