The development and progression of human cancers are continuously and dynamically regulated by intrinsic and extrinsic factors. As a converging point of multiple oncogenic pathways, signal transducer and activator of transcription 3 (STAT3) is constitutively activated both in tumor cells and tumor-infiltrated immune cells. Activated STAT3 persistently triggers tumor progression through direct regulation of oncogenic gene expression. Apart from its oncogenic role in regulating gene expression in tumor cells, STAT3 also paves the way for human cancer growth through immunosuppression. Activated STAT3 in immune cells results in inhibition of immune mediators and promotion of immunosuppressive factors. Therefore, STAT3 modulates the interaction between tumor cells and host immunity. Accumulating evidence suggests that targeting STAT3 may enhance anti-cancer immune responses and rescue the suppressed immunologic microenvironment in tumors. Taken together, STAT3 has emerged as a promising target in cancer immunotherapy.

Signal Transducers and Activators of Transcription (STATs) are a family of transcription factors that regulate cell proliferation, differentiation, apoptosis, immune and inflammatory responses, and angiogenesis. Cumulative evidence has established that STAT3 has a critical role in the development of multiple cancer types. Because it is constitutively activated during disease progression and metastasis in a variety of cancers, STAT3 has promise as a drug target for cancer therapeutics. Recently, STAT3 was found to have an important role in maintaining cancer stem cells in vitro and in mouse tumor models, suggesting STAT3 is integrally involved in tumor initiation, progression and maintenance. STAT3 has been traditionally considered as nontargetable or undruggable, and the lag in developing effective STAT3 inhibitors contributes to the current lack of FDA-approved STAT3 inhibitors. Recent advances in cancer biology and drug discovery efforts have shed light on targeting STAT3 globally and/or specifically for cancer therapy. In this review, we summarize current literature and discuss the potential importance of STAT3 as a novel target for cancer prevention and of STAT3 inhibitors as effective chemopreventive agents.

Angiogenesis has an important role in tumour cell growth and metastasis. Anisomycin has been shown to inhibit tumour cell growth. However, whether anisomycin can inhibit angiogenesis of tumours has not been reported. The present study demonstrated that there was a positive correlation between tumour angiogenesis and the number of CD44 + /CD133 + serous human ovarian cancer stem cells (HuOCSCs). Subsequently, it was confirmed that anisomycin significantly inhibited the proliferation, invasion, tumorigenic ability and tumour angiogenesis of HuOCSCs. Gene expression profiling by cDNA microarrays revealed that the expression levels of vascular endothelial cell markers, platelet-derived growth factors, Notch pathway components and 27 tumour angiogenesis-related genes were significantly decreased in the anisomycin-treated group compared with the control group. Further experiments demonstrated that the expression levels of endogenous long non-coding RNA (lncRNA) maternally expressed 3 (Meg3) were significantly decreased in anisomycin-treated HuOCSCs, whereas the expression levels of microRNA (miR)-421 were significantly increased. The results of luciferase reporter assays indicated that, when miR-421 was overexpressed in cells, the luciferase activities of wild-type platelet derived growth factor receptor α (PDGFRA) 3' untranslated region and Meg3 reporter plasmids were significantly decreased. Overexpression of miR-421 in HuOCSCs significantly enhanced the anisomycin-mediated inhibition of HuOCSC proliferation. Taken together, the present results demonstrated that anisomycin inhibited the activation downstream of the Notch1 pathway by attenuating the molecular sponge effect of the lncRNA-Meg3/miR-421/PDGFRA axis, ultimately inhibiting angiogenesis, proliferation and invasion in ovarian cancer cells.

Hesperidin (HD) is a common flavanone glycoside isolated from citrus fruits and possesses great potential for cardiovascular protection. Hesperetin (HT) is an aglycone metabolite of HD with high bioavailability. Through the docking simulation, HD and HT have shown their potential to bind to two cellular proteins: transmembrane serine protease 2 (TMPRSS2) and angiotensin-converting enzyme 2 (ACE2), which are required for the cellular entry of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). Our results further found that HT and HD suppressed the infection of VeroE6 cells using lentiviral-based pseudo-particles with wild types and variants of SARS-CoV-2 with spike (S) proteins, by blocking the interaction between the S protein and cellular receptor ACE2 and reducing ACE2 and TMPRSS2 expression. In summary, hesperidin is a potential TMPRSS2 inhibitor for the reduction of the SARS-CoV-2 infection.