1 INTRODUCTION
Pancreatic cancer is a highly lethal malignancy, which is characterized by the close parallel between cancer incidence and death rates, asymptomatic development, and an extremely poor prognosis (Kleeff et al., 2016; Aier, Semwal, Sharma, & Varadwaj, 2019). According to global cancer statistics, pancreatic cancer is the seventh leading cause of cancer-associated mortality worldwide in 2018 (Bray et al., 2018). Due to the rapid cancer progression without specific symptoms, most patients with pancreatic cancer are diagnosed at advanced stages. The 5-year relative survival rate in patients diagnosed with the disease remains as low as 9% in the United States (Siegel, Miller, & Jemal, 2020). Therefore, pancreatic cancer is commonly fatal. Pancreatic cancer is predicted to become the second leading death cause from cancer in the United States by 2030 (Rahib et al., 2014). However, over the past few decades, therapeutic drugs for pancreatic cancer have yielded very limited success in clinical trials (Roth, Cardin, & Berlin, 2020; Kamisawa, Wood, Itoi, & Takaori, 2016). Moreover, drug resistance is a serious problem in the treatment of pancreatic cancer (Binenbaum, Na’ara, & Gil, 2015). Thus, development of effective targeted agents for treating pancreatic cancer is urgently needed.
STAT3 is a pro-oncogenic transcription factor, and is regarded as a therapeutic target in numerous human cancers, including lung cancer, colorectal cancer, breast cancer, prostate cancer, ovarian cancer and so on (Dutta, Sabri, Li, & Li, 2015; Banerjee & Resat, 2016; Ouédraogo et al., 2017; Liang et al., 2020). STAT3 regulates the expression of its downstream target genes related to cell proliferation, survival, migration, invasion, epithelial-mesenchymal transition, tumor angiogenesis, the Warburg effect, cancer stem cell properties, and immune evasion closely connected with cancer progression and malignancy (Carpenter & Lo, 2014; Kamran, Patil, & Gude, 2013; Wendt, Balanis, Carlin, & Schiemann, 2014; Gao et al., 2017; Demaria et al., 2010; Galoczova, Coates, & Vojtesek, 2018; Wang et al., 2004). Persistently activated STAT3 also mediates cancer-associated inflammation that can initiate and promote malignant transformation, and genetic alterations in pre-neoplastic and malignant cells can also induce and maintain a tumor-promoting inflammatory microenvironment that further facilitates tumorigenesis and malignant progression (Fan, Mao, & Yang, 2013; Yu, Lee, Herrmann, Buettner, & Jove, 2014; Yu, Pardoll, & Jove, 2009). In addition to the tumor cells, STAT3, which is a converging point of numerous oncogenic pathways, is also constitutively activated in tumor-infiltrated immune cells, resulting in suppression of immune mediators and promotion of immunosuppressive factors (Wang, Shen, Wang, Shen, & Zhou, 2018; Yu, Kortylewski, & Pardoll, 2007). In addition, accumulating evidence also indicates that feedback activation of STAT3 plays a critical role in mediating the drug resistance that often follows the targeted cancer therapies and conventional chemotherapies (Lee et al., 2017; Zhao et al., 2016). Taken together, inhibiting STAT3 activation is an attractive and promising approach for cancer prevention, therapy and drug sensitization (Johnson, O’Keefe, & Grandis, 2018; Furtek, Backos, Matheson, & Reigan, 2016). Overexpression and hyperactivation of the oncoprotein STAT3 frequently occur in pancreatic cancer (Scholz et al., 2003; Fofaria & Srivastava, 2015; He et al., 2018; Wei et al., 2003). STAT3 is also a major mediator in KRAS -driven pancreatic cancer that lacks clinically actionable targets (Li, Grivennikov, & Karin, 2011; D’Amico et al., 2018). Hence, inhibiting aberrantly activated STAT3 is a promising strategy for pancreatic cancer therapy.
To identify natural compounds that block the transcriptional activity of STAT3, we utilized the STAT3-luciferase (STAT3-luc) reporter system containing STAT3-binding elements to screen potentially active compounds from our in-house natural product library including more than 1000 compounds and identified a natural compound termed Trienomycin A (TA), a secondary metabolite produced by the moss-soil-derived actinomyceteStreptomyces cacaoi subsp. asoensis H2S5, that has anti-neuroinflammatory and anti-tumor activities (Tang et al., 2018; Liu et al., 2020). However, very little is known about the impact of TA on pancreatic cancer, and the underlying molecular mechanisms of TA are also unclear. In this study, we found that TA memorably repressed the transcriptional activity of STAT3. We further demonstrated TA inhibited the phosphorylation of tyrosine at the 705 site of STAT3 and the protein expression of downstream target genes that related to cell proliferation, growth, migration and invasion, by binding to STAT3 protein. Furthermore, we found that TA significantly reduced the growth of pancreatic tumor in vivo , and that TA had no obvious toxic effects on the normal tissues (heart, liver, spleen, lung and kidney) of mice. Collectively, our data give the first evidence that TA is a potential STAT3 pathway inhibitor with significant activity against pancreatic cancer, and it is worthwhile to be further studied.