ALTERED SENSITIVITY TO RETINOID-INDUCED APOPTOSIS ASSOCIATED WITH CHANGES IN THE SUBCELLULAR-DISTRIBUTION OF BCL-2

In the acute promyelocytic leukemia cell line NB4, Bcl-2 downregulatio n occurred as a late event of retinoid-induced differentiation. In the maturation-resistant NB4-R1 subclone, retinoids failed to downregulat e Bcl-2 even in the situation of apoptosis massively induced by pan-ag onists and RXR-selective agonists. We observed that NB4 and NB4-R1 cel ls differed with respect to the intracellular localization of Bcl-2 wh ich showed a perinuclear localization in NB4-R1 cells, while Bar was b roadly expressed in the cytoplasm and to only a minor extent in the pe rinuclear area. Therefore, the distinct intracellular localization of Bcl-2 and Bar was in general nonoverlaping. Bcl-2 remained massively e xpressed until cell disruption. Bar was not significantly upregulated in cells committed to death. However, Bar localization changed from a diffuse pattern to concentrate in few specific cytoplasmic area at a s tage preceding the formation of apoptotic bodies. A human Bcl-2 transg ene was transiently overexpressed in NB4-R1 cells which showed increas ed resistance to apoptosis induced by retinoids. Stably transfected cl ones of NB4-R1 cells showed an increased expression of Bcl-2 and a mar ked resistance to apoptosis. Interestingly, the overexpression of Bcl- 2 restored a pattern of uniform Bcl-2 labeling in the cytoplasm and, r emarkably, the colocalization of Bcl-2 with Bar. This work demonstrate s that the ability of retinoid-induced cells to undergo apoptosis depe nds on the level of expression and the functional interaction between Bcl-2 and Bar. (C) 1997 Academic Press.