Pharmacological Inhibition of STAT3 Attenuates Pancreatic Cancer Cell Invasion and Migration By Interacting with TGF-Β/Smad Signaling

Background Pancreatic ductal adenocarcinoma (PDAC) is a gastrointestinal malignancy with extremely poor prognosis, primarily due to the infiltration and metastasis of tumor cells. Many studies have shown that the TGF-β/Smad signaling-mediated EMT pathway is a key driving factor of infiltration and metastasis of pancreatic cancer cells, and aberrant STAT3 activation is vital to pancreatic cancer cell metastasis. In this study, a small molecule antagonist, Stattic, was used for targeted STAT3 inhibition, its interaction with TGF-β/Smad signaling was observed, and EMT of pancreatic cancer cells was inhibited to decrease its metastasis and infiltration. Methods The effects of Stattic on the invasion, infiltration, and EMT of pancreatic cancer cells were studied. We also evaluated the effects of Stattic on the STAT3 and TGF-β/Smad signaling pathways and studied the interactions between STAT3 and TGF-β/Smad signaling using overexpression and siRNA techniques. Results Stattic inhibited the invasion and infiltration of pancreatic cancer cells in a dose- and time-dependent manner, which was related to EMT inhibition and subsequent downregulation of extracellular matrix components. Stattic downregulated the expression of metalloproteases, such as MMP-2, and also inhibited the expression of EMT-TFs, such as Snail1 and Slug, and downregulated downstream HIF1α and VEGF expression to inhibit angiogenesis. Mechanistically, in in vitro cell models and in vivo nude mouse tumorigenicity models, Stattic treatment significantly downregulated the protein levels of the matrix synthesis-promoting factor TGF-β and the phosphorylation and nuclear import of its downstream Smad2/3. These results suggest that Stattic inactivates TGF-β/Smad signaling to antagonize EMT and eventually inhibit the invasion and infiltration of pancreatic cancer cells. Stattic is a small molecule inhibitor of STAT3 that can downregulate STAT3 activity, which is enhanced by Smad4-dependent TGF-β/Smad signaling. Interestingly, IL-6 or STAT3 overexpression promotes EMT but in a non-TGF-β/Smad-dependent manner, and is regulated by antagonism between STAT3 and Smad4. Conclusions The application of Stattic pharmacologically to inhibit STAT3 decreases the invasion and infiltration of pancreatic cancer cells, which may be achieved through interactions with TGF-β/Smad signaling and EMT antagonism.