Abnormalities of copper accumulation in cell lines established from nine different alleles of mottled are the same as those found in Menkes disease

Interference between copper transport systems and platinum drugs

Cisplatin, or cis-diamminedichloridoplatinum(II) cis-[PtCl₂(NH₃)₂], is a platinum-based anticancer drug largely used for the treatment of various types of cancers, including testicular, ovarian and colorectal carcinomas, sarcomas, and lymphomas. Together with other platinum-based drugs, cisplatin triggers malignant cell death by binding to nuclear DNA, which appears to be the ultimate target. In addition to passive diffusion across the cell membrane, other transport systems, including endocytosis and some active or facilitated transport mechanisms, are currently proposed to play a pivotal role in the uptake of platinum-based drugs. In this review, an updated view of the current literature regarding the intracellular transport and processing of cisplatin will be presented, with special emphasis on the plasma membrane copper permease CTR1, the Cu-transporting ATPases, ATP7A and ATP7B, located in the trans-Golgi network, and the soluble copper chaperone ATOX1. Their role in eliciting cisplatin efficacy and their exploitation as pharmacological targets will be addressed.

A Role for The ATP7A Copper Transporter in Tumorigenesis and Cisplatin Resistance

The ATP7A protein is a ubiquitously expressed copper-translocating P-type ATPase that controls cytoplasmic copper concentrations by mediating cellular copper egress. In vitro studies have previously demonstrated that ATP7A abundance in various tumor cell lines is correlated with increased resistance to cisplatin, a widely-used chemotherapy agent. However, to date no studies have examined a role for ATP7A in tumor growth or cisplatin sensitivity in vivo. In this study, we deleted ATP7A in H-RAS transformed tumorigenic mouse embryonic fibroblasts (MEF^RAS7A-). Interestingly, loss of ATP7A was found to markedly suppress tumorigenesis in MEF^RAS7A- cells relative to wild type parental cells. This was associated with hyperaccumulation of copper and sensitivity to reactive oxygen species and hypoxia. Tumor grafts lacking ATP7A were markedly more sensitive to cisplatin chemotherapy compared to ATP7A-expressing control tumors. These findings identify ATP7A at the nexus between tumorigenesis and cisplatin resistance pathways, underscoring its potential as a therapeutic target for regulating both tumor growth and the efficacy of cisplatin treatment.

ATP7A-related copper transport diseases-emerging concepts and future trends

This Review summarizes recent advances in understanding copper-transporting ATPase 1 (ATP7A), and examines the neurological phenotypes associated with dysfunction of this protein. Involvement of ATP7A in axonal outgrowth, synapse integrity and neuronal activation underscores the fundamental importance of copper metabolism to neurological function. Defects in ATP7A cause Menkes disease, an infantile-onset, lethal condition. Neonatal diagnosis and early treatment with copper injections enhance survival in patients with this disease, and can normalize clinical outcomes if mutant ATP7A molecules retain small amounts of residual activity. Gene replacement rescues a mouse model of Menkes disease, suggesting a potential therapeutic approach for patients with complete loss-of-function ATP7A mutations. Remarkably, a newly discovered ATP7A disorder-isolated distal motor neuropathy-has none of the characteristic clinical or biochemical abnormalities of Menkes disease or its milder allelic variant occipital horn syndrome (OHS), instead resembling Charcot-Marie-Tooth disease type 2. These findings indicate that ATP7A has a crucial but previously unappreciated role in motor neuron maintenance, and that the mechanism underlying ATP7A-related distal motor neuropathy is distinct from Menkes disease and OHS pathophysiology. Collectively, these insights refine our knowledge of the neurology of ATP7A-related copper transport diseases and pave the way for further progress in understanding ATP7A function.

Gene expression profiling in chronic copper overload reveals upregulation ofPrnpandApp

The level at which copper becomes toxic is not clear. Several studies have indicated that copper causes oxidative stress; however, most have tested very high levels of copper exposure. We currently have only a limited understanding of the protective systems that operate in cells chronically exposed to copper. Additionally, the limits of homeostatic regulation are not known, making it difficult to define the milder effects of copper excess. Furthermore, a robust assay to facilitate the diagnosis of copper excess and to distinguish mild, moderate, and severe copper overload is needed. To address these issues, we have investigated the effects on steady-state gene expression of chronic copper overload in a cell culture model system using cDNA microarrays. For this study we utilized cells from genetic models of copper overload: fibroblast cells from two mouse mutants, C57BL/6- Atp7aMobrand C57BL/6- Atp7aModap. These cell lines accumulate copper to abnormally high levels in normal culture media due to a defect in copper export from the cell. We identified 12 differentially expressed genes in common using our outlier identification methods. Surprisingly, our results show no evidence of oxidative stress in the copper-loaded cells. In addition, candidate components perhaps responsible for a copper-specific homeostatic response are identified. The genes that encode for the prion protein and the amyloid-β precursor protein, two known copper-binding proteins, are upregulated in both cell lines.

Copper transporting P-type ATPases and human disease

Copper transporting P-type ATPases, designated ATP7A and ATP7B, play an essential role in mammalian copper balance. Impaired intestinal transport of copper, resulting from mutations in the ATP7A gene, lead to Menkes disease in humans. Defects in a similar gene, the copper transporting ATPase ATP7B, result in Wilson disease. This ATP7B transporter has two functions: transport of copper into the plasma protein ceruloplasmin, and elimination of copper through the bile. Variants of ATP7B can be functionally assayed to identify defects in each of these functions. Tissue expression studies of the copper ATPases and their copper chaperone ATOX1 indicate that there is not complete overlap in expression. Other chaperones may be important for the transport of copper into ATP7A and ATP7B.

Drug targets in Menkes disease - prospective developments

Menkes disease (MNK) is an X-linked recessive disorder characterised by a copper-transporting ATPase defect. In the affected cells, copper transport from the cytosol to the Golgi apparatus is disturbed, resulting in a reduction of copper efflux. Orally-administered copper, which accumulates in the intestine, cannot be absorbed and thus a copper deficiency arises. The characteristic features of MNK are progressive neurological degeneration, connective tissue disorders and hair abnormalities, which are caused by a reduction in the activity of several copper-dependent enzymes, due to concomitant copper deficiency. Subcutaneous injections of copper-histidine complex, which currently forms the accepted mode of treatment, prevent the neurological degeneration in some patients when the treatment is initiated soon after birth. However, when the treatment is started later, the neurological degenerative processes are not prevented. Moreover, the treatment does not improve the connective tissue disorders that are caused by the low activity of lysyl oxidase. In order to solve these problems, a form of the treatment aimed at delivering copper into the Golgi apparatus should be studied. An attempt is made in this review to present what is currently known about MNK and its variants, the efficacy and problems of currently accepted treatments and finally therapeutic targets in MNK.

Fibroblast silver loading for the diagnosis of Menkes disease

Menkes disease is a genetic disorder of copper metabolism. Copper uptake and retention assays on fibroblast or amniotic fluid cell cultures have been used for pre- and postnatal diagnosis. These copper loading tests are complicated by the use of 64Cu, which is not commonly available and has a very short (12.8 hours) physical half life. Besides copper, silver is also a substrate for the bacterial homologue of the Menkes transport protein. We report here that loading tests using radioactive silver (110mAg), instead of copper, can be used for the diagnosis of Menkes disease. 110mAg is commercially available and has a convenient physical half life of 250 days, which makes it suitable for use in diagnostic laboratories. Our studies support the hypothesis that reduction of divalent to monovalent copper is an essential step preceding transport.