5-Fluorouracil (5-FU) is a widely used chemotherapeutic agent that inhibits the growth and initiates the apoptosis of epithelial tumors, including squamous cell carcinoma of the head and neck region. However, resistance to this drug is often observed in a clinical setting. The primary mode of action of 5-FU is believed to be the inhibition of thymidylate synthase. Overexpression of the enzymes involved in thymidine synthesis has been shown in some cases to be associated with resistance. However, the detailed mechanisms of resistance of squamous cell carcinoma are not fully understood. In the present study, we examined the involvement of survival signaling pathways in the resistance of squamous carcinoma cells to 5-FU. 5-FU induced the activation of the ERK and Akt kinases in UM-SCC-23 human squamous carcinoma cells, indicating that this anticancer drug activates survival signaling pathways as well as apoptotic signals. In 5-FU-resistant UM-SCC-23 cells established by our group, ERK and Akt signals were constitutively activated. U0126 is an inhibitor of MEK, which is an upstream activator for ERK. U0126 failed to sensitize resistant UM-SCC-23 cells to 5-FU-induced apoptotic cell death. This is in sharp contrast to LY294002, which is an inhibitor of phosphatidylinositol 3-kinase, an upstream activator for Akt. LY294002 drastically enhanced 5-FU-induced apoptotic cell death in resistant UM-SCC-23 cells. These results indicate that the Akt survival signal plays an important role in the resistance of squamous carcinoma cells to 5-FU treatment, and suggest that the modification of Akt activity might provide a new strategy for human 5-FU-resistant squamous carcinoma therapy.

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