Copper is an essential dietary trace metal required for a number of physiological and biochemical processes, but is highly toxic in excess. Copper can exist in either the reduced (Cu⁺) or oxidised (Cu²⁺) state and like iron can act as an important cofactor in a number of fundamental redox reactions. Despite its obvious importance to human metabolism, relatively little is known about the mechanisms involved in the intestinal absorption of copper. It has been suggested that the divalent metal transporter DMT1 might act as a common uptake pathway for a number of metal species (Gunshin et al. 1997). However, work in our laboratory has demonstrated that DMT1 primarily acts as a Fe²⁺ transporter with a much reduced affinity for other metals (Tandy et al. 2000). An alternative copper uptake pathway (CTR1) has been identified, which when expressed in HEK293 cells functions as a copper transporter (Lee et al. 2002). The purpose of our present work was to investigate the effects of copper on the expression of these two transporters in human intestinal Caco-2 cells.

Caco-2 cells were cultured for 21 days (the final 24 h in the presence or absence of 100 mM Cu²⁺) in 25 cm² flasks and were used to isolate total RNA, which was subjected to RT-PCR for CTR1, DMT1 and β-actin (control). In parallel flasks, cellular protein was isolated for Western blotting.

CTR1 and DMT1 (non-IRE isoform) mRNA were unchanged by copper treatment, whereas DMT1 (+IRE) mRNA was decreased (control, 0.81 ± 0.10 a.u.; +Cu, 0.40 ± 0.11 a.u.; means ± S.E.M.; n = 5; P < 0.02, Student’s unpaired t test). DMT1 protein levels were also decreased by copper treatment (control, 111.5 ± 6.4 a.u.; +Cu, 52.9 ± 10.8; n = 4; P < 0.01). Regulation of DMT1 (+IRE) mRNA by cellular iron status is thought to occur via differential interaction between an iron responsive element (IRE) in the 3â untranslated region and cytoplasmic iron regulatory proteins (IRP). Recent evidence suggests that copper may also be able to regulate IRP binding to IRE by replacing iron in the iron-sulphur cluster present in IRP1 (Oshiro et al. 2002).

This work was funded by BBSRC grant 90/D13400.