LRP1 receptor-mediated immunosuppression of α-MMC on monocytes

Alpha-MMC is a type I ribosome-inactivating protein purified from bitter gourd that has strong anti-tumour and antiviral activity. Alpha-MMC also has immunosuppressive effects, but the mechanism of these immunosuppressive effects remains unclear. It is reported that the binding of α-MMC to its specific cell membrane LRP1 receptor is key to its biological effects. In this study, we investigated the effect of α-MMC on cytotoxicity and cytokine release regulation in three immune cells, human monocyte THP-1 cells, B-lymphocyte WIL2 cells and T-lymphocyte H9 cells, and explored the correlation between this effect and LRP1 receptor distribution on these three cell types. We demonstrate that α-MMC has a significant effect of apoptosis induction and cytokine release in THP-1 cells but has no effect on WIL2-S and H9 cells. Specifically, at a non-cytotoxic dose (80 μg/ml), α-MMC regulates THP-1 cells by inhibiting IL-1β, IL-2, IL-8, IL-9, IL-12, MIP-1α/β, MCP-1 and TNF-α expression and enhancing IL-1ra and RANTES expression, resulting in the inhibition of cellular immune function. Subsequent experiments showed that the cytokine expression regulated by α-MMC can be blocked by silencing the LRP1 receptor of α-MMC. Further research indicated that phosphorylation of 9 signalling proteins of the MAPK pathway was significantly regulated by α-MMC and was blocked by LRP1 silencing. We conclude that the regulation of cytokine expression induced by α-MMC in monocyte THP-1 cells is mediated by the LRP1 receptor, likely via the MAPK signalling pathway. Our results suggest that the inhibition effect on monocytes/macrophages mediates the immunosuppressive function of α-MMC. Due to the selective cytotoxicity and cytokine release regulation of α-MMC in monocytes/macrophages, α-MMC may be used for killing Tumour-Associated Macrophages (M2 subtypes) or inhibiting their cytokine release in the tumour microenvironment.