THYROID-HORMONE EFFLUX FROM MONOLAYER-CULTURES OF HUMAN FIBROBLASTS AND HEPATOCYTES - EFFECT OF LIPOPROTEINS AND OTHER THYROXINE TRANSPORT PROTEINS

We have previously shown that human skin fibroblasts exposed to prefor med low density lipoprotein (LDL)-thyroxine (T-4) complexes internaliz e more T-4 than they do when exposed to T-4 alone. The system is set t o function when the LDL receptor is up-regulated by reducing the intra cellular concentration of cholesterol, and the LDL concentration outsi de the cell is in the range of the kDa of the receptor. High density l ipoproteins (HDL), albumin (HSA), transthyretin (TTR), and thyroxine-b inding globulin (TBG) interfere with, rather than facilitate, T-4 entr y. Of the three classes of lipoproteins (VLDL, LDL, and HDL), HDL is t he major carrier of thyroid hormones. While LDL delivers cholesterol ( and T-4) to cells, HDL is the scavenger of cholesterol. We thus hypoth esized that HDL could also facilitate thyroid hormone exit from cells. This hypothesis was tested on two human cell lines: skin fibroblasts and hepatocytes (Hep G2), using physiological concentrations of HDL or , as control, physiological concentrations of LDL, HSA, TTR, and TBG o r buffer. Because cell surface receptors for HDL are regulated by intr acellular cholesterol in a manner opposite to that of LDL receptors, w e evaluated the effect of HDL land other proteins) in three states: no rmal, high, and low intracellular cholesterol content (i.e. normal, hi gh, and low expression of HDL receptors). In both cell lines and with either T-4 or T-3, we found that: 1) HDL as well as the other proteins tested increased the efflux and augmented both the initial rate of ex it and the equilibrium value. 2) The efflux did not saturate over a wi de range of protein concentrations. 3) The effect of HDL, LDL, and the other proteins on the fractional efflux rate of thyroid hormones rema ined the same irrespective of the intracellular cholesterol content (a nd, therefore, irrespective of the expression of either LDL or HDL rec eptors). 4) HSA, TTR, and TBG were, on a mass basis, equipotent and mo re efficient than lipoproteins. However, the effect of Lipoproteins - whose Ka for T-4 is comparable to that of HSA - was disproportionately high. On a molar basis, LDL (about 80% of the weight being accounted for by lipids) was more effective than HDL2 (about 60% lipids) and HDL , was more effective than HDL3 (about 40% lipids), suggesting that the disproportionate effect of lipoproteins was due to transfer of the ly pophylic thyroid hormones to the lipid moiety of Lipoproteins. 5. A mi xture of HDL and LDL gave the same efflux rate as a mixture of HSA, TT R, and TBG. The data indicate that the efflux of T-4 and T-3 from cell s is rapid and appears not to be mediated by a particular lipoprotein. The disproportionately large effect of lipoproteins, which are low af finity thyroid hormone carriers, compared with nonlipoprotein carriers , and the greater effect of LDL compared with HDL, might indicate that the lipoproteins establish a nonspecific physical contact with the pl asma membrane and that their hydrophobic nature favors the release of the similarly hydrophobic thyroid hormones.