p120E4F-1: A Novel Candidate Factor for Mediating Bmi-1 Function in Hematopoietic Stem Cells.
Bibliographic record
Abstract
Abstract We have recently demonstrated that the Polycomb group (Pc-G) gene bmi-1 is essential for the self-renewal/proliferation of both normal and leukemic hematopoietic stem cells (HSCs). Interestingly, none of the gene products with which Bmi-1 interacts are expressed in this cellular compartment. Therefore, it has been postulated that the proliferative function of Bmi-1 in HSCs is mediated through its interaction with non-identified partners. A yeast two-hybrid assay, using Bmi-1 as bait to screen a fetal liver cDNA library, led to the isolation of p120E4F-1, previously identified as a negative regulator of cell proliferation. Mutational analysis showed that this interaction occurs through the HTH motif of Bmi-1, shown to be essential for its ability to extend the replicative lifespan of fibroblasts. The interaction between endogenous Bmi-1 and p120E4F-1 has been confirmed in mammalian cells by co-immunoprecipitation experiments. Curiously, Bmi-1 and p120E4F-1 interact specifically in the cytoplasmic compartment. In addition, increasing the protein levels of Bmi-1 leads to a reduction in p120E4F-1 protein levels, without affecting cellular localization or mRNA levels. Using overexpression and small interfering RNA (siRNA) -mediated knock-down experiments, we show that the overexpression of Bmi-1 into NIH3T3 cells enhances cell growth, whereas reduction of endogenous Bmi-1 results in significant growth suppression. Conversely, overexpression of p120E4F-1 led to an inhibition of cell proliferation, whereas reduction of p120E4F-1 level led to an hyperproliferation. Complementation studies also demonstrates that the proliferative defect induced by the overexpression of p120E4F-1 can be partially rescued by overexpression of Bmi-1 and that the proliferative defect induced by Bmi-1 knock-down is suppressed by the reduction of p120E4F-1 protein level. Moreover, we show that both the HTH and the RING1 domain of Bmi-1 are necessary to abrogate the anti-proliferative effect of p120E4F-1. Taken together, these data demonstrate that Bmi-1 and p120E4F-1 interact physically and genetically and suggest that Bmi-1 down regulates p120E4F-1 through a post transcriptional mechanism. As Bmi-1 is a RING finger domain containing protein, we postulate that it might be acting as an E3 ubiquitin ligase on p120E4F-1. To investigate this possibility we used the ts20 mutant cell line expressing a thermolabile ubiquitin-activating enzyme (E1) that is inactivated at elevated temperature, preventing ubiquitination and subsequent degradation. We found that endogenous p120E4F-1 accumulates markedly when cells are shifted to the restrictive temperature. Moreover, introduction of Bmi-1 specific siRNA prevents accumulation p120E4F-1 at the nonpermissive temperature, suggesting that p120E4F-1 is subject to ubiquitination and that Bmi-1 is required for its degradation. Altogether our data reveal an unsuspected cytoplasmic function for Bmi-1 in the regulation of cell cycle progression through its interaction with p120E4F-1.
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How this classification was reachedexpand
Full frame machine prediction
Teacher imitationNot calibrated prevalence, not ground truth. Human validation pending. The Gemma side is a direct model label for every work in the frame, read from the title-only record. The Codex side is a classifier learned from the 10,348 direct Codex labels and calibrated to design-weighted sample rates; fields without enough sample support carry no Codex call. Candidate is the union of the two sides; consensus is their intersection. These outputs are machine_predicted_unvalidated and are not human labels.
Distilled classifier scores by category (both heads)
| Category | Codex | Gemma |
|---|---|---|
| Metaresearch | 0.000 | 0.000 |
| Meta-epidemiology (narrow) | 0.001 | 0.000 |
| Meta-epidemiology (broad) | 0.000 | 0.000 |
| Bibliometrics | 0.000 | 0.000 |
| Science and technology studies | 0.000 | 0.000 |
| Scholarly communication | 0.000 | 0.000 |
| Open science | 0.000 | 0.000 |
| Research integrity | 0.000 | 0.000 |
| Insufficient payload (model declined to judge) | 0.002 | 0.001 |
Machine scores (provisional)
The two teacher heads of the student model, read on this work. A score orders the frame for review; it never asserts a category, and the validation status ships verbatim with every row.
Baseline scores from an immature model (maturity gate not passed, 7 training rounds). Scores rank; they never assert a category.
score_only:v0-immature-baseline · verbatim from the scoring run: score_only means the number may rank works, and no category label ships from itClassification
machine, unvalidatedMachine predicted; a candidate call from one source (direct Gemma or distilled Codex), not a consensus.
How this classification was reached, model by model and score by score, is at the end of the page under "How this classification was reached".