TSH control of PKA catalytic subunit activity in thyroid cell cultures

The protein expression and the enzyme activity of the catalytic subunit (C) of the cAMP-dependent protein kinases were studied in porcine thyroid cell primary cultures stimulated with two doses of TSH (0.1 mU/ml and 1 mU/ml) for 1 to 3 days. In TSH-stimulated cells the desensitization of the catalytic subunit activity was accompanied by a simultaneous and parallel decrease of its immunoreactivity. The loss of catalytic subunit was rapid and reached its maximum after 1 day of culture. It is similar in the two subcellular compartments: cytosol and particulate extracts. Contrary to the observed loss of the C subunit protein molecules in TSH-stimulated cells, the expression of the Cbeta subunit mRNA in these cells was increased fivefold compared to controls, while no significant change was observed on the Calpha subunit mRNA. These results suggest that TSH controls the Cbeta subunits of PKA at two levels: at the transcriptional level it increases Cbeta mRNA expression, and at the translational or posttranslational level TSH decreases the amount and the activity of the Cbeta protein molecules.