Glutamate promotes CCL2 expression to recruit tumor-associated macrophages by restraining EZH2-mediated histone methylation in hepatocellular carcinoma

Glutamate is well-known as metabolite for maintaining the energy and redox homeostasis in cancer, moreover it is also the primary excitatory neurotransmitter in the central nervous system. However, whether glutamatergic signaling can regulate hepatocellular carcinoma (HCC) progression and the specific regulatory mechanisms are unknown. In the present study, we found that glutamate and its receptor NMDAR2B were significantly elevated in HCC patients, which predicts poor prognosis. Glutamate could upregulate CCL2 expression on hepatoma cells and further enhance the capability of tumor cells to recruit tumor-associated macrophages (TAMs). Mechanistically, glutamate could facilitate CCL2 expression through NMDAR pathway by decreasing the expression of EZH2, which regulates the H3K27me3 levels on the CCL2 promoter, rather than affecting DNA methylation. Moreover, inhibiting glutamate pathway with MK801 could significantly delay tumor growth, with reduced TAMs in implanted Hepa1-6 mouse HCC models. Our work suggested that glutamate could induce CCL2 expression to promote TAM infiltration by negatively regulating EZH2 levels in hepatoma cells, which might serve as a potential prognostic marker and a therapeutic target for HCC patients.