p21-activated kinase 4 inhibition protects against liver ischemia/reperfusion injury: Role of nuclear factor erythroid 2-related factor 2 phosphorylation

Background and Aims: p21-activated kinase 4 (PAK4), an oncogenic protein, has emerged as a promising target for anticancer drug development. Its role in oxidative stress conditions, however, remains elusive. We investigated the effects of PAK4 signaling on hepatic ischemia/reperfusion (I/R) injury. Approach and Results: Hepatocyte- and myeloid-specific Pak4 knockout (KO) mice and their littermate controls were subjected to a partial hepatic I/R (HIR) injury. We manipulated the catalytic activity of PAK4, either through genetic engineering (gene knockout, overexpression of wild-type [WT] or dominant-negative kinase) or pharmacological inhibitor, coupled with a readout of nuclear factor erythroid 2-related factor 2 (Nrf2) activity, to test the potential function of PAK4 on HIR injury. PAK4 expression was markedly up-regulated in liver during HIR injury in mice and humans. Deletion of PAK4 in hepatocytes, but not in myeloid cells, ameliorated liver damages, as demonstrated in the decrease in hepatocellular necrosis and inflammatory responses. Conversely, the forced expression of WT PAK4 aggravated the pathological changes. PAK4 directly phosphorylated Nrf2 at T369, and it led to its nuclear export and proteasomal degradation, all of which impaired antioxidant responses in hepatocytes. Nrf2 silencing in liver abolished the protective effects of PAK4 deficiency. A PAK4 inhibitor protected mice from HIR injury. Conclusions: PAK4 phosphorylates Nrf2 and suppresses its transcriptional activity. Genetic or pharmacological suppression of PAK4 alleviates HIR injury. Thus, PAK4 inhibition may represent a promising intervention against I/R-induced liver injury.