The acute phase protein alpha-1-antitrypsin inhibits transferrin uptake in PLC/PRF/5 cells and increases release of hepatitis B virus surface antigen and alpha-fetoprotein.

Abstract

The present study was designed to investigate whether the acute phase protein alpha-1-antitrypsin (alpha1-AT), which has an inhibitory effect on transferrin (tf) receptor-mediated iron uptake in K562 and THP1 cells, has a similar effect in PLC/PRF/5 cells. This hepatic cell line is of specific interest because it is infected with hepatitis B virus (HBV). Therefore, we addressed the additional question whether alpha1-AT has any effect on cellular protein synthesis and replication of HBV in PLC/PRF/5 cells. Cells were incubated with various concentrations of alpha1-AT, dexamethasone, IL-6 and desferrioxamine. HBs-AG, alpha-fetoprotein and albumin concentrations in culture media were measured using commercially available methods. For equilibrium inhibition binding experiments, cells were incubated with 85-182 pmol/l [125I]tf. To study the potential effect of alpha1-AT on DNA synthesis we measured the incorporation of [3H]thymidine into DNA. In equilibrium saturation binding experiments, [125I]tf bound to PLC/PRF/5 cells with K(D) 17.45 +/- 4.57 nM and a maximum density of binding sites of 267,285 +/- 39,915 sites/cell. In inhibition studies alpha1-AT demonstrated an apparently monophasic inhibition of [125I]tf to its receptor. At concentrations > 30 micromol/l alpha1-AT inhibited the growth of PLC/PRF/5 cells up to approximately 50%. The inhibitory effect of alpha1-AT on DNA synthesis was not as potent as that on growth. At the highest concentration of 100 micromol/l, alpha1-AT produced a 35% maximum inhibition of [3H]thymidine incorporation. Incubating PLC/PRF/5 cells with corticosteroids enhanced HBs-AG release significantly. Interestingly, alpha1-AT showed the same pattern of effects on cell metabolism and HBs-AG release as the corticosteroids. When we incubated the cells with 50 micromol/l alpha1-AT, alpha-fetoprotein production increased significantly and HBs-AG release almost doubled. We have to assume that there is a specific mechanism inducing HBs-AG release by alpha1-AT, as has been shown to be the case with steroids.