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A novel inhibitor of cyclin-Cdk activity detected in transforming growth factor beta-arrested epithelial cells
J M Slingerland1, L Hengst, C H Pan
1Department of Molecular Biology, Scripps Research Institute, La Jolla, California 92037.
Abstract:
Transforming growth factor beta (TGF-beta) is a potent inhibitor of epithelial cell growth. Cyclins E and A in association with Cdk2 have been shown to play a role in the G1-to-S phase transition in mammalian cells. We have studied the effects of TGF-beta-mediated growth arrest on G1/S cyclins E and A. Inhibition of cyclin A-associated kinase by TGF-beta is primarily due to a decrease in cyclin A mRNA and protein. By contrast, while TGF-beta inhibits accumulation of cyclin E mRNA, the reduction in cyclin E protein is minimal. Instead, we find that the activation of cyclin E-associated kinase that normally accompanies the G1-to-S phase transition is inhibited. A novel inhibitor of cyclin-Cdk complexes was detected in TGF-beta-treated cell lysates. Inhibition is mediated by a heat-stable protein that targets both Cdk2 and Cdc2 kinases. In G0-arrested cells, a similar inhibitor of Cdk2 kinase was detected. These data suggest the existence of an inhibitor of cyclin-dependent kinases induced under different conditions to mediate antiproliferative responses.
Insights
Transforming growth factor beta (TGF-beta) inhibits epithelial cell growth by affecting cell cycle regulators. A novel inhibitor protein targeting cyclin-dependent kinases (CDKs) is induced during TGF-beta-mediated growth arrest.
Area of Science:
- Cell Biology
- Molecular Biology
- Cancer Research
Background:
- Transforming growth factor beta (TGF-beta) is a key regulator of cell proliferation and differentiation.
- Cyclins E and A, complexed with Cyclin-Dependent Kinases (CDKs), are critical for the G1 to S phase transition in the cell cycle.
- Dysregulation of cell cycle control is a hallmark of cancer.
Purpose of the Study:
- To investigate the impact of TGF-beta-induced growth arrest on G1/S phase cyclins (Cyclin E and Cyclin A) and their associated kinases.
- To identify potential molecular mechanisms by which TGF-beta mediates its antiproliferative effects.
- To characterize novel inhibitors of cyclin-dependent kinases.
Main Methods:
- Analysis of Cyclin E and Cyclin A mRNA and protein levels in response to TGF-beta treatment.
- Assay of Cyclin E- and Cyclin A-associated kinase activity.
- Detection and characterization of inhibitors of cyclin-Cdk complexes in cell lysates using biochemical methods.
Main Results:
- TGF-beta treatment led to decreased Cyclin A mRNA and protein, inhibiting Cyclin A-associated kinase activity.
- While TGF-beta reduced Cyclin E mRNA, Cyclin E protein levels were minimally affected, but its kinase activity was inhibited.
- A novel heat-stable protein inhibitor of both Cdk2 and Cdc2 kinases was identified in TGF-beta-treated cells and in G0-arrested cells.
Conclusions:
- TGF-beta inhibits epithelial cell proliferation through distinct mechanisms affecting Cyclin A and Cyclin E kinase activity.
- A novel inhibitor protein targeting cyclin-dependent kinases is induced by TGF-beta and during G0 arrest, mediating antiproliferative responses.
- These findings elucidate a novel mechanism of cell cycle regulation by TGF-beta and identify potential therapeutic targets for cancer.