SIGNAL-TRANSDUCTION DURING LIVER-REGENERATION

Following partial hepatectomy (PH), there is a rapid and highly orches trated series of biochemical events which occur prior to cellular prol iferation. Some of these events are presumably intimately linked with the eventual regeneration of the liver, whereas others are likely to b e stress related or required for the continued differentiated function of the liver while regeneration is occurring. The regulation of the A P-1 transcription factor c-Jun during hepatic regeneration has been st udied here. There is a progressive increase in c-Jun-mRNA levels after sham operation, one-third PH, and two-thirds PH. A concomitant increa se in activating protein 1 (AP-1) binding activity is also observed. T he c-Jun protein is a major constituent of the AP-I complex in quiesce nt and early regenerating liver. The activity of c-Jun amino-terminal kinase (JNK), which phosphorylates the activation domain of the c-Jun protein, is markedly stimulated after one-third and two-thirds PH. c-J un amino-terminal kinase-1 is a constituent of this stimulated JNK act ivity after PH. When primary cultures of adult rat hepatocytes are inc ubated with epidermal growth factor or transforming growth factor-alph a, AP-1 transcriptional activity is increased and the activation domai n of the c-Jun protein is further potentiated. Phosphopeptide mapping of the endogenous c-Jun protein in proliferating cultured hepatocytes demonstrates phosphorylation of the c-Jun activation domain. Pretreatm ent of animals prior to PH with a neutralizing antibody to tumour necr osis factor-alpha (TNF alpha), inhibits hepatocyte DNA synthesis and J NK activation. It is concluded that the stimulation of one-third or tw o-thirds PH activates JNK through a mechanism that requires TNF alpha, which phosphorylates the c-Jun activation domain in hepatocytes, resu lting in enhanced transcription of AP-1-dependent genes. Although nucl ear factor-kappa B (NF kappa B) binding activity is induced during liv er regeneration following PH, the physiological consequence of this in duction is unknown. The role of NF kappa B during liver regeneration h as been assessed by delivering to the liver a super-repressor of NF ka ppa B activity using an adenoviral vector expressing a mutated form of I kappa B. This adenovirus (Ad5I kappa B) was almost exclusively expr essed in the liver and inhibited NF kappa B DNA binding activity and t ranscriptional activity in cultured cells as well as in the liver in v ivo. Following PH, Ad5I kappa B, but not a control adenovirus (Ad5 bet a gal), resulted in the induction of apoptosis as demonstrated by hist ological staining and TUNEL analysis. In addition, infection with Ad5I kappa B but not Ad5 beta gal decreased the mitotic index following PH . These two phenomena, increased apoptosis and cell cycle arrest, were associated with liver failure in animals infected with the ad5I kappa B but not Ad5 beta gal as demonstrated by elevated serum bilirubin an d ammonia levels. Thus, the induction of NF kappa B during liver regen eration following PH appears to be a required event to prevent apoptos is and to allow for normal cell cycle progression.