The peroxisome proliferator-activated nuclear receptors (PPAR-α, PPAR-β, and PPAR-γ), which modulate the expression of genes involved in energy homeostasis, cell cycle, and immune function, may play a role in hepatic stellate cell activation. Previous studies focused on the decreased expression of PPAR-γ in hepatic stellate cell activation but did not investigate the expression and role of the PPAR-α and -β isotypes. The aim of this study was to evaluate the expression of the different PPARs during hepatic stellate cell activation in vitro and in situ and to analyze possible factors that might contribute to their expression. In a second part of the study, the effect of a PPAR-β agonist on acute liver injury was evaluated.

The effects of PPAR isotype-specific ligands on hepatic stellate cell transition were evaluated by bromodeoxyuridine incorporation, gel shifts, immunoprecipitation, and use of antisense PPAR-β RNA–expressing adenoviruses. Tumor necrosis factor α–induced PPAR-β phosphorylation and expression was evaluated by metabolic labeling and by using specific P38 inhibitors.

Hepatic stellate cells constitutively express high levels of PPAR-β, which become further induced during culture activation and in vivo fibrogenesis. No significant expression of PPAR-α or -γ was found. Stimulation of the P38 mitogen-activated protein kinase pathway modulated the expression of PPAR-β. Transcriptional activation of PPAR-β by L165041 enhanced hepatic stellate cell proliferation. Treatment of rats with a single bolus of CCl₄ in combination with L165041 further enhanced the expression of fibrotic markers.

PPAR-β is an important signal-transducing factor contributing to hepatic stellate cell proliferation during acute and chronic liver inflammation.