Regulation of heme synthesis and proteasomal activity by copper: Possible implications for Wilson's disease

Wilson's disease (Wd) is a genetic disorder resulting in Cu²⁺ accumulation, and is caused by mutations in the ATP7B gene, the copper transporter. In vivo studies show a correlation between Cu²⁺ accumulation and malfunction of the heme biosynthesis pathway. In this study, we describe multiple effects of Cu²⁺ accumulation on heme synthesis, which, in turn, affect proteasomal activity. Cu²⁺ toxicity was examined in two hepatocellular carcinoma cell lines, HepG2 and Hep3B, with Hep3B cells containing an integrated hepatitis B virus genome. Exposure of HepG2 and Hep3B cells to Cu²⁺ inhibited the enzymes PBGD and ALAD of the heme synthesis pathway and, in parallel, upregulated heme oxygenase-1 (HO-1). Proto-porphyrin IX (PpIX) and the heme pool were reduced as a result of these processes. PpIX synthesis was found to be lower in cells expressing the mutant ATP7B (P1134P), compared to those expressing the WT enzyme. Proteasomal activity was inhibited under Cu²⁺ treatment in HepG2 cells; however, Cu ²⁺ induced marked proteosomal acceleration in Hep3B cells. Under these conditions, Ub-conjugated proteins were gradually accumulated, whereas treatment with bathocuproine disulfonic acid (BCS), a Cu²⁺ chelator, reversed this effect. In conclusion, our data suggest that copper downregulates the heme synthesis pathway in hepatocellular cells and further reduces it in the presence of mutated ATP7B.