Legumain-deficient macrophages regulate inflammation and lipid metabolism in adipose tissues to protect against diet-induced obesity

Adipose tissue macrophages (ATMs) are key players in the development of obesity and associated metabolic inflammation, which contributes to systemic metabolic dysfunction, and understanding the interaction between macrophages and adipocytes is crucial for developing novel macrophage-based strategies against obesity. Here, we found that Legumain (Lgmn), a well-known lysosomal cysteine protease, is expressed mainly in the ATMs of obese mice. To further define the potential role of Lgmn-expressing macrophages in the generation of an aberrant metabolic state, Lgmn^F/F; LysM^Cre mice, which do not express Lgmn in macrophages, were maintained on a high-fat diet (HFD), and metabolic parameters were assessed. Macrophage-specific Lgmn deficiency protects mice against HFD-induced obesity, diminishes the quantity of proinflammatory macrophages in obese adipose tissues, and alleviates hepatic steatosis and insulin resistance. By analysing the transcriptome and proteome of murine visceral white adipose tissue (vWAT) after HFD feeding, we determined that macrophage Lgmn deficiency causes changes in lipid metabolism and the inflammatory response. Furthermore, the reciprocity of macrophage-derived Lgmn with integrin α5β1 in adipocytes was tested via colocalization analyses. It is further demonstrated in macrophage and adipocyte coculture system that macrophage derived Lgmn bound to integrin α5β1 in adipocytes, therefore attenuating PKA activation, downregulating lipolysis-related proteins and eventually exacerbating obesity development. Overall, our study identified Lgmn as a previously unrecognized regulator involved in the interaction between ATMs and adipocytes contributing to diet-induced obesity and suggested that Lgmn is a potential target for treating metabolic disorders.