Copper is an essential element in a variety of biological processes, but it can be toxic when present in excessive amounts. The central regulators of cellular copper metabolism include copper-binding proteins, copper transporters, metal membrane active transporters and copper-dependent enzymes. However, the way in which cupric ions (Cu(2+)) cause cellular changes in proteins and lead to toxic effects is less well-known. The aim of this study is to identify the proteins related to Cu(2+) toxicity or detoxification mechanisms in tilapia (Oreochromis niloticus) using a proteomic approach. A cell line derived from the liver of tilapia, Hepa-T1, was used as a model and exposed to two sub-lethal concentrations of waterborne copper for 96 h. The proteins expressed in Hepa-T1 were investigated by differential protein profiling using two-dimensional gel electrophoresis (2DE). It was found that Cu(2+) (120 and 300 microM) caused the differential expression of 93 different proteins, 18 of which were further verified by real-time quantitative polymerase chain reaction (PCR) analysis. Following analysis with ingenuity pathway software, several proteins were found to be involved in lipid metabolism, tissue connective development and cell cycle control, thus indicating that copper toxicity affects these cellular functions.