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A role for G1/S cyclin-dependent protein kinases in the apoptotic response to ionizing radiation

Carla V Finkielstein1, Lin G Chen, James L Maller

  • 1Howard Hughes Medical Institute and Department of Pharmacology, University of Colorado School of Medicine, Denver, Colorado 80262, USA.

The Journal of Biological Chemistry
|August 15, 2002
PubMed

Insights

Ionizing radiation triggers apoptosis in Xenopus embryos pre-mid-blastula transition. Caspase cleavage of cyclin D1-Cdk4 and cyclin A2-Cdk2 promotes specific apoptotic events, including DNA fragmentation.

Area of Science:

  • Developmental Biology
  • Cellular Biology
  • Radiation Biology

Background:

  • The mid-blastula transition (MBT) in Xenopus development alters embryonic response to ionizing radiation.
  • p27(Xic1) inhibits cyclin D1-Cdk4 and cyclin A2-Cdk2, preventing apoptosis post-MBT.

Purpose of the Study:

  • To investigate the distinct roles of cyclin D1-Cdk4 and cyclin A2-Cdk2 complexes during apoptosis in pre-MBT Xenopus embryos irradiated with ionizing radiation.
  • To elucidate the mechanisms by which these complexes contribute to radiation-induced apoptosis.

Main Methods:

  • Analysis of caspase cleavage products of cyclin D1 and cyclin A2.
  • Assessment of kinase activity and substrate specificity of truncated cyclin A2-Cdk2 (Delta N-cyclin A2-Cdk2).
  • In vitro assays using Xenopus egg extracts to induce DNA fragmentation.

Main Results:

  • Caspase cleavage generates active Delta N-cyclin A2-Cdk2, resistant to p27(Xic1) inhibition and proteasomal degradation.
  • Delta N-cyclin A2-Cdk2 phosphorylates histone H2B, potentially facilitating DNA cleavage.
  • Addition of Delta N-cyclin A2-Cdk2, but not full-length cyclin A2-Cdk2, induces DNA fragmentation in egg extracts.
  • Caspase-generated cyclin D1 products show increased affinity for p27(Xic1), reducing retinoblastoma protein phosphorylation.

Conclusions:

  • Caspase-mediated cleavage of cyclin D1-Cdk4 and cyclin A2-Cdk2 plays a crucial role in promoting specific apoptotic events in response to ionizing radiation.
  • These modified cyclin complexes contribute to DNA fragmentation and altered cell cycle regulation during radiation-induced apoptosis.

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