The immunomodulatory beta-galactoside-specific lectin from mistletoe: partial sequence analysis, cell and tissue binding, and impact on intracellular biosignalling of monocytic leukemia cells

Journal/Book: Anticancer-Res 12 (3), 669-675. 1992;

Abstract: Nanogram quantities of the beta-galactoside-specific lectin frommistletoe (ML-I) that is composed of two different types of subunitsexhibit immunomodulatory potency and enhance cytokine secretion in vitroand in vivo. Partial sequence analysis of the carbohydrate-binding Bchain revealed a ragged N-terminus and overall homologies to the Bsubunit of Ricin D and Ricin E. Two evolutionarily neutral substitutionswere apparent in the otherwise identical N-terminal sequences of the twotoxic chains within the lectin preparation. On the basis of theinfluence of chemical modification by group-specific reagents on ligandbinding, the lectin was biotinylated with biotinyl-N-hydroxysuccinimideester to allow monitoring of cell binding. Monocytic leukemia cells(THP-1) specifically bound the lectin with positive cooperativity at lowlectin concentrations. Radiolabelled lectin could be found in severalorgans and in an experimental solid tumor in biodistribution in mice.Its presence in a notable amount in spleens is especially noteworthywith respect to the already reported immunomodulation. To determineintracellular responses that precede the lectin-dependent augmentationof cytokine secretion, phosphorylation of proteins and phospholipids aswell as Ca(2+)-mobilization were assessed in THP-1 cells. Quantitativeincreases of (32P)-phosphate incorporation were determined for a 28 kDaprotein and for phosphatidylinositol-4,5-biphosphate. Similarly, thefluorescence activity of the intracellular Ca(2+)-indicator fluo-3 iselevated by approximately 25% after lectin treatment. Apparently, cellbinding of the lectin is followed by modulation of biosignallingprocesses. Author.

Keyword(s): AMINO-ACID-SEQUENCE

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