The PML/RARα fusion protein inhibits tumor necrosis factor-α-induced apoptosis in U937 cells and acute promyelocytic leukemia blasts

Abstract

We investigated the effect of the acute promyelocytic leukemia (APL) specific PML/RARα fusion protein on the sensitivity to TNF-α-mediated apoptosis. The U937 leukemia cell line was transduced with PML/RARα cDNA. PML/RARα expression caused a markedly reduced sensitivity to TNF-α, even if apoptosis was triggered by agonistic antibodies to TNF-α receptors I and II (TNF-αRI, II). PML/RARα induced a 10-20-fold decrease of the TNF-α- binding capacity via downmodulation of both TNF-αRI and TNF-αRII: this may mediate at least in part the reduced sensitivity to TNF-α. Furthermore, the fusion protein did not modify Fas expression (CD95) or sensitivity to Fas- mediated apoptosis. The pathophysiological significance of these findings is supported by two series of observations. (a) Fresh APL blasts exhibit no TNF- α binding and are resistant to TNFα-mediated apoptosis. Conversely, normal myeloblasts-pro-myelocytes show marked TNF-αR expression and are moderately sensitive to TNF-α-mediated cytotoxicity. Similarly, blasts from other types of acute myeloid leukemia (AML M1, M2, and M4 FAB types) show an elevated TNF-α binding. (b) The NB4 APL cell line, which is PML/RARα+, shows low TNF-αR expression capacity and is resistant to TNF-α-triggered apoptosis; conversely a PML/RARα-NB4 subclone (NB4.306) exhibits detectable TNF-α- binding capacity and is sensitive to TNF-α-mediated cytotoxicity. These studies indicate that the PML/RARα fusion protein protects against TNF-α- induced apoptosis, at least in part via downmodulation of TNF-αRI/II: this phenomenon may play a significant role in APL, which is characterized by prolonged survival of leukemic blasts.