1. ¹Children's Cancer Institute Australia for Medical Research, Randwick, NSW 2031, Australia
2. ²Iron Metabolism and Chelation Program, Department of Pathology, University of Sydney, New South Wales 2006, Australia
3. ³Centre for Children's Cancer and Blood Disorders, Sydney Children's Hospital, Randwick, NSW 2031, Australia

Correspondence: Professor GM Marshall, E-mail: g.marshall@unsw.edu.au

⁴These authors equally contributed to the study

Received 9 September 2008; Revised 13 November 2008; Accepted 21 November 2008.

Abstract

Increased retinoic acid receptor (RAR₂) gene expression is a hallmark of cancer cell responsiveness to retinoid anticancer effects. Moreover, low basal or induced RAR₂ expression is a common feature of many human cancers, suggesting that RAR₂ may act as a tumour suppressor gene in the absence of supplemented retinoid. We have previously shown that low RAR₂ expression is a feature of advanced neuroblastoma. Here, we demonstrate that the ABC domain of the RAR₂ protein alone was sufficient for the growth inhibitory effects of RAR₂ on neuroblastoma cells. ATP7A, the copper efflux pump, is a retinoid-responsive gene, was upregulated by ectopic overexpression of RAR₂. The ectopic overexpression of the RAR₂ ABC domain was sufficient to induce ATP7A expression, whereas, RAR₂ siRNA blocked the induction of ATP7A expression in retinoid-treated neuroblastoma cells. Forced downregulation of ATP7A reduced copper efflux and increased viability of retinoid-treated neuroblastoma cells. Copper supplementation enhanced cell growth and reduced retinoid-responsiveness, whereas copper chelation reduced the viability and proliferative capacity. Taken together, our data demonstrates ATP7A expression is regulated by retinoic acid receptor and it has effects on intracellular copper levels, revealing a link between the anticancer action of retinoids and copper metabolism.