Protein synthesis-dependent potentiation by thyroxine of antiviral activity of interferon-γ

We have studied the prenuclear signal transduction pathway by which thyroid hormone potentiates the antiviral activity of human interferon-γ (IFN-γ) in HeLa cells, which are deficient in thyroid hormone receptor (TR). The action of thyroid hormone was compared with that of milrinone, which has structural homologies with thyroid hormone. L-Thyroxine (T4), 3,5,3'-L- triiodothyronine (T₃), and milrinone enhanced the antiviral activity of IFN- γ up to 100-fold; a potentiation blocked by cycloheximide. The 5'- deiodinase inhibitor 6-n-propyl-2-thiouracil did not block the T4 effect. 3,3',5,5'-Tetraiodothyroacetic acid prevented the effect of T₄ but not of milrinone. The effects of T₄ and milrinone were blocked by inhibitors of protein kinases C (PKC) and A (PKA) and restored by PKC and PKA agonists; only the effect of T was blocked by genistein, a tyrosine kinase inhibitor. In separate models, milrinone was shown not to interact with nuclear TR-β. T₄ potentiation of the antiviral activity of IFN-γ requires PKC, PKA, and tyrosine kinase activities but not traditional TR.