Evaluation of the "Nickel-Ion Hypothesis" of Cytotoxic Responses in AS52 CHO Cells

Abstract

Eleven nickel compounds representing a range of solubilities and biological activities were tested for toxicity, mutagenicity, and cytosolic and nuclear nickel uptake in AS52 cells. values ranging LC50 from 2-130 ug Ni/ml for particulates and 120-150 ug Ni/ml for the water soluble salts (NiCl2, NiS04, Ni(CH3C00)2) were determined. The Ni(OH)2, NiC03 , and nickel sulphides (Ni3S2 , Ni 7S6 , amorphous NiS) exhibited similar toxicities (LC50's of 2.0, 5.8, 4.1, 8.2, 4.1 ~g Ni/ml respectively), while the nickel oxides were less toxic and showed large variations between the black, Li 2Ni 8010 , and green NiO forms (LC50's of 18.1, 75, 130 ug Ni/ml). Concentrations reducing survival to the range 20-80% were tested for mutagenicity and degree of nickel uptake. Although nickel compounds have been reported to be only weak or equivocal mutagens, the results indicate a low but significant increase in mutation rate at the gpt locus induced by all the nickel compounds tested. The majority of compounds displayed nuclear to cytoplasmic nickel ratios of ≈ 1:4 to 1:2, though this was ≈ 1:20 for nickel salts. NiC03 appeared to be intermediate in behaviour with a ratio of ≈ 1:12. Comparison of the eleven compounds at the same toxicity level (LC50) showed a 75-fold difference in exposure levels but about a 10-fold difference in cytoplasmic and nuclear nickel levels. There appears to be a very good correspondence between previously reported dissolution half times (T50's) of the compounds tested and the cytosolic nickel levels at a given toxicity level. For the water-soluble salts, previous reports have shown that cellular distribution varies from that of particulates due to differences in the manner of uptake. The present work confirms this and suggests that the compounds can be divided into three classes: watersoluble salts producing very low nuclear levels and high cytosolic levels, inert nickel oxides (green NiO and lithium nickel oxide) with relatively low nuclear and cytosolic nickel levels, and the remaining compounds (the major class) with relatively high cytosolic levels and nuclear nickel levels. Overall , the data supports the N i eke 1-Ion Hypothesis which suggests that the Ni 2+ ion is the active agent in nickel toxicity and mutagenicity, and that, as a first approximation, its intracellular concentration is responsible for the observed effects, irrespective of the nickel compound.