P11.15.B TUMOUR-SECRETED PHOSPHOPROTEIN 1 (SPP1) VIA ACTIVATION OF MICROGLIA CONTRIBUTES TO THE FORMATION OF THE BREAST AND LUNG CANCER BRAIN METASTASES

Abstract BACKGROUND Brain metastases (BrMets) occur in 20%-40% of patients with breast and lung cancers and contribute significantly to morbidity and/or mortality. The molecular mechanisms that drive and permit the progression of metastasis in the brain are poorly understood. Identifying components of the metastatic niches and propensities of primary tumors towards the brain microenvironment are essential to improve our understanding of BrMet development, patient management and outcomes. We have previously demonstrated the pivotal role of microglia, brain resident myeloid cells, in progression of malignant gliomas. SPP1 (a secreted phosphoglycoprotein 1) is a tumour derived potent activator of microglia (brain resident myeloid cells). MATERIAL AND METHODS Expression of SPP1 was analysed in public datasets of breast and lung cancer brain metastasis and by qPCR and ELISA in a number of patient derived cell lines. Cancer cells cancer proliferation and cell invasion was evaluated in Matrigel assay in microglia-cancer cell co-cultures. Expression of genes supporting tumour invasion and immunosuppression was determined by qPCR. The effects of the 7aaRGD interfering peptide, or Minocycline (MINO). Intracranial growth of patients derived xenograft (PDX) in nude mice was evaluated histologically and by measuring luminescence of stably labelled malignant cells, and myeloid cell accumulation was determined with immunofluorescence. RESULTS We found upregulated expression of SPP1 is elevated in breast and lung cancer brain metastasis and a number of patient derived cell lines. Cancer cells-derived SPP1 strongly activate microglia in ex vivo co-cultures, increases cancer cell invasion in Matrigel assay and upregulated a number of genes supporting tumour invasion and immunosuppression. Blocking SPP1-mediated cancer-microglia communication with the 7aaRGD interfering peptide and MINO abolished microglia-dependent cancer cell invasion. The intracranial growth of the patient derived xenografts injected to nude mice was established, and staining for microglia markers TMEM119 and Sal1 were performed. CONCLUSION The results highlight the important role of breast and lung cancer BrMet-derived SPP1 in shaping the permissive microenvironment of BrMets, and indicate a potential of the 7aaRGD peptide or minocycline to be new therapeutics in brain metastasis treatment. Studies were supported by the TRANSCAN-3/1/168/2022 ImmuMet project funded by National Center for Research and Development, Poland.