In the original publication [...].

Breast cancer is the most frequently diagnosed cancer, and second most common source of brain metastasis in women. Despite recent advances in detection and therapeutics, patients with breast cancer brain metastases (BCBM) survive only 6-18 months after diagnosis. HER2-enriched breast cancer and triple-negative breast cancer (TNBC) subtypes have the highest propensity to metastasize to the brain. Mechanisms that drive BCBM remain unclear, contributing to limited effective treatments and poor prognoses for HER2-enriched breast cancer and TNBC patients. Truncated glioma-associated oncogene homolog 1 (tGLI1), a gain-of-function GLI1 transcription factor discovered in our lab, promotes BCBM of circulating tumor cells in vivo. We previously reported that tGLI1 enriches the breast cancer stem cell (BCSC) subpopulation, and that tGLI1-positive BCSCs strongly activate astrocytes, the most abundant glial cell type in the brain. The mechanisms by which tGLI1-activated astrocytes promote BCBM have not been investigated. Since tumor-associated astrocytes have been reported to secrete specific cytokines, such as ciliary neurotrophic factor (CNTF), to activate astrocytes and cancer cells by binding to IL-6R and CNTF receptor-alpha (CNTFRα), we determined whether astrocytes activated by tGLI1-positive breast cancer cells secrete elevated levels of CNTF. Results of CNTF ELISA revealed that conditioned media from tGLI1-positive breast cancer cells increases the ability of astrocytes to secrete CNTF compared to GLI1-positive or control cells. Furthermore, we found that exogenous CNTF significantly activates astrocytes and promotes BCSCs, important mediators of tumor progression and metastasis in breast cancer. Analysis of patient datasets reveals CNTFRα is more highly expressed in brain metastases compared to breast cancer and normal breast tissues, suggesting that CNTF secreted by activated astrocytes can promote the growth of BCSCs in the brain. We further found that CNTF and CNTFRα gene signatures are significantly associated with worse brain metastasis-free survival, suggesting an important role for the tGLI1-CNTF-CNTFRα pathway in BCBM development. In summary, our study demonstrates for the first time that astrocytes activated by tGLI1-expressing BCBM secrete CNTF, CNTF further activates astrocytes and promotes BCSCs, and that this tumor-astrocyte interaction could be a novel mechanism for BCBM development and progression. Citation Format: Grace L. Wong, Sherona R. Sirkisoon, Noah R. Aguayo, Daniel L. Doheny, Dongqin Zhu, Angelina T. Regua, Austin Arrigo, Sara G. Manore, Calvin J. Wagner, Alexandra Thomas, Ravi Singh, Fei Xing, Guangxu Jin, Kounosuke Watabe, Hui-Wen Lo. Astrocytes activated by tGLI1-expressing breast cancer brain metastases upregulate CNTF to activate astrocytes and promote breast cancer stem cells [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 2354.

Mechanisms underlying breast cancer brain metastasis (BCBM) are still unclear. In this study, we observed that extracellular vesicles (EVs) secreted from breast cancer cells with increased expression of tGLI1, a BCBM-promoting transcription factor, strongly activated astrocytes. EV-derived microRNA/miRNA microarray revealed tGLI1-positive breast cancer cells highly secreted miR-1290 and miR-1246 encapsulated in EVs. Genetic knockin/knockout studies established a direct link between tGLI1 and both miRNAs. Datamining and analysis of patient samples revealed that BCBM patients had more circulating EV-miRs-1290/1246 than those without metastasis. Ectopic expression of miR-1290 or miR-1246 strongly activated astrocytes whereas their inhibitors abrogated the effect. Conditioned media from miR-1290- or miR-1246-overexpressing astrocytes promoted mammospheres. Furthermore, miRs-1290/1246 suppressed expression of FOXA2 transcription repressor, leading to CNTF cytokine secretion and subsequent activation of astrocytes. Finally, we conducted a mouse study to demonstrate that astrocytes overexpressing miR-1290, but not miR-1246, enhanced intracranial colonization and growth of breast cancer cells. Collectively, our findings demonstrate, for the first time, that breast cancer EV-derived miR-1290 and miR-1246 activate astrocytes in the brain metastatic microenvironment and that EV-derived miR-1290 promotes progression of brain metastases through the novel EV-miR-1290→FOXA2→CNTF signaling axis.

The goal of this study is to identify pharmacological inhibitors that target a recently identified novel mediator of breast cancer brain metastasis (BCBM), truncated glioma-associated oncogene homolog 1 (tGLI1). Inhibitors of tGLI1 are not yet available. To identify compounds that selectively kill tGLI1-expressing breast cancer, we screened 1527 compounds using two sets of isogenic breast cancer and brain-tropic breast cancer cell lines engineered to stably express the control, GLI1, or tGLI1 vector, and identified the FDA-approved antifungal ketoconazole (KCZ) to selectively target tGLI1-positive breast cancer cells and breast cancer stem cells, but not tGLI1-negative breast cancer and normal cells. KCZ’s effects are dependent on tGLI1. Two experimental mouse metastasis studies have demonstrated that systemic KCZ administration prevented the preferential brain metastasis of tGLI1-positive breast cancer and suppressed the progression of established tGLI1-positive BCBM without liver toxicities. We further developed six KCZ derivatives, two of which (KCZ-5 and KCZ-7) retained tGLI1-selectivity in vitro. KCZ-7 exhibited higher blood–brain barrier penetration than KCZ/KCZ-5 and more effectively reduced the BCBM frequency. In contrast, itraconazole, another FDA-approved antifungal, failed to suppress BCBM. The mechanistic studies suggest that KCZ and KCZ-7 inhibit tGLI1’s ability to bind to DNA, activate its target stemness genes Nanog and OCT4, and promote tumor proliferation and angiogenesis. Our study establishes the rationale for using KCZ and KCZ-7 for treating and preventing BCBM and identifies their mechanism of action.

Abstract Breast cancer is the second leading cause of brain metastases in women; patients with breast cancer brain metastasis (BCBM) survive a median of 14.1 months following diagnosis. Cancer stem cells are thought to be one of the driving forces behind distant metastasis, treatment resistance, and late-stage recurrence. Despite advances made in understanding breast cancer stem cells (BCSC), it remains challenging to effectively target BCSC underscoring the need to identify and inhibit novel mediators of BCSC for treating BCBM patients. The hedgehog-smoothened pathway is an important mediator of breast cancer stem cells (BCSC); however, FDA-approved therapies targeting smoothened have demonstrated limited clinical efficacy in breast cancer. Truncated glioma-associated oncogene homolog 1 (tGLI1) was discovered in our laboratory as an alternative GLI1 splice variant that functions as a tumor-specific gain-of-function transcription factor and terminal effector of the hedgehog pathway. Our laboratory recently reported that tGLI1 promotes preferential metastasis to the brain in breast cancer by activating BCSC and astrocytes in the tumor microenvironment (Oncogene 39:64–78, 2020). tGLI1 knockdown abrogated BCBM, providing the rationale to therapeutically target tGLI1. This study aimed to determine if tGLI1 can be therapeutically targeted. Cell-based chemical screens followed by validations demonstrated that ketoconazole, an FDA-approved azole antifungal, and novel derivatives specifically inhibit tGLI1 leading to suppression of BCSC in vitro and BCBM in vivo. Mechanistic studies suggest that KCZ-dependent cell kill is, in part, mediated through downregulation of tGLI1 target genes OCT4, Nanog, and VEGFA. Based on these data, we opened a window-of-opportunity study in patients with BCBM to determine if ketoconazole penetrates the blood-brain barrier (BBB) and alters tGLI1 signaling in humans (NCT03796273). Preliminary sample analysis demonstrates ketoconazole crosses the blood-tumor barrier and that tGLI1 expression correlates with tGLI1 signaling in resected samples. Collectively, these data establish tGLI1 as an actionable target for BCBM.

JAK2–STAT3 and TrkA signaling pathways have been separately implicated in aggressive breast cancers; however, whether they are co-activated or undergo functional interaction has not been thoroughly investigated. Herein we report, for the first time that STAT3 and TrkA are significantly co-overexpressed and co-activated in triple-negative breast cancer (TNBC) and HER2-enriched breast cancer, as shown by immunohistochemical staining and data mining. Through immunofluorescence staining–confocal microscopy and immunoprecipitation–Western blotting, we found that TrkA and STAT3 co-localize and physically interact in the cytoplasm, and the interaction is dependent on STAT3-Y705 phosphorylation. TrkA–STAT3 interaction leads to STAT3 phosphorylation at Y705 by TrkA in breast cancer cells and cell-free kinase assays, indicating that STAT3 is a novel substrate of TrkA. β-NGF-mediated TrkA activation induces TrkA–STAT3 interaction, STAT3 nuclear transport and transcriptional activity, and the expression of STAT3 target genes, SOX2 and MYC. The co-activation of both pathways promotes breast cancer stem cells. Finally, we found that TNBC and HER2-enriched breast cancer with JAK2–STAT3 and TrkA co-activation are positively associated with poor overall metastasis-free and organ-specific metastasis-free survival. Collectively, our study uncovered that TrkA is a novel activating kinase of STAT3, and their co-activation enhances gene transcription and promotes breast cancer stem cells in TNBC and HER2-enriched breast cancer.

Mechanisms for breast cancer metastasis remain unclear. Whether truncated glioma-associated oncogene homolog 1 (TGLI1), a transcription factor known to promote angiogenesis, migration and invasion, plays any role in metastasis of any tumor type has never been investigated. In this study, results of two mouse models of breast cancer metastasis showed that ectopic expression of TGLI1, but not GLI1, promoted preferential metastasis to the brain. Conversely, selective TGLI1 knockdown using antisense oligonucleotides led to decreased breast cancer brain metastasis (BCBM) in vivo. Immunohistochemical staining showed that TGLI1, but not GLI1, was increased in lymph node metastases compared to matched primary tumors, and that TGLI1 was expressed at higher levels in BCBM specimens compared to primary tumors. TGLI1 activation is associated with a shortened time to develop BCBM and enriched in HER2-enriched and triple-negative breast cancers. Radioresistant BCBM cell lines and specimens expressed higher levels of TGLI1, but not GLI1, than radiosensitive counterparts. Since cancer stem cells (CSCs) are radioresistant and metastasis-initiating cells, we examined TGLI1 for its involvement in breast CSCs and found TGLI1 to transcriptionally activate stemness genes CD44, Nanog, Sox2, and OCT4 leading to CSC renewal, and TGLI1 outcompetes with GLI1 for binding to target promoters. We next examined whether astrocyte-priming underlies TGLI1-mediated brain tropism and found that TGLI1-positive CSCs strongly activated and interacted with astrocytes in vitro and in vivo. These findings demonstrate, for the first time, that TGLI1 mediates breast cancer metastasis to the brain, in part, through promoting metastasis-initiating CSCs and activating astrocytes in BCBM microenvironment.

Abstract Breast cancer is the leading cause of cancer-related deaths in American women. Despite the current standard-of-care, which implements tumor resection, radiotherapy, and chemotherapy, triple-negative and HER2-positive breast cancer patients often relapse and present with recurrent disease, for which there is no cure. Thus, there is an urgent need to identify new molecular targets to improve patient response to anti-cancer therapies. The JAK2-STAT3 and TrkA receptor tyrosine kinase signaling pathways have been separately implicated in metastatic breast cancers, but information about their crosstalk remains limited. Activated Janus kinase 2 (JAK2) receptor tyrosine kinase phosphorylates signal transducer and activator of transcription 3 (STAT3) transcription factor, inducing STAT3 nuclear translocation and transcriptional activity. Abnormal activation of STAT3-mediated transcription has been identified as a pro-tumorigenic event in breast cancers and promotes progression through induction of breast cancer stem cells, invasion, and angiogenesis. Tropomyosin receptor kinase A (TrkA) frequently forms oncogenic fusion proteins and its overexpression has been shown to drive malignant transformation of breast cancer cells. JAK2 inhibitors, while approved for treatment of myeloproliferative neoplasms, are currently in clinical trials for triple-negative breast cancers. Currently, there are two FDA-approved TrkA inhibitors for treatment of NTRK1 fusion-positive solid tumors, one of which is considered a tumor-agnostic inhibitor. However, whether these two important pathways are co-activated in breast cancers and whether the co-activation is associated with overall progression of breast cancer have not been investigated. Herein, we report that STAT3 and TrkA are more co-activated in triple-negative and HER2-enriched breast cancers, compared to luminal subtypes, as determined by immunohistochemical staining of 33 invasive breast carcinomas and datamining data of over 1,500 breast cancer patients. We also observed that high co-activation of both JAK2-STAT3 and TrkA pathways significantly shortens overall metastasis-free survival of both triple-negative and HER2-enriched breast cancer patients when compared to patients with low co-activation. Similarly, we also find that JAK2-STAT3 and TrkA co-activation also significantly shortens brain, lung, and bone metastasis-free survival in both of these breast cancer subtypes, suggesting a critical role for JAK2-STAT3 and TrkA in distant metastasis of aggressive breast cancers. Taken together, our findings indicate a novel signaling crosstalk between JAK2-STAT3 and TrkA pathways in triple-negative and HER2-enriched breast cancers, and the use of their co-activation as a prognostic indicator for metastatic breast cancers of both subtypes. Citation Format: Angelina T. Regua, Noah R. Aguayo, Sara Abu Jalboush, Daniel L. Doheny, Sara G. Manore, Dongqin Zhu, Grace L. Wong, Austin Arrigo, Calvin J. Wagner, Yang Yu, Karen Baylon, Alexandra Thomas, Michael D. Chan, Jimmy Ruiz, Guangxu Jin, Roy E. Strowd, Peiqing Sun, Linda J. Metheny-Barlow, Jiayuh Lin, Hui-Wen Lo. JAK2/STAT3 and TrkA pathways are frequently co-activated in triple-negative and HER2-enriched breast cancers and the co-activation correlates with an increased potential of metastasis [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 1979.

Abstract Breast cancer is the second leading cause of brain metastases in women; patients with breast cancer brain metastasis (BCBM) survive only 6-18 months following diagnosis. Current standard of care options for BCBM include stereotactic radiosurgery, whole-brain radiotherapy, and surgical resection. Novel agents that act on targetable driver mutations (EGFR, ALK, PI3K, etc) and penetrate the blood-brain barrier have recently shown increasing promise as systemic options for patients with distant lung and melanoma metastases. Nevertheless, brain metastases represent an area of unmet clinical need for which new pathways, novel mechanisms, and innovative therapies are needed. Cancer stem cells are thought to be one of the driving forces behind not only distant metastasis, but also late-stage recurrence. The hedgehog pathway has been identified as an important mediator of breast cancer stem cells (BCSC); however, FDA-approved therapies targeting this pathway have demonstrated limited efficacy in breast cancer clinical trials. Despite advances made in understanding BCSC, it is still challenging to effectively target BCSC underscoring the need to identify and inhibit novel mediators of BCSC for treating BCBM patients. Our laboratory recently reported that truncated glioma-associated oncogene homolog 1 (tGLI1) promotes preferential metastasis to the brain in breast cancer by activating BCSC and astrocytes in the tumor microenvironment (Oncogene 39:64-78, 2020). tGLI1 is an alternatively spliced GLI1 variant that functions as a tumor-specific gain-of-function transcription factor and terminal effector of the hedgehog pathway. We found that tGLI1 knockdown abrogated BCBM, providing the rationale to therapeutically target tGLI1. Cell-based chemical screens followed by validations demonstrated that ketoconazole, an FDA-approved azole antifungal, specifically inhibits tGLI1 leading to suppression of BCSC in vitro and BCBM in vivo. Modification of KCZ side chains produced derivative compounds that retained tGLI1-selectivity in both in vitro models of BCSC and in vivo models of BCBM with increased blood-brain barrier penetrance. Mechanistic studies suggest that KCZ-dependent cell kill is, in part, mediated through downregulation of tGLI1 target genes OCT4, Nanog, and VEGFA. Based on these data, we opened a window-of-opportunity study in patients with BCBM to determine if ketoconazole penetrates the blood-brain barrier and alters tGLI1 signaling in humans (NCT03796273). Preliminary sample analysis has confirmed tGLI1 expression in collected BCBM and glioma samples. To help identify more effective tGLI1 inhibitors, we screened 63 azole compounds for tGLI1-selectivity and identified four additional compounds as potential tGLI1 inhibitors. Collectively, these data establish tGLI1 as an actionable target for BCBM. Citation Format: Daniel L. Doheny, Sherona R. Sirkisoon, Tadas Rimkus, Dongqin Zhu, Noah R. Aguayo, Marlyn Anguelov, Sara Manore, Fei Xing, Linda Metheny-Barlow, Kounosuke Watabe, Alexandra Thomas, Adrianna Henson Masters, Roy E. Strowd, Hui-Wen Lo. Truncated glioma-associated oncogene homolog 1 (tGLI1) is an actionable therapeutic target in breast cancer brain metastases [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 2866.

Triple-negative breast cancer (TNBC) and HER2-positive breast cancer are particularly aggressive and associated with unfavorable prognosis. TNBC lacks effective treatments. HER2-positive tumors have treatment options but often acquire resistance to HER2-targeted therapy after initial response. To address these challenges, we determined whether novel combinations of JAK2-STAT3 and SMO-GLI1/tGLI1 inhibitors synergistically target TNBC and HER2 breast cancer since these two pathways are concurrently activated in both tumor types and enriched in metastatic tumors. Herein, we show that novel combinations of JAK2 inhibitors (ruxolitinib and pacritinib) with SMO inhibitors (vismodegib and sonidegib) synergistically inhibited in vitro growth of TNBC and HER2-positive trastuzumab-resistant BT474-TtzmR cells. Synergy was also observed against breast cancer stem cells. To determine if the combination is efficacious in inhibiting metastasis, we treated mice with intracardially inoculated TNBC cells and found the combination to inhibit lung and liver metastases, and prolong host survival without toxicity. The combination inhibited orthotopic growth, VEGF-A expression, and tumor vasculature of both TNBC and HER2-positive trastuzumab-refractory breast cancer. Lung metastasis of orthotopic BT474-TtzmR xenografts was suppressed by the combination. Together, our results indicated that dual targeting of JAK2 and SMO resulted in synergistic suppression of breast cancer growth and metastasis, thereby supporting future clinical testing.

Breast cancer is the second leading cause of brain metastases in women; patients with breast cancer brain metastasis (BCBM) survive only 6-18 months following diagnosis. Current standard of care options for BCBM include stereotactic radiosurgery, whole-brain radiotherapy, and surgical resection. Cancer stem cells are thought to be one of the driving forces behind not only distant metastasis, but also late-stage recurrence. The hedgehog pathway has been identified as an important mediator of stem cells, however, FDA-approved therapies targeting this pathway have demonstrated limited efficacy in breast cancer clinical trials. Despite our current knowledge of breast cancer stem cells (BCSC), there remains a significant challenge in managing patients with metastatic breast cancer, underscoring the need to identify novel, actionable targets. Our laboratory discovered an alternative splice variant of glioma-associated oncogene homolog 1 (GLI1), termed truncated GLI1 (tGLI1), that is a tumor-specific gain-of-function transcription factor and terminal effector of the hedgehog pathway that is preferentially expressed in most BCBM samples and recurrent gliomas. Recent results established that tGLI1 promotes BCSC and is associated with preferential metastasis to the brain and radioresistance, justifying tGLI1 as an ideal therapeutic target for BCBM patients. To identify tGLI1-targeting agents, we screened 1,520 compounds across three commercial drug libraries and found ketoconazole (KCZ), an FDA-approved imidazole antifungal and component of previously studied anti-neoplastic regimens, selectively killed tGLI1-expressing breast cancer cells with increased efficacy against BCSC in vitro. tGLI1 knockdown abolished the ability of KCZ to target BCSC, indicating that KCZ-mediated suppression of BCSC is dependent on tGLI1. Intracardiac mouse studies showed KCZ selectively inhibited circulating tGLI1-positive breast cancer cells from developing into brain metastases and suppressed the progression of existing brain metastases. Mass spectrometry demonstrated KCZ effectively penetrated the blood-brain barrier (BBB) and blood-tumor barrier (BTB). Mechanistic studies suggest that KCZ-dependent cell kill is, in part, mediated through disruption of the functional tGLI1-STAT3 interaction. Furthermore, modification of KCZ side chains produced derivative compounds that retained tGLI1-selectivity in both in vitro models of BCSC and in vivo models of BCBM with increased BBB penetrance. Collectively, our preclinical results demonstrate that KCZ is an effective inhibitor of BCSC and brain metastasis of tGLI1-positive breast cancer. Based on these promising preclinical data, we opened a window-of-opportunity study in patients with BCBM and recurrent gliomas to determine if KCZ treatment alters tGLI1 signaling in humans (NCT03796273). Citation Format: Daniel L. Doheny, Sherona R. Sirkisoon, Tadas Rimkus, Dongqin Zhu, Noah R. Aguayo, Alexandria Harrison, Marlyn Anguelov, Sara Manore, Fei Xing, Linda J. Metheny-Barlow, Kounosuke Watabe, Alexandra Thomas, Adrianna Henson Masters, Roy Strowd, Hui-Wen Lo. Antifungal ketoconazole inhibits tumor-specific transcription factor tGLI1 leading to suppression of breast cancer stem cells and brain metastasis [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 5025.

Abstract Breast cancer is the second leading cause of brain metastases in women; patients with breast cancer brain metastasis (BCBM) survive an average of 6–18 months following diagnosis. Cancer stem cells are thought to be one of the driving forces behind distant metastasis, treatment resistance, and late-stage recurrence. The hedgehog-smoothened pathway has been identified as an important mediator of breast cancer stem cells (BCSC); however, FDA-approved therapies targeting smoothened have demonstrated limited clinical efficacy in breast cancer. Despite advances made in understanding BCSC, it is still challenging to effectively target BCSC underscoring the need to identify and inhibit novel mediators of BCSC for treating BCBM patients. Our laboratory recently reported that truncated glioma-associated oncogene homolog 1 (tGLI1) promotes preferential metastasis to the brain in breast cancer by activating BCSC and astrocytes in the tumor microenvironment (Oncogene 39:64–78, 2020). tGLI1 was discovered in our laboratory as an alternatively spliced GLI1 that functions as a tumor-specific gain-of-function transcription factor and terminal effector of the hedgehog pathway. We found that tGLI1 knockdown abrogated BCBM, providing the rationale to therapeutically target tGLI1. Cell-based chemical screens followed by validations demonstrated that ketoconazole, an FDA-approved azole antifungal, specifically inhibits tGLI1 leading to suppression of BCSC in vitro and BCBM in vivo. Based on these data, we opened a window-of-opportunity study in patients with BCBM to determine if ketoconazole penetrates the blood-brain barrier (BBB) and alters tGLI1 signaling in humans (NCT03796273). Preliminary sample analysis has confirmed tGLI1 expression in collected BCBM samples. To help identify more effective tGLI1 inhibitors, we screened 63 azole compounds for tGLI1-selectivity and identified four additional compounds as potential tGLI1 inhibitors. Animal studies were performed to compare the efficacy of these four compounds with ketoconazole in suppressing BCBM. Collectively, these data establish tGLI1 as an actionable target for BCBM.

Abstract Despite improvements in early detection and intervention, breast cancer remains the second leading cause of cancer-related death in women and the second most common cancer to metastasize to the brain. Current standard of care options for breast cancer brain metastases (BCBM) include stereotactic radiosurgery, whole-brain radiotherapy, and surgical resection. Local and distant recurrences are common leading to significant morbidity; effective FDA-approved drugs for these patients remain a significant unmet need. Our laboratory discovered an alternative splice variant of glioma-associated oncogene homolog 1 (GLI1), termed truncated GLI1 (tGLI1) that is a tumor-specific gain-of-function transcription factor preferentially expressed in most BCBM samples and recurrent gliomas. Recent results established that tGLI1 promotes breast cancer stem cells (BrCSCs) and is associated with preferential metastasis to the brain and radioresistance, justifying tGLI1 as an ideal therapeutic target for BCBM patients. To identify tGLI1-targeting agents, we screened 1,520 compounds across three commercial drug libraries and found ketoconazole, an FDA-approved azole antifungal and component of previously studied anti-neoplastic regimens, selectively killed tGLI1-expressing breast cancer cells with heightened efficacy against the CSC subpopulation in vitro. tGLI1 knockdown abolished the ability of ketoconazole to target BrCSCs, indicating that ketoconazole effect is dependent on tGLI1. Intracardiac mouse studies showed ketoconazole selectively inhibited circulating tGLI1-positive breast cancer cells from developing into brain metastases and suppressed the progression of existing brain metastases. Mass spectrometry demonstrated ketoconazole effectively penetrated the blood-brain barrier (BBB) and blood-tumor barrier (BTB). Mechanistic studies suggest that ketoconazole-dependent cell kill is, in part, mediated through disruption of the tGLI1-STAT3 interaction. Collectively, our preclinical results demonstrate that ketoconazole is an effective inhibitor of BrCSCs and brain metastasis of tGLI1-positive breast cancer. Based on these promising preclinical data, we opened a window-of-opportunity study in patients with BCBM and recurrent gliomas to determine if ketoconazole treatment alters tGLI1 signaling in humans (NCT03796273).

Breast cancer is the second leading cause of cancer-related mortality in women; metastasis to distant organs results in 90% of deaths for these patients. Cancer stem cells (CSCs) are considered the drivers of metastasis. Despite our current knowledge of breast CSCs, there still remains a significant challenge in managing patients with the metastatic breast cancer, underscoring the need for identifying novel regulators of breast CSCs. The hedgehog pathway is an important mediator of stem cells; however, the effect of truncated glioma-associated oncogene homolog 1 (tGLI1), a nuclear effector of the hedgehog pathway and a gain-of-function GLI1 transcription factor, on breast CSCs has never been investigated. Herein, we investigated whether tGLI1 is implicated in breast CSCs by evaluating tGLI1 expression levels in cells grown as monolayer versus mammospheres, as a representation of the stem cell population, and found tGLI1 to be induced in mammosphere culture. Overexpression of tGLI1 promoted mammosphere-forming ability of breast cancer cells, as well as, increased the breast CSC population defined by CD44high/CD24low expression. Further, tGLI1 overexpression transformed normal mammary epithelial cells resulting in increased mammosphere formation and enhanced anchorage-independent growth of immortalized human mammary epithelial HMLE cells. Functional and biochemical assays further showed that tGLI1 promotes breast CSC self-renewal by transcriptional activation of stemness genes including a novel tGLI1 target gene, OCT4, a recently reported tGLI1 target gene (CD44), and known GLI1 target genes (Nanog and SOX2). Bioinformatic analysis of breast cancer patient datasets revealed that activated tGLI1 is associated with shortened time to develop metastasis to the lung, bone, and brain. Furthermore, tGLI1 activation is enriched in HER2-enriched and triple-negative breast cancers, the subtypes with the highest propensity to metastasize, compared to luminal subtypes. Gene Set Enrichment Analysis showed that high tGLI1 activation is enriched in breast cancer with high gene signatures of breast CSCs, radioresistance, and metastasis. We further validated these results by immunohistochemical staining of paired primary breast tumors with lymph node metastases and found that that expression of tGLI1, but not GLI1, was increased in lymph node metastases and that tGLI1 was expressed at higher levels (84-91%) of lymph node-positive metastatic HER2-enriched and triple-negative breast tumors. Lastly, tGLI1 knockdown resulted in decreased mammosphere formation of breast cancer cells and decreased expression of stemness genes, OCT4, CD44, and Nanog. Taken together, these findings establish a novel role that tGLI1 plays in mediating breast CSCs and implicate tGLI1 in facilitating breast cancer metastasis. Citation Format: Sherona R. Sirkisoon, Richard L. Carpenter, Tadas Rimkus, Daniel Doheny, Dongqin Zhu, Noah R. Aguayo, Marlyn Anguelov, Austin Arrigo, Fei Xing, Michael Chan, Jimmy Ruiz, Linda J. Metheny-Barlow, Roy Strowd, Jiayuh Lin, Boris C. Pasche, Waldemar Debinski, Kounosuke Watabe, Hui-Wen Lo. Tumor-specific gain-of-function tGLI1 transcription factor is a novel mediator of breast cancer stem cells and a novel transcriptional activator of cancer stemness genes [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 3689.

Abstract Breast cancer is the second leading cause of brain metastases in women; patients with breast cancer brain metastasis (BCBM) survive only 6–18 months after diagnosis. Mechanisms for BCBM remain unclear, which contributes to ineffective treatments and dismal prognosis. Truncated glioma-associated oncogene homolog 1 (tGLI1) belongs to the GLI1 family of zinc-finger transcription factors and functions as a tumor-specific gain-of-function mediator of tumor invasion and angiogenesis. Whether tGLI1 plays any role in metastasis of any tumor type remains unknown. Using an experimental metastasis mouse model, via intracardiac implantation, we showed that ectopic expression of tGLI1, but not GLI1, promoted preferential metastasis to brain. Conversely, selective tGLI1 knockdown using tGLI1-specific antisense oligonucleotides led to decreased brain metastasis of intracardially inoculated breast cancer cells. Furthermore, intracranial implantation mouse study revealed tGLI1 enhanced intracranial colonization and growth of breast cancer cells. Immunohistochemical staining of patient samples showed that tGLI1, but not GLI1, was increased in lymph node metastases compared to matched primary tumors, and that tGLI1 was expressed at higher levels in BCBM specimens compared to primary tumors. Whether tGLI1 plays any role in radioresistance is unknown; we found radioresistant BCBM cell lines and patient specimens expressed higher levels of tGLI1 than radiosensitive counterparts, and that tGLI1 promotes radioresistance. Since cancer stem cells (CSCs) are highly metastatic and radioresistant, we examined whether tGLI1 promotes BCBM and radioresistance through activating CSCs. Results showed that tGLI1 transcriptionally activates stemness genes CD44, Nanog, Sox2, and OCT4, leading to stem cell activation. Furthermore, we observed that tGLI1-positive CSCs strongly activated and interacted with astrocytes, the most abundant brain tumor microenvironmental cells known to promote tumor growth, in vitro and in vivo. Collectively, our findings establish a novel role of that tGLI1 plays in promoting breast cancer preferential metastasis to brain, radioresistance, and astrocytes in the metastatic niche.

Tumor suppressor candidate 2 (TUSC2, also known as FUS1) was identified as a candidate tumor suppressor gene located in a region on chromosome 3p21.3 that undergoes allelic loss in lung and breast cancers. Loss of TUSC2 expression has been reported in various cancers and is associated with poor survival. Evidence to date indicates that TUSC2 behaves as a tumor suppressor in lung cancer; however, its role as a tumor suppressor in other tumor types has not been fully established. Since the mechanism for gliomagenesis is still unclear, we investigated the role of TUSC2 in the development and progression of glioblastoma (GBM), the most common and deadliest brain cancer in adults. Here, we found that forced TUSC2 expression suppressed neurosphere-forming capability of glioma stem cells, regardless of molecular subtypes. Forced expression of TUSC2 in GBM cell lines inhibited their ability to form colonies and neurospheres. To further determine whether TUSC2 plays a tumor suppressive role in GBM, we knocked down TUSC2 expression in TUSC2-expressing GBM cells using siRNA and CRISPR/Cas9, and found TUSC2 knockdown to significantly enhance neurosphere formation of GBM cells. Using an orthotopic GBM xenograft mouse model, we further observed that CRISPR/Cas9-mediated TUSC2 knockout significantly promoted the intracranial growth of GBM tumors. To gain inisghts into the mechanisms underlying TUSC2’s tumor suppressive function in GBM, we conducted RNA-Seq using control and TUSC2-knockout GBM cell lines, and identified a number of genes whose expression was altered in response to TUSC2 loss. Ongoing studies are being conducted to validate idenified genes, and elucidate their involvement in gliomagenesis and GBM progression. Furthermore, we speculated that TUSC2 may interact with cellular proteins leading to tumor suppression. To test this hypothesis, we conducted protein interactome analysis using immunoprecipitaton followed by mass spectrometry in which cell lysates from GBM cells and human astrocytes (a cell-of-origin for GBM) were used. This study has identified approximately 40 proteins that differentially interact with TUSC2 in GBM cells versus human astrocytes. Roles of these TUSC2-interacting proteins in GBM suppression and gliomagenesis are being examined in onging studies. Finally, we have generated conditional TUSC2-knockout mice to further address the role that TUSC2 plays in gliomagenesis. Collectively, these findings support a novel role that TUSC2 plays in GBM progression and gliomagenesis, thereby advancing our understanding of GBM pathobiology. Citation Format: Tadas K. Rimkus, Dongqin Zhu, Richard L. Carpenter, Ivy Paw, Austin Arrigo, Sherona Sirkisoon, Daniel Doheny, Noah Aguayo, Jingyun Lee, Guangxu Jin, Eric Spooner, Boris Pasche, Waldemar Debinski, Hui-Wen Lo. Roles of tumor suppressor candidate 2 (TUSC2) in glioblastoma progression and gliomagenesis [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 3481.