[The effect of thyroid hormones on low density lipoprotein receptors in cultured human skin fibroblasts (author's transl)].

The catabolic rate of low density lipoprotein (LDL) has been shown to be delayed in hypothyroidism and increased in hyperthyroidism. LDL catabolism is mainly mediated through the LDL receptor pathway in the parenchymal cells. In the present paper the effect of thyroid hormones in LDL receptors was studied in cultured human skin fibroblasts. Human fibroblasts were established from 2 normal subjects, one heterozygous patient and 2 homozygous patients with familial hypercholesterolemia (FH). Human LDL (density 1.020 approximately 1.050 g/ml) from the serum of healthy subjects was prepared by differential ultracentrifugation. 125I-LDL was prepared by the ICl method of MacFarlane. Prior to incubation with 125I-LDL, cells were preincubated in a medium containing 5% lipoprotein-deficient serum obtained from a myxoedematous patient. Uptake and degradation of LDL by normal cells and FH heterozygous cells increased about 20% with 1.0 microgram/ml of L-triiodo-thyronine (T3) or L-thyroxine (T4). In FH homozygous cells, no increase of LDL receptor activity was observed with T3 or T4. These results suggest that thyroid hormones regulate LDL catabolism through the LDL pathway.