The transcription factor NF-kB has been implicated in both hepatocyte proliferation and death (apoptosis). NF-kB is activated during liver regeneration, a process in which hepatocytes proliferate, but its role in this process is unclear. Previous studies demonstrated that NF-kB is required to prevent activation of apoptosis by TNF, a pleiotropic cytokine that is required for liver regeneration. Inhibition of NF-kB in vitro has been shown to switch the proliferative signal of TNF to an apoptotic signal. Thus it is thought that NF-kB activates transcription of a gene that blocks TNF-mediated apoptosis, which has yet to be identified in hepatocytes. Additionally, NF-kB upregulates IL-6, another cytokine required for liver regeneration. However, it is not clear which liver cell types (hepatocytes or non-parenchymal cells) are responsible for IL-6 production during liver regeneration. I hypothesized that NF-kB activation was required in hepatocytes during liver regeneration for both a proliferative stimulus and to prevent TNF-mediated apoptosis. This hypothesis was tested by creation of a transgenic mouse in which NF-kB activation could be blocked by inducible overexpression of an inhibitor of NF-kB, DN-IkBa, specifically in hepatocytes. Using these mice, I determined that inhibition of NF-kB in hepatocytes during liver regeneration did not affect IL-6 production or hepatocyte proliferation.
Additionally, I found that inhibition of NF-kB did not lead to hepatocyte apoptosis, as previously reported. Instead, I found that the dose of TNF required to activate apoptosis was much higher than that required to activate NF-kB. Furthermore, the primary location of NF-kB activation during liver regeneration was in non-parenchymal cells instead of hepatocytes. These results indicate that NF-kB is not required in hepatocytes during liver regeneration. Further studies are needed to determine the role of NF-kB activation in non-parenchymal cells during liver regeneration.
I also searched for NF-kB regulated anti-apoptotic genes by transfecting a hepatocyte cell line with DN-IkBa. These cells were used in a cDNA subtraction assay to create a library of potentially NF-kB regulated genes. One gene isolated, A20, was previously identified as an anti-apoptotic gene. Preliminary data indicated A20 overexpression could protect AML12 cells from TNF-mediated apoptosis in the absence of NF-kB activity.