Induction of morphological transformation in BALB/3T3 mouse embryo cells by okadaic acid

Differential expression of genes associated with cell proliferation and apoptosis induced by okadaic acid during the transformation process of BALB/c 3T3 cells

Okadaic acid (OA) is a tumor promoter in two-stage carcinogenesis experiments. Nevertheless, the effects of OA on cell transformation, cell proliferation and apoptosis vary widely, and the molecular events underlying these effects of OA are not well understood. In the present study, we examined the promoting activity and the associated effects on cell growth and apoptosis mediated by OA in BALB/c 3T3 cells, and evaluated alterations of gene transcriptional expression by microarray analysis. The promoting activity of OA was estimated by a two-stage transformation assay, in which cells were treated first with a low dose of the initiator N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) and then with OA for 14 days. It showed that OA, at concentrations of 7.8-31.3 ng/ml, enhanced the transformation of MNNG-treated cells. In the promotion phase, cells exposed to OA (7.8 ng/ml) grew slowly for the first 2 days and subsequently died. As determined by Hoechst 33342 fluorescent dye and Annexin-V/PI dual-colored flow cytometry, OA induced morphologically apoptotic cells and increased the percentage of early apoptotic cells. The gene expression profile induced by OA at five time points in the promotion phase was determined by use of a specific mouse toxicological microarray containing 1796 clones, and a total of 177 differentially expressed genes were identified. By gene ontology analysis, 31 of these were determined to be functionally involved with cell growth and/or maintenance. In this group, numerous genes associated with the cell proliferation and cell cycle progression were down-regulated at early and/or middle time points. Among these was a subset of genes associated with apoptosis, in which Bnip3, Cycs, Casp3 and Bag1 genes are involved in the mitochondrial pathway of apoptosis. Ier3, Mdm2 and Bnip3 genes may be p53 targets. Furthermore, real-time PCR confirmed the expression changes of five genes selected at random from the differentially expressed genes. We conclude that OA induces cell growth inhibition and apoptosis in the two-stage, MNNG-initiated transformation of BALB/c 3T3 cells. The results of gene expression profile analysis imply that multiple molecular pathways are involved in OA-induced proliferation inhibition and apoptosis. Mitochondrial and p53-associated apoptotic pathways also may contribute to OA-induced apoptosis.

Lack of transforming activity of fumonisin B1 in BALB/3T3 A31-1-1 mouse embryo cells

The capacity of fumonisin B1 (FB1) to induce morphological transformation of cultured mammalian cells was assessed using BALB/3T3 A31-1-1 mouse embryo cells. FB1 with 90% purity was prepared from Fusarium proliferatum grown on whole corn. Cell growth was not inhibited by 48 hr of exposure at concentrations up to 1000 micrograms/ml. Moderate inhibition was induced by 6 days of exposure. In transformation assays with a 48-hr exposure, increases in transformed foci were observed at some concentrations; however, the responses were not reproducible. Prolonged exposure for up to 4 wk at 10, 100 and 500 micrograms/ml failed to induce increases in transformed foci. Analysis of combined results showed that only the increase induced by a 48-hr exposure at 500 micrograms/ml was significant. A trend test indicated the lack of a dose response for concentrations of 10-1000 micrograms/ml. FB1 seems to lack in vitro transforming activity.