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 fermentation and contribute to systemic Cd toxicity, e.g. in liver and kidneys. We have recently demonstrated that the receptor for human neutrophil gelatinase-associated lipocalin (hNGAL) is expressed in human colon and colon-like Caco-2 BBE cells where it mediates transcytosis of MT and PC. Here we show in colon-like Caco-2 BBE cells that hNGAL receptor (hNGAL-R) dependent toxicity is significantly higher with CdMT than with CdPC3 (2.5-50μM Cd(2+) complexed to MT or PC3 for ≤24h), using MTT assay. Fluorescence-labelled A546-MT, but not A488-PC3 (both 700nM), 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 compared to A488-PC3 (both 700nM). The tubulin polymerization disruptor nocodazole (16.7μM) almost abolished CdMT and CdPC3 toxicity, reduced apical uptake of both A546-MT and A488-PC3, but increased transcytosis efficiency of A546-MT compared to that of A488-PC3 by preventing trafficking of A546-MT to lysosomes. Hence, following hNGAL-R dependent endocytosis of CdMT/CdPC3 in colonic epithelia, a nocodazole-sensitive trafficking pathway may preferentially target CdMT, but not CdPC3, to lysosomes, causing increased colonic epithelial toxicity but reduced systemic toxicity.