MECHANISMS OF UPTAKE OF GALLIUM BY HUMAN NEUROBLASTOMA-CELLS AND EFFECTS OF GALLIUM AND ALUMINUM ON CELL-GROWTH, LYSOSOMAL PROTEASE, AND CHOLINE ACETYL TRANSFERASE-ACTIVITY

We have studied the uptake and removal of gallium, used as an analogue of aluminum, and the effects of aluminum itself on cultured human neu roblastoma cells treated with soluble metal complexes. The prohibitive ly high cost of measurement of the only available radioisotope of alum inum (Al-26) precluded its usage, and so we considered that gallium, w hich is chemically extremely similar, would be the most suitable model . Gallium has been used thus in a number of previous biological studie s and has been found to behave like aluminum in many respects. We have previously shown that AZ-EDTA treatment results in uptake of aluminum and expression of hyperphosphorylated tau, a key component of Alzheim er's disease paired helical filaments. Here we demonstrate that galliu m uptake can occur by two separate methods, both leading to physiologi cally relevant intracellular metal concentrations. Uptake from medium containing bovine transferrin occurred mainly by pinocytosis, but in t he presence of human transferrin (hTf), uptake by transferrin-mediated endocytosis occurred also, despite a very low level of hTf saturation , indicating that Tf-mediated uptake is a very effective method of Ga internalization. The intracellular gallium is relatively stable, thoug h partially removable by (1 mM) EDTA, desferrioxamine, or 1,2-dimethyl -3-hydroxypyrid-4-one. Aluminum and gallium treatment were found to in crease the overall activity of lysosomal proteases, enzymes implicated in amyloid precursor protein cleavage. No effects were detected on ch oline acetyl transferase activity, cell growth, or tritiated thymidine incorporation or on the structure of the cells, as judged by light or electron microscopy. (C) 1998 Academic Press.