TSH-Dependent expression of the LDL receptor-associated protein (RAP) in thyroid epithelial cells

Binding of thyroglobulin (Tg) to the low-density lipoprotein receptor-associated protein (RAP) during the biosynthetic pathway prevents premature Tg interactions with sortilin

OBJECTIVE

Sortilin, a Vps10p family member, is expressed by thyroid epithelial cells (TEC), where it binds to internalized thyroglobulin (Tg) molecules. Premature binding of Tg to sortilin during biosynthesis may cause intracellular retention of Tg. Such a premature interaction may be prevented by one or more inhibitor/s. Because both sortilin and Tg bind to the low-density lipoprotein receptor-associated protein (RAP), we investigated whether RAP serves such a function.

METHODS

Immunofluorescence staining for sortilin, Tg, and RAP was performed in FRTL-5 cells. Co-immunoprecipitation experiments were performed in extracts from FRTL-5 or COS-7 cells, the former co-transfected with Tg and/or RAP and/or sortilin, or in thyroid extracts from RAP KO mice.

RESULTS

Tg and sortilin did not co-localize in FRTL-5 cells following inhibition of protein synthesis, suggesting that newly synthesized, endogenous sortilin and Tg do not interact, in confirmation of which an anti-sortilin antibody did not co-precipitate Tg in FRTL-5 cells. In contrast, Tg co-localized with RAP in FRTL-5 cells. Co-immunoprecipitation of Tg with an anti-sortilin antibody in COS-7 cells transfected with sortilin and Tg was abolished when cells were co-transfected with RAP, indicating that RAP prevents binding of Tg to sortilin during biosynthesis, in confirmation of which an anti-sortilin antibody co-precipitated Tg in thyroid extracts from RAP KO mice to a greater extent than in thyroid extracts from WT mice.

CONCLUSIONS

Tg does not bind prematurely to sortilin because of its interaction with RAP during protein biosynthesis. These findings add new information to the knowledge of thyroid physiology.

Intracellular retention of thyroglobulin in the absence of the low-density lipoprotein receptor-associated protein (RAP) is likely due to premature binding to megalin in the biosynthetic pathway

OBJECTIVE

The low-density lipoprotein receptor associated protein (RAP) is expressed by thyroid epithelial cells (TEC) in a TSH-dependent manner. In the thyroid RAP functions as a molecular chaperone for the thyroglobulin (Tg) endocytic receptor megalin/LRP2, which is retained intracellularly in RAP KO mice rather than being expressed on the apical membrane of TEC, its usual location. RAP binds also to Tg, which is also retained intracellularly in RAP KO mice, thereby suggesting a role of RAP in Tg secretion. Here we investigated whether Tg intracellular retention in the absence of RAP is due to premature Tg-megalin interactions during the biosynthetic pathway or to a direct action of RAP on Tg secretion.

METHODS

We performed immunoprecipitation experiments in thyroid extracts from RAP KO and WT mice. In addition, we investigated Tg secretion in COS-7 cells co-transfected with human RAP (hRAP) and mouse Tg (mTg).

RESULTS

An anti-megalin megalin precipitated greater amounts of Tg in thyroid extracts from RAP KO than from WT mice, suggesting increased intracellular interactions between megalin and Tg in the absence of RAP. COS-7 cells transiently transfected with hRAP, mTg or both, expressed the two proteins accordingly. RAP was found almost exclusively in cell extracts, whereas Tg was found both in extracts and media, as expected from the knowledge that RAP is ER-resident and that Tg is secreted. Regardless of whether cells were transfected with mTg alone or were co-transfected with hRAP, similar proportions of the total Tg synthesized were detected in cell extracts and media.

CONCLUSIONS

The intracellular retention of Tg in the absence of RAP is likely due to its premature interaction with megalin, whereas RAP does not seem to affect Tg secretion directly.

Sortilin is a putative postendocytic receptor of thyroglobulin

The Vps10p family member sortilin is involved in various cell processes, including protein trafficking. Here we found that sortilin is expressed in thyroid epithelial cells (thyrocytes) in a TSH-dependent manner, that the hormone precursor thyroglobulin (Tg) is a high-affinity sortilin ligand, and that binding to sortilin occurs after Tg endocytosis, resulting in Tg recycling. Sortilin was found to be expressed intracellularly in thyrocytes, as observed in mouse, human, and rat thyroid as well as in FRTL-5 cells. Sortilin expression was demonstrated to be TSH dependent, both in FRTL-5 cells and in mice treated with methimazole and perchlorate. Plasmon resonance binding assays showed that Tg binds to sortilin in a concentration-dependent manner and with high affinity, with Kd values that paralleled the hormone content of Tg. In addition, we found that Tg and sortilin interact in vivo and in cultured cells, as observed by immunoprecipitation, in mouse thyroid extracts and in COS-7 cells transiently cotransfected with sortilin and Tg. After incubation of FRTL-5 cells with exogenous, labeled Tg, sortilin and Tg interacted intracellularly, presumably within the endocytic pathway, as observed by immunofluorescence and immunoelectron microscopy, the latter technique showing some degree of Tg recycling. This was confirmed in FRTL-5 cells in which Tg recycling was reduced by silencing of the sortilin gene and in CHO cells transfected with sortilin in which recycling was increased. Our findings provide a novel pathway of Tg trafficking and a novel function of sortilin in the thyroid gland, the functional impact of which remains to be established.

Sequencing of the entire coding region of the receptor associated protein (RAP) in patients with primary hypothyroidism of unknown origin

The LDL receptor-associated protein (RAP) is involved in secretion of thyroglobulin (Tg) from the thyrocyte to the colloid. Disruption of the RAP gene in mice results in a reduced Tg content within the colloid, leading to subclinical hypothyroidism and histological alterations resembling early goiter. Here we studied the entire coding sequence of RAP in genomic DNA samples from 18 patients with primary hypothyroidism not due to thyroid autoimmunity or dysgenesis. The control group included 21 subjects with no evidence of thyroid alterations. Eleven different polymorphisms with amino-acid substitution and 4 different missense polymorphisms without amino-acid substitution were found in various regions of the RAP gene. Only one polymorphism in exone 7 (V311M) was observed exclusively in patients, but it had been previously reported in normal subjects as well. The remaining polymorphisms were found either both in patients and controls or only in controls and had not been previously reported. The frequency of the various polymorphisms did not differ significantly between patients and controls. Based on these findings, we conclude that alterations of the RAP gene are not a common cause of hypothyroidism, although it cannot be excluded that other, rarer alterations with a pathogenic effect exist, and that they should be investigated in a larger number of patients.

Hepatic Niemann-Pick C1-like 1 regulates biliary cholesterol concentration and is a target of ezetimibe

Niemann-Pick C1-like 1 (NPC1L1) is required for cholesterol absorption. Intestinal NPC1L1 appears to be a target of ezetimibe, a cholesterol absorption inhibitor that effectively lowers plasma LDL-cholesterol in humans. However, human liver also expresses NPC1L1. Hepatic function of NPC1L1 was previously unknown, but we recently discovered that NPC1L1 localizes to the canalicular membrane of primate hepatocytes and that NPC1L1 facilitates cholesterol uptake in hepatoma cells. Based upon these findings, we hypothesized that hepatic NPC1L1 allows the retention of biliary cholesterol by hepatocytes and that ezetimibe disrupts hepatic function of NPC1L1. To test this hypothesis, transgenic mice expressing human NPC1L1 in hepatocytes (L1-Tg mice) were created. Hepatic overexpression of NPC1L1 resulted in a 10- to 20-fold decrease in biliary cholesterol concentration, but not phospholipid and bile acid concentrations. This decrease was associated with a 30%-60% increase in plasma cholesterol, mainly because of the accumulation of apoE-rich HDL. Biliary and plasma cholesterol concentrations in these animals were virtually returned to normal with ezetimibe treatment. These findings suggest that in humans, ezetimibe may reduce plasma cholesterol by inhibiting NPC1L1 function in both intestine and liver, and hepatic NPC1L1 may have evolved to protect the body from excessive biliary loss of cholesterol.