Interleukin-34 (IL-34) is a ligand for the CSF-1R and has also two additional 
receptors, PTPRZ1 and syndecan-1. IL-34 plays a role in innate immunity, 
inflammation, and cancer. However, the role of IL-34 in breast cancer is still 
ill-defined. We analyzed IL-34 mRNA expression in breast cancer cell lines and 
breast cancer patients and applied established computational approaches 
(CIBERSORT, ESTIMATE, TIMER, TCIA), to analyze gene expression data from The 
Cancer Genome Atlas (TCGA). Expression of IL-34 was associated with a favorable 
prognosis in luminal and HER2 but not basal breast cancer patients. Gene 
expression of CSF-1 and CSF-1R was strongly associated with myeloid cell 
infiltration, while we found no or only weak correlations between IL-34, PTPRZ1, 
syndecan-1 and myeloid cells. In vitro experiments showed that tyrosine 
phosphorylation of CSF-1R, ERK, and FAK and cell migration are differentially 
regulated by IL-34 and CSF-1 in breast cancer cell lines. Collectively, our data 
suggest that correlation of IL-34 gene expression with survival is dependent on 
the molecular breast cancer subtype. Furthermore, IL-34 is not associated with 
myeloid cell infiltration and directly regulates breast cancer cell migration 
and signaling.