Increasing evidence suggests that the normal genetic programs underlying various developmental processes can often be usurped in pathological conditions such as cancer metastasis55, 56
. The functional role of epithelial-mesenchymal transition, a process crucial for embryonic organogenesis, serves as a leading example of this burgeoning paradigm2, 4, 7
. Not surprisingly, physiological and pathological EMT share many common features and molecular regulators, and EMT studies in the context of both development and cancer may thus provide an integrative understanding of the pathological basis of malignancy.
Elf5 is well known as the master regulator of alveolar cell fate in mammary gland development35, 36
, and recent work from our laboratory suggests a more expansive role for Elf5 in inhibiting MaSC activity37
. However, it was not previously known whether this role of Elf5 in influencing mammary cell fate could also impact the development and progression of mammary tumors. In this report, we delineate for the first time the role of Elf5 in regulating EMT and metastasis. We have used a combination of in vitro
cellular systems, bioinformatic analyses, and multiple in vivo
metastasis models to show that gain of Elf5 promotes a global acquisition of numerous epithelial characteristics and suppresses metastatic progression. Conversely, loss of Elf5 induces dramatic EMT and accelerates lung metastasis. Mechanistically we have shown that Elf5 regulates these cellular programs via its direct transcriptional repression of the key EMT transcription factor SNAIL2
. Finally, clinical dataset analyses suggest that this Elf5-SNAIL2 signaling axis is intact in breast cancer patients and significantly affects patient prognosis, particularly in the ER- cohort. Additional clinical and functional analysis will be needed to investigate the molecular basis of this subtype-specific association and the potential involvement of ELF5-SNAIL2 axis in other subgroups of breast cancers.
Snail2 is a transcription factor conserved across species that has been shown to play an important role in EMT during gastrulation5
. In the breast epithelium, Snail2 is normally expressed in the basal/stem cell-enriched population in both mice and humans57
and has been established as one of the most powerful mediators of EMT in mouse and human invasive carcinoma cells19, 58–60
. Intriguingly, a recent study has demonstrated a critical role for Snail2 in regulating mammary stem cell (MaSC) and CSC activity, particularly in cooperation with the transcription factor Sox920
. While Snail2 expression is known to be lost during the transition from MaSCs to differentiated luminal cells20
, the underlying mechanism of this repression in normal mammary differentiation is unknown. Our current study indicates Elf5 as a crucial negative regulator of Snail2 in both mammary gland development and breast cancer. Adding to our recent observation that loss of Elf5 results in an increase in MaSC activity37
, we have further dissected the role of the Elf5-Snail2 axis in MaSC regulation by gain-of-function overexpression of Elf5. Elf5 overexpression decreased MaSC activity, which was reverted by overexpression of Snail2 (Supplementary Table S4
). Conversely, the increased MaSC phenotype observed in Elf5-KO mammary epithelial cells was reversed by Snail2 knockdown (Supplementary Table S5
). Taken together with our extensive analysis of the Elf5-Snail2 axis in EMT and metastasis, our results suggest that high expression of Elf5 in the differentiated luminal lineage inhibits Snail2 expression and basal cell properties, while loss of Elf5 not only induces EMT and increases Snail2 -dependent MaSC activity, but also promote breast cancer metastasis (). Overall, these findings support previous work by Weinberg and others demonstrating a tight relationship between EMT and stemness20, 21, 40, 41, 61
. Interestingly, GSEA of our microarray data indicated that there is significant enrichment of the CSC gene signature in Elf5-KO mouse mammary glands () and, correspondingly, negative enrichment of the CSC signature in Elf5-overexpressing breast cancer cells (). Future experiments will be needed to investigate the potential role of Elf5 as an inhibitor of CSCs.
Altogether, our analyses have concluded that Elf5, through its direct negative regulation of Snail2, serves as a master enforcer of the epithelial cell fate. In the physiological context, this role promotes the proper identity of alveolar mammary epithelial cells, whereas in the pathological context Elf5 functions as a suppressor of EMT and cancer metastasis. Notably, Elf5 loss has been frequently detected during the early onset of disease progression at the stage of breast hyperplasia. This event may therefore represent one of the driving forces for early stage breast tumor cells to proceed with EMT and subsequent metastatic progression, thus highlighting the recently described Elf5-Snail2 axis as a potential target for early therapeutic intervention in breast cancer progression.