Acute in vitro thyrotropin regulation of lysosomal enzyme activity in the thyroid follicular cell

Abstract

The acute effect of a physiological concentration (1 mU/1) of thyrotropin (TSH) on the activity of four lysosomal enzymes in the thyroid follicular lining cell has been studied by quantitative cytochemical techniques. N-acetyl-β-glucosarninidase (NAG) activity was increased by 14% after 10 min TSH stimulation and NAG and β-galactosidase activities were increased by 24% and 25% respectively ( P < 0.05) after 20 rain stimulation and by 40% and 45% ( P < 0.05) respectively after 30 min stimulation with TSH, indicating an early processing of these carbohydrate residues in thyroglobulin. Acid phosphatase activity, an acid hydrolase unrelated to the hydrolysis of thyroglobulin, was unchanged 30 min after TSH stimulation. Leucyl-β-naphthylaminidase (LNA) activity changed biphasically with peak activities of 7 and 25 min possibly representing an early fusion of endocytotic vesicles and lysosomes and later the release process of the thyroid hormones. The changes in LNA activity and thus membrane permeability were not reflected in the other enzyme activities studied. This may indicate that the TSH regulation of lysosomal enzyme activities could be independent to the endocytotic process, which is known to involve fusion of lysosomes and endocytotic vesicles. In conclusion we have demonstrated for the first time with physiological concentrations of TSH a specific acute regulation of some lysosomal enzyme activities which may be involved in thyroglobulin processing. Further, these effects may be independent of the changes in lysosomal membrane permeability due to formation of secondary lysosomes.