Lipopolysaccharide facilitates immune escape of hepatocellular carcinoma cells via m6A modification of lncRNA MIR155HG to upregulate PD-L1 expression

Recent studies have suggested that the initiation and progression of hepatocellular carcinoma (HCC) are closely associated with lipopolysaccharide (LPS) of intestinal bacteria. However, the role of LPS in immune regulation of HCC remains largely unknown. An orthotopic Hepa1-6 tumor model of HCC was constructed to analyze the effect of LPS on the expression of immune checkpoint molecules PD-1 and PD-L1. Then we verified the regulation of PD-L1 by LPS in HCC cells. Based on the previous finding that lncRNA MIR155HG regulates PD-L1 expression in HCC cells, we analyzed the relationship of LPS signaling pathway molecules with PD-L1 and MIR155HG by bioinformatics. The molecular mechanism of MIR155HG regulating PD-L1 expression induced by LPS was investigated by RNA pull-down followed by mass spectrometry, RNA immunoprecipitation, fluorescence in situ hybridization, and luciferase reporter assay. Finally, the HepG2 xenograft model was established to determine the role of MIR155HG on PD-L1 expression in vivo. We showed that LPS induced PD-1 and PD-L1 expression in mouse tumor tissues and induced PD-L1 expression in HCC cells. Mechanistically, upregulation of METTL14 by LPS promotes the m6A methylation of MIR155HG, which stabilizes MIR155HG relying on the "reader" protein ELAVL1 (also known as HuR)-dependent pathway. Moreover, MIR155HG functions as a competing endogenous RNA (ceRNA) to modulate the expression of PD-L1 by miR-223/STAT1 axis. Our results suggested that LPS plays a critical role in immune escape of HCC through METTL14/MIR155HG/PD-L1 axis. This study provides a new insight for understanding the complex immune microenvironment of HCC.