Transforming growth factor-beta 1 (TGF-beta 1) exerted growth-inhibitory effect on L1210 leukemic cell line, manifested by the decrease in viable and increase in dead cells. The cell death evoked by TGF-beta 1 was both necrotic and apoptotic, quantified by the trypan blue exclusion method and apoptotic index, respectively. The induction of programmed cell death by TGF-beta 1 was confirmed by gel electrophoresis of DNA, where the characteristic 'DNA ladder' resulting from the internucleosomal DNA cleavage was visualized. The enhancement of cell mortality by TGF-beta 1 was associated with the inhibition of ornithine decarboxylase (ODC) expression (measured by the reverse transcriptase-polymerase chain reaction method) and impaired activity of this key enzyme in polyamine synthesis. Orotic acid (OA)--a known tumor promoter--stimulated proliferation of L1210 leukemic cells and diminished the necrotic effect of TGF-beta 1, but it did not change the extent of apoptosis evoked by TGF-beta 1. OA increased the expression of ODC and diminished depressional influence of TGF-beta 1 on transcription and activity of ODC in leukemic cells.
In conclusion: OA is a bioactive compound stimulating the growth of leukemic cells and diminishing the growth-inhibitory effect of TGF-beta 1. ODC gene is probably one of the targets for both OA and TGF-beta 1 influences in L1210 leukemic cells.