leupeptins and 2-3-epoxysuccinic-acid

The cyclin-dependent kinase inhibitor p21(Cip1/WAF1) has been implicated as an inducer of differentiation. However, although expression of p21 is increased in postmitotic cells immediately adjacent to the proliferative compartment, its expression is decreased in cells further along the differentiation program. Expression of the p21 protein was decreased in terminally differentiated primary keratinocytes of mice, and this occurred by a proteasome-dependent pathway. Forced expression of p21 in these cells inhibited the expression of markers of terminal differentiation at both the protein and messenger RNA levels. These inhibitory effects on differentiation were not observed with a carboxyl-terminal truncation mutant or with the unrelated cyclin-dependent kinase inhibitor p16(INK4a), although all these molecules exerted similar inhibition of cell growth. These findings reveal an inhibitory role of p21 in the late stages of differentiation that does not result from the effects of p21 on the cell cycle.

Topics: Acetylcysteine; Adenoviridae; Animals; Animals, Newborn; Cell Cycle; Cell Differentiation; Cells, Cultured; Cyclin-Dependent Kinase Inhibitor p21; Cyclin-Dependent Kinases; Cyclins; Enzyme Inhibitors; Gene Expression Regulation; Keratinocytes; Leupeptins; Membrane Proteins; Mice; Mutation; Promoter Regions, Genetic; Protein Precursors; RNA, Messenger; Succinates; Transfection

The p27 mammalian cell cycle protein is an inhibitor of cyclin-dependent kinases. Both in vivo and in vitro, p27 was found to be degraded by the ubiquitin-proteasome pathway. The human ubiquitin-conjugating enzymes Ubc2 and Ubc3 were specifically involved in the ubiquitination of p27. Compared with proliferating cells, quiescent cells exhibited a smaller amount of p27 ubiquitinating activity, which accounted for the marked increase of p27 half-life measured in these cells. Thus, the abundance of p27 in cells is regulated by degradation. The specific proteolysis of p27 may represent a mechanism for regulating the activity of cyclin-dependent kinases.

Topics: Adenosine Triphosphate; Anaphase-Promoting Complex-Cyclosome; Animals; Cell Cycle Proteins; Cell Line; Cyclin-Dependent Kinase Inhibitor p27; Cyclin-Dependent Kinases; Cysteine Endopeptidases; Electroporation; Enzyme Inhibitors; Humans; Kinetics; Leupeptins; Ligases; Mice; Microtubule-Associated Proteins; Multienzyme Complexes; Proteasome Endopeptidase Complex; Rabbits; Recombinant Proteins; Succinates; Tumor Cells, Cultured; Tumor Suppressor Proteins; Ubiquitin-Conjugating Enzymes; Ubiquitin-Protein Ligase Complexes; Ubiquitin-Protein Ligases; Ubiquitins