Signal transducer and activator of transcription 3 (STAT3) is a cytoplasmic transcription factor that regulates cell proliferation, differentiation, apoptosis, angiogenesis, inflammation and immune responses. Aberrant STAT3 activation triggers tumor progression through oncogenic gene expression in numerous human cancers, leading to promote tumor malignancy. On the contrary, STAT3 activation in immune cells cause elevation of immunosuppressive factors. Accumulating evidence suggests that the tumor microenvironment closely interacts with the STAT3 signaling pathway. So, targeting STAT3 may improve tumor progression, and anti-cancer immune response. In this review, we summarized the role of STAT3 in cancer and the tumor microenvironment, and present inhibitors of STAT3 signaling cascades. [BMB Reports 2019; 52(7): 415-423].

Enhanced expression of the cancer stem cell (CSC) marker, CD133, is closely associated with a higher rate of tumor formation and poor prognosis in hepatocellular carcinoma (HCC) patients. Despite its clinical significance, the molecular mechanism underlying the deregulation of CD133 during tumor progression remains to be clarified. Here, we report on a novel mechanism by which interleukin‐6/signal transducer and activator of transcription 3 (IL‐6/STAT3) signaling up‐regulates expression of CD133 and promotes HCC progression. STAT3 activated by IL‐6 rapidly bound to CD133 promoter and increased protein levels of CD133 in HCC cells. Reversely, in hypoxic conditions, RNA interference silencing of STAT3 resulted in decrease of CD133 levels, even in the presence of IL‐6, with a concomitant decrease of hypoxia‐inducible factor 1 alpha (HIF‐1α) expression. Active STAT3 interacted with nuclear factor kappa B (NF‐κB) p65 subunit to positively regulate the transcription of HIF‐1α providing a mechanistic explanation on how those three oncogenes work together to increase the activity of CD133 in a hypoxic liver microenvironment. Activation of STAT3 and its consequent induction of HIF‐1α and CD133 expression were not observed in Toll‐like receptor 4/IL‐6 double‐knockout mice. Long‐term silencing of CD133 by a lentiviral‐based approach inhibited cancer cell‐cycle progression and suppressed in vivo tumorigenicity by down‐regulating expression of cytokinesis‐related genes, such as TACC1, ACF7, and CKAP5. We also found that sorafenib and STAT3 inhibitor nifuroxazide inhibit HCC xenograft formation by blocking activation of STAT3 and expression of CD133 and HIF‐1α proteins. Conclusion: IL‐6/STAT3 signaling induces expression of CD133 through functional cooperation with NF‐κB and HIF‐1α during liver carcinogenesis. Targeting STAT3‐mediated CD133 up‐regulation may represent a novel, effective treatment by eradicating the liver tumor microenvironment. (Hepatology 2015;62:1160‐1173)

Tumor angiogenesis is required for tumor development and is stimulated by angiogenic inducers like VEGF (vascular endothelial growth factor). Our previous study demonstrated that STAT3 (signal transducer and activator of transcription 3) up-regulates HIF-1α (hypoxia inducible factor-1α) protein stability and enhances HIF-1-mediated VEGF expression in hypoxic solid tumor cells, thus suggesting that the inhibition of STAT3 signaling may have clinical applications. In this study, we examined in vitro and in vivo, whether caffeic acid (CA) or its derivative CADPE [3-(3,4-dihydroxy-phenyl)-acrylic acid 2-(3,4-dihydroxy-phenyl)-ethyl ester] exert anticancer activity by targeting STAT3. It was found that CA or CADPE significantly inhibit STAT3 activity, and that this in turn down-regulates HIF-1α activity. Consequently, sequential blockade of STAT3 and HIF-1α resulted in the down-regulation of VEGF by inhibiting their recruitment to the VEGF promoter. In mice bearing a Caki-I carcinoma, both CA and CADPE retarded tumor growth and suppressed STAT3 phosphorylation, HIF-1α expression, vascularization and STAT3-inducible VEGF gene expression in tumors. Taken together, our results demonstrate that CA and CADPE are potential inhibitors of STAT3 and that they suppress tumor angiogenesis by inhibiting the activity of STAT3, the expression of HIF-1α and VEGF.

Hypoxia-inducible factor 1α (HIF-1α) is rapidly degraded by the ubiquitin-proteasome pathway under normoxic conditions. Ubiquitination of HIF-1α is mediated by interaction with von Hippel-Lindau tumor suppressor protein (pVHL). In our previous report, we found that hypoxia-induced active signal transducer and activator of transcription3 (STAT3) accelerated the accumulation of HIF-1α protein and prolonged its half-life in solid tumor cells. However, its specific mechanisms are not fully understood. Thus, we examined the role of STAT3 in the mechanism of pVHL-mediated HIF-1α stability. We found that STAT3 interacts with C-terminal domain of HIF-1α and stabilizes HIF-1α by inhibition of pVHL binding to HIF-1α. The binding between HIF-1α and pVHL, negative regulator of HIF-1α stability, was interfered dose-dependently by overexpressed constitutive active STAT3. Moreover, we found that the enhanced HIF-1α protein levels by active STAT3 are due to decrease of poly-ubiquitination of HIF-1α protein via inhibition of interaction between pVHL and HIF-1α. Taken together, our results suggest that STAT3 decreases the pVHL-mediated ubiquitination of HIF-1α through competition with pVHL for binding to HIF-1α, and then stabilizes HIF-1α protein levels.

Background: Signal transducer and activator of transcription 3 (STAT3), a multifaceted transcription factor, modulates host immune responses by activating cellular response to signaling ligands. STAT3 has a pivotal role in the pathophysiology of kidney injury by counterbalancing resident macrophage phenotypes under inflammation conditions. However, STAT3’s role in acute kidney injury (AKI), particularly in macrophage migration, and in chronic kidney disease (CKD) through fibrosis development, remains unclear. Methods: Stattic (a JAK2/STAT3 inhibitor, 5 mg/kg or 10 mg/kg) was administered to evaluate the therapeutic effect on LPS-induced AKI (L-AKI) and LPS-induced CKD (L-CKD), with animals sacrificed 6–24 h and 14 days post-LPS induction, respectively. The immune mechanisms of STAT3 blockade were determined by comparing the macrophage phenotypes and correlated with renal function parameters. Also, the transcriptomic analysis was used to confirm the anti-inflammatory effect of L-AKI, and the anti-fibrotic role was further evaluated in the L-CKD model. Results: In the L-AKI model, sequential increases in BUN and blood creatinine levels were time-dependent, with a marked elevation of 0–6 h after LPS injection. Notably, two newly identified macrophage subpopulations (CD11b^highF4/80^low and CD11b^lowF4/80^high), exhibited population changes, with an increase in the CD11b^highF4/80^low population and a decrease in the CD11b^lowF4/80^high macrophages. Corresponding to the FACS results, the tubular injury score, NGAL, F4/80, and p-STAT3 expression in the tubular regions were elevated. STAT3 inhibitor injection in L-AKI and L-CKD mice reduced renal injury and fibrosis. M2-type subpopulation with CD206 in CD11b^lowF4/80^high population increased in the Stattic-treated group compared with that in the LPS-alone group in the L-AKI model. Additionally, STAT3 inhibitor reduced inflammation driven by LPS-stimulated macrophages and epithelial cells injury in the co-culture system. Transcriptomic profiling identified 3 common genes in the JAK-STAT, TLR, and TNF signaling pathways and 11 common genes in the LPS with macrophage response. The PI3K-AKT (IL-6, Akt3, and Pik3r1) and JAK-STAT pathways were determined as potential Stattic targets. Further confirmation through mRNA and protein expressions analyses showed that Stattic treatment reduced inflammation in the L-AKI and fibrosis in the L-CKD mice. Conclusions: STAT3 blockade effectively mitigated inflammation by retrieving the CD11b^lowF4/80^high population, further emphasizing the role of STAT3-associated macrophage-driven inflammation in kidney injury.