Abstract
BackgroundProgrammed cell death ligand 1 (PD-L1) is an important immune-inhibitory protein expressed on cancer cells to mediate cancer escape through interaction with PD-1 expressed on activated T lymphocytes (T cells). Previously, we reported that colon and breast cancer stem cells (CSCs) expressed much higher levels of PD-L1 than their parental cells, suggesting they will be more resistant to immune attack.MethodsWe investigated the underlining mechanism of PD-L1 increase in colon CSCs, with a special focus on the effect of insulin and epithelial growth factor (EGF), the two fundamental components to sustain the metabolism and stemness in the culture of CSCs.ResultsWe found that insulin increased the total and surface PD-L1 levels through PI3K/Akt/mTOR pathway as the increase could be inhibited by the dual inhibitor of the pathway, BEZ235. EGF didn’t affect the total PD-L1 levels of CSCs but increased the cell surface protein levels by flow cytometry analysis, indicating EGF promotes the transport of PD-L1 to the cell surface. Blocking cell surface PD-L1 with a specific antibody resulted in a significant reduction of tumour sphere formation but didn’t interfere with the sphere growth, suggesting that cell surface PD-L1 may act as an adhering molecule for CSCs.ConclusionsApart from the essential roles in metabolism and stemness, insulin and EGF involve in up-regulation of PD-L1 expression in colon CSCs, therefore the inhibition of insulin and EGF/EGFR pathways can be considered for cancer immunotherapy or combined with PD-1/PD-L1 antibody-based cancer immunotherapy to eliminate CSCs.
Highlights
Programmed cell death ligand 1 (PD-L1) is an important immune-inhibitory protein expressed on cancer cells to mediate cancer escape through interaction with Programmed death protein 1 (PD-1) expressed on activated T lymphocytes (T cells)
We found that insulin could increase PD-L1 protein expressions of total and cell surface levels, and this increase was through Phosphoinositide 3-kinase (PI3K)/Akt/Mechanistic target of rapamycin (mTOR) pathway
epithelial growth factor (EGF) helps PD-L1 transport to cell membrane Though we did not observe the increase of total protein after EGF treatment by Western blotting (WB) assays, we found an increase of PD-L1 in the EGF treated cells when we analysed the cells by flow cytometry (Fig. 4a)
Summary
Programmed cell death ligand 1 (PD-L1) is an important immune-inhibitory protein expressed on cancer cells to mediate cancer escape through interaction with PD-1 expressed on activated T lymphocytes (T cells). We reported that colon and breast cancer stem cells (CSCs) expressed much higher levels of PD-L1 than their parental cells, suggesting they will be more resistant to immune attack. Cancer immunotherapy based on blocking immune check-point molecules such as programmed cell death protein 1 (PD-1 or CD279) and its ligand 1 (PD-L1 or CD274 or B7-H1) is a popular topic under intensive investigation. We previously studied the PD-L1 expression in CSCs from breast cancer MCF-7 and colon cancer HCT-116 cells and showed that PD-L1 expression markedly increased in CSCs for these cancers [10]. What factors are responsible for the increase and how they are regulated, especially regarding to the regulation role of fundamental factors in tumour microenvironment such as insulin and epithelial growth factor (EGF) on the expression and transport of PD-L1, were not studied in the study or reported by other researchers
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