SFRP1 decreases WNT-Mediated M2 macrophage marker expression in breast tissue

The Wnt family of secreted proteins are involved in mammary gland development and tumorigenesis. It has recently been shown that Wnt ligands promote M2 macrophage polarization and so we sought to determine the effects of a Wnt signaling antagonist, Secreted Frizzled Related Protein 1 (SFRP1), on M2 marker expression. We measured a murine M2 marker (Arg1) in mice with a targeted deletion of Sfrp1 during different stages of mammary gland development including puberty, pregnancy, and lactation, as well as in response to obesity. Next, to determine whether Wnt signaling/antagonism affects human M2 markers (CD209 and CCL18), we treated a human patient derived explant (PDE) breast tissue sample with exogenous Wnt3a in the presence and absence of rSFRP1. Finally, we expanded our PDE study to 13 patients and performed bulk RNAseq analysis following the treatment described above. We found that in loss of Sfrp1 in the murine mammary gland increased Arg1 expression. Moreover, we showed that Wnt3a increases CD209 and CCL18 mRNA and protein expression in breast PDEs and that their expression is decreased in response to rSFRP1. Our RNAseq analysis unveiled novel genes that were affected by Wnt3a treatment and subsequently reversed when rSFRP1 was added. Validation of these data exhibited that chemokines involved in promoting macrophage polarization and cancer metastasis, including CCL11 and CCL26, were stimulated by Wnt3a signaling and their expression was abrogated by treatment with rSFRP1. Our data suggest that SFRP1 may be an important mediator that tempers Wnt signaling in the tumor microenvironment.