Abstract 1569: pSTAT3+ stromal cells drive the invasive growth of brain metastases
Authors: Dankner Matthew , Maritan Sarah M. , Priego Neibla , Nadaf Javad , Nkili Andy , Zhuang Rebecca , Kruck Georgia , Zuo Dongmei , Nowakowski Alexander , Inglebert Yanis , Savage Paul , Park Morag , Guiot Marie-Christine , McKinney Anne , Muller William J. , Valiente Manuel , Petrecca Kevin , Siegel Peter M. 
Published: June 2022

Abstract
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.