Introduction: Exosomes produced by neoplastic cells can promote the development of a pro-tumorigenic microenvironment via intercellular communication, and in some cases promote metastasis. The role of exosomes in the spread of prostate cancer, however, remains poorly understood. Here, we show that exosomes derived from enzalutamide resistant prostate cancer cells (EnzR exos) mediate intercellular communication with bone marrow-resident cells, alter the expression of extracellular matrix (ECM) proteins, and enhance metastasis in a clinically relevant mouse model of metastatic prostate cancer.

Methods: Exosomes were isolated via ultracentrifugation of conditioned media from enzalutamide resistant CWR-R1 cells. Mice were primed with exosomes via three systemic injections of 10 ug exosomal protein. Exosome-mediated changes in bone marrow ECM composition were assessed via RNA sequencing of CD11b+ bone marrow cells and immunohistochemistry of formalin-fixed, decalcified, paraffin-embedded mouse femurs and tibias. For metastasis studies, intracardiac injection of luciferase-expressing enzalutamide resistant CWR-R1 cells into C.B.-17 SCID mice was used to establish metastatic tumor burden, and animals were monitored via bioluminescence imaging. Exosomal RNA sequencing was performed on EnzR exos and clinical specimens, with exosomes from normal prostate epithelial cells (PNT2) used as a control.

Results: Mice primed with EnzR exos developed significantly enhanced metastatic tumor burden compared to unprimed controls. EnzR exo priming also led to decreased expression of thrombospondin-1 in the bone marrow compartment, and increased expression of versican, as determined by RNA sequencing and immunohistochemistry. Exosomal RNA sequencing revealed that five miRNAs were significantly enriched in EnzR exos compared to PNT2 exosome controls. One of these, miR-4443, was found to target thrombospondin-1. Prostate cancer patient serum samples were also enriched in miR-4443 over healthy controls, and exosomal miR-4443 correlated with disease progression.

Conclusions: These data indicate that exosomes derived from enzalutamide resistant prostate cancer cells can alter ECM composition in the bone marrow compartment in a pro-tumorigenic manner. Evidence for the clinical relevance of these data was observed in both serum samples and bone biopsies from prostate cancer patients, demonstrating abundant serum exosomal miR-4443 and similarly altered bone marrow ECM composition in advanced prostate cancer patients. Exosomal miR-4443 may be a promising diagnostic and prognostic marker, as well as a potential therapeutic target.

Citation Format: Stephen E. Henrich, Kaylin McMahon, Michael Plebanek, Fabio Tavora, Andre De Souza, Anthony Mega, Benedito Carneiro, C. Shad Thaxton. Prostate cancer exosomes alter extracellular matrix composition in bone marrow and enhance metastasis [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 6204.