Reduced T3 deiodination by the human hepatoblastoma cell line HepG2 caused by deficient T3 sulfation

Type I deiodination of T3 sulfate occurs at a Vmax that is 30-fold higher as compared to T3, both in rat and in human liver homogenates. We now present data showing lack of T3 deiodination by a human liver derived hepatoblastoma cell line, HepG2, caused by deficient T3 sulfation. Cellular entry of T3 was assessed by its nuclear binding after whole cell incubation. In spite of the presence of type I deiodinase, as confirmed by T4 and rT3 deiodination in homogenates, no deiodination of T3 could be detected. Since HepG2 cell homogenates also deiodinated chemically synthesized T3 sulfate (T3S) and inhibition of type I deiodination by propylthiouracil (PTU) did not cause T3S accumulation in whole cell incubations, we conclude that (i) HepG2 cells show reduced T3 deiodination caused by deficient T3 sulfation, and (ii) sulfation of T3 is an obligatory step prior to hepatic deiodination.