[Mechanisms of shedding of a soluble form of the TSH receptor].

The thyrotropin (TSH) receptor in human-thyroid glands is cleaved into an extracellular alpha subunit and a transmembrane beta subunit held together by disulfide bridges. An excess of the latter component relative to the former suggests shedding of the ectodomain. Indeed we observed such shedding in cultures of human thyrocytes and permanently transfected L or Chinese hamster ovary cells. Shedding was increased by inhibitors of endocytosis, recycling and lysosomal degradation suggesting that it was dependent on receptor residency at the cell surface. It was slightly increased by TSH and phorbol esters whereas forskolin and 8 bromo cAMP were without effect. The complete inhibition of soluble TSH receptor shedding by the specific inhibitor BB-2116 indicated that the cleavage reaction is catalyzed probably at the cell surface by a matrix metalloprotease-like enzyme. Shedding of the TSH receptor alpha domain is the consequence of two events: cleavage of the proreceptor into alpha and beta subunits and reduction of the disulfide bridge(s). The use of different specific inhibitors including monoclonal antibodies allowed us to implicate the enzyme protein disulfide isomerase in the reduction of TSHR disulfide bounds. The shed alpha subunit probably results in circulating TSH receptor ectodomain detected in human blood. This shedding mechanism might be implicated in the development of autoimmune diseases.