Baicalein and baicalin promote antitumor immunity by suppressing PD-L1 expression in hepatocellular carcinoma cells

Blocking the PD-L1/PD-1 pathway to prevent the immune evasion of tumor cells is a powerful approach for treating multiple cancers, including hepatocellular carcinoma (HCC). Previous studies have shown that baicalein and baicalin are directly cytotoxic to some tumors, here we demonstrate that in addition to direct cytotoxicity, these two flavonoids stimulate the T cell mediated immune response against tumors through reduction of PD-L1 expression in cancer cells. Interestingly, more significant tumor regression was observed in BALB/c mice than in BALB/c-nu/nu mice after baicalein and baicalin treatment. PD-L1 upregulation induced by interferon-gamma (IFN-gamma) was significantly inhibited by these two flavonoids in vitro. Both baicalein and baicalin enhanced the cytotoxicity of T cells to eliminate tumor cells, which was abrogated after HCC cells were transfected with a PD-L1 over-expression plasmid or after T cells were pretreated with an anti-PD-1 blocking antibody. Further mechanistic research indicated that the IFN-gamma-induced expression and promoter activity of PD-L1 were suppressed by these two flavonoids, and these effects were mediated by STAT3 activity inhibition. Therefore, baicalein and baicalin decreased STAT3 activity, further downregulated IFN-gamma-induced PD-L1 expression and subsequently restored T cell sensitivity to kill tumor cells. Our findings provide novel insight into the anticancer effects of baicalein and baicalin through which tumor growth is inhibited by PD-L1 expression downregulation and suggest that these flavonoids have great potential for clinical treatment.