Milrinone mimics thyroid hormone potentiation of the antiviral activity of human interferon-γ in hela cells

Milrinone (Mil) is a bipyridine inotropic agent which has structural homologies with thyroid hormone and thyromimetic effects in several tissues. We have examined the action of Mil and amrinone (Am) in a system in which Lthyroxine (T₄) and 3,5,3'-triiodo-L-thyronine (T3) enhance the antiviral action of recombinant human interferon-y (IFN-y) in cultured cells. Mil or Am (107-10"5 M) were added to HeLa cells either for 24 h prior to 24 h of IFN-y, or together with IFN-y for 24 h. Antiviral effect was then assayed. Mil and Am alone had no antiviral effect. Mil (10⁷ M) enhanced IFN-ys action only when added prior to IFN-y whereas Mil (10⁵ M) also enhanced IFN-y action during coincubation with IFN-y. Am was ineffective. The effect of Mil was blocked by cycloheximidQ (25 ng/ml) when both agents were applied prior to IFN-y treatment. Mil action was not blocked by tetraiodothyroacetic acid (tetrac) or triiodothyroacetic acid (triac), in contrast to prior studies with T4 and these analogues in this assay. In studies of T₃-binding to the receptor TR, Mil did not displace T₃. In cells cotransfected with TRa or TR expression plasmid, Mil neither transactivated, nor blocked, T₃-mediated transcriptional activation. Thus, milrinone mimicked thyroid hormone's potentiation of IFN-ys antiviral effect in a protein synthesis-dependent manner, but the bipyridine's action was not mediated by the thyroid hormone receptor.