The effects of cyclosporine and α-difluoromethylornithine on the induction of ornithine decarboxylase activity in colon cancer cells in vitro

Previous studies in our laboratory have demonstrated that cyclosporine (CsA) and α-difluoromethylornithine (DFMO) inhibit the growth of mouse (MC-26) colon carcinoma in vitro and in vivo. The mechanism of DFMO inhibition is known; however, the mechanism by which CsA acts remains unclear. Enzyme kinetic studies failed to demonstrate a direct effect of CsA on ODC activity; DFMO, as expected, caused significant inhibition of ODC activity. Significant induction of ODC activity occurred in MC-26 cells six hours after replating in fresh media, as compared with baseline activity obtained immediately after trypsinization. There was a super-induction of ODC activity at six hours caused by the addition of CsA (8.3 x 10^-3 mM). The increase in ODC activity was effectively abolished by the addition of emetine but not actinomycin D. These findings suggest that new protein synthesis is required for the elaboration of ODC induction measured at six hours. The presence of CsA (8.3 x 10^-3 mM) did not significantly alter ODC activity at 24 hours. However, both emetine and actinomycin D significantly inhibited ODC activity at 24 hours. These data indicate that the synthesis of new mRNA and subsequently new protein are required to maintain the increases in ODC activity that were observed six hours after replating.