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Functional interference between retinoic acid or steroid hormone receptors and the oncoprotein Fli-1
T G Darby1, J D Meissner, A Rühlmann
1Max Delbrück Zentrum, Berlin, Germany.
Abstract:
The Fli-1 protein is a member of the ets proto-oncogene family, whose overexpression is a consequence of Friend murine leukemia virus (F-MuLV) integration in Friend erythroleukemic cells. We present evidence that Fli-1 and the retinoic acid receptor (RAR alpha) can reciprocally repress one another's transcriptional activation. Overexpression of Fli-1 inhibits the retinoic acid-induced activation of genes carrying a functional retinoic acid response element (RARE). Conversely, RAR alpha is able to repress Fli-1-mediated transcriptional activation. Transfection analysis of RAR alpha and Fli-1 mutants in cultured cells demonstrate that the DNA binding domain of RAR alpha and the N-terminal region of Fli-1 are required for repression. Gel retardation analysis demonstrates that RAR alpha cannot bind to the Fli-1 binding site in the E74 promoter and the expression of Fli-1 does not affect RAR alpha binding to DNA. Furthermore, the data suggest an indirect interaction between Fli-1 and RAR alpha mediated by a 'bridging' factor(s) present in nuclear extracts from RM10 erythroleukemia cells. Fli-1 also interferes with the action of receptors for thyroid or glucocorticoid hormone in several hematopoietic cell lines. The RA-induced differentiation and decrease of cell proliferation was blocked in myeloblastic leukemia HL-60 cells overexpressing the N-terminal region of Fli-1 at physiological concentrations of RA. These data suggest that accumulation of Fli-1 can oppose the transcriptional activity of hormone receptors in hematopoietic cells.
Insights
Fli-1 protein overexpression blocks retinoic acid receptor (RAR alpha) activity, inhibiting gene activation and cell differentiation. This interaction, mediated by bridging factors, impacts hormone receptor function in hematopoietic cells.
Area of Science:
- Molecular Biology
- Oncology
- Cell Biology
Background:
- Fli-1, an ets proto-oncogene, is overexpressed due to Friend murine leukemia virus (F-MuLV) integration in erythroleukemia.
- Retinoic acid receptor alpha (RAR alpha) plays a crucial role in cellular differentiation and gene regulation.
Purpose of the Study:
- To investigate the reciprocal regulatory relationship between Fli-1 and RAR alpha.
- To elucidate the mechanism by which Fli-1 affects RAR alpha-mediated transcriptional activation and cellular responses.
Main Methods:
- Transfection analysis using RAR alpha and Fli-1 mutants.
- Gel retardation assays to assess DNA binding.
- Studies in HL-60 cells to evaluate effects on differentiation and proliferation.
Main Results:
- Fli-1 overexpression inhibits retinoic acid-induced gene activation via retinoic acid response elements (RAREs).
- RAR alpha represses Fli-1-mediated transcription; DNA binding domains are crucial for this repression.
- Fli-1 and RAR alpha interact indirectly via bridging factors in nuclear extracts.
- Fli-1 interferes with thyroid and glucocorticoid hormone receptor activity.
- Overexpression of Fli-1 blocks RA-induced differentiation and proliferation in HL-60 cells.
Conclusions:
- Fli-1 and RAR alpha exhibit reciprocal repression of transcriptional activity.
- Fli-1 can antagonize the function of nuclear hormone receptors in hematopoietic cells.
- Fli-1 accumulation may contribute to the dysregulation observed in certain leukemias.