Regulation of Cyclin E Protein Levels through E2F-Mediated Inhibition of Degradation

Abstract

Cyclin E is a key cell cycle regulator, whose accumulation is believed to bepositively modulated chiefly at the transcriptional level. We show that forcedexpression of E2F family members specifically stabilizes cyclin E protein by reducingits conjugation with ubiquitin, leading to increased steady-state levels. The stabilizedprotein shows enhanced association with cdk2 and higher kinase activity, indicatingthat cyclin E stabilization bears functional consequences. Although the activity ofE2F on the cyclin E protein does not entail increased cyclin E gene transcription, itdoes require the E2F transactivation capacity, as demonstrated by E2F and DP-1mutant analysis. However, such activity does not require E2F binding to pRb.Furthermore, E2F stabilizes cyclin E even in non-proliferating cells. Our results bearsignificance for the understanding of tumor progression, in light of the well-knownautoregulatory loop between E2F and cyclin E and the disregulation of E2F that isone consequence of the almost universal impairment of the pRb pathway in cancer.Constitutive pRb inactivation leads to enhanced E2F activity through loss of binding.We propose that such increased E2F activity stabilizes cyclin E and contributes toestablish the high and persistent levels of the protein commonly found in humanneoplasias.