Matrix metalloproteinase-12 (MMP12) is a proinflammatory macrophage-secreted protein with immunomodulatory functions that affects neutrophil infiltration, cytokine release, macrophage recruitment, and proliferation. We have previously demonstrated that the genetic deletion of MMP12 in a cardiometabolic mouse model ameliorates obesity-induced low-grade inflammation, white adipose tissue dysfunction, and atherosclerosis. Based on the various beneficial metabolic effects of MMP-12 deletion, we hypothesized that loss of MMP-12 also positively affects whole-body energy metabolism and/or brown adipose tissue (BAT) function in a cardiometabolic mouse model. To investigate the effects of MMP12 deletion on whole-body energy metabolism and/or BAT function, we used low-density lipoprotein receptor (Ldlr)/Mmp12 double knockout (DKO) fed a high-fat, sucrose- and cholesterol-enriched diet. DKO mice housed at 22°C showed increased energy expenditure and decreased BAT size and triglyceride (TG) content. Untargeted proteomic analyses revealed the upregulation of proteins and pathways related to mitochondrial function, glucose metabolism, and fatty acid oxidation in the BAT of DKO mice, whereas the abundance of proteins and pathways associated with inflammation was reduced. In addition, DKO mice exhibited reduced macrophage infiltration in BAT, with the infiltrating macrophages showing lower expression of lipid-associated marker genes. Loss of MMP12 was associated with reduced compactness and sphericity of the mitochondria in the BAT. Following an acute cold exposure, DKO mice had decreased circulating lipid concentrations, especially very low-density lipoprotein-TG and LDL-cholesterol, and increased expression of thermogenic genes. We conclude that MMP12 may play a detrimental role in whole-body energy homeostasis and thermogenesis, as it triggers macrophage infiltration, inflammation, and mitochondrial dysfunction in BAT.
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