Pancreatic cancer belongs to the group of extremely aggressive human cancers; conventional treatments have little impact. In 2010, it accounts for only 3% of new cancer cases in the United States, and the fourth leading cause of cancer death. Only 6 percent of patients will survive 5 years after diagnosis 
. Various factors, which include its aggressive nature, lack of early screening, absence of therapeutic targets, and lack of effective treatments, make the pancreatic cancer become one of the most difficult cancers to treat. In recent years, the clinicians and cancer scientists have made some significant advances into the management of the disease, especially discovery and development of targeted therapeutics 
Many oncogenic molecular pathways including EGF/EGFR, Ras-Raf-MEK, PI3K/Akt, JAK/STAT, p16INK4A/retinoblastoma, Smad4/TGF-β, and hedgehog signaling pathways, have been reported to be involved in the pathogenesis of pancreatic cancer 
. Among them, STAT3 is thought by many researchers as a suitable therapeutic target for drug discovery because constitutive activation of STAT3 alone is sufficient to induce the relevant disease, the inhibition of STAT3 signaling could suppress and reverse the development of relevant disease, and the molecular mechanism of tumorigenesis caused by STAT3 pathway has been well defined.
Chemoprevention was first defined by Sporn in 1976 and refers to the use of natural or synthetic agents to reverse, suppress or prevent carcinogenic progression 
. It has been proved as a rationale and promising strategy by several recent epidemiological studies in preventing cancer in high-risk populations. Because natural compounds are generally cheaper and safer than synthetic ones, there is growing interest in the possible therapeutic potential of natural products against cancer. Many epidemiological, preclinical, and clinical studies have demonstrated the cancer-preventive effects of green tea 
. The chemotherapeutic and chemopreventive effects of green tea have been attributed to the polyphenol components, especially EGCG, which is the most abundant polyphenol in green tea and accounts for more than 40% of the total polyphenol mixture 
. In the recent few decades, it was under intensive investigation by using animal models of carcinogenesis and cultured tumor cell lines 
. EGCG has demonstrated remarkable chemopreventive and chemotherapeutic potential against various types of cancers, e.g. skin, lung, breast, colon, prostate, stomach, and pancreas, by modulating the intracellular signaling network 
In the present study, we provide strong evidence that EGCG can inhibit cell viability and induce apoptosis of pancreatic cancer cells. First, we found that the expression and activation of STAT3 were inhibited by EGCG, while the induction of caspase-3 activity and PARP cleavage were enhanced. Moreover, this compound also inhibited the invasion and migration of pancreatic cancer cells, which has been reported to be implicated with STAT3 
. These results demonstrate that EGCG has a marked anti-cancer effect on pancreatic cancer at least in part by the inhibition of STAT3 signaling pathway. Second, we found that the STAT3 shRNA can alone reduce cell motility and viability of cancer cells. Furthermore, STAT3 shRNA can enhance the inhibitory effects of EGCG on cell motility and viability in pancreatic cancer cells, suggesting that EGCG can influence some other gene/pathway besides STAT3.
We also found that EGCG could suppress the expression of STAT3-downstream genes, which include the angiogenic protein VEGF, cell proliferative Cyclin D1, oncogenic transcription factor c-Myc, and several anti-apoptotic proteins, including survivin and Bcl-XL
. Some genes are prominent targets for both NF-κB and STAT3, such as Cyclin D1, Bcl-XL
and c-Myc, while survivin is STAT3-dependant. VEGF is also controlled by STAT3 and might be indirectly regulated by NF-κB via HIF-1α 
. The EGCG-medicated inhibition of Cyclin D1, VEGF, and BclXL
transcription is consistent with previous reports 
, which might result from the suppression of EGCG against both the NF-κB and STAT3 pathways 
. Similarly, the EGCG inhibition on Wnt signaling and PI3K/Akt could also contribute to the down-regulation of cMyc and survivin, respectively 
Pancreatic cancer is poorly treated by conventional chemotherapies including gemcitabine due to the profound chemoresistance through widely expressed HMGA1 
. CP690550 (Tasocitinib), an orally active immunosuppressant, is being developed by Pfizer for the treatment of inflammatory bowel disease, dry eyes, rheumatoid arthritis, ankylosing spondylitis, psoriasis, psoriatic arthritis, and for the prevention of transplant rejection 
. CP690550 specifically inhibits JAK3, which has a pivotal role in cytokine signal transduction that governs lymphocyte survival, proliferation, differentiation, and apoptosis. Recent studies have demonstrated the anticancer activity of CP690550 in various cancers 
. In this study, we found that gemcitabine, CP690550 and EGCG alone inhibited cell viability, induced apoptosis and attenuated STAT3-regulated gene transcription in AsPC-1 and PANC-1 cells. EGCG further enhanced the effects of gemcitabine or CP690550 on cell viability, apoptosis and on the expression of STAT3-target genes. Our results provide a new application method, in which the use of EGCG can enhance the therapeutic effects of anticancer drugs while possibly reducing their side effects.
In conclusion, our findings provide unprecedented insights into the STAT3 signaling pathway by which EGCG inhibits viability, invasion and migration, and induces apoptosis in pancreatic cancer cells. Inhibition of STAT3 by shRNA could suppress viability of cancer cells, and down-regulate the STAT3-target genes. Most importantly, EGCG further enhanced the therapeutic potential of gemcitabine and CP690550 against pancreatic cancer.