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

Increased cell proliferation most likely plays a key role in peroxisome pro liferator-induced liver cancer. Recently, Kupffer cells were shown to be re sponsible for Wy-14,643-induced cell proliferation. However, the mechanism by which peroxisome proliferators activate Kupffer cells is unknown. Since gut-derived endotoxin is a known activator of Kupffer cells, the hypothesis that it is involved was evaluated. Increased cell proliferation and peroxi some induction were unaffected by gut sterilization, Moreover, endotoxin wa s not detectable in portal blood following treatment with Wy-14,643, Theref ore, it is concluded that gut-derived endotoxin is not responsible for Kupf fer cell activation. To test the hypothesis that Kupffer cells are activate d by Wy-14,643 directly, Kupffer cell superoxide production was measured fo llowing treatment in vitro, Wy-14,643 increased superoxide production in a dose-dependent manner (0.1 and 50 mu M) with half-maximal stimulation at 2. 5 mu M Diethylhexylphthalate (DEHP) and ethylhexanol did not increase super oxide production even at doses 50 times higher than Wy-14,643; however, mon oethylhexylphthalate (MEHP) activated superoxide production as effectively as Wy-14,643 with half-maximal stimulation at 5 mu M. Treatment with Wy-14, 643 for 21 days caused a 2-fold increase in Kupffer cell superoxide product ion while DEHP did not. Pretreatment of Kupffer cells with staurosporine (0 .01-10 pM) completely blocked generation of superoxide demonstrating that p rotein kinase C is required. Moreover, Wy-14,643 increased Kupffer cell pro tein kinase C activity 3-fold. Pretreatment of Kupffer cells with the amino acid glycine (0.01-3 mM), which blunts calcium signaling, inhibited Wy-14, 643-stimulated superoxide production and increased protein kinase C activit y completely. These data are consistent with the hypothesis that potent per oxisome proliferators (Wy-14,643 and MEHP) directly activate Kupffer cell p roduction of oxidants via mechanisms involving protein kinase C, Further, p eroxisome proliferator treatments that sustain elevated rates of cell proli feration (e.g. Wy-14,643) activate Kupffer cell superoxide production follo wing long-term dietary treatment supporting the hypothesis that Kupffer cel l-derived oxidants are involved in peroxisome proliferator-induced neoplasi a.