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Thyrotropin Activates Guanosine 5'-Diphosphate/Guanosine 5'-Triphosphate Exchange on the Rate-Limiting Endocytic Catalyst, Rab5a, in Human Thyrocytes in Vivo and in Vitro

Université Catholique de Louvain, Christian de Duve Institute of Cellular Pathology, Cell Biology Unit, B-1200 Brussels, Belgium

Address all correspondence and requests for reprints to: Marie-France van den Hove, Université Catholique de Louvain, Christian de Duve Institute of Cellular Pathology, Cell Biology Unit, 75 avenue Hippocrate, B-1200 Brussels, Belgium. E-mail: vandenhove{at}cell.ucl.ac.be.

Context: 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.

Design: 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.

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

Main Outcome Measurements: 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.

Conclusion: 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.