Emerging roles of metal solute carriers in cancer mechanisms and treatment.

The literature concerning the roles of metal transporting solute carriers in the development and progression of human cancer, and in the delivery of metal-containing anticancer drugs, chemical carcinogens and imaging agents, is reviewed. A range of different solute carrier families, including members from the SLC2A, SLC11A, SLC22A, SLC25A, SLC30A, SLC31A, SLC39A, SLC40A, SLC47A and SLCO1B families, and various metal substrates, including arsenic, copper, gadolinium, iron, platinum and zinc, have been implicated in these cancer-related transport processes. For example, the transport of platinum-based anticancer drugs has been reported to be influenced by the expression and activities of OCT1-3 (SLC22A1-3), OCTN1/2 (SLC22A4/5), CTR1/2 (SLC31A1/2) and MATE1/2 (SLC47A1/2) solute carriers. As another example, solute carriers mediate control over the availability of endogenous metal ions, such as copper, iron and zinc, may have key roles in regulating tumour angiogenesis, cell proliferation, epithelial-to-mesenchymal transition and aberrant MAPK and STAT-3 signal transduction in cancer. In conclusion, emerging mechanisms involving metal transporting solute carriers are being defined and seem likely to make major contributions to cancer development and progression, and to the delivery of anticancer and tumour imaging agents.

Oxaliplatin transport mediated by organic cation/carnitine transporters OCTN1 and OCTN2 in overexpressing human embryonic kidney 293 cells and rat dorsal root ganglion neurons.

The organic cation/carnitine transporters OCTN1 and OCTN2 are related to other organic cation transporters (OCT1, OCT2, and OCT3) known for transporting oxaliplatin, an anticancer drug with dose-limiting neurotoxicity. In this study, we sought to determine whether OCTN1 and OCTN2 also transported oxaliplatin and to characterize their functional expression and contributions to its neuronal accumulation and neurotoxicity in dorsal root ganglion (DRG) neurons relative to those of OCTs. [(14)C]Oxaliplatin uptake, platinum accumulation, and cytotoxicity were determined in OCTN-overexpressing human embryonic kidney (HEK) 293 cells and primary cultures of rat DRG neurons. Levels of mRNA and functional activities of rat (r)Octns and rOcts in rat DRG tissue and primary cultures were characterized using reverse transcription-polymerase chain reaction and uptake of model OCT/OCTN substrates, including [(3)H]1-methyl-4-phenylpyridinium (MPP(+)) (OCT1-3), [(14)C]tetraethylammonium bromide (TEA(+)) (OCT1-3 and OCTN1/2), [(3)H]ergothioneine (OCTN1), and [(3)H]l-carnitine (OCTN2). HEK293 cells overexpressing rOctn1, rOctn2, human OCTN1, and human OCTN2 showed increased uptake and cytotoxicity of oxaliplatin compared with mock-transfected HEK293 controls; in addition, both uptake and cytotoxicity were inhibited by ergothioneine and L-carnitine. The uptake of ergothioneine mediated by OCTN1 and of L-carnitine mediated by OCTN2 was decreased during oxaliplatin exposure. rOctn1 and rOctn2 mRNA was readily detected in rat DRG tissue, and they were functionally active in cultured rat DRG neurons, more so than rOct1, rOct2, or rOct3. DRG neuronal accumulation of [(14)C]oxaliplatin and platinum during oxaliplatin exposure depended on time, concentration, temperature, and sodium and was inhibited by ergothioneine and to a lesser extent by L-carnitine but not by MPP(+). Loss of DRG neuronal viability during oxaliplatin exposure was inhibited by ergothioneine but not by L-carnitine or MPP(+). OCTN1 and OCTN2 both transport oxaliplatin and are functionally expressed by DRG neurons. OCTN1-mediated transport of oxaliplatin appears to contribute to its neuronal accumulation and treatment-limiting neurotoxicity more so than OCTN2 or OCTs.

Oxaliplatin-induced loss of phosphorylated heavy neurofilament subunit neuronal immunoreactivity in rat DRG tissue.

BACKGROUND: Oxaliplatin and related chemotherapeutic drugs cause painful chronic peripheral neuropathies in cancer patients. We investigated changes in neuronal size profiles and neurofilament immunoreactivity in L5 dorsal root ganglion (DRG) tissue of adult female Wistar rats after multiple-dose treatment with oxaliplatin, cisplatin, carboplatin or paclitaxel. RESULTS: After treatment with oxaliplatin, phosphorylated neurofilament heavy subunit (pNF-H) immunoreactivity was reduced in neuronal cell bodies, but unchanged in nerve fibres, of the L5 DRG. Morphometric analysis confirmed significant changes in the number (-75%; P < 0.0002) and size (-45%; P < 0.0001) of pNF-H-immunoreactive neurons after oxaliplatin treatment. pNF-H-immunoreactive neurons had overlapping size profiles and co-localisation with neurons displaying cell body immunoreactivity for parvalbumin, non-phospho-specific neurofilament medium subunit (NF-M) and non-phospho-specific neurofilament heavy subunit (NF-H), in control DRG. However, there were no significant changes in the numbers of neurons with immunoreactivity for parvalbumin (4.6%, P = 0.82), NF-M (-1%, P = 0.96) or NF-H (0%; P = 0.93) after oxaliplatin treatment, although the sizes of parvalbumin (-29%, P = 0.047), NF-M (-11%, P = 0.038) and NF-H (-28%; P = 0.0033) immunoreactive neurons were reduced. In an independent comparison of different chemotherapeutic agents, the number of pNF-H-immunoreactive neurons was significantly altered by oxaliplatin (-77.2%; P < 0.0001) and cisplatin (-35.2%; P = 0.03) but not by carboplatin or paclitaxel, and their mean cell body area was significantly changed by oxaliplatin (-31.1%; P = 0.008) but not by cisplatin, carboplatin or paclitaxel. CONCLUSION: This study has demonstrated a specific pattern of loss of pNF-H immunoreactivity in rat DRG tissue that corresponds with the relative neurotoxicity of oxaliplatin, cisplatin and carboplatin. Loss of pNF-H may be mechanistically linked to oxaliplatin-induced neuronal atrophy, and serves as a readily measureable endpoint of its neurotoxicity in the rat model.

Contributions of rat Ctr1 to the uptake and toxicity of copper and platinum anticancer drugs in dorsal root ganglion neurons.

Dorsal root ganglion (DRG) neurons are affected by platinum-induced neurotoxicity and neurodegenerative processes associated with disturbed copper homeostasis and transport. This study aimed to understand the role of copper transporter 1 (Ctr1) in the uptake and toxicity of copper and platinum drugs in cultured rat DRG neurons, and the functional activities of rat Ctr1 (rCtr1) as a membrane transporter of copper and platinum drugs. Heterologous expression of rCtr1 in HEK293 cells (HEK/rCtr1 cells) increased the uptake and cytotoxicity of copper, oxaliplatin, cisplatin and carboplatin, in comparison to isogenic vector-transfected control cells. Cultured rat DRG neurons endogenously expressed rCtr1 protein on their neuronal cell body plasma membranes and cytoplasm, and displayed substantial capacity for taking up copper, but were resistant to copper toxicity. The uptake of copper by both cultured rat DRG neurons and HEK/rCtr1 cells was saturable and inhibited by cold temperature, silver and zinc, consistent with it being mediated by rCtr1. Cultured rat DRG neurons accumulated platinum during their exposure to oxaliplatin and were sensitive to oxaliplatin cytotoxicity. The accumulation of platinum by both cultured rat DRG neurons and HEK/rCtr1 cells, during oxaliplatin exposure, was saturable and temperature dependent, but was inhibited by copper only in HEK/rCtr1 cells. In conclusion, rCtr1 can transport copper and platinum drugs, and sensitizes cells to their cytotoxicities. DRG neurons display substantial capacity for accumulating copper via a transport process mediated by rCtr1, but appear able to resist copper toxicity and use alternative mechanisms to take up oxaliplatin.

Neuronal expression of copper transporter 1 in rat dorsal root ganglia: association with platinum neurotoxicity.

PURPOSE: We report the neuronal expression of copper transporter 1 (CTR1) in rat dorsal root ganglia (DRG) and its association with the neurotoxicity of platinum-based drugs. METHODS: CTR1 expression was studied by immunohistochemistry and RT-PCR. The toxicity of platinum drugs to CTR1-positive and CTR1-negative neurons was compared in DRG from animals treated with maximum tolerated doses of oxaliplatin (1.85 mg/kg), cisplatin (1 mg/kg) or carboplatin (8 mg/kg) twice weekly for 8 weeks. RESULTS: Abundant CTR1 mRNA was detected in DRG tissue. CTR1 immunoreactivity was associated with plasma membranes and cytoplasmic vesicular structures of a subpopulation (13.6 +/- 3.1%) of mainly large-sized (mean cell body area, 1,787 +/- 127 microm(2)) DRG neurons. After treatment with platinum drugs, the cell bodies of these CTR1-positive neurons became atrophied, with oxaliplatin causing the greatest percentage reduction in the mean cell body area relative to controls (42%; P < 0.05), followed by cisplatin (18%; P < 0.05) and carboplatin causing the least reduction (3.2%; P = NS). CTR1-negative neurons, with no immunoreactivity or only diffuse cytoplasmic staining, showed less treatment-induced cell body atrophy than CTR1-positive neurons. CONCLUSIONS: CTR1 is preferentially expressed by a subset of DRG neurons that are particularly vulnerable to the toxicity of platinum drugs. These findings, together with its neuronal membrane localization, are suggestive of CTR1-related mechanisms of platinum drug neuronal uptake and neurotoxicity.