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The oncogene heregulin induces apoptosis in breast epithelial cells and tumors
E J Weinstein1, S Grimm, P Leder
1Department of Genetics, Harvard Medical School and Howard Hughes Medical Institute, Boston, Massachusetts 02115, USA.
Abstract:
The products of a growing number of genes have been shown to display seemingly contradictory functions; namely, the induction of tumorigenesis and the induction of apoptosis. Heregulin's involvement in oncogenesis occurs through its interactions with members of the EGF receptor tyrosine kinase family. Recently one isoform of heregulin, beta2b, was isolated in an in vitro screen for dominant, apoptosis-inducing genes in kidney epithelial cells. Here we show that heregulin is also capable of mediating apoptosis in human and murine mammary tumor cell lines and murine tumors. Furthermore, through transfection of the human breast cancer cell line MCF-7 with the truncated transmembrane/cytoplasmic segment of the heregulin gene, we show that the intracellular region of the heregulin precursor is sufficient for induction of apoptosis. Through the use of DNA fragmentation assays we also show that apoptosis occurs in cell lines established from heregulin-induced mammary gland tumors. TdT addition of digoxigenin labeled nucleotides to 3' OH ends of DNA breaks recapitulated these results in the actual tumors. Finally, over-expression of heregulin is shown to lead to the down-regulation of Bcl-2, an inhibitor of apoptosis. Conversely, the transfection of Bcl-2 into MCF-7 cells inhibits heregulin-mediated programmed cell death.
Insights
Heregulin, a gene product, can induce both tumorigenesis and apoptosis. This study demonstrates heregulin mediates apoptosis in mammary tumor cells and identifies its intracellular region as sufficient for this effect, impacting Bcl-2 regulation.
Area of Science:
- Molecular Biology
- Cancer Research
- Cell Biology
Background:
- Gene products can exhibit dual functions, promoting tumorigenesis and apoptosis.
- Heregulin interacts with EGF receptor tyrosine kinases in oncogenesis.
- Heregulin beta2b isoform was previously identified as an apoptosis-inducing gene.
Purpose of the Study:
- To investigate heregulin's role in mediating apoptosis in mammary tumor cells.
- To determine if the intracellular region of heregulin is sufficient for apoptosis induction.
- To elucidate the mechanism of heregulin-mediated apoptosis, including its effect on Bcl-2.
Main Methods:
- Transfection of human breast cancer cell line MCF-7 with heregulin gene segments.
- DNA fragmentation assays to detect apoptosis.
- Analysis of Bcl-2 expression levels in response to heregulin.
- TdT-mediated dUTP nick end labeling (TUNEL) assay on tumors.
Main Results:
- Heregulin mediates apoptosis in human and murine mammary tumor cell lines and tumors.
- The intracellular region of the heregulin precursor is sufficient to induce apoptosis.
- Apoptosis was confirmed in cell lines derived from heregulin-induced mammary tumors.
- Heregulin overexpression leads to down-regulation of Bcl-2, an apoptosis inhibitor.
- Bcl-2 transfection inhibits heregulin-mediated apoptosis.
Conclusions:
- Heregulin is a potent inducer of apoptosis in mammary cancer cells and tumors.
- The intracellular domain of heregulin plays a critical role in mediating programmed cell death.
- Heregulin-induced apoptosis is associated with the down-regulation of the anti-apoptotic protein Bcl-2.