Resistance to interferons in melanoma cells does not correlate with the expression or activation of signal transducer and activator of transcription 1 (Stat1)

Increased IRF9-STAT2 signaling leads to adaptive resistance toward targeted therapy in melanoma by restraining GSDME-dependent pyroptosis

Melanoma is the leading cause of cutaneous malignancy death. BRAF inhibitors (BRAFis) have been developed as target therapies because nearly half of melanoma patients have activating mutations in the BRAF oncogene. However, the fast-developed resistance of BRAFis limits its treatment efficacy. Understanding the molecular mechanism of resistance is vital to increase the success of clinical treatment. We searched three datasets (GSE42872, GSE52882, and GSE106321) from the Gene Expression Omnibus (GEO), which analyzed the mRNA expression profile in melanoma cells under BRAFis treatment, and the differentially expressed genes (DEGs) were identified. Among all the DEGs, increased expression of IRF9 and STAT2 was distinguished and verified to be upregulated in BRAFis-treated melanoma cells. Furthermore, IRF9 or STAT2 overexpression led to less sensitivity, while IRF9 or STAT2 knockdown increased sensitivity to BRAFis treatment. In a subcutaneous xenograft tumor model, we demonstrated that IRF9 or STAT2 overexpression slowed BRAFis-induced tumor shrank, but IRF9 or STAT2 knockdown led to BRAFis-induced tumor shrank more quickly. More interestingly, we discovered that IRF9-STAT2 signaling controlled GSDME-dependent pyroptosis by restoring GSDME transcription. These results suggest that targeting IRF9/STAT2 may lead to more promising effective treatments to prevent melanoma resistance to BRAFis by inducing pyroptosis.

Lentiviral miR30-based RNA interference against heparanase suppresses melanoma metastasis with lower liver and lung toxicity

Aim: To construct short hairpin RNAs (shRNAs) and miR30-based shRNAs against heparanase (HPSE) to compare their safety and their effects on HPSE down-modulation in vitro and in vivo to develop a more ideal therapeutic RNA interference (RNAi) vector targeting HPSE.

Methods: First, we constructed shRNAs and miR30-based shRNAs against HPSE (HPSE-shRNAs and HPSE-miRNAs) and packed them into lentiviral vectors. Next, we observed the effects of the shRNAs on knockdown for HPSE expression, adhesion, migration and invasion abilities in human malignant melanoma A375 cells in vitro. Furthermore, we compared the effects of the shRNAs on melanoma growth, metastasis and safety in xenograft models.

Results: Our data showed that these artificial miRNAs targeting HPSE could be effective RNAi agents mediated by Pol II promoters in vitro and in vivo, although these miRNAs were not more potent than the HPSE-shRNAs. It was noted that obvious lung injuries, rarely revealed previously, as well as hepatotoxicity could be caused by lentivirus-mediated shRNAs (LV shRNAs) rather than lentivirus-mediated miRNAs (LV miRNAs) in vivo. Furthermore, enhanced expression of pro-inflammatory cytokines IL-6 and TGF-β1 and endogenous mmu-miR-21a-5p were detected in lung tissues of shRNAs groups, whereas the expression of mmu-let-7a-5p, mmu-let-7b-5p and mmu-let-7c-5p were down-regulated.

Conclusion: These findings suggest that artificial miRNAs display an improved safety profile of lowered lung injury or hepatotoxicity relative to shRNAs in vivo. The mechanism of lung injuries caused by shRNAs may be correlated with changes of endogenous miRNAs in the lung. Our data here increase the flexibility of a miRNA-based RNAi system for functional genomic and gene therapy applications.

IRF9 is a key factor for eliciting the antiproliferative activity of IFN-alpha

A number of tumors are still resistant to the antiproliferative activity of human interferon (IFN)-alpha. The Janus kinases/Signal Transducers and Activators of Transcription (JAK-STAT) pathway plays an important role in initial IFN signaling. To enhance the antiproliferative activity of IFN-alpha, it is important to elucidate which factors in the JAK-STAT pathway play a key role in eliciting this activity. In human ovarian adenocarcinoma OVCAR3 cells sensitive to both IFN-alpha and IFN-gamma, only IFN regulatory factor 9 (IRF9)-RNA interference (RNAi) completely inhibited the antiproliferative activity of IFN-alpha among the intracellular JAK-STAT pathway factors. Conversely, Stat1-RNAi did not inhibit the antiproliferative activity of IFN-alpha, whereas it partially inhibited that of IFN-gamma. As a cell death pathway, it is reported that tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) induces apoptosis through TRAIL-receptor (R) 1 and TRAIL-R2. In IFN-alpha-treated OVCAR3 cells, IRF9-RNAi inhibited transcription of TRAIL whereas Stat1-RNAi did not, suggesting that the transcription of TRAIL induced by IFN-alpha predominantly required IRF9. Furthermore, IFN-stimulated response element-like motifs of TRAIL bound to IFN-stimulated gene factor 3 (ISGF3) complex after IFN-alpha treatment. Subsequently, TRAIL-R2-RNAi inhibited both antiproliferative activities of IFN-alpha and TRAIL, suggesting that TRAIL-R2 mediated both IFN-alpha and TRAIL signals to elicit their antiproliferative activities. Finally, IRF9 overexpression facilitated IFN-alpha-induced apoptosis in T98G (human glioblastoma multiforme) cells, which were resistant to IFN-alpha. Thus, this study suggests that IRF9 is the key factor for eliciting the antiproliferative activity of IFN-alpha and TRAIL may be one of the potential mediators.

Modulation of SOCS protein expression influences the interferon responsiveness of human melanoma cells

Background

Endogenously produced interferons can regulate the growth of melanoma cells and are administered exogenously as therapeutic agents to patients with advanced cancer. We investigated the role of negative regulators of interferon signaling known as suppressors of cytokine signaling (SOCS) in mediating interferon-resistance in human melanoma cells.

Methods

Basal and interferon-alpha (IFN-α) or interferon-gamma (IFN-γ)-induced expression of SOCS1 and SOCS3 proteins was evaluated by immunoblot analysis in a panel of n = 10 metastatic human melanoma cell lines, in human embryonic melanocytes (HEM), and radial or vertical growth phase melanoma cells. Over-expression of SOCS1 and SOCS3 proteins in melanoma cells was achieved using the PINCO retroviral vector, while siRNA were used to inhibit SOCS1 and SOCS3 expression. Tyr⁷⁰¹-phosphorylated STAT1 (P-STAT1) was measured by intracellular flow cytometry and IFN-stimulated gene expression was measured by Real Time PCR.

Results

SOCS1 and SOCS3 proteins were expressed at basal levels in melanocytes and in all melanoma cell lines examined. Expression of the SOCS1 and SOCS3 proteins was also enhanced following stimulation of a subset of cell lines with IFN-α or IFN-γ. Over-expression of SOCS proteins in melanoma cell lines led to significant inhibition of Tyr⁷⁰¹-phosphorylated STAT1 (P-STAT1) and gene expression following stimulation with IFN-α (IFIT2, OAS-1, ISG-15) or IFN-γ (IRF1). Conversely, siRNA inhibition of SOCS1 and SOCS3 expression in melanoma cells enhanced their responsiveness to interferon stimulation.

Conclusions

These data demonstrate that SOCS proteins are expressed in human melanoma cell lines and their modulation can influence the responsiveness of melanoma cells to IFN-α and IFN-γ.

Resistance to IFN-alpha-induced apoptosis is linked to a loss of STAT2

Type I IFNs (IFN-alpha/beta) are pleitropic cytokines widely used in the treatment of certain malignancies, hepatitis B and C, and multiple sclerosis. IFN resistance is a challenging clinical problem to overcome. Hence, understanding the molecular mechanism by which IFN immunotherapy ceases to be effective is of translational importance. In this study, we report that continuous IFN-alpha stimulation of the human Jurkat variant H123 led to resistance to type I IFN-induced apoptosis due to a loss of signal transducers and activators of transcription 2 (STAT2) expression. The apoptotic effects of IFN-alpha were hampered as STAT2-deficient cells were defective in activating the mitochondrial-dependent death pathway and ISGF3-mediated gene activation. Reconstitution of STAT2 restored the apoptotic effects of IFN-alpha as measured by the loss of mitochondrial membrane potential, cytochrome c release from mitochondria, caspase activation, and ultimately cell death. Nuclear localization of STAT2 was a critical event as retention of tyrosine-phosphorylated STAT2 in the cytosol was not sufficient to activate apoptosis. Furthermore, silencing STAT2 gene expression in Saos2 and A375S.2 tumor cell lines significantly reduced the apoptotic capacity of IFN-alpha. Altogether, we show that STAT2 is a critical mediator in the activation of type I IFN-induced apoptosis. More importantly, defects in the expression or nuclear localization of STAT2 could lessen the efficacy of type I IFN immunotherapy.

Mechanisms of resistance to interferon-gamma-mediated cell growth arrest in human oral squamous carcinoma cells

Interferon-gamma (IFNgamma) has an antiproliferative effect on a variety of tumor cells. However, many tumor cells resist treatment with IFNs. Here, we show that IFNgamma fails to inhibit the growth of some types of oral squamous cell carcinoma (OSCC) cells that possess a fully functional IFNgamma/STAT1 (signal transducer and activator of transcription-1) signaling pathway. IFNgamma inhibited the growth of the HSC-2, HSC-3, and HSC-4 OSCC cell lines. However, Ca9-22 cells were resistant to IFNgamma despite having intact STAT1-dependent signaling, such as normal tyrosine phosphorylation, DNA binding activity, and transcriptional activity of STAT1. The growth inhibition of HSC-2 cells resulted from S-phase arrest of the cell cycle. IFNgamma inhibited cyclin A2 (CcnA2)-associated kinase activity, which correlated with the IFNgamma-mediated down-regulation of CcnA2 and Cdk2 expression at both the transcriptional and post-transcriptional level in HSC-2 cells but not in Ca9-22 cells. RNAi-mediated knockdown of CcnA2 and Cdk2 resulted in growth inhibition in both cell lines. These results indicate that the resistance of OSCC to IFNgamma is not due simply to the deficiency in STAT1-dependent signaling but results from a defect in the signaling component that mediates this IFNgamma-induced down-regulation of CcnA2 and Cdk2 expression at the transcriptional and post-transcriptional levels.

Virus-induced unfolded protein response attenuates antiviral defenses via phosphorylation-dependent degradation of the type I interferon receptor

Phosphorylation-dependent ubiquitination and degradation of the IFNAR1 chain of the type I interferon (IFN) receptor is regulated by two different pathways, one of which is ligand independent. We report that this ligand-independent pathway is activated by inducers of unfolded protein responses (UPR), including viral infection, and that such activation requires the endoplasmic reticulum-resident protein kinase PERK. Upon viral infection, activation of this pathway promotes phosphorylation-dependent ubiquitination and degradation of IFNAR1, specifically inhibiting type I IFN signaling and antiviral defenses. Knockin of an IFNAR1 mutant insensitive to virus-induced turnover or conditional knockout of PERK prevented IFNAR1 degradation, whether UPR-induced or virus-induced, and restored cellular responses to type I IFN and resistance to viruses. These data suggest that specific activation of the PERK component of UPR can favor viral replication. Interfering with PERK-dependent IFNAR1 degradation could therefore contribute to therapeutic strategies against viral infections.

Requirement of catalytically active Tyk2 and accessory signals for the induction of TRAIL mRNA by IFN-beta

Tumor necrosis factor (TNF)-related apoptosis-inducing ligand/Apo2 ligand (TRAIL/Apo2L) mRNA was induced preferentially by interferon (IFN)-beta but not IFN-alpha in human fibrosarcoma and primary fibroblast cells. To characterize the signaling components mediating the IFN subtype-specific induction of this gene, we used mutant cell lines lacking individual components involved in signaling by type I IFNs. TRAIL was not induced by IFN-beta in mutant cell lines U2A, U3A, U4A, U5A, and U6A, which lack, respectively, IFN regulatory factor-9 (IRF-9), Stat1, Jak1, IFNAR-2.2, and Stat2, indicating transcription factor IFN-stimulated gene factor 3 (ISGF3) was essential for the induction of this gene. TRAIL was not induced by IFN-beta in U1A (Tyk2 null) or U1A.R930 cells (that express a kinase-deficient point mutant of Tyk2) but was induced in U1A.wt-5 cells (U1A cells expressing wild-type Tyk2), indicating that Tyk2 protein and kinase activity were both required for induction of the gene. Biochemical and genetic analyses revealed the requirement of transcription factor NF-kappa B and phosphoinositide 3-kinase (PI3K) but not extracellular signal-regulated kinase (ERK) for the induction of TRAIL by IFN-beta. Furthermore, the antiproliferative but not antiviral effects of IFN-beta required catalytically active Tyk2, suggesting that expression of genes, such as TRAIL, may play an important role in mediating the biologic effects of IFNs.

Oncolytic viruses in cancer therapy

Oncolytic virotherapy is a promising form of gene therapy for cancer, employing nature’s own agents to find and destroy malignant cells. The purpose of this review is to provide an introduction to this very topical field of research and to point out some of the current observations, insights and ideas circulating in the literature. We have strived to acknowledge as many different oncolytic viruses as possible to give a broader picture of targeting cancer using viruses. Some of the newest additions to the panel of oncolytic viruses include the avian adenovirus, foamy virus, myxoma virus, yaba-like disease virus, echovirus type 1, bovine herpesvirus 4, Saimiri virus, feline panleukopenia virus, Sendai virus and the non-human coronaviruses. Although promising, virotherapy still faces many obstacles that need to be addressed, including the emergence of virus-resistant tumor cells.

An update on malignant melanoma vaccine research: insights into mechanisms for improving the design and potency of melanoma therapeutic vaccines

Currently, cancer vaccine therapy for melanoma has a 2-fold focus. On the one hand, advances have been aimed at improving the effectiveness of melanoma vaccines based on a greater understanding of melanoma tumor cell biology. On the other hand, there is increasing evidence that the immune system, our defense against tumors, also inadvertently plays a supportive role in promoting the development and progression of tumors. Hence, two opposing forces 'hanging in the balance' dictate patients' responses to melanoma: tumor cell biology and the status of the immune system. Recent developments in our understanding of both of these aspects have provided new leads and insights for novel ways to improve vaccine design and add to the melanoma vaccine armory. As the focus of immunotherapy shifts its aim towards the tumor microenvironment, we are now developing the ability to program the immune responses raised by vaccination against melanoma. The aim here is to prevent myeloid and regulatory T-cell-mediated immune suppression as well as to counteract tumor-derived factors capable of suppressing immune responses. A redirected strategy for vaccine immunotherapy is proposed based on our greater understanding of tumor immunity. Using a combination therapy of immune-potentiating melanoma vaccines together with adjuvants for overcoming the immunosuppressive forces will allow us to activate protective immunity against melanoma. Other cancer vaccines (i.e. colon or renal) are already offering reasons for hope and expectation that vaccine immunotherapy will also produce successful outcomes for patients with melanoma.

STAT5 contributes to interferon resistance of melanoma cells

BACKGROUND

Malignant melanoma is a highly aggressive neoplastic disease whose incidence is increasing rapidly. In recent years, the use of interferon alpha (IFNalpha) has become the most established adjuvant immunotherapy for melanoma of advanced stage. IFNalpha is a potent inhibitor of melanoma cell proliferation, and the signal transducer and activator of transcription STAT1 is crucial for its antiproliferative action. Although advanced melanomas clinically resistant to IFNalpha are frequently characterized by inefficient STAT1 signaling, the mechanisms underlying advanced-stage interferon resistance are poorly understood.

RESULTS

Here, we demonstrate that IFNalpha activates STAT5 in melanoma cells and that in IFNalpha-resistant cells STAT5 is overexpressed. Significantly, the knockdown of STAT5 in interferon-resistant melanoma cells restored the growth-inhibitory response to IFNalpha. When STAT5 was overexpressed in IFNalpha-sensitive cells, it counteracted interferon-induced growth inhibition. The overexpressed STAT5 diminished IFNalpha-triggered STAT1 activation, most evidently through upregulation of the inhibitor of cytokine-signaling CIS.

CONCLUSIONS

Our data demonstrate that overexpression and activation of STAT5 enable melanoma cells to overcome cytokine-mediated antiproliferative signaling. Thus, overexpression of STAT5 can counteract IFNalpha signaling in melanoma cells, and this finally can result in cytokine-resistant and progressively growing tumor cells. These findings have significant implications for the clinical failure of IFNalpha therapy of advanced melanoma because they demonstrate that IFNalpha induces the activation of STAT5 in melanoma cells, and in STAT5-overexpressing cells, this contributes to IFNalpha resistance.

Expression of STAT1 and STAT2 in malignant melanoma does not correlate with response to interferon-alpha adjuvant therapy

Interferon-alpha (IFN-alpha) is used as an adjuvant therapy in patients with malignant melanoma and who have undergone surgical resection of high-risk lesions. Defective expression or activation of STAT1 or STAT2 has been shown to correlate with IFN-alpha or resistance in vitro; however, recent data from our laboratory suggest that the anti-tumor effects of IFN-alpha are dependent on STAT1 signaling within host immune cells. We measured STAT1 and STAT2 expression in 28 melanoma biopsies (8 cutaneous lesions; 1 lung metastasis; 19 nodal metastases) obtained from patients prior to the initiation of adjuvant IFN-alpha therapy. Disease recurrence following IFN-alpha treatment did not correlate with the staining intensity of either STAT1 (P = 0.61) or STAT2 (P = 0.52). Tumors with minimal STAT1 or STAT2 expression (< 20% positive) were present in four patients with tumor-positive lymph nodes, who exhibited prolonged relapse-free survival (> 44 months) following adjuvant therapy. Conversely, high levels of STAT1 were present in a patient who recurred during the course of IFN-alpha therapy. A case study of one patient who experienced recurrent disease during IFN-alpha treatment revealed that STAT1 levels were greater in the recurrent tumor when compared to the original lesion. These studies provide direct evidence to suggest that levels of STAT1 and STAT2 within the tumor do not influence a patient's response to adjuvant IFN-alpha.

Identification of proteins regulated by interferon-? in resistant and sensitive malignant melanoma cell lines

Treatment of patients with malignant melanoma with interferon-alpha achieves a response in a small but significant subset of patients. Currently, although much is known about interferon biology, little is known about either the particular mechanisms of interferon-alpha activity that are crucial for response or why only some patients respond to interferon-alpha therapy. Two melanoma cell lines (MeWo and MM418) that are known to differ in their response to the antiproliferative activity of interferon-alpha, have been used as a model system to investigate interferon-alpha action. Using a proteomics approach based on two-dimensional polyacrylamide gel electrophoresis and mass spectrometry, several proteins induced in response to interferon-alpha have been identified. These include a number of gene products previously known to be type I interferon responsive (tryptophanyl tRNA synthetase, leucine aminopeptidase, ubiquitin cross-reactive protein, gelsolin, FUSE binding protein 2 and hPNPase) as well as a number of proteins not previously reported to be induced by type I interferon (cathepsin B, proteasomal activator 28alpha and alpha-SNAP). Although the proteins upregulated by interferon-alpha were common between the cell lines when examined at the level of Western blotting, the disparity in the basal level of cathepsin B was striking, raising the possibility that the higher level in MM418 may contribute to the sensitivity of this cell line to interferon-alpha treatment.

Regulation of Gene Expression by Pegylated IFN-α2b and IFN-α2b in Human Peripheral Blood Mononuclear Cells

The pleiotropic biologic effects of interferon (IFN) are mediated through regulation of the expression of numerous IFN-sensitive genes. Peripheral blood mononuclear cells (PBMCs) obtained from healthy donors were analyzed to study the immunoregulatory and antiviral messenger RNAs (mRNAs) and proteins regulated by pegylated IFN-alpha2b (PEG-IFN-alpha2b) and IFN-alpha2b. A dose-dependent and time-dependent response for multiple IFN-regulated genes was observed. IFN-dependent protein production and secretion were correlated with IFN-regulated mRNA induction. Overall regulation of gene expression patterns for PEG-IFN-alpha2b and IFN-alpha2b was comparable, even though the antiviral activity of PEG-IFN-alpha2b demonstrated a longer biologic halflife in vitro compared with IFN-alpha2b. To study the heterogeneity of responses, PBMCs obtained from over 25 healthy donors were analyzed. Within a particular donor dataset, gene-specific and dose-dependent responses to PEG-IFN-alpha2b treatment, demonstrated in both the amplitude of transcriptional upregulation and the duration of sustained mRNA upregulation, were observed. However because of donor heterogeneity, the amplitude of a given transcriptional response could not be predicted for a specific dose of PEG-IFN-alpha2b. Notably, mRNA levels of oligoadenylate synthetase (OAS), double-stranded RNA (dsRNA)-activated protein kinase (PKR), IP-10, IFN-stimulated gene 54 (ISG54), and ISG15 were upregulated after 120 h of continuous PEG-IFN-alpha2b treatment. These results suggest that the use of antiviral and immunoregulatory protein mRNA levels as markers to assess the therapeutic efficacy of IFN-alpha2b and PEG-IFN-alpha2b against viral and neoplastic diseases in clinical trials is promising but will require further analysis using clinical patient samples.

The antitumor effects of interferon-alpha are maintained in mice challenged with a STAT1-deficient murine melanoma cell line

INTRODUCTION

Interferon-alpha (IFN-alpha) is currently administered to patients with metastatic malignant melanoma and those who are at risk for recurrence following surgery for high-risk lesions. Signal transducer and activator of transcription 1 (STAT1) is a transcription factor that is activated by IFN-alpha and is thought to mediate the majority of its antitumor effects. Loss of STAT1 has been found in IFN-resistant melanoma cells. We developed a murine melanoma cell line in a STAT1-deficient mouse. We also transfected B16 melanoma cells with a wild-type form of STAT1 to induce its overexpression. Using the resulting cell lines and STAT1-deficient mice, we tested whether IFN-alpha could exert an antitumor effect on melanoma cells in the absence of STAT1-mediated signal transduction.

MATERIALS AND METHODS

A melanoma tumor was induced in STAT1-deficient mice via the application of DMBA (tumor initiator) followed by croton oil (tumor promoter). Immunohistochemical analysis confirmed that the resulting tumor was a malignant melanoma. Immunoblot analysis, intracellular flow cytometry, and gel-shift analysis were used to confirm the lack of STAT1 in the derivative cell line (AGS-1). In addition, the STAT1 protein was overexpressed in B16 melanoma cells by stable transfection with a plasmid construct encoding wild-type STAT1. The effects of IFN-alpha on these cell lines were studied in vitro and in vivo.

RESULTS

STAT1 was not expressed in the AGS-1 murine melanoma cell line. Treatment with IFN-alpha did not lead to activation of STAT1. Cell proliferation assays revealed that while IFN-alpha did not exert an antiproliferative effect on this cell line, it was capable of prolonging the survival of STAT1-competent C57BL/6 mice bearing 1 x 10(6) AGS-1 tumor cells in the intraperitoneal position (n = 20, P < 0.05), as compared to PBS-treated controls. Also, the survival of IFN-alpha-treated mice (as compared to PBS-treated controls) was not affected by the overexpression of STAT1 in B16 tumor cells.

CONCLUSIONS

This data suggests that IFN-alpha can enhance survival in an animal model where STAT1-mediated signal transduction and gene regulation is absent within the tumor but is present within the host. This data also indicates that the overexpression of STAT1 within the tumor does not significantly enhance the effects of exogenously administered IFN-alpha in this model. These findings indicate that the bulk of the antitumor actions of IFN-alpha may be derived from its effects on host tissues.

Interferon resistance promotes oncolysis by influenza virus NS1-deletion mutants

NS1 protein of influenza virus is a virulence factor that counteracts Type I interferon (IFN)-mediated antiviral response by the host. A recombinant influenza A virus that lacks the NS1 protein only replicates efficiently in systems that contain defective IFN pathways. We demonstrate that the conditional replication properties of NS1-modified influenza A virus mutants can be exploited for the virus-mediated oncolysis of IFN-resistant tumor cells. IFN resistance in analyzed tumor cell lines correlated with a reduced expression of STAT1. Addition of exogenous IFNalpha or supernatant of virus-infected endothelial cells inhibited viral oncolysis in IFN-sensitive but not in IFN-resistant cell lines. The oncolytic potential of NS1-modified influenza A virus mutants could be exploited in vivo in a SCID mouse model of a subcutaneously-implanted human IFN-resistant melanoma. The data indicate that IFN-resistant tumors are a suitable target for oncolysis induced by NS1-modified influenza virus mutants. STAT1 might serve as a marker to identify these IFN-resistant tumors.

Interferon-γ-Mediated Growth Regulation of Melanoma Cells: Involvement of STAT1-Dependent and STAT1-Independent Signals

Interferon-gamma, a known inhibitor of tumor cell growth, has been used in several protocols for the treatment of melanoma. We have studied the molecular events underlying interferon-gamma-induced G0/G1 arrest in four metastatic melanoma cell lines with different responsiveness to interferon-gamma. The growth arrest did not result from enhanced expression of cyclin-dependent kinase inhibitors p21 and p27. Instead, it correlated with downregulation of cyclin E and cyclin A and inhibition of their associated kinase activities. We show that interferon-gamma-induced growth inhibition could be abrogated by overexpression of dominant negative STAT1 (signal transducer and activator of transcription 1) in the melanoma cell line A375, suggesting that STAT1 plays a crucial part for the anti-proliferative effect. Erythropoietin stimulation of a chimeric receptor led to a concentration-dependent STAT1 activation and concomitant growth arrest when it contained the STAT recruitment motif Y440 of the interferon-gamma receptor 1. In contrast, dose-response studies for interferon-gamma revealed a discrepancy between levels of STAT1 activation and the extent of growth inhibition; whereas STAT1 was activated by low doses of interferon-gamma (10 U per mL), growth inhibitory effects were only visible with 100-fold higher concentrations. Our results suggest the presence of additional signals emanating from the interferon-gamma receptor, which may counteract the anti-proliferative function of STAT1.

Anticancer activity of sodium stibogluconate in synergy with IFNs

Cancer cell resistance limits the efficacy of IFNs. In this study, we show that sodium stibogluconate (SSG) and IFN-alpha synergized to overcome IFN-alpha resistance in various human cancer cell lines in culture and eradicated IFN-alpha-refractory WM9 human melanoma tumors in nude mice with no obvious toxicity. SSG enhanced IFN-alpha-induced Stat1 tyrosine phosphorylation, inactivated intracellular SHP-1 and SHP-2 that negatively regulate IFN signaling, and induced cellular protein tyrosine phosphorylation in cancer cell lines. These effects are consistent with inactivation of phosphatases as the basis of SSG anticancer activity. Characterization of SSG by chromatography revealed that only selective compounds in SSG were effective protein tyrosine phosphatase inhibitors. These observations suggest the potential of SSG as a clinically usable protein tyrosine phosphatase inhibitor in cancer treatment and provide insights for developing phosphatase-targeted therapeutics.

Inhibition of tumor cell growth by murine splenic adherent cells stimulated with IFN-gamma

We have previously reported that the growth of lymphocytes and tumor cells with lymphocyte lineage was strongly inhibited by a part of cloned macrophage hybridomas. This growth inhibition was accomplished by cell-to-cell contact and found to be attributed to lipid-like molecule(s) in a macrophage hybridoma cell membrane fraction. Instead of macrophage hybridomas, in the present study we utilized splenic adherent cells (SACs) that had been stimulated with IFN-gamma to see whether they inhibited tumor cell growth or not. The results demonstrated that IFN-gamma-stimulated but not unstimulated SACs showed a significant growth inhibition of BW-5147 tumor cells. This growth inhibition was not mainly mediated by prostaglandin E2 secreted from macrophages, since the inhibition was not reduced in the presence of indomethacin. Furthermore, as was reported previously in the case of macrophage hybridomas, the inhibitory activity resides in a lipid fraction of IFN-gamma-stimulated SAC membrane.