miRNA dynamics in tumor-infiltrating myeloid cells modulating tumor progression in pancreatic cancer.

Myeloid cells including tumor-associated macrophages (TAM) and myeloid-derived suppressor cells (MDSC) are known as important mediators of tumor progression in solid tumors such as pancreatic cancer. Infiltrating myeloid cells have been identified not only in invasive tumors, but also in early pre-invasive pancreatic intraepithelial precursor lesions (PanIN). The functional dynamics of myeloid cells during carcinogenesis is largely unknown. We aimed to systematically elucidate phenotypic and transcriptional changes in infiltrating myeloid cells during carcinogenesis and tumor progression in a genetic mouse model of pancreatic cancer. Using murine pancreatic myeloid cells isolated from the genetic mouse model at different time points during carcinogenesis, we examined both established markers of macrophage polarization using RT-PCR and FACS as well as transcriptional changes focusing on miRNA profiling. Myeloid cells isolated during carcinogenesis showed a simultaneous increase of established markers of M1 and M2 polarization during carcinogenesis, indicating that phenotypic changes of myeloid cells during carcinogenesis do not follow the established M1/M2 classification. MiRNA profiling revealed distinct regulations of several miRNAs already present in myeloid cells infiltrating pre-invasive PanIN lesions. Among them miRNA-21 was significantly increased in myeloid cells surrounding both PanIN lesions and invasive cancers. Functionally, miRNA-21-5p and -3p altered expression of the immune-modulating cytokines CXCL-10 and CCL-3 respectively. Our data indicate that miRNAs are dynamically regulated in infiltrating myeloid cells during carcinogenesis and mediate their functional phenotype by facilitating an immune-suppressive tumor-promoting micro-milieu.