Thyrotrophin, forskolin and ionomycin increase cathepsin B mRNA concentrations in rat thyroid cells in culture.

In order to study the regulation of cathepsin B expression in the thyroid, cathepsin B mRNA concentrations were measured in rat thyroid cells (FRTL5) in culture. Northern blot analysis demonstrated that cathepsin B mRNA concentrations were increased in FRTL5 cells cultured for up to 6 days in TSH. The effect of TSH on cathepsin B mRNA concentrations was dose dependent over the range 25-150 mu units/ml. Cytoplasmic dot-blot analysis was used to characterize this effect further. The TSH-induced increase in cathepsin B mRNA concentrations (approximately fivefold over that in untreated cells) was partially mimicked by forskolin (approximately threefold) and ionomycin, while phorbol ester decreased cathepsin B mRNA concentrations. Similar changes were observed for thyroglobulin and actin mRNA concentrations. TSH had no effect on cathepsin B enzymatic activity or immunoreactive protein concentration. These results demonstrate (1) that cathepsin B expression in the thyroid is regulated in parallel with that of thyroglobulin and actin, and (2) that cyclic AMP- and Ca2+-dependent processes stimulate gene expression, while phorbol ester treatment inhibits gene expression in FRTL5 cells.