Kupffer cell oxidant production is central to the mechanism of peroxisome proliferators

Increased cell proliferation most likely plays a key role inperoxisome proliferator-induced liver cancer. Recently, Kupffercells were shown to be responsible for Wy-14,643-induced cellproliferation. However, the mechanism by which peroxisome proliferatorsactivate Kupffer cells is unknown. Since gut-derived endotoxinis a known activator of Kupffer cells, the hypothesis that itis involved was evaluated. Increased cell proliferation andperoxisome induction were unaffected by gut sterilization. Moreover,endotoxin was not detectable in portal blood following treatmentwith Wy-14,643. Therefore, it is concluded that gut-derivedendotoxin is not responsible for Kupffer cell activation. Totest the hypothesis that Kupffer cells are activated by Wy-14,643directly, Kupffer cell superoxide production was measured followingtreatment in vitro. Wy-14,643 increased superoxide productionin a dose-dependent manner (0.1 and 50 µM) with half-maximalstimulation at 2.5 µM. Diethylhexylphthalate (DEHP) andethylhexanol did not increase superoxide production even atdoses 50 times higher than Wy-14,643; however, monoethylhexylphthalate(MEHP) activated superoxide production as effectively as Wy-14,643with half-maximal stimulation at 5 µM. Treatment withWy-14,643 for 21 days caused a 2-fold increase in Kupffer cellsuperoxide production while DEHP did not. Pretreatment of Kupffercells with staurosporine (0.01–10 pM) completely blockedgeneration of superoxide demonstrating that protein kinase Cis required. Moreover, Wy-14,643 increased Kupffer cell proteinkinase C activity 3-fold. Pretreatment of Kupffer cells withthe amino acid glycine (0.01–3 mM), which blunts calciumsignaling, inhibited Wy-14,643-stimulated superoxide productionand increased protein kinase C activity completely. These dataare consistent with the hypothesis that potent peroxisome proliferators(Wy-14,643 and MEHP) directly activate Kupffer cell productionof oxidants via mechanisms involving protein kinase C. Further,peroxisome proliferator treatments that sustain elevated ratesof cell proliferation (e.g. Wy-14,643) activate Kupffer cellsuperoxide production following long-term dietary treatmentsupporting the hypothesis that Kupffer cell-derived oxidantsare involved in peroxisome proliferator-induced neoplasia.