Diminished activity of peroxisome proliferator-activated receptor (PPAR) is implicated in activation of hepatic stellate cells (HSC), a critical event in development of liver fibrosis. In the present study, we investigated PPAR regulation by tumor necrosis factor (TNF) in a HSC cell line designated as BSC. In BSC, TNF decreased both basal and ligand (GW1929)-induced PPAR mRNA levels without changing its protein expression. Nuclear extracts from BSC treated with TNF showed decreased binding of PPAR to PPAR-responsive element (PPRE) as determined by electrophoretic mobility shift assay. In BSC transiently transfected with a PPAR1 expression vector and a PPRE-luciferase reporter gene, TNF decreased both basal and GW1929-induced transactivation of the PPRE promoter. TNF increased activation of extracellular signal-regulated kinases (ERK) 1/2 and c-Jun NH₂-Terminal Kinase (JNK), previously implicated in phosphorylation of Ser⁸² of PPAR1 and resultant negative regulation of PPAR transactivity. In fact, TNF failed to inhibit transactivity of a Ser⁸² Ala PPAR1 mutant in BSC. TNF-mediated inhibition of PPAR transactivity was not blocked with a Ser³² Ala/Ser³⁶ Ala mutant of inhibitory NFB (IB). These results suggest that TNF inhibits PPAR transactivity in cultured HSC at least in part by diminished PPAR-PPRE (DNA) binding and ERK1/2-mediated phosphorylation of Ser⁸² of PPAR1, but not via the NFB pathway.