Inhibition of STAT3 reverses drug resistance acquired in temozolomide-resistant human glioma cells

The alkylating agent temozolomide (TMZ) is an effective drug used for the treatment of malignant gliomas. However, tumor relapse combined with the development of drug resistance remains a significant problem. To clarify the mechanism of the resistance of glioma cells to TMZ chemotherapy, TMZ-resistant glioma cell lines (TR cells) were generated using U373 and U251 human glioma cells, and TMZ-resistance was confirmed via viability and apoptosis assays. The TMZ-resistance of TR cells was not associated with the TMZ-resistance molecule O(6)-methylguanine-DNA-methyltransferase. Notably, the expression level of signal transducers and activators of transcription 3 (STAT3) and serine 727-phosphorylated STAT3 (pSTAT3-Ser727) was highly increased in TR cells, while that of 705-phosphorylated STAT3 (pSTAT3-Tyr705) was decreased. The inhibition of STAT3 expression by small interfering RNA enhanced TR cell TMZ sensitivity. These results suggest that STAT3 contributes to TMZ-resistance in gliomas and is a potential target for the reversal of TMZ-resistance in patients with a recurrent glioma.