The role of constitutively activated STAT3 in B16 melanoma cells

A Novel Systemic siDR6 Delivery System Based on DP7-C for the Treatment of Metastatic Lung Cancer

Background

The treatment of metastatic lung cancer, a common complication of many primary cancers, has historically been a significant clinical challenge. Once lung metastasis occurs, patients' survival is often significantly shortened. Therefore, prevention and treatment of lung metastases is an important aspect of cancer treatment. In this study, a simple, low-toxicity, cholesterol-modified cationic cell-penetrating peptide DP7 (DP7-C), in combination with siDR6 was used for intravenous administration for the treatment of lung metastases.

Methods

Initially, clinical databases were analyzed to determine the expression levels of death receptor 6 (DR6) in metastatic tumors and the correlation between DR6 expression and patient survival times. The DP7-C/siDR6 micelles were prepared by a self-assembly method. By cultivating 293T, B16F10 and LL2 cells, the in vitro experiments were performed to assess the transfection efficiency, safety and anti-cancer ability of DP7-C/siDR6, while its targeting efficiency and prevention of lungs were investigated by mouse experiments. Furthermore, the therapeutic efficacy of DP7-C/siDR6 was demonstrated in the LL2 model of lung cancer in situ, the B16F10 model of artificial lung metastasis, and the 4T1 model of spontaneous lung metastasis.

Results

The clinical data analysis revealed that DR6 was highly expressed in the majority of metastatic tumors and that patients with high DR6 expression exhibited significantly shorter survival times. The DP7-C/siDR6 showed high transfection efficiency, and it could inhibit tumor cell growth by suppressing the STAT3 signaling pathway. Subsequent mouse experiments demonstrated that intravenous administration of DP7-C/siDR6 resulted in efficient lung targeting. The inhibition of DR6 expression on lung endothelial cells was found to prevent metastasis-induced primary necrosis of lung endothelial cells, thereby preventing tumor metastasis. And the DP7-C/siDR6 treatment showed excellent therapeutic efficacy in the tumor models.

Conclusion

The systemic delivery of DP7-C micelles carrying siDR6 provide an alternative therapeutic strategy to halt cancer lung metastasis.

Rolling Circle Amplification-Based Self-Assembly to Form a "GPS-Nanoconveyor" for In Vitro Targeted Imaging and Enhanced Gene/Chemo (CRISPR/DOX) Synergistic Therapy

The GPS-Nanoconveyor (MA-NV@DOX-Cas13a) is a targeted nanoplatform designed for the imaging and gene/chemotherapy synergistic treatment of melanoma. It utilizes rolling circle amplification (RCA) products as a scaffold to construct a DNA "Nanoconveyor" (NV), which incorporates a multivalent aptamer (MA) as a "GPS", encapsulates doxorubicin (DOX) in the transporter, and equips it with CRISPR/Cas13a ribonucleoproteins (Cas13a RNP). Carrying MA enhances the ability to recognize the overexpressed receptor nucleolin on B16 cells, enabling targeted imaging and precise delivery of MA-NV@DOX-Cas13a through receptor-mediated endocytosis. The activation of signal transducer and activator of transcription 3 (STAT3) in cancer cells triggers cis-cleavage of CRISPR/Cas13a, initiating its trans-cleavage function. Additionally, deoxyribonuclease I (DNase I) degrades MA-NV, releasing DOX for intracellular imaging and as a chemotherapeutic agent. Experiments demonstrate the superior capabilities of this versatile nanoplatform for cellular imaging and co-treatment while highlighting the advantages of these nanodrug delivery systems in mitigating DOX side effects.

Next-generation bromodomain inhibitors of the SWI/SNF complex enhance DNA damage and cell death in glioblastoma

Glioblastoma (GBM) is an aggressive brain cancer with a poor prognosis. While surgical resection is the primary treatment, adjuvant temozolomide (TMZ) chemotherapy and radiotherapy only provide slight improvement in disease course and outcome. Unfortunately, most treated patients experience recurrence of highly aggressive, therapy-resistant tumours and eventually succumb to the disease. To increase chemosensitivity and overcome therapy resistance, we have modified the chemical structure of the PFI-3 bromodomain inhibitor of the BRG1 and BRM catalytic subunits of the SWI/SNF chromatin remodelling complex. Our modifications resulted in compounds that sensitized GBM to the DNA alkylating agent TMZ and the radiomimetic bleomycin. We screened these chemical analogues using a cell death ELISA with GBM cell lines and a cellular thermal shift assay using epitope tagged BRG1 or BRM bromodomains expressed in GBM cells. An active analogue, IV-129, was then identified and further modified, resulting in new generation of bromodomain inhibitors with distinct properties. IV-255 and IV-275 had higher bioactivity than IV-129, with IV-255 selectively binding to the bromodomain of BRG1 and not BRM, while IV-275 bound well to both BRG1 and BRM bromodomains. In contrast, IV-191 did not bind to either bromodomain or alter GBM chemosensitivity. Importantly, both IV-255 and IV-275 markedly increased the extent of DNA damage induced by TMZ and bleomycin as determined by nuclear γH2AX staining. Our results demonstrate that these next-generation inhibitors selectively bind to the bromodomains of catalytic subunits of the SWI/SNF complex and sensitize GBM to the anticancer effects of TMZ and bleomycin. This approach holds promise for improving the treatment of GBM.

Interferon-gamma induces melanogenesis via post-translational regulation of tyrosinase

Melanogenesis (melanin pigment production) in melanocytes is canonically stimulated by the alpha melanocyte stimulating hormone (αMSH), which activates the cyclic-AMP-mediated expression of the melanocyte inducing transcription factor (MITF) and its downstream melanogenic genes, including the principal rate-limiting melanogenic enzyme tyrosinase (TYR). Here, we report that interferon-gamma (IFNG; type II interferon), but not interferon-alpha (a type I interferon), induces a noncanonical melanogenic pathway in mouse and human melanocytic cells. Inhibition of IFNG pathway by the JAK1/2 inhibitor ruxolitinib or knocking out Stat1 gene abrogated the IFNG-induced melanogenesis. Interestingly, IFNG-induced melanogenesis was independent of MITF. IFNG markedly increased the TYR protein expression but did not affect the mRNA expression, suggesting a post-translational regulatory mechanism. In contrast, IFNG had no effect on the expression of other melanogenesis-related proteins, for example, tyrosinase-related protein 1 (TYRP1) and dopachrome tautomerase (DCT). Glycosidase digestion assays revealed that IFNG treatment increased the mature glycosylated form of TYR, but not its de novo synthesis. Moreover, cycloheximide chase assay showed that degradation of TYR was decreased in IFNG-treated cells. These results suggest that the IFNG-STAT1 pathway regulates melanogenesis via regulation of the post-translational processing and protein stability of TYR.

Brahma-Related Gene-1 (BRG1) promotes the malignant phenotype of glioblastoma cells

Glioblastoma multiforme (GBM) is an aggressive malignant brain tumour that is resistant to existing therapeutics. Identifying signalling pathways deregulated in GBM that can be targeted therapeutically is critical to improve the present dismal prognosis for GBM patients. In this report, we have identified that the BRG1 (Brahma‐Related Gene‐1) catalytic subunit of the SWI/SNF chromatin remodelling complex promotes the malignant phenotype of GBM cells. We found that BRG1 is ubiquitously expressed in tumour tissue from GBM patients, and high BRG1 expression levels are localized to specific brain tumour regions. Knockout (KO) of BRG1 by CRISPR‐Cas9 gene editing had minimal effects on GBM cell proliferation, but significantly inhibited GBM cell migration and invasion. BRG1‐KO also sensitized GBM cells to the anti‐proliferative effects of the anti‐cancer agent temozolomide (TMZ), which is used to treat GBM patients in the clinic, and selectively altered STAT3 tyrosine phosphorylation and gene expression. These results demonstrate that BRG‐1 promotes invasion and migration, and decreases chemotherapy sensitivity, indicating that it functions in an oncogenic manner in GBM cells. Taken together, our findings suggest that targeting BRG1 in GBM may have therapeutic benefit in the treatment of this deadly form of brain cancer.

STAT3 inhibitory stattic enhances immunogenic cell death induced by chemotherapy in cancer cells

BACKGROUND

Induction of immunogenic cell death (ICD) is considered a promising strategy for cancer immunotherapy. Stattic is an inhibitor of STAT3, which is found constitutively active in many cancers and plays a major role in cancer progression.

OBJECTIVES

In the present study, we proposed to evaluate whether stattic can enhance the effects of chemotherapy in the induction of ICD in cancer cells harboring hyperactive STAT3.

METHODS

The growth inhibitory effects of stattic and chemo agents including doxorubicin (DOX) and oxaliplatin (OXP) were evaluated using MTT assay in B16F10 and CT26 cell lines. Flow cytometry was applied to study cell apoptosis and calreticulin (CRT) surface exposure. The levels of high mobility group box 1 (HGMB1), heat shock protein70 (HSP70) and interleukin-12 (IL-12) were measured using ELISA.

RESULTS

Treatment of B16F10 and CT26 cells with stattic in combination with DOX resulted in synergistic antitumor effects with combination index being 0.82 and 0.87, respectively. Interestingly, we found a higher level of ICD markers including CRT expression as well as HMGB1 and HSP70 secretion in the cells received combination therapy of stattic and DOX as compared with monotherapies. Moreover, exposure of dendritic cells (DCs) to conditioned media (CM) from cancer cells treated with stattic and/or DOX resulted in secretion of IL-12, which is an indicator of DCs maturation and induction of Th1 response. OXP and stattic monotherapy induced ICD in CT26 cells and stimulated IL-12 secretion by DCs; however, we did not observe a significant increase in the level of ICD in CT26 cells and IL-12 secretion by DCs when CT26 cells were treated with stattic and OXP combination as compared with monotherapy groups.

CONCLUSION

These findings indicate that STAT3 inhibitory stattic can increase ICD induced by DOX. Graphical abstract.

UV-B-activated B16 melanoma cells or HaCaT keratinocytes accelerate signaling pathways associated with melanogenesis via ANGPTL 2 induction, an activity antagonized by Chrysanthemum extract

Sunburn causes inflammation, which increases melanin production in skin and causes hyperpigmentation. Angiopoietin-like protein (ANGPTL) 2 is an inflammatory mediator induced in sun-exposed skin areas. However, whether ANGPTL2 functions in melanin production remains unclear. To assess this possibility, we overexpressed Angptl2 in the melanoma line B16 and in the keratinocyte line HaCaT. Relative to controls, Angptl2-expressing B16 cells produced higher melanin levels via tyrosinase induction. Accordingly, Angptl2-expressing HaCaT cells secreted relatively high levels of both endothelin-1 (ET-1) and α-melanocyte-stimulating hormone (α-MSH). Moreover, treatment with an extract from Chrysanthemum indicum × Erigeron annuus (CE) suppressed ANGPTL2 expression and repressed tyrosinase induction in melanocytes and of α-MSH and ET-1 in keratinocytes. Our data suggest that ANGPTL2 expression in keratinocytes and melanin-producing cells accelerates pigment production and that treatment of skin with a CE extract could prevent melanin accumulation.

The critical role that STAT3 plays in glioma-initiating cells: STAT3 addiction in glioma

Glioma-Initiating Cells (GICs) are thought to be responsible for tumor initiation, progression and recurrence in glioblastoma (GBM). In previous studies, we reported the constitutive phosphorylation of the STAT3 transcription factor in GICs derived from GBM patient-derived xenografts, and that STAT3 played a critical role in GBM tumorigenesis. In this study, we show that CRISPR/Cas9-mediated deletion of STAT3 in an established GBM cell line markedly inhibited tumorigenesis by intracranial injection but had little effect on cell proliferation in vitro. Tumorigenesis was rescued by the enforced expression of wild-type STAT3 in cells lacking STAT3. In contrast, GICs were highly addicted to STAT3 and upon STAT3 deletion GICs were non-viable. Moreover, we found that STAT3 was constitutively activated in GICs by phosphorylation on both tyrosine (Y705) and serine (S727) residues. Therefore, to study STAT3 function in GICs we established an inducible system to knockdown STAT3 expression (iSTAT3-KD). Using this approach, we demonstrated that Y705-STAT3 phosphorylation was critical and indispensable for GIC-induced tumor formation. Both phosphorylation sites in STAT3 promoted GIC proliferation in vitro. We further showed that S727-STAT3 phosphorylation was Y705-dependent. Targeted microarray and RNA sequencing revealed that STAT3 activated cell-cycle regulator genes, and downregulated genes involved in the interferon response, the hypoxia response, the TGFβ pathway, and remodeling of the extracellular matrix. Since STAT3 is an important oncogenic driver of GBM, the identification of these STAT3 regulated pathways in GICs will inform the development of better targeted therapies against STAT3 in GBM and other cancers.

MicroRNA203a suppresses glioma tumorigenesis through an ATM-dependent interferon response pathway

Glioblastoma (GBM) is a deadly and incurable brain tumor. Although microRNAs (miRNAs) play critical roles in regulating the cancer cell phenotype, the underlying mechanisms of how they regulate tumorigenesis are incompletely understood. We previously showed that miR-203a is expressed at relatively low levels in GBM patients, and ectopic miR-203a expression in GBM cell lines inhibited cell proliferation and migration, increased sensitivity to apoptosis induced by interferon (IFN) or temozolomide in vitro, and inhibited GBM tumorigenesis in vivo. Here we show that ectopic expression of miR-203a in GBM cell lines promotes the IFN response pathway as evidenced by increased IFN production and IFN-stimulated gene (ISG) expression, and high basal tyrosine phosphorylation of multiple STAT proteins. Importantly, we identified that miR-203a directly suppressed the protein levels of ataxia-telangiectasia mutated (ATM) kinase that negatively regulates IFN production. We found that high ATM expression in GBM correlates with poor patient survival and that ATM expression is inversely correlated with miR-203a expression. Knockout of ATM expression and inhibition of ATM function in GBM cell lines inhibited cell proliferation and migration, increased sensitivity to apoptosis induced by therapeutic agents in vitro, and markedly suppressed GBM tumor growth and promoted animal survival. In contrast, restoring ATM levels in GBM cells ectopically expressing miR-203a increased tumorigenicity and decreased animal survival. Our study suggests that low miR-203a expression in GBM suppresses the interferon response through an ATM-dependent pathway.

Self-Associating Poly(ethylene oxide)-block-poly(α-carboxyl-ε-caprolactone) Drug Conjugates for the Delivery of STAT3 Inhibitor JSI-124: Potential Application in Cancer Immunotherapy

Constitutive activation of signal transducer and activator of transcription 3 (STAT3) in tumor cells and tumor associated dendritic cells (DCs) plays a major role in the progression of cancer. JSI-124 (cucurbitacin I) is a potent inhibitor of STAT3; however, its poor solubility and nonspecificity limit its effectiveness in cancer immunotherapy. In order to achieve a nanocarrier for solubilization and passive targeting of JSI-124 to tumor cells and tumor associated DCs, the drug was chemically conjugated to pendent COOH groups of self-associating poly(ethylene oxide)-block-poly(α-carboxylate-ε-caprolactone) (PEO-b-PCCL). Developed PEO-b-P(CL-JSI-124) conjugates self-assembled to polymeric micelles of 40 nm size range with negligible drug release under physiological mimicking conditions. The conjugation of JSI-124 to PEO-b-PCCL was confirmed by ¹H NMR, thin layer chromatography (TLC), and HPLC with a conjugation of 8.9% w/w of the polymer. As expected, JSI-124 nanoconjugates showed lower potency in p-STAT3 inhibition and direct anticancer activity in B16-F10 melanoma cells. Interestingly, JSI-124 nanoconjugates were more powerful than free drug in reducing the level of p-STAT3 in tumor exposed bone marrow derived dendritic cells (BMDCs). The JSI-124 nanoconjugates were also significantly more active than free drug in reversing the immunosuppressive effect of B16-F10 tumor and led to significantly better phenotypical and functional stimulation of tumor exposed immature BMDCs in the presence of immune adjuvants like LPS and CpG. Our findings points to great promise for PEO-b-P(CL-JSI-124) micelles for modulation of immunosuppressive microenvironment in melanoma tumors, implicating application of this strategy in cancer immunotherapy.

Signal transducer and activator of transcription 3 in myeloid-derived suppressor cells: an opportunity for cancer therapy

Cancer progression is in part determined by interactions between cancer cells and stromal cells in the tumor microenvironment (TME). The identification of cytotoxic tumor-infiltrating lymphocytes has instigated research into immune stimulating cancer therapies. Although a promising direction, immunosuppressive mechanisms exerted at the TME hamper its success. Myeloid-derived suppressor cells (MDSCs) have come to the forefront as stromal cells that orchestrate the immunosuppressive TME. Consequently, this heterogeneous cell population has been the object of investigation. Studies revealed that the transcription factor signal transducer and activator of transcription 3 (STAT3) largely dictates the recruitment, activation and function of MDSCs in the TME. Therefore, this review will focus on the role of this key transcription factor during the MDSC's life cycle and on the therapeutic opportunities it offers.

Renalase Expression by Melanoma and Tumor-Associated Macrophages Promotes Tumor Growth through a STAT3-Mediated Mechanism

To sustain their proliferation, cancer cells overcome negative-acting signals that restrain their growth and promote senescence and cell death. Renalase (RNLS) is a secreted flavoprotein that functions as a survival factor after ischemic and toxic injury, signaling through the plasma calcium channel PMCA4b to activate the PI3K/AKT and MAPK pathways. We show that RNLS expression is increased markedly in primary melanomas and CD163(+) tumor-associated macrophages (TAM). In clinical specimens, RNLS expression in the tumor correlated inversely with disease-specific survival, suggesting a pathogenic role for RNLS. Attenuation of RNLS by RNAi, blocking antibodies, or an RNLS-derived inhibitory peptide decreased melanoma cell survival, and anti-RNLS therapy blocked tumor growth in vivo in murine xenograft assays. Mechanistic investigations showed that increased apoptosis in tumor cells was temporally related to p38 MAPK-mediated Bax activation and that increased cell growth arrest was associated with elevated expression of the cell-cycle inhibitor p21. Overall, our results established a role for the secreted flavoprotein RNLS in promoting melanoma cell growth and CD163(+) TAM in the tumor microenvironment, with potential therapeutic implications for the management of melanoma. Cancer Res; 76(13); 3884-94. ©2016 AACR.

MiR-21: an environmental driver of malignant melanoma?

Since the mid-1950’s, melanoma incidence has been rising steadily in industrialized Caucasian populations, thereby pointing to the pivotal involvement of environmental factors in melanomagenesis. Recent evidence underlines the crucial role of microRNA (miR) signaling in cancer initiation and progression. Increased miR-21 expression has been observed during the transition from a benign melanocytic lesion to malignant melanoma, exhibiting highest expression of miR-21. Notably, common BRAF and NRAS mutations in cutaneous melanoma are associated with increased miR-21 expression. MiR-21 is an oncomiR that affects critical target genes of malignant melanoma, resulting in sustained proliferation (PTEN, PI3K, Sprouty, PDCD4, FOXO1, TIPE2, p53, cyclin D1), evasion from apoptosis (FOXO1, FBXO11, APAF1, TIMP3, TIPE2), genetic instability (MSH2, FBXO11, hTERT), increased oxidative stress (FOXO1), angiogenesis (PTEN, HIF1α, TIMP3), invasion and metastasis (APAF1, PTEN, PDCD4, TIMP3). The purpose of this review is to provide translational evidence for major environmental and individual factors that increase the risk of melanoma, such as UV irradiation, chemical noxes, air pollution, smoking, chronic inflammation, Western nutrition, obesity, sedentary lifestyle and higher age, which are associated with increased miR-21 signaling. Exosomal miR-21 induced by extrinsic and intrinsic stimuli may be superimposed on mutation-induced miR-21 pathways of melanoma cells. Thus, oncogenic miR-21 signaling may be the converging point of intrinsic and extrinsic stimuli driving melanomagenesis. Future strategies of melanoma treatment and prevention should thus aim at reducing the burden of miR-21 signal transduction.

Fad104, a positive regulator of adipocyte differentiation, suppresses invasion and metastasis of melanoma cells by inhibition of STAT3 activity

Metastasis is the main cause of death in patients with cancer, and understanding the mechanisms of metastatic processes is essential for the development of cancer therapy. Although the role of several cell adhesion, migration or proliferation molecules in metastasis is established, a novel target for cancer therapy remains to be discovered. Previously, we reported that fad104 (factor for adipocyte differentiation 104), a regulatory factor of adipogenesis, regulates cell adhesion and migration. In this report, we clarify the role of fad104 in the invasion and metastasis of cancer cells. The expression level of fad104 in highly metastatic melanoma A375SM cells was lower than that in poorly metastatic melanoma A375C6 cells. Reduction of fad104 expression enhanced the migration and invasion of melanoma cells, while over-expression of FAD104 inhibited migration and invasion. In addition, melanoma cells stably expressing FAD104 showed a reduction in formation of lung colonization compared with control cells. FAD104 interacted with STAT3 and down-regulated the phosphorylation level of STAT3 in melanoma cells. These findings together demonstrate that fad104 suppressed the invasion and metastasis of melanoma cells by inhibiting activation of the STAT3 signaling pathway. These findings will aid a comprehensive description of the mechanism that controls the invasion and metastasis of cancer cells.

The curcumin analog EF24 targets NF-κB and miRNA-21, and has potent anticancer activity in vitro and in vivo

EF24 is a curcumin analog that has improved anticancer activity over curcumin, but its therapeutic potential and mechanism of action is unknown, which is important to address as curcumin targets multiple signaling pathways. EF24 inhibits the NF-κB but not the JAK-STAT signaling pathway in DU145 human prostate cancer cells and B16 murine melanoma cells. EF24 induces apoptosis in these cells apparently by inhibiting miR-21 expression, and also enhances the expression of several miR-21 target genes, PTEN and PDCD4. EF24 treatment significantly suppressed the growth of DU145 prostate cancer xenografts in immunocompromised mice and resulted in tumor regression. EF24 enhanced the expression of the miR-21 target PTEN in DU145 tumor tissue, but suppressed the expression of markers of proliferating cells (cyclin D1 and Ki67). In syngeneic mice injected with B16 cells, EF24 treatment inhibited the formation of lung metastasis, prolonged animal survival, inhibited miR-21 expression and increased the expression of miR-21 target genes. Expression profiling of miRNAs regulated by EF24 in vitro and in vivo showed that the antitumor activity of EF24 reflected the enhanced expression of potential tumor suppressor miRNAs as well as the suppressed expression of oncogenic miRNAs, including miR-21. Taken together, our data suggest that EF24 is a potent anticancer agent and selectively targets NF-κB signaling and miRNA expression, indicating that EF24 has significant potential as a therapeutic agent in various cancers.

Melittin derived peptides for nanoparticle based siRNA transfection

Traditional transfection agents including cationic lipids and polymers have high efficiency but cause cytotoxicity. While cell penetrating peptide based transfection agents exhibit improved cytotoxicity profiles, they do not have the efficiency of existing lipidic agents due to endosomal trapping. As a consequence, we propose an alternative method to efficient peptide based siRNA transfection by starting with melittin, a known pore-forming peptide. By incorporating modifications to decrease cytotoxicity and improve siRNA binding, we have developed p5RHH, which can complex siRNA to form nanoparticles of 190 nm in diameter. p5RHH exhibits high efficiency with GFP knockdown at concentrations as low as 5 nM, with negligible cytotoxicity. To date, p5RHH has shown the ability to transfect B16 cells, Human Umbilical Vein Endothelial Cells, and RAW264.7 cells with high efficiency. These in vitro models demonstrate that p5RHH mediated transfection can block cancer cell proliferation, angiogenesis, and foam cell formation. Moreover, p5RHH/siRNA nanoparticles maintain their size and transfection efficiency in the presence of serum proteins suggesting the potential for use of p5RHH in vivo. These data suggest that our strategy for development of siRNA transfecting peptides can provide an avenue to safe and effective siRNA therapeutics.

MicroRNA miR-21 regulates the metastatic behavior of B16 melanoma cells

MicroRNA-21 (miR-21) is overexpressed in many human tumors and has been linked to various cellular processes altered in cancer. miR-21 is also up-regulated by a number of inflammatory agents, including IFN, which is of particular interest considering the close relationship between inflammation and cancer. Because miR-21 appears to be overexpressed in human melanoma, we examined the role of miR-21 in cancer development and metastasis in B16 mouse melanoma cells. We found that miR-21 is a member of an IFN-induced miRNA subset that requires STAT3 activation. To characterize the role of miR-21 in melanoma behavior, we transduced B16 cells with lentivirus encoding a miR-21 antagomir and isolated miR-21 knockdown B16 cells. miR-21 knockdown or IFN treatment alone inhibited B16 cell proliferation and migration in vitro, and in combination they had an enhanced effect. Moreover, miR-21 knockdown sensitized B16 cells to IFN-induced apoptosis. In B16 cells miR-21 targeted tumor suppressor (PTEN and PDCD4) and antiproliferative (BTG2) proteins. To characterize the role of miR-21 in vivo, empty vector- and antagomiR-21-transduced B16 melanoma cells were injected via tail vein into syngeneic C57BL/6 mice. Although empty vector-transduced B16 cells produced large lung metastases, miR-21 knockdown cells only formed small lung lesions. Importantly, miR-21 knockdown tumor-bearing mice exhibited prolonged survival compared with empty vector tumor-bearing mice. Thus, miR-21 regulates the metastatic behavior of B16 melanoma cells by promoting cell proliferation, survival, and migration/invasion as well as by suppressing IFN action, providing important new insights into the role of miR-21 in melanoma.

IFN induces miR-21 through a signal transducer and activator of transcription 3-dependent pathway as a suppressive negative feedback on IFN-induced apoptosis

The microRNA miR-21 is overexpressed in many human cancers, wherein accumulating evidence indicates that it functions as an oncogene. Here, we report that the cytokine IFN rapidly induces miR-21 expression in human and mouse cells. Signal transducer and activator of transcription 3 (STAT3) was implicated in this pathway based on the lack of IFN effect on miR-21 expression in prostate cancer cells with a deletion in the STAT3 gene. STAT3 ablation abrogated IFN induction of miR-21, confirming the important role of STAT3 in regulating miR-21. Chromatin immunoprecipitation analysis showed that STAT3 directly bound the miR-21 promoter in response to IFN. Experiments in mouse embryo fibroblasts with a genetic deletion of the p65 NF-κB subunit showed that IFN-induced miR-21 expression was also dependent on NF-κB. STAT3 silencing blocked both IFN-induced p65 binding to the miR-21 promoter and p65 nuclear translocation. Thus, IFN-induced miR-21 expression is coregulated by STAT3 and NF-κB at the level of the miR-21 promoter. Several cell death regulators were identified as downstream targets of miR-21, including PTEN and Akt. Functional experiments in prostate cancer cells directly showed that miR-21 plays a critical role in suppressing IFN-induced apoptosis. Our results identify miR-21 as a novel IFN target gene that functions as a key feedback regulator of IFN-induced apoptosis.