unc-51-like autophagy activating kinase 1 and 2 (Ulk1/2) regulate autophagy initiation under various stress conditions. However, the physiological functions of these Ser/Thr kinases are not well characterized. Here, we show that mice with liver-specific double knockout (LDKO) of Ulk1 and Ulk2 (Ulk1/2 LDKO) are viable, but exhibit overt hepatomegaly phenotype. Surprisingly, Ulk1/2 LDKO mice display normal autophagic activity in hepatocytes upon overnight fasting, but are strongly resistant to acetaminophen (APAP)-induced liver injury. Further studies revealed that Ulk1/2 are also dispensable for APAP-induced autophagy process, but are essential for the maximum activation of c-Jun N-terminal kinase (JNK) signaling both in vivo and in isolated primary hepatocytes during APAP treatment. Mechanistically, APAP-induced inhibition of mechanistic target of rapamycin complex 1 releases Ulk1 from an inactive state. Activated Ulk1 then directly phosphorylates and increases the kinase activity of mitogen-activated protein kinase kinase 4 and 7 (MKK4/7), the upstream kinases and activator of JNK, and mediates APAP-induced liver injury. Ulk1-dependent phosphorylation of MKK7 was further confirmed by a context-dependent phosphorylation antibody. Moreover, activation of JNK and APAP-induced cell death was markedly attenuated in Mkk4/7 double knockdown hepatocytes reconstituted with an Ulk1-unphosphorylatable mutant of MKK7 compared to those in cells rescued with wild-type MKK7.
Conclusion: Together, these findings reveal an important role of Ulk1/2 for APAP-induced JNK activation and liver injury, and understanding of this regulatory mechanism may offer us new strategies for prevention and treatment of human APAP hepatotoxicity. (Hepatology 2018;67:2397-2413).
© 2017 by the American Association for the Study of Liver Diseases.