Abstract

CCAAT/enhancer binding protein alpha (C/EBP) causes growth arrest via direct interaction with the cyclin-dependent kinases cdk2 and cdk4. In this paper, we present evidence showing that C/EBP enhances a proteasome-dependent degradation of cdk4 during growth arrest in liver of newborn mice and in cultured cells. Overexpression of C/EBP 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/EBP 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/EBP. 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/EBP, but this complex is weak or undetectable in livers expressing C/EBP. Our studies show that C/EBP disrupts the cdk4–cdc37–Hsp90 complex via direct interaction with cdk4 and reduces protein levels of cdk4 by increasing proteasome-dependent degradation of cdk4.