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.

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 the desmoplastic reaction of the tumor stroma, the formation of which is orchestrated by transforming growth factor (TGF)-β. Recent data from pancreatic cancer mouse models have shown that the TGF-β pathway is controlled by transcriptionally active p73 (TAp73) through secretion of biglycan (Bgn) via intermittent expression of the TGF-β signaling intermediates, Smad3 and Smad4. Genetic knockout of TP73, and, as a consequence, deficient induction of Smad3/Dpc4 and secretion of Bgn led to activation of TGF-β signaling through a (Smad-independent) ERK pathway, favoring epithelial-mesenchymal transition (EMT) and cell motility. Except for BGN, these functions of TAp73 have recently been shown to also operate in human PDAC cells and are reminiscent of what we previously observed for the small GTPase, RAC1b. This prompted us to hypothesize that TAp73 and RAC1b are part of the same tumor-suppressive pathway in human PDAC cells. The two objectives of this study, therefore, were to reveal i) if the regulatory interactions between TAp73 and Bgn previously discovered in murine PDAC-derived cells also operate in their human counterparts, and ii) if RAC1b collaborates with TAp73 in these tumor-suppressive activities in human PDAC cells. Using a variety of experimental approaches, including mutual rescue experiments, we were able to show that the previously proposed tumor-suppressive TAp73-Smad4-Bgn signaling also operates in human cells and that RAC1b is as an upstream activator of this pathway. Our findings highlight the complex role of TGF-β in pancreatic tumorigenesis and might have implications for therapeutic approaches targeting this growth factor for inhibition.