Reduced expression of mature TGFβ1 correlates with the suppression of rat hepatocyte apoptosis by the peroxisome proliferator, nafenopin

Abstract

Non-genotoxic carcinogens cause cancer without damaging the DNA. Peroxisome proliferators (PPs) are a class of potent rodent non-genotoxic hepatocarcinogens that may act by perturbing hepatocyte growth regulation. Previously, we have shown that although cultured rat hepatocytes degenerate rapidly in culture, their survival can be reversibly maintained by the PP nafenopin. This prolonged survival is associated with a decrease in the number of hepatocytes displaying the chromatin condensation characteristic of apoptosis. The addition of the negative growth regulator TGFβ-1 induced high levels of hepatocye apoptosis but nafenopin was able to suppress this TGFβ-1 induced apoptosis. These data suggested that increased levels of mature TGFβ-1 may be involved in the signalling of the apoptosis seen in degenerating hepatocyte cultures. To test this hypothesis, we carried out Western blot analyses using a anti-TGFβ1 antibody. There was an increase ( p = 0.014) in expression of mature TGFβ1 in degenerating rat hepatocyte cultures compared with hepatocyte cultures surviving in the presence of nafenopin. However, there was a concommitant decrease ( p = 0.024) in TGFβ1-latency activated protein (TGFβ1-LAP), the precursor of the active, mature form. Immunocytochemistry confirmed that TGFβ1 /TGFβ1-LAP expression was predominantly in the hepatocytes displaying apoptotic morphology although expression was detected also in non-parenchymal liver cells. The immunocytochemistry data indicate that TGFβ1 is involved during the onset of hepatocyte apoptosis and that the PP nafenopin can impinge on this cell death pathway. TGFβ1-LAP, probably produced mainly by the non-parenchymal liver cells, may be processed less efficiently to the mature, active form in the presence of nafenopin, although more data are required to confirm this hypothesis.