Figure 4. Reprogramming of non-genomic estrogen signaling by SOX2: A new driver of breast cancer stem-like cellular states with tumor-initiating capacity. SOX2 overexpression confers a tumor-initiating advantage to ERα-positive MCF-7 breast cancer cells in an estrogen-dependent manner, i.e. when s.c. injected into ovariectomized nude mice implanted with estrogen pellets, SOX2-overexpressing MCF-7/Rep cells formed more and larger tumors than MCF-7 parental cells, which mostly failed to produce tumor outgrowths in the absence of Matrigel. The previously unrecognized ability of the stemness factor SOX2 to promote estrogen-dependent tumorigenesis of ERα-positive breast cancer cells appears to depend on the activation of non-genomic estrogen signaling. E₂ activates nuclear ERα (genomic pathway) and ERα in or near the membrane (non-genomic pathway). Membrane-associated ERα associates with growth factor signaling components (e.g., EGFR/HER2, INSR, IGF-1R), thus allowing E₂ to activate the growth factor signaling by activating key molecules such as PI3K or Ras and downstream molecules such as AKT, mTOR, MEK, and MAPK. The crosstalk between the non-genomic estrogen signaling and the growth factor signaling is bidirectional, because a variety of kinases including MAPKs, AKT, and mTOR coordinately regulate the phosphorylation of specific sites of the ERα, leading to ligand (E₂)-independent ERα activation. Moreover, phosphorylation of ERα co-regulatory proteins by growth factor kinases regulates the ERα signaling pathway to enhance the nuclear genomic ERα-mediated response. Our findings strongly suggest that reprogramming of non-genomic estrogen signaling by the stemness factor SOX2 is a key molecular feature that determines the tumor-initiating capacity of breast cancer stem-like cellular states.