Differential transcytosis and toxicity of the hNGAL receptor ligands cadmium-metallothionein and cadmium-phytochelatin in colon-like Caco-2 cells: Implications for in vivo cadmium toxicity

The environmental toxicant cadmium (Cd) enters the food chain. A substantial proportion of Cd in nutrients of plant origin is present as Cd-metallothionein (CdMT) and Cd-phytochelatin (CdPC) complexes, which may be absorbed and transcytosed intact by colonic enterocytes following bacterial fermentationand contribute to systemic Cd toxicity, e. g. in liver and kidneys. We have recently demonstrated thatthe receptor for human neutrophil gelatinase-associated lipocalin (hNGAL) is expressed in human colonand colon-like Caco-2 BBE cells where it mediates transcytosis of MT and PC. Here we show in colon-likeCaco2 BBE cells that hNGAL receptor (hNGAL-R) dependent toxicity is significantly higher with CdMTthan with CdPC3(2.5-50 M Cd2+ complexed to MT or PC3for = 24 h), using MTT assay. Fluorescencelabelled A546-MT, but not A488-PC3(both 700 mu M), co-localizes with the lysosomal marker cathepsin-B, as determined by confocal microscopy. In transwell experiments with confluent monolayers, transcytosis efficiency (i. e. the ratio of basal delivery to apical decrease) of A546-MT is decreased comparedto A488-PC3(both 700 nM). The tubulin polymerization disruptor nocodazole (16.7 mu M) almost abolished CdMT and CdPC3toxicity, reduced apical uptake of both A546-MT and A488-PC3, but increasedtranscytosis efficiency of A546-MT compared to that of A488-PC3by preventing trafficking of A546-MTto lysosomes. Hence, following hNGAL-R dependent endocytosis of CdMT/CdPC3in colonic epithelia, anocodazolesensitive trafficking pathway may preferentially target CdMT, but not CdPC3, to lysosomes, causing increased colonic epithelial toxicity but reduced systemic toxicity. (C) 2014 Elsevier Ireland Ltd. All rights reserved.