Copper Transport in Mammalian Cells: Special Care for a Metal with Special Needs

Abstract

Copper plays an essential role in human physiology. It is required for respiration, radical defense, neuronal myelination, angiogenesis, and many other processes. Copper has distinct physicochemical properties that pose uncommon challenges for its transport across biological membranes. Only small amounts of copper are present in biological fluids, and essentially none of it exists in a free ion form. These properties and the low redox potential of copper dictate special structural and mechanistic features in copper transporters. This minireview discusses molecular mechanisms through which copper enters and exits human cells. Dietary Acquisition and Excretion of CopperDietary copper is acquired via the small intestine through a process that is not fully understood. Earlier studies showing the major involvement of the mammalian transporter CTR1 in cellular copper uptake (1) led to the assumption that in enterocytes CTR1 mediates the acquisition of dietary copper at the apical membrane. Recent studies questioned this assumption. Cell-surface labeling revealed that the majority (and perhaps all) of surface hCTR1 3 in Caco-2 cells, a model for enterocytes, is located at the basolateral border, i.e. the blood side, and thus cannot mediate apical copper entry (2). Basolateral CTR1 is also observed in renal cells and hepatocytes. Furthermore, in mice with an intestine-specific deletion of CTR1, enterocyte copper levels are higher (and not lower) than in normal mice, arguing against the direct role of CTR1 in apical uptake (3). It seems most likely that in enterocytes some other transporter plays a major role in copper uptake from the fluid that exits the stomach (Fig. 1). The existence of such pathways is supported by the observation that fibroblasts from CTR1 knock-out mice retain ϳ25-30% of their copper uptake (4). Divalent metal transporter DMT1 was suggested as a candidate for copper uptake (5). Other systems not thought of as major copper transporters may accept copper as a surrogate substrate and contribute to the overall copper absorption. Endocytic mechanisms may also contribute to apical copper entry. It is not yet clear how the copper pool from apical dietary uptake in enterocytes interacts with the copper that enters from the blood (Fig. 1).In intestinal cells and tissue, CTR1 was detected at the plasma membrane and in vesicles (2, 6). Thus, CTR1 in enterocytes may take up copper from the blood (as in other cell types) and also from the vesicles in which dietary copper has been sequestered (Fig. 1). This latter role is suggested by the finding that, although the intestinal deletion of CTR1 results in high copper levels in enterocytes, this copper is not available (3). It is tempting to speculate that CTR1 is uniquely positioned to make copper bioavailable either because it has a specific recognition mechanism and/or it is directly coupled to the copper chaperones that distribute copper for further utilization.After copper crosses the membrane, it is delivered to acceptor proteins by cytosolic metallochaperone...

Unification of the Copper(I) Binding Affinities of the Metallo-chaperones Atx1, Atox1, and Related Proteins

“…It appears that, for some Cu I -ATPases at least, metal-binding sites in the transmembrane domain may also independently receive Cu I from copper chaperones (9). The overall molecular structure and binding site of each MBD is similar to that of Atox1 (10).…”

Ctr1 Is an Apical Copper Transporter in Mammalian Intestinal Epithelial Cells in Vivo That Is Controlled at the Level of Protein Stability

“…20 and reviewed in Ref. 32). One possibility may be the wide variability in different laboratory Caco-2 cell lines that leads to differences in gene expression, tight junction integrity, and other variables that could lead to changes in Ctr1 trafficking (33)(34)(35).…”

“…20 and reviewed in Ref. 32), the results presented here based on light microscopy, confocal microscopy, and biotinylation analyses of mouse intesti- Total protein extracts of 100 g were subjected to immunoblotting with anti-Ctr1 (1:1,000) and anti-actin (1:5,000) antibodies. B, HEK293T cells were incubated with BCS or copper at the concentrations indicated in the presence of cycloheximide for 8 h and subjected to immunoblotting as described in A.…”

Dynamic internalization and recycling of a metal ion transporter: Cu homeostasis and CTR1, the human Cu+ uptake system

“…The acute regulatory response described in the present work might be expected to play a role in the pulsatile appearance of the micronutrient in the diet. A balance is normally maintained between the uptake ( predominantly by CTR1), the intracellular system responsible for its distribution to target proteins (through GSH and the metallochaperones), and the efflux transporters, Cu-activated ATPases ATP7A and ATP7B (Kaplan and Lutsenko, 2009). Internalization of CTR1 in the presence of excess extracellular Cu is reversible and CTR1 returns to the plasma membrane when extracellular Cu levels are reduced (Molloy and Kaplan, 2009).…”