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 protooncogene 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.
Bibliographical noteFunding Information:
The authors wish to acknowledge Becky Burkhard for construction of the pCHC-D-raf plasmid, Michelle Laciak, David Chism and Debabrata Maji for tissue culture help, and Ronit Yarden for preparation of RNA samples for Northern analyses. Additionally, the authors thank Drs Susan Chrysogelos and Michael Johnson for critical review of this manuscript and helpful discussions. This work was supported in part by Public Health Service Award CA50376 from the National Cancer Institute to F Kern, by CA71465 to D El-Ashry, and by the Lombardi Cancer Center Macromolecular Synthesis and Sequencing Core Facility and Flow Cytometry/Cell Sorting Core Facility (Public Health Service Grant 2P30-CA-51008). D El-Ashry was supported by a Career Development Award from the Department of Defense (USAMRDC, DAMD17-94-J-4172) and D Miller was supported by a Pre-doctoral Fellowship Award from the Department of Defense (USAMRDC, DAMD17-94-J-4167).
- Breast cancer
- Raf-1 kinase