Copper Accumulations as a Uremic Toxin in CKD

Background: 95% of copper (Cu) in the body is excreted into the bile; meanwhile, the rest is excreted by the kidney. Excess Cu accumulation is known to produce reactive oxygen species in Wilson's disease, and the higher level of serum Cu, the more likely to develop kidney injury. However, it remains unknown whether the relationship between Cu accumulation and kidney injury. We demonstrated that serum Cu level was negatively correlated with serum creatinine level in diabetic rats; thus, we hypothesize that Cu might be accumulated as a uremic toxin with progression of kidney injury. In the present study, we investigated serum Cu levels in patients with CKD and explored how Cu accumulation occurs in CKD and induces kidney injury. Methods: 115 patients with CKD who admitted to our hospital were enrolled to conduct a retrospective clinical study to investigate the relationship between serum Cu and CKD. Low-dose cisplatin (5 mg/kg per week x 4 weeks) was injected into wild-type mice to induce CKD. Serum and urinary levels of Cu, gene expressions of Cu transporters in liver and kidney, and expression pattern of Cu transporters in the liver and kidney were analyzed. LLC-PK1 cells, cultured proximal tubular cells (PTC) were co-incubated with Cu to investigate if Cu accumulation causes PTC damages. Results: Serum Cu level was increased in CKD stage 4 and 5 when compared to earlier stages. Under 65-year-old and female showed higher level of serum Cu. ATP7B and CTR1 gene expressions in the livers were reduced and serum Cu levels were increased in CKD rodents. The increased level of serum Cu was associated with the increase in its urinary excretion. CTR1, a transporter responsible for Cu intake into PTC, was expressed on basolateral plasma membrane of PTC under basal condition, but CTR1 was translocated into nucleus in the CKD rodents. Co-incubation with Cu increased α-SMA, TGF-β, and cleaved caspase3, suggesting that high level of cooper causes PTC injury and fibrosis. Conclusion: We identified that Cu was accumulated with progression of kidney impairment in patients with CKD and high amount of Cu induced PTC apoptosis and profibrotic reaction, indicating that Cu can be considered a uremic toxin. Liver metabolism and excretion of Cu is suppressed with the decrease in liver ATP7B and CTR1 in the context of CKD, which leads to increase urinary Cu levels that induces PTC damages and renal fibrosis.