THE PML RAR-ALPHA FUSION PROTEIN INHIBITS TUMOR NECROSIS FACTOR-ALPHA-INDUCED APOPTOSIS IN U937 CELLS AND ACUTE PROMYELOCYTIC LEUKEMIA BLASTS/

We investigated the effect of the acute promyelocytic Leukemia (APL) s pecific PML/RAR alpha fusion protein on the sensitivity to TNF-alpha-m ediated apoptosis, The U937 leukemia cell line was transduced with PML /RAR alpha cDNA. PML/RAR alpha expression caused a markedly reduced se nsitivity to TNF-alpha, even if apoptosis was triggered by agonistic a ntibodies to TNF-alpha receptors I and II (TNF-alpha RI, II). PML/RAR alpha induced a 10-20-fold decrease of the TNF-alpha-binding capacity via downmodulation of both TNF-alpha RI and TNF-alpha RII: this may me diate at least in part the reduced sensitivity to TNF-alpha. Furthermo re, the fusion protein did not modify Fas expression (CD95) or sensiti vity to Fas-mediated apoptosis The pathophysiological significance of these findings is supported by two series of observations. (a) Fresh A PL blasts exhibit no TNF-alpha binding and are resistant to TNF-alpha- mediated apoptosis, Conversely normal myeloblasts-promyelocytes show m arked TNF-alpha R expression and are moderately sensitive to TNF-alpha -mediated cytotoxicity. Similarly, blasts from other types of acute my eloid leukemia (AML M1, M2, and M4 FAB types) show an elevated TNF-alp ha binding. (b) The NB4 APL cell line, which is PML/RARa(+), shows low TNF-alpha R expression capacity and is resistant to TNF-alpha-trigger ed apoptosis; conversely a PML/RAR alpha(-) NB4 subclone (NB4.306) exh ibits detectable TNF-alpha-binding capacity and is sensitive to TNF-al pha-mediated cytotoxicity. These studies indicate that the PML/RAR alp ha fusion protein protects against TNF-alpha-induced apoptosis, at lea st in part via downmodulation of TNF-alpha RI/II: this phenomenon may play a significant role in APL, which is characterized by prolonged su rvival of leukemic blasts.