Rapid activation of latent transcription factor complexes reflects initiating signals in liver regeneration

Liver regeneration following partial hepatectomy represents a physiologic response to a growth stimulus occurring in the intact animal. Understanding what growth factors and cytokines trigger liver regeneration will provide insights into recovery from hepatic injury mediated by viruses and toxins, and promote an understanding of normal cellular growth control. The modification of pre-existing latent transcription factors in the remnant liver by extracellular signals immediately post-hepatectomy provides a mechanism for the transcriptional activation of primary or immediate early growth response genes, thereby establishing a transcriptional cascade. Two factors activated within minutes to hours post-hepatectomy in a protein synthesis-independent fashion include PHF/NF-kappaB and Stat3. Interestingly, these factors are commonly activated by cytokines such as TNFalpha, IL-1 and IL-6 suggesting that there may be a connection between cytokine release and the onset of liver regeneration. In addition to these known transcription factor complexes, we have used a reporter gene assay in transgenic mice to attempt to identify promoter sequences that are responsible for the transcriptional activation of the liver-restricted IGFBP-1 immediate early gene within minutes posthepatectomy. Studies so far indicate that an upstream region of 800 bp is able to confer both tissue-restricted expression and induction during liver regeneration. Identification of the transcriptional activators or liver regeneration factors responsible for this induction will result in further dissection of the initiating signals.