Hepatic ischemia reperfusion injury (IRI), linked to leukocyte recruitment and subsequent release of cytokines and free radicals, remains a significant problem in organ transplantation. Leukocyte transmigration across vascular barriers depends upon coordinated focal matrix degradation events. We previously demonstrated that matrix metalloproteinase-9 (MMP-9) is an important mediator of leukocyte traffic to inflamed liver. Here, we extend our studies to the analysis of MMP-12, a macrophage metalloelastase implicated in tumorigenesis. Methods and Results: C57BL6 mice treated with a selective MMP-12 inhibitor (MMP-12-inh; 5mg/Kg; gavage 6hrs prior to surgery, IV at reperfusion) and C57BL6 vehicle (control) treated mice were subjected to 90 minutes of warm partial-liver ischemia followed by reperfusion. MMP-12 mRNA was almost undetectable in naÏve livers and it was significantly upregulated in C57BL6 livers from 6h to 48h after IRI (p<0.05). Administration of MMP-12-inh to C57BL6 mice depressed the serum AST (2475±1190 vs. 4670±994, p<0.05) and ALT (3963±1504 vs. 7455±2041, p<0.05) levels (IU/L) at 6h post IRI, as compared with controls. Moreover, MMP-12-inh therapy significantly improved histological preservation after IRI; MMP-12-inh treated livers showed modest signs of vascular congestion and necrosis, contrasting with extensive necrosis observed in the respective control cohorts. Additionally, Mac-1 macrophage (2.0±2.2cells vs. 7.2±2.6vs /HPF; p<0.01) and Ly6G neutrophil (2.96±2.29 vs. 6.23±2.67/HPF; p<0.03) infiltration was markedly reduced in MMP-12 inhibitor treated mice post-IRI, compared to controls. Besides cleaving extracellular matrix components, MMPs can also activate other MMPs. Interestingly, MMP-12 inhibition resulted in a significant decrease of MMP-9 activity (2781±2202 vs. 9772±2160; AU; p<0.002) after hepatic IRI. To further investigate if MMP-12 inhibition affects MMP-9 activity, peritoneal macrophages were stimulated with LPS in the presence of several concentrations of MMP-12 inhibitor. Indeed, MMP-12 inhibition significantly decreased MMP-9 activity in LPS-stimulated macrophages (p<0.05). Conclusion: Our data suggest that MMP-12 is a relevant mediator of the pathogenic events underlying liver IRI. They also support the view that MMP-12 could serve as a new target for therapies directed at reducing hepatic IRI, thereby improving the outcome of liver transplantation. This is the first study to document a function for MMP-12 in hepatic IRI.

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