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TGF-beta induced G(1) cell cycle arrest requires the activity of the proteasome pathway. Transforming growth factor
Fan Zhang1, Mia Mönkkönen, Stina Roth
1Haartman Institute, Department of Virology and Molecular Cancer Biology Program, Biomedicum Helsinki, University of Helsinki, Central Hospital Laboratory Diagnostics, Finland.
Abstract:
Transforming growth factor-beta (TGF-beta) induces a potent G(1)/S-phase cell cycle arrest of epithelial cells by inhibiting the activities of cyclin D- and cyclin E-associated kinase complexes. Downregulation of the kinase activities is mediated by induction of cyclin dependent kinase (CDK) inhibitor p15(Ink4b) which blocks CDK4 and CDK6 kinases and leads to binding of p27(Kip1) to CDK2-cyclin E complex. Levels of several of these factors are controlled by the ubiquitin-proteasome pathway. We demonstrate here that proteasomal inhibitors release the cells from TGF-beta imposed G(1)-phase arrest and instigate the entry of the cells into S-phase. Proteasomal inhibitors are shown to specifically increase the activity of the cyclin D-kinase complex by increasing the levels of p27(Kip1) and cyclin D and by maintaining CDK4/6 protein levels leading to phosphorylation of the retinoblastoma protein without increasing cyclin E-associated kinase activity. The results indicate caution in the potential therapeutic use of the proteasome inhibitors due to unscheduled initiation of DNA replication in the presence of a physiological growth inhibitor.
Insights
Proteasome inhibitors can release cells from TGF-beta-induced G1 arrest, promoting DNA replication. This suggests caution for therapeutic use due to unscheduled cell cycle entry.
Area of Science:
- Cell Biology
- Molecular Biology
- Cancer Research
Background:
- Transforming growth factor-beta (TGF-beta) is a key regulator of epithelial cell cycle progression.
- TGF-beta induces G1/S-phase arrest by inhibiting cyclin-dependent kinase (CDK) activities.
- The ubiquitin-proteasome pathway plays a role in regulating cell cycle factors.
Purpose of the Study:
- To investigate the effect of proteasome inhibitors on TGF-beta-induced cell cycle arrest.
- To elucidate the mechanisms by which proteasome inhibitors affect cell cycle regulators.
Main Methods:
- Treatment of epithelial cells with proteasome inhibitors and TGF-beta.
- Analysis of cell cycle progression using S-phase entry assays.
- Western blot analysis to assess protein levels and kinase activities of key cell cycle regulators (e.g., cyclins, CDKs, CDK inhibitors).
Main Results:
- Proteasome inhibitors reversed TGF-beta-induced G1 arrest, leading to S-phase entry.
- Proteasome inhibitors increased cyclin D-CDK4/6 activity by stabilizing p27(Kip1) and cyclin D, and maintaining CDK4/6 levels.
- Retinoblastoma protein phosphorylation was observed without a significant increase in cyclin E-CDK2 activity.
Conclusions:
- Proteasome inhibitors can override TGF-beta-mediated cell cycle arrest.
- Therapeutic application of proteasome inhibitors requires careful consideration due to the risk of unscheduled DNA replication in the presence of growth inhibitors.