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 ethylene glycol monomethyl ether (EGME), a member of the glycol ether family, as compared to its reactive metabolite 2-methoxy-acetaldehyde (MALD). Since disruption of equilibrium between cell proliferation and cell death is thought to play a key role in multistage carcinogenesis, we investigated, in Syrian hamster embryo (SHE) cells exposed to various doses of EGME and MALD, impairment in apoptosis rate and in ornithine decarboxylase (ODC) metabolism. The activity of this rate-limiting enzyme of polyamine biosynthesis is closely related to cell proliferation and cell transformation. At the end-point, comparative action of the two products on SHE cell morphological transformation frequency was evaluated. One-stage exposure of SHE cells to 2 mM EGME 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 protocol (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 on TPA inhibiting action. Western blotting analysis showed that sequential treatment (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 that obtained with TPA alone; this was accompanied by an increase of ODC protein 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 cooperative events as apoptosis inhibition and ODC superinduction, followed by the increase of SHE-cell transformation frequency, are highly indicative of a carcinogenic potential for the metabolite, MALD. (C) 1999 Elsevier Science Ireland Ltd. All rights reserved.