title: Triiodothyronine (T3)-associated upregulation and downregulation of nuclear T3 binding in the human fibroblast cell (MRC-5)--stimulation of malic enzyme, glucose-6-phosphate-dehydrogenase, and 6-phosphogluconate-dehydrogenase by insulin, but not by T3.
abstract: The specific nuclear binding of triiodothyronine (T3) (NBT3) and the activity of malic enzyme (ME), glucose-6-phosphate-dehydrogenase (G6PD), and 6-phosphogluconate-dehydrogenase (6PGD) were studied in the human fibroblast cell (MRC-5). The overall apparent binding affinity (Ka) was 2.7 x 10(9) L.mol-1 estimated from kinetic studies of nuclear T3 binding, and 2.5 x 10(9) L.mol-1 estimated from equilibrium studies. The scatchard plots were curvilinear and composed of a high-affinity binding site with Ka1 3.4 +/- 0.7 x 10(9) L.mol-1 and maximal binding capacity (MBC) MBC1 57.0 +/- 11.9 fmol/mg DNA and a low-affinity binding site with Ka2 2.9 +/- 1.1 x 10(8) L.mol-1 and MBC2 124.7 +/- 22.1 fmol/mg DNA (n = 6). Incubation of cells with 6 nmol/L T3 for 20 hours reduced NBT3 to 62.2% +/- 15.7% (P less than .01, n = 11). The Ka estimated from kinetic studies was reduced to 6.7 x 10(7) L.mol-1, and the scatchard plots were linear, with Ka 4.5 +/- 1.6 x 10(8) L.mol-1 and MBC 137.0 +/- 44.6 fmol/mg DNA (n = 3) of the same magnitude as the low-affinity binding site in cells incubated without T3 (NS). The reduction in NBT3 was reversible and maximal at T3 concentrations saturating the high-affinity binding site and more than 58% of the total nuclear binding sites. The MRC-5 cell cytosol contained ME, G6PD, and 6PGD activities.(ABSTRACT TRUNCATED AT 250 WORDS)
authors.affiliation: Department of Clinical Chemistry, Odense University Hospital, Denmark.
authors.forename: L E
authors.initials: LE
authors.lastname: Matzen
authors.forename: S R
authors.initials: SR
authors.lastname: Kristensen
authors.forename: J
authors.initials: J
authors.lastname: Kvetny
mesh_terms: D002460:Cell Line; D002467:Cell Nucleus / Q000378:metabolism; D015536:Down-Regulation / Q000502:physiology; D005347:Fibroblasts / Q000378:metabolism*; D005954:Glucosephosphate Dehydrogenase / Q000378:metabolism*; D006801:Humans; D007328:Insulin / Q000494:pharmacology*; D008291:Malate Dehydrogenase / Q000378:metabolism*; D010734:Phosphogluconate Dehydrogenase / Q000378:metabolism*; D014284:Triiodothyronine / Q000378:metabolism / Q000502:physiology*; D015854:Up-Regulation / Q000502:physiology
chemical_list: D007328:Insulin; D014284:Triiodothyronine; D008291:Malate Dehydrogenase; D010734:Phosphogluconate Dehydrogenase; D005954:Glucosephosphate Dehydrogenase