RAPID ACTIVATION OF POST-HEPATECTOMY FACTOR NUCLEAR FACTOR KAPPA-B INHEPATOCYTES, A PRIMARY RESPONSE IN THE REGENERATING LIVER

The liver represents one of the few organs in the intact animal that h as the capacity to regenerate following in jury or partial hepatectomy . One of the earliest responses that has been detected in the remnant liver is the activation of post-hepatectomy factor(s) (PHF), a kappa B site DNA binding activity. We reasoned that understanding the molecul ar nature of PHF might provide insight into what triggers liver regene ration. We found that PHF is rapidly activated and turned over in the regenerating liver, demonstrating peak activity at 30 min post-hepatec tomy and virtual disappearance by 1 h. As determined by supershift, cr oss-linking, and crosslinking/immunoprecipitation analyses, PHF contai ns intact p50/p65 nuclear factor kappa B (NF-kappa B) subunits. To exp lore the basis for activation of PHF/NF-kappa B in the regenerating li ver, we determined the level of individual Rel family subunits in the nuclei of normal and regenerating liver cells. We found evidence for n uclear translocation of p65/RelA, but other Rel family proteins includ ing p50/NF-kappa B1 and p52/NF-kappa B2 are present at a low level in the nuclei of cells at a constitutive level pre- and post-hepatectomy and appear not to form DNA binding homodimers. The level of I kappa B- alpha falls slightly then increases at 3 h post-hepatectomy in concert with the induction of its mRNA. As demonstrated by the induction of I kappa B-alpha mRNA in hepatocytes in situ and identification of PHF/N F-kappa B in cultured hepatocytes, PHF/NF-kappa B is localized primari ly in hepatocytes in the regenerating liver. This represents one of th e few examples of NF-kappa B activation in the intact animal in a non- hematopoietic cell type. The activation of PHF/NF-kappa B suggests a m echanism by which hepatocytes regulate their mitogenic program during liver regeneration.