THYROID-HORMONE ANALOGS POTENTIATE THE ANTIVIRAL ACTION OF INTERFERON-GAMMA BY 2 MECHANISMS

L-thyroxine (L-T-4) potentiates the antiviral activity of human interf eron-gamma (IFN-gamma) in HeLa cells. We have added thyroid hormone an d analogues to cells either 1) for 24 h pretreatment prior to 24 h of IFN-gamma (1.0 IU/ml), 2) for 24 h cotreatment with IFN-gamma, 3) for 4 h, after 20 h cell incubation with IFN-gamma, alone, or 4) for 24 h pretreatment and 24 h cotreatment with IFN-gamma. The antiviral effect of IFN-gamma was then assayed. L-T-4 potentiated the antiviral action of IFN-gamma by a reduction in virus yield of more than two logs, the equivalent of a more than 100-fold potentiation of the IFN's antivira l effect. 3,3',5-L-triiodothyronine (L-T-3) was as effective as L-T-4 when coincubated for 24 h with IFN-gamma but was less effective than L -T-4 when coincubated for only 4 h. D-T-4, D-T-3, 3,3',5-triiodothyroa cetic acid (triac), tetraiodothyroacetic acid (tetrac), and 3,5-diiodo thyronine (T-2) were inactive. When preincubated with L-T-4 for 24 h p rior to IFN-gamma treatment, tetrac blocked L-T-4 potentiation, but, w hen coincubated with L-T-4 for 4 h after 20 h IFN-gamma, tetrac did no t inhibit the L-T-4 effect. 3,3',5'-L-triiodothyronine (rT(3)) also po tentiated the antiviral action of IFN-gamma, but only in the preincuba tion model. Furthermore, the effects of rT(3) preincubation and L-T-3 coincubation were additive, resulting in 100-fold potentiation of the IFN-gamma effect. When F-T-4, L-T-3, or rT(3), plus cycloheximide (5 m u g/ml), was added to cells for 24 h and then removed prior to 24 h IF N-gamma exposure, the potentiating effect of the three iodothyronines was completely inhibited. In contrast, IFN-gamma potentiation by 4 h o f L-T-4 or L-T-3 coincubation was not inhibited by cycloheximide (25 m u g/ml). These studies demonstrate two mechanisms by which thyroid hor mone can potentiate IFN-gamma's effect: 1) a protein synthesis-depende nt mechanism evidenced by enhancement of IFN-gamma's antiviral action by L-T-4, L-T-3, or rT(3) preincubation, and inhibition of enhancement by tetrac and cycloheximide, and 2) a protein synthesis-independent ( posttranslational) mechanism, not inhibited by tetrac or cycloheximide , demonstrated by 4 h coincubation of L-T-4 or L-T-3, but not rT(3), w ith IFN-gamma. The protein synthesis-dependent pathway is responsive t o rT(3), a thyroid hormone analogue generally thought to have little e ffect on protein synthesis. A posttranslational mechanism by which the antiviral action of IFN-gamma can be regulated has not previously bee n described. (C) 1996 Wiley-Liss, Inc.