Hop bitter acids inhibit PD-L1 expression and tumorigenicity of hepatocellular carcinoma cells

Bitter acids (BA) are the major component of the female inflorescences of the hop plant humulus lupulus. Asides from its main application in the beer brewery process recent studies revealed that BA are multipotent bioactive compounds that exhibit anti-carcinogenic effects.

The aim of the present study was assess the effect of BA on hepatocellular carcinoma (HCC).

Methods and Results: Three different human HCC cell lines (Hep3B, HepG2, PLC) were incubated with a BA extract purified with supercritical CO2. Beginning at a concentration of 25µg/ml BA induced cytotoxic effects in all 3 HCC cell lines as evidenced by morphological changes and LDH release. Already at lower concentrations BA lead to a dose dependent inhibition of proliferation and migration in functional in vitro analysis. At a concentrations as low as 5µg/ml BA inhibited ERK-phosphorylation, and led to a down-regulation of AP–1 and NFkappaB activity in HCC cells. Moreover, we observed that BA reduced the expression of programmed cell death–1 ligand (PD-L1), which plays a crucial role in tumor evasion from host immunity, and the expression of which is associated with poor prognosis in HCC-patients. In contrast, BA induced PD-L1 expression in primary human hepatocytes in a dose-dependent manner. Of note, we have previously shown that PD-L1 on hepatocytes exhibits anti-inflammatory effects.

Conclusions: Bitter acids have the potential to ameliorate different pro-tumorigenic mechanisms known to promote HCC progression. Further, they seem to affect the immune system beneficially via differential effects on PD-L1 expression in cancerous and non-cancerous cells. Since previous studies have shown the safety of even long term application of hop extracts in man, our data suggest the potential use of bitter acids as a functional nutrient for both the prevention and treatment of HCC.