Peroxisome proliferators (PPs) are rodent nongenotoxic hepatocarcinogens that induce peroxisome proliferation and DNA synthesis, and suppress apoptosis in rodent hepatocytes. PPs act through the PP-activated receptor alpha (PPARalpha); tumour necrosis factor alpha (TNFalpha) and hepatic nonparenchymal cells (NPCs), the major source of TNF alpha in the liver, have also been implicated in mediating the rodent hepatic response to PPs. Here we investigate the interaction between PPARalpha and NPCs in regulating the response to PPs. Using normal hepatocyte cultures containing around 20% NPCs, the PP nafenopin (50 microM) induced DNA synthesis and suppressed transforming growth factor beta1-induced apoptosis. However, when the NPCs were removed by differential centrifugation, nafenopin did not induce DNA synthesis or suppress apoptosis in the pure hepatocytes. Reconstitution of the normal hepatocyte cultures by mixing together the pure hepatocytes and the previously separated NPCs in the same proportions as the original cell preparation (17.7+/-8.7% NPCs) restored the response to nafenopin. Interestingly, nafenopin was still able to induce beta-oxidation in the pure hepatocyte cultures, consistent with NPCs being required for PP-induced growth but not for peroxisome proliferation. Next, we evaluated the role of PPARalpha in the hepatocyte dependency upon NPCs. Interestingly, NPCs isolated from PPARalpha-null mice, like those isolated from the wild-type NPCs, restored the hepatocyte response to nafenopin. However, as expected, PPARalpha-null hepatocytes remained non-responsive to PPs, irrespective of the genotype of the added NPCs. These data support a role for NPCs in facilitating a response of hepatocytes to PPs that is ultimately dependent on the presence of PPARalpha in the hepatocyte.