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Fat paradox in liver disease
1Department of Pathology, Keck School of Medicine of the University of Southern California and the Department of Veterans Affairs Greater Los Angeles Healthcare System, Los Angeles, CA, USA. htsukamo@usc.edu
The Keio Journal of Medicine
|February 3, 2006
Summary
Fat accumulation in liver cells (hepatocytes) is key in alcoholic and non-alcoholic fatty liver disease. However, this fat accumulation is essential for keeping liver cells (hepatic stellate cells) quiescent, revealing a paradox in liver fat metabolism.
Area of Science:
- Hepatology
- Molecular Biology
- Cellular Metabolism
Background:
- Alcoholic liver disease (ALD) and non-alcoholic fatty liver disease (NAFLD) share a common hallmark: lipid accumulation in hepatocytes, leading to steatosis.
- Hepatic stellate cells (HSC) normally store vitamin A and triglycerides, but activation leads to lipid depletion.
- Recent research indicates that adipogenic/lipogenic transcriptional regulation (PPARγ, LXRa, SREBP-1c) is crucial for HSC quiescence.
Purpose of the Study:
- To investigate the contrasting roles of lipid accumulation in hepatocytes and HSC in ALD and NAFLD.
- To explore the significance of adipogenic transcriptional regulation in maintaining HSC quiescence versus inducing hepatocyte steatosis.
- To understand the implications of this 'fat paradox' for liver fibrosis in ALD and NAFLD.
Main Methods:
- Analysis of gene expression of adipogenic transcription factors (PPARγ, LXRa, SREBP-1c) in steatotic liver models (ALD and NAFLD).
- Assessment of HSC phenotype (quiescent vs. activated) in relation to lipid content and transcription factor expression.
- Experimental manipulation of HSCs using adipocyte differentiation cocktails and ectopic expression of transcription factors.
Main Results:
- Adipogenic transcription factors are upregulated in steatotic livers of ALD and NAFLD mouse models, contrasting with normal livers.
- Adipogenic regulation is essential for HSC quiescence but simultaneously promotes steatosis in hepatocytes.
- Despite steatotic conditions, HSC activation and subsequent fibrosis still occur in ALD and NAFLD models.
Conclusions:
- A 'fat paradox' exists where adipogenic regulation promotes HSC quiescence and hepatocyte steatosis, highlighting their interdependent homeostatic control.
- Activated HSC in steatotic livers may exhibit defective insulin signaling or lipogenic regulation.
- Understanding this paradox is crucial for developing targeted therapies for ALD and NAFLD-associated fibrosis.