Abstract
Morniga-G, the Gal-specific black mulberry (Morus nigra) lectin, displays high affinity for T (CD176) and Tn (CD175) antigens, frequently expressed at the cancer cell surface. The effects of Morniga-G were investigated on a Tn-positive leukemic Jurkat cell line. The lectin, used in a concentration range between 5–20 μg/mL, induced cell death in leukemic Jurkat cells. Microscopic and cytofluorometric analyses indicated that Jurkat cell death was essentially apoptotic, associated with an increase in the ceramide content and a depolarization of the mitochondrial transmembrane potential. This lectin-mediated cell death was inhibited by the pan caspase-inhibitor zVAD. In addition, cleavage of caspases 8, 9, and 3 was observed in Morniga-G-treated Jurkat cells whereas Jurkat cell lines that are deficient in caspase 8–10, caspase 9, or FADD, survived to the lectin-mediated toxicity. Furthermore, in the presence of TRAIL- or DR5-blocking mononoclonal antibodies, Jurkat cells became resistant to Morniga-G, suggesting that the lectin triggers cell death via the TRAIL/DR5 pathway. In silico computer simulations suggest that Morniga-G might facilitate both the DR5 dimerization and the building of TRAIL/DR5 complexes. Finally, upon treatment of Jurkat cells with benzyl-GalNAc, an O-glycosylation inhibitor, a decrease in Tn antigen expression associating with a reduced Morniga-G toxicity, was observed. Taken together, these results suggest that Morniga-G induces the cell death of Tn-positive leukemic cells via concomitant O-glycosylation-, caspase-, and TRAIL/DR5-dependent pathways.
Highlights
Glycosylation is known to contribute to different recognition and activation cell events and to the cell death processes, altogether occurring during normal functioning of the cellular immune system [1,2,3,4]
Plant lectins with different monosaccharide-binding specificities, e.g., the Man/Glc-specific Concanavalin A (Con A) and the GlcNAc-specific pokeweed (Phytolacca americana) mitogen pokeweed mitogen (PWM), are known for a long time to readily activate resting lymphocytes
The in vitro effect of Gal/GalNAc/Tn-specific lectin Morniga-G was evaluated on resting human lymphocytes, and results were compared to Con A and Morniga-M, the Man-specific lectin from Morus nigra
Summary
Glycosylation is known to contribute to different recognition and activation cell events and to the cell death processes, altogether occurring during normal functioning of the cellular immune system [1,2,3,4]. Previous results demonstrated that plant lectins displaying very similar monosaccharide-binding specificity and three-dimensional structure differ in their capacity to recognize subtle alterations in the glycosylation induced by the lymphocyte activation process [5]. In this respect, two closely structurally-related Man-specific lectins, artocarpin from. Tn and T consist of the first and second step in the O-glycosylation biosynthesis pathway, respectively, just before a further elongation giving longer Gal/GalNAc-containing O-glycans Both types of tumor-associated carbohydrate antigens (TACAs) have been identified in 70–90% of colon, stomach, bladder, lung, ovary, prostate, and cervix cancers whereas they have not been, or only slightly, expressed in healthy tissues and organs [9,10,12]. Morniga-G, the Gal/GalNAc-specific lectin from black mulberry, displays high affinity for both T and Tn antigens, in cell-free systems [13]
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