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Updated: Apr 26, 2026
Analysis of Cell Cycle Position in Mammalian Cells
Published on: January 21, 2012
RB family tumor suppressor activity may not relate to active silencing of E2F target genes
Tinke L Vormer1, Kamila Wojciechowicz1, Marleen Dekker1
1Division of Biological Stress Response, The Netherlands Cancer Institute, Amsterdam, the Netherlands.
Abstract:
The retinoblastoma protein pRB and its two homologs p130 and p107 form the family of pocket proteins and play a major role in cell-cycle regulation and suppression of human and mouse tumorigenesis. Pocket proteins regulate the activity of E2F transcription factors during G1-S transition. Two mechanisms have been described: (i) pocket protein binding blocks the transactivation domain of activator E2Fs, inhibiting E2F-dependent transcription and (ii) E2F-bound pocket proteins can recruit chromatin remodeling proteins containing an LxCxE motif (x encoding any amino acid), resulting in active repression of E2F target genes. To investigate the importance of pRB's LxCxE-interacting motif in cell-cycle control and tumor suppression, we generated mouse embryonic fibroblasts and mice expressing a mutant pRB protein carrying an asparagine for phenylalanine substitution at position 750, abrogating LxCxE binding. Because p130 may compensate for loss of pRB, we studied pRB(N750F) activity in the presence and absence of p130. The pRB-LxCxE interaction was not required for cell-cycle arrest upon mitogen deprivation and cell-cell contact, but did contribute to RAS(V12)- and radiation-induced cell-cycle arrest. Remarkably, the pRB-LxCxE interaction was not required for suppression of in vitro and in vivo transformation, even in the absence of p130. These results indicate that pRB's tumor suppressor activity is not effectuated by active silencing of E2F target genes, but rather by regulation of activator E2Fs or another unidentified mechanism. Furthermore, the in vitro response of pocket protein-perturbed cells to mitogen deprivation and cell-cell contact seems a better predictor of tumor development than the response to ectopic RAS(V12) expression. Cancer Res; 74(18); 5266-76. ©2014 AACR.
Insights
The retinoblastoma protein pRB’s interaction with LxCxE is not essential for its tumor suppressor function, suggesting alternative mechanisms beyond active gene silencing are involved in cell-cycle control and preventing cancer. This finding impacts our understanding of pocket protein roles in tumorigenesis.
Area of Science:
- Molecular Biology
- Cell Biology
- Cancer Research
Background:
- Pocket proteins, including retinoblastoma protein (pRB), p130, and p107, are crucial regulators of cell-cycle progression and tumor suppression.
- Pocket proteins control E2F transcription factors during the G1-S phase transition via two main mechanisms: blocking E2F transactivation domains and recruiting chromatin remodelers through an LxCxE motif for active repression.
- The specific role of pRB's LxCxE-interacting motif in cell-cycle control and tumor suppression requires further investigation, especially considering potential compensation by p130.
Purpose of the Study:
- To investigate the functional importance of the pRB LxCxE-interacting motif in cell-cycle regulation and tumor suppression.
- To elucidate whether pRB's tumor suppressor activity relies on active silencing of E2F target genes mediated by the LxCxE motif.
Main Methods:
- Generation of mouse embryonic fibroblasts and mice expressing a mutant pRB protein (pRB(N750F)) with abrogated LxCxE binding.
- Assessment of pRB(N750F) activity in both the presence and absence of p130.
- Evaluation of cell-cycle arrest in response to mitogen deprivation, cell-cell contact, oncogenic RAS(V12) expression, and radiation.
Main Results:
- The pRB-LxCxE interaction was dispensable for cell-cycle arrest induced by mitogen deprivation and cell-cell contact.
- This interaction contributed to, but was not essential for, cell-cycle arrest triggered by RAS(V12) and radiation.
- Crucially, the pRB-LxCxE interaction was not required for suppressing in vitro and in vivo transformation, even when p130 was absent.
Conclusions:
- pRB's tumor suppressor activity is independent of its ability to recruit chromatin remodelers via the LxCxE motif, suggesting alternative mechanisms like direct E2F regulation.
- The study indicates that pRB's tumor suppressor function is not mediated by active silencing of E2F target genes.
- Cellular responses to mitogen deprivation and cell-cell contact in pocket protein-perturbed cells may serve as more reliable indicators of tumor development potential than responses to ectopic RAS(V12) expression.
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