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.
Botta
Roberta
R
Department of Endocrinology, University of Pisa, Via Paradisa 2, 56124, Pisa, Italy.
Lisi
Simonetta
S
Pinchera
Aldo
A
Giorgi
Franco
F
Marcocci
Claudio
C
Taddei
Anna Rita
AR
Fausto
Anna Maria
AM
Bernardini
Nunzia
N
Ippolito
Chiara
C
Mattii
Letizia
L
Persani
Luca
L
de Filippis
Tiziana
T
Calebiro
Davide
D
Madsen
Peder
P
Petersen
Claus Munck
CM
Marinò
Michele
M
D033942:Adaptor Proteins, Vesicular Transport / Q000235:genetics / Q000502:physiology*; D000818:Animals; D019556:COS Cells; D002522:Chlorocebus aethiops; D004705:Endocytosis; D005260:Female; D000882:Haplorhini; D008713:Methimazole / Q000494:pharmacology; D051379:Mice; D008810:Mice, Inbred C57BL; D010472:Perchlorates / Q000494:pharmacology; D051381:Rats; D013954:Thyroglobulin / Q000502:physiology*; D013961:Thyroid Gland / Q000502:physiology*; D013974:Thyroxine / Q000097:blood; D000099183:Sortilin
D033942:Adaptor Proteins, Vesicular Transport; D008713:Methimazole; D010472:Perchlorates; D013954:Thyroglobulin; D013974:Thyroxine; D000099183:Sortilin; C494474:perchlorate