Augmentation of antitumor activity of 5-fluorouracil by interferon α is associated with up-regulation of p27(Kip1) in human hepatocellular carcinoma cells

Several clinical trials have demonstrated the effectiveness of combination therapy with 5-fluorouracil (5-FU) and IFN-α in colon cancer, hepatocellular carcinoma (HCC), and other malignancies. In our preliminary clinical studies, we have observed outstanding effects with this combination therapy in patients with advanced HCC. However, the underlying mechanism by which IFN-α modulates the effects of 5-FU is unknown. We, therefore, conducted a mechanistic study using two HCC cell lines, PLC/PRF/5 and HuH7. IFN-α significantly enhanced the growth inhibitory effect of 5-FU in PLC/PRF/5 cells but not in HuH7 cells, and the isobolographic analysis indicated that this effect was synergistic. Flow cytometric analysis showed a delay in the progression of G₀-G₁ to S phase in PLC/PRF/5, and a sustained induction of the cyclin-dependent kinase inhibitor p27(Kip1) and down- regulation of cyclin D1 was observed. Moreover, increased expression of p27(Kip1) was associated with reduced CDK-2-associated kinase activity. Another difference in the two cell types was that PLC/PRF/5 expressed abundant IFN receptors, but HuH7 did not. Apoptosis assays were not helpful in explaining the mechanism. Our results suggest that the synergistic effects of 5-FU and IFN-α may in part be attributable to alterations in cell cycle progression via up-regulation of p27(Kip1).