RAPID ACTIVATION OF THE STAT3 TRANSCRIPTION COMPLEX IN LIVER-REGENERATION

Liver regeneration in response to partial hepatectomy is a physiologic al growth response observed in the intact animal. Understanding the ea rly signals that trigger liver regeneration is of vital importance to understand the liver's response to injury. It has been observed that p roduction of several growth factors and cytokines, including epidermal growth factor (EGF) and interleukin-6 (IL-6), can activate members of the signal transducers and activators of transcription (Stat) family of transcription factors resulting in tyrosine phosphorylation of thes e factors, nuclear translocation, and an active DNA binding transcript ional complex. Because Stat3 participates in the regulation of primary growth response genes, we wondered if it is induced in the early phas e of liver re-generation. We found that Stat3 DNA-binding activity is in eased in the remnant liver within 30 minutes of partial hepatectomy and peaks at more than 30-fold at 3 hours. This induction is not obse rved after sham surgery. The induction of Stat3 appears to be part of the initial response of the remnant Liver to partial hepatectomy, beca use it occurs in the presence of cycloheximide-mediated protein synthe sis blockade. Activation of Stat3 is unusual, because it extends beyon d the immediate-early time period and remains near peak level at 5 hou rs posthepatectomy. Although insulin-treated H35 cells activate many o f the same immediate-early genes as regenerating liver, Stat3 is not i nduced in these cells. Because Stat factors are known to be inactivate d by protein tyrosine phosphatases (PTPase), we showed that a PTPase i s able to eliminate the DNA binding of hepatic Stat3. It is likely tha t Stat3 contributes to the transcriptional activation of a subset of i mmediate-early genes that ae induced of a subset of immediate-early ge nes that are induced over a prolonged time in the G1 phase of hepatic cells following partial hepatectomy. The identification of Stat3 as an early factor in liver regeneration provides clues to the activation o f signal transduction pathways in the remnant liver within the first m inutes.