APC/C prevents a noncanonical order of cyclin/CDK activity to maintain CDK4/6 inhibitor-induced arrest

Proc Natl Acad Sci U S A. 2024 Jul 23;121(30):e2319574121. doi: 10.1073/pnas.2319574121. Epub 2024 Jul 18.

Abstract

Regulated cell cycle progression ensures homeostasis and prevents cancer. In proliferating cells, premature S phase entry is avoided by the E3 ubiquitin ligase anaphasepromoting complex/cyclosome (APC/C), although the APC/C substrates whose degradation restrains G1-S progression are not fully known. The APC/C is also active in arrested cells that exited the cell cycle, but it is not clear whether APC/C maintains all types of arrest. Here, by expressing the APC/C inhibitor, EMI1, we show that APC/C activity is essential to prevent S phase entry in cells arrested by pharmacological cyclin-dependent kinases 4 and 6 (CDK4/6) inhibition (Palbociclib). Thus, active protein degradation is required for arrest alongside repressed cell cycle gene expression. The mechanism of rapid and robust arrest bypass from inhibiting APC/C involves CDKs acting in an atypical order to inactivate retinoblastoma-mediated E2F repression. Inactivating APC/C first causes mitotic cyclin B accumulation which then promotes cyclin A expression. We propose that cyclin A is the key substrate for maintaining arrest because APC/C-resistant cyclin A, but not cyclin B, is sufficient to induce S phase entry. Cells bypassing arrest from CDK4/6 inhibition initiate DNA replication with severely reduced origin licensing. The simultaneous accumulation of S phase licensing inhibitors, such as cyclin A and geminin, with G1 licensing activators disrupts the normal order of G1-S progression. As a result, DNA synthesis and cell proliferation are profoundly impaired. Our findings predict that cancers with elevated EMI1 expression will tend to escape CDK4/6 inhibition into a premature, underlicensed S phase and suffer enhanced genome instability.

Keywords: anaphase promoting complex/cyclosome (APC/C); breast cancer; cell cycle arrest; cyclin-dependent kinase 4/6 (CDK4/6); genome instability.

MeSH terms

  • Anaphase-Promoting Complex-Cyclosome / genetics
  • Anaphase-Promoting Complex-Cyclosome / metabolism
  • Cell Cycle Checkpoints / drug effects
  • Cell Cycle Proteins / genetics
  • Cell Cycle Proteins / metabolism
  • Cell Line, Tumor
  • Cyclin-Dependent Kinase 4* / antagonists & inhibitors
  • Cyclin-Dependent Kinase 4* / genetics
  • Cyclin-Dependent Kinase 4* / metabolism
  • Cyclin-Dependent Kinase 6* / genetics
  • Cyclin-Dependent Kinase 6* / metabolism
  • Cyclins / genetics
  • Cyclins / metabolism
  • E2F Transcription Factors / genetics
  • E2F Transcription Factors / metabolism
  • F-Box Proteins
  • Humans
  • Piperazines / pharmacology
  • Pyridines / pharmacology
  • S Phase / drug effects

Substances

  • Cyclin-Dependent Kinase 6
  • Cyclin-Dependent Kinase 4
  • Anaphase-Promoting Complex-Cyclosome
  • CDK4 protein, human
  • Pyridines
  • CDK6 protein, human
  • Piperazines
  • palbociclib
  • FBXO5 protein, human
  • Cell Cycle Proteins
  • E2F Transcription Factors
  • Cyclins
  • F-Box Proteins