MELATONIN ENHANCES TEMOZOLOMIDE-INDUCED APOPTOSIS IN GLIOBLASTOMA AND NEUROBLASTOMA CELLS.

Background: The combination of temozolomide (TMZ) and paclitaxel (PTX) is the most commonly used chemotherapy regimen for glioblastoma, but there is no specific treatment for neuroblastoma due to the acquired multidrug resistance. Approximately half of treated glioblastoma patients develop resistance to TMZ and experience serious side effects. Melatonin (MEL), a multifunctional hormone long known for its antitumor effects, has a great advantage in combination cancer therapy thanks to its ability to affect tumors differently than normal cells.

Aim: This study aims to evaluate the in vitro inhibitory effects of MEL in combination with TMZ on cancer cell viability and to elucidate the underlying mechanisms in the glioblastoma and neuroblastoma cell lines.

Materials and methods: C6 (Rattus norvegicus) and N1E-115 (Mus musculus) cancer cell lines and C8-D1A (mice) healthy cell lines were used. Cell proliferation was evaluated using the MTT test. IC50 values were determined by probit analysis. Two concentrations of TMZ (IC50 and 1/2 IC50) were used to induce cytotoxicity in the C6 and N1E-115 cell lines, both alone and in combination with PXT and MEL (all at IC50). The viable, dead, and apoptotic cells were determined by image-based cytometry using Annexin V/PI staining. The gene expression related to signaling pathways was assessed by the quantitative reverse transcription polymerase chain reaction (qRT-PCR), and key proteins were identified by the Western blot analysis.

Results: MTT assay showed that the combination of TMZ and MEL significantly reduces the viability of both glioblastoma and neuroblastoma cells compared to the vehicle-treated controls. Notably, MEL combined with 1/2 IC50 TMZ showed a significant death rate of cancer cells compared to controls and PTX. According to qRT-PCR data, the TMZ + MEL combination resulted in the upregulation of the genes of antioxidative enzymes (Sod1 and Sod2) and DNA repair genes (Mlh1, Exo1, and Rad18) in both cell lines. Moreover, the levels of Nfkb1 and Pik3cg were significantly reduced following the TMZ + MEL treatment. The combination of MEL with TMZ also enhanced the cell cycle arrest and increased the expression of p53 and pro-apoptotic proteins (Bax and caspase-3), while significantly decreasing the expression of anti-apoptotic protein Bcl-2.

Conclusions: Our findings indicate that the combination of MEL with a low dose of TMZ may serve as an upstream inducer of apoptosis. This suggests the potential development of a novel selective therapeutic strategy as an alternative to TMZ for the treatment of both glioblastoma and neuroblastoma.