A Sialic Acid-binding Lectin (SBL) Dependent Apoptosis is Triggered by Sialylated-glycoconjugates in GEM of P388 Cells

Abstract

SBL was originally isolated from frog (Rana catesbeiana) oocytes as a novel sialic acid-binding lectin (SBL) that displayed strong anti-cancer activity. SBL was later shown to be identical to a ribonuclease from oocytes of the same species. The administration of SBL induced apoptosis in mouse leukemia P388 cells but did not kill umbilical vein endothelial or fibroblast cells derived from normal tissues. The cytotoxicity of SBL was inhibited by desialylation of P388 cells and the co-presence of free bovine submaxillary mucin and heparan sulphate. SBL was observed to be incorporated into cells after attachment to cholesterol-rich microdomains and SBL induced apoptosis through the caspase-3 pathway following activation of caspase-8. Addition of the cholesterol remover, methyl-β-cyclodextrin reduced SBL-induced apoptosis with caspase-3 activity. Mass spectrometric and flow cytometric analyses appeared that both a heat shock cognate protein (Hsc70) and a heat shock protein (Hsp70) on the cell membrane bound to SBL and their inhibitor, quercetin significantly reduced SBL-induced apoptosis. Taken together, these findings suggest that sialyl-glycoconjugates present in cholesterol-rich microdomains form complexes with Hsc70 and/or Hsp70 that act as triggers for SBL to induce apoptosis through a pathway involving the activation of caspase-3 and caspase-8.