BMI-1 activates hepatic stellate cells to promote the epithelialmesenchymal transition of colorectal cancer cells

BACKGROUND Activated hepatic stellate cells(aHSCs)are the major source of cancer-associated fibroblasts in the liver.Although the crosstalk between aHSCs and colorectal cancer(CRC)cells supports liver metastasis(LM),the mechanisms are largely unknown.AIM To explore the role of BMI-1,a polycomb group protein family member,which is highly expressed in LM,and the interaction between aHSCs and CRC cells in promoting CRC liver metastasis(CRLM).METHODS Immunohistochemistry was carried out to examine BMI-1 expression in LM and matched liver specimens of CRC.The expression levels of BMI-1 in mouse liver during CRLM(0,7,14,21,and 28 d)were detected by Western blotting(WB)and the quantitative polymerase chain reaction(qPCR)assay.We overexpressed BMI-1 in HSCs(LX2)by lentivirus infection and tested the molecular markers of aHSCs by WB,qPCR,and the immunofluorescence assay.CRC cells(HCT116 and DLD1)were cultured in HSC-conditioned medium(LX2 NC CM or LX2 BMI-1 CM).CM-induced CRC cell proliferation,migration,epithelial-mesenchymal transition(EMT)phenotype,and transforming growth factor beta(TGF-β)/SMAD pathway changes were investigated in vitro.A mouse subcutaneous xenotransplantation tumor model was established by co-implantation of HSCs(LX2 NC or LX2 BMI-1)and CRC cells to investigate the effects of HSCs on tumor growth and the EMT phenotype in vivo.RESULTS Positive of BMI-1 expression in the liver of CRLM patients was 77.8%.The expression level of BMI-1 continued to increase during CRLM in mouse liver cells.LX2 overexpressed BMI-1 was activated,accompanied by increased expression level of alpha smooth muscle actin,fibronectin,TGF-β1,matrix metalloproteinases,and interleukin 6.CRC cells cultured in BMI-1 CM exhibited enhanced proliferation and migration ability,EMT phenotype and activation of the TGF-β/SMAD pathway.In addition,the TGF-βR inhibitor SB-505124 diminished the effect of BMI-1 CM on SMAD2/3 phosphorylation in CRC cells.Furthermore,BMI-1 overexpressed LX2 HSCs promoted tumor growth and the EMT phenotype in vivo.CONCLUSION High expression of BMI-1 in liver cells is associated with CRLM progression.BMI-1 activates HSCs to secrete factors to form a prometastatic environment in the liver,and aHSCs promote proliferation,migration,and the EMT in CRC cells partially through the TGF-β/SMAD pathway.