Question: 

The lipocalin-2/24p3 receptor (24p3R) is expressed in rodent distal nephron and mediates protein endocytosis (Langelueddecke et al. 2012). Here we investigated the expression and function of 24p3R in mouse intestine.

Methods: 

Expression of 24p3R in mouse intestine was determined by immunofluorescence microscopy, and by RT-PCR and immunoblotting in Caco-2 cells or CHO cells transfected with 24p3R. Endo-/transcytosis of peptide-metal complexes were assayed by internalization of fluorescence-labelled peptide ligands in cells grown on plastic or as monolayers on Transwell inserts. Toxicity of metal-peptide complexes was determined by MTT assay.

Results: 

24p3R was detected in all segments of mouse intestine, however, the highest 24p3R expression was observed in apical cell poles of colonic epithelia. Similarly, colonocyte-like Caco-2 cells showed higher expression of 24p3R than duodenocyte-like cells by surface immunofluorescence staining and immunoblotting of plasma membranes. Caco-2 cells or 24p3R-transfected CHO cells endocytosed fluorescence-labelled metallothioneins (MT), phytochelatins (PC) and transferrin (Tf). Cell death (<=30%) was observed with MT or PC (20–30mM; 48h) complexed to the toxic metal ion cadmium. Transwell experiments showed rapid (0.5–4h) uptake of apical fluorescent PC and Tf (1–5mM). About 10% of apical peptides was detected basolaterally after 1h for Tf and 8h for PC.

Conclusions: 

The distribution of 24p3R is similar in kidney and intestinal epithelia where 24p3R contributes to receptor-mediated endocytosis of metal-peptide complexes. However, only high concentrations of toxic metal-peptide complexes induce cell death in colonic cells and transcytosis may contribute to absorption of intact metal-peptide complexes into the circulation.

Funded by DFG TH345/11-1.

Langelueddecke, C. et al. (2012) The lipocalin-2 (24p3/NGAL) receptor is expressed in the distal nephron and mediates protein endocytosis. J. Biol. Chem. 287:159–169