ACTIVATION OF CYCLIC-AMP-DEPENDENT KINASE IS REQUIRED BUT MAY NOT BE SUFFICIENT TO MIMIC CYCLIC-AMP-DEPENDENT DNA-SYNTHESIS AND THYROGLOBULIN EXPRESSION IN DOG THYROID-CELLS

Thyrotropin (TSH), via a cyclic AMP (cAMP)-dependent pathway, induces cytoplasmic retractions, proliferation, and differentiation expression in dog thyroid cells, The role of cAMP-dependent protein kinase (PKA) in the induction of these events was assessed by microinjection into living cells, Microinjection of the heat-stable inhibitor of PKA (PKI) inhibited the effects of TSH, demonstrating that activation of PKA wa s required in this process, Overexpression of the catalytic (C) subuni t of PKA brought about by microinjection of the expression plasmid pC alpha ev or of purified C subunit itself was sufficient to mimic the c AMP-dependent cytoplasmic changes and thyroperoxidase mRNA expression but not to induce DNA synthesis and thyroglobulin (Tg) expression, The cAMP-dependent morphological effect was not observed when C subunit w as coinjected with the regulatory subunit (RI or RII subunit) of PKA, To mimic the cAMP-induced PKA dissociation into free C and R subunits, the C subunit was coinjected with the regulation-deficient truncated RI subunit (RI Delta 1-95) or with wild-type RT or native RII subunits , followed by incubation with TSH at a concentration too low to stimul ate the cAMP-dependent events by itself, Although the cAMP-dependent m orphology changes were still observed, neither DNA synthesis nor Tg ex pression was stimulated in these cells, Taken together, these data sug gest that in addition to PRA activation, another cAMP-dependent mechan ism could exist and play an important role in the transduction of the cAMP signal in thyroid cells.