INVIVO INHIBITION OF CYCLIN-B DEGRADATION AND INDUCTION OF CELL-CYCLEARREST IN MAMMALIAN-CELLS BY THE NEUTRAL CYSTEINE PROTEASE INHIBITOR N-ACETYLLEUCYLLEUCYLNORLEUCINAL

The cytotoxic neutral cysteine protease inhibitor N-acetylleucylleucyl norleucinal (ALLN) inhibits cell-cycle progression in CHO cells, affec ting the G1/S and metaphase-anaphase transition points, as well as S p hase. Mitotic arrest induced by ALLN is associated with the inhibition of cyclin B degradation. At mitosis-inhibiting concentrations of ALLN , cells undergo nuclear-envelope breakdown, spindle formation, chromos ome condensation, and congression to the metaphase plate. However, nor mal anaphase events do not occur, and cells arrest in a metaphase conf iguration for a prolonged period. Steady-state levels of cyclin B incr ease to greater than normal mitotic levels, and cyclin B is not degrad ed for an extended period. Histone H1 kinase activity remains elevated during mitotic arrest. Duration of mitotic arrest depends on ALLN con centration; high concentrations (>50 mug/ml) produce a prolonged mitot ic arrest, whereas at lower concentrations, cells are transiently dela yed through mitosis (up to 4-12 hr), after which they undergo aberrant cell division resulting in randomly sized daughter cells containing v ariable amounts of DNA. Cyclin B degradation fails to occur, and histo ne H1 kinase remains activated for the duration of mitotic arrest at a ll ALLN concentrations.