YangXin Fu

Slug is a direct Notch target required for initiation of cardiac cushion cellularization

Snail family proteins are key regulators of epithelial-mesenchymal transition, but their role in endothelial-to-mesenchymal transition (EMT) is less well studied. We show that Slug, a Snail family member, is expressed by a subset of endothelial cells as well as mesenchymal cells of the atrioventricular canal and outflow tract during cardiac cushion morphogenesis. Slug deficiency results in impaired cellularization of the cardiac cushion at embryonic day (E)–9.5 but is compensated by increased Snail expression at E10.5, which restores cardiac cushion EMT. We further demonstrate that Slug, but not Snail, is directly up-regulated by Notch in endothelial cells and that Slug expression is required for Notch-mediated repression of the vascular endothelial cadherin promoter and for promoting migration of transformed endothelial cells. In contrast, transforming growth factor β (TGF-β) induces Snail but not Slug. Interestingly, activation of Notch in the context of TGF-β stimulation results in synergistic up-regulation of Snail in endothelial cells. Collectively, our data suggest that combined expression of Slug and Snail is required for EMT in cardiac cushion morphogenesis.

Smooth Muscle α-Actin Is a Direct Target of Notch/CSL

Intercellular signaling mediated by Notch receptors is essential for proper cardiovascular development and homeostasis. Notch regulates cell fate decisions that affect proliferation, survival, and differentiation of endothelial and smooth muscle cells. It has been reported that Jagged1–Notch interactions may participate in endocardial cushion formation by inducing endothelial-to-mesenchymal transformation. Here, we show that Notch directly regulates expression of the mesenchymal and smooth muscle cell marker smooth muscle &agr;-actin (SMA) in endothelial and vascular smooth muscle cells via activation of its major effector, CSL. Notch/CSL activation induces SMA expression during endothelial-to-mesenchymal transformation, and Notch activation is required for expression of SMA in vascular smooth muscle cells. CSL directly binds a conserved cis element in the SMA promoter, and this consensus sequence is required for Notch-mediated SMA induction. This is the first evidence of the requirement for Notch activation in the regulation of SMA expression.

Notch Initiates the Endothelial-to-Mesenchymal Transition in the Atrioventricular Canal through Autocrine Activation of Soluble Guanylyl Cyclase

The heart is the most common site of congenital defects, and valvuloseptal defects are the most common of the cardiac anomalies seen in the newborn. The process of endothelial-to-mesenchymal transition (EndMT) in the cardiac cushions is a required step during early valve development, and Notch signaling is required for this process. Here we show that Notch activation induces the transcription of both subunits of the soluble guanylyl cyclase (sGC) heterodimer, GUCY1A3 and GUCY1B3, which form the nitric oxide receptor. In parallel, Notch also promotes nitric oxide (NO) production by inducing Activin A, thereby activating a PI3-kinase/Akt pathway to phosphorylate eNOS. We thus show that the activation of sGC by NO through a Notch-dependent autocrine loop is necessary to drive early EndMT in the developing atrioventricular canal (AVC).

Differential Regulation of Transforming Growth Factor β Signaling Pathways by Notch in Human Endothelial Cells

Notch and transforming growth factor β (TGFβ) play critical roles in endothelial-to-mesenchymal transition (EndMT), a process that is essential for heart development. Previously, we have shown that Notch and TGFβ signaling synergistically induce Snail expression in endothelial cells, which is required for EndMT in cardiac cushion morphogenesis. Here, we report that Notch activation modulates TGFβ signaling pathways in a receptor-activated Smad (R-Smad)-specific manner. Notch activation inhibits TGFβ/Smad1 and TGFβ/Smad2 signaling pathways by decreasing the expression of Smad1 and Smad2 and their target genes. In contrast, Notch increases SMAD3 mRNA expression and protein half-life and regulates the expression of TGFβ/Smad3 target genes in a gene-specific manner. Inhibition of Notch in the cardiac cushion of mouse embryonic hearts reduces Smad3 expression. Notch and TGFβ synergistically up-regulate a subset of genes by recruiting Smad3 to both Smad and CSL binding sites and cooperatively inducing histone H4 acetylation. This is the first evidence that Notch activation affects R-Smad expression and that cooperative induction of histone acetylation at specific promoters underlies the selective synergy between Notch and TGFβ signaling pathways.

Notch3 induces epithelial-mesenchymal transition and attenuates carboplatin-induced apoptosis in ovarian cancer cells.

OBJECTIVE Notch3 is implicated in chemoresistance of ovarian cancer, yet the molecular mechanism underlying Notch3-mediated drug resistance remains to be elucidated. Here, we investigated the role of Notch3 in carboplatin-induced apoptosis in ovarian cancer cells. METHODS Ovarian cancer cell line OVCA429 cells were stably transduced with an empty vector or a retroviral vector expressing the Notch3 intracellular domain (NICD3, the constitutively active form of Notch3) to generate OVCA429/vector and OVCA429/NICD3 cells. Epithelial-mesenchymal transition (EMT) was determined by morphological change and expression of the EMT markers. Carboplatin-induced cytotoxicity was determined by the neutral red uptake assay. Apoptosis was determined by Annexin V staining and Western blotting. Carboplatin-induced phosphorylation of extracellular signal-regulated kinase (ERK) was identified by a phospho-kinase array and confirmed by Western blotting. RESULTS Activation of Notch3 in OVCA429 cells causes a spindle and fibroblast-like morphology, induces the expression of smooth muscle α-actin, Slug and Snail, but decreases the expression of E-cadherin, indicating that Notch3 activation induces EMT in OVCA429 cells. Furthermore, Notch3 activation renders OVCA429 cells more resistant to carboplatin-induced cytotoxicity and attenuates carboplatin-induced apoptosis in these cells. Our results indicate that phosphorylation of ERK is a positive regulator of carboplatin-induced apoptosis in OVCA429 cells. Interestingly, carboplatin-induced ERK phosphorylation is inhibited by Notch3 activation. CONCLUSIONS Notch3 activation induces EMT and attenuates carboplatin-induced apoptosis in OVCA429 cells. ERK phosphorylation plays a pro-apoptotic role in carboplatin-induced apoptosis in OVCA429 cells. Interestingly, Notch3 activation attenuates carboplatin-induced ERK phosphorylation in these cells.

TGFβ and EGF synergistically induce a more invasive phenotype of epithelial ovarian cancer cells.

The epithelial-mesenchymal transition (EMT) is associated with progression and metastasis of epithelial ovarian cancer (EOC). Snail and Slug (two members of the Snail family of transcription factors) down-regulate the expression of the adhesion molecule E-cadherin and thus function as positive regulators of EMT. Their expression is associated with a more invasive phenotype of EOC. However, how their expression in EOC cells is regulated needs to be further defined. Here, we show that transforming growth factor β (TGFβ) and epidermal growth factor (EGF) synergistically induce the expression of Slug and Snail at both mRNA and protein levels in an EOC cell line OVCA429 cells. Using specific chemical inhibitors, we demonstrate that Slug and Snail expression induced by TGFβ is mediated by TGFβ/ALK5 pathway, and EGF-induced expression of Slug and Snail is MEK1/2-dependent. Interestingly, TGFβ-induced Slug expression is also MEK1/2-dependent. Further, we demonstrate that combined TGFβ and EGF stimulation is more potent than either alone in repressing the expression of E-cadherin. Functionally, combined stimulation of TGFβ and EGF enhances the mobility of OVCA429 cells and induces the production of MMP2 by OVCA429 cells more potently than either alone. Taken together, our data demonstrate that TGFβ and EGF signaling pathways synergistically induce EMT and render EOC cells a more invasive phenotype.

RUNX3 maintains the mesenchymal phenotype after termination of the notch signal

Notch is a critical mediator of endothelial-to-mesenchymal transition (EndMT) during cardiac cushion development. Slug, a transcriptional repressor that is a Notch target, is an important Notch effector of EndMT in the cardiac cushion. Here, we report that the runt-related transcription factor RUNX3 is a novel direct Notch target in the endothelium. Ectopic expression of RUNX3 in endothelium induces Slug expression and EndMT independent of Notch activation. Interestingly, RUNX3 physically interacts with CSL, the Notch-interacting partner in the nucleus, and induces Slug in a CSL-dependent, but Notch-independent manner. Although RUNX3 may not be required for the initial induction of Slug and EndMT by Notch, because RUNX3 has a much longer half-life than Slug, it sustains the expression of Slug thereby maintaining the mesenchymal phenotype. CSL binds to the Runx3 promoter in the atrioventricular canal in vivo, and inhibition of Notch reduces RUNX3 expression in the cardiac cushion of embryonic hearts. Taken together, our results suggest that induction of RUNX3 may be a mechanism to maintain Notch-transformed mesenchymal cells during heart development.

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.

Notch activation augments nitric oxide/soluble guanylyl cyclase signaling in immortalized ovarian surface epithelial cells and ovarian cancer cells

Nitric oxide (NO) is generated by tumor, stromal and endothelial cells and plays a multifaceted role in tumor biology. Many physiological functions of NO are mediated by soluble guanylyl cyclase (sGC) and NO/sGC signaling has been shown to promote proliferation and survival of ovarian cancer cells. However, how NO/sGC signaling is modulated in ovarian cancer cells has not been studied. The evolutionarily conserved Notch signaling pathway plays an oncogenic role in ovarian cancer. Here, we report that all three ovarian cancer cell lines we examined express a higher level of GUCY1B3 (the β subunit of sGC) compared to non-cancerous immortalized ovarian surface epithelial (IOSE) cell lines. Interestingly, the highest expression of GUCY1B3 in ovarian cancer OVCAR3 cells is concurrent with the expression of Notch3. In IOSE cells, forced activation of Notch3 increases the expression of GUCY1B3, NO-induced cGMP production, and the expression of cGMP-dependent protein kinase (PKG), thereby enhancing NO- and cGMP-induced phosphorylation of vasodilator-stimulated phosphoprotein (VASP, a direct PKG substrate protein). In contrast, inhibition of Notch by DAPT reduces GUCY1B3 expression and NO-induced cGMP production and VASP phosphorylation in OVCAR3 cells. Finally, we confirmed that inhibition of sGC by ODQ decreases growth of ovarian cancer cells. Together, our work demonstrates that Notch is a positive regulator of NO/sGC signaling in IOSE and ovarian cancer cells, providing the first evidence that Notch and NO signaling pathways interact in IOSE and ovarian cancer cells.

Elevated β-catenin activity contributes to carboplatin resistance in A2780cp ovarian cancer cells.

Ovarian cancer is the fifth leading cause of cancer-related mortalities in women. Epithelial ovarian cancer (EOC) represents approximately 90% of all ovarian malignancies. Most EOC patients are diagnosed at advanced stages and current chemotherapy regimens are ineffective against advanced EOC due to the development of chemoresistance. It is important to better understand the molecular mechanisms underlying acquired resistance to effectively manage this disease. In this study, we examined the expression of the Wnt/β-catenin signaling components in the paired cisplatin-sensitive (A2780s) and cisplatin-resistant (A2780cp) EOC cell lines. Our results showed that several negative regulators of Wnt signaling are downregulated, whereas a few Wnt ligands and known Wnt/β-catenin target genes are upregulated in A2780cp cells compared to A2780s cells, suggesting that Wnt/β-catenin signaling is more active in A2780cp cells. Further analysis revealed nuclear localization of β-catenin and higher β-catenin transcriptional activity in A2780cp cells compared to A2780s cells. Finally, we demonstrated that chemical inhibition of β-catenin transcriptional activity by its inhibitor CCT036477 sensitized A2780cp cells to carboplatin, supporting a role for β-catenin in carboplatin resistance in A2780cp cells. In conclusion, our data suggest that increased Wnt/β-catenin signaling activity contributes to carboplatin resistance in A2780cp cells.