Characterization of cadmium uptake in human intestinal crypt cells HIEC in relation to inorganic metal speciation

Cadmium (Cd) uptake was studied under inorganic exposure conditions in normal human intestinal crypt cells HIEC. The uptake time course of 0.3 μM Cd in a serum-free chloride medium was analyzed according to a first order equation with rapid initial (U0) and maximal (Umax) accumulation values of 14.1 ± 1.4 pmol/mg protein and 41.4 ± 2.0 pmol/mg protein, respectively. The presence of a 300-fold excess of unlabeled Cd dramatically decreased tracer uptake, showing the involvement of specific mechanism(s) of transport. Our speciation studies revealed the preferential uptake of the free ion Cd2+, but also suggested that CdCln2−n species may contribute to Cd accumulation. Specific mechanisms of transport of very high and similar affinity (Km ∼ 5 μM) have been characterized under both chloride and nitrate exposure conditions, but a two-fold higher capacity (Vmax) was estimated in the nitrate medium used to increase [Cd2+] over chlorocomplex formation. A clear inhibition of 109Cd uptake was observed at external acidic pH under both exposure media. An La-inhibitible 46% increase in 109Cd uptake was obtained in nominally Ca-free nitrate medium, whereas Zn provided additional inhibition. These results show different kinetic parameters for Cd uptake as a function of inorganic metal speciation. Cd2+ uptake would not involve the H+-coupled symport NRAMP2 but would be related instead to the Ca and/or Zn pathways. Because proliferative crypt cells play a critical role in the renewal process of the entire intestinal epithelium, studies on the impact of Cd on HIEC cell functions clearly deserve further investigation.