Many breast cancers express the estrogen receptor (ER) and need estrogen to grow. Additionally, the growth of many breast cancers depends on cancer stem cells (CSCs), our understanding of which is just beginning to emerge. In 2010, a research group led by Dr. Charlotte Kuperwasser at Tufts University School of Medicine in Boston discovered that, just as the tumors that originate from them, human breast CSCs are regulated by estrogen via the FGF9/TBX3 signaling pathway, the same pathway that regulates normal embryonic breast stem cells (Fillmore et al. 2010). The findings could accelerate the development of breast cancer therapies.
Estrogen stimulates the expansion of human breast CSCs
Human breast CSCs appear to be concentrated within specific cell subsets that form colonies (“tumorspheres”) relatively easily and are resistant to chemotherapy. Kuperwasser and her team found that when ER-expressing (ER+) human breast cancer MCF7 cells are cultured in an estrogen-enriched medium, the number of ER+ CSCs and tumorspheres increases eight-fold and seven-fold respectively. This does not happen if the culture medium has an estrogen inhibitor. Additionally, ovariectomized NOD scid – NOD.CB17-Prkdcscid/J (001303) – mice engrafted with MCF7 cells develop tumors 100 times more efficiently if the cells are pre-treated with estrogen.
Estrogen expands breast CSCs through FGF9 and TBX3
In mice, estrogen indirectly mediates normal mammary gland maturation by stimulating cells to produce two paracrine factors – FGF9 and TBX3. The Kuperwasser team found that estrogen treatment likewise induces the expansion of cultured human breast cancer cells and increases the secretion of TBX3 and FGF9 (and other FGF family members) in these cultures. Furthermore, the cultures become enriched in breast CSCs, increasing the efficiency with which they form tumorspheres and/or tumors in NOD scid mice. In contrast, inhibiting FGF9 receptors has the opposite effect. It also prevents tumor development in NOD scid mice injected with estrogen-stimulated MCF7 cells. The effect of the FGF9 signaling pathway is similar whether the breast cancer cells are ER+, ER-, or patient-derived.
TBX3 expression increases significantly in MCF7 cells cultured with either estrogen or FGF9, but moreso in cells cultured with both estrogen and FGF9, and not at all in cells cultured with either TBX3 or FGF9 receptor inhibitors. Knocking down TBX3 in breast cancer cells (whether ER+ cells cultured with estrogen, ER- cells, or patient-derived) significantly impedes the formation of tumorspheres and/or the expansion of CSCs. Additionally, the number of CSCs increases nine-fold in MCF7 cells forced to over-express TBX3, resulting in a 100-fold increase in their ability to initiate tumors in NOD scid mice.
These results indicated that cultured MCF7 CSCs expand through an estrogen-FGF9/TBX3 signaling pathway and that TBX3 is sufficient to promote normal and cancer stem-like cell phenotypes.
Estrogen/FGF9/TBX3 signaling operates in many human breast cancers
Kuperwasser and her team found that TBX3 expression is high in many subtypes of breast cancer and positively correlated with chemo-resistance, ER+ status and breast cancer recurrence. These data were obtained by quantifying TBX3 and FGFR3 expression using gene expression microarrays from more than 18,000 cancer patients. Additionally, in a majority of primary tumors, ER-alpha expression levels positively correlated with those of FGFR3.
The Kuperwasser team's results indicated that estrogen stimulates the expansion of breast CSCs via FGF9/FGFR and TBX3 signaling, suggesting that targeting this pathway may be a promising therapy for breast cancer.
Fillmore CM, Gupta PB, Rudnick JA, Caballero S, Keller PJ, Lander ES, and Kuperwasser C. PNAS Early Edition. Estrogen expands breast cancer stem-like cells through paracrine FGF/Tbx3 signaling. Published online November 22, 2010, doi:10.1073/pnas.1007863107.