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

Abstract

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 μM RA treatment for 1–4 days. In flow cytometry analysis, a block in the G₁ phase was noted in all the pCR3‐B23 and pCR3 cells after 2 days of 10 μM RA treatment and continued to be observed at all times measured up to 6 days. Smaller peaks of apoptotic cells with less than G₁ DNA content were observed in pCR3‐B23 as compared with pCR3 cells after 4–6 days of 10 μM RA treatment. As measured by expressions of differentiation markers and the functional assessment of the ability to reduce nitrobluetetrazolium, 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 μM RA for 3–4 days. IRF‐1 was induced after 6 hr of 10 μM 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 μM; 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.