Enhancement of Thyroxine Entry into Low Density Lipoprotein (LDL) Receptor-Competent Fibroblasts by LDL: An Additional Mode of Entry of Thyroxine into Cells

Having demonstrated that plasma low density lipoproteins (LDL) bind T4 through a specific interaction with their sole apolipoprotein, apoB-100, we tested the hypothesis that cells could internalize the LDL-T4 complex via cell surface LDL receptors. These receptors are down-regulated by cholesterol loading and up-regulated by cholesterol deficiency. We, therefore, studied the uptake of [125I]T4 or [125I]T3 by human skin fibroblasts grown in 10% lipoprotein-deficient serum in the absence or presence of LDL. At concentrations of LDL (12.5 and 22 micrograms protein/ml) that gave significant binding of T4 but did not exceed LDL receptor capacity, both the initial rate of saturable T4 uptake and the uptake at equilibrium increased by 27-63%. No significant increase occurred at a LDL concentration of 1.6 micrograms protein/ml (less than 3% occupancy), whereas there was a 20 to 31% reduction at 125 micrograms/ml (approximately 5 times the saturation dose for the LDL receptor). These changes were confirmed with several different LDL preparations and were mimicked by isolated apoB-100 and apoE, the sole ligands for the LDL receptors (apoB/E receptors). T4 uptake did not increase in normal fibroblasts with down-regulated LDL receptors or in LDL receptor-deficient fibroblasts from a patient with familial hypercholesterolemia. In the latter cell line the uptake of T4 (and T3) in the absence of LDL was indistinguishable from that of normal fibroblasts. T3 uptake in normal fibroblasts was not enhanced by LDL. The specificity of the LDL effect was shown by the finding that T4-binding globulin, prealbumin, or serum albumin, at concentrations giving 10-90% T4 bound, failed to increase T4 uptake. Instead, each of these major thyroid hormone-binding plasma proteins caused a dose-dependent decrease in T4 entry. It is concluded that at least two modes of entry into fibroblasts are available for T4. The first is the cell surface thyroid hormone-binding sites, which recognize the free hormone and are present in both normal and LDL receptor-negative fibroblasts. The second, and additional, mode of entry is via the LDL receptors, which recognize the T4-LDL complex, are absent in LDL receptor-negative fibroblasts, are reduced in down-regulated fibroblasts, and are unavailable for T3, owing to the low affinity of T3 for LDL.