Peroxisome proliferator activated receptor-alpha expression in human liver

Area of Science:

• Molecular biology
• Toxicology
• Genetics

Background:

• Peroxisome proliferator-activated receptor alpha (PPAR alpha) is a nuclear receptor involved in regulating gene expression.
• PPAR alpha mediates the effects of peroxisome proliferators, which cause pathological changes in mice but not humans.
• Human liver exhibits refractoriness to the effects of peroxisome proliferators observed in rodent models.

Purpose of the Study:

• To investigate the molecular mechanisms underlying the differential response to peroxisome proliferators between human and mouse liver.
• To compare the expression levels and DNA-binding activity of PPAR alpha in human versus mouse liver.
• To identify potential reasons for the refractoriness of human liver to peroxisome proliferator-induced effects.

Main Methods:

• RNase protection assays were used to quantify hepatic PPAR alpha mRNA levels relative to beta-actin mRNA in humans and mice.
• Human PPAR alpha cDNA was isolated and analyzed for functional coding potential, including assessment of alternative RNA splicing.
• Supershift analysis with specific antisera and a peroxisome proliferator response element (CYP4A6-Z) was performed on liver lysates to measure PPAR alpha DNA binding activity.

Main Results:

• Hepatic PPAR alpha mRNA levels were found to be approximately one order of magnitude lower in humans compared to mice.
• Human liver samples showed significant levels of PPAR alpha splice variants lacking exon 6, suggesting impaired receptor expression due to alternative splicing.
• PPAR alpha DNA binding activity was >10-fold lower in human liver lysates than in mouse liver lysates, with human PPAR alpha binding being less prominent than other unidentified proteins.

Conclusions:

• Low levels of PPAR alpha expression in human liver, potentially exacerbated by alternative splicing, may be insufficient for effective transcriptional activation.
• The reduced PPAR alpha expression and DNA binding activity in humans likely contribute to the refractoriness of human liver to peroxisome proliferator-induced pathological effects.
• Humans possess the genetic potential for a functional PPAR alpha receptor, but its expression levels in the liver limit its biological activity compared to mice.