A factor in serum specifically inhibits the secretion of ferritin L. (A) Fetal bovine serum inhibits secretion of ferritin L. Fetal bovine serum was added to the media of AdX-FL–infected HepG2 cells in increasing concentrations (0%, 5%, 10%, 20%, and 30%) from the outset of the starvation period, during radiolabeling, and throughout the chase period. The bands were quantified as described in “Materials and methods.” The relative synthesis and secretion of ferritin L compared with the sample without serum are plotted in the graph. Increasing concentrations of serum are associated with a slight decrease in synthesis and marked drop in secretion of ferritin L. (B-C) Serum does not affect the secretion of albumin or apolipoprotein A-I. AdX-FL–infected HepG2 cells were incubated with media containing either 0% or 30% fetal bovine serum as in panel A. The cell lysate (labeled C) and media (M) were divided into 2 aliquots and immunoprecipitated with antibodies against albumin (B) or apolipoprotein A-1 (C). No difference in the secretion of albumin or apolipoprotein A-1 occurs with 30% serum. (D) Human serum inhibits ferritin L secretion. AdX-FL–infected HepG2 cells were incubated in media containing 0% or 40% human serum as in panel A. The cell lysates (labeled C) and media (M) were immunoprecipitated using polyclonal antibodies against ferritins. The secretion of ferritin L is markedly reduced with human serum.