Cyclin E-induced S phase without activation of the pRb/E2F pathway

Jiri Lukas, Thomas Herzinger, Klaus Hansen, Maria Cristina Moroni, Dalia Resnitzky, Kristian Helin, Steven I. Reed, Jiri Bartek

Research output: Contribution to journalArticlepeer-review


In cells of higher eukaryotes, cyclin D-dependent kinases Cdk4 and Cdk6 and, possibly, cyclin E-dependent Cdk2 positively regulate the G 1 to S- phase transition, by phosphorylating the retinoblastoma protein (pRb), thereby releasing E2F transcription factors that control S-phase genes. Here we performed microinjection and transfection experiments using rat R12 fibroblasts, their derivatives conditionally overexpressing cyclins D1 or E, and human U-2-OS cells, to explore the action of G 1 cyclins and the relationship of E2F and cyclin E in S-phase induction. We demonstrate that ectopic expression of cyclin E, but not cyclin D1, can override G 1 arrest imposed by either the p16(INK4a) Cdk inhibitor specific for Cdk4 and Cdk6 or a novel phosphorylation-deficient mutant pRb. Several complementary approaches to assess E2F activation, including quantitative reporter assays in live cells, showed that the cyclin E-induced S phase and completion of the cell division cycle can occur in the absence of E2F-mediated transactivation. Together with the ability of cyclin E to overcome a G 1 block induced by expression of dominant-negative mutant DP-1, a heterodimeric partner of E2Fs, these results provide evidence for a cyclin E-controlled S phase-promoting event in somatic cells downstream of or parallel to phosphorylation of pRb and independent of E2F activation. They furthermore indicate that a lack of E2F-mediated transactivation can be compensated by hyperactivation of this cyclin E-controlled event.

Original languageEnglish
Pages (from-to)1479-1492
Number of pages14
JournalGenes and Development
Issue number11
Publication statusPublished - Jun 1 1997


  • Cyclin D1
  • Cyclin E
  • E2F
  • p16
  • pRb phosphorylation
  • S phase

ASJC Scopus subject areas

  • Genetics
  • Developmental Biology


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