The mechanism of the premetastatic niche facilitating colorectal cancer liver metastasis generated from myeloid-derived suppressor cells induced by the S1PR1–STAT3 signaling pathway

The tumor-derived factors involved in the expansion and accumulation of myeloid-derived suppressor cells (MDSCs) in metastatic dissemination of colorectal cancer (CRC) to the liver has not been studied. Immunohistochemistry was used to detect sphingosine-1-phosphate receptor 1 (S1PR1) and signal transducer and activator of transcription-3 (STAT3) in human colorectal tumors. IL-6 and interferon-γ were detected by enzyme-linked immunosorbent assay (ELISA). Tumor growth, invasion, and migration were evaluated by MTT, transwell, and wound healing assays, respectively. Subcutaneous tumor-bearing and CRC liver metastasis (CRLM) nude mouse models were constructed. The percentage of MDSCs was measured using multicolor flow cytometry. Western blot assay was used to evaluate S1PR1 and p-STAT3 expression in MDSCs after separation from the liver and tumor by magnetic antibody. T-cell suppression assay was detected by carboxyfluorescein succinimidyl ester (CFSE). Aberrant co-expressed S1PR1 and p-STAT3 was correlated with metachronous liver metastasis and poor prognosis in CRC. A mutual activation loop between S1PR1 and STAT3 can enhance CRC cell proliferation, migration, and invasion in vitro and in vivo. The expression of p-STAT3 and its downstream proteins can be regulated by S1PR1. p-STAT3 was the dependent signaling pathway of S1PR1 in the promotion of cell growth and liver metastasis in CRC. The level of IL-6 and the associated MDSCs stimulated by the S1PR1–STAT3 correlated with the number of liver metastatic nodes in the CRLM mouse models and patients. Increased CD14+HLA-DR−/low MDSCs from CRLM patients inhibited autologous T-cell proliferation and predict poor prognosis. The S1PR1–STAT3–IL-6–MDSCs axis operates in both tumor cells and MDSCs involved in the promotion of growth and liver metastasis in CRC. MDSCs induced by S1PR1–STAT3 in CRC cells formed the premetastatic niche in the liver can promote organ-specific metastasis.