Abstract 1248: ERGi-USU-6 salt derivatives, a selective inhibitor for ERG positive prostate cancer cells

Background: Approximately half of prostate cancer patients harbor the oncogenic TMPRSS2-ERG gene fusion expressing the ERG oncoprotein. Our lead compound, ERGi-USU, is a potent small molecule inhibitor that binds the atypical kinase RIOK2 leading to the inhibition of the growth of ERG positive cancer cells reducing ERG levels. We have developed a new and more effective derivative, ERGi-USU-6. We have reported the new salts formula EB3P0-16 of ERGi-USU-6 inhibiting the growth of ERG positive prostate cancer cells at IC50=89nM achieving the range of FDA approved drugs. Encouraged by this success we have designed and tested a broad array of new salt formulations to further improve the biological activity of ERGi-USU-6.

Methods: From fifty potential salt formulations, we prioritized candidate salts based on the structure, solubility, ionic hydrophobic/hydrophilic characteristics. Series of salts were synthesized by chemical reactions with ERGI-USU-6. Selected formulas include, bisulfate, butyrate, carbonate, salicylate, tartrate salts. The different stages of salt formulation included the testing of a) preformulation, b) pre-formulation processing, c) physicochemical properties, d) solubility, e) pKa, f) partition coefficient, g) pH- solubility profiling, h) prodrug formulation, and i) polymorphism. The IC50 values for ERG protein inhibition was compared to the parental compound and were calculated in a 12-step dilution range and were repeated thrice. The protein levels were quantified by measuring protein levels in a wide range using iBright instrument.

Results: A total of 21 salt formulas were prioritized and synthesized. The result confirmed the lead characteristics of EB3P0-16. However, among the new formulas four other salt derivatives showed improved ERG protein inhibition compared to the parental compound. Also, we gained new insights in designing the next generation of formula by the exclusion of chemical structures that did not lead to improvement.

Conclusion: This study confirmed EB3P0-16 as the lead formula of ERGi-USU-6. We found four new salt formulas with improved activity achieving ERG protein inhibition within the effective concentration range. The significance of our findings opens the possibility of new deigns towards the pre-clinical testing of ERG inhibitors.

Disclaimer: The opinions or assertions contained herein are the private ones of the author/speaker and are not to be construed as official or reflecting the views of the Department of Defense, the Uniformed Services University of the Health Sciences or any other agency of the U.S. Government. The identification of specific products, scientific instrumentation, or organization is considered an integral part of the scientific endeavor and does not constitute endorsement or implied endorsement on the part of the author, DoD, or any component agency.

Citation Format: Binil Eldhose, Charles P. Xavier, Mallesh Pandrala, Albert Dobi, Sanjay V. Malhotra. ERGi-USU-6 salt derivatives, a selective inhibitor for ERG positive prostate cancer cells [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2021; 2021 Apr 10-15 and May 17-21. Philadelphia (PA): AACR; Cancer Res 2021;81(13_Suppl):Abstract nr 1248.

Abstract 5240: Effective inhibition of TMPRSS2-ERG positive prostate cancer cells by a new formula of ERGi-USU-6

Background: Prostatic adenocarcinoma is among the leading cause of cancer-related deaths among men in the United States. The Erythroblast Transformation-Specific-Related Gene, ERG is dormant in normal prostate epithelium. Due to gene fusions, the ERG oncogene is frequently activated by androgenic signals in prostate cancer. ERG disrupts the normal differentiation, promotes epithelial-mesenchymal transition, migratory and invasive properties of cancer cells. Approximately 35% of metastatic castration-resistant prostate cancers harbor ERG oncogene. Due to the failure of androgen axis directed therapies, there is an urgent need to develop inhibitors to targeting prostate cancer-causing genes, such as ERG. We have identified a potent small-molecule inhibitor ERGi-USU that is remarkably selective for inhibiting the growth of ERG positive cancer cells through direct binding to the RIOK2 atypical kinase, a putative upstream regulator of ERG. We completed a structure-activity relationship (SAR) study and compound development resulting in the potent derivative, ERGi-USU-6. Our current objective is to improve the therapeutic properties of ERGi-USU-6, by new salt formulations.

Methods: Evaluation of the five selected salt formulations of ERGi-USU-6 were performed by assessing the growth of ERG positive prostate cancer cell line (VCaP) and by monitoring ERG and RIOK2 protein levels. Selectivity was assessed by monitoring the growth, endogenous ERG and RIOK2 levels in normal ERG positive human umbilical vein derived endothelial cells (HUVEC). The IC50 values for cell growth, compared to the parental compound were calculated in a 12-step dilution range performed in triplicates and independently repeated three times. Cell growth was measured by quantitative Cell Glow assay monitoring viable cells using Perkin Elmer Envision assay instrument and protein levels were quantified by measuring bioluminescence in IBright instrument in a wide dynamic range.

Results: One new salt formula with improved activity were identified, demonstrating improved cell growth (IC50=89nM vs. parental IC50=139), ERG protein and RIOK2 protein inhibition. None of the salt formulas of ERGi-USU-6 showed any effect on the primary cultures of the ERG positive normal endothelial cells (HUVEC) in the effective concentration range.

Conclusion: The first evaluation of salt formulas of ERGi-USU-6 may open the possibilities for preclinical assessments of this remarkably cancer-selective compound.

Citation Format: Binil Eldhose, Charles P. Xavier, Mallesh Pandrala, Sanjay V. Malhotra, Albert Dobi. Effective inhibition of TMPRSS2-ERG positive prostate cancer cells by a new formula of ERGi-USU-6 [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 5240.

Abstract 5305: Reexpression of LSAMP, a gene frequently deleted in African American prostate cancers, suppresses tumor growth and β-catenin activity

Introduction: African American (AA) men have the highest prostate cancer incidence and mortality rates in the US. The biological contribution to this disparity, however, is not well understood. LSAMP inactivation has been implicated in several cancers, and was recently identified in prostate cancer. A higher frequency of LSAMP inactivation has been observed in AA prostate cancer, and this aberration has been associated with a significantly greater risk of disease progression. In the characterization of LSAMP in prostate cancer cells lines, we found the copy number to be variable and the expression to be low or undetectable. LNCaP, MDA PCa 2B, and DU 145 prostate cancer cell lines were stably transduced to express LSAMP in an inducible or constitutive manner. LSAMP expression in these cell lines resulted in reduced cell proliferation, and induced a reversion to indolent cell-cell, and cell-extra-cellular-matrix adhesion characteristics, consistent with its tumor suppressive role. LSAMP expression also resulted in the down-regulation of receptor tyrosine kinases EPHA3, FGFR2, and FGFR4, and reduced activation of their downstream ERK and AKT pathways. Several Integrins were also up-regulated upon LSAMP expression. Additionally, β-catenin localization was altered, suggesting a potential reduction in transcriptional activity. We assessed the tumor suppressive function of LSAMP further, using in vitro assays and in vivo mouse models.

Methods: LSAMP expressing and control DU 145 cells were used to investigate the tumor suppressive function of LSAMP in vivo. Athymic nude mice were injected either subcutaneously, to determine effect of LSAMP expression on prostate tumor growth rates, or intravenously, to determine effect of LSAMP expression on tumor formation. We performed the TOPflash/FOPflash luciferase reporter assay to determine whether LSAMP expression modulates transcriptional activity of β-catenin in vitro.

Results: LSAMP expression resulted in a significant inhibition of tumor growth in the subcutaneous xenograft model. In the intravenous xenograft model, LSAMP expression resulted in a reduced incidence of distant metastases. Consistent with the negative modulation of signal transduction, and β-catenin localization previously observed, LSAMP expression resulted in a reduction of β-catenin transcriptional activity in vitro.

Conclusion: These studies provide in vivo evidence of the suppressive function of LSAMP in prostate tumors, corroborating previous in vitro and clinical findings. LSAMP expression reduced tumor growth rates, and incidence of distant metastases. LSAMP expression also reduced β-catenin transcriptional activity in vitro. These findings provide further support for a biological mechanism underlying the aggressive prostate cancer phenotype observed with LSAMP inactivation.

Citation Format: Kevin Babcock, Taduru Sreenath, Charles P. Xavier, Inger L. Rosner, Shiv Srivastava, Albert Dobi, Shyh-Han Tan. Reexpression of LSAMP, a gene frequently deleted in African American prostate cancers, suppresses tumor growth and β-catenin activity [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 5305.

Identification of a Small Molecule That Selectively Inhibits ERG-Positive Cancer Cell Growth

Oncogenic activation of the ETS-related gene (ERG) by recurrent gene fusions (predominantly TMPRSS2-ERG) is one of the most validated and prevalent genomic alterations present in early stages of prostate cancer. In this study, we screened small-molecule libraries for inhibition of ERG protein in TMPRSS2-ERG harboring VCaP prostate cancer cells using an In-Cell Western Assay with the highly specific ERG-MAb (9FY). Among a subset of promising candidates, 1-[2-Thiazolylazo]-2-naphthol (NSC139021, hereafter ERGi-USU) was identified and further characterized. ERGi-USU selectively inhibited growth of ERG-positive cancer cell lines with minimal effect on normal prostate or endothelial cells or ERG-negative tumor cell lines. Combination of ERGi-USU with enzalutamide showed additive effects in inhibiting growth of VCaP cells. A screen of kinases revealed that ERGi-USU directly bound the ribosomal biogenesis regulator atypical kinase RIOK2 and induced ribosomal stress signature. In vivo, ERGi-USU treatment inhibited growth of ERG-positive VCaP tumor xenografts with no apparent toxicity. Structure-activity-based derivatives of ERGi-USU recapitulated the ERG-selective activity of the parental compound. Taken together, ERGi-USU acts as a highly selective inhibitor for the growth of ERG-positive cancer cells and has potential for further development of ERG-targeted therapy of prostate cancer and other malignancies.Significance: A highly selective small-molecule inhibitor of ERG, a critical driver of early stages of prostate cancer, will be imperative for prostate cancer therapy. Cancer Res; 78(13); 3659-71. ©2018 AACR.

Abstract 1183: Structure-activity studies and biological evaluations of ERGi-USU, a highly selective inhibitor for ERG-positive prostate cancer cells

Introduction and objectives: While new prostate cancer (CaP) treatments (Abiraterone and Enzalutamide) have improved survival in castration-resistant prostate cancer (CRPC), their benefits are short-lived and drug resistance develops likely due to numerous adaptive mutations. Therefore it is a critical to develop effective novel inhibitors to target other signaling pathways that promote or contribute to prostate tumorigenesis. Accumulating evidence has established the androgen regulated TMPRSS2-ERG fusion as a common oncogenic driver that contributes to the early development and progression of over half of CaP. Therefore, ERG oncoprotein and ERG dependent pathways are promising targets for CaP therapy in early stages when cancer is most responsive to treatment. We previously identified a small molecule inhibitor, ERGi-USU, which selectively inhibits ERG protein and cell growth in ERG positive tumor cell lines and mouse xenograft models. In an effort to further develop ERGi-USU with enhanced efficacy we performed detailed structure-activity relationship (SAR) evaluation of ERGi-USU core structure and developed new derivatives.

Methods: Based on SAR of the core structure of ERGi-USU, 48 new derivatives were designed and synthesized by substitutions with alkyl, alkoxy, cycloalkyl, heterocycloalkyl, aryl, heteroaryl or hydroxyl groups. The new ERGi-USU derivatives were evaluated for inhibition of cell growth and ERG protein levels in the TMPRSS2-ERG fusion harboring CaP cell line, VCaP. Four of these compounds have been selected for evaluation of ERG selectivity by defining IC50 in ERG positive malignant cells (VCaP, KG1, MOLT-4 and COLO320), ERG negative CaP cell line (LNCaP) or ERG positive normal primary endothelium-derived cells (HUVEC).

Result: Like parental compound, four new ERGi-USU derivatives exhibited inhibition of cell growth and ERG protein levels in ERG positive VCaP, KG1, MOLT-4 and COLO320 cell lines, with no or minimal effects on LNCaP and HUVEC cells. One of the new derivatives (ERGi-USU#6) showed increased efficacy for cell growth inhibition (IC50=0.074µM) compared to the parental ERGi-USU (IC50=0.200µM). Other three new compounds showed similar IC50 as the ERGi-USU.

Conclusion: Comprehensive evaluation of ERGi-USU derivatives along with parental compound has continued to underscore selective inhibition of ERG positive tumor cells by these small molecules.

Citation Format: Ahmed A. Mohamed, Charles P. Xavier, Gauthaman Sukumar, Samuel D. Banister, Vineet Kumar, Shyh-Han Tan, Shilpa Katta, Lakshmi Ravindranath, Muhammad Jamal, Taduru Sreenath, David G. McLeod, Gyorgy Petrovics, Albert Dobi, Meera Srivastava, Sanjay Malhotra, Clifton Dalgard, Shiv Srivastava. Structure-activity studies and biological evaluations of ERGi-USU, a highly selective inhibitor for ERG-positive prostate cancer cells [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 1183. doi:10.1158/1538-7445.AM2017-1183

Synergistic Activity with NOTCH Inhibition and Androgen Ablation in ERG-Positive Prostate Cancer Cells

The oncogenic activation of the ETS-related gene (ERG) due to gene fusions is present in over half of prostate cancers in Western countries. Because of its high incidence and oncogenic role, ERG and components of ERG network have emerged as potential drug targets for prostate cancer. Utilizing gene expression datasets, from matched normal and prostate tumor epithelial cells, an association of NOTCH transcription factors with ERG expression status was identified, confirming that NOTCH factors are direct transcriptional targets of ERG. Inhibition of ERG in TMPRSS2-ERG-positive VCaP cells led to decreased levels of NOTCH1 and 2 proteins and downstream transcriptional targets and partially recapitulated the phenotypes associated with ERG inhibition. Regulation of NOTCH1 and 2 genes by ERG were also noted with ectopic ERG expression in LNCaP (ERG-negative prostate cancer) and RWPE-1 (benign prostate-derived immortalized) cells. Furthermore, inhibition of NOTCH by the small-molecule γ-secretase inhibitor 1, GSI-1, conferred an increased sensitivity to androgen receptor (AR) inhibitors (bicalutamide and enzalutamide) or the androgen biosynthesis inhibitor (abiraterone) in VCaP cells. Combined treatment with bicalutamide and GSI-1 showed strongest inhibition of AR, ERG, NOTCH1, NOTCH2, and PSA protein levels along with decreased cell growth, cell survival, and enhanced apoptosis. Intriguingly, this effect was not observed in ERG-negative prostate cancer cells or immortalized benign/normal prostate epithelial cells. These data underscore the synergy of AR and NOTCH inhibitors in reducing the growth of ERG-positive prostate cancer cells.Implications: Combinational targeting of NOTCH and AR signaling has therapeutic potential in advanced ERG-driven prostate cancers. Mol Cancer Res; 15(10); 1308-17. ©2017 AACR.

Autoantibodies against oncogenic ERG protein in prostate cancer: potential use in diagnosis and prognosis in a panel with C-MYC, AMACR and HERV-K Gag

Overdiagnosis and overtreatment of prostate cancer (CaP) is attributable to widespread reliance on PSA screening in the US. This has prompted us and others to search for improved biomarkers for CaP, to facilitate early detection and disease stratification. In this regard, autoantibodies (AAbs) against tumor antigens could serve as potential candidates for diagnosis and prognosis of CaP. Towards this, our goals were: i) To investigate whether AAbs against ERG oncoprotein (overexpressed in 25-50% of Caucasian American and African American CaP) are present in the sera of CaP patients; ii) To evaluate an AAb panel to enhance CaP detection. The results using an enzyme-linked immunosorbent assay (ELISA) showed that anti-ERG AAbs are present in a significantly higher proportion in the sera of CaP patients compared to healthy controls (p = 0.0001). Furthermore, a panel of AAbs against ERG, AMACR and human endogenous retrovirus-K Gag successfully differentiated CaP patient sera from healthy controls (AUC = 0.791). These results demonstrate for the first time that anti-ERG AAbs are present in the sera of CaP patients. In addition, the data also suggest that AAbs against ERG together with AMACR and HERV-K Gag may be a useful panel of biomarkers for diagnosis and prognosis of CaP.

Abstract 5058: Silencing of NOTCH signaling enhances the sensitivity of ERG positive prostate cancer cells to AR inhibitors

Introduction: Androgen receptor (AR) signaling plays a critical role in all the stages of prostate cancer (CaP) ranging from organ confined to castration-resistant (CRPC) phases. Although androgen deprivation therapy (ADT) remains the mainstay treatment for advanced CaP, the inevitable transition from androgen- sensitive to CRCP presents the most significant challenge in CaP therapy. Androgen dependent expression of oncogenic ETS related gene (ERG) in half of all CaP in western countries plays critical role in the tumorigenesis of CaP through regulation of cancer specific signaling pathways. We found that NOTCH transcription factors are common targets of ERG in ERG positive cancer cells. NOTCH signaling pathway is an important signaling pathway in the development of drug-resistant tumor growth. In the current study we evaluated the combinatorial effects of NOTCH and AR inhibitors in the context of ERG positive prostate cancer cells.

Methods: ERG, NOTCH1, NOTCH2 and downstream targets of NOTCH transcription factors were analyzed by Western blot assays. Dose and time kinetics of combining NOTCH inhibitor (γ-Secretase inhibitor 1, GSI-1) and AR inhibitors (Bicalutamide, Enzalutamide, and Abiraterone) were assessed in a panel of ERG positive or ERG negative CaP cells. Trypan blue exclusion, methylthiazole tetrazolium (MTT), or ApoTox-Glo™ Triplex assays were used to asses cell proliferation, apoptosis and drug cytotoxicity.

Results: Prostate cancer cell lines with endogenous or ectopic expression of ERG showed upregulation of NOTCH1 and NOTCH2. The NOTCH inhibitor, GSI-1 conferred an increased sensitivity to all tested AR inhibitors (Bicalutamide, Enzalutamide, and Abiraterone) with bicalutamide showing the most robust inhibition of AR, ERG, NOTCH1, NOTCH2, PSA, decreased cell growth and enhanced apoptosis in ERG positive VCaP cells. This observation was not seen in ERG negative LNCaP cells or in ERG positive primary endothelial cells.

Conclusions: NOTCH inhibitor enhanced sensitivity of AR inhibitors in ERG positive VCaP cells growth. The combination of the GSI-1 with AR inhibitors has shown synergistic effect when compared to single agent treatment. Taken together, our study suggests that NOTCH inhibitors may enhance the actions of AR inhibitors in the treatment of ERG positive prostate cancers. Inhibition of AR and NOTCH signaling may offer new opportunities in assessing ERG targeted therapy for prostate cancer.

Citation Format: Ahmed A. Mohamed, Shyh-Han Tan, Shilpa Katta, Charles P. Xavier, Lakshmi Ravindranath, Wei Huang, Hua Li, Meera Srivastava, Shashwat Sharad, Taduru Sreenath, Gyorgy Petrovics, Albert Dobi, Shiv Srivastava. Silencing of NOTCH signaling enhances the sensitivity of ERG positive prostate cancer cells to AR inhibitors. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 5058. doi:10.1158/1538-7445.AM2015-5058

Inactivation of Wnt signaling by a human antibody that recognizes the heparan sulfate chains of glypican-3 for liver cancer therapy

Wnt signaling is important for cancer pathogenesis and is often up-regulated in hepatocellular carcinoma (HCC). Heparan sulfate proteoglycans (HSPGs) function as coreceptors or modulators of Wnt activation. Glypican-3 (GPC3) is an HSPG that is highly expressed in HCC, where it can attract Wnt proteins to the cell surface and promote cell proliferation. Thus, GPC3 has emerged as a candidate therapeutic target in liver cancer. While monoclonal antibodies to GPC3 are currently being evaluated in preclinical and clinical studies, none have shown an effect on Wnt signaling. Here, we first document the expression of Wnt3a, multiple Wnt receptors, and GPC3 in several HCC cell lines, and demonstrate that GPC3 enhanced the activity of Wnt3a/β-catenin signaling in these cells. Then we report the identification of HS20, a human monoclonal antibody against GPC3, which preferentially recognized the heparan sulfate chains of GPC3, both the sulfated and nonsulfated portions. HS20 disrupted the interaction of Wnt3a and GPC3 and blocked Wnt3a/β-catenin signaling. Moreover, HS20 inhibited Wnt3a-dependent cell proliferation in vitro and HCC xenograft growth in nude mice. In addition, HS20 had no detectable undesired toxicity in mice. Taken together, our results show that a monoclonal antibody primarily targeting the heparin sulfate chains of GPC3 inhibited Wnt/β-catenin signaling in HCC cells and had potent antitumor activity in vivo.

CONCLUSION

An antibody directed against the heparan sulfate of a proteoglycan shows efficacy in blocking Wnt signaling and HCC growth, suggesting a novel strategy for liver cancer therapy.

Casein kinase 1δ functions at the centrosome and Golgi to promote ciliogenesis

CK1δ acts at the centrosome and Golgi to support polarized transport for ciliogenesis. It controls distribution of ciliary effectors Rab11, Rab8, CEP290, PCM1, and IFT20 and also promotes MT nucleation at the Golgi and positioning and integrity of the Golgi. Interaction of CK1δ with AKAP450 mediates Golgi MT nucleation and ciliogenesis.

Inhibition of casein kinase 1 delta (CK1δ) blocks primary ciliogenesis in human telomerase reverse transcriptase immortalized retinal pigmented epithelial and mouse inner medullary collecting duct cells-3. Mouse embryonic fibroblasts (MEFs) and retinal cells from Csnk1d (CK1δ)-null mice also exhibit ciliogenesis defects. CK1δ catalytic activity and centrosomal localization signal (CLS) are required to rescue cilia formation in MEFs^{Csnk1d null}. Furthermore, expression of a truncated derivative containing the CLS displaces full-length CK1δ from the centrosome and decreases ciliary length in control MEFs, suggesting that centrosomal CK1δ has a role in ciliogenesis. CK1δ inhibition also alters pericentrosomal or ciliary distribution of several proteins involved in ciliary transport, including Ras-like in rat brain-11A, Ras-like in rat brain-8A, centrosomal protein of 290 kDa, pericentriolar material protein 1, and polycystin-2, as well as the Golgi distribution of its binding partner, A-kinase anchor protein 450 (AKAP450). As reported for AKAP450, CK1δ was required for microtubule nucleation at the Golgi and maintenance of Golgi integrity. Overexpression of an AKAP450 fragment containing the CK1δ-binding site inhibits Golgi-derived microtubule nucleation, Golgi distribution of intraflagellar transport protein 20 homologue, and ciliogenesis. Our results suggest that CK1δ mediates primary ciliogenesis by multiple mechanisms, one involving its centrosomal function and another dependent on its interaction with AKAP450 at the Golgi, where it is important for maintaining Golgi organization and polarized trafficking of multiple factors that mediate ciliary transport.

Secreted Frizzled-related protein potentiation versus inhibition of Wnt3a/β-catenin signaling

Wnt signaling regulates a variety of cellular processes during embryonic development and in the adult. Many of these activities are mediated by the Frizzled family of seven-pass transmembrane receptors, which bind Wnts via a conserved cysteine-rich domain (CRD). Secreted Frizzled-related proteins (sFRPs) contain an amino-terminal, Frizzled-like CRD and a carboxyl-terminal, heparin-binding netrin-like domain. Previous studies identified sFRPs as soluble Wnt antagonists that bind directly to Wnts and prevent their interaction with Frizzleds. However, subsequent observations suggested that sFRPs and Frizzleds form homodimers and heterodimers via their respective CRDs, and that sFRPs can stimulate signal transduction. Here, we present evidence that sFRP1 either inhibits or enhances signaling in the Wnt3a/β-catenin pathway, depending on its concentration and the cellular context. Nanomolar concentrations of sFRP1 increased Wnt3a signaling, while higher concentrations blocked it in HEK293 cells expressing a SuperTopFlash reporter. sFRP1 primarily augmented Wnt3a/β-catenin signaling in C57MG cells, but it behaved as an antagonist in L929 fibroblasts. sFRP1 enhanced reporter activity in L cells that were engineered to stably express Frizzled 5, though not Frizzled 2. This implied that the Frizzled expression pattern could determine the response to sFRP1. Similar results were obtained with sFRP2 in HEK293, C57MG and L cell reporter assays. CRDsFRP1 mimicked the potentiating effect of sFRP1 in multiple settings, contradicting initial expectations that this domain would inhibit Wnt signaling. Moreover, CRDsFRP1 showed little avidity for Wnt3a compared to sFRP1, implying that the mechanism for potentiation by CRDsFRP1 probably does not require an interaction with Wnt protein. Together, these findings demonstrate that sFRPs can either promote or suppress Wnt/β-catenin signaling, depending on cellular context, concentration and most likely the expression pattern of Fzd receptors.

sFRP-1 binds via its netrin-related motif to the N-module of thrombospondin-1 and blocks thrombospondin-1 stimulation of MDA-MB-231 breast carcinoma cell adhesion and migration

Secreted frizzled-related protein (sFRP)-1 is a Wnt antagonist that inhibits breast carcinoma cell motility, whereas the secreted glycoprotein thrombospondin-1 stimulates adhesion and motility of the same cells. We examined whether thrombospondin-1 and sFRP-1 interact directly or indirectly to modulate cell behavior. Thrombospondin-1 bound sFRP-1 with an apparent K(d)=48nM and the related sFRP-2 with a K(d)=95nM. Thrombospondin-1 did not bind to the more distantly related sFRP-3. The association of thrombospondin-1 and sFRP-1 is primarily mediated by the amino-terminal N-module of thrombospondin-1 and the netrin domain of sFRP-1. sFRP-1 inhibited α3β1 integrin-mediated adhesion of MDA-MB-231 breast carcinoma cells to a surface coated with thrombospondin-1 or recombinant N-module, but not adhesion of the cells on immobilized fibronectin or type I collagen. sFRP-1 also inhibited thrombospondin-1-mediated migration of MDA-MB-231 and MDA-MB-468 breast carcinoma cells. Although sFRP-2 binds similarly to thrombospondin-1, it did not inhibit thrombospondin-1-stimulated adhesion. Thus, sFRP-1 binds to thrombospondin-1 and antagonizes stimulatory effects of thrombospondin-1 on breast carcinoma cell adhesion and motility. These results demonstrate that sFRP-1 can modulate breast cancer cell responses by interacting with thrombospondin-1 in addition to its known effects on Wnt signaling.