SEK1/MKK4-mediated SAPK/JNK signaling participates in embryonic hepatoblast proliferation via a pathway different from NF-κB-induced anti-apoptosis

Mice lacking the stress-signaling kinase SEK1 die from embryonic day 10.5 (E10.5) to E12.5. Although a defect in liver formation is accompanied with the embryonic lethality of sek1^-1- mice, the mechanism of the liver defect has remained unknown. In the present study, we first produced a monoclonal antibody specifically recognizing murine hepatoblasts for the analysis of liver development and further investigated genetic interaction of sek1 with tumor necrosis factor-α receptor 1 gene (tnfr1) and protooncogene c-jun, which are also responsible for liver formation and cell apoptosis. The defective liver formation in sek1^-1- embryos was not protected by additional tnfr1 mutation, which rescues the embryonic lethality of mice lacking NF-κB signaling components. There was a progressive increase in the hepatoblast cell numbers of wild-type embryos from E10.5 to E12.5. Instead, impaired hepatoblast proliferation was observed in sek1^-1- rivers from E10.5, though fetal liver-specific gene expression was normal. The impaired phenotype in sek1^-1- livers was more severe than in c-jun^-1- embryos, and sek1^-1- c-jun^-1- embryos died more rapidly before E8.5. The hepatoblast proliferation required no hematopoiesis, since liver development was not impaired in AML1^-1- mice that lack hematopoietic functions. Stimulation of stress-activated protein kinase/c-Jun N-terminal kinase by hepatocyte growth factor was attenuated in sek1^-1- livers. Thus, SEK1 appears to play a crucial role in hepatoblast proliferation and survival in a manner apparently different from NF-κB or c-Jun.