Direct uptake of copper from plasma ceruloplasmin by cells that do and do not express CTR1 (996.2)

Abstract

The main Cu‐binding protein of blood plasma, ceruloplasmin (Cp), has mainly been studied for its role in mediating cellular iron efflux but also has other functions, and as shown by i.v. infusion of 67Cu‐Cp, its Cu enters most major organs. To determine whether it delivers Cu directly to cells and the mechanisms involved, we incubated purified radio‐copper‐labeled and unlabeled Cp with cells and measured rates of Cu uptake and accumulation. Uptake was demonstrated for human mammary epithelial cell monolayers (PMC42 cells) with tight junctions, and with mouse embryonic fibroblasts expressing/not expressing Ctr1. Cp‐Cu was internalized. It could not be washed from the cell surface even with acidic buffers, and was found in the cytosol. Inhibitors of endocytosis did not impair uptake. Cu deficient cells exposed to Cp increased their cellular Cu contents; and holo‐Cp was converted to apo‐Cp during cell treatment, with no loss of total Cp protein. Uptake was almost the same in cells not expressing Ctr1. Excess ionic Cu(II) and Cu(I) virtually eliminated Cp‐Cu uptake. The presence of Cu(II) and Fe(III) reductase activities on the cell surface was verified. We conclude that Cp‐Cu delivers Cu to cells by a process involving cell surface reduction of Cp‐Cu(II), release of apo‐Cp, and uptake of the resulting Cu(I) by Ctr1 and an additional as yet unidentified Cu(I) transporter. Grant Funding Source: Supported by PHS Grant R15 GM100464