POTENTIATION BY THYROXINE OF INTERFERON-GAMMA-INDUCED ANTIVIRAL STATEREQUIRES PKA AND PKC ACTIVITIES

Added to HeLa cells previously exposed to recombinant human interferon (IFN)-gamma for 20 h, thyroid hormone [L-thyroxine (T-4)] in physiolo gical concentrations potentiates the antiviral action of IFN-gamma by more than 100-fold in 4 h. We examined protein kinase activities for t heir contributions to the mechanism of this posttranslational effect o f thyroid hormone. Added concurrently with thyroid hormone, the protei n kinase C (PKC) inhibitor CGP-41251 (5 nM) blocked T-4 potentiation o f IFN-gamma action. Coincubated with CGP-41251, phorbol 12-myristate 1 3-acetate (PA) reversed the effect of the inhibitor on thyroid hormone action. U-73122 (10 nM), a phospholipase C inhibitor, also blocked ho rmone potentiation. KT-5720 (500 nM), a protein kinase A (PKA) inhibit or, completely inhibited the T-4 effect, whereas 8-bromoadenosine 3',5 '-cyclic monophosphate (8-BrcAMP) restored hormone action in the prese nce of KT-5720. In the absence of T-4, 8-BrcAMP and PMA, added togethe r to cells in the 4-h paradigm, fully reproduced hormone potentiation of the antiviral effect of IFN-gamma. Incubated individually with IFN- gamma-treated cells, the two agonists had no potentiating action. Thyr oid hormone apparently must activate both PKA and PKC in the nongenomi c pathway of IFN-gamma action to enhance antiviral activity in HeLa ce lls.