Suppression of mouse hepatocyte apoptosis by peroxisome proliferators: role of PPAR alpha and TNF alpha

Peroxisome proliferators (PPs) are a diverse group of nongenotoxic chemical s that in rodents cause hepatic peroxisome proliferation, liver enlargement , increased replicative DNA synthesis and suppression of apoptosis. The eff ects of PPs in vivo can be reproduced in vitro where PPs can induce mouse h epatocyte DNA synthesis and suppress both spontaneous apoptosis and that in duced by transforming growth factor beta (TGF beta). In vitro, high concent rations (> 500 U/ml) of exogenous tumour necrosis factor alpha (TNF alpha) [M. Rolfe, N.H. James, R.A. Roberts, TNF alpha suppresses apoptosis and ind uces S-phase in rodent hepatocytes: a mediator of the hepatocarcinogenicity of peroxisome proliferators?, Carcinogenesis 18 (1997) 2277-2280] are also able to stimulate hepatocyte DNA synthesis and suppress apoptosis, implica ting TNF alpha in mediating or permitting the liver growth response to PPs. Here, using cultured mouse hepatocytes isolated from PPAR alpha null mice, we have examined the role of the peroxisome proliferator activated recepto r cw (PPAR alpha) in mediating the suppression of apoptosis caused by PPs. In addition we have investigated further the role of TNF alpha in mediating the rodent response to PPs. The PP nafenopin (50 mu M) was unable to stimu late DNA synthesis measured by bromodeoxyuridine incorporation in these PPA R alpha null mouse hepatocytes (96% of control), unlike epidermal growth fa ctor, a growth factor used as a positive control. In assays of apoptosis us ing H33258 staining of chromatin condensation, nafenopin was unable to supp ress either spontaneous or TGF beta 1-induced apoptosis. In contrast, high concentrations of TNF alpha (> 500 U/ml) were able to both stimulate DNA sy nthesis (204% of control) and suppress apoptosis in PPAR alpha null hepatoc ytes (40% and 38% of control for spontaneous and TGF beta 1-induced apoptos is respectively). However, TNF alpha could not stimulate beta-oxidation of palmitoyl CoA in either PPAR alpha null mouse or B6C3F1 (PPAR alpha wild ty pe) mouse hepatocytes. These data confirm the dependence of the response to PPs on PPAR alpha by demonstrating that PPAR alpha mediates the suppressio n of hepatocyte apoptosis in response to PPs. In addition, the data provide evidence that high concentrations of TNF alpha can modulate DNA synthesis and apoptosis in the absence of PPs and PPAR alpha. Thus, in vivo, physiolo gical levels of TNF alpha may be permissive for a PPAR alpha-dependent grow th response to PPs. (C) 2000 Elsevier Science B.V. All rights reserved.