Apoptosis inhibition and ornithine decarboxylase superinduction as early epigenetic events in morphological transformation of Syrian hamster embryo cells exposed to a 2-methoxyacetaldehyde, a metabolite of 2-methoxyethanol

We have conducted a study to determine the carcinogenic potential of ethyle ne glycol monomethyl ether (EGME), a member of the glycol ether family, as compared to its reactive metabolite 2-methoxy-acetaldehyde (MALD). Since di sruption of equilibrium between cell proliferation and cell death is though t to play a key role in multistage carcinogenesis, we investigated, in Syri an hamster embryo (SHE) cells exposed to various doses of EGME and MALD, im pairment in apoptosis rate and in ornithine decarboxylase (ODC) metabolism. The activity of this rate-limiting enzyme of polyamine biosynthesis is clo sely related to cell proliferation and cell transformation. At the end-poin t, comparative action of the two products on SHE cell morphological transfo rmation frequency was evaluated. One-stage exposure of SHE cells to 2 mM EG ME and 100 mu M MALD for 5 h did not change basal apoptotic level, whereas 0.16 mu M phorbol ester (TPA) decreased it. Using two-stage exposure protoc ol (1 h xenobiotic followed by 5 h TPA), MALD strongly inhibited apoptosis more than did TPA alone: the parent compound EGME did not have any effect o n TPA inhibiting action. Western blotting analysis showed that sequential t reatment (MALD/TPA) increased Bcl-2 oncoprotein expression, whereas Bcl-x(L ) and Bar proteins were not changed. The same staged exposure of SHE cells to MALD/TPA strongly induced ODC activity, and the rate was higher than tha t obtained with TPA alone; this was accompanied by an increase of ODC prote in level. This ODC superinduction was not observed with EGME/TPA treatment. In long-term SHE-cell morphological transformation assay, staged exposure to MALD (800 mu M or 1 mM for 24 h) followed by TPA applications increased the number of transformed colonies at the seventh day. Such early cooperati ve events as apoptosis inhibition and ODC superinduction, followed by the i ncrease of SHE-cell transformation frequency, are highly indicative of a ca rcinogenic potential for the metabolite, MALD. (C) 1999 Elsevier Science Ir eland Ltd. All rights reserved.