TAp73 Inhibits EMT and Cell Migration in Pancreatic Cancer Cells Through Promoting SMAD4 Expression and SMAD4 Dependent Inhibition of ERK Activation

Pancreatic ductal adenocarcinoma (PDAC) is a fatal disease due to early metastatic spread, late diagnosis and the lack of efficient therapies. A major driver of cancer progression and hurdle to successful treatment is transforming growth factor (TGF)-β. Recent data from pancreatic cancer mouse models have shown that transcriptionally active p73 (TAp73), a p53 family member, inhibits tumor progression through promoting tumor suppressive canonical TGF-β/Smad signaling, while preventing non-canonical TGF-β signaling through extracellular signal-regulated kinases (ERK)1/2. Here, we have studied whether this mechanism also operates in human PDAC. Using the PDAC-derived tumor cell lines PANC-1 and HPAFII, we show that TAp73 induces the expression of the epithelial marker and invasion suppressor E-cadherin and the common-mediator Smad, SMAD4, while at the same time suppressing expression of the EMT master regulator SNAIL and basal and TGF-β1-induced activation of ERK1 and ERK2. Using dominant-negative and RNA interference-based inhibition of SMAD4 function we went on to show that inhibition of ERK activation by TAp73 is mediated through SMAD4. Intriguingly, both SMAD4 and the α isoform of TAp73 - but not the isoform - interfered with cell migration as shown by xCELLigence technology. Our findings highlight the role of TAp73-SMAD4 signaling in tumor suppression of human PDAC and identify direct inhibition of basal and TGF-β-stimulated pro-invasive ERK activation as an underlying mechanism.