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 I mU/ml) for 1 to 3 days. In TSH-stimulated cells the desensitization of the catalyt ic subunit activity was accompanied by a simultaneous and parallel decrease of its immunoreactivity, The loss of catalytic subunit was rapid and reach ed its maximum after 1 day of culture. It is similar in the two subcellular compartments: cytosol and particulate extracts. Contrary to the observed l oss of the C subunit protein molecules in TSH-stimulated cells, the express ion of the CP subunit mRNA in these cells was increased fivefold compared t o controls, while no significant change was observed on the C alpha subunit mRNA. These results suggest that TSH controls the CP subunits of PKA at tw o levels: at the transcriptional level it increases CP mRNA expression, and at the translational or posttranslational level TSH decreases the amount a nd the activity of the Cp protein molecules. (C) 1999 Academic Press.