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Constitutive Raf-1 kinase activity in breast cancer cells induces both estrogen-independent growth and apoptosis
D El-Ashry1, D L Miller, S Kharbanda
1Lombardi Cancer Research Center, Department of Biochemistry and Molecular Biology, Georgetown University Medical Center, Washington, DC 20007, USA.
Abstract:
Overexpression of many growth factor receptors, as well as growth factors, has been shown to confer varying degrees of estrogen-independent growth on estrogen receptor (ER) positive breast cancer cells. The proto-oncogene Raf-1 is a key intermediate in the signal transduction pathway of many of these growth factor receptors, and when constitutively activated in fibroblasts is transforming. To examine the effects of Raf-1 kinase activity on the estrogen-dependent growth of human breast cancer cells, ER + MCF-7 breast cancer cells were stably transfected with an expression construct directing the expression of an amino-truncated protein having constitutive kinase activity. Expression of constitutively activated Raf in MCF-7 cells is incompatible with growth in the presence of estrogen; that is, cells down-regulate expression of the transfected Raf. Constitutive Raf activity does allow for growth of the cells in the absence of estrogen, suggesting that activation of growth factor signaling pathways through Raf may confer a selective advantage for growth of breast cancer cells under estrogen-deprived conditions. In addition, the high levels of Raf activity induce apoptosis in cells grown under either condition. This is a novel activity for Raf, and may occur because the levels of the constitutive Raf are extremely high in these cells.
Insights
Constitutively activated Raf-1 kinase in estrogen receptor-positive breast cancer cells promotes growth without estrogen but induces cell death at high levels. This suggests Raf-1 signaling may offer a survival advantage in estrogen-deprived conditions.
Area of Science:
- Oncology
- Molecular Biology
- Cell Signaling
Background:
- Estrogen receptor (ER)-positive breast cancer growth is often linked to growth factor signaling.
- Proto-oncogene Raf-1 is a crucial mediator in growth factor receptor signal transduction pathways.
Purpose of the Study:
- To investigate the impact of Raf-1 kinase activity on estrogen-dependent human breast cancer cell growth.
- To explore the role of activated Raf-1 in estrogen-independent proliferation and cell survival.
Main Methods:
- Stable transfection of ER-positive MCF-7 breast cancer cells with a constitutively active, amino-truncated Raf-1 construct.
- Assessment of cell growth, proliferation, and apoptosis under estrogen-present and estrogen-deprived conditions.
Main Results:
- Constitutive Raf-1 activity was incompatible with estrogen-dependent growth, leading to down-regulation of the transfected Raf.
- Activated Raf-1 supported cell growth in the absence of estrogen, indicating a potential selective advantage under estrogen deprivation.
- High levels of Raf activity induced apoptosis in MCF-7 cells regardless of estrogen presence, a novel finding for Raf.
Conclusions:
- Activation of growth factor signaling via Raf-1 may confer a survival advantage for breast cancer cells in estrogen-deprived environments.
- Elevated Raf activity can lead to apoptosis, suggesting a complex role in breast cancer cell fate.
- This study highlights a novel apoptotic function of high-level Raf activity in breast cancer cells.