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p130 mediates TGF-beta-induced cell-cycle arrest in Rb mutant HT-3 cells
Hyun Ho Choi1, Hyun Soon Jong, Sang Hyun Song
1Cancer Research Center, Seoul National University College of Medicine, Seoul, Republic of Korea.
Objective:
The retinoblastoma proteins include Rb and the functionally and structurally related proteins p107 and p130. It has been reported that HT-3 cells are sensitive to TGF-beta growth inhibition, despite the Rb mutation. The purpose of this study was to elucidate the growth-inhibitory mechanism of TGF-beta in Rb mutant HT-3 cells.
Methods:
Growth inhibition by TGF-beta in cervical carcinoma cell lines was evaluated by counting cell numbers. Cell-cycle distribution was determined by staining DNA with propidium iodide (PI) and measured using a flow cytometer. The level of each protein expression was determined by Western blot analysis. To evaluate the assembly of cdk2/p21, cdk2/cyclin E, and E2F-4/p130 complexes by TGF-beta, immunoprecipitation was performed.
Results:
TGF-beta inhibited the proliferation of HT-3 cells expressing mutant Rb protein and efficiently induced cell-cycle arrest at G(1) phase. p21 protein level was elevated in TGF-beta-treated HT-3 cells, while other G(1) regulatory protein levels were unaltered. TGF-beta markedly enhanced the binding of p21 with cdk2 but decreased that of cdk2 with cyclin E and inhibited the phosphorylation of p130 but did not change Rb and p107 protein status. We also found that E2F-1 protein level was lower in TGF-beta-treated cells and suggest that this might be the result of enhanced binding between E2F-4 and p130.
Conclusions:
Our results demonstrate that p130, instead of Rb, can mediate growth inhibition by TGF-beta in Rb mutant HT-3 cells.
Insights
Transforming growth factor-beta (TGF-beta) inhibits Rb mutant HT-3 cell proliferation by inducing G(1) cell-cycle arrest. The protein p130, not Rb, mediates this TGF-beta growth inhibition in Rb mutant cells.
Area of Science:
- Cell Biology
- Molecular Biology
- Cancer Research
Background:
- Retinoblastoma (Rb) proteins, including Rb, p107, and p130, regulate cell growth.
- HT-3 cells with a mutated Rb gene are reportedly sensitive to TGF-beta.
- The precise mechanism of TGF-beta growth inhibition in Rb-mutant cells remains unclear.
Purpose of the Study:
- To investigate the mechanism of TGF-beta-induced growth inhibition in Rb-mutant HT-3 cells.
- To determine which retinoblastoma protein family member mediates the response to TGF-beta.
Main Methods:
- Cell proliferation was assessed by cell counting.
- Cell-cycle distribution was analyzed using flow cytometry.
- Protein expression, complex formation (e.g., p21/cdk2, E2F-4/p130), and phosphorylation were evaluated via Western blot and immunoprecipitation.
Main Results:
- TGF-beta treatment induced G(1) cell-cycle arrest in HT-3 cells.
- p21 protein levels increased, enhancing p21/cdk2 binding and reducing cdk2/cyclin E interaction.
- TGF-beta inhibited p130 phosphorylation and reduced E2F-1 levels, suggesting p130 involvement.
Conclusions:
- p130, rather than the mutated Rb protein, is the key mediator of TGF-beta-induced growth inhibition in Rb-mutant HT-3 cells.
- This finding highlights a compensatory mechanism in cell-cycle regulation.