Cell surface protein disulfide-isomerase is involved in the shedding of human thyrotropin receptor ectodomain

J Couet, S de Bernard, H Loosfelt, B Saunier… - Biochemistry, 1996 - ACS Publications
J Couet, S de Bernard, H Loosfelt, B Saunier, E Milgrom, M Misrahi
Biochemistry, 1996ACS Publications
In human thyroid glands the TSH receptor undergoes a cleavage reaction which yields to an
extracellular α subunit and a membrane spanning β subunit linked together by disulfide
bridges. A similar reaction is observed in transfected L cells although some uncleaved
monomers persist in these cells. We have recently shown that the α subunit of the TSH
receptor undergoes partial shedding in human thyroid cells and heterologous cells
permanently transfected with an expression vector encoding the receptor. This shedding is a …
In human thyroid glands the TSH receptor undergoes a cleavage reaction which yields to an extracellular α subunit and a membrane spanning β subunit linked together by disulfide bridges. A similar reaction is observed in transfected L cells although some uncleaved monomers persist in these cells. We have recently shown that the α subunit of the TSH receptor undergoes partial shedding in human thyroid cells and heterologous cells permanently transfected with an expression vector encoding the receptor. This shedding is a two-step process. The first step consists in the cleavage of the proreceptor at the cell surface probably by a matrix metalloprotease and the second step in the reduction of the disulfide bridge(s) (Couet, J., Sar, S., Jolivet, A., Vu Hai, M. T., Milgrom, E., & Misrahi, M. 1996, J. Biol. Chem. 271, 4545−4552). We have used the transfected L cells to study the second step involved in sTSHR shedding. The membrane impermeant sulfhydryl reagent DTNB (5,5‘-dithiobis(2-nitrobenzoic acid)) allowed us to confirm that the reduction of the TSH receptor disulfide bonds occurred at the cell surface. The antibiotic bacitracin even at low concentrations also elicited a marked inhibition of TSH receptor shedding. This led us to implicate the enzyme protein disulfide isomerase (PDI, EC 5.3.4.1) in this process. We thus tested the inhibitory activity of specific monoclonal antibodies raised against PDI. All antibodies elicited a marked inhibition of sTSHR shedding. This confirmed that cell surface PDI is involved in the shedding of the TSH receptor ectodomain. The shed α subunit may be at the origin of circulating TSH receptor ectodomain detected in human blood.
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