Tumor cell invasion and resistance to therapy are the most intractable biological characteristics of cancer and, therefore, the most challenging for current cancer research and treatment paradigms. Refractory cancers, including pancreatic cancer and glioblastoma, show an inextricable association between the highly invasive behavior of tumor cells and their resistance to chemotherapy, radiotherapy and targeted therapies. These aggressive properties of cancer share distinct cellular pathways that are connected to each other by several molecular hubs. There is increasing evidence to show that glycogen synthase kinase (GSK)‐3β is aberrantly activated in various cancer types and this has emerged as a potential therapeutic target. In many but not all cancer types, aberrant GSK3β sustains the survival, immortalization, proliferation and invasion of tumor cells, while also rendering them insensitive or resistant to chemotherapeutic agents and radiation. Here we review studies that describe associations between therapeutic stimuli/resistance and the induction of pro‐invasive phenotypes in various cancer types. Such cancers are largely responsive to treatment that targets GSK3β. This review focuses on the role of GSK3β as a molecular hub that connects pathways responsible for tumor invasion and resistance to therapy, thus highlighting its potential as a major cancer therapeutic target. We also discuss the putative involvement of GSK3β in determining tumor cell stemness that underpins both tumor invasion and therapy resistance, leading to intractable and refractory cancer with dismal patient outcomes.

Glycogen synthase kinase (GSK)3β is a multifunctional serine/threonine protein kinase with more than 100 substrates and interacting molecules. GSK3β is normally active in cells and negative regulation of GSK3β activity via phosphorylation of its serine 9 residue is required for most normal cells to maintain homeostasis. Aberrant expression and activity of GSK3β contributes to the pathogenesis and progression of common recalcitrant diseases such as glucose intolerance, neurodegenerative disorders and cancer. Despite recognized roles against several proto-oncoproteins and mediators of the epithelial–mesenchymal transition, deregulated GSK3β also participates in tumor cell survival, evasion of apoptosis, proliferation and invasion, as well as sustaining cancer stemness and inducing therapy resistance. A therapeutic effect from GSK3β inhibition has been demonstrated in 25 different cancer types. Moreover, there is increasing evidence that GSK3β inhibition protects normal cells and tissues from the harmful effects associated with conventional cancer therapies. Here, we review the evidence supporting aberrant GSK3β as a hallmark property of cancer and highlight the beneficial effects of GSK3β inhibition on normal cells and tissues during cancer therapy. The biological rationale for targeting GSK3β in the treatment of cancer is also discussed at length.

The major obstacles to treatment of pancreatic cancer are the highly invasive capacity and resistance to chemo- and radiotherapy. Glycogen synthase kinase 3β (GSK3β) regulates multiple cellular pathways and is implicated in various diseases including cancer. Here we investigate a pathological role for GSK3β in the invasive and treatment resistant phenotype of pancreatic cancer. Pancreatic cancer cells were examined for GSK3β expression, phosphorylation and activity using Western blotting and in vitro kinase assay. The effects of GSK3β inhibition on cancer cell survival, proliferation, invasive ability and susceptibility to gemcitabine and radiation were examined following treatment with a pharmacological inhibitor or by RNA interference. Effects of GSK3β inhibition on cancer cell xenografts were also examined. Pancreatic cancer cells showed higher expression and activity of GSK3β than non-neoplastic cells, which were associated with changes in its differential phosphorylation. Inhibition of GSK3β significantly reduced the proliferation and survival of cancer cells, sensitized them to gemcitabine and ionizing radiation, and attenuated their migration and invasion. These effects were associated with decreases in cyclin D1 expression and Rb phosphorylation. Inhibition of GSK3β also altered the subcellular localization of Rac1 and F-actin and the cellular microarchitecture, including lamellipodia. Coincident with these changes were the reduced secretion of matrix metalloproteinase-2 (MMP-2) and decreased phosphorylation of focal adhesion kinase (FAK). The effects of GSK3β inhibition on tumor invasion, susceptibility to gemcitabine, MMP-2 expression and FAK phosphorylation were observed in tumor xenografts. The targeting of GSK3β represents an effective strategy to overcome the dual challenges of invasiveness and treatment resistance in pancreatic cancer.

We previously showed that NADPH oxidase organizer 1 (Noxo1), a component of NADPH oxidase 1 (NOX1), is a TNF-α-induced tumor-promoting factor in gastric tumorigenesis. However, the mechanism of NOX1-induced reactive oxygen species (ROS) signaling for the gastric tumorigenesis has not been understood. Here, we showed that expression of NOX1 complex components, including Noxo1, but not other NOX family members was significantly upregulated in both mouse models for gastritis and gastric tumors, which was associated with increased ROS levels. We also found that NF-κB directly regulated NOXO1 expression in TNF-α-stimulated gastric cancer cells, suggesting that inflammation induces NOX1 complex activation through TNF-α/NF-κB pathway. Notably, in situ hybridization indicated that Noxo1 mRNA was detected in proliferating cells of gastritis and gastric tumors, and pharmacological inhibition of NOX activity significantly suppressed the proliferation of MKN45 gastric cancer cells and gastric hyperplasia of K19-C2mE mice. These results suggest that NOX1/ROS signaling has an important role in increased proliferation of stomach epithelial cells in the inflamed mucosa. Moreover, we found that expression of SOX2, a marker of gastric epithelial stem cells, was increased by NOX1/ROS signaling. Furthermore, disruption of Noxo1 in K19-C2mE mice significantly suppressed gastritis-associated metaplastic hyperplasia, a potent preneoplastic lesion, which was associated with decreased number of SOX2-positive cells. These results indicate that inflammation-induced Noxo1 expression is responsible for development of metaplastic hyperplasia in the stomach through an increase in SOX2-expressing undifferentiated epithelial cells. Therefore, inhibition of the NOX1/ROS signaling pathway is a possible strategy for prevention and therapy for gastric cancer development.

Background & Aims The activation of Wnt/β-catenin signaling causes the development of gastric and colon cancers. Sox17 represses Wnt/β-catenin signaling and is down-regulated in colon cancer. This study was designed to elucidate the role of Sox17 during the course of gastrointestinal tumorigenesis. Methods Sox17 expression was examined in gastrointestinal tumors of mouse models and humans. The roles of Sox17 in gastric tumorigenesis were examined by cell culture experiments and by construction of Sox17 transgenic mice. Results Sox17 was induced in K19-Wnt1/C2mE mouse gastric tumors and K19-Wnt1 preneoplastic lesions, where Wnt/β-catenin signaling was activated. Consistently, Wnt activation induced Sox17 expression in gastric cancer cells. In contrast, Sox17 was rarely detected by immunohistochemistry in gastric and colon cancers, whereas strong nuclear staining of Sox17 was found in >70% of benign gastric and intestinal tumors. Treatment with a demethylating agent induced Sox17 expression in gastric cancer cells, thus indicating the down-regulation of Sox17 by methylation. Moreover, transfection of Sox17 in gastric cancer cells suppressed both the Wnt activity and colony formation efficiency. Finally, transgenic expression of Sox17 suppressed dysplastic tumor development in K19-Wnt1/C2mE mouse stomach. Conclusions Sox17 plays a tumor suppressor role through suppression of Wnt signaling. However, Sox17 is induced by Wnt activation in the early stage of gastrointestinal tumorigenesis, and Sox17 is down-regulated by methylation during malignant progression. It is therefore conceivable that Sox17 protects benign tumors from malignant progression at an early stage of tumorigenesis, and down-regulation of Sox17 contributes to malignant progression through promotion of Wnt activity.