Induction of apoptosis in cultured hepatocytes and in the regressing liver by transforming growth factor-beta 1 occurs without activation of an endonuclease.

In previous studies in vivo apoptotic liver cells were found to be positive for transforming growth factor-beta 1 (TGF-beta 1). In hepatocyte cultures TGF-beta 1 induced rounding up and fragmentation of the cells into multiple vesicles. As revealed by the DNA specific stain H33258 the chromatin of these cells condensed and segregated into masses at the nuclear membrane, followed by nuclear fragmentation. Ultrastructurally the cytoplasm was well preserved as demonstrated by the presence of intact cell organelles. These features strongly suggest that occurrence of apoptosis. Furthermore we administered TGF-beta 1 in vivo using an experimental model in which regression of the liver was initiated by a short preceding treatment with the hepatomitogen cyproterone acetate (CPA). Two doses of 1 nM TGF-beta 1/kg each augmented the incidence of apoptotic hepatocytes 5-fold. These studies strongly suggest that TGF-beta 1 is involved in the initiation of apoptosis in the liver In TGF-beta 1 treated hepatocytes both from the liver and monolayer culture no DNA fragmentation into oligosomes could be detected. Comparison of nuclei in which endonuclease was activated by Ca2+ with apoptotic nuclei revealed no obvious similarities, as demonstrated by FACS analysis, H33258 staining and electron microscopy. Thus, apoptosis induced by a growth inhibitor obviously occurs without activation of an endonuclease.