While vascular endothelial growth factor (VEGF) expression in breast tumors has been correlated with a poor outcome in the pathogenesis of breast cancer, the expression, localization, and function of VEGF receptors VEGFR1 (also known as FLT1) and VEGFR2 (also known as KDR or FLK1), as well as neuropilin 1 (NRP1), in breast cancer are controversial.
We investigated the expression and function of VEGF and VEGF receptors in breast cancer cells. We observed that VEGFR1 expression was abundant, VEGFR2 expression was low, and NRP1 expression was variable. MDA-MB-231 and MCF-7 breast cancer cells, transfected with antisense VEGF cDNA or with siVEGF ( VEGF-targeted small interfering RNA), showed a significant reduction in VEGF expression and increased apoptosis as compared to the control cells. Additionally, specifically targeted knockdown of VEGFR1 expression by siRNA (siVEGFR1) significantly decreased the survival of breast cancer cells through down-regulation of protein kinase B (AKT) phosphorylation, while targeted knockdown of VEGFR2 or NRP1 expression had no effect on the survival of these cancer cells. Since a VEGFR1-specific ligand, placenta growth factor (PGF), did not, as expected, inhibit the breast cancer cell apoptosis induced by siVEGF, and since VEGFR1 antibody also had no effects on the survival of these cells, we examined VEGFR1 localization. VEGFR1 was predominantly expressed internally in MDA-MB-231 and MCF-7 breast cancer cells. Specifically, VEGFR1 was found to be colocalized with lamin A/C and was expressed mainly in the nuclear envelope in breast cancer cell lines and primary breast cancer tumors. Breast cancer cells treated with siVEGFR1 showed significantly decreased VEGFR1 expression levels and a lack of VEGFR1 expression in the nuclear envelope.
This study provides, to our knowledge for the first time, evidence of a unique survival system in breast cancer cells by which VEGF can act as an internal autocrine (intracrine) survival factor through its binding to VEGFR1. These results may lead to an improved strategy for tumor therapy based on the inhibition of angiogenesis.
Shalom Avraham and colleagues' study provides evidence of a survival system in breast cancer cells by which VEGF acts as an internal autocrine survival factor through its binding to VEGFR1.
One woman in eight will develop breast cancer during her lifetime. Most of these women live for many years after their diagnosis and many are cured of their cancer. However, sometimes the cancer grows inexorably and spreads (metastasizes) around the body despite the efforts of oncologists. Characteristics of the tumor known as prognostic factors can indicate whether this spreading is likely to happen. Large tumors that have metastasized have a poorer prognosis than small tumors that are confined to the breast. The expression of specific proteins within the tumor also provides prognostic information. One protein whose expression is associated with a poor prognosis is vascular endothelial growth factor (VEGF). VEGF stimulates angiogenesis—the growth of new blood vessels. Small tumors get the nutrients needed for their growth from existing blood vessels but large tumors need to organize their own blood supply. They do this, in part, by secreting VEGF. This compound binds to proteins (receptors) on the surface of endothelial cells (the cells lining blood vessels), which then send a signal into the cell instructing it to make new blood vessels. Angiogenesis inhibitors, including molecules that block the activity of VEGF receptors, are being developed for the treatment of cancer.
Some breast cancer cell lines (cells isolated from breast cancers and grown in the laboratory) make VEGF and VEGF receptors (VEGFR1, VEGFR2, and neuropilin 1 [NRP1]). But, although some studies have reported an association between VEGFR1 expression in breast tumors and a poor prognosis, other studies have found no expression of VEGFR1 in breast tumors. Consequently, the role of VEGF receptors in breast cancer is unclear. In this study, the researchers analyzed the expression and function of VEGF and its receptors in breast cancer cells to investigate whether and how VEGF helps these cells to survive.
The researchers first examined the expression of VEGF receptors in several human breast cancer cell lines. All of them expressed VEGFR1, some expressed NRP1, but VEGFR2 expression was universally low. They then investigated the function of VEGF and its receptors in two human breast cancer cell lines (MDA-MB-231 and MCF-7). In both cell lines, blocking the expression of VEGF or of VEGFR1 (but not of the other two receptors) reduced cell survival by stimulating a specific process of cell death called apoptosis. Unexpectedly, adding VEGF to the cultures did not reverse the effect of blocking VEGF expression, a result that suggests that VEGF and VEGFR1 do not affect breast cancer cell survival by acting at the cell surface. Accordingly, when the researchers examined where VEGFR1 occurs in the cell, they found it on the membranes around the nucleus of the breast cancer cell lines and not on the cell surface; several primary breast tumors and normal breast tissue had the same localization pattern. Finally, the researchers showed that inhibitors of VEGF action that act at the cell surface did not affect the survival of the breast cancer cell lines.
These findings suggest that VEGF helps breast cancer cells to survive in a unique way: by binding to VEGFR1 inside the cell. In other words, whereas VEGF normally acts as a paracrine growth factor (it is released by one cell and affects another cell), in breast cancer cells it might act as an internal autocrine (intracrine) survival factor, a factor that affects the cells in which it is produced. These findings need confirming in more cell lines and in primary breast cancers but could have important implications for the treatment of breast cancer. Inhibitors of VEGF and VEGFR1 that act inside the cell (small molecule drugs) might block breast cancer growth more effectively than inhibitors that act at the cell surface (for example, proteins that bind to the receptor), because internally acting inhibitors might both kill the tumor directly and have antiangiogenic effects, whereas externally acting inhibitors could only have the second effect.
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