High Molecular Weight Kininogen but Not Factor XII Deficiency Attenuates Acetaminophen-Induced Liver Injury in Mice

Acetaminophen (APAP) toxicity is a common cause of acute liver failure (ALF). Dysregulation of coagulation is well documented in ALF patients. Recent reports indicate that small subset of ALF patients (~10%) exhibit spontaneous bleeding, while thrombotic complications are more frequent. Animal studies demonstrated that the inhibition of tissue factor-initiated thrombin generation attenuates APAP-induced liver injury in mice via both fibrin- and protease activated receptor 1-dependent mechanisms. Activation of FXII leads to two events: propagation of coagulation by activation of FXI (intrinsic pathway) and activation of plasma prekallikrein to kallikrein, with subsequent cleavage of high molecular weight kininogen (HK) into bradykinin (BK) and cleaved HK fragments (contact pathway).

There is a growing interest in therapeutic targeting of FXII or FXI to prevent thrombosis, primarily because this goal may be attainable without incurring a significant bleeding risk. In this study, we evaluated if the intrinsic coagulation pathway contributes to the activation of coagulation and liver injury in a mouse model of APAP-induced ALF. We postulated that targeting FXII would attenuate coagulation and reduce liver injury without affecting hemostasis. In addition, FXII deficiency could provide a further protection by preventing activation of contact pathway and subsequent reduction of inflammatory response.

We used 12 weeks old FXII, FXI, prekallkrein and HK deficient male mice and their respective WT controls. Mice were fasted for 16 hours followed by a single intraperitoneal injection of APAP (400 mg/kg) or sterile saline. Livers and plasma samples were collected 6 and 24 hours after injections. Hepatocellular necrosis was determined on liver sections stained with H&E. Plasma levels of alanine transaminase (ALT- marker of liver injury), thrombin-antithrombin (TAT) complexes, plasmin-α₂ antiplasmin (PAP) complexes and interleukin-6 were analyzed using commercially available assays. HK cleavage was determined using BK ELISA, Western blot, and mass spectrometry analysis.

Consistently with previously published data, plasma levels of ALT, TAT and IL-6, as well as liver injury scores were significantly elevated in APAP-challenged mice compared to saline-injected WT mice at both 6 and 24 hours. Surprisingly, neither FXII, FXI, nor prekallikrein deficiency had statistically significant effects on any of these parameters in APAP-challenged mice at either time point. Interestingly, however, plasma levels of ALT were significantly attenuated in HK-/- mice (n=23-34) compared to HK+/+ (n=19-31) controls at both 6 (1278 ± 184 vs. 1850 ± 235 U/L; p<0.05) and 24 hours (1546 ± 302 vs. 3857 ± 493 U/L; p<0.01) after injection of APAP. The attenuation of liver injury was observed despite the lack of a significant effect on APAP-induced coagulation activation (plasma TAT levels [mean±SEM]: 72.9 ± 8.2 μg/L in HK^+/+ vs. 70.8 ± 4.3 μg/L in HK^-/-). HK deficiency also reduced plasma levels of IL-6 (75.8%; p<0.001), number of infiltrating neutrophils in liver (59.8%; p<0.001) and liver necrotic area (28.1%; p<0.001) 24 hours after APAP challenge. Importantly, the protective effects of HK deficiency were completely reversed by the injection of HK protein into APAP-treated HK-/- mice.

It has been previously shown that plasminogen deficiency protects against APAP-induced liver injury. Indeed, APAP administration activated the fibrinolytic system in WT mice as demonstrated by increased plasma levels of active tissue plasminogen activator (19.5 fold; p<0.01) and PAP complexes (64.4 fold; p<0.01). Using Western blotting analysis, BK ELISA, and mass spectrometry analysis we demonstrated that plasmin efficiently cleaves HK in a buffer system as well as in mouse and human plasma. Importantly, plasmin mediated cleavage of HK resulted in the release of BK and generation of cleaved HK fragments.

In summary, our data indicate that the extrinsic but not the intrinsic coagulation pathway drives the coagulation-mediated pathologies associated with APAP-induced liver injury in mice. HK contributes to the liver injury independently of thrombin generation. We propose that in APAP-challenged mice, HK cleavage and downstream hepatotoxicity, are mediated by plasmin rather than FXIIa-dependent generation of kallikrein.