MOLECULAR MECHANISMS OF TRANSFORMATION OF C3H 10T1/2 C18 MOUSE EMBRYOCELLS AND DIPLOID HUMAN FIBROBLASTS BY CARCINOGENIC METAL-COMPOUNDS/

Carcinogenic arsenic, nickel, and chromium compounds induced morpholog ical and neoplastic transformation but no mutation to ouabain resistan ce in 10T1/2 mouse embryo cells, lead chromate also did not induce mut ation to ouabain or 6-thioguanine resistance in Chinese hamster ovary cells. The mechanism of metal-induced morphological transformation was likely not due to the specific base substitution mutations measured i n ouabain resistance mutation assays, and for lead chromate, likely no t due to this type of base substitution mutation or to frameshift muta tions. Preliminary data indicate increases in steady-state levels of c -myc RNA in arsenic-, nickel-, and chromium-transformed cell lines. We also showed that carcinogenic nickel, chromium, and arsenic compounds and N-methyl-N-nitro-N-nitrosoguanidine (MNNG) induced stable anchora ge independence (At) in diploid human fibroblasts (DHF) but no focus f ormation or immortality. Nickel subsulfide and lead chromate induced A l but not mutation to 6-thioguanine resistance. The mechanism of induc tion of Al by metal salts in DHF was likely not by the type of base su bstitution or frameshift mutations measured in these assays. MNNG indu ced Al, mutation to 6-thioguanine resistance, and mutation to ouabain resistance,and might induce Al by base substitution or frameshift muta tions. Dexamethasone, aspirin, and salicylic acid inhibited nickel sub sulfide, MNNG, and 12-O-tetradecanoylphorbol-13-acetate (TPA)-induced Al in DHF, suggesting that arachidonic acid metabolism and oxygen radi cal generation play a role in induction of Al. We propose that nickel compounds stimulate arachidonic acid metabolism, consequent oxygen rad ical generation, and oxygen radical attack upon DNA. Intracellular red uction of Cr(VI) to Cr(V) or other species that generate oxygen radica ls leads to CF(V) or oxygen radical attack upon DNA. Arsenite causes c hromosome breaks. We propose that arsenic, nickel, and chromium compou nds then cause small deletions or mutations in the 5' or 3' regulatory regions of the c-myc and other protooncogenes, resulting in stabiliza tion of c-myc RNA and higher steady-state levels of c-myc RNA and prot ein. We also postulate that nickel-induced oxygen radical generation, Cr(V) ions or oxygen radicals generated by chromium, and arsenite indu ce inactivating mutations or deletions in tumor suppressor genes. Arse nic, nickel, or chromium compound-induced neoplastic transformation is postulated to proceed through a combination of activation of c-myc an d/or other protooncogenes and inactivation of tumor suppressor.