Cytotoxic and other bioactivities of a novel and known sesquiterpene lactones isolated from Vernonia leopoldi (Sch. Bip. ex Walp.) Vatke in breast cancer cell lines

Vernonia leopoldi (Sch. Bip. ex Walp.) Vatke (Asteraceae) is one of the widely used anti-cancer traditional medicinal plants in Ethiopia, despite the lack of data to support its therapeutic efficacy. Here we describe the isolation of compounds from the plant and the investigation of their cytotoxicity and other bioactivities. We identified the novel sesquiterpene lactone (SL) 11ß,13-dihydrovernodalol along with the three other SLs (vernomenin, vernolepin, and 11ß,13-dihydrovernodalin) and three flavonoids (apigenin, eriodyctiol, and luteolin) isolated from this plant for the first time. The structures of all the compounds were established based on extensive analysis of nuclear magnetic resonance spectroscopic data and confirmed by high-resolution electrospray ionization mass spectrometry. We then studied the biological activities of the SLs and found that all were cytotoxic at low μM ranges against MCF-7 and JIMT-1 breast cancer cells as well as against the normal-like MCF-10A breast epithelial cells evaluated in a spectrophotometric assay. All the SLs significantly reduced JIMT-1 cell migration after 72 h of treatment with 2 μM concentrations in a wound healing assay. Treatment with all SLs reduced the aldehyde dehydrogenase expressing cancer stem cell sub-population of the JIMT-1 cells significantly, evaluated by flow cytometry. Only 11ß,13-dihydrovernodalin resulted in a significant inhibition of tumor necrosis factor-α-induced translocation of nuclear factor κB to the cell nucleus. In addition, we show that the reporter fluorophore nitrobenzoxadiazole (NBD) can successfully be conjugated with an SL and that this SL-NBD conjugate is taken up efficiently in JIMT-1 cells. Therefore, the overall bioactivities of the SL compounds and specifically their effects against the stemness of breast cancer cells make them prime candidates for further in-depth investigation.

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Vernonia leopoldi (Sch. Bip. ex Walp.) Vatke is a traditional anticancer medicinal plant in Ethiopia.

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Sesquiterpene lactones (SLs) and flavonoids are isolated from V. leopoldi for the first time.

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A novel SL, named 11ß,13-dihydrovernodalol, was discovered.

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All the SLs reduce stemness and inhibit cell migration of cancer cells.

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A novel fluorophore-conjugated SL was synthesized for the study of SL uptake and localization in cells.

Stereoretentive Nucleophilic Substitution at the Tetrasubstituted Carbon of Galiellalactone

The fungal metabolite galiellalactone (1) was, as its acetate 4, discovered to undergo a substitution reaction with cysteine derivatives. By studying the reaction mechanism and the intermediates formed, and in an effort to expand the chemical diversity of the galiellalactonoids, a mild and general method of preparing ether, thioether, and amine analogues of galiellalactone was developed. The reaction is a formal stereoretentive nucleophilic substitution at an oxygenated tertiary carbon. NMR analysis of the progressing reaction shows that it involves an initial allylic substitution to form a new Michael acceptor, followed by the addition of a second equivalent of the nucleophile to this and, finally, a retro Michael reaction. This restores the original galiellalactone system with a double bond between C-2a and C-3, but with a new substituent at C-7b. As galiellalactone is a promising STAT3 inhibitor, this novel transformation facilitates the semisynthesis of a wide variety of new analogues for structure-activity relationship studies.

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.

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.