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A transactivator of c-myc is coordinately regulated with the proto-oncogene during cellular growth
1Department of Pathology, Georgetown University School of Medicine, Washington, DC 20007, USA.
Abstract:
A recently cloned nuclear protein, which binds a far upstream element (FUSE) of the human c-myc proto-oncogene, stimulates promoter driven expression in undifferentiated cells. In concert with a loss of c-myc expression, both FUSE binding protein (FBP) mRNA and protein levels disappeared in HL60 cells after PMA-induced differentiation, due to a drop in the rate of transcription that was measured by nuclear runoff. During the differentiation of these cells, the brief half-lives of FBP mRNA (3 h) and protein (1.5 h) did not change, allowing for the rapid down-regulation of nuclear protein levels, as detected by immunohistochemical staining. Like c-myc, FBP is expressed in proliferating cells from a variety of lineages, but not in quiescent cells. When T cells and fibroblasts were stimulated to transit from G0 into the cell cycle, there was a dramatic rise of both FBP mRNA and DNA sequence specific nuclear FBP binding activity, which correlated with the appearance of c-myc mRNA. In contrast to the transient expression of many other immediate early growth response genes, both FBP and c-myc expression were sustained for more than 24 h. In fibroblasts, the coordinate expression of FBP and c-myc throughout all phases of the cell cycle is consistent with FBP's role as a growth-dependent regulator of c-myc expression.
Insights
FUSE binding protein (FBP) regulates c-myc expression in proliferating cells. FBP levels decrease during cell differentiation, indicating its role as a growth-dependent regulator of c-myc.
Area of Science:
- Molecular Biology
- Cell Biology
- Gene Regulation
Background:
- The human c-myc proto-oncogene is crucial for cell proliferation.
- A nuclear protein binding the FUSE element of c-myc has been identified.
- This protein stimulates gene expression in undifferentiated cells.
Purpose of the Study:
- To investigate the role of FUSE binding protein (FBP) in c-myc regulation during cell differentiation and proliferation.
- To understand the expression dynamics of FBP and its correlation with c-myc.
- To determine FBP's function as a growth-dependent regulator.
Main Methods:
- PMA-induced differentiation of HL60 cells.
- Nuclear runoff assays to measure transcription rates.
- Measurement of FBP mRNA and protein half-lives.
- Immunohistochemical staining.
- Stimulation of T cells and fibroblasts to enter the cell cycle.
- Analysis of FBP and c-myc mRNA expression levels.
Main Results:
- FBP mRNA and protein levels decreased in HL60 cells upon PMA-induced differentiation due to reduced transcription.
- FBP mRNA and protein exhibit short half-lives (3h and 1.5h, respectively) during differentiation, facilitating rapid down-regulation.
- FBP expression, similar to c-myc, is detected in proliferating cells but not quiescent cells.
- Stimulation of T cells and fibroblasts into the cell cycle led to increased FBP mRNA and binding activity, correlating with c-myc mRNA appearance.
- Both FBP and c-myc expression were sustained for over 24 hours in cycling fibroblasts, unlike transient immediate early genes.
Conclusions:
- FBP plays a significant role in regulating c-myc expression.
- FBP is rapidly down-regulated during cell differentiation.
- FBP acts as a growth-dependent regulator of c-myc, with coordinate expression observed throughout the cell cycle in fibroblasts.