Rapid onset of nucleolar disintegration preceding cell cycle arrest in roscovitine‐induced apoptosis of human MCF‐7 breast cancer cells

The aim of our study was to explore the antiproliferative and pro‐apoptotic action of roscovitine (ROSC) on human breast cancer MCF‐7 cells. We examined the effect of ROSC on cell proliferation, cell cycle progression, nucleolar morphology, posttranslational modifications of histones as well as on induction of apoptosis. The effects of ROSC on the argyrophilic nucleolar organizer regions (AgNORs) and nucleolar RNA of MCF‐7 cells were marked: ROSC treatment changed the pattern of AgNORs in a time‐dependent manner. The disintegration of nucleoli manifested by increasing number of nucleolar fragments already began at 6 hr posttreatment. This was accompanied by a redistribution of the nucleolin from the nucleolus beginning after 6 hr and preceded a decrease of histone acetylation and phosphorylation. Inhibition of DNA synthesis and accumulation of G2/M‐arrested cells starting 6 hr posttreatment coincided with a strong increase of the p53 level and with an appearance of a few cells committed to undergo apoptosis. However, all these changes preceded the main wave of apoptosis, which occurred after 24 hr ROSC treatment as assessed by determination of the frequency of Annexin binding, activation of caspases as well as of DNA fragmentation. Onset of PARP‐1 cleavage detected by immunoblotting and by immunohistochemistry 6 hr or 9 hr posttreatment, respectively, preceded for a few hours the DNA fragmentation detected in situ by TUNEL assay. Reconstitution of MCF‐7 cells with caspase‐3 did not change the kinetics of ROSC‐induced apoptosis. Our results show that disintegration of nucleoli is an early marker of ROSC‐induced changes. Cell cycle arrest precedes the main wave of apoptosis. © 2003 Wiley‐Liss, Inc.