Indocyanine green rapidly inhibits triiodothyronine-induced gene transcription

Indocyanine green (ICG), a commonly used diagnostic drug, interferes with thyroid hormone transport into cells. We tested whether gene transcription triggered by exposing cells to triiodothyronine (T3) could be inhibited by ICG. First, HuH-7, MCF-7 and U2OS cells transiently expressing a T3-responsive luciferase gene (12DR4-luc2CP) and a thyroid hormone receptor (TRα or TRβ) were treated with 10 ng/mL T3 with or without 4 (a concentration achievable in human serum), 20 or 100 μM ICG for 3 h. Then, TRβDR4luc cells (U2OS cells stably expressing both 12DR4-luc2CP and TRβ) were treated with 10 ng/mL T3 for 48 h, with 4 or 20 μM ICG added simultaneously with T3 or either 24 or 45 h after T3. Finally, U2OS cells transiently expressing 12DR4-luc2CP and either TRα or TRβ were incubated for 3 h in serum from Graves’ disease patients before or after treatment, with or without 4 μM ICG. The following results were obtained: In TRα- or TRβ-overexpressing MCF-7 and U2OS (but not HuH-7) cells, 4 μM ICG inhibited T3-induced gene transcription. In TRβDR4luc cells, 4 and 20 μM ICG added at 24 or 45 h after T3 significantly lowered T3-induced luciferase activity. In the final experiment, cells placed in hyperthyroid serum, as compared to those in euthyroid serum taken from the same patient after treatment, exhibited higher luciferase activity, which was diminished by the addition of ICG. These results indicate that ICG may promptly reduce T3-induced transcription even when added subsequently to T3, suggesting its potential usefulness against thyrotoxic crisis.