Efflux of N1-acetylspermidine from hepatoma fosters macrophage-mediated immune suppression to dampen immunotherapeutic efficacy

Metabolic reprogramming is a hallmark of tumor progression. Here, we examined the metabolic profile of hepatocellular carcinoma (HCC), a disease that responds poorly to immune checkpoint blockade (ICB). Polyamine metabolism increased in HCC samples. Of the polyamine spectrum analyzed, N1-acetylspermidine (N1-Ac-Spd) accumulated in HCC tissue as compared with nontumoral liver tissue and was elevated in paired plasma. Injection of N1-Ac-Spd promoted tumor progression in preclinical models and compromised the efficacy of ICB. Inflammatory macrophages increased expression of the spermidine/spermine N1-acetyltransferase 1, SAT1, in hepatoma cells, leading to increased N1-Ac-Spd efflux via the polyamine transporter protein SLC3A2. Mechanistically, N1-Ac-Spd efflux activated SRC signaling in a charge-dependent manner, which in turn induced CCL1⁺ macrophage polarization, the recruitment of CCR8⁺ regulatory T cells, and an immunosuppressive tumor microenvironment (TME). In vivo interventions targeting SLC3A2, SAT1, or CCL1 enhanced the antitumor effects of ICB therapy. Our findings provide insight into the mechanisms whereby metabolic reprogramming fosters an immunosuppressive TME, with implications for the treatment of HCC.