Acquisition of dietary copper: a role for anion transporters in intestinal apical copper uptake

Abstract

Copper is an essential micronutrient in humans and is required for a wide range of physiological processes, including neurotransmitter biosynthesis, oxidative metabolism, protection against reactive oxygen species, and angiogenesis. The first step in the acquisition of dietary copper is absorption from the intestinal lumen. The major human high-affinity copper uptake protein, human copper transporter hCTR1, was recently shown to be at the basolateral or blood side of both intestinal and renal epithelial cell lines and thus does not play a direct role in this initial step. We sought to functionally identify the major transport pathways available for the absorption of dietary copper across the apical intestinal membrane using Caco2 cells, a well-established model for human enterocytes. The initial rate of apical copper uptake into confluent monolayers of Caco2 cells is greatly elevated if amino acids and serum proteins are removed from the growth media. Uptake from buffered saline solutions at neutral pH (but not at lower pH) is inhibited by either d- or l-histidine, unaltered by the removal of sodium ions, and inhibited by ∼90% when chloride ions are replaced by gluconate or sulfate. Chloride-dependent copper uptake occurs with Cu(II) or Cu(I), although Cu(I) uptake is not inhibited by histidine, nor by silver ions. A well-characterized inhibitor of anion exchange systems, DIDS, inhibited apical copper uptake by 60–70%, while the addition of Mn(II) or Fe(II), competitive substrates for the divalent metal transporter DMT1, had no effect on copper uptake. We propose that anion exchangers play an unexpected role in copper absorption, utilizing copper-chloride complexes as pseudo-substrates. This pathway is also observed in mouse embryonic fibroblasts, human embryonic kidney cells, and Cos-7 cells. The special environment of low pH, low concentration of protein, and protonation of amino acids in the early intestinal lumen make this pathway especially important in dietary copper acquisition.

Substrate Profile and Metal-ion Selectivity of Human Divalent Metal-ion Transporter-1

“…Copper transport by DMT1 therefore could be further tested in additional mammalian expression systems or in rodent models in which DMT1 is lacking or overexpressed. Meanwhile, there is strong evidence that absorption and cellular uptake of copper is served by alternative transport systems, among which are the copper transporter 1 (70,71) and an anion transporter that appears capable of transporting copper-chloride complexes (72).…”

Rate and Regulation of Copper Transport by Human Copper Transporter 1 (hCTR1)

“…hCTR1 also suppresses phenotypes of copper deficiency in Drosophila CTR mutants (9) and mediates copper transport when expressed in insect cells (10). Studies in cultured cells from mouse CTR1 knockouts indicate that CTR1 is the principal high affinity transporter of copper in mammals (11), although other pathways of copper entry clearly exist (12,13). It is generally believed that hCTR1 transports Cu ϩ and not Cu 2ϩ ions (14).…”

Phenotypic Copper Resistance in Aerobic Intestinal Bacteria From Children With Different Levels of Copper-Exposure

“…Intestinal copper absorption occurs very efficiently, and copper excess is removed by the liver and excretion with bile into feces, in the order of 4.5 mg/day [6] . However, most of the copper secreted with the bile is reabsorbed by enterocytes [34,35] , and the copper concentration in Chinese children's blood is around 1 mcg/ml [36] . In addition, the intestinal copper absorption is inversely proportional to the copper content in the diet, being reduced to about one-tenth in high-copper diets [37] .…”