Inhibition of the expression of ornithine decarboxylase and c-Myc by cell-permeant ceramide in difluoromethylornithine-resistant leukaemia cells

摘要

pCeramide has emerged as a novel lipid mediator in cell growth and apoptosis. In difluoromethylornithine-resistant L1210 cells stimulated to growth from quiescence, the cell-permeant analogues of ceramide iN/i-acetylsphingosine (Csub2/sub-ceramide) and iN/i-hexanoylsphingosine (Csub6/sub-ceramide) inhibited the induction of ornithine decarboxylase (ODC) activity with ICsub50/sub of 8.3 and 1.5 iμ/iM respectively. This effect was strictly related to the ability to inhibit cell growth and [sup3/supH]thymidine incorporation. The suppression of cell growth was also associated with apoptosis. The addition of bacterial sphingomyelinase resulted in a significant, but limited, reduction of ODC induction and [sup3/supH]thymidine incorporation. Bacterial lipopolysaccharide, which may act as a ceramide analogue, also inhibited the induction of the enzyme. Moreover, Csub6/sub-ceramide largely prevented the accumulation of ODC mRNA and its precursor, ODC heterogeneous nuclear RNA, that accompanied the induction of ODC activity. A slight increase in ODC turnover was also observed. The DNA-binding activity of some transcription factors known to bind and transactivate the ODC gene was investigated by gel mobility-shift assay under the same experimental conditions. However, only the binding of Myc/Max was negatively affected by the treatment with Csub6/sub-ceramide. Furthermore, the amount of immunoreactive c-Myc, which increased after stimulation of the cells to growth, was strongly reduced by Csub6/sub-ceramide. These results suggest that the inhibition of c-Myc and ODC expression may be early events in the response of leukaemia cells to ceramide./p