FBF1 maintains stem cell-like properties in breast cancer via PI3K/AKT/SOX2 axis

Considerable evidence suggests that tumor initiation, malignancy, metastasis and recurrence occur due to emergence of cancer stem cells (CSCs). Fas binding factor 1 (FBF1) is a multifunctional protein that plays essential roles in the regulation of development and cell fate decisions. However, the function in maintaining stem cell-like properties of breast cancer remains elusive.

Tissue microarray was used to evaluate FBF1 expression. Cancer stemness assays were performed in FBF1 silencing and overexpressing cells in vitro and in a xenograft model in vivo. RNA sequencing, immunofluorescence and immunoprecipitation assays were performed to explore the underlying mechanism. Clinical expression and significance of FBF1 and stemness-associated factors were explored by analyzing datasets.

We report that FBF1 was highly expressed in breast cancer and significantly correlated with clinical progression. Silencing FBF1 in MDA-MB-231 cells restrained CSCs properties, including side population, sphere formation and migration, whereas ectopic FBF1 expression increased the side population proportion, enhanced the sphere formation ability, and promoted the expression of core stemness genes, such as SOX2, OCT4, KLF4 and NANOG, as well as facilitated metastasis of T47D breast cancer cells. Furthermore, mice bearing FBF1-overexpressed T47D xenografts had higher tumorigenic frequency and stronger metastasis potential. In addition, exploration of the underlying mechanism indicated that FBF1 binds PI3K which then activates PI3K-AKT phosphorylation cascades. Then the activated p-AKT interacts with stemness marker SOX2, elevates SOX2 and OCT4 activity, and finally forms PI3K/AKT/SOX2 axis, which mediates stem cell-like identities. Moreover, PI3K inhibitors abolished FBF1-mediated signaling pathway and diminished breast cancer stemness in vitro and in vivo. In 24 human breast cancer samples, we found a good positive correlation between the expression of FBF1 and p-AKT, as well as between FBF1 and SOX2 as determined by IHC. Clinical data showed that FBF1 expression was positively correlated with the expression of POU5F1 (OCT4), AKT1 and was negatively correlated with PTEN, which is a negative regulator of PI3K/AKT signaling.

Collectively, we identified a potential CSCs regulator and suggested a novel mechanism by which FBF1 governs cancer cell stemness. This study thus introduces an effective target for the diagnosis and treatment of breast cancer.