EVIDENCE FOR EXTRACELLULARLY ACTING CATHEPSINS MEDIATING THYROID-HORMONE LIBERATION IN THYROID EPITHELIAL-CELLS

Thyroglobulin (Tg) is the major secretory product of thyroid epithelia l cells and is stored in the lumen of thyroid follicles at high concen trations. Thyroid hormone liberation is assumed to occur separately fr om this storage compartment within lysosomes. However, for the transfe r of Tg to lysosomes, mechanisms to solubilize the luminal content mus t precede its endocytosis, because part of the luminal Tg occurs in a covalently cross-linked form. Here, by immunoprecipitation and immunob lotting we show that the majority of procathepsin B or L and a fractio n of mature cathepsin B are released from porcine thyrocytes in. vitro . Released cathepsins were detectable on the cell surface of the thyro cytes by immunocytochemistry and amounted to 27% of the total cathepsi n B. Cytochemical studies revealed the proteolytic activity of catheps in B at neutral pH on the cell surface of thyrocytes. Therefore, the p ossibility of extracellular proteolysis by cathepsins was investigated by incubating plasma membrane preparations, conditioned media, or lys osomes with Tg. The liberation of thyroid hormones was quantitated by RIA, and the degradation of Tg was determined by SDS-PAGE. Extracellul ar and plasma membrane-associated proteases rapidly mediated up to 54% of the total T-4 liberation by limited proteolysis of Tg at neutral p H under conditions where cysteine proteases were reactivated. We propo se that released and proteolytically active cysteine proteases, i.e. c athepsins B and L, provide thyrocytes with a pathway of limited extrac ellular proteolysis of Tg before endocytosis.