Modulating Chemosensitivity of Tumors to Platinum-Based Antitumor Drugs by Transcriptional Regulation of Copper Homeostasis

Platinum (Pt)-based antitumor agents have been effective in treating many human malignancies. Drug importing, intracellular shuffling, and exporting—carried out by the high-affinity copper (Cu) transporter (hCtr1), Cu chaperone (Ato x1), and Cu exporters (ATP7A and ATP7B), respectively—cumulatively contribute to the chemosensitivity of Pt drugs including cisplatin and carboplatin, but not oxaliplatin. This entire system can also handle Pt drugs via interactions between Pt and the thiol-containing amino acid residues in these proteins; the interactions are strongly influenced by cellular redox regulators such as glutathione. hCtr1 expression is induced by acute Cu deprivation, and the induction is regulated by the transcription factor specific protein 1 (Sp1) which by itself is also regulated by Cu concentration variations. Copper displaces zinc (Zn) coordination at the zinc finger (ZF) domains of Sp1 and inactivates its DNA binding, whereas Cu deprivation enhances Sp1-DNA interactions and increases Sp1 expression, which in turn upregulates hCtr1. Because of the shared transport system, chemosensitivity of Pt drugs can be modulated by targeting Cu transporters. A Cu-lowering agent (trientine) in combination with a Pt drug (carboplatin) has been used in clinical studies for overcoming Pt-resistance. Future research should aim at further developing effective Pt drug retention strategies for improving the treatment efficacy.

Targeting drug transport mechanisms for improving platinum-based cancer chemotherapy

INTRODUCTION

Platinum (Pt)-based antitumor agents remain important chemotherapeutic agents for treating many human malignancies. Elevated expression of the human high-affinity copper transporter 1 (hCtr1), resulting in enhanced Pt drug transport into cells, has been shown to be associated with improved treatment efficacy. Thus, targeting hCtr1 upregulation is an attractive strategy for improving the treatment efficacy of Pt-based cancer chemotherapy.

AREA COVERED

Regulation of hCtr1 expression by cellular copper homeostasis is discussed. Association of elevated hCtr1 expression with intrinsic sensitivity of ovarian cancer to Pt drugs is presented. Mechanism of copper-lowering agents in enhancing hCtr1-mediated cis-diamminedichloroplatinum (II) (cisplatin, cDDP) transport is reviewed. Applications of copper chelation strategy in overcoming cDDP resistance through enhanced hCtr1 expression are evaluated.

EXPERT OPINION

While both transcriptional and post-translational mechanisms of hCtr1 regulation by cellular copper bioavailability have been proposed, detailed molecular insights into hCtr1 regulation by copper homeostasis remain needed. Recent clinical study using a copper-lowering agent in enhancing hCtr1-mediated drug transport has achieved incremental improvement in overcoming Pt drug resistance. Further improvements in identifying predictive measures in the subpopulation of patients that can benefit from the treatment are needed.

IGF1R predicts better survival in high-grade serous epithelial ovarian cancer patients and correlates with hCtr1 levels

Aim: To evaluate the potential of IGF1R as a prognostic marker for high-grade serous ovarian cancer (HGSOC) patients. Patients & methods: The expression levels of IGF1R and drug transporters (ABCB1, hCtr1) were measured longitudinally in chemo-naive and chemo-treated tumor samples from 19 HGSOC patients, and their correlation with the clinical outcome was examined. Results: IGF1R expression was significantly upregulated in treated tumor samples, which positively correlated with hCtr1 levels. Patients with metastatic tumors with IGF1R expression higher than median showed better overall survival (median not reached) and disease-free survival (26.7 months) than those with less than median expression (overall survival: 27.5 months [p = 0.029]; disease-free survival: 11.9 months [p = 0.014]). Conclusion: IGF1R prognosticates prolonged survival in HGSOC patients, possibly due to its positive correlation with hCtr1.

Exploring the Gating Mechanism of the Human Copper Transporter, hCtr1, Using EPR Spectroscopy

Ctr1 is a membrane-spanning homotrimer that facilitates copper uptake in eukaryotic cells with high affinity. While structural details of the transmembrane domain of human Ctr1 have been elucidated using X-ray crystallography and cryo-EM, the transfer mechanisms of copper and the conformational changes that control the gating mechanism remain poorly understood. The role of the extracellular N-terminal domains is particularly unclear due to the absence of a high-resolution structure of the full-length hCtr1 protein and limited biochemical and biophysical characterization of the transporter in solution and in cell. In this study, we employed distance electron paramagnetic resonance to investigate the conformational changes of the extracellular N-terminal domain of full-length hCtr1, both in vitro and in cells, as a function of Cu(I) binding. Our results demonstrate that at specific Cu(I) concentrations, the extracellular chains move closer to the lumen to facilitate copper transfer. Additionally, while at these concentrations the intracellular part is penetrating the lumen, suggesting a ball-and-chain gating mechanism. Moreover, this phenomenon was observed for both reconstituted protein in micelles and in native cell membranes. However, the measured distance values were slightly different, suggesting that the membrane's characteristics and therefore its lipid composition also impact and even regulate the gating mechanism of hCtr1.