Inhibition of constitutive signal transducer and activator of transcription 3 activation by novel platinum complexes with potent antitumor activity

Tumor-Associated Macrophage Targeting of Nanomedicines in Cancer Therapy

The tumor microenvironment (TME) is pivotal in tumor growth and metastasis, aligning with the "Seed and Soil" theory. Within the TME, tumor-associated macrophages (TAMs) play a central role, profoundly influencing tumor progression. Strategies targeting TAMs have surfaced as potential therapeutic avenues, encompassing interventions to block TAM recruitment, eliminate TAMs, reprogram M2 TAMs, or bolster their phagocytic capabilities via specific pathways. Nanomaterials including inorganic materials, organic materials for small molecules and large molecules stand at the forefront, presenting significant opportunities for precise targeting and modulation of TAMs to enhance therapeutic efficacy in cancer treatment. This review provides an overview of the progress in designing nanoparticles for interacting with and influencing the TAMs as a significant strategy in cancer therapy. This comprehensive review presents the role of TAMs in the TME and various targeting strategies as a promising frontier in the ever-evolving field of cancer therapy. The current trends and challenges associated with TAM-based therapy in cancer are presented.

Platinum(IV) Complexes as Inhibitors of STAT3 and Regulators of the Tumor Microenvironment To Control Breast Cancer

Interplay between breast cancer (BC) cells and the tumor microenvironment (TME) influences the outcome of cancer treatment. Aberrant activation of signal transducer and activator of transcription 3 (STAT3) promotes the interaction and causes immunosuppression and drug resistance. Platinum(IV) complexes SPP and DPP bearing pterostilbene-derived axial ligand(s) were synthesized to inhibit the JAK2-STAT3 pathway in BC cells and regulate the TME. These complexes exerted remarkable antiproliferative activity against the triple-negative BC cells, suppressed the expression of phosphorylated STAT3 and STAT3-related cyclooxygenase-2 and IL-6, and activated caspase-3 and cleaved poly ADP-ribose polymerase, preventing the repair of DNA lesions and inducing apoptosis. Furthermore, DPP promoted the maturation and antigen presentation of dendritic cells, repressed the proliferation and differentiation of myeloid-derived suppressor cells and regulatory T cells, and facilitated the expansion of T cells. As a consequence, DPP showed excellent anticancer activity against BC with almost no general toxicity in vivo as a potential chemoimmunotherapeutic agent.

STAT family of transcription factors in breast cancer: Pathogenesis and therapeutic opportunities and challenges

Breast cancer is the most commonly diagnosed cancer and second-leading cause of cancer deaths in women. Breast cancer stem cells (BCSCs) promote metastasis and therapeutic resistance contributing to tumor relapse. Through activating genes important for BCSCs, transcription factors contribute to breast cancer metastasis and therapeutic resistance, including the signal transducer and activator of transcription (STAT) family of transcription factors. The STAT family consists of six major isoforms, STAT1, STAT2, STAT3, STAT4, STAT5, and STAT6. Canonical STAT signaling is activated by the binding of an extracellular ligand to a cell-surface receptor followed by STAT phosphorylation, leading to STAT nuclear translocation and transactivation of target genes. It is important to note that STAT transcription factors exhibit diverse effects in breast cancer; some are either pro- or anti-tumorigenic while others maintain dual, context-dependent roles. Among the STAT transcription factors, STAT3 is the most widely studied STAT protein in breast cancer for its critical roles in promoting BCSCs, breast cancer cell proliferation, invasion, angiogenesis, metastasis, and immune evasion. Consequently, there have been substantial efforts in developing cancer therapeutics to target breast cancer with dysregulated STAT3 signaling. In this comprehensive review, we will summarize the diverse roles that each STAT family member plays in breast cancer pathobiology, as well as, the opportunities and challenges in pharmacologically targeting STAT proteins and their upstream activators in the context of breast cancer treatment.

Central Roles of STAT3-Mediated Signals in Onset and Development of Cancers: Tumorigenesis and Immunosurveillance

Since the time of Rudolf Virchow in the 19th century, it has been well-known that cancer-associated inflammation contributes to tumor initiation and progression. However, it remains unclear whether a collapse of the balance between the antitumor immune response via the immunological surveillance system and protumor immunity due to cancer-related inflammation is responsible for cancer malignancy. The majority of inflammatory signals affect tumorigenesis by activating signal transducer and activation of transcription 3 (STAT3) and nuclear factor-κB. Persistent STAT3 activation in malignant cancer cells mediates extremely widespread functions, including cell growth, survival, angiogenesis, and invasion and contributes to an increase in inflammation-associated tumorigenesis. In addition, intracellular STAT3 activation in immune cells causes suppressive effects on antitumor immunity and leads to the differentiation and mobilization of immature myeloid-derived cells and tumor-associated macrophages. In many cancer types, STAT3 does not directly rely on its activation by oncogenic mutations but has important oncogenic and malignant transformation-associated functions in both cancer and stromal cells in the tumor microenvironment (TME). We have reported a series of studies aiming towards understanding the molecular mechanisms underlying the proliferation of various types of tumors involving signal-transducing adaptor protein-2 as an adaptor molecule that modulates STAT3 activity, and we recently found that AT-rich interactive domain-containing protein 5a functions as an mRNA stabilizer that orchestrates an immunosuppressive TME in malignant mesenchymal tumors. In this review, we summarize recent advances in our understanding of the functional role of STAT3 in tumor progression and introduce novel molecular mechanisms of cancer development and malignant transformation involving STAT3 activation that we have identified to date. Finally, we discuss potential therapeutic strategies for cancer that target the signaling pathway to augment STAT3 activity.

STAT3 pathway in cancers: Past, present, and future

Signal transducer and activator of transcription 3 (STAT3), a member of the STAT family, discovered in the cytoplasm of almost all types of mammalian cells, plays a significant role in biological functions. The duration of STAT3 activation in normal tissues is a transient event and is strictly regulated. However, in cancer tissues, STAT3 is activated in an aberrant manner and is induced by certain cytokines. The continuous activation of STAT3 regulates the expression of downstream proteins associated with the formation, progression, and metastasis of cancers. Thus, elucidating the mechanisms of STAT3 regulation and designing inhibitors targeting the STAT3 pathway are considered promising strategies for cancer treatment. This review aims to introduce the history, research advances, and prospects concerning the STAT3 pathway in cancer. We review the mechanisms of STAT3 pathway regulation and the consequent cancer hallmarks associated with tumor biology that are induced by the STAT3 pathway. Moreover, we summarize the emerging development of inhibitors that target the STAT3 pathway and novel drug delivery systems for delivering these inhibitors. The barriers against targeting the STAT3 pathway, the focus of future research on promising targets in the STAT3 pathway, and our perspective on the overall utility of STAT3 pathway inhibitors in cancer treatment are also discussed.

Highlighted STAT3 as a potential drug target for cancer therapy

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.

Targeting STAT3 signaling in kidney disease

The Janus kinase/signal transducer and activator of transcription (JAK/STAT) signaling pathway is a multifaceted transduction system that regulates cellular responses to incoming signaling ligands. STAT3 is a central member of the JAK/STAT signaling cascade and has long been recognized for its increased transcriptional activity in cancers and autoimmune disorders but has only recently been in the spotlight for its role in the progression of kidney disease. Although genetic knockout and manipulation studies have demonstrated the salutary benefits of inhibiting STAT3 activity in several kidney disease models, pharmacological inhibition has yet to make it to the clinical forefront. In recent years, significant effort has been aimed at suppressing STAT3 activation for treatment of cancers, which has led to the development of a wide variety of STAT3 inhibitors, but only a handful have been tested in kidney disease models. Here, we review the detrimental role of dysregulated STAT3 activation in a variety of kidney diseases and the current progress in the treatment of kidney diseases with pharmacological inhibition of STAT3 activity.

PI3k and Stat3: Oncogenes that are Required for Gap Junctional, Intercellular Communication

Gap junctional, intercellular communication (GJIC) is interrupted in cells transformed by oncogenes such as activated Src. The Src effector, Ras, is required for this effect, so that Ras inhibition restores GJIC in Src-transformed cells. Interestingly, the inhibition of the Src effector phosphatidyl-inositol-3 kinase (PI3k) or Signal Transducer and Activator of Transcription-3 (Stat3) pathways does not restore GJIC. In the contrary, inhibition of PI3k or Stat3 in non-transformed rodent fibroblasts or epithelial cells or certain human lung carcinoma lines with extensive GJIC inhibits communication, while mutational activation of PI3k or Stat3 increases GJIC. Therefore, it appears that oncogenes such as activated Src have a dual role upon GJIC; acting as inhibitors of communication through the Ras pathway, and as activators through activation of PI3k or Stat3. In the presence of high Src activity the inhibitory functions prevail so that the net effect is gap junction closure. PI3k and Stat3 constitute potent survival signals, so that their inhibition in non-transformed cells triggers apoptosis which, in turn, has been independently demonstrated to suppress GJIC. The interruption of gap junctional communication would confine the apoptotic event to single cells and this might be essential for the maintenance of tissue integrity. We hypothesize that the GJIC activation by PI3k or Stat3 may be linked to their survival function.

The role of STAT3 in leading the crosstalk between human cancers and the immune system

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.

Regulation of HC11 mouse breast epithelial cell differentiation by the E-cadherin/Rac axis

It was previously demonstrated that differentiation of some established breast epithelial cell lines requires confluence and stimulation with hydrocortisone, insulin and prolactin inducers. We and others previously demonstrated that E-cadherin engagement, which is favored under conditions of confluence, increases the levels and activity of the Rac small GTPase. To investigate the functional relationship between the transforming ability of Rac and its role as an integral component of the differentiative E-cadherin signaling pathway, we introduced a mutationally activated form of Rac, Rac^V12, into the mouse breast epithelium-derived cell line, HC11. Our results demonstrate that the strength of the Rac signal is key for the outcome of the differentiation process; cRac1 is critically required for differentiation, and at low levels, mutationally activated Rac^V12 is able to increase differentiation, presumably reinforcing the E-cadherin/Rac differentiative signal. However, high Rac^V12 expression blocked differentiation concomitant with E-cadherin downregulation, while inducing neoplastic transformation. Therefore, the intensity of the Rac signal is a central determinant in the balance between cell proliferation vs differentiation, two fundamentally opposed processes, a finding which could also have important therapeutic implications.

Direct inhibition of STAT signaling by platinum drugs contributes to their anti-cancer activity

Platinum-based chemotherapeutics are amongst the most powerful anti-cancer drugs. Although their exact mechanism of action is not well understood, it is thought to be mediated through covalent DNA binding. We investigated the effect of platinum-based chemotherapeutics on signaling through signal transducer and activator of transcription (STAT) proteins, which are involved in many oncogenic signaling pathways.

We performed in vitro experiments in various cancer cell lines, investigating the effects of platinum chemotherapeutics on STAT phosphorylation and nuclear translocation, the expression of STAT-modulating proteins and downstream signaling pathways. Direct binding of platinum to STAT proteins was assessed using an AlphaScreen assay. Nuclear STAT3 expression was determined by immunohistochemistry and correlated with disease-free survival in retrospective cohorts of head and neck squamous cell carcinoma (HNSCC) patients treated with cisplatin-based chemoradiotherapy (n= 65) or with radiotherapy alone (n = 32).

At clinically relevant concentrations, platinum compounds inhibited STAT phosphorylation, resulting in loss of constitutively activated STAT proteins in multiple distinct cancer cell lines. Platinum drugs specifically inhibited phospho-tyrosine binding to SH2 domains, thereby blocking STAT activation, and subsequently downregulating pro-survival- and anti-apoptotic- target genes. Importantly, we found that active STAT3 in tumors directly correlated with response to cisplatin-based chemoradiotherapy in HNSCC patients (p = 0.006).

These findings provide insight into a novel, non-DNA-targeted mechanism of action of platinum drugs, and could be leveraged into the use of STAT expression as predictive biomarker for cisplatin chemotherapy and to potentiate other therapeutic strategies such as immunotherapy.

Radiotherapy in the age of cancer immunology: Current concepts and future developments

Major advances in the knowledge of cancer biology and its interactions with tumor immune environment led to the emergence, in the last five years of new immunotherapy-based treatment strategies in cancer patients. At the same time, improvement in radiation technique and progress in radiobiology allowed in the last decade to expand the applications of radiotherapy in a growing number of settings. At present, there are strong theoretical basis to propose immune-enhanced radiation therapy that may represent in the future a new paradigm of treatment, combining the intrinsic power of radiotherapy to elicit a specific, systemic, tumor-directed immune response with modern highly conformal and precise dose delivery, in order to maximize response at the major site of disease and obtain durable disease control. The aim of this review is to describe the principal mechanisms of immune modulation of response to radiation and investigational strategies to harness the potential of radiation-inducible immune response: radiation therapy is expected to be not just a local treatment but the cornerstone of a multimodal strategy that might achieve long-lasting tumor remission at the primary site and systemic efficacy metastatic lesions.

An oxidative stress-based mechanism of doxorubicin cytotoxicity suggests new therapeutic strategies in ABC-DLBCL

Diffuse large B-cell lymphomas (DLBCLs) contain 2 major molecular subtypes; namely, the germinal center B-cell-like (GCB) and the activated B-cell-like (ABC) DLBCLs. It is well documented that ABC-DLBCL cases have a significantly poorer survival response than GCB-DLBCLs in both the CHOP (cyclophosphamide, vincristine, doxorubicin, and prednisone) and the rituximab (R)-CHOP eras. However, the underlying cause of this subtype disparity is poorly understood. Nevertheless, these clinical observations raise the possibility for an ABC-DLBCL-specific resistance mechanism that is directed toward 1 of the CHOP components and is inadequately addressed by rituximab. Here, we report that the main cytotoxic ingredient in CHOP, doxorubicin (Dox), has subtype-specific mechanisms of cytotoxicity in DLBCLs resulting from differences in the subcellular distribution pattern. Specifically, in cell line models of ABC-DLBCL, Dox is often enriched in the cytoplasm away from the nuclear DNA. As a result, Dox-induced cytotoxicity in ABC-DLBCLs is often dependent on oxidative stress, rather than DNA damage response. These findings are corroborated by gene signature analysis, which demonstrates that basal oxidative stress status predicts treatment outcome among patients with ABC-DLBCL, but not patients with GCB-DLBCL. In terms of redox-related resistance mechanism, our results suggest that STAT3 confers Dox resistance in ABC-DLBCLs by reinforcing an antioxidant program featuring upregulation of the SOD2 gene. Furthermore, a small-molecule STAT3 inhibitor synergizes with CHOP to trigger oxidative stress and kill ABC-DLBCL cells in preclinical models. These results provide a mechanistic basis for development of novel therapies that target either STAT3 or redox homeostasis to improve treatment outcomes for ABC-DLBCLs.

Signal transducer and activator of transcription 3 as a therapeutic target for cancer and the tumor microenvironment

Signal transducer and activator of transcription 3 (STAT3) is a cytoplasmic transcription factor that modulates the transcription of a variety of genes to regulate important biological functions, including cell proliferation, differentiation, survival, angiogenesis, and immune response. Constitutive activation of STAT3 is important in oncogenic signaling and occurs at high frequency in human cancers, including diverse solid tumors and hematologic malignancies. Moreover, it is associated with a poor prognosis. The tumor microenvironment has recently been recognized as a key condition for cancer progression, invasion, angiogenesis, metastasis, and drug resistance by activation of STAT3 signaling. Therefore, understanding the biology associated with STAT3-mediated signaling cascades in the tumor microenvironment may offer the therapeutic potential to treat human cancers. This review presents an overview of the critical roles of STAT3 in the tumor microenvironment related to cancer biology and discusses recent advancements in the development of anticancer drugs that therapeutically inhibit STAT3 signaling cascades.

CPA-7 influences immune profile and elicits anti-prostate cancer effects by inhibiting activated STAT3

BACKGROUND

Platinum-based chemotherapy is emerging as the first line of treatment for castration resistant prostate cancer. Among the family of platinum (IV)-based compounds, a member known as CPA-7 inhibits the growth of multiple cancer cell lines. However, how and to what extent CPA-7 elicits its anti-prostate cancer effects in vivo is largely unknown.

METHODS

In this study, we firstly assessed the potential toxicity of the synthesized CPA-7 in a prostate cancer model as well as in normal mice. Next, we evaluated the in vitro effects of CPA-7 on the growth of prostate cancer cells using cell counting assay, and calculated the tumor sizes and cumulative survival rate of the tumor bearing mice by Kaplan-Meier method during CPA-7 treatment. Then we measured the expression level of the activated form of STAT3 (one targets of CPA-7) and its transcriptive activity post CPA-7 treatment by synergistically using western blot, IHC, and firefly luciferase reporter assays. Finally, effects of CPA-7 on immune cell trafficking in the tumor draining lymph nodes and in the spleens are evaluated with flow cytometry.

RESULTS

Treatment with CPA-7 significantly inhibited growth of prostate cancer cells in vitro, and also in mice resulting in a prolonged survival and a decreased recurrence rate. These therapeutic effects are due, at least in part, to functional depletion of STAT3 in prostate tumor tissue as well as in the surrounding areas of tumor cell invasion. CPA-7 treatment also resulted in a reduced level of regulatory T cells and increased levels of cytotoxic T and T helper cells in the spleen and in tumor infiltrating lymph nodes. This favorable effect on immune cell trafficking may account for the amnestic immune response against recurrent prostate cancer.

CONCLUSIONS

CPA-7 is a promising new therapeutic agent for prostate cancer that both inhibits tumor cell proliferation and stimulates anti-tumor immunity. It has potential as first line treatment and/or as an adjuvant for refractory prostate cancer.

Hydroxamic Acid and Benzoic Acid-Based STAT3 Inhibitors Suppress Human Glioma and Breast Cancer Phenotypes In Vitro and In Vivo

STAT3 offers an attractive target for cancer therapy, but small-molecule inhibitors with appealing pharmacologic properties have been elusive. Here, we report hydroxamic acid-based and benzoic acid-based inhibitors (SH5-07 and SH4-54, respectively) with robust bioactivity. Both inhibitors blocked STAT3 DNA-binding activity in vitro and in human glioma, breast, and prostate cancer cells and in v-Src-transformed murine fibroblasts. STAT3-dependent gene transcription was blocked along with Bcl-2, Bcl-xL, Mcl-1, cyclin D1, c-Myc, and survivin expression. Nuclear magnetic resonance analysis of STAT3-inhibitor complexes defined interactions with the SH2 and DNA-binding domains of STAT3. Ectopic expression of the SH2 domain in cells was sufficient to counter the STAT3-inhibitory effects of SH4-54. Neither compound appreciably affected STAT1 or STAT5 DNA-binding activities, STAT3-independent gene transcription, or activation of a panel of oncogenic kinases in malignant cells. Each compound decreased the proliferation and viability of glioma, breast, and prostate cancer cells and v-Src-transformed murine fibroblasts harboring constitutively active STAT3. Further, in mouse xenograft models of glioma and breast cancer, administration of SH5-07 or SH4-54 effectively inhibited tumor growth. Our results offer preclinical proof of concept for SH5-07 and SH4-54 as candidates for further development as cancer therapeutics.

Bioactive sesquiterpene lactones and other compounds isolated from Vernonia cinerea

Four new sesquiterpene lactones, 8α-(2'Z-tigloyloxy)-hirsutinolide (1), 8α-(2'Z-tigloyloxy)-hirsutinolide-13-O-acetate (2), 8α-(4-hydroxytigloyloxy)-hirsutinolide (3), and 8α-hydroxy-13-O-tigloyl-hirsutinolide (4), along with seven known derivatives (5-11), three norisoprenoids (12-14), a flavonoid (15), and a linoleic acid derivative (16), were isolated from the chloroform partition of a methanol extract from the combined leaves and stems of Vernonia cinerea. Their structures were established by 1D and 2D NMR, UV, and MS analyses. Compounds 1-16 were evaluated for their inhibitory effects against the viability of U251MG glioblastoma and MDA-MB-231 breast cancer cells that harbour aberrantly-active STAT3, compared to normal NIH3T3 mouse fibroblasts that show no evidence of activated STAT3. Among the isolates, compounds 2 and 7 inhibited the aberrant STAT3 activity in glioblastoma or breast cancer cells. Further, compounds 7 and 8 inhibited viability of all three cell lines, compounds 2, 4, and 9 predominantly inhibited the viability of the U251MG glioblastoma cell line.

The high-mobility group A1a/signal transducer and activator of transcription-3 axis: an achilles heel for hematopoietic malignancies?

Although HMGA1 (high-mobility group A1; formerly HMG-I/Y) is an oncogene that is widely overexpressed in aggressive cancers, the molecular mechanisms underlying transformation by HMGA1 are only beginning to emerge. HMGA1 encodes the HMGA1a and HMGA1b protein isoforms, which function in regulating gene expression. To determine how HMGA1 leads to neoplastic transformation, we looked for genes regulated by HMGA1 using gene expression profile analysis. Here, we show that the STAT3 gene, which encodes the signaling molecule signal transducer and activator of transcription 3 (STAT3), is a critical downstream target of HMGA1a. STAT3 mRNA and protein are up-regulated in fibroblasts overexpressing HMGA1a and activated STAT3 recapitulates the transforming activity of HMGA1a in fibroblasts. HMGA1a also binds directly to a conserved region of the STAT3 promoter in vivo in human leukemia cells by chromatin immunoprecipitation and activates transcription of the STAT3 promoter in transfection experiments. To determine if this pathway contributes to HMGA1-mediated transformation, we investigated STAT3 expression in our HMGA1a transgenic mice, all of which developed aggressive lymphoid malignancy. STAT3 expression was increased in the leukemia cells from our transgenics but not in control cells. Blocking STAT3 function induced apoptosis in the transgenic leukemia cells but not in controls. In primary human leukemia samples, there was a positive correlation between HMGA1a and STAT3 mRNA. Moreover, blocking STAT3 function in human leukemia or lymphoma cells led to decreased cellular motility and foci formation. Our results show that the HMGA1a-STAT3 axis is a potential Achilles heel that could be exploited therapeutically in hematopoietic and other malignancies overexpressing HMGA1a.

Constitutively activated STAT3 frequently coexpresses with epidermal growth factor receptor in high-grade gliomas and targeting STAT3 sensitizes them to Iressa and alkylators

PURPOSE

The goals of this study are to elucidate the relationship of the oncogenic transcription factor signal transducer and activator of transcription 3 (STAT3) with glioma aggressiveness and to understand the role of high STAT3 activity in the resistance of malignant gliomas and medulloblastomas to chemotherapy.

EXPERIMENTAL DESIGN

Immunohistochemical staining and biochemical methods were used to examine the extent of STAT3 activation and EGFR expression in primary specimens and cell lines, respectively. Cellular response to drug treatments was determined using cell cytotoxicity and clonogenic growth assays.

RESULTS

We found STAT3 to be constitutively activated in 60% of primary high-grade/malignant gliomas and the extent of activation correlated positively with glioma grade. High levels of activated/phosphorylated STAT3 were also present in cultured human malignant glioma and medulloblastoma cells. Three STAT3-activating kinases, Janus-activated kinase 2 (JAK2), EGFR, and EGFRvIII, contributed to STAT3 activation. An inhibitor to JAK2/STAT3, JSI-124, significantly reduced expression of STAT3 target genes, suppressed cancer cell growth, and induced apoptosis. Furthermore, we found that STAT3 constitutive activation coexisted with EGFR expression in 27.2% of primary high-grade/malignant gliomas and such coexpression correlated positively with glioma grade. Combination of an anti-EGFR agent Iressa and a JAK2/STAT3 inhibitor synergistically suppressed STAT3 activation and potently killed glioblastoma cell lines that expressed EGFR or EGFRvIII. JSI-124 also sensitized malignant glioma and medulloblastoma cells to temozolomide, 1,3-bis(2-chloroethyl)-1-nitrosourea, and cisplatin in which a synergism existed between JSI-124 and cisplatin.

CONCLUSION

STAT3 constitutive activation, alone and in concurrence with EGFR expression, plays an important role in high-grade/malignant gliomas and targeting STAT3/JAK2 sensitizes these tumors to anti-EGFR and alkylating agents.

Targeting the cytoplasmic and nuclear functions of signal transducers and activators of transcription 3 for cancer therapy

Signal transducers and activators of transcription (STAT) are a highly conserved family of transcription factors that are activated by phosphorylation in the cytoplasm, after which they translocate to the nucleus to regulate gene expression. Among the seven STATs, STAT3 is of particular interest due to its constitutive phosphorylation in a large proportion of human cancers and its ability to induce neoplastic transformation. Inhibition of STAT3 can reverse tumor growth in experimental systems while having few effects in normal cells. These findings have implicated STAT3 as a potentially important target for therapeutic intervention. In addition to its well-described role as a transcription factor, STAT3 has been found recently to have important effects in the cytoplasm. Collectively, these functions of STAT3 directly contribute to tumorigenesis, invasion, and metastasis. Given the potential importance of STAT3 as a target for cancer therapy, molecules have been developed that can block STAT3 function at a variety of steps. These drugs show promise as anticancer agents in model systems of a variety of common human cancers. Thus, elucidating the functions of STAT3 and developing agents to inhibit this protein remain important scientific and clinical challenges.

Abrogation of signal transducer and activator of transcription 3 reactivation after Src kinase inhibition results in synergistic antitumor effects

PURPOSE

The Src family of kinases (SFKs) regulate multiple signal transduction cascades and influence proliferation, motility, survival, and angiogenesis. Dasatinib inhibits SFKs, which leads to cytotoxicity, cell cycle arrest, apoptosis, and decreased invasion of cancer cells. Signal transducer and activator of transcription 3 (STAT3) is a latent transcription factor that regulates survival and proliferation. Dasatinib results in rapid and durable inhibition of c-Src, whereas STAT3 undergoes only transient inactivation. We hypothesized that the reactivation of STAT3 after dasatinib treatment represents the engagement of a compensatory signal for cell survival that blocks the antitumor effects of SFK inhibition.

EXPERIMENTAL DESIGN

The effects of upstream inhibitors on STAT3 activation were assessed with western blotting and a quantitative bioplex phosphoprotein assay. We used the 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay to determine the cytotoxicity and propidium iodine/annexin V staining with fluorescence-activated cell sorting (FACS) analysis to evaluate cell cycle change and apoptosis. The combination index was calculated by the Chou-Talalay equation. Cytokines were quantitated using a multiplexed, particle-based FACS analysis.

RESULTS

C-Src and several downstream molecules were rapidly and durably inhibited by dasatinib. However, STAT3 was reactivated by 24 h. The addition of JAK inhibitors during dasatinib incubation resulted in sustained inhibition of STAT3, although JAK activation by dasatinib was not shown. Combined SFK and JAK inhibition resulted in synergistic cytotoxicity due to increased apoptosis.

CONCLUSIONS

The reactivation of STAT3 during dasatinib treatment is caused by the engagement of a compensatory pathway that suppresses the antitumor effects of SFK inhibition and allows cancer cell survival. Abrogation of this pathway resulted in synergistic cytotoxicity. Given that STAT3 reactivation occurred in 14 of 15 solid tumor cell lines, dasatinib combined with Janus-activated kinase inhibitors may have widespread application in cancer treatment.

Regulation of metastases by signal transducer and activator of transcription 3 signaling pathway: clinical implications

Numerous cytokines, growth factors, and oncogenic proteins activate signal transducer and activator of transcription 3 (Stat3), which has been recognized as one of the common pathways in cancer cells. Stat3 signaling affects the expression and function of a variety of genes that are critical to cell survival, cell proliferation, invasion, angiogenesis, and immune evasion. Evidently, the Stat3 signaling pathway regulates cancer metastasis and constitutes a potential preventive and therapeutic target for cancer metastasis.

SD-1029 inhibits signal transducer and activator of transcription 3 nuclear translocation

PURPOSE

Signal transducer and activator of transcription 3 (Stat3) proteins have important roles in cancer cell survival and proliferation. Recent studies show that aberrant Stat3 activation promotes tumor growth and survival in several human cancers, and thus, presents an attractive pathway for the development of targeted anticancer therapy. Stat3 is a DNA-binding transcription factor, and thus, its function depends on cytoplasmic to nuclear translocation. To discover novel inhibitors of the Stat3 signaling pathway, we designed a cell-based screening assay capable of identifying compounds that inhibit Stat3 nuclear translocation and activity.

EXPERIMENTAL DESIGN

Cell-based fluorescence microscope screening and quantitative measurement of enhanced green fluorescent protein-Stat3 nuclear translocation assays were used to identify novel Stat3 inhibitors. The effects of identified Stat3 inhibitors on Janus kinase (Jak), Stat3 expression, and activation were determined by Western blotting and kinase in vitro autophosphorylation assay. The effects of identified Stat3 inhibitors on cell growth was evaluated by cell proliferation assay and apoptosis assay.

RESULTS

Among the National Cancer Institute Diversity set, a 2,000-member library of bioactive small molecules, we identified SD-1029 as a micromolar inhibitor of IL-6 or oncostatin-induced Stat3 nuclear translocation. Biochemical analysis shows that SD-1029 inhibits tyrosyl phosphorylation of Stat3 implicating SD-1029 as an inhibitor of Jak. Further analysis shows that this compound inhibits tyrosyl phosphorylation of the Jak2 isoenzyme. The antiapoptotic proteins Bcl-X(L) and survivin, target proteins of activated Stat3, are down-regulated by SD-1029 resulting in the induction of apoptosis in several human breast and ovarian cancer cell lines. SD-1029 also enhances apoptosis induced by paclitaxel in ovarian cancer cells.

CONCLUSIONS

These results show that SD-1029 directly abrogates the Jak-Stat3 signaling pathway in human cancer cells expressing constitutively active Stat, and add to the growing literature that validates this pathway as a viable target for further drug development. Finally, SD-1029 may represent a suitable prototype for structural optimization and exploration as a therapeutic lead.

Resveratrol inhibits Src and Stat3 signaling and induces the apoptosis of malignant cells containing activated Stat3 protein

Resveratrol is a naturally occurring phytoalexin with antioxidant and antiinflammatory properties. Recent studies suggest that resveratrol possesses anticancer effects, although its mechanism of action is not well understood. We now show that resveratrol inhibits Src tyrosine kinase activity and thereby blocks constitutive signal transducer and activator of transcription 3 (Stat3) protein activation in malignant cells. Analyses of resveratrol-treated malignant cells harboring constitutively-active Stat3 reveal irreversible cell cycle arrest of v-Src-transformed mouse fibroblasts (NIH3T3/v-Src), human breast (MDA-MB-231), pancreatic (Panc-1), and prostate carcinoma (DU145) cell lines at the G0-G1 phase or at the S phase of human breast cancer (MDA-MB-468) and pancreatic cancer (Colo-357) cells, and loss of viability due to apoptosis. By contrast, cells treated with resveratrol, but lacking aberrant Stat3 activity, show reversible growth arrest and minimal loss of viability. Moreover, in malignant cells harboring constitutively-active Stat3, including human prostate cancer DU145 cells and v-Src-transformed mouse fibroblasts (NIH3T3/v-Src), resveratrol treatment represses Stat3-regulated cyclin D1 as well as Bcl-xL and Mcl-1 genes, suggesting that the antitumor cell activity of resveratrol is in part due to the blockade of Stat3-mediated dysregulation of growth and survival pathways. Our study is among the first to identify Src-Stat3 signaling as a target of resveratrol, further defining the mechanism of antitumor cell activity of resveratrol and raising its potential application in tumors with an activated Stat3 profile.

Persistent activation of stat3 signaling induces survivin gene expression and confers resistance to apoptosis in human breast cancer cells

PURPOSE

Signal transducer and activator of transcription 3 (Stat3) protein is persistently activated in breast cancer and promotes tumor cell survival. To gain a better understanding of the role of constitutive Stat3 signaling in breast cancer progression, we evaluated the expression profile of potential Stat3-regulated genes that may confer resistance to apoptosis.

EXPERIMENTAL DESIGN

Stat3 signaling was blocked with antisense oligonucleotides in human MDA-MB-435s breast cancer cells and Affymetrix GeneChip microarray analysis was done. The candidate Stat3 target gene Survivin was further evaluated in molecular assays using cultured breast cancer cells and immunohistochemistry of breast tumor specimens.

RESULTS

Survivin, a member of the inhibitor of apoptosis protein family, was identified as a potential Stat3-regulated gene by microarray analysis. This was confirmed in Survivin gene promoter studies and chromatin immunoprecipitation assays showing that Stat3 directly binds to and regulates the Survivin promoter. Furthermore, direct inhibition of Stat3 signaling blocked the expression of Survivin protein and induced apoptosis in breast cancer cells. Direct inhibition of Survivin expression also induced apoptosis. Increased Survivin protein expression correlates significantly (P = 0.001) with elevated Stat3 activity in primary breast tumor specimens from high-risk patients who were resistant to chemotherapy treatment.

CONCLUSIONS

We identify Survivin as a direct downstream target gene of Stat3 in human breast cancer cells that is critical for their survival in culture. Our findings suggest that activated Stat3 signaling contributes to breast cancer progression and resistance to chemotherapy by, at least in part, inducing expression of the antiapoptotic protein, Survivin.