Zinc treatment of Wilson's disease

Therapeutic implications of impaired nuclear receptor function and dysregulated metabolism in Wilson's disease

Copper is an essential trace element that is required for the activity of many enzymes and cellular processes, including energy homeostasis and neurotransmitter biosynthesis; however, excess copper accumulation results in significant cellular toxicity. The liver is the major organ for maintaining copper homeostasis. Inactivating mutations of the copper-transporting P-type ATPase, ATP7B, result in Wilson's disease, an autosomal recessive disorder that requires life-long medicinal therapy or liver transplantation. Current treatment protocols are limited to either sequestration of copper via chelation or reduction of copper absorption in the gut (zinc therapy). The goal of these strategies is to reduce free copper, redox stress, and cellular toxicity. Several lines of evidence in Wilson's disease animal models and patients have revealed altered hepatic metabolism and impaired hepatic nuclear receptor activity. Nuclear receptors are transcription factors that coordinate hepatic metabolism in normal and diseased livers, and several hepatic nuclear receptors have decreased activity in Wilson's disease and Atp7b-/- models. In this review, we summarize the basic physiology that underlies Wilson's disease pathology, Wilson's disease animal models, and the possibility of targeting nuclear receptor activity in Wilson's disease patients.

Elevated copper impairs hepatic nuclear receptor function in Wilson's disease

Wilson's disease (WD) is an autosomal recessive disorder that results in accumulation of copper in the liver as a consequence of mutations in the gene encoding the copper-transporting P-type ATPase (ATP7B). WD is a chronic liver disorder, and individuals with the disease present with a variety of complications, including steatosis, cholestasis, cirrhosis, and liver failure. Similar to patients with WD, Atp7b⁻/⁻ mice have markedly elevated levels of hepatic copper and liver pathology. Previous studies have demonstrated that replacement of zinc in the DNA-binding domain of the estrogen receptor (ER) with copper disrupts specific binding to DNA response elements. Here, we found decreased binding of the nuclear receptors FXR, RXR, HNF4α, and LRH-1 to promoter response elements and decreased mRNA expression of nuclear receptor target genes in Atp7b⁻/⁻ mice, as well as in adult and pediatric WD patients. Excessive hepatic copper has been described in progressive familial cholestasis (PFIC), and we found that similar to individuals with WD, patients with PFIC2 or PFIC3 who have clinically elevated hepatic copper levels exhibit impaired nuclear receptor activity. Together, these data demonstrate that copper-mediated nuclear receptor dysfunction disrupts liver function in WD and potentially in other disorders associated with increased hepatic copper levels.

Excretion of copper complexed with thiomolybdate into the bile and blood in LEC rats

Copper (Cu) accumulating in a form bound to metallothionein (MT) in the liver of Long-Evans rats with a cinnamon-like coat color (LEC rats), an animal model of Wilson disease, was removed with ammonium tetrathiomolybdate (TTM), and the fate of the Cu complexed with TTM and mobilized from the liver was determined. TTM was injected intravenously as a single dose of 2, 10 or 50 mg TTM/kg body weight into LEC and Wistar (normal Cu metabolism) rats, and then the concentrations of Cu and molybdenum (Mo) in the bile and plasma were monitored with time after the injection. In Wistar rats, most of the Mo was excreted into the urine, only a small quantity being excreted into the bile, while Cu excreted into the urine decreased. However, in LEC rats, Cu and Mo were excreted into the bile and blood, and the bile is recognized for the first time as the major route of excretion. The Cu excreted into both the bile and plasma was accompanied by an equimolar amount of Mo. The relative ratio of the amounts of Cu excreted into the bile and plasma was 40/60 for the low and high dose groups, and 70/30 for the medium dose group. The systemic dispositions of the Cu mobilized from the liver and the Mo complexed with the Cu were also determined for the kidneys, spleen and brain together with their urinal excretion. Although Mo in the three organs and Cu in the kidneys and spleen were increased or showed a tendency to increase, Cu in the brain was not increased at all doses of TTM.