Although the involvement of protein arginine methyltransferase 1 (PRMT1) in tumorigenesis has been reported, its roles in breast cancer progression and metastasis has not been elucidated. Here we identified PRMT1 as a key regulator of the epithelial-mesenchymal transition (EMT) in breast cancer. We showed that the EMT program induced by PRMT1 endowed the human mammary epithelial cells with cancer stem cell properties. Moreover, PRMT1 promoted the migratory and invasive behaviors in breast cancer cells. We also demonstrated that abrogation of PRMT1 expression in breast cancer cells abated metastasis in vivo in mouse model. In addition, knockdown of PRMT1 arrested cell growth in G1 tetraploidy and induced cellular senescence. Mechanistically, PRMT1 impacted EMT process and cellular senescence by mediating the asymmetric dimethylation of arginine 3 of histone H4 (H4R3me2as) at the ZEB1 promoter to activate its transcription, indicating the essential roles of this epigenetic control both in EMT and in senescence. Thus, we unraveled a dual function of PRMT1 in modulation of both EMT and senescence via regulating ZEB1. This finding points to the potent value of PRMT1 as a dual therapeutic target for preventing metastasis and for inhibiting cancer cell growth in malignant breast cancer patients.

The tumor premetastatic niche initiated by primary tumors is constructed by multiple molecular factors and cellular components and provides permissive condition that allows circulating tumor cells to successfully metastasize. Myeloid-derived suppressor cells (MDSCs), a population of immature cells in pathological conditions, play a critical role in the formation of the premetastatic niche. However, few researches are focused on the function of monocytic MDSCs (mo-MDSCs), a subtype of MDSCs, in the construction of the niche. Here, we show that the number of mo-MDSCs is significantly increased in the premetastatic lungs of tumor-bearing mice, thus promoting tumor cell arrest and metastasis. Before the arrival of tumor cells, the lung-recruited mo-MDSCs produced IL-1β, thereby increasing E-selectin expression and promoting tumor cell arrest on endothelial cells. Depletion of mo-MDSCs in the premetastatic lungs decreased IL-1β production, resulting in reduced E-selectin expression. In addition, compared with alveolar macrophages and interstitial macrophages, mo-MDSCs were the major source of IL-1β expression in the premetastatic lungs. Cytokine array analyses and transwell experiments revealed that CCL12 recruits mo-MDSCs to premetastatic lungs. CCL12 knockdown in tumor-bearing mice significantly decreased mo-MDSC infiltration into the premetastatic lungs, leading to reduced E-selectin expression. Overall, the permissive conditions produced by the infiltrated mo-MDSCs correlated with increased tumor cell arrest and metastasis. These results reveal a novel role of mo-MDSCs in constructing the premetastatic niche. Thus, inhibition of mo-MDSCs infiltration may change the premetastatic niche to normal condition and attenuate tumor metastasis.

During malignant progression, primary tumors rebuild leukocyte profile and suppress the host anti-tumor immune response. Tumor-associated neutrophils (TAN) increased in the cancer patients and emerged as an important participant and regulator of immune responses. The aim of this study is to investigate the role of circulating TAN (cTAN) in the metastatic process of advanced malignancy. We tested circulating neutrophils from patients (n = 180) with various types of cancer using flow cytometry analyses. We also used B16F10 cell-implanted C57BL/6 tumor-bearing mice model to simulate the advanced malignancy. Peripheral neutrophils were isolated by ficoll density gradient centrifugation, and in vitro tumor-leukocyte co-culture model was used to test tumor cell survival under leukocyte challenge condition. Here, we showed that neutrophils increased in the peripheral blood under the pathological condition of advanced malignancy both in cancer patients and in tumor-bearing mice. In mouse model, the malignantly increased neutrophils were identified as TAN according to the gene transcriptional analyses. We also showed that cTAN enhance tumor metastasis and cTAN could inhibit the activation of the peripheral leukocytes and rescue tumor cells from leukocyte challenge. In conclusion, our finding suggests that the abundance of cTAN in advanced cancer patients contributes to the circulating tumor cell survival by suppressing peripheral leukocyte activation.

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