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Effects of Difluoromethylornithine and Dicyclohexylammonium Sulfate on the Transformed State of AKR-MCA Cells¹

Bristol Baylor Laboratory, Baylor College of Medicine, Houston, Texas 77030

The effect of two inhibitors of polyamine biosynthesis, difluoromethylornithine and dicyclohexylammonium sulfate, on the transformed fibroblastic cell line AKR-MCA and its parental counter part AKR-2B was investigated. Treatment of monolayer AKR-MCA cells with either agent results in morphological changes akin to AKR-2B; the cells appear to be flattened with a polygonal shape. The ability of the inhibitors to alter the phenotype is lost when the cells are cocultured with polyamines. More specifically, putrescine and spermidine abrogate the effects of difluoromethylornithine while only spermidine is effective in reversing the dicyclohexylammonium sulfate induced phenomenon. Further evidence that these enzyme inhibitors are reversing the transformed state of AKR-MCA is obtained from soft agarose experiments. AKR-MCA cells will generate colonies only in the absence of either difluoromethylornithine or dicyclohexylammonium sulfate.

Polyamine levels were determined in parental AKR-2B and AKR-MCA cells. The levels of putrescine and spermine were similar in both cell types. In contrast, significantly more (P 0.01) spermidine was expressed by the malignant line [7.3 ± 0.8 (SD) nmol/10⁶ cells] when compared with the untransformed AKR-2B (5.4 ± 0.8 nmol/10⁶) cells. Intracellular putrescine and spermidine were sensitive to difluoromethylornithine, dicyclohexylammonium sulfate, and dimethylformamide, a planar, polar solvent which has been reported to "normalize" the transformed phenotype. AKR-MCA treated with difluoromethylornithine or dimethylformamide manifested time dependent reductions in both polyamines which preceded morphological changes. Dicyclohexylammonium sulfate similarly caused a 70% reduction in spermidine, but in contrast to the other agents there was a marked accumulation of putrescine. These data concur with the established molecular actions of the two enzyme inhibitors as blockers of ornithine decarboxylase (difluoromethylornithine) and spermidine synthase (dicyclohexylammonium sulfate). The normalizing capacity of dimethylformamide was not compromised by cotreatment with putrescine or spermidine.

Both difluoromethylornithine and dicyclohexylammonium sulfate inhibited the growth of monolayer AKR-2B and AKR-MCA. In view of the well documented cytostatic effects of polyamine inhibitors, it is suggested that a decrease in growth by these agents triggers a more normal phenotype in AKR-MCA cells.

¹ This work was supported by NIH grants CA34432 and CA38173 (M.G.B.).

² To whom requests for reprints should be addressed, at the Bristol-Baylor Laboratory, Department of Pharmacology, Baylor College of Medicine, Houston, TX 77030.

Received 7/21/86. Revised 1/12/87. Revised 4/24/87. Accepted 4/30/87.