l-Cysteine-mediated modulation of copper trafficking in prostate cancer cells: an in vitro and in vivo investigation with 64Cu and 64Cu-PET

Abstract

Copper imbalance is implicated in many diseases, including cancer. Copper in blood is mainly transported by carrier proteins but a small fraction is bound to low molecular weight species, possibly amino acids. Their roles in cellular copper delivery are unknown. Our aim was to test whether accumulation of ⁶⁴Cu into cancer-derived cells can be influenced by copper-binding serum amino acids. In vitro cellular accumulation of ⁶⁴Cu was measured in Hank's Balanced Salt Solution in the presence of 100 μM L-histidine, L-methionine, L-cysteine and L-threonine. L-Cysteine markedly increased ⁶⁴Cu accumulation and retention in DU145, PC3 and SK-OV-3 cells, while some other cell lines did not show an effect. This effect was not due to ⁶⁴Cu delivery in the form of a ⁶⁴Cu–cysteine complex, nor to reduction of ⁶⁴Cu(II) to ⁶⁴Cu(I) by L-cysteine. Pre-incubation of cells with L-cysteine increased ⁶⁴Cu accumulation, even if L-cysteine was removed from HBSS before ⁶⁴Cu was added. The effect of L-cysteine on ⁶⁴Cu accumulation was not mediated by increased glutathione synthesis. Despite the demonstrable in vitro effect, pre-injection of L-cysteine precursor N-acetyl-cysteine (NAC) in vivo did not enhance ⁶⁴Cu delivery to DU145 xenografts in mice. Instead, it decreased ⁶⁴Cu accumulation in the DU145 tumour and in brain, as assessed by PET imaging. We conclude that ⁶⁴Cu is not delivered to DU145 cancer cells in vitro as a complex with amino acids but its cellular accumulation is enhanced by L-cysteine or NAC influx to cells. The latter effect was not demonstrable in vivo in the DU145 xenograft.