Transformation of rat tracheal epithelial cells to immortal growth variants by particulate and soluble nickel compounds.

The cytotoxicity and transforming activity of nickel subsulfide, nickel oxide and nickel sulfate was studied by assays of colony-forming efficiency and of transformation of rat tracheal epithelial (RTE) cells to enhanced growth variants (EGVs) and immortal growth variants (IGVs). Nickel subsulfide caused dose-dependent cytotoxicity between 1 and 5 micrograms/ml, whereas the cytotoxic range of nickel oxide and nickel sulfate was 50-200 micrograms/ml and 60-130 micrograms/ml, respectively. At lower concentrations, nickel sulfate caused modest (up to 126%) growth stimulation. During the initial 24-h treatment period, internalized nickel subsulfide particles were observed in phagocytic vesicles in cells near the periphery of all RTE cell colonies, whereas nickel oxide particles were not internalized but had adhered to both the cells and the tissue culture dish. After 7-10 days of the transformation assay, nickel subsulfide particles were no longer visible, but nickel oxide particles remained on the dish for the duration of the 5 week assay. During weeks 3-5 of the transformation assay, internalized nickel oxide particles were observed in non-vacuolated cells at the periphery of the colonies. All 3 nickel compounds significantly (p < 0.05) increased the transformation frequency of RTE cells to EGVs at moderately cytotoxic concentrations; the order of potency was Ni3S2 > NiO = NiSO4. MNNG, the positive control, was twice as active as nickel subsulfide at 1/3 the concentration and 1/6 the duration of treatment. EGVs induced by MNNG, nickel subsulfide and nickel sulfate were cloned and converted to IGVs at frequencies of 44, 24 and 43%, respectively. In contrast, EGVs transformed by nickel oxide rarely converted to IGVs (13%). All nickel-induced IGVs were immunohistochemically epithelial, mitotically active, aneuploid and exhibited high plating efficiencies. Our results suggest that respiratory epithelial cells are targets for the transforming capabilities of several nickel compounds but that the potency and mechanism of transformation by various forms of nickel may be different according to the physico-chemical properties of each compound.