Choline supplementation inhibits diethanolamine-induced morphological transformation in Syrian hamster embryo cells: Evidence for a carcinogenic mechanism

Abstract

DEA, an amino alcohol, and its fatty acid condensates are widely used in commerce. DEA is hepatocarcinogenic in mice, but shows no evidence of mutagenicity or clastogenicity in a standard testing battery. However, it increased the number of morphologically transformed colonies in the Syrian hamster embryo (SHE) cell morphologic transformation assay. The goal of this work was to test the hypothesis that DEA treatment causes morphologic transformation by a mechanism involving altered cellular choline homeostasis. As a first step, the ability of DEA to disrupt the uptake and intracellular utilization of choline was characterized. SHE cells were cultured in medium containing DEA (500 μg/ml), and 33P-phosphorus or 14C-choline was used to label phospholipid pools. After 48 h, SHE cells were harvested, lipids were extracted, and radioactive phospholipids were quantified by autoradiography after thin layer chromatographic separation. In control cells, phosphatidylcholine (PC) was the major phospholipid, accounting for 43 ± 1% of total phospholipid synthesis. However, with DEA treatment, PC was reduced to 14 ± 2% of total radioactive phospholipids. DEA inhibited choline uptake into SHE cells at concentrations ≥ 50 μg/ml, reaching a maximum 80% inhibition at 250-500 μg/ml. The concentration dependence of the inhibition of PC synthesis by DEA (0, 10, 50, 100, 250, and 500 μg/ml) was determined in SHE cells cultured over a 7-day period under the conditions of the transformation assay and in the presence or absence of excess choline (30 mM). DEA treatment decreased PC synthesis at concentrations ≥ 100 μg/ml, reaching a maximum 60% reduction at 500 μg/ml. However, PC synthesis was unaffected when DEA-treated cells were cultured with excess choline. Under 7- day culture conditions, 14C-DEA was incorporated into SHE lipids, and this perturbation was also inhibited by choline supplementation. Finally, DEA (10- 500 μg/ml) transformed SHE cells in a concentration-dependent manner, whereas with choline supplementation, no morphologic transformation was observed. Thus, DEA disrupts intracellular choline homeostasis by inhibiting choline uptake and altering phospholipid synthesis. However, excess choline blocks these biochemical effects and inhibits cell transformation, suggesting a relationship between the two responses. Overall, the results provide a plausible mechanism to explain the morphologic transformation observed with DEA and suggest that the carcinogenic effects of DEA may be caused by intracellular choline deficiency.