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 (TNF alpha), and non-parenchymal liver cells have been implicated in mediating the hepatic response to peroxisome proliferators. Here, we have investigated the dependency of the hepatocyte response to peroxisome proliferators on non-parenchymal cells, a major source of hepatic cytokines. Addition of non-parenchymal cells, or conditioned medium from non-parenchymal cell cultures, increased DNA synthesis (220% and 270% of control, respectively) and suppressed transforming growth factor beta(1)-induced hepatocyte apoptosis (32% and 54% of control, respectively). Removal of non-parenchymal cells from 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 purified hepatocytes. TNF alpha was detected in the medium of non-parenchymal cell (3-15 pg/ml) and normal hepatocyte cultures (25-100 pg/ml) by bioassay using L929 cells. However, the contribution of TNF alpha released from non-parenchymal cells was small compared with that released spontaneously by hepatocytes. Nafenopin significantly increased the release of TNF alpha from non-parenchymal cells to 56 +/- 18 pg/ml, but had little effect on TNF alpha release by hepatocytes. However, the concentration of exogenous TNF alpha required 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 and this may be mediated, at least in part by TNF alpha. However, the levels of TNF alpha released spontaneously or in response to peroxisome proliferators are insufficient per se to induce a growth response.