Phorbol ester (12-O-tetradecanoylphorbol 13-acetate) prevents ornithine decarboxylase inhibition and apoptosis in L1210 leukemic cells exposed to TGF-β1

Abstract Previous studies have shown that growth suppression and apoptosis of leukemic cells exposed to TGF- β 1 is associated with the inhibition of ornithine decarboxylase (ODC) — the key enzyme of polyamine pathway. The aim of the present study was to evaluate the influence of 12- O -tetradecanoylphorbol 13-acetate (TPA) — a potent ODC inducer on antiproliferative and apoptotic effects of TGF- β 1 in L1210 leukemic cells. Cells were incubated in 2%FCS/RPMI1640 medium, supplemented with TGF- β 1 (2 ng/ml), TPA (100 ng/ml) or α-difluoromethyl-ornithine (DFMO) (5 mM). Cell proliferation, apoptosis and necrosis were evaluated using [methyl- 3 H] thymidine, electron microscopy, electrophoresis of DNA and trypan blue exclusion. Expression and activity of ODC were determinated by RT-PCR and measurement of 14 CO 2 release from L-1- 14 C ornithine, respectively. TGF- β 1 inhibited proliferation and induced apoptotic and necrotic cell death in L1210 leukemic cells. The above effects were associated with the inhibition of ODC expression and activity, measured 2 and 4 hr after TGF- β 1 administration, respectively. The presence of DFMO, an irreversible inhibitor of ODC, led to apoptotic fragmentation of DNA, similar to that observed in TGF- β 1 -treated cultures. Administration of TPA simultaneously with TGF- β 1 significantly reduced antiproliferative, apoptotic and necrotic effects of TGF- β 1 , and prevented its inhibitory action on ODC expression and activity. It is concluded that: down-regulation of ODC expression may be one of the early events associated with TGF- β 1 -evoked suppression of growth and apoptosis; ODC is involved in the mechanism of protective action of TPA on TGF- β 1 -related growth inhibition of L1210 leukemic cells.