Role of hepatic non-parenchymal cells in the response of rat hepatocytes to the peroxisome proliferator nafenopin in vitro

Induction of liver cancer by peroxisome proliferators such as nafenopin is frequently associated with increased liver growth, increased DNA synthesis and suppression of apoptosis, The cytokine, tumour necrosis factor alpha (T NF alpha), and non-parenchymal liver cells have been implicated in mediatin g the hepatic response to peroxisome proliferators. Here, we have investiga ted the dependency of the hepatocyte response to peroxisome proliferators o n non-parenchymal cells, a major source of hepatic cytokines. Addition of n on-parenchymal cells, or conditioned medium from non-parenchymal cell cultu res, increased DNA synthesis (220% and 270% of control, respectively) and s uppressed transforming growth factor beta (1)-induced hepatocyte apoptosis (32% and 54% of control, respectively). Removal of non-parenchymal cells fr om normal hepatocyte cultures prevented both the nafenopin- and TNF alpha - induced increase in DNA synthesis and suppression of hepatocyte apoptosis; this response was restored by returning non-parenchymal cells to the purifi ed hepatocytes. TNF alpha was detected in the medium of non-parenchymal cel l (3-15 pg/ml) and normal hepatocyte cultures (25-100 pg/ml) by bioassay us ing L929 cells. However, the contribution of TNF alpha released from non-pa renchymal cells was small compared with that released spontaneously by hepa tocytes. Nafenopin significantly increased the release of TNF alpha from no n-parenchymal cells to 56 +/- 18 pg/ml, but had little effect on TNF alpha release by hepatocytes. However, the concentration of exogenous TNF alpha r equired to elicit a response in hepatocytes was 100 pg/ml and above. These data provide evidence that hepatic non-parenchymal cells are permissive for the growth response of hepatocytes in vitro to peroxisome proliferators an d this may be mediated, at least in part by TNF alpha. However, the levels of TNF alpha released spontaneously or in response to peroxisome proliferat ors are insufficient per se to induce a growth response.