Hepatocyte-specific inhibition of NF-κB leads to apoptosis after TNF treatment, but not after partial hepatectomy

The transcription factor NF-κB has been implicated in both hepatocyte proliferation and apoptosis. Mice deficient in the p65 subunit of NF-κB die during gestation from hepatocyte apoptosis (1). Inhibition of NF-κB in hepatocyte cell lines blocked TNF-induced proliferation and sensitized these cells to apoptosis (2, 3). Furthermore, while inactive in adult, quiescent liver, NF-κB is rapidly activated after partial hepatectomy (PH), a surgical procedure that stimulates a process of compensatory proliferation known as liver regeneration (4, 5). NF-κB can be activated by multiple inflammatory stimuli, including cytokines such as TNF and IL-1, as well as bacterial endotoxin. These agents activate NF-κB by signaling kinases to phosphorylate the inhibitor of NF-κB, IκBα, which targets it for ubiquitination and subsequent degradation by the proteasome (6). Degradation of IκBα releases NF-κB to translocate from the cytosol to the nucleus, where it can activate transcription of genes containing appropriate NF-κB binding sites. NF-κB transcription can be inhibited by overexpression of a nondegradable IκBα mutant that no longer contains N-terminal phosphorylation sites. Several groups have used this “super-repressor” IκBα to show that inhibition of NF-κB sensitizes multiple cell types, including hepatocytes, to apoptosis (2, 7–10). TNF signaling through its type 1 receptor (TNFR1) plays a particularly important role in NF-κB activation in the liver. The lethal hepatocyte apoptosis observed in p65 knockout mice can be rescued by crossing these mice into a TNF or TNFR1-null background (11, 12). TNFR1/p65 double-knockout mice die shortly after birth due to massive acute inflammation of the liver. While TNFR1 knockout mice develop normally, they are severely impaired for liver regeneration and are deficient in activation of NF-κB after PH as well as downstream events such as IL-6 upregulation and STAT3 activation (13). Using a hepatocyte progenitor cell line, we showed that inhibition of NF-κB directly blocked TNF-mediated increases in IL-6 and STAT3 (3). However, other work has suggested that in vivo it is the liver nonparenchymal cells (NPCs) (i.e., resident macrophages, or Kupffer cells) that are responsible for IL-6 release (14, 15). Thus the cell-specific role of NF-κB in IL-6 production and hepatocyte proliferation during liver regeneration has yet to be clarified. NF-κB can be activated in both hepatocytes and NPCs during liver regeneration (4, 5). Iimuro et al. (9) found increased hepatocyte apoptosis in the regenerating liver of rats injected before PH with an adenoviral vector containing the super-repressor IκBα. However, the viral vector by itself also caused increased TNF