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Abstract

Group 3 medulloblastoma (MB) is the most aggressive type of MB with the least favourable outcome. Although enhanced STAT3 signaling is implicated in acquired chemoresistance of multiple cancers, a role for STAT3 in MB chemoresistance is not known. Here, I evaluated if IL-6/STAT3 signaling contributes to acquired chemoresistance in Group 3 MB. Group 3 MB cells that are initially chemosensitive were rendered chemoresistant either by incremental drug selection, or by conditioning with IL-6 family cytokines. IL-6 cytokines robustly stimulated the activation of JAK/STAT3 activity, and the chemoresistant variants exhibited increased STAT3 phosphorylation when transiently treated or conditioned with IL-6. Abrogation of STAT3 or IL6R expression in the chemoresistant cells successfully restored the chemosensitivity, highlighting the requirement of IL-6/STAT3 signaling in acquired chemoresistance. MB cells rendered chemoresistant following IL-6 conditioning secreted high levels of IL-6, indicating that an IL-6 autocrine feedback loop forms an important stimulus able to sustain enhanced STAT3 signaling. Furthermore, IL-6 secreted by chemoresistant cells also stimulated phosphorylated STAT3 in treatment-naïve chemosensitive cells, suggesting acquired chemoresistance may be propagated through the tumour microenvironment via a paracrine mechanism. I postulated that immune cells in the TME could initiate paracrine cytokine signaling. Indeed, microglia co-cultured with MB cells secreted cytokines, including IL-6, that phosphorylated STAT3 and enhanced MB chemoresistance. Unexpectedly, IL6R-/- MB cells co-cultured with microglia also exhibited enhanced STAT3 phosphorylation and chemoresistance, suggesting involvement of cytokines in addition to IL-6. I then demonstrated that IL-6 family cytokines effectively induced STAT3 phosphorylation and chemoresistance. Gp130 encodes the common β subunit for the IL-6 family cytokine receptors required for activating JAK/STAT3 signaling. Abrogation of gp130 expression in MB effectively blocked STAT3 signaling and IL-6 cytokine mediated chemoresistance. Furthermore, combination chemotherapy that included inhibitors targeting gp130, JAKs or STAT3 effectively circumvented IL-6 mediated chemoresistance in Group 3 MB. Analysis of multi-layered GEO databases of MB unveiled certain gene expression changes in the IL-6/STAT3 signaling axis that correlated with poor outcomes associated with Group 3 MB. Overall, elucidation of the role of IL-6/STAT3 signaling in cell survival and acquired chemoresistance revealed strategies for molecular targeted therapies to combat chemoresistance in Group 3 MB.