REDUCED EXPRESSION OF MATURE TGF-BETA-1 CORRELATES WITH THE SUPPRESSION OF RAT HEPATOCYTE APOPTOSIS BY THE PEROXISOME PROLIFERATOR, NAFENOPIN

Non-genotoxic carcinogens cause cancer without damaging the DNA. Perox isome proliferators (PPs) are a class of potent rodent non-genotoxic h epatocarcinogens that may act by perturbing hepatocyte growth regulati on. Previously, we have shown that although cultured rat hepatocytes d egenerate rapidly in culture, their survival can be reversibly maintai ned by the PP nafenopin. This prolonged survival is associated with a decrease in the number of hepatocytes displaying the chromatin condens ation characteristic of apoptosis. The addition of the negative growth regulator TGF beta-1 induced high levels of hepatocye apoptosis but n afenopin was able to suppress this TGF beta-1 induced apoptosis. These data suggested that increased levels of mature TGF beta-1 may be invo lved in the signalling of the apoptosis seen in degenerating hepatocyt e cultures. To test this hypothesis, we carried out Western blot analy ses using a anti-TGF beta 1 antibody. There was an increase (p = 0.014 ) in expression of mature TGF beta 1 in degenerating rat hepatocyte cu ltures compared with hepatocyte cultures surviving in the presence of nafenopin. However, there was a concommitant decrease (p = 0.024) in T GF beta 1-latency activated protein (TGF beta 1-LAP), the precursor of the active, mature form. Immunocytochemistry confirmed that TGF beta 1/TGF beta 1-LAP expression was predominantly in the hepatocytes displ aying apoptotic morphology although expression was detected also in no n-parenchymal liver cells. The immunocytochemistry data indicate that TGF beta 1 is involved during the onset of hepatocyte apoptosis and th at the PP nafenopin can impinge on this cell death pathway. TGF beta 1 -LAP, probably produced mainly by the non-parenchymal liver cells, may be processed less efficiently to the mature, active form in the prese nce of nafenopin, although more data are required to confirm this hypo thesis.