Effect of Plant Lectin Viscumin on Macrophages Derived from THP-1 Cell Line

The plant lectin viscumin is a toxin belonging to the RIPII protein family. The A-chain of the toxin specifically hydrolyzes the N-glycosidic bond at the adenosine 4324 position in 28S ribosomal RNA, thereby halting protein synthesis in cells. The B-chain of the toxin binds specific glycoside residues on the cell surface and mediates endocytosis of the toxin into the cell. The cytotoxic effect of viscumin varies significantly between different cell lines, with cells of hematopoietic origin being the most sensitive. Therefore, the aim of this study is to investigate the cytotoxic effect of viscumin on proinflammatory macrophages obtained from the acute monocytic leukemia cell line THP-1. It was shown that 50% inhibition of macrophage viability, as measured by MTT, is achieved at a concentration of 4.3 nM viscumin, which is 3 orders of magnitude less than for resistant lines according to literature data. At the same time, it was shown that the hypersensitivity of macrophages to the toxin is not related to the number of viscumin binding sites on the cell surface. The number of binding sites on macrophages, estimated by flow cytometry, is no more than 2 million per cell, which is 16 times less than the number of binding sites for ricin (a related toxin of the RIPII family that uses the same glycoside residue for endocytosis) on RIPII-resistant cell lines. It has also been shown that after 6 hours of incubation with an excessive concentration of viscumin (100 nM), the proportion of intracellular toxin is 8% of the amount that entered the cell in the first 30 minutes. At the same time, this amount of toxin A-chains results in the inactivation of only 6.5% of the ribosomes detected by real-time PCR, and the proportion of viable cells is less than 20%. However, literature data show that inactivation of 20% of ribosomes in the viscumin-resistant Caco-2 cell line does not result in a significant decrease in viability. These results suggest that cellular mechanisms other than translational arrest are the major contributors to the increased sensitivity of cells to the toxin.