Heterogeneity in the Metastatic Microenvironment: JunB-Expressing Microglia Cells as Potential Drivers of Melanoma Brain Metastasis Progression

SIMPLE SUMMARY: Brain metastasis is a devastating but common consequence of advanced stage melanomas. The interactions between cancer cells and non-cancerous cells in the tumor microenvironment alleviate cancer progression. Previously, we found that brain metastatic melanoma cells reprogrammed microglia into tumor-promoting cells, but the mechanism is yet to be clarified. Here, we identified the transcription factor JunB to be dramatically upregulated in microglia following their exposure to melanoma. We recognized a diverse range (high and low) of JunB expression levels in melanoma-associated microglia, and therefore sought to establish two different microglia populations, expressing high or low levels of JunB, and to delineate their involvement in melanoma brain metastasis progression. High-JunB microglia demonstrated pro-tumor properties, while low-JunB microglia demonstrated anti-tumor properties. Moreover, we described the mechanism by which JunB upregulation is induced by melanoma cells. Microglia highly expressing JunB may serve as a target for brain-metastasizing melanoma therapy. ABSTRACT: Reciprocal signaling between melanoma brain metastatic (MBM) cells and microglia reprograms the phenotype of both interaction partners, including upregulation of the transcription factor JunB in microglia. Here, we aimed to elucidate the impact of microglial JunB upregulation on MBM progression. For molecular profiling, we employed RNA-seq and reverse-phase protein array (RPPA). To test microglial JunB functions, we generated microglia variants stably overexpressing JunB (JunB(hi)) or with downregulated levels of JunB (JunB(lo)). Melanoma-derived factors, namely leukemia inhibitory factor (LIF), controlled JunB upregulation through Janus kinase (JAK)/signal transducer and activator of transcription 3 (STAT3) signaling. The expression levels of JunB in melanoma-associated microglia were heterogeneous. Flow cytometry analysis revealed the existence of basal-level JunB-expressing microglia alongside microglia highly expressing JunB. Proteomic profiling revealed a differential protein expression in JunB(hi) and JunB(lo) cells, namely the expression of microglia activation markers Iba-1 and CD150, and the immunosuppressive molecules SOCS3 and PD-L1. Functionally, JunB(hi) microglia displayed decreased migratory capacity and phagocytic activity. JunB(lo) microglia reduced melanoma proliferation and migration, while JunB(hi) microglia preserved the ability of melanoma cells to proliferate in three-dimensional co-cultures, that was abrogated by targeting leukemia inhibitory factor receptor (LIFR) in control microglia–melanoma spheroids. Altogether, these data highlight a melanoma-mediated heterogenous effect on microglial JunB expression, dictating the nature of their functional involvement in MBM progression. Targeting microglia highly expressing JunB may potentially be utilized for MBM theranostics.