High-density lipoprotein-facilitated entry of thyroid hormones into cells: a mechanism different from the low-density lipoprotein-facilitated entry.
In human skin fibroblasts incubated with interstitial fluid concentrations of low-density lipoproteins (LDL) (22 microg/mL) that had been preincubated with [(125)I] thyroid hormone (TH), thyroxine (T(4)) whole-cell saturable up-take (CSU) is approximately 40% greater than in control fibroblasts. This effect of the LDL, which requires upregulation of low-density lipoprotein-receptors (LDL-R; K(d) approximately 20 microg/mL), is less for [(125)I]triiodothyronine (T(3)). We have evaluated high-density lipoproteins (HDL), and assessed whether lipoproteins target TH to the nucleus. In some experiments, fibroblasts were cholesterol-deprived (chol(-)) or cholesterol-enriched (chol(+)) to upregulate LDL-R or high-density lipoprotein-receptors (HDL-R), respectively. In chol(-) fibroblasts, 25 microg/mL of LDL increased both T(4) CSU and T(4) nuclear saturable uptake (NSU) by 48% or 30%, respectively; the latter becoming appreciable at approximately 180 minutes. Interstitial fluid concentrations of HDL (280 microg/mL or approximately 50-fold greater than K(d) of the HDL-R) inhibited both T(4) and T(3) CSU even in chol(+) fibroblasts. However, when chol(+) fibroblasts were incubated first with HDL, and after cell washings with [(125)I]T(4) or [(125)I]T(3), T(4) or T(3) CSU increased by 24% or 12%, respectively. The corresponding increase in chol(-) fibroblasts was 8% or 0%, and in nonmanipulated fibroblasts was 15% or 4%. Unlike LDL, the magnitude of the increase in T(4) or T(3) NSU caused by HDL matched the corresponding CSU, and was already evident at 30 minutes. In conclusion, cells have a LDL-facilitated, LDL-R-mediated mode of entry of TH (T(4) >> T(3)) that targets relatively late only part of TH to the nucleus, and an HDL-facilitated mode of entry (T(4) > T(3)) that targets immediately to the nucleus all of the TH, suggesting entry of TH in free form. This effect of HDL represents facilitated diffusion of TH through the cell membrane.
Benvenga
Salvatore
S
Sezione di Endocrinologia, Dipartimento Clinico Sperimentale di Medicina e Farmacologia, University of Messina School of Medicine, Policlinico Universitario, Messina, Italy. s.benvenga@me.nettuno.it
Alesci
Salvatore
S
Trimarchi
Francesco
F
D002467:Cell Nucleus / Q000378:metabolism; D002478:Cells, Cultured; D006801:Humans; D008075:Lipoproteins, HDL / Q000494:pharmacology*; D008077:Lipoproteins, LDL / Q000494:pharmacology*; D013963:Thyroid Hormones / Q000493:pharmacokinetics*; D013974:Thyroxine / Q000493:pharmacokinetics
D008075:Lipoproteins, HDL; D008077:Lipoproteins, LDL; D013963:Thyroid Hormones; D013974:Thyroxine