van den Hove MF, Croizet-Berger K, Tyteca D, Selvais C, de Diesbach P, Courtoy PJ. Thyrotropin activates guanosine 5'-diphosphate/guanosine 5'-triphosphate exchange on the rate-limiting endocytic catalyst, Rab5a, in human thyrocytes in vivo and in vitro.
J Clin Endocrinol Metab 2007;
92:2803-10. [PMID:
17473071 DOI:
10.1210/jc.2006-2351]
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Abstract
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.
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