Over-expression of nucleophosmin/B23 decreases the susceptibility of human leukemia HL-60 cells to retinoic acid-induced differentiation and apoptosis

Stable clones of HL-60 cells in which nucleophosmin/B23 was over-expressed were established. Less percentages (4-20%) of nucleophosmin/B23 over-expressed (pCR3-B23) cells exhibited the morphological characteristic of apoptosis as compared with control vector-transfected (pCR3) cells (6-53%) during the 10 mu M RA treatment for 1-4 days. In flow cytometry analysis, a block in the G(1) phase was noted in ail the pCR3-B23 and pCR3 cells after 2 days of 10 mu M RA treatment and continued to be observed at all times measured up to 6 days. Smaller peaks of apoptotic cells with less than G(1) DNA content were observed in pCR3-B23 as compared with pCR3 cells after 4-6 days of 10 mu M RA treatment. As measured by expressions of differentiation markers and the functional assessment of the ability to reduce nitroblue-tetrazolium, our results further showed that over-expression of nucleophosmin/B23 decreased the response of the cells to RA-induced differentiation. Less cleavage of PARP and in vitro caspase-3 activity were observed in PCR3-B23 cells as compared with pCR3 cells treated with 10 mu M RA for 3-4 days, IRF-1 was induced after 6 hr of 10 mu M RA treatment in the pCR3-B23 and pCR3 cells. Significantly more nucleophosmin/B23 was co-immunoprecipitated with IRF-1 from pCR3-B23 cells than from pCR3 cells during RA treatment (10 mu M; 24 hr, 96 hr). The IRF-1 transcriptional activity was found to be attenuated in pCR3-B23 cells as compared with pCR3 cells during the treatment of cells with RA, Nucleophosmin/B23, through interacting with IRF-1, plays an important role in the control of the susceptibility of cells to RA-induced differentiation and apoptosis. Int. J. Cancer 88:392-400, 2000. (C) 2000 Wiley-Liss. Inc.