Mouse hepatocyte response to peroxisome proliferators: dependency on hepatic nonparenchymal cells and peroxisome proliferator activated receptor alpha (PPAR alpha)

Peroxisome proliferators (PPs) are rodent nongenotoxic hepatocarcinogens th at induce peroxisome proliferation and DNA synthesis, and suppress apoptosi s in rodent hepatocytes. PPs act through the PP-activated receptor alpha (P PAR alpha); tumour necrosis factor alpha (TNF alpha) and hepatic nonparench ymal cells (NPCs), the major source of TNF alpha in the liver, have also be en implicated in mediating the rodent hepatic response to PPs. Here we inve stigate the interaction between PPARa and NPCs in regulating the response t o PPs. Using normal hepatocyte cultures containing around 20% NPCs, the PP nafenopin (50 muM) induced DNA synthesis and suppressed transforming growth factor beta (1)-induced apoptosis. However, when the NPCs were removed by differential centrifugation, nafenopin did not induce DNA synthesis or supp ress apoptosis in the pure hepatocytes. Reconstitution of the normal hepato cyte cultures by mixing together the pure hepatocytes and the previously se parated NPCs in the same proportions as the original cell preparation (17.7 +/- 8.7% NPCs) restored the response to nafenopin. Interestingly, nafenopi n was still able to induce beta -oxidation in the pure hepatocyte cultures, consistent with NPCs being required for PP-induced growth but not for pero xisome proliferation. Next, we evaluated the role of PPAR alpha in the hepa tocyte dependency upon NPCs. Interestingly, NPCs isolated from PPAR alpha - null mice, like those isolated from the wild-type NPCs, restored the hepato cyte response to nafenopin. However, as expected, PPARa-null hepatocytes re mained non-responsive to PPs, irrespective of the genotype of the added NPC s. These data support a role for NPCs in facilitating a response of hepatoc ytes to PPs that is ultimately dependent on the presence of PPAR alpha in t he hepatocyte.