Thyrotropin activates guanosine 5'-diphosphate/guanosine 5'-triphosphate exchange on the rate-limiting endocytic catalyst, Rab5a, in human thyrocytes in vivo and in vitro.
BACKGROUND
We have previously reported that the TSH receptor/cAMP cascade enhances the coordinate expression of the rate-limiting endocytic catalysts, Rab5a and Rab7, which respectively promote thyroglobulin (Tg) internalization and transfer to lysosomes, thereby accelerating thyroid hormone secretion.


OBJECTIVE
We address whether TSH further controls Rab5a activity by promoting its GTP-bound state.


METHODS
We compared Rab5a activation in seven pairs of hyperactive and corresponding quiescent thyroid tissues; TSH effect was reproduced on polarized cultures of normal human thyrocytes.


METHODS
We studied seven euthyroid patients bearing hyperactive autonomous adenomas; normal thyroid tissue for culture.


METHODS
Rab5a GDP/GTP exchange factor activity [Rab5a-guanine nucleotide exchange factor (GEF)], expression of Rabex-5 (a Rab5a-GEF), and function of thyrocytes in vitro were the main outcome measures.


RESULTS
In autonomous adenomas, constitutive activation increased both total activity and sedimentability (membrane recruitment) of Rab5a-GEF, compared with perinodular tissues. Increased Rab5a-GEF activity correlated with increased expression of Rabex-5 and Rab5a, as well as with Tg store depletion. In polarized human thyrocyte monolayers, TSH did not affect total Rab5a-GEF activity after 2 h but promoted its membrane recruitment; after 4 d, TSH increased both Rab5a-GEF activity and Rabex-5 expression and recruitment onto membranes where Rabex-5 coimmunoprecipitated with Rabaptin-5 and Rab5a. Sedimentable Rab5a-GEF perfectly correlated with apical endocytosis and lysosomal transfer of 125I-Tg, and with basolateral secretion of 125I-derived hormones.


CONCLUSIONS
This study provides the first clinical and experimental evidence that regulation of the activity of a rate-limiting endocytic catalyst finely tunes a tightly controlled cellular function that ultimately governs whole body metabolism.
van den Hove
Marie-France
MF
Université Catholique de Louvain, Christian de Duve Institute of Cellular Pathology, Cell Biology Unit, B-1200 Brussels, Belgium. vandenhove@cell.ucl.ac.be
Croizet-Berger
Karine
K
Tyteca
Donatienne
D
Selvais
Charlotte
C
de Diesbach
Philippe
P
Courtoy
Pierre J
PJ
D016764:Cell Polarity / Q000502:physiology; D002478:Cells, Cultured; D004705:Endocytosis / Q000187:drug effects / Q000502:physiology*; D020662:Guanine Nucleotide Exchange Factors / Q000378:metabolism; D006153:Guanosine Diphosphate / Q000378:metabolism; D006160:Guanosine Triphosphate / Q000378:metabolism*; D006801:Humans; D066298:In Vitro Techniques; D007457:Iodine Radioisotopes; D021381:Protein Transport / Q000187:drug effects / Q000502:physiology; D013347:Subcellular Fractions / Q000378:metabolism; D013961:Thyroid Gland / Q000166:cytology / Q000378:metabolism*; D013972:Thyrotropin / Q000493:pharmacokinetics*; D020696:rab5 GTP-Binding Proteins / Q000378:metabolism*
D020662:Guanine Nucleotide Exchange Factors; D007457:Iodine Radioisotopes; C108382:RABGEF1 protein, human; D006153:Guanosine Diphosphate; D006160:Guanosine Triphosphate; D013972:Thyrotropin; D020696:rab5 GTP-Binding Proteins