Mechanistic Studies of TGFβ-induced Cancer Stem Cell Formation and Mammary Tumorigenesis through an ILEI/LIFR Signaling Axis

The dual functionalities of Transforming Growth Factor Beta (TGFb) as either a tumor promoting or suppressing cytokine is highly dependent on cellular context, specifically during malignant transformation and metastatic progression of normal mammary epithelial cells. Our lab has extensively studied the pathologic function of TGFb in mammary tumorigenesis and metastasis through its role in activating epithelial-mesenchymal transitions (EMT) and cancer stem cell (CSC) phenotypes. We have demonstrated that TGFb induces EMT and CSC phenotypes in mammary epithelial cells through a non-canonical, translational regulatory mechanism involving an hnRNP complex containing hnRNP E1 that inhibits translation of a cohort of mRNAs. We hypothesized that TGFb-mediated EMT and CSC formation through hnRNP E1 functional silencing is dependent on ILEI translation and its downstream functional output. Through extensive in vitro biochemical, biophysical and cell/molecular analysis we identified a novel mechanism of ILEI signaling through its extracellular ligand-binding and activation of Leukemia Inhibitor Factor Receptor (LIFR). LIFR is highly implicated in embryonic stem cell maintenance through coordination of several key signaling pathways such as Erk/MAPK, PI3K/Akt, and Jak/STAT. We determined that mammary epithelial cells confer EMT and CSC properties through elevated STAT3 activity by TGFb-induced ILEI translation and subsequent autocrine LIFR activation. Disruption of this pathway by shRNA silencing of ILEI or LIFR in tumorigenic cell lines results in in vivo abrogation of tumorigenesis and metastasis. Interestingly, we can also demonstrate that TGFb transcriptionally upregulates LIFR in mammary epithelial cells through both canonical (SMAD3) and non-canonical (ILEI/STAT3) pathways in a kinetically coordinated manner. Cellular studies indicate that disruption of either TGFb/SMAD and/or TGFb/ILEI/STAT3 pathways alters LIFR transcription and stemness properties that are correlated with enhanced proliferation, invasion, and migration phenotypes. In total, we have established a cellular mechanism by which TGFb induces mammary EMT and CSC programs in a feed-forward manner through the upregulation of both a ligand (ILEI) and its extracellular receptor (LIFR). These findings provide a novel platform for targeted therapies against mammary tumorigenesis and metastatic spread