Notch and TGFβ form a positive regulatory loop and regulate EMT in epithelial ovarian cancer cells

Epithelial-mesenchymal transition (EMT) plays a critical role in the progression of epithelial ovarian cancer (EOC). However, the mechanisms that regulate EMT in EOC are not fully understood. Here, we report that activation of Notch1 induces EMT in EOC cells as evidenced by downregulation of E-cadherin and cytokeratins, upregulation of Slug and Snail, as well as morphological changes. Interestingly, activation of Notch1 increases TGFβ/Smad signaling by upregulating the expression of TGFβ and TGFβ type 1 receptor. Time course experiments demonstrate that inhibition of Notch by DAPT (a γ-secretase inhibitor) decreases TGFβ-induced phosphorylation of receptor Smads at late, but not at early, timepoints. These results suggest that Notch activation plays a role in sustaining TGFβ/Smad signaling in EOC cells. Furthermore, inhibition of Notch by DAPT decreases TGFβ induction of Slug and repression of E-cadherin and knockdown of Notch1 decreases TGFβ-induced repression of E-cadherin, indicating that Notch is required, at least in part, for TGFβ-induced EMT in EOC cells. On the other hand, TGFβ treatment increases the expression of Notch ligand Jagged1 and Notch target gene HES1 in EOC cells. Functionally, the combination of Notch1 activation and TGFβ treatment is more potent in promoting motility and migration of EOC cells than either stimulation alone. Taken together, our results indicate that Notch and TGFβ form a reciprocal positive regulatory loop and cooperatively regulate EMT and promote EOC cell motility and migration.