Role of Copper in the Genesis of Liver Cancer
By Kalipatnapu N. Rao, PhD, and Patricia K. Eagon, PhD
Many chemicals are carcinogenic to liver in rodents, some of which are also suspected to cause liver cancer in humans. These chemicals include common household and industrial agents as well as agricultural chemicals, pesticides, herbicides, and several therapeutic drugs that have wide human exposure and use. Experimental rodent models of liver cancer developed in our laboratory, as well as human liver specimens, demonstrate alterations in copper metabolism and accumulation of excess copper. Copper is an essential element and its levels are tightly regulated in the body. However, excess copper levels are toxic and may contribute to the development of liver cancer.1 Removal of excess copper by chelating agents or by zinc treatment inhibits the development of liver tumors. Human liver tumors also show accumulation of excess copper, similar to that seen in rodent models. Thus, it may be possible to reduce the incidence of human liver cancer by the use of copper-chelating agents or by a diet high in zinc.
Background
Liver cancer is one of the most common cancers world-wide and is known to affect several million individuals. In the United States, it is less common than in other areas of the world. However, recent evidence suggests that the incidence of this cancer in this country is increasing. The reason for this increased incidence is not known. Liver cancer takes several years to develop and the exact mechanism(s) by which liver cancer develops is not understood, even though several causative agents are suspected in the genesis of this cancer. There is no successful curative treatment for advanced liver cancer. Although liver transplantation, radiation, and chemotherapy are often attempted, these efforts usually meet with limited success. Unless we understand the mechanisms of this disease, the ideal therapy will continue to be elusive. Experiments conducted with rodents suggest that exposure to certain chemicals through the environment such as food, water, and air might initiate liver cancer. Once initiated, diet, as well as other physiological and environmental factors, is likely to influence the development of tumors further.
Environmental Chemicals and Liver Cancer
Several man-made and industrial chemicals that have wide human exposure are inducers of liver cancer in rodents. These chemicals include pesticides, herbicides, plasticizers, hypolipidemic agents such as clofibrate and gemfibrozil, and several others. These compounds, when given at high doses, are known to cause liver cancer in rodents. One theory regarding the mechanisms by which these agents cause liver cancer has received recent attention. It is known that these chemicals do not damage DNA, so identification of their specific actions was perplexing. It appears that one mechanism of action is the promotion of a high and sustained level of cell division. In addition, these chemicals change pathways of metabolism by altering enzyme activities and levels of chemicals, which results in metabolic imbalance. Sooner or later, an error will occur, and one or more cells will be altered. Each of these actions appears to inhibit the growth of normal liver cells and favor the growth of cancer cells, leading to the development of tumors.
It is also interesting to note that some of these chemicals mimic estrogens and alter sex steroid hormone levels in the blood of experimental rats. Therefore, these chemicals may also be classified as endocrine disrupters. Elevated sex steroid hormones such as estrogen may result in the transformation of a normal liver cell to a cancer cell and promote its growth to tumor.
The question becomes whether it is possible to reverse the actions of these chemicals and retard the growth of cancer cells. There may be several ways to achieve this goal. Of course, the primary goal would be to understand the metabolic changes and develop a strategy to reverse these processes in order to prevent malignant transformation. Our recent research has shown that in rats, exposure to these agents results in an accumulation of copper in the livers of experimental animals. As outlined below, copper has been shown to have a significant role in cellular damage and in the genesis of tumors in certain situations. Our studies confirmed that during the progression of liver cancer, the genes that precisely regulate copper levels in the body are suppressed, resulting in abnormal accumulation of copper in liver, liver tumors (if present), and possibly in other tissues as well. Copper is an essential element required for normal body functions; however, copper in excess is quite toxic to the liver cell. But somehow the cancer cell is able to overcome the toxic effects of excess copper and exhibit malignant growth and progression. These findings suggest that one strategy to reverse this process might be the removal of excess copper by copper chelators such as trientine or penicillamine, or by zinc administration—agents that appear to inhibit the development of liver tumors. Although these studies have only been performed to date in animal models, we find these results exciting and see potential for the development of preventive and therapeutic strategies.
Copper Toxicity
As stated above, copper is an essential element, which must be generally supplied through the diet. The recommended daily copper intake for humans is 1.5-3.0 mg. Copper is a cofactor for many enzymes. Copper deficiency is associated with reduced hemoglobin production and reduced elastin formation. Copper does not usually accumulate in excess in healthy individuals. The development of chronic effects occurs very infrequently because acute toxic episodes usually limit the excess intake of copper. However, copper toxicity can result from accidental or intentional exposure. Excessive copper intake causes intestinal disturbances, dizziness, headache, and a metallic taste in the mouth, and, if excessive, death. Respiratory collapse, hemolytic anemia, hemoglobinemia, and hepatic and renal failure were also documented in cases of excessive copper intake. Copper poisoning by drinking water is very uncommon.
Copper Disorders
Genetic disorders of copper metabolism include Wilson’s disease and Menkes’ disease.2 Wilson’s disease is an autosomal recessive disease and its prevalence rate is high. In this disease, copper accumulates in many tissues. Clinical manifestations include neurologic and liver disorders. Characteristic findings are Kayser-Fleischer rings in the eye and low serum ceruloplasmin, a copper-containing protein. Copper accumulation in the liver is caused by disturbance in biliary excretion of copper. A decrease in expression of the Wilson’s disease gene, which encodes a protein P-type ATPase, causes a block in the incorporation of copper into ceruloplasmin and also limits its excretion through the bile. Hepatic sequelae of Wilson’s disease demonstrate a broad spectrum of chronic, chronic active, and acute liver diseases, cirrhosis, and fulminant hepatic failure. Copper also plays an important role in iron metabolism as well. Copper deficiency impairs iron absorption, resulting in anemia. Ceruloplasmin has peroxidase activity that oxidizes ferrous iron to the ferric state prior to its binding by plasma transferrin. Elevated copper levels in the liver sometimes coincide with elevated iron in the liver. Zinc treatment prevents the absorption of copper from the intestine. If they are not treated, patients with Wilson’s disease die either due to progressive liver failure or neurological disease. Patients are treated with a copper-chelating agent such as penicillamine or trientine, zinc, and/or a copper-restricted diet. Patients should avoid foods high in copper such as shellfish, chocolate, mushrooms, nuts, and liver. Occasionally, liver transplantation is necessary for selected patients. Orally administered zinc inhibits copper absorption through the intestine. Therefore, Wilson’s disease patients are treated with zinc to prevent copper absorption from the intestine and prevent excess copper deposition in the body. If excess copper is not removed by the above treatments the accumulated copper causes cirrhosis of the liver, neurological degeneration and ultimately death. The defect in copper excretion is attributed to a lack of Wilson’s disease gene expression. The Wilson Disease gene has recently been cloned. This discovery holds promise to cure this disease as well as liver cancer. Only 11 cases of hepatocellular carcinoma have been reported in patients with Wilson’s disease. This low rate may be due to death from hepatic failure precluding the development of liver cancer or to the success of treatments to reduce copper levels.
Copper and Liver Cancer
Does excess copper cause liver cancer? If so, is it possible to prevent the development of liver cancer by removing the excess copper? The high incidence of cancer among coppersmiths suggests a primary carcinogenic role for the copper ion. A higher incidence of stomach cancers are noted in regions where soil zinc-to-copper ratios are found to be reduced. There are several experimental animal models available to study the role of copper in the genesis of liver cancer. Long-Evans rats with cinnamon-like coat (LEC), Bedlington terriers, and toxic milk mutation mice are known to have increased copper accumulation in the liver. LEC rats appear to closely mimic human Wilson’s disease. LEC rats suffer from spontaneous hepatitis with hemolytic anemia and jaundice, developing around 4 months after birth, which results in death in 50% of rats due to fulminant hepatitis. The rats that recover from fulminant hepatitis develop chronic hepatitis and a high rate of liver cancer (95%). LEC rats accumulate copper in liver and have a decreased serum ceruloplasmin. In LEC rats, the gene which is homologous with Wilson’s disease gene for humans decreases. Copper-restricted or high-zinc diets or treatment with copper-removing agents inhibits the growth of liver tumors in LEC rats. Thus, the role of copper as a causative agent of liver cancer is established. However, the mechanism by which copper induces liver cancer is not known. It is suggested that copper overload inhibits the growth of normal liver cells, giving a growth advantage to cancer cells that subsequently progress to tumors. It is also known that copper causes a variety of damages to liver cells, and, thus, may provoke the liver cells to carcinogenic changes or to increased cell growth.
During the course of our studies with liver cancer induced in rats by several hypolipidemic agents, we observed a significant alteration in copper metabolism.3 These animals showed accumulation of copper in liver tumors, and a significant decrease in serum ceruloplasmin. Expression of the genes that control copper metabolism such as Wilson’s disease gene and ceruloplasmin gene also decreased in liver tumors, confirming our contention that the induction of liver cancer in these rats by hypolipidemic chemicals causes alterations in copper metabolism as seen in patients with Wilson’s disease and LEC rats. In our initial experiments with rats treated with hypolipidemic agents, we observed that removal of excess copper by trientine or zinc diet partially prevented the liver pathology that leads to the progression of liver cancer and prevented deposition of copper. These results clearly underscored our contention that an essential element like copper, when deposited in excess, can cause cancer.
We next asked several important questions with respect to human liver cancer. Does human liver cancer tissue also demonstrate accumulation of excess copper? Does excess copper in the liver cause liver cancer? Concomitant with accumulation of excess copper, does the tumor show a significant decrease in zinc levels? Does the suppression of Wilson’s disease gene and ceruloplasmin gene expression precede the development of human liver tumors? To answer these important questions, we examined several human liver tumors, the tissue around the tumors, and normal liver tissues to determine levels of copper, zinc, and iron. The human liver cancer also possessed a several-fold accumulation of copper and a significant decrease in zinc levels, as was seen in the rats treated in our laboratory with hypolipidemic agents. Currently, experiments are under way to determine whether human liver tumors also exhibit suppression of Wilson’s disease and ceruloplasmin genes. If indeed copper plays an important role in the development of human liver cancer, then it may be possible to ameliorate the growth of liver cancer by copper-chelating agents or by zinc supplementation, as demonstrated unequivocally in LEC rats and in our experimental rats with liver cancer. The results obtained in these studies have potentially far-reaching clinical implications in the management of patients with liver cancer. (Dr. Rao is Professor of Pathology and Chief, Toxicology Laboratory; Dr. Eagon is Associate Professor of Medicine, University of Pittsburgh Medical Center, Pittsburgh, PA.)
References
1. Toxicity of Metals. LW Chang, ed.; L. Magos, T. Suzuki, Associate Eds. 1996; CRC Press, Inc.
2. Harris ZL, Gitlin JD. Am J Clin Nutr 1996;63:836S-841S.
3. Eagon PK, Teepe AG, et al. Carcinogenesis 1999;20:1091-1096.
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