Induction of apoptosis in human hepatoma cells by alpha-fetoprotein.

Abstract

We have investigated the effects of purified human alpha-fetoprotein (AFP) on the growth of the human hepatocarcinoma-cells HepG2 in culture. Cancer-derived AFP (cAFP), isolated from the culture medium of AFP-secreting HepG2 cells and embryonal AFP (eAFP), isolated from human cord serum, were used for these studies. Both AFP pre parations studied were shown to induce strong dose-dependent inhibition of HepG2 cell proliferation and complete growth arrest at high protein concentrations (more than 0.1 mg/ml). To test whether AFP may trigger an endogenous suicide program in hepatoma cells, we examined whether DNA fragmentation preceded cell death. After exposure of the cells of the high AFP dose (1.0 mg/ml), DNA fragmentation was detected as early as 2 h after treatment, and 70% of cells were apoptotic by 24 h. DNA fragmentation was shown to precede other signs of cell death for several hours. Typical morphological changes of apoptosis were observed after 4 h of exposure of cells to high AFP doses. Low concentrations of cAFP and eAFP (less than 0.1 mg/ml) failed to induce growth inhibition of HepG2 cells, rather showing a weak stimulative effect, demonstrating a biphasic AFP activity. Cell pretreatment with the transcriptional inhibitor actinomycin D had no measurable influence on AFP cytotoxicity. These findings demonstrate that protein synthesis is not required for this mechanism of cell death. The charcoal-treated ligand-free eAFP (eAFPp) had a dose-dependent growth-inhibitory activity, similar to intact protein, but slightly less intensive. The similar growth-inhibitory activities of cAFP, eAFP and eAFPp, which have a significant difference in bound-ligand content, provide evidence that the main role in cell growth regulation may be attributed to the protein moiety of the entire AFP molecule, but not to its ligands. These biologically active AFP ligands could, however, modulate AFP-growth-regulating activity. Growth factor deprivation distinctly enhanced the cytostatic activity of high AFP concentrations and also increased the mitogenic activity of low AFP levels, showing the interdependence of the growth-regulative activity of AFP and growth factors. The findings of this study demonstrated that AFP is directly introduced into the intracellular pathways of cell growth regulation and programmed cell death.