Related Experiment Videos
Disruption of TGF-beta growth inhibition by oncogenic ras is linked to p27Kip1 mislocalization
1Whitehead Institute for Biomedical Research, Nine Cambridge Center, Massachusetts 02142, USA.
Abstract:
Expression of oncogenic Ras in epithelial tumor cells is linked to the loss of transforming growth factor-beta (TGF-beta) anti-proliferative activity, and was proposed to involve inhibition of Smad2/3 nuclear translocation. Here we studied several epithelial cell lines expressing oncogenic N-RasK61 and show that TGF-beta-induced nuclear translocation of and transcriptional activation by Smad2/3 were unaffected. In contrast, oncogenic Ras mediated nuclearto-cytoplasmic mislocalization of p27KiP1 (p27) and of the cyclin-dependent kinase (CDK) CDK6, but not CDK2. Concomitantly, oncogenic Ras abrogated the ability of TGF-beta to release p27 from CDK6, to enhance its binding to CDK2 and to inhibit CDK2 activity. Inactivation of Ras by a specific antagonist restored the growth inhibitory response to TGF-beta with concurrent normalization of p27 and CDK6 localization. Therefore, the disruption of TGF-beta-mediated growth inhibition by oncogenic Ras appears to be due to lack of inhibition of CDK2, caused by the sequestration of p27 and CDK2 in different subcellular compartments and by the loss of TGF-beta-induced partner switching of p27 from CDK6 to CDK2.
Insights
Oncogenic Ras disrupts transforming growth factor-beta (TGF-beta) tumor suppression by mislocalizing p27KiP1 (p27) and CDK6, preventing CDK2 inhibition. Restoring Ras function normalizes these proteins and TGF-beta
Area of Science:
- Cell Biology
- Cancer Biology
- Molecular Oncology
Background:
- Oncogenic Ras expression in epithelial tumors correlates with loss of TGF-beta's anti-proliferative effects.
- This loss was hypothesized to stem from inhibited Smad2/3 nuclear translocation.
Purpose of the Study:
- To investigate the mechanism by which oncogenic Ras disrupts TGF-beta-mediated growth inhibition in epithelial cells.
- To determine the role of p27KiP1 (p27) and cyclin-dependent kinases (CDKs) in this process.
Main Methods:
- Utilized epithelial cell lines expressing oncogenic N-RasK61.
- Assessed TGF-beta-induced Smad2/3 nuclear translocation and transcriptional activity.
- Analyzed the subcellular localization of p27 and CDK CDK6/CDK2.
- Evaluated the effect of Ras inactivation on TGF-beta signaling and cell growth.
Main Results:
- Oncogenic Ras did not affect TGF-beta-induced Smad2/3 nuclear translocation or transcriptional activity.
- Oncogenic Ras caused the mislocalization of p27 and CDK6 from the nucleus to the cytoplasm.
- Oncogenic Ras blocked TGF-beta's ability to release p27 from CDK6, enhance p27-CDK2 binding, and inhibit CDK2 activity.
- Inactivating Ras restored TGF-beta's growth inhibitory response and normalized p27/CDK6 localization.
Conclusions:
- Oncogenic Ras disrupts TGF-beta-mediated growth inhibition by preventing CDK2 inactivation.
- This occurs due to p27 and CDK2 sequestration in different cellular compartments, hindering p27's partner switching from CDK6 to CDK2.
- Targeting Ras signaling may restore TGF-beta sensitivity in epithelial tumors.