STAT3 mediates cancer stem‐like tumorsphere formation and PD‐L1 expression to contribute radioresistance in HBV‐positive hepatocellular carcinoma

Abstract

We proposed that cancer stem cells (CSCs) survived and presented resistance to radiotherapy (RT) in hepatocellular carcinoma (HCC) cells. Interleukin 6 (IL‐6) has been reported to be particularly involved in HCC tumorigenesis. Therefore, we intended to validate that IL‐6 downstream STAT3‐mediated CSCs formation and immune checkpoint PD‐L1 expression in HCC, thus contributing to radioresistance. HBV‐positive HCC tumorspheres were formed and exposed with X‐ray irradiation, cell viability of which was measured consequently. Specific inhibitors targeting EGFR (by gefitinib), STAT3 (by BBI608), and HCC‐targeted therapy sorafenib were investigated to suppress tumorsphere formation. Reverse transcription‐quantitative polymerase chain reaction (RT‐qPCR) was used for detecting STAT3‐downstream PD‐L1 and anti‐apoptosis MCL1 and BCL2 gene expression in the PLC5‐derived tumorspheres and STAT3‐knockdown PLC5. We found that RT significantly inhibited HBV‐positive Hep3B and PLC5 cell viability but not for HCC‐derived stem‐like tumorspheres cultured by EGF, IL‐6, bFGF, and HGF. It revealed that tumorspheres presented radioresistance compared with the parental cells. Specifically, RT induces IFNs, EGF, and IL‐6 expression, resulting in STAT3 phosphorylation. Kaplan–Meier plotter indicated that highly EGF (p = .0024), IL‐6 (p = .12), and FGF2 (p = .0041) were associated with poor survival probability in patients with HBV‐positive HCC. We further demonstrated that BBI608 and sorafenib significantly suppressed PLC5 cell viability and PLC5‐derived tumorsphere formation. To investigate the mechanism of CSC‐presented radioresistance, STAT3 and STAT3‐downstream genes, including PD‐L1 and anti‐apoptosis MCL1 and BCL2, were detected using qPCR. We demonstrated higher STAT3, PD‐L1, MCL1, and BCL2 in Hep3B‐ and PLC5‐derived CSCs compared to PLC5. In addition, knockdown of STAT3 reduced cell proliferation in PLC5 cells, resulting in down‐regulation of IL‐6‐mediated PD‐L1 and BCL‐2. Meanwhile, we found that knockdown of STAT3 significantly improved RT‐mediated suppression of tumorsphere formation. In conclusion, we found that CSCs presented radioresistance and figured out which may be mediated by STAT3 in HBV‐positive HCC.