The environment of increased concentration of docosahexaenoic acid in glioblastoma may suppress the anti-tumor effect of macrophages

Regarding glioblastoma, there has been controversy over whether a large number of infiltrating macrophages act as anti-tumor effectors or not. It has been exhibited that intratumoral lipid environments have a possible influence on anti-tumor immunity. Necrosis of glioblastoma and non-necrotic tissues of astrocytic tumors were analyzed to compare the amount of free docosahexaenoic acid (DHA) content. The apoptosis inducible effects of DHA on macrophages derived from peripheral blood monocytes and cultured glioma cells were evaluated by flow cytometry. The influence of DHA on the anti-tumor effects of macrophages was assessed at 0, 30, 60, and 90 mM of DHA by 51Cr releasing assay and MTT assay. The mean concentration of free DHA in necrotic tissues (757.6 mmol/kg) was 5 times higher than that in non-necrotic tissues (147.2 mmol/kg). The DHA concentration of 30 mM induced apoptosis in macrophages, however, glioma cells were not affected even at a DHA concentration of 60 mM. Macrophages pre-exposed to DHA for 24 h decreased the cytotoxicity to U251MG cells as shown by 51Cr releasing assay. Total viability of co-cultured macrophages and U251MG cells showed an increase at high concentrations of DHA (60 and 90 mM) according to 24 h MTT assay, although each separate culture did not. The DHA concentration in necrosis of glioblastoma was sufficient for macrophages to cause apoptosis and suppress their anti-tumor effects. The results suggest that liberated DHA in necrosis can induce apoptosis in macrophages and inhibit their functions.