Glioblastoma multiforme (GBM) is the most common primary central nervous system tumor. Even with standard and aggressive treatment strategies the 5-year survival rate is less than 30%. Also, previous results reveal that glutathione S-transferase M3 (GSTM3) is highly expressed in brain tissue and represents the predominant activity of GSTs, a group of enzymes that mainly facilitates detoxification, in the human brain. The compound studied here, ME-E5, is extracted from cultured Vibrio psychroerythrus, a marine bacterial species.
In this study, the cytotoxic activity of MB-E5 in U87MG and GBM8401 glioblastoma cell lines were investigated using MTT assay and TUNEL assay. Tumor sphere assay were performed to determine the impact of MB-E5 on in vitro neuron cancer stem cell growth. Immunofluorescence assay and western blotting were used to evaluate the level of autophagy-related proteins and the PI3K/Akt pathway. gstm3 gene expression levels were measured in TMZ-resistant glioblastoma cell line by semi-quantitative polymerase chain reaction (PCR).
MTT assays show that MB-E5 exhibits higher cytotoxic activity than Temozolomide (TMZ), a first-line drug in the treatment of gliomas, in GBM cell lines. Neurosphere formation is significantly reduced at low MB-E5 levels. TUNEL assay and western blot analysis show that MB-E5 induces apoptosis and reduces glioblastoma cell survival rate by inhibiting the PI3K/AKT/mTOR signaling pathway. Also, immunofluorescence assay and western blotting results indicate that MB-E5 promotes the expression of autophagy marker LC3-II. Furthermore, MB-E5 demonstrates higher cytotoxicity against TMZ resistant-GBM8401 cells. The results of semi-quantitative PCR indicate a decrease in the expression of gstm3 after 3 days of 200 µM TMZ treatment, but the expression level recovers after 15 days of continuous TMZ treatment in the survival population of GBM cell lines. Moreover, GSTM3 mRNA expression is reduced after MB-E5 treatment.
These results suggest that the marine-derived bioactive compound MB-E5 may be effective as a cancer therapeutic agent in glioblastoma. We also theorize that MB-E5 may reduce gstm3, which could play a key role in TMZ-resistant glioblastoma cells.
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Legal entity responsible for the study
Doctoral Degree Program in Marine Biotechnology, National Sun Yat-sen University, Taiwan
Kaohsiung Chang Gung Memorial Hospital and Chang Gung University College of Medicine, Kaohsiung, Taiwan
All authors have declared no conflicts of interest.