ABSTRACT
Transferrin, as the major iron-transport protein in serum and other body fluids, has a central role in managing iron the body receives. Liver is a major site of transferrin synthesis, and in this study we present evidence that liver synthesis of human transferrin is suppressed by both the toxic metal lead and bacterial lipopolysaccharide, an inducer of the hepatic acute phase response. The responses of intact endogenous transferrin in the human hepatoma cell line HepG2 and chimeric human transferrin-chloramphenicol acetyltransferase genes in transgenic mice were examined. In HepG2 cells, 35S-transferrin protein synthesis and mRNA levels were suppressed by 100 microM and 10 microM lead acetate as early as 24 h after the initial treatment. Yet, synthesis of two proteins known to respond in the hepatic acute phase reaction, complement C3 and albumin, was not altered by the lead treatment. In transgenic mouse liver, lead suppressed expression of chimeric human transferrin genes at both the protein and mRNA levels, but LPS only suppressed at the protein level. The study indicates that lead suppresses human transferrin synthesis by a mechanism that differs from the hepatic acute phase response and that lead may also affect iron metabolism in humans by interfering with transferrin levels.
Subject(s)
Lead/toxicity , Lipopolysaccharides/toxicity , Liver/drug effects , Organometallic Compounds/toxicity , Transferrin/biosynthesis , Acute-Phase Reaction/metabolism , Albumins/biosynthesis , Animals , Carcinoma, Hepatocellular , Chloramphenicol O-Acetyltransferase/genetics , Complement C3/biosynthesis , Humans , Liver/enzymology , Liver/metabolism , Liver Neoplasms , Mice , Mice, Transgenic , RNA, Messenger/biosynthesis , RNA, Messenger/drug effects , Recombinant Fusion Proteins/biosynthesis , Recombinant Fusion Proteins/drug effects , Recombinant Fusion Proteins/genetics , Transferrin/drug effects , Transferrin/genetics , Tumor Cells, CulturedABSTRACT
The iron-binding protein transferrin has major roles in transporting, delivering, and sequestering ferric ions acquired by body tissues. Yet, during aging, serum transferrin levels decrease in humans. Likewise, in transgenic mice carrying chimeric human transferrin transgenes, liver expression of transferrin transgenes decreases with age. The aging regulation is due to decreased gene transcription. Electrophoretic mobility shift assays and antibody-recognition have revealed the binding of 5' regulatory elements of the human transferrin gene by three YY1 proteins, called YY1, YY1-a, and YY1-b, and an Sp1-a transcription factor. An age-related increase in YY1-a and YY1-b binding activities and a decrease in Sp1-like binding activity were shown. Since Sp1 is a positive transcription factor and YY1 can be a negative transcription factor, the alterations in their binding with age could cause the decreased transcription of the human transferrin transgene, and also the age-related decreased serum transferrin levels in humans.
Subject(s)
Aging/physiology , DNA-Binding Proteins/metabolism , Sp1 Transcription Factor/metabolism , Transcription Factors/metabolism , Transferrin/genetics , Animals , Base Sequence , Erythroid-Specific DNA-Binding Factors , Female , Humans , Male , Mice , Mice, Transgenic , Molecular Sequence Data , Transgenes , YY1 Transcription FactorABSTRACT
The major iron-transport protein in serum is transferrin (TF) which also has the capacity to transport other metals. This report presents evidence that synthesis of human TF can be regulated by the metal lead. Transgenic mice carrying chimeric human TF-chloramphenicol acetyl transferase (CAT) genes received lead or sodium salts by intraperitoneal injections or in drinking water. Transgene expression in liver was suppressed 31 to 50% by the lead treatment. Lead regulates human TF transgenes at the mRNA level since liver CAT enzyme activity, CAT protein, and TF-CAT mRNA levels were all suppressed. The dosages of lead did not alter synthesis of the other liver proteins, mouse TF and albumin, as measured by Northern blot analysis of total liver RNA and rocket immunoelectrophoresis of mouse sera. Moderate levels of lead exposure were sufficient to evoke the human TF transgene response; blood lead levels in mice that received lead acetate in drinking water ranged from 30 micrograms/dl to 56 micrograms/dl. In addition to suppressing expression of TF-CAT genes in transgenic mice, lead also suppressed synthesis of TF protein in cultured human hepatoma HepG2 cells. The regulation of human TF apparently differs from the regulation of mouse TF which is unresponsive to lead exposure.
