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Liver regeneration: molecular mechanisms of growth control
1Department of Pathology, Duke University, Durham, North Carolina 27710.
Abstract:
The molecular signals controlling liver regeneration are becoming rapidly defined. Control of growth in regenerating liver has advanced from elusive serum factors and nutrient effects to identification of entirely new growth factors with apparent liver specificity as well as establishment of meaningful gene expression patterns for growth factors already known. Based on studies with hepatocyte cultures and gene expression in regenerating liver, the substances EGF, TGF alpha, HBGF-1 (aFGF), and two new substances (HPTA/HGF and Hepatopoietin B) have been defined as complete mitogens for hepatocytes and implicated in control of liver growth. The amino acid sequence of HPTA/HGF recently became clear and revealed interesting structural homologies in a molecule that might become the largest known growth factor. The plasticity of growth responses seen in liver may be controlled by these factors as well as by comitogenic substances such as norepinephrine which, although nonmitogenic per se, can initiate growth in hepatocytes exposed to the above mitogenic growth factors or mitogenic inhibitors such as TGF beta. The role of the latter in cessation of DNA synthesis in liver regeneration will be discussed, presenting the positive and negative evidence that constitutes the TGF beta paradox of liver regeneration.
Insights
Key growth factors like EGF and Hepatocyte Growth Factor (HGF) drive liver regeneration. Norepinephrine and TGF-beta also play complex roles in controlling hepatocyte growth and DNA synthesis.
Area of Science:
- Hepatology
- Molecular Biology
- Cellular Growth Signaling
Background:
- Liver regeneration involves complex molecular signals.
- Previous research focused on serum factors and nutrient effects.
- Advancements include identifying specific growth factors and gene expression patterns.
Purpose of the Study:
- To define molecular signals controlling liver regeneration.
- To identify key mitogens and their roles in hepatocyte growth.
- To explore the roles of comitogenic and inhibitory substances.
Main Methods:
- Studies using hepatocyte cultures.
- Analysis of gene expression in regenerating liver.
- Investigation of growth factor sequences and structural homologies.
Main Results:
- Epidermal Growth Factor (EGF), Transforming Growth Factor alpha (TGF alpha), Heparin-Binding Growth Factor-1 (HBGF-1/aFGF), Hepatocyte Growth Factor/Hepatocyte Prostatum-Derived Growth Factor (HGF/HPTA), and Hepatopoietin B identified as mitogens.
- HGF/HPTA sequence revealed structural homologies.
- Norepinephrine acts as a comitogen, initiating growth with mitogens.
- Transforming Growth Factor beta (TGF beta) implicated in the cessation of DNA synthesis.
Conclusions:
- Specific growth factors are crucial for liver regeneration.
- Comitogenic and inhibitory signals modulate hepatocyte proliferation.
- The TGF beta paradox in liver regeneration warrants further investigation.