Critical analysis of the potential for targeting STAT3 in human malignancy

The STAT3 inhibitor galiellalactone inhibits the generation of MDSC-like monocytes by prostate cancer cells and decreases immunosuppressive and tumorigenic factors

BACKGROUND

The transcription factor signal transducer and activator of transcription 3 (STAT3) is implicated in cancer drug resistance, metastasis, and immunosuppression and has been identified as a promising therapeutic target for new anticancer drugs. Myeloid-derived suppressor cells (MDSCs) play a major role in the suppression of antitumor immunity and STAT3 is involved in the accumulation, generation, and function of MDSCs. Thus, targeting STAT3 holds the potential of reversing immunosuppression in cancer. This study aims to investigate the effect of the small molecule STAT3 inhibitor galiellalactone on prostate cancer cell- induced generation of MDSCs from monocytes and the effect on immunosuppressive factors and inflammatory cytokines.

METHODS

Primary human monocytes were cocultured with prostate cancer cells (DU145, PC3, and LNCaP-IL6) or with conditioned medium (CM) from prostate cancer cells in the presence or absence of the STAT3 inhibitor galiellalactone. Monocytes were analyzed by flow cytometry for an MDSC-like phenotype (CD14⁺ HLA-DR^-/lo ). The secretion and gene expression of immunosuppressive factors and inflammatory cytokines from prostate cancer cells and monocytes were investigated.

RESULTS

Galiellalactone blocked the prostate cancer cell-induced generation of MDSC-like monocytes with an immunosuppressive phenotype ex vivo. Monocytes cultured with CM from prostate cancer cells showed increased expression of phosphorylated STAT3. Prostate cancer cells increased the expression of interleukin1β (IL1β), IL10, and IL6 in monocytes which was inhibited by galiellalactone. In addition, galiellalactone decreased indoleamine 2,3-dioxygenase gene expression in monocytes. Galiellalactone reduced the levels of IL8 and granulocyte macrophage-colony stimulating factor in prostate cancer cells per se.

CONCLUSION

The STAT3 inhibitor galiellalactone may prevent the prostate cancer cell-induced generation of MDSCs and reverse the immunosuppressive mechanisms caused by the interplay between prostate cancer cells and MDSCs. This is a potential new immunotherapeutic approach for the treatment of prostate cancer.

Adipose-derived mesenchymal stem cells promote osteosarcoma proliferation and metastasis by activating the STAT3 pathway

Osteosarcoma is the most common primary bone malignancy in children and young adults, but the role of adipose-derived mesenchymal stem cells (ADSCs) in the rapid progression of osteosarcoma is still unclear. Here, we found that ADSCs promoted tumour growth and invasion by increasing matrix metalloproteinase 2/9 (MMP2/9) expression in tumour cells. The persistent activation of signal transducer and activator of transcription 3 (STAT3) has been shown to directly promote tumour growth by mediating a wide spectrum of cellular responses, and STAT3 activation was detected in osteosarcoma cells co-cultured with ADSCs or treated with ADSC-conditioned medium. Furthermore, siRNA-mediated STAT3 inhibition in osteosarcoma cells decreased cell proliferation and invasion and down-regulated MMP2/9 expression. In addition, a nude mouse model of osteosarcoma was established by injecting luciferase-labelled MG63 cells into the tibia. As shown in in vivo bioluminescence images, ADSCs promoted tumour cell proliferation, invasion progression and metastasis. STAT3 inhibition attenuated tumour growth and metastasis and prolonged the survival of these mice. After the siRNA treatment, the MMP2, MMP9 and Ki67 levels decreased. Based on these data, stromal ADSCs promote osteosarcoma progression by increasing STAT3 signalling-mediated MMP2/9 expression.

Novel Molecular Targets for Chemoprevention in Malignancies of the Head and Neck

Cancers of the head and neck region are among the leading causes of cancer-related mortalities worldwide. Oral leukoplakia and erythroplakia are identified as precursor lesions to malignancy. Patients cured of an initial primary head and neck cancer are also susceptible to developing second primary tumors due to cancerization of their mucosal field. Multi-step acquisition of genetic mutations leading to tumorigenesis and development of invasive cancer has been previously described. Recently, whole exome sequencing of tumor specimens has helped to identify driver mutations in this disease. For these reasons, chemoprevention or the use of systemic or biologic agents to prevent carcinogenesis is an attractive concept in head and neck cancers. Nonetheless, despite extensive clinical research in this field over the past couple decades, no standard of care option has emerged. This review article reports on targeted interventions that have been attempted in clinical trials to date, and focuses on novel molecular pathways and drugs in development that are worthy of being tested for this indication as part of future endeavors.

Theaflavins suppress tumor growth and metastasis via the blockage of the STAT3 pathway in hepatocellular carcinoma

Theaflavins, the major black tea polyphenols, have been reported to exhibit promising antitumor activities in several human cancers. However, the role of theaflavins in hepatocellular carcinoma (HCC) is still unknown. In this study, we found that theaflavins could significantly inhibit proliferation, migration, and invasion, and induce apoptosis in HCC cells in vitro. Furthermore, we found that theaflavins inhibited the growth and metastasis of HCC in an orthotopic model and a lung metastasis model. Immunohistochemical analyses and terminal deoxynucleotidyl transferase dUTP nick end-labeling assays showed that theaflavins could suppress proliferation and induce apoptosis in vivo. Theaflavins also suppressed constitutive and inducible signal transducer and activator of transcription 3 (STAT3) phosphorylation. The downstream proteins regulated by STAT3, including the antiapoptotic proteins (Bcl-2 and Survivin) and the invasion-related proteins (MMP-2, MMP-9), were also downregulated after theaflavins treatment. Theaflavins induced apoptosis by activating the caspase pathway. Together, our results suggest that theaflavins suppress the growth and metastasis of human HCC through the blockage of the STAT3 pathway, and thus may act as potential therapeutic agents for HCC.

Optimization of pyrazole-containing 1,2,4-triazolo-[3,4-b]thiadiazines, a new class of STAT3 pathway inhibitors

Structure-activity relationship studies of a 1,2,4-triazolo-[3,4-b]thiadiazine scaffold, identified in an HTS campaign for selective STAT3 pathway inhibitors, determined that a pyrazole group and specific aryl substitution on the thiadiazine were necessary for activity. Improvements in potency and metabolic stability were accomplished by the introduction of an α-methyl group on the thiadiazine. Optimized compounds exhibited anti-proliferative activity, reduction of phosphorylated STAT3 levels and effects on STAT3 target genes. These compounds represent a starting point for further drug discovery efforts targeting the STAT3 pathway.

STAT3 as a Chemoprevention Target in Carcinogen-Induced Head and Neck Squamous Cell Carcinoma

Head and neck squamous cell carcinoma (HNSCC) is a frequently fatal disease due, in large part, to a high rate of second primary tumor (SPT) formation. The 4-nitroquinoline 1-oxide (4-NQO) mouse model of oral carcinogenesis provides a robust system in which to study chemopreventive agents in the context of chemically induced HNSCC tumors. STAT3 is a potent oncogene that is hyperactivated by tyrosine phosphorylation early in HNSCC carcinogenesis and is a rational therapeutic target. We recently reported that loss-of-function of the STAT3 phosphatase PTPRT promotes STAT3 activation in HNSCC tumors and preclinical models and may serve as a predictive biomarker of response to STAT3 inhibitors, including the small-molecule Stattic. We therefore investigated the hypothesis that Ptprt-knockout (KO) mice would be more susceptible to 4-NQO-induced oral carcinogenesis and more sensitive to Stattic-mediated chemoprevention compared with wild-type (WT) mice. Herein, we demonstrate that Ptprt WT and KO mice develop similar spectra of HNSCC disease severity upon 12 weeks of 4-NQO administration, with no apparent effect of Ptprt genotype on carcinogenesis or treatment outcome. Targeting of STAT3 with Stattic resulted in a chemopreventive effect against 4-NQO-induced oral cancer (P = 0.0402). While these results do not support a central role for PTPRT in 4-NQO-induced HNSCC carcinogenesis, further investigation of STAT3 as a chemoprevention target in this cancer is warranted. Cancer Prev Res; 9(8); 657-63. ©2016 AACR.

Preclinical Characterization of 3β-(N-Acetyl l-cysteine methyl ester)-2aβ,3-dihydrogaliellalactone (GPA512), a Prodrug of a Direct STAT3 Inhibitor for the Treatment of Prostate Cancer

The transcription factor STAT3 is a potential target for the treatment of castration-resistant prostate cancer. Galiellalactone (1), a direct inhibitor of STAT3, prevents the transcription of STAT3 regulated genes. In this study we characterized 6 (GPA512, Johansson , M. ; Sterner , O. Patent WO 2015/132396 A1, 2015 ), a prodrug of 1. In vitro studies showed that 6 is rapidly converted to 1 in plasma and is stable in a buffer solution. The pharmacokinetics of 6 following a single oral dose indicated that the prodrug was rapidly absorbed and converted to 1 with a tmax of 15 min. Oral administration of 6 in mice increased the plasma exposure of the active parent compound 20-fold compared to when 1 was dosed orally. 6 treated mice bearing DU145 xenograft tumors had significantly reduced tumor growth compared to untreated mice. The favorable druglike properties and safety profile of 6 warrant further studies of 6 for the treatment of castration-resistant prostate cancer.

Overexpression of STAT3/pSTAT3 was associated with poor prognosis in gastric cancer: a meta-analysis

Signal transducer and activator of transcription 3 (STAT3) and phospho-STAT3 (pSTAT3) play important roles in the development of gastric cancer. STAT3 is often associated with cell survival, proliferation, and transformation. The prognostic value of STAT3/pSTAT3 in patients with gastric cancer remains controversial in numerous published studies. The aim of this study was to summarize recent findings relevant to the prognostic role of STAT3 and pSTAT3 in patients with gastric cancer. A meta-analysis was performed by searching Web of Knowledge, EMBASE, and PubMed to identify studies on the prognostic impact of STAT3/pSTAT3 in gastric cancers in August 2014. In all, 10 studies were included in the analysis. Data were collected for comparing survival rates in patients with high STAT3 levels compared to those with low levels. Pooled hazard ratios (HRs) and 95% confidence intervals (CIs) were calculated. Sensitivity analysis was conducted, and publication bias was evaluated. Eventually, 1667 cases of gastric cancer were subjected to the final analysis. Among patients with gastric cancer, poor survival was predicted by higher expressions of STAT3 (HR=2.30; 95% CI=1.13-4.68; P=0.02) and pSTAT3 (HR=1.75; 95% CI=1.17-2.61; P=0.006). Moreover, overexpression of STAT3 was associated with poor tumor stage. Additionally, our analysis did not show any statistically significant effect of publication bias regarding STAT3 or pSTAT3. The results of this meta-analysis demonstrated that overexpression of STAT3 and pSTAT3 was associated with poor prognosis in gastric cancer.

Frequent promoter hypermethylation of PTPRT increases STAT3 activation and sensitivity to STAT3 inhibition in head and neck cancer

STAT3 overactivation is a common event in many cancers, including head and neck squamous cell carcinoma (HNSCC), where STAT3 represents a promising therapeutic target. HNSCC is not characterized by frequent kinase mutations, in contrast to some malignancies where mutational activation of kinases upstream of STAT3 is common. Instead, STAT3 may be activated by loss-of-function (LOF) of negative regulators of STAT3, including by promoter hypermethylation of PTPRT. Here we first analyzed The Cancer Genome Atlas (TCGA) data and determined that the PTPRT promoter is frequently hypermethylated in several cancers, including HNSCC (60.1% of tumors analyzed) in association with downregulation of PTPRT mRNA expression and upregulation of pSTAT3 expression. These findings were confirmed in an independent cohort of HNSCC tumors by methylation-specific PCR (MSP) and immunohistochemistry. We demonstrate that PTPRT promoter methylation and gene silencing is reversible in HNSCC cells, leading to PTPRT-specific downregulation of pSTAT3 expression. We further show that PTPRT promoter methylation is significantly associated with sensitivity to STAT3 inhibition in HNSCC cells, suggesting that PTPRT promoter methylation may serve as a predictive biomarker for responsiveness to STAT3 inhibitors in clinical development.

2-Guanidinoquinazolines as new inhibitors of the STAT3 pathway

Synthesis and SAR investigation of 2-guanidinoquinazolines, initially identified in a high content screen for selective STAT3 pathway inhibitors, led to a more potent analog (11c) that demonstrated improved anti-proliferative activity against a panel of HNSCC cell lines.

Differential roles of STAT3 in the initiation and growth of lung cancer

Signal transducer and activator of transcription 3 (STAT3) is linked to multiple cancers, including pulmonary adenocarcinoma. However, the role of STAT3 in lung cancer pathogenesis has not been determined. Using lung epithelial-specific inducible knockout strategies, we demonstrate that STAT3 has contrasting roles in the initiation and growth of both chemically and genetically induced lung cancers. Selective deletion of lung epithelial STAT3 in mice before cancer induction by the smoke carcinogen, urethane, resulted in increased lung tissue damage and inflammation, K-Ras oncogenic mutations and tumorigenesis. Deletion of lung epithelial STAT3 after establishment of lung cancer inhibited cancer cell proliferation. Simultaneous deletion of STAT3 and expression of oncogenic K-Ras in mouse lung elevated pulmonary injury, inflammation and tumorigenesis, but reduced tumor growth. These studies indicate that STAT3 prevents lung cancer initiation by maintaining pulmonary homeostasis under oncogenic stress, whereas it facilitates lung cancer progression by promoting cancer cell growth. These studies also provide a mechanistic basis for targeting STAT3 to lung cancer therapy.

Galiellalactone is a direct inhibitor of the transcription factor STAT3 in prostate cancer cells

The transcription factor STAT3 is constitutively active in several malignancies including castration-resistant prostate cancer and has been identified as a promising therapeutic target. The fungal metabolite galiellalactone, a STAT3 signaling inhibitor, inhibits the growth, both in vitro and in vivo, of prostate cancer cells expressing active STAT3 and induces apoptosis of prostate cancer stem cell-like cells expressing phosphorylated STAT3 (pSTAT3). However, the molecular mechanism of this STAT3-inhibiting effect by galiellalactone has not been clarified. A biotinylated analogue of galiellalactone (GL-biot) was synthesized to be used for identification of galiellalactone target proteins. By adding streptavidin-Sepharose beads to GL-biot-treated DU145 cell lysates, STAT3 was isolated and identified as a target protein. Confocal microscopy revealed GL-biot in both the cytoplasm and the nucleus of DU145 cells treated with GL-biot, appearing to co-localize with STAT3 in the nucleus. Galiellalactone inhibited STAT3 binding to DNA in DU145 cell lysates without affecting phosphorylation status of STAT3. Mass spectrometry analysis of recombinant STAT3 protein pretreated with galiellalactone revealed three modified cysteines (Cys-367, Cys-468, and Cys-542). Here we demonstrate with chemical and molecular pharmacological methods that galiellalactone is a cysteine reactive inhibitor that covalently binds to one or more cysteines in STAT3 and that this leads to inhibition of STAT3 binding to DNA and thus blocks STAT3 signaling without affecting phosphorylation. This further validates galiellalactone as a promising direct STAT3 inhibitor for treatment of castration-resistant prostate cancer.

EGCG inhibits the growth and tumorigenicity of nasopharyngeal tumor-initiating cells through attenuation of STAT3 activation

A subset of cancer cells, termed cancer stem cells (CSCs) or tumor-initiating cells (TICs) could initiate tumors and are responsible for tumor recurrence and chemotherapeutic resistance. In this study, we enriched TICs in nasopharyngeal carcinoma (NPC) by the spheres formation and characterized the stem-like signatures such as self-renewal, proliferation, chemoresistance and tumorigenicity. By this method, we investigated that epigallocathechin gallate (EGCG), the major polyphenol in green tea could target TICs and potently inhibit sphere formation, eliminate the stem-like properties and enhance chemosensitivity in NPC through attenuation of STAT3 activation, which could be important in regulating the stemness expression in NPC. Our results demonstrated that STAT3 pathway plays an important role in mediating tumor-initiating capacities in NPC and suggest that inactivation of STAT3 with EGCG may represent a potential preventive and therapeutic approach for NPC.