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Increased expression of p27Kip1 arrests neuroblastoma cell growth
1Division of Clinical Sciences, National Cancer Institute, Bethesda, Maryland 20892-1928, USA.
Background And Procedure:
To investigate the molecular mechanisms by which retinoic acid (RA) alters cell growth, the expression and activity of components of the cell cycle machinery were analyzed.
Results And Conclusions:
Within 2 days of RA treatment, and prior to the arrest of NB cells in the G1 phase of the cell cycle, there was a complete downregulation of GI cyclin/cdk activities. Protein levels for the G1 cyclin/cdk were essentially unchanged during this time, although there was a decrease in the steady state levels of hyperphosphorylated Rb and p60N-MYC proteins. The cdk inhibitors, p21Cip1 and p27Kip1 were constitutively expressed in KCNR, while p15 INK4B and p16 INK4A mRNA were undetected. Within 24 hr of RA treatment, there was a 4-fold increase in the expression of p27Kip1, although p27 mRNA levels were unchanged. Levels of p21Cip1 were unaltered. Coincident with the decrease in kinase activity there was an increase in p27 bound to G1 cyclin/cdk. The increase in p27 was not due to an increase in transcription. In other cell systems, increased expression of c-MYC has been shown to lead to a decrease in p27 levels that is regulated at the post-transcriptional level (sequestration). To determine whether increased levels of N-MYC could affect the level of p27, we evaluated the expression of p27 in a series of N-MYC transfected cells and found that constitutive overexpression of N-MYC led to a decrease in the steady-state levels of p27 and in p27 bound to G1 cyclin/cdk complexes. Using adenoviral vectors expressing p27, we found that infection leads to increased p27 expression, which causes a decrease in cdk activity and an accumulation of cells in G1.
Insights
Retinoic acid (RA) treatment downregulates G1 cyclin/cdk activity in neuroblastoma cells by increasing the expression of p27Kip1, a cyclin-dependent kinase inhibitor, leading to G1 cell cycle arrest. This mechanism involves post-transcriptional regulation and is influenced by N-MYC levels.
Area of Science:
- Molecular Biology
- Cell Cycle Regulation
- Cancer Research
Background:
- Retinoic acid (RA) is a crucial signaling molecule involved in cell differentiation and growth.
- Neuroblastoma (NB) cells are a pediatric cancer where RA is used therapeutically.
- Understanding RA's impact on the cell cycle is key to its therapeutic application.
Purpose of the Study:
- To elucidate the molecular mechanisms by which retinoic acid (RA) influences cell growth.
- To investigate the role of cell cycle machinery components in RA-mediated effects.
- To determine how RA affects cyclin-dependent kinase (cdk) activity and expression of cdk inhibitors.
Main Methods:
- Analysis of cell cycle machinery expression and activity in response to RA treatment.
- Western blotting to assess protein levels of cyclins, cdks, and cdk inhibitors (p21Cip1, p27Kip1, p15INK4B, p16INK4A).
- Assessment of hyperphosphorylated Rb and p60N-MYC protein levels.
- Evaluation of p27 expression in N-MYC transfected cells and using adenoviral vectors.
Main Results:
- RA treatment led to a complete downregulation of G1 cyclin/cdk activities within 2 days, preceding G1 arrest.
- Protein levels of G1 cyclin/cdks remained unchanged, but hyperphosphorylated Rb and p60N-MYC decreased.
- p27Kip1 expression significantly increased within 24 hours of RA treatment, independent of mRNA levels.
- Increased p27Kip1 correlated with decreased cdk activity and G1 cell cycle accumulation.
- Overexpression of N-MYC reduced steady-state levels of p27 and its binding to G1 cyclin/cdk complexes.
Conclusions:
- RA induces G1 cell cycle arrest in neuroblastoma cells primarily through the upregulation of the cdk inhibitor p27Kip1.
- The increase in p27Kip1 is regulated post-transcriptionally and is influenced by N-MYC levels.
- Targeting p27Kip1 expression or its interaction with cyclin/cdk complexes represents a potential therapeutic strategy in neuroblastoma.