YKL-40 downregulation is a key factor to overcome temozolomide resistance in a glioblastoma cell line

The frequent recurrence of glioblastoma multiforme (GBM) after standard treatment with temozolomide (TMZ) is a crucial issue to be solved in the clinical field. O-6-methylguanine-DNA methyltransferase (MGMT) is considered one of the major mechanisms involved in TMZ resistance. However, some important mechanisms for TMZ resistance other than MGMT have recently been identified. In the present study, we established a TMZ-resistant (TMZ-R) U87 glioblastoma cell line in vitro and in vivo and investigated novel targeting molecules other than MGMT in those cells. The TMZ-R U87 glioblastoma cell line was established in vitro and in vivo. TMZ-R U87 cells showed a more invasive activity and a shorter survival time in vivo. Gene expression analysis using DNA microarray and quantitative PCR (qPCR) demonstrated that YKL-40, MAGEC1 and MGMT mRNA expression was upregulated 100-, 83- and 6-fold, respectively in the TMZ-R U87 cell line. Western blot analysis and qPCR demonstrated that STAT3 phosphorylation, STAT3 target genes and stem cell and mesenchymal marker genes were upregulated to a greater extent in the TMZ-resistant cell line. Notably, short hairpin (sh)RNA-based inhibition against the YKL-40 gene resulted in moderate growth inhibition in the resistant cells in vitro and in vivo. Additionally, YKL-40 gene inhibition exhibited significant suppression of the invasive activity and particularly partially restored the sensitivity to TMZ. Therefore, YKL-40 may be a novel key molecule in addition to MGMT, that is responsible for TMZ resistance in glioblastoma cell lines and could be a new target to overcome TMZ resistance in recurrent glioblastomas in the future.