Spermidine enhances macrophages anti-inflammatory and regenerative functions by improving mitochondrial fitness

Background: Liver fibrosis is an aberrant wound-healing process in response to chronic hepatic injury. Functional plasticity of macrophages is crucial in this pathological process. How the pro-fibrotic function of macrophages is sustained metabolically and whether it can be targeted for therapy remain to be explored. Purpose: This study investigates the impact of spermidine, a natural polyamine, on macrophage function and its potential in treating hepatic fibrosis. Methods: Chronic liver disease datasets and single-cell RNA sequencing (scRNA-seq) data were analyzed for the characteristics of spermidine metabolism in hepatic macrophage function and liver fibrosis. Mice undergoing liver fibrosis caused by carbon tetrachloride (CCl4) were treated with spermidine through daily drinking water. Macrophage-specific PGC1α knockout mice were used to determine the requirement of mitochondrial fitness in the anti-fibrotic function of macrophages. Furthermore, spermidine-treated macrophages were adoptively transferred to mice with liver fibrosis to test their therapeutic potential. Results: Analysis of chronic liver disease datasets revealed a dysregulated polyamine metabolism in diseased liver tissues, particularly in distinct subsets of macrophages. Spermidine supplementation protected mice from CCl4-induced liver fibrosis through endowing macrophages a persistent anti-inflammatory and pro-resolving function that is characterized by elevated matrix metalloproteinase expression and enhanced mitochondrial function. Spermidine-treated macrophages (SPDMs) exhibited increased mitochondrial mass, augmented oxidative phosphorylation, and altered fatty acid metabolism. The markers characteristic of tolerogenic and liver-regenerative macrophages were upregulated in SPDMs in a manner dependent on peroxisome proliferator-activated receptor-γ coactivator (PGC) -1α, a key regulator of mitochondrial homeostasis. Interestingly, oleate generated by stearoyl-CoA desaturase 1 (SCD1) was essential for SPDMs to acquire the increased mitochondrial fitness. Notably, adoptive transfer of SPDMs to fibrotic mice significantly attenuated disease progression. The anti-fibrotic effect was compromised in mice with myeloid cell-specific deletion of PGC1α, highlighting the importance of mitochondrial biogenesis in mediating macrophage phenotype plasticity. Conclusions: Our study implicates dysregulation of spermidine metabolism in hepatic macrophages in the development of chronic liver diseases. We demonstrated an anti-fibrotic function of spermidine that is attributed to its action on macrophages. PGC1α-mediated mitochondrial fitness is required for spermidine to confer macrophages an enhanced anti-inflammatory and anti-fibrotic capacity. Meanwhile, we provide novel insights into the role of fatty acid metabolism in modifying the biological function of macrophages. This study opens new avenues for treating fibrotic diseases by targeting macrophage plasticity through spermidine-mediated metabolic reprogramming and demonstrate the potential of spermidine-trained macrophages as approaches for inflammatory and fibrotic conditions.