5-Aminolevulinic Acid Suppresses Prostaglandin E2 Production by Murine Macrophages and Enhances Macrophage Cytotoxicity Against Glioma

Background: 5-Aminolevulinic acid (5-ALA) induces the accumulation of a large amount of protoporphyrin IX (PpIX) in tumors, which has been used in the treatment of several cancers. 5-ALA is commonly used for fluorescence-guided tumor resection in clinical neurosurgery and for photodynamic therapy based on the generation of cytotoxic oxygen.

Objective: The purpose of this study was to identify the mechanisms of 5-ALA-induced immune response in macrophages in malignant glioma.

Methods: Intracellular levels of 5-ALA-induced PpIX in C3H/HeN murine peritoneal macrophages were measured by the median fluorescence intensity using flow cytometry and confocal laser scanning microscopy. Macrophages were cultured in vitro with or without 0.5 mM 5-ALA, 0.1 μg/mL lipopolysaccharide, and 20% glioma-conditioned medium. Levels of immunosuppressive prostaglandin E₂ (PGE₂), interleukin-10, and transforming growth factor β were measured using enzyme immunoassay in the culture supernatant. In addition, macrophages and RSV-M mouse glioma cells were co-cultured in vitro with cell culture inserts with or without 5-ALA (0.1 and 0.5 mM) and lipopolysaccharide (0.1 μg/mL).

Results: We found that 5-ALA-induced PpIX accumulated in macrophages and significantly suppressed PGE₂ production and expression of both cyclooxygenase-2 and microsomal prostaglandin E synthase-1. 5-ALA treatment also suppressed PGE₂ production by glioma-conditioned medium. 5-ALA suppressed RSV-M glioma cell proliferation in a concentration-dependent manner.

Conclusions: These results indicate that 5-ALA suppressed PGE₂ production by macrophages via the downregulation of cyclooxygenase-2 and microsomal prostaglandin E synthase-1 expression levels. This is a novel mechanism to induce effective immune response against glioma in macrophages.