Cathepsin K in thyroid epithelial cells: sequence, localization and possible function in extracellular proteolysis of thyroglobulin

Extracellular proteolysis of thyroglobulin at the apical surface of thyroid epithelial cells results in liberation of thyroxine, and is mediated by lysosomal cysteine proteases such as cathepsins B and L. Here, we report on the expression of the cysteine protease Cathepsin K in thyroid epithelial cells. The cDNA for porcine thyroid Cathepsin K showed homologies ranging from 71% to 94% to the cDNA of Cathepsin K from various species and cell types. The deduced amino acid sequence of porcine thyroid Cathepsin K predicted a 37 kDa preproenzyme, with the active site residues Cys-140, His-277 and Asn-297, and one potential N-glycosylation site. The localization of Cathepsin K was not restricted to lysosomes. Rather, secreted Cathepsin K was predominantly found within the follicular lumen and in association with the apical plasma membrane of thyroid epithelial cells. Enzyme cytochemistry showed that cell-surface associated Cathepsin K was proteolytically active at neutral pH. In vitro, recombinant Cathepsin K liberated thyroxine from thyroglobulin by limited proteolysis at neutral pH. We postulate that its localization enables Cathepsin K to contribute to the extracellular proteolysis of thyroglobulin, i.e. thyroid hormone liberation, at the apical surface of thyroid epithelial cells in situ.