Cisplatin stabilizes a multimeric complex of the human Ctr1 copper transporter: requirement for the extracellular methionine-rich clusters

Distinct mechanisms for Ctr1-mediated copper and cisplatin transport

The Ctr1 family of integral membrane proteins is necessary for high affinity copper uptake in eukaryotes. Ctr1 is also involved in cellular accumulation of cisplatin, a platinum-based anticancer drug. Although the physiological role of Ctr1 has been revealed, the mechanism of action of Ctr1 remains to be elucidated. To gain a better understanding of Ctr1-mediated copper and cisplatin transport, we have monitored molecular dynamics and transport activities of yeast Saccharomyces cerevisiae Ctr1 and its mutant alleles. Co-expression of functional Ctr1 monomers fused with either cyan or yellow fluorescent protein resulted in fluorescence resonance energy transfer (FRET), which is consistent with multimer assembly of Ctr1. Copper near the K(m) value of Ctr1 enhanced FRET in a manner that correlated with cellular copper transport. In vitro cross-linking of Ctr1 confirmed that copper-induced FRET reflects conformational changes within pre-existing Ctr1 complexes. FRET assays in membrane-disrupted cells and protein extracts showed that intact cell structure is necessary for Ctr1 activity. Despite Ctr1-dependent cellular accumulation, cisplatin did not change Ctr1 FRET nor did it attenuate copper-induced FRET. A Ctr1 allele defective in copper transport enhanced cellular cisplatin accumulation. N-terminal methionine-rich motifs that are dispensable for copper transport play a critical role for cisplatin uptake. Taken together, our data reveal functional roles for structural remodeling of the Ctr1 multimeric complex in copper transport and suggest distinct mechanisms employed by Ctr1 for copper and cisplatin transport.

The roles of copper transporters in cisplatin resistance

Platinum-based antitumor agents have been effective in the treatments of many human malignancies but the ultimate success of these agents is often compromised by development of drug resistance. One mechanism associated with resistance to platinum drugs is reduced intracellular accumulation owing to impaired drug intake, enhanced outward transport, or both. Mechanisms for transporting platinum drugs were not known until recent demonstrations that import and export transporters involved in maintenance copper homeostasis are also involved in the transport of these drugs. Ctr1, the major copper influx transporter, has been convincingly demonstrated to transport cisplatin and its analogues, carboplatin, and oxaliplatin. Evidence also suggests that the two copper efflux transporters ATP7A and ATP7B regulate the efflux of cisplatin. These observations are intriguing, because conventional thinking of the inorganic physiologic chemistry of cisplatin and copper is quite different. Hence, understanding the underlying mechanistic aspects of these transporters is critically important. While the mechanisms by which hCtr1, ATP7A and ATP7B transport copper ions have been studied extensively, very little is known about the mechanisms by which these transporters shuffle platinum-based antitumor agents. This review discusses the identification of copper transporters as platinum drug transporters, the structural-functional and mechanistic aspects of these transporters, the mechanisms that regulate their expression, and future research directions that may eventually lead to improved efficacy of platinum-based-based drugs in cancer chemotherapy through modulation of their transporters' activities.

The human cytochrome c oxidase assembly factors SCO1 and SCO2 have regulatory roles in the maintenance of cellular copper homeostasis

Human SCO1 and SCO2 are metallochaperones that are essential for the assembly of the catalytic core of cytochrome c oxidase (COX). Here we show that they have additional, unexpected roles in cellular copper homeostasis. Mutations in either SCO result in a cellular copper deficiency that is both tissue and allele specific. This phenotype can be dissociated from the defects in COX assembly and is suppressed by overexpression of SCO2, but not SCO1. Overexpression of a SCO1 mutant in control cells in which wild-type SCO1 levels were reduced by shRNA recapitulates the copper-deficiency phenotype in SCO1 patient cells. The copper-deficiency phenotype reflects not a change in high-affinity copper uptake but rather a proportional increase in copper efflux. These results suggest a mitochondrial pathway for the regulation of cellular copper content that involves signaling through SCO1 and SCO2, perhaps by their thiol redox or metal-binding state.

The internalization and degradation of human copper transporter 1 following cisplatin exposure

The human copper transporter 1 (hCTR1), the major transporter responsible for copper influx, mediates one component of the cellular accumulation of cisplatin (DDP). Both copper and DDP cause rapid down-regulation of hCTR1 expression in human ovarian carcinoma cells. In this study, we investigated the mechanism of this effect using digital deconvolution microscopy and Western blot analysis of cells stained with antibodies directed at both ends of the protein. Treatment of 2008 cells with DDP in combination with inhibitors of various endosomal pathways (amiloride, cytochalasin D, nystatin, and methyl-beta-cyclodextrin) showed that hCTR1 degradation was blocked by amiloride and cytochalasin D, indicating that hCTR1 was internalized primarily by macropinocytosis. Expression of transdominant-negative forms of dynamin I and Rac showed that loss of hCTR1 was not dependent on pathways regulated by either of these proteins. DDP-induced loss of hCTR1 was blocked by the proteasome inhibitors lactacystin, proteasome inhibitor 1, and MG132. This study confirms that DDP triggers the rapid loss of hCTR1 from ovarian carcinoma cells at clinically relevant concentrations. The results indicate that DDP-induced loss of hCTR1 involves internalization from the plasma membrane by macropinocytosis followed by proteasomal degradation. Because hCTR1 is a major determinant of early DDP uptake, prevention of its degradation offers a potential approach to enhancing tumor sensitivity.

Platinum transporters and drug resistance

Cisplatin, a platinum coordinated complex, is a widely used antineoplastic agent for the treatment of metastatic tumors of the testis, metastatic ovarian tumors, lung cancer, advanced bladder cancer and many other solid tumors. The cytotoxic action of the drug is often thought to be associated with its ability to bind DNA to form cisplatin-DNA adducts. The development of resistance to cisplatin during treatment is common and constitutes a major obstacle to the cure of sensitive tumors. Although to understand the clinically relevant mechanisms of resistance, many studies have been aimed at clarifying the biochemical/molecular alterations of cisplatin-resistance cells, these studies did not conclusively identify the basis of cellular resistance to cisplatin. In this review, cisplatin resistance was discussed in terms of the relevant transporters, such as copper transporters (CTRs), organic cation transporters (OCTs) and multi-drug resistance related transporters (MDRs). These transporters seem to be contributed to cisplatin resistance through the reduction of drug accumulation in the cell. Better understanding the mechanism of cisplatin resistance associated with transporters will provide the useful informations for overcoming the cisplatin resistance.

The Copper Transporter (Ctr) Family of Cu+ Uptake Systems

Copper (Cu(+)) transporters of the Ctr family are sequence diverse eukaryotic proteins that function by an unknown mechanism of action. We have conducted bioinformatic analyses of sequenced Ctr proteins. Multiple paralogues are found in single organisms, and these may be either closely or distantly related to each other. Protein phylogeny generally correlates with organismal source and protein size with proteins of each cluster being derived from a specific eukaryotic kingdom and exhibiting characteristic domain arrangements. Some homologues exhibit repeats of the basic 3 TMS unit. Regions of conserved hydrophobicity and amphipathicity suggest functional roles, particularly for putative TMSs 2 and 3 which possess a nearly fully conserved M X(3) M motif in putative TMS2. We propose that this motif comprises the transmembrane Cu(+)-binding site in oligomeric channels that take up Cu(+) by a passive, membrane potential-dependent mechanism.

Role of copper transporters in the uptake and efflux of platinum containing drugs

Cellular mechanisms for the uptake, intracellular distribution and efflux of the platinum (Pt) containing compounds cisplatin (DDP), carboplatin (CBDCA) and oxaliplatin (LOHP) are unknown. Current data suggest that specialized transporters/carriers mediate the transport of Pt drugs across the cellular membranes. Specific roles for the copper (Cu) transporters CTR1, ATP7A and ATP7B have been demonstrated during recent years. The finding that in cultured cells and tumor samples a correlation can be found between the expression of Cu transporters and the degree of the acquired resistance to Pt drug suggests that the Cu transporters are important constituents of the program that regulates sensitivity to Pt drugs. A model is presented that describes the function of Cu transporters in the regulation of Pt drug uptake and efflux.