Abstract LB-354: A novel orally-bioavailable small-molecule inhibitor of Stat3 regresses human breast and lung cancer xenografts

Constitutive activation of STAT3, a member of the Signal Transducer and Activator of Transcription (STAT) family of proteins, occurs with a high frequency in diverse human tumors and is a molecular abnormality promoting carcinogenesis and tumor progression. Although STAT3 has been targeted for anticancer drug development, there is presently no STAT3 drug in the clinic. To identify potent therapeutics for tumors harboring constitutively-active STAT3, the key dimerization step in STAT3 activation was targeted in small-molecule drug development. A novel, orally bio-available small-molecule inhibitor has been identified that binds STAT3 SH2 domain with a KD of 504 nM. BP-1-102 blocks constitutively-active STAT3 signaling and selectively inhibits growth, survival, migration and invasion of diverse STAT3-dependent tumor cells in vitro. BP-1-102-mediated inhibition of aberrantly-active STAT3 in tumor cells suppresses the expression of c-Myc, Cyclin D1, Bcl-xL, Survivin, and vascular endothelial growth factor (VEGF). Further, treatment with BP-1-102 of breast cancer cells harboring aberrantly-active STAT3 disrupts STAT3-NFκB cross-talk and blocks the phosphorylation of focal adhesion kinase and paxillin, while enhancing E-cadherin expression. Intravenous or oral administration of BP-1-102 furnishes micromolar or microgram levels in tumor tissues and inhibits growth of human breast and lung tumor xenografts. Data together identify BP-1-102 as a small-molecule STAT3 inhibitor with a great potential for clinical development. The oral bioavailability represents a substantial advancement in the discovery of small-molecule STAT3 inhibitors as novel anticancer drugs.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr LB-354. doi:1538-7445.AM2012-LB-354

Abstract LB-346: EGF receptor, Jaks and Stat3 crosstalk promote enhanced colony-forming, motility, migration, and invasive phenotype of cisplatin-resistant ovarian cancer cells

Majority of the ovarian cancer patients who initially respond to cisplatin therapy develop recurrent and resistant disease that is mechanistically not well understood. To define the biological presentations and the molecular phenotype of cisplatin resistant ovarian cancer, we subjected the sensitive A2780S ovarian cancer line to multiple rounds of cisplatin treatments, followed by recovery and derived the lines, A2780S/CP1 (S/CP1), A2780S/CP3 (S/CP3), and A2780S/CP5 (S/CP5), which are resistant to 1, 3, and 5 μM cisplatin, respectively. In vitro studies show the three resistant lines have increased colony-forming ability and altered morphology consistent with enhanced motility, migration, and invasiveness. The malignant phenotype progresses with increasing resistance and is associated with hyperactive epidermal growth factor receptor (EGFR)/extracellular signal-regulated kinase 1/2 and Janus kinases (Jaks), and aberrant Signal Transducer and Activator of Transcription (Stat) 3 activation. Elevated expression of survivin and vascular endothelial growth factor (VEGF), and enhanced matrix metalloproteinase activities are evident in the resistant cells, together with increased vimentin and snail expression, and E-cadherin downregulation, indicative of epithelial-mesenchymal transition (EMT). The inhibition of EGFR or Stat3 activity in the resistant cells repressed the enhanced colony-forming, motility, and migratory phenotype, downregulated survivin, VEGF and vimentin expression, and sensitized the resistant cells to cisplatin. In vivo studies using intra-peritoneal xenograft models in nude mice revealed the resistant lines have a high tumor incidence and formed small, cisplatin-resistant tumor nodules at several locations on the small-intestine and colon, compared to the sensitive A2780S line that exhibited low tumor incidence and formed exclusively ovarian tumors. Treatment with EGFR or Stat3 inhibitor sensitized the resistant tumor nodules to cisplatin. Studies together indicate hyperactive EGFR and Jak signaling predominantly through the induction of aberrantly-active Stat3 promote EMT during cisplatin resistance development. Collectively, these events serve as critical mediators of the invasive phenotype associated with cisplatin resistant ovarian cancer and hence, are targets for preventing the development of resistance and the recurrent disease during cisplatin therapy in ovarian cancer.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr LB-346. doi:1538-7445.AM2012-LB-346

Small molecule STAT5-SH2 domain inhibitors exhibit potent antileukemia activity

A growing body of evidence shows that Signal Transducer and Activator of Transcription 5 (STAT5) protein, a key member of the STAT family of signaling proteins, plays a pivotal role in the progression of many human cancers, including acute myeloid leukemia and prostate cancer. Unlike STAT3, where significant medicinal effort has been expended to identify potent direct inhibitors, Stat5 has been poorly investigated as a molecular therapeutic target. Thus, in an effort to identify direct inhibitors of STAT5 protein, we conducted an in vitro screen of a focused library of SH2 domain binding salicylic acid-containing inhibitors (∼150) against STAT5, as well as against STAT3 and STAT1 proteins for SH2 domain selectivity. We herein report the identification of several potent (K(i) < 5 μM) and STAT5 selective (>3-fold specificity for STAT5 cf. STAT1 and STAT3) inhibitors, BP-1-107, BP-1-108, SF-1-087, and SF-1-088. Lead agents, evaluated in K562 and MV-4-11 human leukemia cells, showed potent induction of apoptosis (IC(50)'s ∼ 20 μM) which correlated with potent and selective suppression of STAT5 phosphorylation, as well as inhibition of STAT5 target genes cyclin D1, cyclin D2, C-MYC, and MCL-1. Moreover, lead agent BP-1-108 showed negligible cytotoxic effects in normal bone marrow cells not expressing activated STAT5 protein. Inhibitors identified in this study represent some of the most potent direct small molecule, nonphosphorylated inhibitors of STAT5 to date.

Abstract A121: Direct SH2 domain-targeting inhibitors of Stat3: Potent anticancer agents and mitigators of drug resistance

As a master regulator of cell signaling and tumorigenesis, signal transducer and activator of transcription 3 (Stat3) protein has emerged at the forefront of anti-cancer drug development. Abnormal Stat3 activity has been demonstrated in a wide variety of human cancers including leukemia, lymphoma, multiple myeloma, glioblastoma and cancers of the pancreas, breast, prostate, and ovary. Constitutive Stat3 activation interferes with normal cell cycling and causes the accumulation of anti-apoptotic proteins. This renders malignant cells resistant to naturally occurring apoptotic cues and allows them subject to proliferate rapidly. Cancer cells become reliant on increased levels of Stat3 activity are vulnerable to therapeutic intervention through Stat3 inhibition. In healthy cells, Stat3 activity is transient and non-essential, thus, inhibiting Stat3 presents an avenue for the development of novel cancer therapeutic agents.

Our approach involves the interruption of several critical Stat3 functions by occupation of the SH2 domain with small molecule inhibitors. Stat3's SH2 domain is a key component in the Stat3 signaling pathway as it not only facilitates activation of monomeric Stat3 but also moderates the formation of the transcriptionally active Stat3:Stat3 homodimer.

Our research groups have conducted a thorough structure-activity relationship on a known Stat3-SH2 domain binder (S3I-201) and have discovered several more potent and more drug-like Stat3 inhibitors. Most recently, we have utilized a tetrapodal scaffold that has allowed more complete occupation of the SH2 domain and resulted in greatly improved binding affinity. These novel compounds effectively displace an SH2 domain-binding peptide probe, prevent Stat3 phosphorylation in cell line models and suppress Stat3 target gene expression at near-nanomolar concentrations. Our latest Stat3 inhibitors are effective across a wide variety of human cancers and exhibit a 10–20-fold improvement in cellular EC50 values over the parent compound, S3I-201.

Remarkable activity is observed in mouse xenograft models of human breast cancer where nearly complete inhibition of tumor growth is observed at a dosing of 3 mg/kg daily. Furthermore, recent experiments demonstrate the same potent activity when the drug is administered by oral gavage, with plasma drug concentrations reaching 20 μM. Preliminary investigations have also shown that our lead compounds can re-sensitize malignant cells that are resistant to conventional chemotherapeutics agents.

We present our newest library of Stat3 inhibitors, which holds great promise in the fight against cancer.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference: Molecular Targets and Cancer Therapeutics; 2011 Nov 12-16; San Francisco, CA. Philadelphia (PA): AACR; Mol Cancer Ther 2011;10(11 Suppl):Abstract nr A121.

Abstract B220: Artificially inducing protein-membrane anchorage: Introducing a new therapeutic modality

We aim to develop an innovative therapeutic modality of inhibiting aberrant protein function through suppression of activation and/or nuclear translocation. Nature has developed prenylation, a post-translational modification that covalently attaches a hydrophobic prenyl group to a protein to facilitate protein-membrane association to the plasma membrane. We hypothesize that mimicking Nature by artificially inducing protein-membrane anchorage through the use of a rationally designed protein-membrane anchor (PMA), we can simultaneously inhibit the activation and nuclear translocation of oncogenic proteins. Our aim is to explore the therapeutic potential of the protein-membrane anchor and potentially develop a novel drug modality that can be utilized by cancer patients.

Our proof-of-concept PMA was design to target signal transducer and activator of transcription 3 (Stat3) protein.

Constitutively-active Stat3 directly contributes to the progression of cancer and is present in numerous human cancers. A number of studies have shown that down-regulation of this oncogene via iRNA knockdown induces cellular apoptosis. Thus, Stat3 is an attractive target for the development of potent anti-cancer therapeutics for cancer.

Our proto-type PMA 1 was composed of two binding modules: a recognition motif to bind the protein and an anchor to sequester the protein complex to the membrane. The PMA was comprised of a potent Stat3 recognition sequence GpYLPQTV-NH2 covalently attached to a cholesterol membrane anchor.

We tested the ability of our PMA to anchor Stat3 to the cell membrane in MDA-MB-231 breast cancer cells which are known to have constitutively-active Stat3. We immunostained these cells with membrane stain FM-4–64 (red), anti-Stat3 antibody (green) and DAPI (nucleus, blue). In the absence of PMA 1, there was strong Stat3 nuclear presence. Most excitingly, in the presence of 25 μM concentration PMA 1, we observed complete sequestration of Stat3 to the cell membrane through PMA-Stat3 association. Currently, we are designing and synthesizing more drug-like, nonphosphorylated PMAs that are less prone to metabolic degradation. We will conduct further studies to determine the biochemical and biological utility of these PMAs.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference: Molecular Targets and Cancer Therapeutics; 2011 Nov 12-16; San Francisco, CA. Philadelphia (PA): AACR; Mol Cancer Ther 2011;10(11 Suppl):Abstract nr B220.

Abstract A125: Silencing Stat3 signaling in human cancers: Identifying potent small molecule inhibitors of Stat3 function

Stat3 is essential for transducing signals from extracellular stimuli, but also functions as a nuclear transcription factor required for regulating genes involved in proliferation, apoptosis, angiogenesis and invasion, in addition to genes encoding cytokines, chemokines and growth factors. In contrast to the transient nature of Stat3 activation in normal cells, many human cancers, including breast, prostate, ovarian, brain and multiple myeloma (MM) harbor constitutive Stat3 activity. Stat3 downstream target genes are critical to the dysregulated biological processes that promote tumor cell growth, survival and induce chemoresistance, thus targeting Stat3 signaling represents an important therapeutic target in cancer therapy.

We have rationally designed and developed Stat3 inhibitors that disrupt transcriptionaly active Stat3-Stat3 homo-dimers, suppress Stat3 activation (phosphorylation), inhibit Stat3-target gene expression (c-Myc, Bcl-xL, survivin) and potently induce apoptosis in tumor cells harboring aberrant Stat3 activity. Moreover, lead compound BP-1–102, a salicylic acid containing small molecule, induced strong antitumor effects on human breast cancer (MDA-MB-231) xenografts and in MM preclinical tumor models. Most notably, given via oral gavage, BP-1–102 strongly inhibited the growth of human breast tumor xenografts, identifying it as a most potent orally bioavailable Stat3-targeting inhibitor.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference: Molecular Targets and Cancer Therapeutics; 2011 Nov 12-16; San Francisco, CA. Philadelphia (PA): AACR; Mol Cancer Ther 2011;10(11 Suppl):Abstract nr A125.

Abstract A201: The potent STAT3/5 inhibitor, BP-1–102, demonstrates significant antitumor activity against Waldenstrom macroglobulinemia

The STAT family of cytoplasmic transcription factors function to transmit signals from external stimuli such as growth factors and cytokines. STAT activation is normally transient, however it has now been extensively demonstrated that several members of the STAT family, particularly STAT3 and STAT5, are aberrantly activated in a number of human cancers. STAT3 induces the expression of genes known to be involved in tumorigenesis, implicating its dysregulation in a number of hallmark oncogenic processes including tumor cell survival, proliferation, angiogenesis, and drug resistance. Aberrant STAT3 signaling is prevalent in hematologic malignancies including Waldenstrom Macroglobulinemia (WM), a rare form of B cell non-Hodgkin lymphoma. Analysis of the gene and protein expression profiles of WM patients suggests that aberrant STAT3 signaling plays a critical role in WM, providing rationale for the therapeutic use of STAT3 inhibitors.

We have demonstrated pre-clinical efficacy of a novel, highly specific and potent small molecule STAT3/5 inhibitor, BP-1–102, in both hyper-IgM secreting B cell lymphoma cell lines (Mec-1 and RL) and two WM cell lines (MWCL-1 and BCWM-1). BP-1–102 is a phosphotyrosine mimetic designed to directly bind the STAT3 SH2 domain (required for STAT3 phosphorylation, dimerization and nuclear localization), and therefore functions to suppress transcriptional activation of STAT3 target genes. We have confirmed that BP-1–102 directly interacts with STAT3's SH2 domain and is one of the most effective disruptors of STAT3 activity described to date. Treatment of cell lines with low M doses of BP-1–102 induced dose-dependent decreases in constitutive and cytokine-stimulated (IL6/IL10) STAT3 phosphorylation (pSTAT3) as well as pSTAT3 nuclear localization. We further compared the potency of BP-1–102 against STAT5 and the potentially pro-apoptotic STAT1 compared to STAT3 using phospho-flow cytometry. BP-1–102 effectively inhibited GM-CSF-induced pSTAT5 in AML2 cells at low doses (&lt; 12.5 M) but only weakly inhibited IFN -induced pSTAT1 in U937 leukemic cells at similar dose concentrations confirming the selectivity of BP-1–102 for STAT3/5. Using a STAT3 dependent luciferase reporter construct, we confirmed repression of STAT3 transcriptional activity which correlated with a dose-dependent decrease in the expression of STAT3 target genes (Mcl-1, Bcl-xL, Survivin and c-Myc). Inhibition of pSTAT3 resulted in decreased cell viability as assessed by MTT assay (IC50: 6–10μM). In addition, treatment of cells with BP-1–102 resulted in caspase-dependent apoptosis which correlated with caspase-3 and PARP cleavage. Interestingly, co-culture of Mec-1, RL and BCWM-1 with bone marrow stroma cells (BMSC) reduced the cytotoxicity of BP-1–102 suggesting BMSC-conferred resistance, while BP-1–102 was equally cytotoxic in MWCL-1 regardless of either culture condition. Preliminary investigation suggests that the efflux system, used by cells to extrude toxic substances and linked to drug resistance in cancer, may be responsible for the observed BMSC-mediate resistance to BP-1–102.

Collectively, these findings demonstrate a critical role for STAT3 signaling in the molecular pathology of WM and provide the rationale for further development of STAT3 inhibitors for the treatment of WM.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference: Molecular Targets and Cancer Therapeutics; 2011 Nov 12-16; San Francisco, CA. Philadelphia (PA): AACR; Mol Cancer Ther 2011;10(11 Suppl):Abstract nr A201.

An activatable multimodal/multifunctional nanoprobe for direct imaging of intracellular drug delivery

Multifunctional nanoparticles integrated with imaging modalities (such as magnetic resonance and optical) and therapeutic drugs are promising candidates for future cancer diagnostics and therapy. While targeted drug delivery and imaging of tumor cells have been the major focus in engineering nanoparticle probes, no extensive efforts have been made towards developing sensing probes that can confirm and monitor intracellular drug release events. Here, we present quantum dot (Qdot)-iron oxide (IO) based multimodal/multifunctional nanocomposite probe that is optically and magnetically imageable, targetable and capable of reporting on intracellular drug release events. Specifically, the probe consists of a superparamagnetic iron oxide nanoparticle core (IONP) decorated with satellite CdS:Mn/ZnS Qdots where the Qdots themselves are further functionalized with STAT3 inhibitor (an anti-cancer agent), vitamin folate (as targeting motif) and m-polyethylene glycol (mPEG, a hydrophilic dispersing agent). The Qdot luminescence is quenched in this nanocomposite probe ("OFF" state) due to combined electron/energy transfer mediated quenching processes involving IONP, folate and STAT3 agents. Upon intracellular uptake, the probe is exposed to the cytosolic glutathione (GSH) containing environment resulting in restoration of the Qdot luminescence ("ON" state), which reports on uptake and drug release. Probe functionality was validated using fluorescence and MR measurements as well as in vitro studies using cancer cells that overexpress folate receptors.

Antagonism of the Stat3-Stat3 protein dimer with salicylic acid based small molecules

More than 50 new inhibitors of the oncogenic Stat3 protein were identified through a structure-activity relationship (SAR) study based on the previously identified inhibitor S3I-201 (IC₅₀ =86 μM, K(i) >300 μM). A key structural feature of these inhibitors is a salicylic acid moiety, which, by acting as a phosphotyrosine mimetic, is believed to facilitate binding to the Stat3 SH2 domain. Several of the analogues exhibit higher potency than the lead compound in inhibiting Stat3 DNA binding activity, with an in vitro IC₅₀ range of 18.7-51.9 μM, and disruption of Stat3-pTyr peptide interactions with K(i) values in the 15.5-41 μM range. One agent in particular exhibited potent inhibition of Stat3 phosphorylation in both breast and multiple myeloma tumor cells, suppressed the expression of Stat3 target genes, and induced antitumor effects in tumor cells harboring activated Stat3 protein.

The R(h)oads to Stat3: Stat3 activation by the Rho GTPases

The signal transducer and activator of transcription-3 (Stat3) is a member of the STAT family of cytoplasmic transcription factors. Overactivation of Stat3 is detected with high frequency in human cancer and is considered a molecular abnormality that supports the tumor phenotype. Despite concerted investigative efforts, the molecular mechanisms leading to the aberrant Stat3 activation and Stat3-mediated transformation and tumorigenesis are still not clearly defined. Recent evidence reveals a crosstalk close relationship between Stat3 signaling and members of the Rho family of small GTPases, including Rac1, Cdc42 and RhoA. Specifically, Rac1, acting in a complex with the MgcRacGAP (male germ cell RacGAP), promotes tyrosine phosphorylation of Stat3 by the IL6-receptor family/Jak kinase complex, as well as its translocation to the nucleus. Studies have further revealed that the mutational activation of Rac1 and Cdc42 results in Stat3 activation, which occurs in part through the upregulation of IL6 family cytokines that in turn stimulates Stat3 through the Jak kinases. Interestingly, evidence also shows that the engagement of cadherins, cell to cell adhesion molecules, specifically induces a striking increase in Rac1 and Cdc42 protein levels and activity, which in turn results in Stat3 activation. In this review we integrate recent findings clarifying the role of the Rho family GTPases in Stat3 activation in the context of malignant progression.

A functional nuclear epidermal growth factor receptor, SRC and Stat3 heteromeric complex in pancreatic cancer cells

Evidence is presented for the nuclear presence of a functional heteromeric complex of epidermal growth factor (EGFR), Src and the Signal Transducer and Activator of Transcription (Stat)3 proteins in pancreatic cancer cells. Stat3 remains nuclear and associated with Src or EGFR, respectively, upon the siRNA knockdown of EGFR or Src, demonstrating the resistance of the complex to the modulation of EGFR or Src alone. Significantly, chromatin immunoprecipitation (ChIP) analyses reveal the nuclear EGFR, Src and Stat3 complex is bound to the c-Myc promoter. The siRNA knockdown of EGFR or Src, or the pharmacological inhibition of Stat3 activity only marginally suppressed c-Myc expression. By contrast, the concurrent modulation of Stat3 and EGFR, or Stat3 and Src, or EGFR and Src strongly suppressed c-Myc expression, demonstrating that the novel nuclear heteromeric complex intricately regulates the c-Myc gene. The prevalence of the transcriptionally functional EGFR, Src, and Stat3 nuclear complex provides an additional and novel mechanism for supporting the pancreatic cancer phenotype and explains in part the insensitivity of pancreatic cancer cells to the inhibition of EGFR, Src or Stat3 alone.

Abstract 2031: A functional nuclear epidermal growth factor receptor, Src and Stat3 heteromeric complex in pancreatic cancer cells

Pancreatic cancer is an extremely lethal malignancy whose prognosis has not improved appreciably in the last decade. The disease is usually diagnosed only after it has reached an advanced or metastatic state but even local, surgically resectable, tumors have a fairly poor prognosis. This is further complicated by the fact that little is known about the underlying mechanisms of pancreatic cancer, as evidenced by a lack of any efficacious targeted therapies. While hyperactive epidermal growth factor receptor (EGFR), Src tyrosine kinase and Signal Transducer and Activator of Transcription (Stat)3 have been detected in pancreatic cancer and implicated in the disease, the exact mechanisms by which they promote the disease phenotype are not well understood. We present evidence of a functional heteromeric complex of EGFR, c-Src and Stat3 that is detected at the plasma membrane and cytoplasm, and inside the nucleus of pancreatic cancer cells. The inhibition of c-Src, but not EGFR kinase, reduced the level of nuclear EGFR levels. Furthermore, the inhibition of Src or EGFR kinase alone decreased nuclear Stat3 levels, while the greatest decrease occurred upon the combined inhibition of EGFR and Src kinase activities. Significantly, evidence shows that the nuclear EGFR, Src and Stat3 complex functions as a transcriptional complex. Thus, the siRNA knockdown of EGFR or Src, or the pharmacological inhibition of aberrant Stat3 activity only moderately decreased c-Myc, iNOS, and VEGF expression. By contrast, the knockdown of EGFR or Src together with the inhibition of Stat3 activity strongly suppressed c-Myc expression in Panc-1 and Colo-357 cells. Chromatin immunoprecipitation (ChIP) studies further validated these findings and showed that the nuclear EGFR, Src, and Stat3 complex is bound to the c-Myc promoter. Taken together, these data identify a novel and functional heteromeric EGFR, Src, and Stat3 complex that promotes the induction of c-Myc in pancreatic cancer cells. Our study provides additional insight into the mechanisms by which hyperactive EGFR, Src and Stat3 pathway promotes the pancreatic cancer phenotype.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr 2031. doi:10.1158/1538-7445.AM2011-2031

Abstract 1983: A cell-permeable Stat3 SH2 domain mimetic inhibitor of Stat3 activation inhibits growth of human breast tumor xenografts

Human tumors harbor multiple molecular aberrations that have become the driving force for uncontrolled cell proliferation and survival, and other tumor-associated processes. Constitutive activation of the Signal Transducer and Activator of Transcription (Stat) 3 protein is a frequent occurrence in many human cancers, including breast cancer. Given the role of aberrant Stat3 activity in human tumors, small-molecule Stat3 inhibitors would be useful novel therapeutics and tools for probing Stat3-mediated tumor processes. We herein report that a 28-mer peptide, SPI, composed of the amino acid residues 588-615 of the Stat3 SH2 domain, replicates Stat3 biochemical properties and functions as a Stat3 inhibitor. Studies show SPI and Stat3 (or Stat3 SH2 domain) both bind with similar affinities to known Stat3-binding phosphotyrosine (pY) peptide motifs, including the epidermal growth factor receptor (EGFR) peptides, pY1068EGFR and pY1086EGFR, and the high-affinity, interleukin-6 receptor (IL-6R)/gp130-derived peptide, GpYLPQTV-NH2. Consequently, SPI potently and selectively inhibits Stat3 SH2 domain:pY interactions and disrupts the binding of Stat3 to the IL-6R/gp130 peptide, GpYLPQTV-NH2. Fluorescence imaging studies show SPI is cell membrane-permeable. Treatment with SPI of malignant cells that harbor aberrant Stat3 activity resulted in the selective inhibition of constitutive Stat3 phosphorylation, and of Stat3 DNA-binding and transcriptional activities. By contrast, similar treatments have little or no effects on the induction of the Stat1, Stat5, and Erk1/2MAPK pathways in malignant cells or in EGF-stimulated fibroblasts. In contrast to normal cells, human breast, pancreatic, prostate, and non-small cell lung cancer cells harboring constitutively-active Stat3 are highly sensitive to SPI and undergo extensive morphology changes, concomitant with the loss of viability and apoptosis. Significantly, treatment with SPI or its inverse sequence version strongly inhibited the growth of human breast tumor xenografts in mice, which is associated with the down-regulation of the expression of the known Stat3-regulated genes, Cyclin D1, Bcl-xL and Survivin. Our study identifies SPI as a novel molecular probe for interrogating Stat3 signaling and that functions as a selective Stat3 inhibitor, which induces antitumor response in vivo.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr 1983. doi:10.1158/1538-7445.AM2011-1983

Abstract 1532: Small-molecule Stat3 inhibitor sensitizes Cisplatin-resistant ovarian cancer cells to Cisplatin-induced apoptosis

Ovarian Cancer causes the highest number of gynecologic neoplasm related deaths in the United States. The disease and its progression to drug resistance remain poorly understood. While the aberrant activation of the Signal Transducer and Activator of Transcription (Stat) 3 protein is prevalent in ovarian cancer and is detected in chemotherapy resistance, its exact role in the development and maintenance of the drug resistance phenotype remains poorly understood. We sought to investigate the molecular mechanisms by which constitutive Stat3 activity may support Cisplatin resistance in ovarian cancer. Compared to the sensitive A2780S ovarian cancer line, which does not harbor aberrant Stat3 activity, the SKOV3 line harboring constitutively-active Stat3 was partially sensitive to Cisplatin and showed evidence of moderate apoptosis in response to Cisplatin treatment. Compared to the sensitive A2780S cells, the partly-resistant, SKOV3 cells over-express c-Myc, Survivin, and Bcl-xL, and the XIAP anti-apoptotic factor, as well as VEGF pro-angiogenic factor and the matrix metalloproteinases, MMP-2 and MMP-9. The inhibition of constitutively-active Stat3 in SKOV3 cells by the small-molecule inhibitor, S3I-201 suppressed the expression of the anti-apoptotic and the pro-angiogenic factors, and induced moderate degree of apoptosis. Significantly, treatment with S3I-201 enhanced the sensitivity of SKOV3 cells to Cisplatin. Furthermore, the combined treatment of SKOV3 cells with Cisplatin and S3I-201 strongly inhibited colony formation and cell migration in vitro. Our study together identifies potential mechanisms by which aberrant Stat3 activity might support Cisplatin resistance in ovarian cancer, raising the potential that the combination therapy with Stat3 inhibitors would be effective therapeutic and chemo-sensitizing modalities for ovarian cancer and the recurrent disease.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr 1532. doi:10.1158/1538-7445.AM2011-1532

Abstract 1956: Enhanced colony-forming ability, and motility and migratory properties of Cisplatin-resistant ovarian cancer cells

Ovarian cancer causes the highest gynecologic neoplasm-related deaths in the United States. Although the majority of ovarian cancer patients respond to Cisplatin therapy, most develop recurrent and resistant disease, raising the need to understand the molecular mechanisms that support the resistant phenotype and to identify novel effective therapeutic and chemo-sensitizing modalities. In this study, we developed Cisplatin resistance in vitro to address the molecular and biological mechanisms that may be presented by the recurrent disease. The resistant lines, A2780S/CP3 and A2780S/CP5, which were derived from the Cisplatin-sensitive line, A2780S and are resistant to 3 and 5 μM Cisplatin, respectively, showed marked changes in morphology, compared to the parental cells. Notably, both resistant lines showed enhanced colony-forming ability and significantly enhanced cell motility and migratory properties. Molecular mechanistic analysis indicated the Cisplatin resistance was associated with hyperactive epidermal growth factor receptor (EGFR)-extracellular signal-regulated kinase (Erk)1/2MAPK and EGFR-Signal Transducer and Activator of Transcription (Stat)3 pathways, the overexpression of Survivin and vascular endothelial growth factor receptor (VEGF), and F-actin and Cortactin, with F-actin localization that is predominantly at the cellular extensions. Treatment of the Cisplatin-resistant A2780S/CP5 line with the EGFR inhibitor, Iressa or the Stat3 inhibitor, S3I-201 inhibited the colony-forming ability of these cells, without suppressing their viability, suggesting EGFR-Erk1/2 and EGFR-Stat3 activation are required for the proliferation of the resistant cells. Furthermore, inhibition of EGFR or Stat3 activation significantly suppresses the migration of A2780S/CP3 cells, while the motility of both A2780S/CP3 and A2780S/CP5 lines are sensitive to EGFR inhibition. These results together suggest that Cisplatin resistance in ovarian cancer is mediated in part through the activation of the EGF receptor-mediated Erk and Stat3 pathways that promote increased cell proliferation, motility and migration. These findings raise the possibility that EGFR or Stat3 inhibitors combined with Cisplatin may be an effective therapeutic approach for Cisplatin-resistant ovarian cancer.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr 1956. doi:10.1158/1538-7445.AM2011-1956

A STAT3-mediated metabolic switch is involved in tumour transformation and STAT3 addiction

The pro-oncogenic transcription factor STAT3 is constitutively activated in a wide variety of tumours that often become addicted to its activity, but no unifying view of a core function determining this widespread STAT3-dependence has yet emerged. We show here that constitutively active STAT3 acts as a master regulator of cell metabolism, inducing aerobic glycolysis and down-regulating mitochondrial activity both in primary fibroblasts and in STAT3-dependent tumour cell lines. As a result, cells are protected from apoptosis and senescence while becoming highly sensitive to glucose deprivation. We show that enhanced glycolysis is dependent on HIF-1α up-regulation, while reduced mitochondrial activity is HIF-1α-independent and likely caused by STAT3-mediated down-regulation of mitochondrial proteins. The induction of aerobic glycolysis is an important component of STAT3 pro-oncogenic activities, since inhibition of STAT3 tyrosine phosphorylation in the tumour cell lines down-regulates glycolysis prior to leading to growth arrest and cell death, both in vitro and in vivo. We propose that this novel, central metabolic role is at the core of the addiction for STAT3 shown by so many biologically different tumours.

Design, synthesis, and in vitro characterization of novel hybrid peptidomimetic inhibitors of STAT3 protein

Aberrant activation of oncogenic signal transducer and activator of transcription 3 (STAT3) protein signaling pathways has been extensively implicated in human cancers. Given STAT3's prominent dysregulatory role in malignant transformation and tumorigenesis, there has been a significant effort to discover STAT3-specific inhibitors as chemical probes for defining the aberrant STAT3-mediated molecular events that support the malignant phenotype. To identify novel, STAT3-selective inhibitors suitable for interrogating STAT3 signaling in tumor cells, we explored the design of hybrid molecules by conjugating a known STAT3 inhibitory peptidomimetic, ISS610 to the high-affinity STAT3-binding peptide motif derived from the ILR/gp-130. Several hybrid molecules were examined in in vitro biophysical and biochemical studies for inhibitory potency against STAT3. Lead inhibitor 14aa was shown to strongly bind to STAT3 (K(D)=900 nM), disrupt STAT3:phosphopeptide complexes (K(i)=5 μM) and suppress STAT3 activity in in vitro DNA binding activity/electrophoretic mobility shift assay (EMSA). Moreover, lead STAT3 inhibitor 14aa induced a time-dependent inhibition of constitutive STAT3 activation in v-Src transformed mouse fibroblasts (NIH3T3/v-Src), with 80% suppression of constitutively-active STAT3 at 6h following treatment of NIH3T3/v-Src. However, STAT3 activity recovered at 24h after treatment of cells, suggesting potential degradation of the compound. Results further showed a suppression of aberrant STAT3 activity in NIH3T3/v-Src by the treatment with compound 14aa-OH, which is the non-pTyr version of compound 14aa. The effect of compounds 14aa and 14aa-OH are accompanied by a moderate loss of cell viability.

Identification of Purine-Scaffold Small-Molecule Inhibitors of Stat3 Activation by QSAR Studies

To facilitate the discovery of clinically useful Stat3 inhibitors, computational analysis of the binding to Stat3 of the existing Stat3 dimerization disruptors and quantitative structure-activity relationships (QSAR) were pursued, by which a pharmacophore model was derived for predicting optimized Stat3 dimerization inhibitors. The 2,6,9-trisubstituted-purine scaffold was functionalized in order to access the three subpockets of the Stat3 SH2 domain surface and to derive potent Stat3-binding inhibitors. Select purine scaffolds showed good affinities (K(D), 0.8-12 μM) for purified, nonphosphorylated Stat3 and inhibited Stat3 DNA-binding activity in vitro and intracellular phosphorylation at 20-60 μM. Furthermore, agents selectively suppressed viability of human prostate, breast and pancreatic cancer cells, and v-Src-transformed mouse fibroblasts that harbor aberrant Stat3 activity. Studies herein identified novel small-molecule trisubstituted purines as effective inhibitors of constitutively active Stat3 and of the viability of Stat3-dependent tumor cells, and are the first to validate the use of purine bases as templates for building novel Stat3 inhibitors.

THAP5 is a DNA-binding transcriptional repressor that is regulated in melanoma cells during DNA damage-induced cell death

THAP5 was originally isolated as a specific interactor and substrate of the mitochondrial pro-apoptotic Omi/HtrA2 protease. It is a human zinc finger protein characterized by a restricted pattern of expression and the lack of orthologs in mouse and rat. The biological function of THAP5 is unknown but our previous studies suggest it could regulate G2/M transition in kidney cells and could be involved in human cardiomyocyte cell death associated with coronary artery disease (CAD). In this report, we expanded our studies on the properties and function of THAP5 in human melanoma cells. THAP5 was expressed in primary human melanocytes as well as in all melanoma cell lines that were tested. THAP5 protein level was significantly induced by UV irradiation or cisplatin treatment, conditions known to cause DNA damage. The induction of THAP5 correlated with a significant increase in apoptotic cell death. In addition, we show that THAP5 is a nuclear protein that could recognize and bind a specific DNA motif. THAP5 could also repress the transcription of a reporter gene in a heterologous system. Our work suggests that THAP5 is a DNA-binding protein and a transcriptional repressor. Furthermore, THAP5 has a pro-apoptotic function and it was induced in melanoma cells under conditions that promoted cell death.

A cell-permeable Stat3 SH2 domain mimetic inhibits Stat3 activation and induces antitumor cell effects in vitro

Given the role of constitutively active Signal Transducer and Activator of Transcription (Stat) 3 in human tumors, Stat3 inhibitors would be useful as novel therapeutics and as tools for probing Stat3-mediated tumor processes. We herein report that a 28-mer peptide, SPI, derived from the Stat3 SH2 domain, replicates Stat3 biochemical properties. Studies show SPI and Stat3 (or Stat3 SH2 domain) bind with similar affinities to known Stat3-binding phosphotyrosine (pY) peptide motifs, including those of the epidermal growth factor receptor (EGFR) and the high-affinity, IL-6R/gp130-derived pY-peptide, GpYLPQTV-NH(2). Consequently, SPI functions as a potent and selective inhibitor of Stat3 SH2 domain:pTyr interactions and disrupts the binding of Stat3 to the IL-6R/gp130 peptide, GpYLPQTV-NH(2). Fluorescence imaging and immunofluorescence staining/laser-scanning confocal microscopy show SPI is cell membrane-permeable, associates with the cytoplasmic tail of EGFR in NIH3T3/hEGFR, and is present in the cytoplasm, but strongly localized at the plasma membrane and in the nucleus in malignant cells harboring persistently active Stat3. Moreover, SPI specifically blocks constitutive Stat3 phosphorylation, DNA binding activity, and transcriptional function in malignant cells, with little or no effect on the induction of Stat1, Stat5, and Erk1/2(MAPK) pathways, or on general pTyr profile at the concentrations that inhibit Stat3 activity. Significantly, treatment with SPI of human breast, pancreatic, prostate, and non-small cell lung cancer cells harboring constitutively active Stat3 induced extensive morphology changes, associated with viability loss and apoptosis. Our study identifies SPI as a novel molecular probe for interrogating Stat3 signaling and that functions as a selective inhibitor of Stat3 activation with antitumor cell effects.

Abstract 2528: Enhanced sensitivity of pancreatic cancer cells to the concurrent inhibition of aberrant Stat3 and EGFR or Src

Pancreatic cancer is a lethal disease with little understanding of the etiology and no effective therapies. While aberrant epidermal growth factor receptor (EGFR), Src and Stat3 are detected in pancreatic cancer and implicated in the disease, their exact roles in the support of the disease phenotype remain poorly defined. Moreover, the potential for a cross-talk between EGFR, Src and Stat3 poses a challenge to any therapy that targets only one of these entities. In this study, we sought to define the EGFR, Src and Stat3 signaling integration and to investigate the contributory roles of the three entities to the pancreatic cancer phenotype. We found in Panc-1 and Colo-357 lines that phospho-Y845EGFR, pY1068EGFR and pY1086EGFR levels are responsive to c-Src inhibition, in contrast to pY1173EGFR that is solely EGFR kinase-dependent. Treatment of Panc-1 and Colo-357 cells with the EGFR inhibitor, Iressa (ZD 1839) or the Src inhibitor, Dasatinib (Das) suppressed the constitutively-active Stat3 activity, suggesting that both EGFR and Src activities promote aberrant Stat3 activation in pancreatic cancer cells. However, the early suppression of aberrantly-active Stat3 by the EGFR and Src inhibition in Panc-1 cells is countered by a Janus kinase (Jaks)-dependent re-activation, suggesting that Jaks activity could represent a compensatory mechanism for Stat3 induction. The abrogation of c-Src activity by either Das treatment or the over-expression of a kinase-dead c-Src mutant suppressed the levels of phospho-FAK, phospho-p130Cas, phospho-cortactin, phospho-p120catenin, and phospho-paxillin, suggesting that Src activity induces the mediators of motility and migration in pancreatic cancer cells. By contrast, neither EGFR nor Src inhibition modulated the baseline enhanced phospho-ErkMAPK or phospho-Akt levels in Panc-1 and Colo-357 cells, consistent with the observation that the inhibition of EGFR or Src alone only weakly suppressed cell growth and survival of pancreatic cancer cells. By contrast, the concurrent inhibition of Stat3 and EGFR or Src induced greater viability loss and apoptosis, and diminished the migration/invasion of pancreatic cancer cells in vitro. Of therapeutic significance, the concurrent inhibition of Stat3 and EGFR or Src, as compared to mono-targeting modality, induced a stronger human pancreatic tumor growth inhibition in xenografts models. We infer that the large tumor growth inhibition in vivo is due to the simultaneous suppression of the abnormal functions of Stat3 and EGFR or Src. These studies strongly suggest that the concurrent targeting of Stat3 and EGFR or Src could be a beneficial therapeutic approach for pancreatic cancer.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 2528.

Abstract 3684: Developing STAT3 protein inhibitors as adjuvant therapeutics: Promising synergistic effects in human cancers

Signal Transducer and Activator of Transcription 3 (Stat3) protein is a cytosolic transcription factor that relays signals from receptors in the plasma membrane directly to the nucleus, and is routinely hyper-activated in many human cancers and diseases. STAT3 induces anti-apoptotic gene expression programs (e.g. Bcl-xL) and the over-expression of cell cycle regulators (e.g. cyclin D1) that contribute significantly to the resistance of cancer to current chemotherapeutic strategies. Since most cancer drugs aim to initiate apoptosis, tumor cells containing activated STAT3 have an intrinsic resistance to current treatment strategies. It has therefore been postulated that STAT3 inhibitors could play a significant role in the future of adjuvant cancer therapies by sensitizing human tumors to traditional chemotherapy. By examining the protein-protein interaction interface and employing computational modeling, we have thus developed a series of small molecule inhibitors of the transcriptionally active STAT3-STAT3 homo-dimer complex. Lead inhibitors showed potent anti-STAT3 activity in vitro and in tumor cell lines, as well as in malignant cells taken from leukemia patients. More specifically, these compounds displayed single digit micromolar activity against breast, prostate, pancreatic and leukemia cell lines and showed negligible cytotoxic effects on healthy cells treated with high µM concentrations of compounds. Preliminary adjuvant studies with a series of clinically relevant therapeutics have shown impressive synergistic effects in leukemia cell lines, as well as in patient tumor cells.

Note: This abstract was not presented at the AACR 101st Annual Meeting 2010 because the presenter was unable to attend.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 3684.

Abstract 4135: Cisplatin resistance promotes cell motility and migration through EGF receptor-mediated Erk and Stat3 pathways in human ovarian cancer cells

Ovarian cancer causes the highest gynecologic neoplasm-related deaths in the United States. A common feature of ovarian cancer is the wide dissemination of the disease in the peritoneum at the time of presentation by the patient, which poses an added challenge to therapy in terms of the ability to completely eliminate the disease tissue. Moreover, disease recurrence is common and frequently associated with drug resistance. Cisplatin has been the most active drug for the treatment of ovarian cancer for the last decades. Although the majority of ovarian cancer patients respond to Cisplatin therapy, most develop recurrent and resistant disease, raising the need to understand the molecular mechanisms that support the resistant phenotype and to identify novel effective therapeutic and chemo-sensitizing modalities. Here, we developed Cisplatin resistance in vitro to address the molecular and biological mechanisms that may be presented by the recurrent disease. The cell characteristic changes and the molecular basis of resistance to Cisplatin were investigated using the Cisplatin-resistant human ovarian cancer cell lines, A2780S/CP1, A2780S/CP3 and A2780S/CP5, which were derived from the parental, Cisplatin-sensitive line, A2780S and are resistant to 1, 3 and 5 μM Cisplatin, respectively. Cell morphology under bright-field microscope showed marked changes in the Cisplatin-resistant cells compared to that of the parental cells. Notably, wound healing and migration assays showed significantly enhanced cell motility and migration properties in Cisplatin-resistant clones. Furthermore, resistance to Cisplatin enhanced the ability of cells to form colonies. Molecular mechanistic analysis indicated that Cisplatin resistance in A2780S/CP3 and A2780S/CP5 cells dramatically activated the EGF receptor (EGFR)-Erk1/2MAPK and EGFR-Stat3 pathways, and upregulated the expression of cellular survival genes such as Bcl-xL and Survivin. Accordingly, treatment of the Cisplatin-resistant A2780S/CP3 and A2780S/CP5 lines with the EGFR inhibitor, Iressa or the Stat3 inhibitor, S3I-201 significantly attenuated colony survival and cell migration. Moreover, resistance to Cisplatin led to the upregulation of the expression of F-actin and cortactin, and promoted the assembly of F-actin at the front of lamellipodia in A2780S/CP3 and A2780S/CP5 lines. These results suggest that Cisplatin resistance in ovarian cancer is mediated in part through the activation of the EGF receptor-mediated Erk and Stat3 pathways that promote cell survival, motility and migration. These findings raise the possibility that EGFR or Stat3 inhibitors combined with Cisplatin may be an effective therapeutic approach for Cisplatin-resistant ovarian cancer.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 4135.

Coordination complex SH2 domain proteomimetics: an alternative approach to disrupting oncogenic protein-protein interactions

We report the first application of coordination complexes as functional proteomimetics of the Src homology 2 (SH2) phosphopeptide-binding domain. As a proof-of-concept, functionalized bis-dipicolylamine (BDPA) copper(ii) complexes are shown to disrupt oncogenic Stat3-Stat3 protein complexes and elicit promising anti-tumour activity.

Enhanced sensitivity of pancreatic cancer cells to concurrent inhibition of aberrant signal transducer and activator of transcription 3 and epidermal growth factor receptor or Src

Many molecular aberrations occur in pancreatic cancer. Although aberrant epidermal growth factor receptor (EGFR), Src, and signal transducer and activator of transcription 3 (Stat3) are implicated in pancreatic cancer, therapies that target only one of these entities are undermined by signaling cross-talk. In the human pancreatic cancer lines, Panc-1 and Colo-357, pY845EGFR, pY1068EGFR, pY1086EGFR, and pY1173EGFR levels and pY416c-Src are concurrently elevated with aberrantly active Stat3 in a complex signaling cross-talk. Thus, understanding the signaling integration would facilitate the design of effective multiple-targeted therapeutic modalities. In Panc-1 and Colo-357 lines, pY845EGFR, pY1068EGFR, and pY1086EGFR levels are responsive to c-Src inhibition in contrast to pY1173EGFR, which is EGFR kinase-dependent. Constitutively active Stat3 is sensitive to both EGFR and Src inhibition, but the early suppression of aberrantly active Stat3 in response to the inhibition of EGFR and Src is countered by a Janus kinase (Jaks)-dependent reactivation, suggesting that Jaks activity is a compensatory mechanism for Stat3 induction. The inhibition of EGFR, Src, or Stat3 alone induced weak biological responses. By contrast, the concurrent inhibition of Stat3 and EGFR or Src induced greater viability loss and apoptosis and decreased the migration/invasion of pancreatic cancer cells in vitro. Significantly, the concurrent inhibition, compared with monotargeting modality, induced stronger human pancreatic tumor growth inhibition in xenografts. We infer that the tumor growth inhibition in vivo is caused by the simultaneous suppression of the abnormal functions of Stat3 and EGFR or Src. These studies strongly suggest that the concurrent targeting of Stat3 and EGFR or Src could be a beneficial therapeutic approach for pancreatic cancer.

A novel small-molecule disrupts Stat3 SH2 domain-phosphotyrosine interactions and Stat3-dependent tumor processes

The molecular modeling of the phosphotyrosine (pTyr)-SH2 domain interaction in the Stat3:Stat3 dimerization, combined with in silico structural analysis of the Stat3 dimerization disruptor, S3I-201, has furnished a diverse set of analogs. We present evidence from in vitro biochemical and biophysical studies that the structural analog, S3I-201.1066 directly interacts with Stat3 or the SH2 domain, with an affinity (K(D)) of 2.74microM, and disrupts the binding of Stat3 to the cognate pTyr-peptide, GpYLPQTV-NH(2), with an IC(50) of 23microM. Moreover, S3I-201.1066 selectively blocks the association of Stat3 with the epidermal growth factor receptor (EGFR), and inhibits Stat3 tyrosine phosphorylation and nuclear translocation in EGF-stimulated mouse fibroblasts. In cancer cells that harbor aberrant Stat3 activity, S3I-201.1066 inhibits constitutive Stat3 DNA-binding and transcriptional activities. By contrast, S3I-201.1066 has no effect on Src activation or the EGFR-mediated activation of the Erk1/2(MAPK) pathway. S3I-201.1066 selectively suppresses the viability, survival, and malignant transformation of the human breast and pancreatic cancer lines and the v-Src-transformed mouse fibroblasts harboring persistently active Stat3. Treatment with S3I-201.1066 of malignant cells harboring aberrantly active Stat3 down-regulated the expression of c-Myc, Bcl-xL, Survivin, the matrix metalloproteinase 9, and VEGF. The in vivo administration of S3I-201.1066-induced significant antitumor response in mouse models of human breast cancer, which correlates with the inhibition of constitutively active Stat3 and the suppression of known Stat3-regulated genes. Our studies identify a novel small-molecule that binds with a high affinity to Stat3, blocks Stat3 activation and function, and thereby induces antitumor response in human breast tumor xenografts harboring persistently active Stat3.

Targeting STAT3 in cancer: how successful are we?

BACKGROUND

Aberrant activation of the signal transducer and activator of transcription (STAT)3 occurs in many human tumors. Moreover, studies utilizing genetic and pharmacological approaches to modulate constitutive STAT3 activity have provided compelling evidence for the critical role of aberrant STAT3 activity in malignant transformation and tumor progression, and thereby validated STAT3 as a novel cancer drug target.

OBJECTIVE

This review is intended to be a full coverage of the efforts to develop direct STAT3 inhibitors and will provide a discussion on the inhibitory modalities developed to date.

METHODS

Review of the literature focused on the modalities and mechanisms that directly target and inhibit the STAT protein or its functions.

RESULTS/CONCLUSION

While a variety of STAT3 inhibitors have been identified that induce antitumor cell effects in vitro and in vivo, the landscape remains murky. With a few exceptions, most of the STAT3 inhibitors reported to date have not undergone an in vivo efficacy, pharmacology or toxicity testing. Also, there is no evidence, per the published literature of an impending clinical development for the few agents that were reported to exhibit in vivo efficacy. Overall, there is the need for a reassessment of the ongoing strategies to target STAT3 intended not only for refinement, but also for incorporating some new technologies to strengthen our efforts and ensure the success - sooner, rather than later - of identifying suitable anti-STAT3 agents for development into clinically useful anticancer therapeutics.

Molecular characterization of highly tumorigenic cell lines used in a xenograph model to investigate cellular therapy for the treatment of refractory ovarian cancer

e16516

Background: Because current therapies for ovarian cancer (OC) have little impact on the long-term survival, there is a compelling need to develop innovative strategies. Our aim was to characterize OC cell lines which can be utilized to test cellular therapy in combination with cytokines or chemotherapies to elicit a graph versus tumor response to treat refractory OC patients. Methods: SKOV-3-RFP, a red fluorescent protein (RFP) expressing line, was passaged through nude mice to create more tumorigenic lines compared to parental SKOV-3 cells. Three SKOV-3-derived lines were established and characterized for IFNα-2b sensitivity, E-cadherinand CCN1gene expression, and STAT3 activation. The cells were grown in the presence of IFNα-2b to determine proliferative effects. Tumors were harvested when mice became moribund; ascitic fluid (AF) and solid tumor (ST) tissue were snap-frozen. Expression of E-cadherinand CCN1, which have been implicated to play role(s) in OC pathobiology, were analyzed in ST harvested from the mice and from SKOV3-derived lines (SKOV3-RFP, AF1, AF2, and AF3). We investigated the activation status in the SKOV3-derived cells of STAT3, which has been associated with malignant transformation and tumor progression. Results: Parental SKOV3-RFP cells when injected at a dose of 5x106 gave a tumor incidence of 4/6 in 14 weeks. AF1 and AF2, when injected at a dose of 1x106 resulted in 100% tumor incidence in 5-weeks (n = 5). These two lines were also more resistant to IFNα-2b compared to RFP. All SKOV3-derived lines and ST expressed E-cadherin by RT-PCR. None of the SKOV3-derived lines expressed CCN1; however, one of the two AF1 ST tested expressed CCN1. Both AF2 ST evaluated showed weak expression of CCN1. In vitro DNA-binding with electrophoretic mobility shift assay (EMSA), showed AF1 and AF3 cells harbor constitutively-active STAT3, whereas the RFP line does not. Conclusions: The SKOV3-derived lines that we developed will be a better model to test novel OC treatment regiments because these lines exhibit increased resistance to IFNα-2b, are more tumorigenic in a xenograph model, show aberrant STAT3 activation, and tumors harvested from these lines express genes that make these lines more aggressive.

No significant financial relationships to disclose.

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.

An oxazole-based small-molecule Stat3 inhibitor modulates Stat3 stability and processing and induces antitumor cell effects

Stat3 is hyperactivated in many human tumors and represents a valid target for anticancer drug design. We present a novel small-molecule Stat3 inhibitor, S3I-M2001, and describe the dynamics of intracellular processing of activated Stat3 within the context of the biochemical and biological effects of the Stat3 inhibitor. S3I-M2001 is an oxazole-based peptidomimetic of the Stat3 Src homology (SH) 2 domain-binding phosphotyrosine peptide that selectively disrupts active Stat3:Stat3 dimers. Consequently, hyperactivated Stat3, which hitherto occurs as "dotlike" structures of nuclear bodies, undergoes an early aggregation into nonfunctional perinuclear aggresomes and a late-phase proteasome-mediated degradation in malignant cells treated with S3I-M2001. Thus, S3I-M2001 inhibited Stat3-dependent transcriptional regulation of tumor survival genes, such as Bcl-xL. Furthermore, Stat3-dependent malignant transformation, survival, and migration and invasion of mouse and human cancer cells harboring persistently activated Stat3 were inhibited by S3I-M2001. Finally, S3I-M2001 inhibited growth of human breast tumor xenografts. The study identifies a novel Stat3 inhibitor, S3I-M2001, with antitumor cell effects mediated in part through a biphasic loss of functional Stat3. The study represents the first on intracellular Stat3 stability and processing following inhibition by a small molecule that has significant antitumor activity.

Ligand-independent phosphorylation of Y869 (Y845) links mutant EGFR signaling to stat-mediated gene expression

Activating mutants of EGFR have been identified in a subset of non-small-cell lung cancers. To investigate mutant-driven signaling, we focused on Y869, a residue in the same activation loop where the L858R and L861Q mutations are located. We observed ligand-independent phosphorylation of Y869 in 32D cells EGFR(L858R) and EGFR(L861Q). The EGFR tyrosine kinase inhibitor (TKI) erlotinib inhibited Y869 P-EGFR in intact cells as well as in a cell-free kinase reaction. Expression of kinase domain of EGFR(L858R) and EGFR(L861Q) exhibited auto-phosphorylation of Y869; this was inhibited by EGFR TKIs but not by Src kinase inhibitor. P-Y859 of EGFR-mediated downstream component, STAT5, was also analyzed. Y694 P-STAT5 was eliminated by erlotinib treatment. Analysis of immune-complexes showed constitutive association of mutant EGFRs with STAT5 and Src which was unaffected by erlotinib or PP1. On the other hand, 32D-EGFR(WT) exhibited constitutive STAT5 phosphorylation and association of EGFR with JAK2. In these cells, a JAK2 inhibitor abrogated P-STAT5 whereas mutant EGFRs did not associate with JAK2. Expression of c-myc was regulated by EGFR/STAT5 signaling in cells expressing EGFR(L858R) and EGFR(L861Q). Our results suggest that ligand-independent and Src activity-independent phosphorylation of Y869 in mutant EGFR regulates STAT5 activation and c-myc expression.

Selective chemical probe inhibitor of Stat3, identified through structure-based virtual screening, induces antitumor activity

S3I-201 (NSC 74859) is a chemical probe inhibitor of Stat3 activity, which was identified from the National Cancer Institute chemical libraries by using structure-based virtual screening with a computer model of the Stat3 SH2 domain bound to its Stat3 phosphotyrosine peptide derived from the x-ray crystal structure of the Stat3beta homodimer. S3I-201 inhibits Stat3.Stat3 complex formation and Stat3 DNA-binding and transcriptional activities. Furthermore, S3I-201 inhibits growth and induces apoptosis preferentially in tumor cells that contain persistently activated Stat3. Constitutively dimerized and active Stat3C and Stat3 SH2 domain rescue tumor cells from S3I-201-induced apoptosis. Finally, S3I-201 inhibits the expression of the Stat3-regulated genes encoding cyclin D1, Bcl-xL, and survivin and inhibits the growth of human breast tumors in vivo. These findings strongly suggest that the antitumor activity of S3I-201 is mediated in part through inhibition of aberrant Stat3 activation and provide the proof-of-concept for the potential clinical use of Stat3 inhibitors such as S3I-201 in tumors harboring aberrant Stat3.

Isoform selective inhibition of STAT1 or STAT3 homo-dimerization via peptidomimetic probes: Structural recognition of STAT SH2 domains

The identification of constitutively activated STAT (Signal Transducers and Activators of Transcription) proteins in aberrant cell signaling pathways has led to investigations targeting the selective disruption of specific STAT isoforms directly associated with oncogenisis. We have identified, through the design of a library of peptidomimetic inhibitors, agents that selectively disrupt STAT1 or STAT3 homo-dimerization at low micromolar concentrations. ISS840 has 20-fold higher inhibition of STAT1 homo-dimerization (IC(50) value of 31 microM) relative to STAT3 (IC(50) value of 560 microM).

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.

Differential effects of Stat3 inhibition in sparse vs confluent normal and breast cancer cells

The signal transducer and activator of transcription-3 (Stat3) is persistently activated in many cancers such as cancer of the breast and is required for transformation by a number of oncogenes. Signaling through Stat3 is determined by a key phosphorylation at tyr-705. We previously demonstrated that cell-to-cell adhesion brought about through cell aggregation or confluence of cultured cells causes a dramatic increase in Stat3 tyr705 phosphorylation and consequently Stat3 activity in both normal and tumor cells. To examine the role of Stat3 at specific time-points relative to confluence, we used two different approaches of Stat3 inhibition: (1). Introduction of high levels of peptide analogues, which block the Stat3-SH2 domain, to inhibit Stat3 binding to and phosphorylation by growth factor receptors. (2). Treatment with two platinum compounds which bind the Stat3 protein and inhibit its activity without affecting its phosphorylation directly. The results demonstrate that Stat3 downregulation in vSrc transformed NIH3T3 cells or in breast cancer lines harboring activated Src induces apoptosis, which is evident at all densities but is more pronounced at post-confluence. In normal cells on the other hand, Stat3 inhibition at post-confluence caused apoptosis while in sparsely growing cells it induced merely a growth retardation.

Mechanisms of disease: Insights into the emerging role of signal transducers and activators of transcription in cancer

Members of the signal transducers and activators of transcription (STAT) pathway, which were originally identified as key components linking cytokine signals to transcriptional events in cells, have recently been demonstrated to have a major role in cancer. They are cytoplasmic proteins that form functional dimers with each other when activated by tyrosine phosphorylation. Activated STAT proteins translocate to the nucleus to regulate expression of genes by binding to specific elements within gene promoters. Constitutive activation of the STAT family members Stat3 and Stat5, and/or loss of Stat1 signaling, is found in a large group of diverse tumors. Increasing evidence demonstrates that STAT proteins can regulate many pathways important in oncogenesis including cell-cycle progression, apoptosis, tumor angiogenesis, tumor-cell invasion and metastasis, and tumor-cell evasion of the immune system. Based on these findings, a growing effort is underway to target STAT proteins directly and indirectly for cancer therapy. This review will highlight STAT signaling pathways, STAT target genes involved in cancer, evidence for STAT activation in human cancers, and therapeutic strategies to target STAT molecules for anticancer therapy.

Targeting Stat3 blocks both HIF-1 and VEGF expression induced by multiple oncogenic growth signaling pathways

Vascular endothelial growth factor (VEGF) upregulation is induced by many receptor and intracellular oncogenic proteins commonly activated in cancer, rendering molecular targeting of VEGF expression a complex challenge. While VEGF inducers abound, only two major transcription activators have been identified for its promoter: hypoxia inducible factor-1 (HIF-1) and signal transducer and activator of transcription (Stat3). Both HIF-1 expression and Stat3 activity are upregulated in diverse cancers. Here, we provide evidence that Stat3 is required for both basal and growth signal-induced expression of HIF-1. Moreover, induction of VEGF by diverse oncogenic growth stimuli, including IL-6R, c-Src, Her2/Neu, is attenuated in cells without Stat3 signaling. We further demonstrate that Stat3 regulates expression of Akt, which is required for growth signal-induced HIF-1 upregulation. Targeting Stat3 with a small-molecule inhibitor blocks HIF-1 and VEGF expression in vitro and inhibits tumor growth and angiogenesis in vivo. Furthermore, tumor cells' in vivo angiogenic capacity induced by IL-6R, which simultaneously activates Jak/STAT and PI3K/Akt pathways, is abrogated when Stat3 is inhibited. Activation of Stat3 signaling by various growth signaling is prevalent in diverse cancers. Results presented here demonstrate that Stat3 is an effective target for inhibiting tumor VEGF expression and angiogenesis.

STAT proteins as novel targets for cancer drug discovery

Signal transducer and activator of transcription (STAT) proteins are latent cytoplasmic transcription factors that were discovered in the context of cytokine and growth factor signalling. Normal STAT signalling is tightly controlled with finite kinetics, which is in keeping with standard cellular responses. However, persistent STAT activation has also been observed and is frequently associated with malignant transformation. Constitutive activation of STAT proteins, notably of Stat3 and Stat5, is detected in many human tumour cells and cells transformed by oncoproteins that activate tyrosine kinase signalling pathways. It is well-established that constitutively active Stat3 is one of the molecular abnormalities that has a causal role in oncogenesis. Aberrant Stat3 promotes uncontrolled growth and survival through dysregulation of gene expression, including cyclin D1, c-Myc, Bcl-xL, Mcl-1 and survivin genes, and thereby contributes to oncogenesis. Moreover, recent studies reveal that persistently active Stat3 induces tumour angiogenesis by upregulation of vascular endothelial growth factor induction, and modulates immune functions in favour of tumour immune evasion. Overall, studies have validated Stat3 as a novel target for cancer therapy, and hence provided the rationale for developing small-molecule Stat3 inhibitors. This review will discuss current evidence for the critical role of aberrant STAT signalling in malignant transformation, and examine the validity as well as the therapeutic potential of Stat3 as a cancer target. An update on the efforts to develop novel Stat3 inhibitors for therapeutic application will also be provided.

A novel platinum compound inhibits constitutive Stat3 signaling and induces cell cycle arrest and apoptosis of malignant cells

Previous studies have established constitutive activation of Stat3 protein as one of the molecular changes required for tumorigenesis. To develop novel therapeutics for tumors harboring constitutively active Stat3, compounds from the NCI 2000 diversity set were evaluated for inhibition of Stat3 DNA-binding activity in vitro. Of these, a novel platinum (IV) compound, IS3 295, interacted with Stat3 and inhibited its binding to specific DNA-response elements. Further analysis suggested noncompetitive-type kinetics for the inhibition of Stat3 binding to DNA. In human and mouse tumor cell lines with constitutively active Stat3, IS3 295 selectively attenuated Stat3 signaling, thereby inducing cell growth arrest at G0/G1 phase and apoptosis. Moreover, in transformed cells, IS3 295 repressed expression of cyclin D1 and bcl-xL, two of the known Stat3-regulated genes that are overexpressed in malignant cells, suggesting that IS3 295 mediates anti-tumor cell activity in part by blocking Stat3-mediated sub-version of cell growth and apoptotic signals. Together, our findings provide evidence for the inhibition of Stat3 activity and biological functions by IS3 295 through interaction with Stat3 protein. This study represents a significant advance in small molecule-based approaches to target Stat3 and suggests potential new applications for platinum (IV) complexes as modulators of the Stat3 pathway for cancer therapy.