Liver copper storage and transport during development: implications for cytotoxicity.

Copper is an essential trace element for many biological processes. Its functions range from influencing specific gene expression to serving as a cofactor or prosthetic group for several enzymes. Intakes of copper at doses that exceed physiologic demands are normally met with efficient homeostatic mechanisms. Ceruloplasmin, albumin, and transcuprein, and to a lesser extent certain amino acids, are major copper-transporting constituents in circulating plasma. After its hepatic uptake, copper may be stored within hepatocytes, secreted into plasma, or excreted in bile. The biliary route represents the major excretory pathway of copper and largely accounts for its hepatic turnover. Copper retained by hepatocytes is mostly bound to specific metal-binding proteins, primarily metallothionein, or incorporated into several cuproenzymes. Copper incorporation into metallothionein and certain cuproproteins appears to require prior binding of copper to glutathione, thus defining a relation between copper metabolism and the intracellular availability of glutathione. Hepatic metallothionein concentrations can be modulated by dietary copper; changes in metallothionein and in copper status are significant throughout development. Binding of copper to metallothionein provides a temporary storage for cytoplasmic copper, preventing it from occurring as (potentially toxic) free ionic metal. In its unbound form, copper can generate hydroxyl radicals. Because metallothionein exhibits a high reactivity toward these radicals, it is increasingly recognized to play a protective role against copper-induced cytotoxicity. We discuss some of the possible toxicologic implications that may arise from changes in hepatic copper and metallothionein status during development.

Responses induced by tacrine in neuronal and non-neuronal cell lines.

Alzheimer's disease (AD) is associated with a reduction in cholinergic activity as a result of specific neuronal loss. Current potential treatments for the disease include both cholinomimetic drugs and anticholinesterase inhibitors. One of the drugs approved by the FDA is tacrine (9-amine-1,2,3,4 tetrahydroacridine; THA), a strong acetylcholinesterase (AChE) inhibitor. We have studied the effects of tacrine on glial and neuronal cells in culture assessing cell survival and viability and morphology. Lactate dehydrogenase (LDH) activity and methylthiazol-diphenyl-tetrazolium (MTT) reduction were used as toxicity indicators. We found that tacrine toxicity on rat B12 glial cells and mouse Neuro 2A cells was strongly dependent on its concentration (up to 500 microM) and time of exposure. The toxic effect was not prevented by serum factors nor by bovine serum albumin. Fluorescein-conjugated phalloidin was used to examine the arrangement of actin filaments at substrate adhesion regions and cell-cell contacts. Primary events following exposure to tacrine included changes in cell morphology, disappearance of actin filament bundles, and disruption of focal adhesion contacts. At concentrations between 10 and 50 microM, tacrine induced neurite outgrowth in Neuro 2A cells, an effect that was not observed in B12 cells, suggesting that certain tacrine effects could be specific for neuronal cells. Although similar trends of response were observed for both cell types, some differences between undifferentiated and differentiated cells were apparent.