Signal transduction during liver regeneration

Area of Science:

• Hepatology and Molecular Biology
• Cellular signaling pathways in liver regeneration
• Transcription factor regulation

Background:

• Partial hepatectomy (PH) initiates a complex cascade of biochemical events preceding hepatocyte proliferation.
• Understanding the regulation of transcription factors like c-Jun and nuclear factor-kappa B (NFκB) is crucial for elucidating liver regeneration mechanisms.
• The roles of c-Jun amino-terminal kinase (JNK) and TNFα in this process require further investigation.

Purpose of the Study:

• To investigate the regulation and role of the AP-1 transcription factor c-Jun during hepatic regeneration after PH.
• To determine the involvement of JNK signaling and TNFα in c-Jun activation and subsequent AP-1-dependent gene transcription.
• To elucidate the physiological function of NFκB induction during liver regeneration following PH.

Main Methods:

• Analysis of c-Jun mRNA levels, AP-1 binding activity, and JNK activity following varying degrees of PH in rats.
• In vitro studies using primary rat hepatocytes treated with growth factors to assess AP-1 activity and c-Jun phosphorylation.
• In vivo experiments using neutralizing antibodies against TNFα and adenoviral vectors expressing a super-repressor of NFκB (Ad5IκB) to evaluate their effects on liver regeneration, apoptosis, and cell cycle progression.

Main Results:

• PH progressively increased c-Jun mRNA and AP-1 binding activity, with concomitant stimulation of JNK activity, including JNK1.
• TNFα neutralization inhibited hepatocyte DNA synthesis and JNK activation post-PH, indicating its requirement for JNK-mediated c-Jun phosphorylation and AP-1 transcription.
• Inhibition of NFκB by Ad5IκB administration post-PH led to increased apoptosis, cell cycle arrest, and liver failure, suggesting NFκB is critical for preventing these adverse outcomes.

Conclusions:

• JNK activation following PH is mediated by TNFα, leading to c-Jun phosphorylation and enhanced transcription of AP-1-dependent genes, crucial for liver regeneration.
• NFκB induction during liver regeneration is essential for preventing apoptosis and allowing normal cell cycle progression, thereby maintaining liver function post-PH.