The hemopexin domain of matrix metalloproteinase-9 attenuates lipopolysaccharide-induced interleukin-6 secretion in liver

Matrix metalloproteinase-9 (MMP-9) has been implicated in modulating hepatic inflammation, as MMP-9 deficiency exacerbates liver damage and inflammatory responses in sepsis models. However, the mechanisms underlying its anti-inflammatory properties, particularly in the context of lipopolysaccharide (LPS) induced liver inflammation, remain poorly understood. Plasma and liver cytokine levels in MMP-9 catalytic-deficient mice were measured after the administration of 4.5 mg/kg LPS injections for 4 and 24 h. LPS-induced inflammatory responses were examined in mouse primary hepatocytes, hepatic, and macrophage cell lines by analyzing the secretion and expression of interleukin-6 (IL-6) in the gain- and loss-of-function of MMP-9. In vitro models were established by lentiviral infection, including MMP-9 knockout, Tet-On inducible wild-type MMP-9, and MMP-9 with catalytic domain mutation or hemopexin (PEX) domain deletion in wild-type parent cells. Following LPS treatment, MMP-9 catalytic-deficient mice demonstrated a significant increase in IL-6 levels, along with a notable decrease in MMP-9 protein expression relative to wild-type mice, in both plasma and liver tissue. Overexpression of MMP-9 significantly attenuated LPS-induced IL-6 secretion in hepatocytes. Notably, even the overexpression of a catalytically inactive MMP-9 mutant retained this suppressive effect. In contrast, deletion of the PEX domain abolished the inhibitory effect of MMP-9 on IL-6 secretion, indicating that the PEX domain, rather than its catalytic activity, is essential for this regulatory function. Moreover, MMP-9 overexpression markedly suppressed IL-6 secretion in response to LPS stimulation, whereas its genetic deletion significantly enhanced IL-6 secretion in macrophages. This study demonstrated that MMP-9 could partially protect the liver from LPS-induced damage by decreasing pro-inflammatory cytokine levels via its PEX domain, indicating the anti-inflammatory potential of the MMP-9 PEX domain against endotoxin.