C/EBPα triggers proteasome-dependent degradation of cdk4 during growth arrest

• Published in: The EMBO journal 2002-03, Vol.21 (5), p.930-941
• Main Authors: Wang, Hongmei, Goode, Triona, Iakova, Polina, Albrecht, Jeffrey H., Timchenko, Nikolai A.
• Format: Article
• Language: English
• Subjects: cdk4; cell cycle; EBPα; EMBO06; EMBO31; proteasome
• ISSN: 0261-4189; 1460-2075
• DOI: 10.1093/emboj/21.5.930

CCAAT/enhancer binding protein alpha (C/EBPα) causes growth arrest via direct interaction with the cyclin‐dependent kinases cdk2 and cdk4. In this paper, we present evidence showing that C/EBPα enhances a proteasome‐dependent degradation of cdk4 during growth arrest in liver of newborn mice and in cultured cells. Overexpression of C/EBPα in several biological systems leads to a reduction of cdk4 protein levels, but not mRNA levels. Experiments with several tissue culture models reveal that C/EBPα enhances the formation of cdk4–ubiquitin conjugates and induces degradation of cdk4 through a proteasome‐dependent pathway. As a result, the half‐life of cdk4 is shorter and protein levels of cdk4 are reduced in cells expressing C/EBPα. Gel filtration analysis of cdk4 complexes shows that a chaperone complex cdk4–cdc37–Hsp90, which protects cdk4 from degradation, is abundant in proliferating livers that lack C/EBPα, but this complex is weak or undetectable in livers expressing C/EBPα. Our studies show that C/EBPα disrupts the cdk4–cdc37–Hsp90 complex via direct interaction with cdk4 and reduces protein levels of cdk4 by increasing proteasome‐dependent degradation of cdk4.