The STAT3 inhibitor GPB730 enhances the sensitivity to enzalutamide in prostate cancer cells

Enzalutamide is a second-generation anti-androgen which has shown increased survival in patients with metastatic prostate cancer. However, some patients do not respond to this therapy or will develop resistance to treatment over time. Signal Transducer and Activator of Transcription 3 (STAT3) is known to be involved in castration-resistant prostate cancer and to interact with androgen receptor (AR)-signaling. This study aims to investigate the combination enzalutamide and the small molecule STAT3 inhibitor GPB730 for enhanced therapeutic effect in advanced prostate cancer in vitro. The prostate cancer cell lines LNCaP (androgen dependent) and C4-2 (androgen insensitive) were used. The effect of enzalutamide and GPB730, alone and in combination, was investigated on viability and IC₅₀ values calculated. Enzalutamide and GPB730 treated LNCaP and C4-2 cells were subjected to western blot and QPCR analyses in order to investigate the expression of AR, STAT3 and down-stream targets. C4-2 were less sensitive to growth inhibition by enzalutamide than LNCaP cells. GPB730 enhanced the growth inhibitory effect of enzalutamide in LNCaP and C4-2 cells. The addition of GPB730 to enzalutamide decreased the IC₅₀ values for enzalutamide by 3.3-fold for LNCaP and by 12-fold for C4-2. In C4-2 cells, GPB730 alone decreased PSA expression and enhanced the enzalutamide induced decrease in NKX3.1 expression. GPB730 and enzalutamide in combination enhanced inhibition of c-myc and survivin expression. This study suggests that enzalutamide may be combined with the STAT3 inhibitor GPB730 in order to enhance the efficacy of enzalutamide, offering a new therapeutic approach in advanced prostate cancer.

Inhibition of STAT3 augments antitumor efficacy of anti-CTLA-4 treatment against prostate cancer

There is an urgent need for new treatment options in metastatic drug-resistant prostate cancer. Combining immunotherapy with other targeted therapies may be an effective strategy for advanced prostate cancer. In the present study, we sought to investigate to enhance the efficacy of anti-CTLA-4 therapy against prostate cancer by the combination with STAT3 inhibition.

Male C57BL6 mice were subcutaneously inoculated with the murine prostate cancer cell line RM-1. Tumor progression was monitored following treatment with vehicle, the small molecule STAT3 inhibitor GPB730, anti-CTLA-4 or GPB730 + anti-CTLA-4. Treatment with anti-CTLA-4 or anti-CTLA-4 + GPB730 significantly inhibited tumor growth and enhanced survival compared to vehicle. Combining anti-CTLA-4 treatment with GPB730 resulted in a significantly prolonged survival compared to anti-CTLA-4 alone. GPB730 significantly increased infiltration of CD45 + cells in tumors of anti-CTLA-4-treated mice compared to anti-CTLA-4 alone. The levels of tumor-infiltrating Tregs were significantly decreased and the CD8:Treg ratio significantly increased by GPB730 treatment in combination with anti-CTLA-4 compared to anti-CTLA-4 alone. Immunohistochemical analysis showed a significant increase in CD45-positive cells in anti-CTLA-4 and anti-CTLA-4 + GPB730-treated tumors compared to vehicle or GPB730 monotherapy. Plasma levels of IL10 were significantly increased by anti-CTLA-4 compared to vehicle but no increase was observed when combining anti-CTLA-4 with GPB730.

In conclusion, STAT3 inhibition by GPB730 enhances the antitumoral activity of anti-CTLA-4 and decreases the intratumoral Treg frequency in a prostate cancer mouse model. These results support the combination of STAT3 inhibition with anti-CTLA-4 therapy to increase clinical responses in patients with prostate cancer.

Abstract A73: Human granulocytic myeloid-derived suppressor cells (G-MDSCs) in metastatic breast cancer patients is a heterogeneous population with angiogenic potential in vivo

MDSCs are potent immunosuppressive myeloid cells that have been implicated in various diseases, including cancer. In humans, MDSCs are divided into Mo-GMDSCs and G-MDSCs subgroups, depending on their surface phenotype and function. While their immunosuppressive properties have been extensively studied, knowledge about their origin and their tumor-promoting functions per se remains scarce. In this study, we demonstrate that G-MDSCs are significantly enriched in the peripheral blood of locoregional recurrent metastatic breast cancer (LRR/MBC) compared to healthy donors. The G-MDSCs display a heterogeneous population with a morphology representing one blast-like and one polymorphonuclear (PMN) population. In a breast cancer xenograft model, co-transplanting G-MDSCs sorted from LRR/MBC together with breast cancer cells significantly promoted angiogenesis and tumor growth. Gene expression profiling analysis revealed that G-MDSCs from LRR/MBC rather clustered with neutrophils from healthy donors, sharing similar expression in genes relevant for angiogenesis, lymphangiogenesis and immunosuppression, but surprisingly not with G-MDSCs from sepsis patients. We conclude that enrichment of G-MDSCs in metastatic breast cancer represents a heterogeneous population of activated neutrophils that can promote angiogenesis and tumor progression, and immature blasts of yet unknown character.

Citation Format: Meliha Mehmeti, Caroline Bergenfelz, Daniel Bexell, Robert Carlsson, Rebecka Hellsten, Anna-Maria Larsson, Niklas Loman, Kristian Riesbeck, Jonas Ahl, Camilla Rydberg-Millrud, Gesine Paul-Visse, Lisa Rydén, Fredrika Killander, Karin Leandersson. Human granulocytic myeloid-derived suppressor cells (G-MDSCs) in metastatic breast cancer patients is a heterogeneous population with angiogenic potential in vivo [abstract]. In: Proceedings of the AACR Special Conference on Tumor Immunology and Immunotherapy; 2018 Nov 27-30; Miami Beach, FL. Philadelphia (PA): AACR; Cancer Immunol Res 2020;8(4 Suppl):Abstract nr A73.

The STAT3 inhibitor galiellalactone inhibits the generation of MDSC-like monocytes by prostate cancer cells and decreases immunosuppressive and tumorigenic factors

BACKGROUND

The transcription factor signal transducer and activator of transcription 3 (STAT3) is implicated in cancer drug resistance, metastasis, and immunosuppression and has been identified as a promising therapeutic target for new anticancer drugs. Myeloid-derived suppressor cells (MDSCs) play a major role in the suppression of antitumor immunity and STAT3 is involved in the accumulation, generation, and function of MDSCs. Thus, targeting STAT3 holds the potential of reversing immunosuppression in cancer. This study aims to investigate the effect of the small molecule STAT3 inhibitor galiellalactone on prostate cancer cell- induced generation of MDSCs from monocytes and the effect on immunosuppressive factors and inflammatory cytokines.

METHODS

Primary human monocytes were cocultured with prostate cancer cells (DU145, PC3, and LNCaP-IL6) or with conditioned medium (CM) from prostate cancer cells in the presence or absence of the STAT3 inhibitor galiellalactone. Monocytes were analyzed by flow cytometry for an MDSC-like phenotype (CD14⁺ HLA-DR^-/lo ). The secretion and gene expression of immunosuppressive factors and inflammatory cytokines from prostate cancer cells and monocytes were investigated.

RESULTS

Galiellalactone blocked the prostate cancer cell-induced generation of MDSC-like monocytes with an immunosuppressive phenotype ex vivo. Monocytes cultured with CM from prostate cancer cells showed increased expression of phosphorylated STAT3. Prostate cancer cells increased the expression of interleukin1β (IL1β), IL10, and IL6 in monocytes which was inhibited by galiellalactone. In addition, galiellalactone decreased indoleamine 2,3-dioxygenase gene expression in monocytes. Galiellalactone reduced the levels of IL8 and granulocyte macrophage-colony stimulating factor in prostate cancer cells per se.

CONCLUSION

The STAT3 inhibitor galiellalactone may prevent the prostate cancer cell-induced generation of MDSCs and reverse the immunosuppressive mechanisms caused by the interplay between prostate cancer cells and MDSCs. This is a potential new immunotherapeutic approach for the treatment of prostate cancer.

Expression of tSTAT3, pSTAT3727 , and pSTAT3 705 in the epithelial cells of hormone-naïve prostate cancer

BACKGROUND

The signal transducer and activator of transcription 3 (STAT3) pathway is observed to be constitutively activated in several malignancies including prostate cancer (PCa). In the present study, we investigated the expression of total STAT3 (tSTAT3) and two forms of activated phosphorylated STAT3 (pSTAT3⁷²⁷ and pSTAT3⁷⁰⁵ ) in tissue microarrays (TMA) of two cohorts of localized hormone-naïve PCa patients and analyzed associations between the expression and disease outcome.

METHODS

The expression of tSTAT3, pSTAT3⁷²⁷ , and pSTAT3⁷⁰⁵ was scored in the nuclei and cytoplasm of prostatic gland epithelial cells in two TMAs of paraffin-embedded prostatic tissue. The TMAs consisted of tissue originated from hormone-naïve radical prostatectomy patients from two different sites: Malmö, Sweden (n = 300) and Dublin, Ireland (n = 99).

RESULTS

The nuclear expression levels of tSTAT3, pSTAT3⁷²⁷ , and pSTAT3⁷⁰⁵ in the epithelial cells of benign glands were significantly higher than in the cancerous glands. Cytoplasmic tSTAT3 levels were also higher in benign glands. Patients with low pSTAT3⁷²⁷ and pSTAT3⁷⁰⁵ levels in the cancerous glands showed reduced times to biochemical recurrence, compared with those with higher levels. No significant trends in nuclear nor in cytoplasmic tSTAT3 were observed in relation to biochemical recurrence in the Malmö cohort. Higher cytoplasmic tSTAT3 was associated with reduced time to biochemical recurrence in the Dublin cohort. Adding the tSTAT3 and pSTAT3 expression data to Gleason score or pathological T stage did not improve their prognostic values.

CONCLUSIONS

Low pSTAT3⁷²⁷ and pSTAT3⁷⁰⁵ expression in epithelial cells of cancerous prostatic glands in hormone-naïve PCa was associated with faster disease progression. However, pSTAT3 and tSTAT3 expression did not improve the prognostic value of Gleason score or pathological T stage and may not be a good biomarker in the early hormone naïve stages of PCa.

Abstract A040: Targeting the prostate cancer stem cell niche with the STAT3 inhibitor galiellalactone

Introduction and Objectives: Cancer stem-like cells (CSCs) represent a small subpopulation of largely quiescent cells that reside within tumors. Several studies have demonstrated that this population is more resistant to current therapies and is therefore directly responsible for tumor recurrence. Recent evidence suggests that the transcription factor STAT3 is crucial to the survival of stem cells in prostate cancer (PCa). Thus, inhibition of activated STAT3 (pSTAT3) is a valid strategy to selectively target PCa CSCs. We have previously shown that pSTAT3 blockade, by galielallactone (GL), not only reduces proliferation and induces apoptosis of prostate cancer cells in vitro, but it also inhibits the growth of prostate tumors and the metastatic spread to regional and distal lymph nodes, in vivo. Here we performed experiments aimed at studying the effect of the STAT3 inhibitor, GL, on PCa CSCs, as a promising therapeutic approach for prostate cancer patients.

Materials and Methods: The expression of stem, basal, and luminal cell surface markers (CD133/CD44/CD24) was analyzed by FACS on DU145 and PC3 cells following treatment with GL. DU145 cells were sorted, based on the expression of the stem cell surface marker CD133, and then plated for further analysis. Expression of pSTAT3 was studied by IHC, on sorted cells, and the effect of GL on clonogenic recovery was determined. Furthermore, the effect of GL on sphere formation efficiency was studied on the CD133 population. The compound galiellalactone was obtained from Glactone Pharma (Sweden).

Results: Our results show that the CSCs population (CD133+/CD44+) expresses high levels of activated pSTAT3 compared to the CD44+/CD24+ cell population. Treatment with GL decreased the number of CSCs in DU145 cells after 48h, while the number of CD44+/CD24+ cells was not significantly affected. Importantly, treatment with GL had no effect on CD133+/CD44+ or CD44+/CD24+ populations in PC3 cells, which do not express pSTAT3. Treatment with GL significantly reduced the ability of CSCs to form colonies and to generate prostate-derived spheres. Interestingly, the CSCs population (CD133+) was more sensitive to GL in terms of colony- and sphere-forming efficiency compared to the CD133- population.

Conclusion: This study demonstrates that the STAT3 inhibitor galiellalactone can specifically target the prostate cancer stem-like cell population in vitro, implying that pSTAT3 inhibition by GL could represent a valid therapeutic strategy to antagonize CSCs in human prostate cancer. Future experiments will be aimed at validating these data and relating the clonogenic inhibition to the effects of pSTAT3 blockade by GL on tumor initiation by prostate cancer stem-like cells in vivo.

Citation Format: Giacomo Canesin, Anne T. Collins, Rebecka Hellsten, Norman J. Maitland, Anders Bjartell. Targeting the prostate cancer stem cell niche with the STAT3 inhibitor galiellalactone [abstract]. In: Proceedings of the AACR Special Conference: Prostate Cancer: Advances in Basic, Translational, and Clinical Research; 2017 Dec 2-5; Orlando, Florida. Philadelphia (PA): AACR; Cancer Res 2018;78(16 Suppl):Abstract nr A040.

Treatment with the WNT5A-mimicking peptide Foxy-5 effectively reduces the metastatic spread of WNT5A-low prostate cancer cells in an orthotopic mouse model

Prostate cancer patients with high WNT5A expression in their tumors have been shown to have more favorable prognosis than those with low WNT5A expression. This suggests that reconstitution of Wnt5a in low WNT5A-expressing tumors might be an attractive therapeutic approach. To explore this idea, we have in the present study used Foxy-5, a WNT5A mimicking peptide, to investigate its impact on primary tumor and metastasis in vivo and on prostate cancer cell viability, apoptosis and invasion in vitro. We used an in vivo orthotopic xenograft mouse model with metastatic luciferase-labeled WNT5A-low DU145 cells and metastatic luciferase-labeled WNT5A-high PC3prostate cancer cells. We provide here the first evidence that Foxy-5 significantly inhibits the initial metastatic dissemination of tumor cells to regional and distal lymph nodes by 90% and 75%, respectively. Importantly, this effect was seen only with the WNT5A-low DU145 cells and not with the WNT5A-high PC3 cells. The inhibiting effect in the DU145-based model occurred despite the fact that no effects were observed on primary tumor growth, apoptosis or proliferation. These findings are consistent with and supported by the in vitro data, where Foxy-5 specifically targets invasion without affecting apoptosis or viability of WNT5A-low prostate cancer cells. To conclude, our data indicate that the WNT5A-mimicking peptide Foxy-5, which has been recently used in a phase 1 clinical trial, is an attractive candidate for complimentary anti-metastatic treatment of prostate cancer patients with tumors exhibiting absent or low WNT5A expression.

Abstract 3900: STAT3 inhibition with Galiellalactone effectively targets the prostate cancer stem-like cell population

Introduction and objectives Cancer stem-like cells (CSCs) represent a small subpopulation of largely quiescent cells that reside within tumors. Several studies have demonstrated that this small population is more resistant to current therapies and is therefore directly responsible for tumor recurrence. The transcription factor STAT3 has been recently related to the regulation of the stem cell niche in prostate cancer (PCa), and inhibition of activated STAT3 (pSTAT3) seems to be a valid strategy to selectively target PCa CSCs. We have previously shown that pSTAT3 blockade by Galielallactone (GL) not only reduces proliferation and induces apoptosis of prostate cancer cells in vitro, but also inhibits the growth of prostate tumors and the metastatic spread to regional and distal lymph nodes in vivo. Here we performed experiments aimed at studying the effect of the inhibitor GL on PCa CSCs, in order to increase our understanding on this compound as a promising therapeutic approach for prostate cancer patients.

Materials and methods The expression of stem, basal and luminal cell surface markers (CD133/CD44/CD24) was analyzed by FACS on DU145 and PC3 cells treated with GL. DU145 cells were sorted based on expression of the above markers and then plated for further analysis, including expression of pSTAT3 by IHC and cell proliferation by the WST1 assay. The effect of GL on clonogenic recovery of PCa CSCs was studied by colony formation with sorted cells.

Results Our results show that the CSC populations (CD133+/CD44+) express high levels of activated pSTAT3 compared to the CD44+/CD24+ cell population. Treatment with GL decreased the number of CSCs in DU145 cells after 48h, whilst the number of CD44+/CD24+ cells was less affected. Importantly, treatment with GL had no effect on CD133+/CD44+ or CD44+/CD24+ populations in PC3 cells, which don’t express pSTAT3. Of note, both CD133+/CD44+ and CD44+/CD24+ cells responded to treatment with GL, as proliferation decreased in a dose-dependent manner. Treatment with GL significantly reduced the ability of CSCs to form colonies in a concentration-dependent manner.

Conclusion This study demonstrates that the STAT3 inhibitor Galiellalactone can specifically target the prostate cancer stem-like cell population in vitro, implying that pSTAT3 inhibition by GL could represent a valid therapeutic strategy to antagonize CSCs in human prostate cancer. Future experiments will be aimed at validating these data and relating the clonogenic inhibition to the effects of pSTAT3 blockade by GL on tumor initiation by prostate cancer stem-like cells in vivo.

Citation Format: Giacomo Canesin, Anne T. Collins, Rebecka Hellsten, Norman J. Maitland, Anders Bjartell. STAT3 inhibition with Galiellalactone effectively targets the prostate cancer stem-like cell population [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 3900. doi:10.1158/1538-7445.AM2017-3900

Expression of STAT3 in Prostate Cancer Metastases

STAT3 and its upstream activator IL6R have been implicated in the progression of prostate cancer and are possible future therapeutic targets. We analyzed 223 metastatic samples from rapid autopsies of 71 patients who had died of castration-resistant prostate cancer (CRPC) to study protein and gene expression of pSTAT3 and IL6R. Immunohistochemical analysis revealed that 95% of metastases were positive for pSTAT3 and IL6R, with varying expression levels. Bone metastases showed significantly higher expression of both pSTAT3 and IL6R in comparison to lymph node and visceral metastases. STAT3 mRNA levels were significantly higher in bone than in lymph node and visceral metastases, whereas no significant difference in IL6R mRNA expression was observed. Our study strongly supports the suggested view of targeting STAT3 as a therapeutic option in patients with metastatic CRPC.

PATIENT SUMMARY

We studied the levels of two proteins (pSTAT3 and IL6R) in metastases from patients who died from castration-resistant prostate cancer. We found high levels of pSTAT3and IL6R in bone metastases, suggesting that these proteins could be used as targets for new anticancer drugs.

Preclinical Characterization of 3β-(N-Acetyl l-cysteine methyl ester)-2aβ,3-dihydrogaliellalactone (GPA512), a Prodrug of a Direct STAT3 Inhibitor for the Treatment of Prostate Cancer

The transcription factor STAT3 is a potential target for the treatment of castration-resistant prostate cancer. Galiellalactone (1), a direct inhibitor of STAT3, prevents the transcription of STAT3 regulated genes. In this study we characterized 6 (GPA512, Johansson , M. ; Sterner , O. Patent WO 2015/132396 A1, 2015 ), a prodrug of 1. In vitro studies showed that 6 is rapidly converted to 1 in plasma and is stable in a buffer solution. The pharmacokinetics of 6 following a single oral dose indicated that the prodrug was rapidly absorbed and converted to 1 with a tmax of 15 min. Oral administration of 6 in mice increased the plasma exposure of the active parent compound 20-fold compared to when 1 was dosed orally. 6 treated mice bearing DU145 xenograft tumors had significantly reduced tumor growth compared to untreated mice. The favorable druglike properties and safety profile of 6 warrant further studies of 6 for the treatment of castration-resistant prostate cancer.

Abstract C179: Preclinical characterization of GPA512: A prodrug of a direct STAT3 inhibitor for the treatment of prostate cancer

Background: The transcription factor STAT3 is a promising target for the treatment of castration-resistant prostate cancer (CRPC) as STAT3 is implicated in drug resistance and castration resistance as well as metastatic spread, tumor growth and immune escape. Galiellalactone (GL) is a direct inhibitor of STAT3 that prevents DNA binding, inhibits proliferation of prostate cancer cells expressing active STAT3 and induces apoptosis by down-regulation of STAT3 activated genes. In this study we aimed to characterize a prodrug of GL, GPA512, with improved drug-like properties and to demonstrate its effect on tumor growth in a xenograft model of prostate cancer following oral administration.

Methods: Stability studies of prodrugs based on GL were performed in 0.1 M PBS buffer (pH 7.4) and in plasma at 37°C. In vitro efficacy of prodrugs was studied by WST-1 proliferation assay in DU145 prostate cancer cells expressing active STAT3. The systemic exposure of GL in mice was studied following a single oral dose of GPA512 or GL (both 10 mg/kg). The plasma concentrations of GL were determined by LC-MS/MS. For the xenograft study NMRI-nude male mice were inoculated subcutaneously with DU145 cells and once tumors were established the mice were divided in two groups with ten mice in each. Mice were treated orally with 40 mg/kg GPA512 daily five times/week for four weeks. Tumor growth was measured by caliper and at the end of the study tumors were harvested for subsequent analyses using immunohistochemistry and mRNA expression analysis.

Results: In vitro studies showed that the prodrug GPA512 is rapidly converted to GL in plasma and that GPA512 is stable in buffer solution and has similar inhibitory effects on proliferation as GL on DU145 cells. The pharmacokinetics of GPA512 following a single oral dose indicated that the compound was rapidly absorbed and converted to GL with a tmax of 15 min. Oral administration of GPA512 in mice increased the plasma exposure (AUC) of the active parent compound 20-fold compared to when GL was dosed orally. GPA512 treated mice bearing subcutaneous DU145 tumors had significantly smaller tumors compared to mice treated with vehicle. No adverse effects or weight loss were observed. Analysis of tumors showed decreased cell proliferation and increased amount of apoptotic cells in GPA512 treated mice compared to control. The mRNA expression of STAT3 regulated anti-apoptotic gene MCl-1 was significantly reduced by GPA512 treatment.

Conclusions: The drug-like properties and safety profile of the prodrug GPA512 and galiellalactone's direct inhibition of STAT3, warrant further studies of GPA512 as a drug candidate for treatment of patients with CRPC.

Citation Format: Rebecka Hellsten, Zilma Escobar, Anders Bjartell, Giacomo Canesin, Susan Evans Axelsson, Olov Sterner, Martin Johansson. Preclinical characterization of GPA512: A prodrug of a direct STAT3 inhibitor for the treatment of prostate cancer. [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference: Molecular Targets and Cancer Therapeutics; 2015 Nov 5-9; Boston, MA. Philadelphia (PA): AACR; Mol Cancer Ther 2015;14(12 Suppl 2):Abstract nr C179.

Abstract 4476: Targeting Wnt5a and STAT3 pathways for the treatment of prostate cancer

Background and Aims

Wnt5a protein levels are upregulated in prostate cancer (PCa) compared to benign tissue and patients with high Wnt5a protein levels have a better outcome after radical prostatectomy compared to patients with low Wnt5a levels (Syed Khaja AS, 2011; 2012). Thus, the reconstitution of the Wnt5a signaling pathway could be a promising therapeutic approach in PCa, as already shown in breast cancer (Säfholm A, 2008). Constitutively active STAT3 (pSTAT3) has been correlated to PCa progression and disruption of its signaling pathway could represent a promising strategy for the treatment of patients with advanced PCa (Hellsten R, 2008). We have analyzed the expression of Wnt5a and pSTAT3 in a cohort of patients with metastatic Castration Resistant Prostate Cancer (mCRPC). Using in vitro and in vivo techniques we have explored the effect of the Wnt5a-mimicking peptide FOXY5 for the reconstitution of the Wnt5a pathway in PCa, and we have studied the effect of the STAT3 inhibitor Galiellalactone (GL) on PCa cell viability, apoptosis, invasion, tumor growth and metastases.

Methods

Wnt5a and pSTAT3 protein expression was analyzed by IHC on a TMA construct with duplicate cores of malignant tissues from 38 patients with mCRPC. Statistical and correlation analyses were performed using the SPSS version 21. Cell viability was determined by MTT assay; apoptotic cells were visualized with M30 CytoDeath antibody 24h after treatment with FOXY5 or GL. Cell invasion was analyzed using matrigel pre-coated cell culture inserts in the presence or absence of FOXY5 or GL. For the animal study, NMRI nude mice were injected orthotopically with 1×106 DU145-Luc cells, and treated IP with GL every day or with FOXY5 every second day. Primary tumor growth and metastatic spread were evaluated weekly by in vivo luminescence using the IVIS Lumina II system. 9 weeks after injection animals were sacrificed and organs were analyzed for the presence of metastases.

Results

IHC analyses of TMA showed high pSTAT3 and low Wnt5a expression in CRPC patients, with a small but significant correlation (r = 0.4646; p = 0.0038). Our results show that GL inhibits viability and induces apoptosis of DU145-Luc cells in vitro, and that it reduces tumor growth and metastatic spread to lymph nodes in vivo. FOXY5 treatment has no effects on cell viability or on primary tumor growth, but it significantly reduces cell invasion in vitro and metastatic spread to lymph nodes in vivo.

Conclusions

Our results confirm that the reconstitution of the Wnt5a pathway and the inhibition of the STAT3 pathway are a promising therapeutic approach for the treatment of mCRPC. We found that GL and FOXY5 are good candidates for the treatment of mCRPC, as they inhibit tumor growth and metastatic spread to lymph nodes. Since these two compounds act through different mechanisms, our future studies will explore the possibility of a combination of GL and FOXY5 for the treatment of PCa.

Citation Format: Giacomo Canesin, Susan Evans-Axelsson, Rebecka Hellsten, Nicholas Don-Doncow, Tommy Andersson, Anders Bjartell. Targeting Wnt5a and STAT3 pathways for the treatment of prostate cancer. [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 4476. doi:10.1158/1538-7445.AM2015-4476

The STAT3 Inhibitor Galiellalactone Effectively Reduces Tumor Growth and Metastatic Spread in an Orthotopic Xenograft Mouse Model of Prostate Cancer

Signal transducer and activator of transcription 3 (STAT3) is known to be involved in the progression of prostate cancer (PCa) and is a key factor in drug resistance and tumor immunoescape. As a result, it represents a promising target for PCa therapy. We studied the effects of the STAT3 inhibitor galiellalactone (GL) on tumor growth and metastatic spread in vitro and in vivo. The effect of GL on cell viability, apoptosis, and invasion was studied in vitro using androgen-independent DU145 and DU145-Luc cell lines. For in vivo studies, mice were injected orthotopically with DU145-Luc cells and treated with daily intraperitoneal injections of GL for 6 wk. GL significantly reduced the growth of the primary tumor and the metastatic spread of PCa cells to regional and distal lymph nodes in vivo. Treatment with GL also resulted in decreased cell proliferation and increased apoptosis compared with controls. In vitro, GL reduces the viability and invasive abilities of DU145-Luc cells and induces apoptosis. Our results showed that tumor growth and early metastatic dissemination of PCa can be significantly reduced by GL, indicating its potential use as a therapeutic compound in advanced metastatic PCa.

PATIENT SUMMARY

In this study, we tested the STAT3 inhibitor galiellalactone (GL) in an animal model of PCa. We found that mice treated with GL had smaller primary tumors and decreased lymph node metastases compared with mice treated with vehicle. GL has potential for treating advanced metastatic PCa.

Therapeutic Targeting of Nuclear γ-Tubulin in RB1-Negative Tumors

In addition to its cytosolic function, γ-tubulin is a chromatin-associated protein. Reduced levels of nuclear γ-tubulin increase the activity of E2 promoter-binding factors (E2F) and raise the levels of retinoblastoma (RB1) tumor suppressor protein. In tumor cells lacking RB1 expression, decreased γ-tubulin levels induce cell death. Consequently, impairment of the nuclear activity of γ-tubulin has been suggested as a strategy for targeted chemotherapy of RB1-deficient tumors; thus, tubulin inhibitors were tested to identify compounds that interfere with γ-tubulin. Interestingly, citral increased E2F activity but impaired microtubule dynamics while citral analogues, such citral dimethyl acetal (CDA), increased E2F activity without affecting microtubules. The cytotoxic effect of CDA on tumor cells was attenuated by increased expression of either RB1 or γ-tubulin, and increased by reduced levels of either RB1 or γ-tubulin. Mechanistic study, in silico and in vitro, demonstrated that CDA prevents GTP binding to γ-tubulin and suggested that the FDA-approved drug dimethyl fumarate is also a γ-tubulin inhibitor. Finally, in vivo growth of xenograft tumors carrying defects in the RB1 signaling pathway were inhibited by CDA treatment. These results demonstrate that inhibition of γ-tubulin has the potential to specifically target tumor cells and may aid in the design of safer and more efficient chemotherapeutic regimes.

IMPLICATIONS

The in vivo antitumorigenic activity of γ-tubulin inhibitors paves the way for the development of a novel broad range targeted anticancer therapy that causes fewer side effects.

Galiellalactone is a direct inhibitor of the transcription factor STAT3 in prostate cancer cells

The transcription factor STAT3 is constitutively active in several malignancies including castration-resistant prostate cancer and has been identified as a promising therapeutic target. The fungal metabolite galiellalactone, a STAT3 signaling inhibitor, inhibits the growth, both in vitro and in vivo, of prostate cancer cells expressing active STAT3 and induces apoptosis of prostate cancer stem cell-like cells expressing phosphorylated STAT3 (pSTAT3). However, the molecular mechanism of this STAT3-inhibiting effect by galiellalactone has not been clarified. A biotinylated analogue of galiellalactone (GL-biot) was synthesized to be used for identification of galiellalactone target proteins. By adding streptavidin-Sepharose beads to GL-biot-treated DU145 cell lysates, STAT3 was isolated and identified as a target protein. Confocal microscopy revealed GL-biot in both the cytoplasm and the nucleus of DU145 cells treated with GL-biot, appearing to co-localize with STAT3 in the nucleus. Galiellalactone inhibited STAT3 binding to DNA in DU145 cell lysates without affecting phosphorylation status of STAT3. Mass spectrometry analysis of recombinant STAT3 protein pretreated with galiellalactone revealed three modified cysteines (Cys-367, Cys-468, and Cys-542). Here we demonstrate with chemical and molecular pharmacological methods that galiellalactone is a cysteine reactive inhibitor that covalently binds to one or more cysteines in STAT3 and that this leads to inhibition of STAT3 binding to DNA and thus blocks STAT3 signaling without affecting phosphorylation. This further validates galiellalactone as a promising direct STAT3 inhibitor for treatment of castration-resistant prostate cancer.

Abstract C229: Targeting STAT3 in prostate cancer: Identification of STAT3 as a direct target of the fungal metabolite galiellalactone

Introduction: The transcription factor Signal Transducer and Activator 3 (STAT3) is known to affect tumor growth in several malignancies. STAT3 is frequently found to be activated by phosphorylation (p-STAT3) in castration-resistant prostate cancer and constitutes a promising therapeutic target. The fungal metabolite galiellalactone (GL), a potent STAT3-signaling inhibitor, inhibits the growth, both in vitro and in vivo, of prostate cancer cells expressing activated STAT3 and inhibits growth and induces apoptosis of prostate cancer stem cell-like cells expressing p-STAT3. GL inhibits STAT3 signaling without blocking phosphorylation by a mechanism that we here aim to elucidate.

Materials and Methods: A biotinylated analogue of galiellalactone (GL-biot) was synthesized for identification of GL target proteins. The p-STAT3 expressing human prostate cancer cell line DU145 was incubated with GL-biot, followed by streptavidin beads to isolate proteins bound to GL-biot. Bound proteins were identified using Western blot analysis. Competition assays were performed by pre-incubating DU145 cell lysates with GL prior to GL-biot treatment. Localization of GL-biot and co-localization of GL-biot with STAT3 was assessed using confocal microscopy. Electrophoretic mobility shift assay (EMSA) was performed on GL treated DU145 cell lysates for analysis of STAT3 binding to DNA.

Results: The inhibitory activity of GL-biot on proliferation and inhibition of STAT3-signaling was similar to that of GL. By adding streptavidin beads to GL-biot-treated DU145 cell lysates, STAT3 was isolated and identified as a target protein. Pretreatment with GL prior to addition of GL-biot prevented the binding of GL-biot to STAT3 in a dose dependent manner. GL-biot was observed to interact also with STAT5 but not STAT1. Confocal microscopy revealed GL-biot in both the cytoplasm and nucleus of DU145 cells treated with GL-biot, appearing to co-localize with STAT3 in the nucleus. DU145 cell lysates subjected to EMSA with STAT3 probes showed a dose dependent decrease in STAT3-DNA binding when treated with GL.

Conclusions: Our results show that GL binds directly to STAT3 and inhibits DNA binding thus preventing the transcriptional activity of STAT3. This further validates GL as a promising STAT3-inhibitor for the treatment of castration-resistant prostate cancer.

Citation Information: Mol Cancer Ther 2013;12(11 Suppl):C229.

Citation Format: Nicholas Don-Doncow, Zilma Escobar, Martin Johansson, Eduardo Muñoz, Olov Sterner, Anders Bjartell, Rebecka Hellsten. Targeting STAT3 in prostate cancer: Identification of STAT3 as a direct target of the fungal metabolite galiellalactone. [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference: Molecular Targets and Cancer Therapeutics; 2013 Oct 19-23; Boston, MA. Philadelphia (PA): AACR; Mol Cancer Ther 2013;12(11 Suppl):Abstract nr C229.

Molecular pathways in the progression of hormone-independent and metastatic prostate cancer

Once prostate cancer becomes castration resistant, cancer cells may rapidly gain the ability to invade and to metastasize to lymph nodes and distant organs. The progression through hormone-dependent to hormone-independent/castration-resistant and metastatic PCa is poorly understood. In this review paper, we provide an overview on the cellular and molecular mechanisms underlying the process of tumor cell invasion and metastasis in prostate cancer. We specifically present the most recent findings on the role of multiple cellular signaling pathways including androgen receptor (AR), mitogen-activated protein kinases (MAPK), Akt, transforming growth factor b (TGFb interleukin-6 (IL-6) and vascular endothelial growth factor (VEGF) in the development of hormone-independent/castration-resistant prostate cancer. In addition, we also discuss the recent findings on signatures of gene expression during prostate cancer progression. Our overviews on the novel findings will help to gain better understanding of the complexity of molecular mechanisms that may play an essential role in the development of castration-resistant and metastatic prostate cancer. It will also shed light on the identification of specific targets and the design of effective therapeutic drug candidates.

Abstract 3554: The fungal metabolite galiellalactone inhibits ALDH positive prostate cancer cells

Introduction: Tumor stem cells have shown resistance to conventional cancer therapy and may be a cause for recurrent cancer, making tumor stem cell targeted therapy attractive. The fungal metabolite galiellalactone is suggested to be a Stat3 specific inhibitor and we have previously shown that galiellalactone inhibits prostate cancer (PCa) cell growth, both in vitro and in vivo, in PCa cells expressing phosphorylated Stat3 (Hellsten et al, 2008). Aldehyde dehydrogenase (ALDH) has shown promise as a marker of cancer stem cells and in this study we investigated the ALDH activity in human PCa cell lines and the effects of galiellalactone on ALDH+ PCa cells.

Methods: The human PCa cell lines DU145, LNCaP, and long-term interleukin-6 stimulated LNCaP (LNCaP-IL6) cells were used. Galiellalactone was prepared by synthesis (Johansson & Sterner, 2002). PCa cells were subjected to Aldefluor assay followed by flow cytometry to detect ALDH+ subpopulations. WST-1 proliferation assay was performed to study the effect of galiellalactone on proliferation of ALDH+ and ALDH- cells sorted from LNCaP-IL6 cells.

Results: In DU145 cells 2 ± 0.25% of the cells were ALDH+ and in LNCaP-IL6 cells, 2 ± 0.36% were ALDH+. No ALDH+ cells were detected in LNCaP cells. Treatment with 5, 10, 25 or 50 µM galiellalactone for 24h decreased the ALDH+ cell population in DU145 and LNCaP-IL6 cells in a dose dependent manner: 25 µM galiellalactone decreased the ALDH+ population in DU145 cells by 40% and by 75% in LNCaP-IL6 cells. In LNCaP-IL6 cells a dose dependent decrease in ALDH+ cells was observed after 72h (65% decrease at 25 µM galiellalactone). However, in DU145 cells no difference in the relative amount of ALDH+ cells was observed in galiellalactone treated DU145 cells compared to untreated control cells after 72h. The proliferation was decreased by galiellalactone in a dose dependent manner in all cell populations isolated from LNCaP-IL6 cells. The proliferation of ALDH+ cells isolated from LNCaP-IL6 cells was inhibited by 75%, ALDH- cells by 75% and unsorted LNCaP-IL6 cells by 80% after treatment with 25 µM galiellalactone for 48h.

Conclusions: ALDH+ subpopulations, possibly representing tumor stem cell populations were detected in human PCa cell lines, and were clearly inhibited by galiellalactone. ALDH is associated with cell growth and drug resistance, and targeting ALDH+ PCa cells with galiellalactone may be a novel potent treatment against therapy resistant PCa. The effect and mechanism of galiellalactone treatment of ALDH+ PCa cells will be further investigated.

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 3554.

Galiellalactone is a novel therapeutic candidate against hormone-refractory prostate cancer expressing activated Stat3

BACKGROUND

Signal transducer and activator of transcription 3 (Stat3) is constitutively active (phosphorylated) in several forms of cancer, including prostate cancer (PCa). Stat3 signaling may be an interesting target for cancer therapy since inhibition of this pathway mediates growth inhibition and apoptosis of these cells. In this study we investigated the in vitro and in vivo effects of the fungal metabolite galiellalactone, a direct inhibitor of Stat3, on PCa cells.

METHODS

The human PCa cell lines DU145, PC-3, and LNCaP were used. Nude mice with subcutaneous PCa cell xenografts were subjected to daily intraperitoneal injections of galiellalactone for 3 weeks. The effect of galiellalactone on the induction of apoptosis of cultured PCa cells was investigated by Western blot analysis, immunocytochemistry, and annexin V staining. Effects of galiellalactone on Stat3 signaling were investigated by a luciferase reporter gene assay. Expression of Stat3 associated proteins and mRNA was investigated by Western blot and real-time quantitative PCR analysis.

RESULTS

Galiellalactone induced apoptosis of p-Stat3 positive PCa cells (androgen-insensitive DU145 and PC-3) but not in cells lacking p-Stat3 (androgen-sensitive LNCaP). Galiellalactone inhibited Stat3-mediated luciferase activity (IC(50) approximately 5 microM) and reduced the expression of Bcl-2, Bcl-x(L), c-myc, and cyclin D1. Furthermore, galiellalactone significantly suppressed DU145 xenograft growth in vivo (42% growth reduction; P<0.002) and reduced the relative mRNA expression of Bcl-x(L) and Mcl-1.

CONCLUSIONS

Galiellalactone induced growth inhibition and apoptosis in androgen-insensitive PCa cells expressing p-Stat3. We suggest that galiellalactone is a potential anti-tumor lead against hormone-refractory PCa with constitutively active Stat3.

A highly conserved protein secreted by the prostate cancer cell line PC-3 is expressed in benign and malignant prostate tissue

In this study we characterize a novel gene on human chromosome 9 and its translation product, PC3-secreted microprotein (PSMP). The gene contains three exons that encode a protein of 139 amino acid residues, including a predicted signal peptide of 36 residues. The molecule is homologous to beta-microseminoprotein (MSP), a protein of unknown function, secreted at high concentration by the prostate gland. These two proteins have only 23% sequence identity, but their common origin is revealed by a preserved pattern of Cys residues. In contrast to MSP, which shows poor conservation between species, PSMP is very conserved. High transcript levels were detected in the prostate cancer cell line PC-3. Antiserum raised against PSMP detected a protein with an apparent molecular mass of 18 kDa in culture medium conditioned by PC-3 cells, but in cell lysates the antiserum also recognized a molecular species of 16 kDa, suggesting that PSMP undergoes post-translational modification. Xenografted PC-3 cell tumors in athymic nude mice showed strong staining for both PSMP protein and mRNA. Studies on human prostate cancer specimens showed immunohistochemical staining of both tumor and benign glandular cells. Our results suggest that PSMP is an important protein with significance in prostate cancer.