Invasive growth of brain metastases is linked to CHI3L1 release from pSTAT3-positive astrocytes

BACKGROUND

Compared to minimally invasive brain metastases (MI BrM), highly invasive (HI) lesions form abundant contacts with cells in the peritumoral brain parenchyma and are associated with poor prognosis. Reactive astrocytes (RAs) labeled by phosphorylated STAT3 (pSTAT3) have recently emerged as a promising therapeutic target for BrM. Here, we explore whether BrM invasion pattern is influenced by pSTAT3+ RAs and may serve as a predictive biomarker for STAT3 inhibition.

METHODS

We used immunohistochemistry to identify pSTAT3+ RAs in HI and MI human and patient-derived xenograft (PDX) BrM. Using PDX, syngeneic, and transgenic mouse models of HI and MI BrM, we assessed how pharmacological STAT3 inhibition or RA-specific STAT3 genetic ablation affected BrM growth in vivo. Cancer cell invasion was modeled in vitro using a brain slice-tumor co-culture assay. We performed single-cell RNA sequencing of human BrM and adjacent brain tissue.

RESULTS

RAs expressing pSTAT3 are situated at the brain-tumor interface and drive BrM invasive growth. HI BrM invasion pattern was associated with delayed growth in the context of STAT3 inhibition or genetic ablation. We demonstrate that pSTAT3+ RAs secrete Chitinase 3-like-1 (CHI3L1), which is a known STAT3 transcriptional target. Furthermore, single-cell RNA sequencing identified CHI3L1-expressing RAs in human HI BrM. STAT3 activation, or recombinant CHI3L1 alone, induced cancer cell invasion into the brain parenchyma using a brain slice-tumor plug co-culture assay.

CONCLUSIONS

Together, these data reveal that pSTAT3+ RA-derived CHI3L1 is associated with BrM invasion, implicating STAT3 and CHI3L1 as clinically relevant therapeutic targets for the treatment of HI BrM.

Machine learning identifies experimental brain metastasis subtypes based on their influence on neural circuits

A high percentage of patients with brain metastases frequently develop neurocognitive symptoms; however, understanding how brain metastasis co-opts the function of neuronal circuits beyond a tumor mass effect remains unknown. We report a comprehensive multidimensional modeling of brain functional analyses in the context of brain metastasis. By testing different preclinical models of brain metastasis from various primary sources and oncogenic profiles, we dissociated the heterogeneous impact on local field potential oscillatory activity from cortical and hippocampal areas that we detected from the homogeneous inter-model tumor size or glial response. In contrast, we report a potential underlying molecular program responsible for impairing neuronal crosstalk by scoring the transcriptomic and mutational profiles in a model-specific manner. Additionally, measurement of various brain activity readouts matched with machine learning strategies confirmed model-specific alterations that could help predict the presence and subtype of metastasis.

Abstract 5105: TIMP1 mediates astrocyte-dependent local immunosuppression in brain metastasis

Brain metastasis is an unmet clinical need, affecting between 10-30% of cancer patients with 200000 to 400000 newly diagnosis per annum in the US. Recently, several clinical trials have reported benefits using immunotherapy to treat brain metastasis. However, variability of the responses is broad and high benefit is found mainly in asymptomatic brain metastasis, while the benefit is dramatically reduced in the clinically relevant stage. Thus, it is currently unknown how to effectively target symptomatic brain metastases with immunotherapy. We previously reported a clinically relevant protumoral program driven by STAT3 activation in a subpopulation of reactive astrocytes in these advanced stages of the disease. Our current study further exploited the heterogeneity within the metastasis-associated microenvironment as a resource to identify novel therapeutic vulnerabilities to improve the benefits of immunotherapies based on immune checkpoint blocking antibodies (ICB) in symptomatic brain metastasis.

Our results demonstrate that reactive astrocytes are strong immunomodulatory cells in brain tumors. We have identified the molecular profile of disease-associated glial cells and defined its connection to modulatory activities on specific lymphocyte populations in experimental brain metastasis as well as human-derived samples. scRNASeq and high content multiplex immunofluorescence allowed us to report a novel local immunomodulatory axis dependent on TIMP1 (astrocytes)/CD63 (CD8 T cells), which is present in brain metastasis patients with high immunoscore and would imply an additional immunosuppressive signal for potential ICB responders in brain metastasis.

Genetic and pharmacologic approaches targeting this STAT3-dependent local immunomodulatory axis have allowed us to define the rationale to combine immune checkpoint blockade with a STAT3 inhibitor, which we previously used in patients. We proved that such combined immunotherapy boost the systemic activation of T cells while also preventing the local blockade. Additionally, our comprehensive strategy includes the possibility to stratify patients that are best qualified to benefit from this therapy by measuring TIMP1 in liquid biopsies from CSF. Even more, our data using Patient Derived Organotypic Cultures (PDOC) from fresh brain metastasis neurosurgeries confirms that our therapeutic strategy might benefit brain metastases generated from any primary source.

In conclusion, we describe an immunosuppressive mechanism in the brain microenvironment that could explain the lack of response to ICB in patients with advanced brain metastasis. Our finding provides the rationale to implement complementary approaches targeting local immunosuppression to increase the benefit of immunotherapy in symptomatic brain metastasis.

Citation Format: Neibla Priego, Pedro García-Gómez, Ana de Pablos-Aragoneses, María Perea-García, Laura Álvaro-Espinosa, Carolina Hernández-Oliver, Elena Martínez-Saez, Ángel Pérez-Núñez, Aurelio Hernández-Laín, Rebeca Sanz-Pamplona, Marc Schmitz, Stephen J. Crocker, Diego Serrano, Asís Palazón, RENACER Red Nacional de Metástasis Cerebral, Manuel Valiente. TIMP1 mediates astrocyte-dependent local immunosuppression in brain metastasis. [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 5105.

Abstract 1569: pSTAT3+ stromal cells drive the invasive growth of brain metastases

Background: Brain metastases (BrM) with highly invasive (HI) growth patterns are associated with shortened local recurrence free- and overall survival compared to minimally invasive (MI) lesions (Dankner et al. 2021). Compared to MI lesions, HI BrM form abundant contacts with cells in the peritumoral brain, particularly GFAP+ reactive astrocytes (RAs). RAs expressing phosphorylated STAT3 (pSTAT3+ GFAP+ cells) have been shown to be required for BrM colonization and outgrowth (Priego et al. 2018). Here, we investigate the role of pSTAT3+ cells in the brain microenvironment in promoting invasive growth.

Methods: We performed immunohistochemistry to identify pSTAT3+ GFAP+ cells in HI and MI human and patient-derived xenograft BrM. We assessed how pharmacological inhibition or genetic ablation of STAT3 affected HI and MI BrM growth in vivo with patient-derived xenograft and syngeneic models of BrM. The secretome of STAT3+ RAs was interrogated to identify STAT3 target genes that could drive invasive cancer growth. scRNA-Seq from patients with highly invasive brain metastases was used to examine the expression of candidate invasion factors in distinct cell types within the brain. Finally, cancer cell invasion was modeled in vitro using a brain slice-tumor co-culture assay.

Results: HI BrM displayed increased pSTAT3+GFAP+ cells compared to MI lesions. Pharmacological STAT3i with Legasil (Silibinin) or genetic ablation of STAT3 specifically in RAs decreased in vivo growth of HI, but not MI, BrM. Brain slice cultures treated with STAT3-activating cytokines induced cancer cell invasion, a response that was ablated with STAT3i. Chi3L1 was identified as a STAT3 target gene expressed abundantly by stromal cells in the BrM microenvironment. Cancer cells treated with recombinant Chi3L1 showed enhanced invasion into brain slice cultures compared to control-treated cells.

Conclusions: pSTAT3+GFAP+ cells are over-represented in HI BrM, rendering HI BrM preferentially sensitive to STAT3i. pSTAT3+ stromal cells functionally contribute to BrM invasion within the brain, in part through Chi3L1. This work nominates HI histopathological growth pattern as a predictive biomarker of response to STAT3i, and highlights Chi3L1 as a novel therapeutic target for the management of HI BrM.

Citation Format: Matthew Dankner, Sarah M. Maritan, Neibla Priego, Javad Nadaf, Andy Nkili, Rebecca Zhuang, Georgia Kruck, Dongmei Zuo, Alexander Nowakowski, Yanis Inglebert, Paul Savage, Morag Park, Marie-Christine Guiot, Anne McKinney, William J. Muller, Manuel Valiente, Kevin Petrecca, Peter M. Siegel. pSTAT3+ stromal cells drive the invasive growth of brain metastases [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 1569.

A clinically compatible drug-screening platform based on organotypic cultures identifies vulnerabilities to prevent and treat brain metastasis

We report a medium-throughput drug-screening platform (METPlatform) based on organotypic cultures that allows to evaluate inhibitors against metastases growing in situ. By applying this approach to the unmet clinical need of brain metastasis, we identified several vulnerabilities. Among them, a blood-brain barrier permeable HSP90 inhibitor showed high potency against mouse and human brain metastases at clinically relevant stages of the disease, including a novel model of local relapse after neurosurgery. Furthermore, in situ proteomic analysis applied to metastases treated with the chaperone inhibitor uncovered a novel molecular program in brain metastasis, which includes biomarkers of poor prognosis and actionable mechanisms of resistance. Our work validates METPlatform as a potent resource for metastasis research integrating drug-screening and unbiased omic approaches that is compatible with human samples. Thus, this clinically relevant strategy is aimed to personalize the management of metastatic disease in the brain and elsewhere.

Brain Microenvironment Heterogeneity: Potential Value for Brain Tumors

Uncovering the complexity of the microenvironment that emerges in brain disorders is key to identify potential vulnerabilities that might help challenging diseases affecting this organ. Recently, genomic and proteomic analyses, especially at the single cell level, have reported previously unrecognized diversity within brain cell types. The complexity of the brain microenvironment increases during disease partly due to the immune infiltration from the periphery that contributes to redefine the brain connectome by establishing a new crosstalk with resident brain cell types. Within the rewired brain ecosystem, glial cell subpopulations are emerging hubs modulating the dialogue between the Immune System and the Central Nervous System with important consequences in the progression of brain tumors and other disorders. Single cell technologies are crucial not only to define and track the origin of disease-associated cell types, but also to identify their molecular similarities and differences that might be linked to specific brain injuries. These altered molecular patterns derived from reprogramming the healthy brain into an injured organ, might provide a new generation of therapeutic targets to challenge highly prevalent and lethal brain disorders that remain incurable with unprecedented specificity and limited toxicities. In this perspective, we present the most relevant clinical and pre-clinical work regarding the characterization of the heterogeneity within different components of the microenvironment in the healthy and injured brain with a special interest on single cell analysis. Finally, we discuss how understanding the diversity of the brain microenvironment could be exploited for translational purposes, particularly in primary and secondary tumors affecting the brain.

BSCI-10. Invasive growth of brain metastases is driven by cancer cell-pSTAT3+ reactive astrocyte crosstalk

Background

Brain metastases (BrM) with a highly invasive (HI) histological growth pattern are associated with poor prognosis compared to minimally invasive (MI) masses. Compared to MI lesions, HI BrM form greater contacts with cells in the peritumoral brain, particularly reactive astrocytes (RAs). RAs expressing phosphorylated STAT3 (pSTAT3+RAs) have been shown to promote BrM colonization. Here, we investigate the role of pSTAT3+RAs in promoting invasive growth of HI BrM.

Methods

We performed immunohistochemistry to identify pSTAT3+RAs in HI and MI human and patient-derived xenograft BrM. We assessed how pharmacological STAT3 inhibition or RA-specific STAT3 genetic ablation affected HI and MI BrM growth in vivo. scRNA-seq data generated from HI BrM astrocytes were integrated with published RA secretome data to identify STAT3 targets expressed by RAs that may drive invasion. Cancer cell invasion was modeled in vitro using a brain slice-tumor co-culture assay.

Results

HI BrM display increased pSTAT3-positivity within RAs when compared to MI lesions. Pharmacological STAT3 inhibition with Legasil (Silibinin) or genetic ablation decreased in vivo growth of HI, but not MI, BrM. Brain slice cultures treated with STAT3-activating cytokines induced cancer cell invasion, a response that was ablated following STAT3 inhibition. Chi3L1 was identified as a STAT3 target expressed by RAs. Cancer cells treated with recombinant Chi3L1 showed greater invasion into brain slice cultures compared to untreated cells.

Conclusions

pSTAT3+RAs are over-represented in HI BrM, rendering HI BrM preferentially sensitive to STAT3 inhibition. pSTAT3+RAs functionally contribute to BrM invasion within the brain, in part through Chi3L1-mediated activity. This work identifies STAT3 and Chi3L1 as clinically relevant therapeutic targets in management of HI BrM.

Brain metastasis models: What should we aim to achieve better treatments?

Brain metastasis is emerging as a unique entity in oncology based on its particular biology and, consequently, the pharmacological approaches that should be considered. We discuss the current state of modelling this specific progression of cancer and how these experimental models have been used to test multiple pharmacologic strategies over the years. In spite of pre-clinical evidences demonstrating brain metastasis vulnerabilities, many clinical trials have excluded patients with brain metastasis. Fortunately, this trend is getting to an end given the increasing importance of secondary brain tumors in the clinic and a better knowledge of the underlying biology. We discuss emerging trends and unsolved issues that will shape how we will study experimental brain metastasis in the years to come.

Abstract 2746: Stat3 labels a subpopulation of reactive astrocytes required for brain metastasis

The diagnosis of brain metastasis involves high morbidity and mortality and remains as an unmet clinical need in spite of being the most common tumor in the brain. Brain metastasis affects between 10-30% of cancer patients with 200.000 new cases yearly only in the US. Increasing evidences point out the relevance of the microenvironment to understand the biology of brain colonization by metastatic cells, however translation strategies targeting it are lacking. We have identified altered signaling pathways in pro-metastatic reactive astrocytes (RA), and translated that into a novel therapeutic application for brain metastasis. Specifically we have identified a subpopulation of RA characterized by activated STAT3 pathway (pSTAT3+). This subpopulation is located in the vicinity of metastatic lesions intermingled with pSTAT3- RA in several experimental models and in 89% of human brain metastases independently of the primary tumor source. The pro-metastatic behaviour of RA is regulated by the activation of the STAT3 pathway, which modulates the immune system locally promoting the survival of cancer cells. Specifically the secretome of pSTAT3+ RA decreases the anti-tumor activity of CD8+ T cells as well as promotes the expansion of the pro-tumor population of CD74+ microglia/macrophage, which infiltrates metastasis cores. Interestingly, the immunosuppressive nature of pSTAT3+ RA is linked to the acquisition of stem cell-like properties, which might reflect the misuse of brain responses to injury instigated by the presence of cancer cells. Genetic and pharmacologic approaches targeting STAT3 in RA impair the progression of brain metastasis, even at advanced stages of the disease. Moreover, a safe and orally bioavailable STAT3 inhibitor reduced brain metastasis in 75% of 18 stage IV lung adenocarcinoma patients with established brain metastases, improving the outcome of the disease by increasing patient survival from 4 to 15 months. Besides increasing therapeutic opportunities for patients with brain metastasis, we have described for the first time the role of reactive astrocytes as regulators of local immunosuppression in brain metastasis as well as the importance of uncovering the heterogeneity within the metastasis-associated microenvironment.

Citation Format: Neibla Priego, Lucía Zhu, Catia Monteiro, Manon Mulders, David Wasilewski, Wendy Bindeman, Laura Doglio, Elena Martínez-Saez, Santiago Ramón y Cajal, Coral Fustero-Torre, Elena Piñeiro-Yáñez, Aurelio Hernández-Laín, Valeria Poli, Javier A. Menéndez, Ricardo Soffietti, Joaquim Bosch-Barrera, Manuel Valiente. Stat3 labels a subpopulation of reactive astrocytes required for brain metastasis [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 2746.

The Potential of Astrocytes as Immune Modulators in Brain Tumors

The neuro-immune axis has emerged as a key aspect to understand the normal function of the Central Nervous System (CNS) as well as the pathophysiology of many brain disorders. As such, it may represent a promising source for novel therapeutic targets. Glial cells, and in particular the extensively studied microglia, play important roles in brain disorders. Astrocytes, in their reactive state, have been shown to positively and negatively modulate the progression of multiple CNS disorders. These seemingly opposing effects, might stem from their underlying heterogeneity, an aspect that has recently come to light. In this article we will discuss the link between reactive astrocytes and the neuro-immune axis with a perspective on their potential importance in brain tumors. Based on the gained knowledge from studies in other CNS disorders, reactive astrocytes are undoubtfully emerging as a key component of the neuro-immune axis, with ability to modulate both the innate and adaptive branches of the immune system. Lastly, we will discuss how we can exploit our improved understanding of the basic biology of astrocytes to further enhance the efficacy of emerging immune-based therapies in primary brain tumors and brain metastasis.

Author Correction: STAT3 labels a subpopulation of reactive astrocytes required for brain metastasis

In the version of this article originally published, the names of three authors were incorrect. The authors were listed as "Coral Fustero-Torres", "Elena Pineiro" and "Melchor Sánchez-Martínez". Their respective names are "Coral Fustero-Torre", "Elena Piñeiro-Yáñez" and "Melchor Sanchez-Martinez". The errors have been corrected in the print, HTML and PDF versions of this article.

STAT3 labels a subpopulation of reactive astrocytes required for brain metastasis

The brain microenvironment imposes a particularly intense selective pressure on metastasis-initiating cells, but successful metastases bypass this control through mechanisms that are poorly understood. Reactive astrocytes are key components of this microenvironment that confine brain metastasis without infiltrating the lesion. Here, we describe that brain metastatic cells induce and maintain the co-option of a pro-metastatic program driven by signal transducer and activator of transcription 3 (STAT3) in a subpopulation of reactive astrocytes surrounding metastatic lesions. These reactive astrocytes benefit metastatic cells by their modulatory effect on the innate and acquired immune system. In patients, active STAT3 in reactive astrocytes correlates with reduced survival from diagnosis of intracranial metastases. Blocking STAT3 signaling in reactive astrocytes reduces experimental brain metastasis from different primary tumor sources, even at advanced stages of colonization. We also show that a safe and orally bioavailable treatment that inhibits STAT3 exhibits significant antitumor effects in patients with advanced systemic disease that included brain metastasis. Responses to this therapy were notable in the central nervous system, where several complete responses were achieved. Given that brain metastasis causes substantial morbidity and mortality, our results identify a novel treatment for increasing survival in patients with secondary brain tumors.

C3G knock-down enhances migration and invasion by increasing Rap1-mediated p38α activation, while it impairs tumor growth through p38α-independent mechanisms

C3G, a Guanine nucleotide Exchange Factor (GEF) for Rap1 and R-Ras, has been shown to play important roles in development and cancer. Previous studies determined that C3G regulates cell death through down-regulation of p38α MAPK activity. Here, we found that C3G knock-down in MEFs and HCT116 cells promotes migration and invasion through Rap1-mediated p38α hyper-activation. These effects of C3G were inhibited by Rap1 knock-down or inactivation. The enhanced migration observed in C3G depleted HCT116 cells was associated with reduction in E-cadherin expression, internalization of ZO-1, actin cytoskeleton reorganization and decreased adhesion. We also found that matrix metalloproteases MMP2 and MMP9 are involved in the pro-invasive effect of C3G down-regulation. Additionally, our studies revealed that both C3G and p38α collaborate to promote growth of HCT116 cells in vitro and in vivo, possibly by enhancing cell survival. In fact, knocking-down C3G or p38α individually or together promoted cell death in vitro, although only the double C3G-p38α silencing was able to increase cell death within tumors. Notably, we found that the pro-tumorigenic function of C3G does not depend on p38α or Rap1 activation. Altogether, our studies uncover novel mechanisms by which C3G controls key aspects of tumorigenesis.

Reactive Astrocytes in Brain Metastasis

Brain metastasis, the secondary growth of malignant cells within the central nervous system (CNS), exceeds the incidence of primary brain tumors (i.e., gliomas) by tenfold and are seemingly on the rise owing to the emergence of novel targeted therapies that are more effective in controlling extracranial disease relatively to intracranial lesions. Despite the fact that metastasis to the brain poses a unmet clinical problem, with afflicted patients carrying significant morbidity and a fatal prognosis, our knowledge as to how metastatic cells manage to adapt to the tissue environment of the CNS remains limited. Answering this question could pave the way for novel and more specific therapeutic modalities in brain metastasis by targeting the specific makeup of the brain metastatic niche. In regard to this, astrocytes have emerged as the major host cell type that cancer cells encounter and interact with during brain metastasis formation. Similarly to other CNS disorders, astrocytes become reactive and respond to the presence of cancer cells by changing their phenotype and significantly influencing the outcome of disseminated cancer cells within the CNS. Here, we summarize the current knowledge on the contribution of reactive astrocytes in brain metastasis by focusing on the signaling pathways and types of interactions that play a crucial part in the communication with cancer cells and how these could be translated into innovative therapies.

p38 MAPK down-regulates fibulin 3 expression through methylation of gene regulatory sequences: role in migration and invasion

p38 MAPKs regulate migration and invasion. However, the mechanisms involved are only partially known. We had previously identified fibulin 3, which plays a role in migration, invasion, and tumorigenesis, as a gene regulated by p38α. We have characterized in detail how p38 MAPK regulates fibulin 3 expression and its role. We describe here for the first time that p38α, p38γ, and p38δ down-regulate fibulin 3 expression. p38α has a stronger effect, and it does so through hypermethylation of CpG sites in the regulatory sequences of the gene. This would be mediated by the DNA methylase, DNMT3A, which is down-regulated in cells lacking p38α, but once re-introduced represses Fibulin 3 expression. p38α through HuR stabilizes dnmt3a mRNA leading to an increase in DNMT3A protein levels. Moreover, by knocking-down fibulin 3, we have found that Fibulin 3 inhibits migration and invasion in MEFs by mechanisms involving p38α/β inhibition. Hence, p38α pro-migratory/invasive effect might be, at least in part, mediated by fibulin 3 down-regulation in MEFs. In contrast, in HCT116 cells, Fibulin 3 promotes migration and invasion through a mechanism dependent on p38α and/or p38β activation. Furthermore, Fibulin 3 promotes in vitro and in vivo tumor growth of HCT116 cells through a mechanism dependent on p38α, which surprisingly acts as a potent inducer of tumor growth. At the same time, p38α limits fibulin 3 expression, which might represent a negative feed-back loop.