The endocytic catalysts, Rab5a and Rab7, are tandem regulators of thyroid hormone production.

Rab proteins are small GTPases that control distinct vesicular transport steps. Along the endocytic pathway, Rab5a is a rate-limiting catalyst of internalization, and Rab7 controls trafficking through late endosomes to lysosomes. The dependence of thyroid hormone production by thyrocytes on thyroglobulin endocytosis and intracellular processing in late endosomes/lysosomes suggests that its rate can be regulated by the expression or function of these endocytic catalysts. We compared the expression level and membrane recruitment of Rab5a and Rab7 in autonomous thyroid adenomas (where the cAMP cascade is constitutively activated) and surrounding quiescent tissues. The concentrations of Rab5a and Rab7, but not of Rab8, were coordinately increased up to 6-fold in adenomas, and correlated with a proportionate decrease in soluble thyroglobulin content (reflecting colloid depletion by accelerated endocytic uptake in hyperactive tissue). In adenomas, a higher proportion of Rab5a and Rab7 was membrane associated, and the equilibrium density of particulate Rab7 and iodine shifted toward lysosomal fractions, indicating that progression along the degradation pathway also was promoted. In cultures of polarized human thyrocytes from normal patients, thyroid-stimulating hormone or forskolin increased, to a similar extent, Rab5a and Rab7 but not Rab8 expression, apical endocytosis of thyroglobulin and lucifer yellow, and basolateral secretion of T(3) and T(4). Taken together, these in vivo and in vitro observations demonstrate that thyroid-stimulating hormone, via cAMP, coordinately enhances the expression of Rab5a and Rab7, which promote Tg endocytosis and transfer to lysosomes, respectively, resulting in accelerated thyroid hormone production.