Endocrine Abstracts

Rap (LDL receptor-associated protein) expression in thyroid epithelial cells: evidence for tsh-dependence in vivo and in vitro

R Botta¹, S Lisi¹, A Pinchera¹, C Segnani², L Cianferotti¹, MA Altea¹, F Menconi¹, L Mattii², GU Corsini³, C Marcocci¹, A Dolfi², N Bernardini² & M Marinò¹

¹Department of Endocrinology and Metabolism, Pisa, Italy; ²Department of Human Morphology and Applied Biology, Pisa, Italy; ³Department of Neuroscience, Pisa, Italy.

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RAP (LDL receptor associated protein) is a 44 kDa endoplasmic reticulum (ER) resident molecular chaperone. In the thyroid RAP is required for expression of megalin, an endocytic receptor responsible for transcytosis of thyroglobulin (Tg), but it also binds to Tg itself, suggesting it may affect thyroid function in various manners. Indeed, findings in RAP KO mice indicate that disruption of the RAP gene results in impaired Tg storage into the colloid, suggesting that RAP serves as a Tg chaperone. Because expression of proteins involved in thyroid homeostasis is TSH-dependent, here we investigated whether also RAP is regulated by TSH. By immunofluorescence RAP was found to be expressed intracellularly in FRTL-5 cells cultured in the presence of TSH, and to co-localize with a known ER-resident protein, namely GRP78 (BiP), showing its correct ER location. RAP expression in FRTL-5 cells was then analyzed following TSH deprivation for 24–72 hours, or culture with various concentrations of TSH. At 24 hours, TSH did not affect RAP levels, as observed by immunofluorescence and Western blotting. However, at 48 and 72 hours TSH up-regulated RAP in a concentration-dependent manner. We then studied RAP expression in vivo, using the model of methimazole and perchlorate treated mice. As expected, treatment resulted in a reduction of serum FT4 and an increase of serum TSH. This was associated with a remarkable increase of RAP expression in thyrocytes, as observed by immunohistochemistry and Western blotting. Therefore, based on findings in FRTL-5 cells and in vivo, we concluded that RAP is expressed by thyrocytes in a TSH-dependent manner, which supports a thyroid specific function of this molecular chaperone.

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Endocrine Abstracts (2006) 11 P812