Inhibitions of protein kinases and protein phosphatases have opposite effects on thyrotropin-stimulated cAMP accumulation in human thyroid cells.

We investigated the effects of inhibitions of protein phosphatases and protein kinases on thyrotropin (TSH) stimulation of cAMP accumulation in human thyroid cells. Okadaic acid (OA) and calyculin-A (CL-A), two potent inhibitors of type-1 (PP-1) and type-2A (PP-2A) protein phosphatases, had a biphasic concentration-dependent response on cAMP formation. An inhibitory effect (41.3% and 47.2% inhibition with OA and CL-A) was first observed at 1 microM OA and 10 nM CL-A, followed by a reduction of this effect with OA (24% inhibition) or by a complete reversal of inhibition with CL-A, at 10-fold higher concentrations of both products. Addition of purified PP-1 and PP-2A to crude membranes from cells preincubated with OA, reversed OA-induced adenylyl cyclase inhibition, confirming that these protein phosphatases regulate TSH-mediated cAMP production. Levels of protein incorporation of 32P were higher with 10 microM OA than with 1 microM OA and did not correlate with the biphasic effect of OA on cAMP production. These results support a dual action of protein phosphorylation in the control of adenylyl cyclase activity stimulated by TSH. H-7, an inhibitor of nucleotide- and calcium/phospholipid-dependent protein kinase (PKC), increased by 197% the stimulation of cAMP accumulation by TSH in thyroid cells. Phorbol 12-myristate 13-acetate (PMA) counteracted the effect of H-7 on cAMP levels, which suggests that PKC is involved in the action of H-7. Moreover, KT5926, an inhibitor of calcium/calmodulin-dependent protein kinase II and myosin light chain kinase, increased basal cAMP levels rather than cAMP levels stimulated by TSH. In light of these results, we suggest that phosphorylation/dephosphorylation cycles regulate basal and TSH-stimulated adenylyl cyclase activities in human thyroid.