Soluble vs insoluble hexavalent chromate. Relationship of mutation to in vitro transformation and particle uptake

Soluble CaCrO4 and insoluble PbCrO4 were tested for induction of mutation to 6-thioguanine (base-substitution, deletion, addition, and frameshift mutations) or ouabain (base-substitution mutations) resistance in Chinese hamster ovary cells and morphological transformation in C3H/10(1/2) mouse embryo cells. CaCrO4 induced dose-dependent cytotoxicity and mutation to 6-thioguanine resistance, but did not induce mutation to ouabain resistance or morphological transformation. Highly cytotoxic amounts of CaCrO4 induced conversion of 10T1/2 cells to adipocytes, but cell lines derived from such cells were not transformed. PbCrO4 was not mutagenic in either mutation assay but induced a dose-dependent, low frequency of focus formation. Cell lines established from these foci had a 3-5-fold increased saturation density, grew in soft agarose, and were tumorigenic in nude mice. Chronic exposure to CaCrO4 or PbCl2 did not induce transformation, PbCl2 was inactive even at acutely cytotoxic concentrations, and sequential treatments with CaCrO4 and PbCl2 did not induce transformation. Light and scanning electron microscopy showed progressive cytoplasmic engulfment of PbCrO4 particles and extensive vacuolization of cells in contact with the particles. No particles were observed inside of vacuoles. We suggest that internalization of PbCrO4 and the associated cellular stress response may be related to PbCrO4-induced neoplastic transformation of 10T1/2 cells.