Dietary and Genetic Effects on Cellular Copper Homeostasis in Bovine and Porcine Tissues

Copper is required for an array of physiological processes and is essential for proper fetal development. In ruminants, copper deficiency is a problem that manifests due to low dietary copper intake or when the diet provides high levels of copper antagonists, such as iron, molybdenum or sulfur. Genetics also contributes to the incidence of copper deficiency, as Simmental cattle have been shown to be more susceptible to copper deficiency than Angus. In pigs, copper deficiency is very rare and weanling pigs oftentimes are supplemented dietary copper as high as 50-fold above their requirement to obtain a pharmacological response. Furthermore, the use of ingredients such as phosphate supplements in nursery pig diets can provide superfluous amounts of dietary iron. Numerous transport and chaperone proteins have been identified in rodents that serve to regulate Cu homeostasis. However, regulation of these proteins has not been studied in cattle or pigs. The purpose of this research was to characterize mRNA of copper regulatory proteins in cattle and pigs, and to characterize the high affinity copper transporter 1 protein in bovine tissues. We examined how these molecular mechanisms are affected by level and source of dietary copper, breed, and high dietary iron in cattle and pigs. We found that severe copper deficiency in the bovine greatly reduced copper status and mRNA expression of cytochrome c oxidase assembly protein 17, an essential copper chaperone protein. In a subsequent study with pregnant Angus and Simmental cows, dietary copper did not affect expression of gene products involved in copper homeostasis in maternal duodenum or liver. However, expression of duodenal copper transporter 1 mRNA in Simmentals was approximately 25% of that of Angus while mRNA expression of Atp7a was approximately 50% of that of Angus. Duodenal copper transporter 1 protein also tended to be lower in Simmental than in Angus. In the same experiment, breed did not affect expression of copper transporters and chaperones in fetal liver. Feeding a diet low in copper reduced copper in the bovine placentome and fetal liver, and consequently mRNA expression of antioxidant 1, cytochrome c oxidase assembly protein 17, and copper metabolism MURR1 domain 1 were up-regulated in liver of low copper fetuses. In a study with weanling pigs, we demonstrated that both level and source of dietary copper affects mRNA expression of copper regulatory proteins. Pigs supplemented 225 mg Cu/kg diet from Cu2(OH)3Cl (tribasic copper chloride) had approximately 50% less mRNA of duodenal copper transporter 1 than those supplemented 225 mg Cu/kg diet from CuSO4 (copper sulfate) and controls. Duodenal metallothionein 1a mRNA was much higher in CuSO4-supplemented pigs than in Cu2(OH)3Cl and control pigs. In the proximal jejunum, copper transporter 1 mRNA was down-regulated while antioxidant 1 mRNA was up-regulated in Cu-supplemented pigs. Interactions between iron and copper were examined in a subsequent study with pigs. Level of dietary iron also affected mRNA expression of copper transporters in liver of weanling pigs. Messenger RNA of the biliary copper exporter Atp7b was down-regulated in the pigs receiving low and high dietary iron, which agreed with the concomitant changes in liver copper concentrations. We also examined iron and copper interactions in the young bovine, utilizing Holstein calves. Data from this study demonstrated that high dietary iron increased copper efflux. Collectively these series of experiments herein address some of the practical issues in regard to copper metabolism in cattle and pigs and provide a better understanding of the cellular regulation that occurs in these species.